By William P. Diamond and David C. ~Oyler 

UNITED STATES DEPARTMENT OF THE INTERIOR 
cap- $&-- :% 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Donald Paul Hodel, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
Diamond, W. P. (William P.) 

Effects of stimulation treatments on coalbeds and surrounding 

strata 

[Report of ~nvestlgat~ons, 90831 

B~bl~ogrdphy p 45-46. 

Supt of Docs no 1 2R 23 9083 

1 Coalhed methane dra~nage 2 Bor~ng I Ovler I)avld C I1 T~tle Ill Ser~es Report of 

~nvestlgations [Un~ted States Bureau of Mlnesl 9083 

TN2B.U43 ITN3051 622s 1622'81 86-607907 
CONTENTS 

Page Page 

Abstract ........................................ 1 Upper Freeport Coalbed. Pennsylvania ........... 27 

Introduction .................................... 2 Lucerne No . 6 Mine. borehole RP-1 ............ 27 

Concepts and limitations of underground Lucerne No . 6 Mine. borehole RP-2 ............ 31 

observations ................................... 2 Lucerne No . 6 Mine. borehole RP-3 ............ 33 

Borehole history and results of underground Lower Kittanning Coalbed. West Virginia ........ 36 

observations ................................... 4 Kitt No . 1 Mine. borehole KE-2 ................ 36 . ............. (Each of the sections listed below on individual Kitt No 1 Mine. borehole DGBH-5 37 

. ................... boreholes includes subsections on borehole Illinois No 6 Coalbed. Illinois 39 

.............. completion. treatment implementation. and Inland Steel Mine. borehole 1-NE 39 

...................... underground observations.) Jawbone Coalbed. Virginia 40 

McClure No . 1 Mine. borehole DG-1A .......... 40 

Blue Creek Coalbed. Alabama ................... 4 Rock Canyon Coalbed. Utah 41 ..................... 

Oak Grove Mine. borehole TW-1 ............... 4 Soldier Canvon Mine . borehole SC- 1 ........... 41 

Oak Grove Mine. borehole TW-2 ............... 6 Summary of observations ........................ 42 

Oak Grove Mine. borehole TW-3 ............... 8 Conclusions ..................................... 45 

Oak &ove Mine. borehole TW-4 ............... 10 References ...................................... 45 

Oak Grove Mine. borehole TW-5 ............... 12 Appendix.-Acquisition and limitations of 

Oak Grove Mine. borehole DHM-5 ............. 15 stimulation treatment data ..................... 47 

Oak Grove Mine. borehole DHM-6 ............. 16 

Pittsburgh Coalbed. Pennsylvania ............... 18 

Vesta No . 5 Mine. borehole USBM-4 ........... 18 

Emerald Mine. borehole EM-5 ................. 20 

Emerald Mine. borehole EM-6 ................ 21 

Emerald Mine. borehole EM-7 ................. 22 

Emerald Mine. borehole EM-8 ................. 22 

Emerald Mine. borehole EM-1 1 ................ 26 

Cumberland Mine. borehole CNG-1034 ......... 26 

ILLUSTRATIONS 

1 . Relationship of mine entry orientation and interception of vertical fracture wings parallel to coal cleat ..... 3 

2 . Treatment record. borehole TW-1 ...................................................................... 4 

3 . Mine map. borehole TW.1. with fracture orientations .................................................... 5 

4 . Schematic cross section parallel to face cleat fractures at borehole TW-1 .................................. 5 

5 . Treatment record. borehole TW-2 ............................................................... 6 

6 . Mine map. borehole TW.2. with fracture orientations .................................................... 7 

7 . Cross section of coal face at borehole TW.2. with fracture orientations .................................... 7 

8 . Comparison of gamma density logs before and after jet slotting. borehole TW-2 ........................... 8 

9 . Treatment record. borehole TW-3 ............................. ....................................... 9 

10 . Mine map. boreholes TW-3 and TW.4. with fracture orientations ......................................... 10 

11 . Treatment record. borehole TW-4 ...................................................................... 11 

12 . Stratigraphic section and near-wellbore underground observations in the Mary Lee-Blue Creek Coalbed 

intervals. borehole TW-5 ............................................................................. 12 

13 . Treatment record. borehole TW-5 .......................... ... .. .................................... 13 

14 . Mine map. borehole TW.5. with fracture orientations .................................................... 14 

15 . Mine map. boreholes DHM-5 and DHM.6. with fracture orientations ..................................... 16 

16 . Treatment record. borehole DHM-6 ..................................................................... 17 

17 . Treatment record. borehole USBM-4 ................................................................... 18 

18 . Mine map. borehole USBM.4. with fracture orientations ................................................. 19 

19 . Cross section of face cleat fracture in entry at location 1 near borehole USBM-4 .......................... 19 

20 . Cross section of butt cleat fractures on rib at location 2 near borehole USBM-4 ........................... 19 

21 . Map of Emerald Mine area with locations of boreholes EM.5. EM.6. EM.7. EM.8. and EM-11 .............. 20 

22 . Mine map. borehole EM.5. with fracture orientations .................................................... 20 

23 . Cross section of fractures on rib closest to borehole EM-5 ................................................ 20 

24 . Treatment record. borehole EM-6 ...................................................................... 21 

25 . Mine map. borehole EM.6. with fracture orientations ................................................... 22 

26 . Cross section of fractures on rib west of borehole EM-6 .................................................. 22 

27 . Treatment record. borehole EM-8 ...................................................................... 23 

28 . Mine map. borehole EM.8. with fracture orientations .................................................... 24 

29 . Cross section of fractures on rib at location 1 near borehole EM-8 ........................................ 25 

30 . Cross section of fractures on rib at location 2 near borehole EM-8 ........................................ 25 

31 . Cross section of fractures on rib at location 3 near borehole EM-8 ........................................ 25 

32 . Cross section of fracture on rib at location 4 near borehole EM-8 ......................................... 25 
33 . Mine map. borehole EM.11. with fracture orientations .................................................. 

34 . Cross section of fractures on rib near borehole EM-11 ................................................... 

35 . MirLe map. borehole CNG.1034. with fracture orientations ............................................... 

36 . Treatment record. borehole RP-1 ....................................................................... 

37 . Mine map. boreholes RP-1 through RP.3. with fracture orientations ...................................... 

38 . Cross section of fractures at location 25 on the southeast rib near borehole RP-1 .......................... 

39 . Mine map. boreholes RP-1 through RP.3. with horizontal distribution of paint on roof ..................... 

40 . Treatment record. borehole RP-2 ....................................................................... 

41 . Cross section of fractures at location 7 on the northwest rib near borehole RP-2 ........................... 

42 . Mine map. boreholes RP-1 through RP.3. with horizontal distribution of paint and sand on top of shale 

parting .............................................................................................. 

43 . Treatment record. borehole RP-3 ...................................................................... 

44 . Cross section of fractures at location 15 on southeast rib and location 16 on northwest rib near borehole RP-3 .... 

45 . Treatment record. borehole KE-2 ....................................................................... 

46 . Mine map. borehole KE.2. with fracture orientation ..................................................... 

47 . Treatment record. borehole DGBH-5 ................................................................... 

48 . Mine map. borehoie DGBH.5. with fracture orientations ................................................. 

49 . Cross section of fractures on east and west ribs near borehole DGBH-5 ................................... 

50 . Treatment record. borehole 1-NE ....................................................................... 

51 . Mine map. borehole 1.NE. with fracture orientations .................................................... 

52 . Treatment record. borehole SC-1 ....................................................................... 

53 . Mine map. borehole SC-1 ............................................................................. 

Page 

26 

26 

27 

28 

29 

30 

31 

32 

33 

TABLES 

1 . Specifications of gelled stimulation fluid. borehole TW-2 ................................................. 6 

2 . Summary of treatment chronology. borehole TW-5 ..................................................... 13 

3 . Summary of fracture observations (paint-coated surfaces). borehole TW-5 ................................. 14 

4 . Summary of treatment chronology. borehole DHM-6 .................................................... 17 

5 . Summary of treatment chronology. borehole USBM-4 ................................................. 18 

6 . Summary of treatment chronology. borehole CNG-1034 .................................................. 27 

7 . Summary of treatment chronology. boreholes RP-1 through RP-3 ........................................ 29 

8 . Orientation and description of vertical fractures. boreholes RP-1 through RP-3 ........................... 30 

9 . Summary of treatment chronology. borehole 1-NE ...................................................... 40 

10 . Summary of stimulation treatment data ................................................................ 43 

11 . Summary of underground observations for 22 intercepted stimulation treatments ......................... 43 

I bbl 

bbllmin 

ft 

ft" 

ft3/d 

gal 

h 

in 

lb 

1bf/in2 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

barrel 

barrel per minute 

foot 

cubic foot 

cubic foot per day 

gallon 

hour 

inch 

pound 

pound (force) per square inch 

1bf/in2(abs) 

1bf/in2(ga) 

1 blgal 

Mft3/d 

min 

MMft3 

pet 

rpm 

std ft3/min 

Yr 

pound (force) per square inch. absolute 

pound (force) per square inch. gauge 

pound (mass) per gallon 

thousand cubic feet per day 

minute 

million cubic feet 

percent 

revolution per minute 

standard cubic foot per minute 

year 
these occurrences have been interpreted to be penetrations into preexisting joints. No 

roof falls or adverse mining conditions were encountered that could be attributed to 

the stimulations. 

LMechanical engineer. 

Pittsburgh Research Center. Bureau of Mines, Pittsburgh. PA 
INTRODUCTION 

The Bureau of Mines has developed several techniques, including the use of horizontal and vertical 

boreholes, to remove gas from coalbeds in advance of 

mining. Horizontal boreholes drilled from underground 

workings as part of the mining cycle have been shown to 

be very effective in providing short-term, immediate relief 

from high methane emissions (1 -6).This technique has 

been widely accepted in the coal mining industry, but it 

does require close coordination to integrate the drilling 

and subsequent gas drainage and disposal into the 

projected mine development plan. 

Vertical boreholes can be placed several years in 

advance of mining to predrain gas from coalbeds over 

relatively large areas ( 7-1 1 ). The vertical borehole 

technique has the additional advantage over horizontal 

boreholes of allowing work to be performed on the surface 

instead of in the more restrictive underground environment. However, except for the relatively large-scale 

vertical borehole programs for both mine safety and for 

commercial production in the Black Warrior Basin of 

Alabama (7, 9, ll), the technique has been underutilized. 

The primary reason for this seems to be a combination of 

the economic conditions in the coal industry, legal 

questions as to the ownership of coalbed gas, and the 

potential of roof damage from the stimulation treatments. 

The question of potential roof damage is the subject of this 

report. 

A total of 22 Government-sponsored stimulation 

treatments in coalbeds have been intercepted by mining. 

Twenty-one of these interceptions were in the eastern 

United States (10 in Pennsylvania, 7 in Alabama, 2 in 

West Virginia, and 1 each in Illinois and Virginia); only 

one in the western United States (Utah) was available for 

investigation. These underground interceptions have 

provided a unique opportunity to observe directly the 

actual effects of the stimulation treatments on coalbeds 

and surrounding strata. This report details observations 

previously presented in various forms by the Government 

and its contractors as well as information not heretofore 

reported, including the results of additional interceptions. 

CONCEPTS AND LIMITATIONS OF UNDERGROUND OBSERVATIONS 

Underground evaluation of areas surrounding boreholes that have been stimulated to increase gas production from coalbeds is an important step in obtaining 

information for the development of efficient stimulation 

treatments that do not adversely affect mining. By 

directly observing the effects of a particular stimulation 

design on a coalbed and surrounding strata and evaluating the observations in conjunction with thorough 

geologic characterization, valuable information can be 

obtained, especially early in the development stages of a 

methane drainage program. It is important to note that 

what is seen underground is dependent upon the area 

exposed by mining and the timing of observations in 

relation to the advance of entries. Obviously once a 

volume of coal is mined, anything contained in that coal is 

forever lost for direct examination. 

Geologic characterization of available underground 

workings in the area to be drilled and stimulated is highly 

recommended before stimulation. This preliminary characterization aids in interpreting the underground 

observations and provides information for placement of 

the borehole and design of the treatment. These geologic 

studies should be continued as the entries are advanced 

toward the stimulated borehole and through the actual 

interception and advance beyond the borehole. The most 

complete characterization of this type was conducted in 

conjunction with Bureau of Mines contract 50333908, the 

final report (12) of which can serve as a model for future 

work. 

The results of previously evaluated treatment minethroughs indicate that it is quite likely that the 

predominant fracture wings will generally follow or 

penetrate the face cleat in the coalbed. The face cleat 

orientation can be determined by measuring cleat 

orientations underground if mine workings are available, 

or it might be inferred either from orientations in coalbeds 

31talic numbers in parentheses refer to items in the list of references at the 

end of this report. 

exposed at the surface or from analysis of surface rock 

joints (13-14). The identification, characterization, and 

orientation measurement of roof joints is also important 

since it appears that many roof penetrations have actually 

been invasions into preexisting roof joints. 

The cleat and roof joint orientation data can be used 

in placing the location of boreholes in relation to the mine 

development plan. This in turn can affect what is seen 

underground and possibly the seriousness of any penetration of existing roof joints. A borehole to be evaluated 

underground could be drilled so that it would be located in 

an entry to be intercepted by mining, allowing direct 

observation of the borehole, which would potentially 

provide the maximum information. Alternatively it could 

be placed in a pillar where less information may be 

gained, but where any roof penetration (which might or 

might not result in roof instability) near the wellbore 

would potentially not extend to the developed entries 

around the pillar. 

If a borehole is to be mined through, it may also be 

possible to place it so that the probable face cleat 

orientation for an induced fracture would be parallel or 

perpendicular to the long axis of the entry (fig. 1). A 

hydraulically induced fracture in the face cleat direction 

and oriented parallel (or nearly parallel) to the long axis 

of the entry would be encountered "head-on" by the 

advancing entry (borehole 1, figure 1). This would provide 

an opportunity to perhaps observe the entire length of the 

fracture from its tip back to the borehole, if it was possible 

to observe the advancing face continuously during 

mining. However, once mining of the entry was completed, the fracture would have been consumed and would 

no longer be available for evaluation. 

Butt cleat induced fractures, which are commonly 

shorter and less extensive than those in the face cleat 

orientation, would perhaps be observable on the ribs 

adjacent to the borehole. Complications could occur if roof 

joints (or other linear geologic discontinuities) are present 

in the entry and if they are parallel to the orientation of 
LEGEND 

- Vertical face cleat fracture 

-- Vertical butt cleat fracture 

Figure 1 .-Relationship of mine entry orientation and interception of vertical fracture wings parallel to coal cleat. 

the face cleat, which commonly is the case. If the 

stimulation treatment penetrated into roof joints, resulting in roof instability, the hazard would potentially be 

exposed over a greater length in the entry if the joints 

paralleled the long axis of the entry than if the roof joint 

only extended across the width of the entry. 

A hydraulic fracture trending in the face cleat 

direction perpendicular to the long axis of an entry would 

presumably be contained in the ribs adjacent to the 

borehole (borehole 2, figure 1) and would be available for 

observation as long as the pillar remained. Butt cleat 

fractures, if present, would have been intercepted head on. 

Since butt cleat fractures are often relatively short, they 

would often be completely mined by a single mining cut 

and never observed. 

When the borehole is located in a pillar or other 

unmined area, there is always some uncertainty as to the 

exact location of the borehole. Knowledge of the exact 

location of the borehole is important in interpreting the 

underground observations, in terms of determining the 

length of the fracture wings and the presumed pathways 

(amount of stairstepping along face and butt cleats away 

from the borehole) of the treatment fluids through the 

coalbed. The uncertainty as to the borehole location 

results from the propensity for a borehole to deviate from 

vertical when drilled, and from inaccuracies in surveys of 

surface location, borehole deviation, and mine workings 

used to determine where the borehole should be underground. 

Even with careful preplanning of the borehole 

location in relation to the expected underground location, 

it may not end up where it is expected. This was observed 

in the case of borehole SC-1 at the Soldier Canyon Mine in 

Utah. The bottom hole location of this borehole (as 

discussed in a later section of this report) was determined 

from a deviation survey to have migrated 35 ft laterally 

from the surface location. This should have placed it near 

the middle of an entry according to the mine plan. A 

thorough inspection of this area after mining failed to 

reveal any trace of the borehole or of the sand proppant 

and stimulation fluids, suggesting that one or more of the 

surveys must have been in error. 

A significant aid in locating and describing the 

pathways of fracturing fluid movements underground is 

the use of fluorescent paint in the treatment fluids. By 

using a permissible ultraviolet light underground, the 

fluorescent paints can usually be readily identified and 

the extent of their pathways mapped. In general, when the 

fluorescent paint has been used, it has been possible to 

trace the movement of the fracturing fluids significantly 

beyond the extent of sand-propped fractures. Many 

different paint colors have been used, with the best results 

observed from orange, yellow, green, and red paint. 

It should be noted that coalbeds sometimes contain 

naturally occurring fluorescent materials that may 

confuse the underground observations if paint similar in 

color to the materials are used. Red and pink fluorescing 

minerals were observed in the Upper Freeport Coalbed at 

the Lucerne No. 6 Mine in Pennsylvania, where red paint 

was used along with several other colors. Green fluorescing resins were observed in the Rock Canyon Coalbed at 

the Soldier Canyon Mine in Utah where green was used as 

the only paint color in the treatment. Blue fluorescent 

paint can be a particular problem, as was seen at the 

Lucerne No. 6 Mine, where hydraulic fluids used in the 

mining machinery also fluoresced blue making it difficult 

to differentiate the two. 

During the stimulations at the Lucerne No. 6 Mine, 

different paint colors were injected at various stages of 

the treatment in an attempt to differentiate the effects of 

the different stages underground. The results of this 

experiment were somewhat varied because of the tendency for the last paint injected along a particular fracture to 

cover and mask any paint injected earlier. 

Care must be taken in comparing underground 

observations of fluorescing paint tagged treatments to 

treatments where paint was not added. This is especially 

important in comparing maximum fracture length. In 5 of 

the 22 mined through treatments, fluorescent paints were 

used along with sand and in each case where any fractures 

were seen (four of five), it was possible to map the fluid 

movements beyond the presence of sand proppant. Paint 

was quite often observed on or in both vertical and 

horizontal fractures. Although it was obvious that 

treatment fluids had flowed through these fractures, it 

was not possible to determine if they previously existed or 

if they had been created by the treatment. Except in a few 

cases most fractures follow paths that look like cleat and 

bedding planes, and the fractures may have existed before 

the treatment began. 

Experience with wells where fluorescing paint has 

been added suggests that, in shallow stimulated coalbeds, 

the major sand-filled fractures in the face and butt cleat 

directions are paralleled by networks of planes along 

which fluids, but not sand, have traveled. In addition, the 

fluids may also stairstep short distances along face and 

butt cleats to form a general fracture trend that does not 

parallel the face or butt cleat direction. Multiple 

horizontal planes of fluid movement have been identified 

with the use of fluorescent paint. As many as four 

horizontal planes have been observed in one case, 

although it is more common to see only one or perhaps two 

planes. 

Vertical fractures may also lie over and connect to 

horizontal planes. These occurrences seem to be of limited 

extent, indicating that they may be fractures actually 

created by the stimulation process and therefore require 

too much creation energy to cover large areas when 

preexisting vertical and horizontal paths for fluid movement already exist. 
BOREHOLE HISTORY AND RESULTS OF UNDERGROUND OBSERVATIONS 

The available borehole completion and stimulation included in the "Summary of Observations" section of this 

treatment data for each of the 22 mined-through report. It should be noted that the coalbed mined and 

stimulation treatments are presented in the following generally completed for gas production at the Oak Grove 

sections along with the results of the underground Mine is termed the Blue Creek Coalbed in this report, 

observations. Data are grouped together for each coalbed which is common usage in the mining district. Stratitreated to aid the reader in locating and interpreting the graphically this coalbed is the bottom bench of the Mary 

results. Summary tables of the most pertinent data are Lee Coalbed (15-1 7). 

BLUE CREEK COALBED, ALABAMA 

OAK GROVE MINE, BOREHOLE TW-1 (18-19) 

Borehole Completion 

Borehole TW-1 was drilled 11 in. in diameter, using 

foam, to a depth of 1,115.25 ft and cased with S5/i-in-OD 

casing to a depth slightly less than 1,115 ft. The bottom of 

casing was placed approximately 2 to 2.5 ft into the top of 

the target Blue Creek Cozlbed, which was later reported 

to be 6 ft thick. The casing was cemented in place with an 

expanding type 16-lb/gal cement containing 18 pct NaC1. 

Approximate bottom hole pressure during cementing was 

1,000 lbf/in2(ga) 2 100 lbf/in2. After the casing was 

cemented in place, a 77/8-in-diameter tricone roller bit was 

used to drill out the float shoe on the bottom of casing and 

to extend the hole to a depth of 1,133.25 ft, approximately 

15 ft below the base of the Blue Creek Coalbed. An 8- to 

10-lb/gal drilling mud was used for this short drilling 

interval, and the hole was circulated with clear water for 

several hours after the completion of drilling to clean the 

hole. At the time of completion, borehole TW-1 was 

approximately 520 ft ahead of mining. 

Treatment Implementation (November 23, 1975) 

The stimulation treatment record for borehole TW-1 

is shown in figure 2. A discussion of the acquisition and 

limitations of stimulation treatment data is given in the 

appendix. The treatment utilized gelled water and sand 

proppant. The stimulation designed was generated with 

computer-based techniques that used formation parameters to estimate the character of the Blue Creek Coalbed. 

A treatment was selected that would theoretically produce 

two vertical fracture wings, each extending 423 ft into the 

coalbed with a sand proppant wing length of 143 ft. 

The borehole was stimulated through 2%-indiameter tubing set at 1,084 ft with a packer installed at 

the top of the wellhead to prevent the stimulation fluid 

from flowing to the surface in the annulus between the 

casing and tubing. No prestimulation cleanup or slotting 

of the coal face with a jetting tool was reported. The 

treatment was initiated with a 2,500-gal gelled water pad 

injected at rates averaging about 2 bbllmin. Surface 

treatment pressures quickly increased to about 1,400 

lbf/in2(ga), leveled off (indicating leakoff into the formation), then gradually increased to 1,775 lbf/in2(ga) where 

formation break occurred with an accompanying sharp 

pressure decline to 600 lbf/in2(ga). 

After the formation break, the injection rate was 

increased to 10.5 bbllmin. The surface pressure increased 

to nearly 1,000 lbf/in2(ga) and then gradualiy decreased to 

about 600 lbf/in2(ga) throughout the remainder of the pad 

injection. An additional 2,500 gal of gelled water 

I 1 I I 1 I I 

Start gel pad rFormation breakdown Shutdown C - a Start clear water flush I 

'I 

I Start sand, I .O lb/gal 

C ,I I:: ................. /@ ................... I :: ........................... 1 11 

I: :. I l$! - I 

I L KEY 

- I c --- Surface pressure - I i ....... I Pumping rate,water i 

I 1 k-.-- (and sand) - I 1 \ -- 

I 

- I .. -----------________ i - I ... .* . ..... 3 ' ;-; :I - I*. - I..' ......: . r* 

TIME, min 

Figure 2.-Treatment record, borehole TW-1 (18). 
coabed was 

to 55 wde, wtte wdest pat ote 

fracture occurring in the center of the coalbed where the 

coal was "softest." The thinner part of the fracture was 

near the roof and floor where the coal was "harder." The 

maximum lateral extent of this fracture was approximately 5 ft near the top of the coalbed to the northeast of the 

borehole (fig. 4). The lateral extent of the fracture 

diminished from the top of the coalbed to the bottom. The 

extent and geometry of this fracture to the southwest was 

not reported. Small patches of sand and gel were found in 

the fracture, indicating partial penetration by the 

stimulation treatment. The presence of "unbroken" gel in 

the fractures observed underground 7 months after the 

treatment is an indication that the viscosity reducing 

agent (breaker) did not perform as designed to reduce the 

gel viscosity within a few hours after the treatment was 

completed. 

A second fracture (or upward extension of the 

mud-filled fracture) filled with the cement used to set the 

casing, was observed in the roof and a short distance into 

the top of the coalbed. The cement-filled fracture in the 

roof was traced 133 ft to the northeast and 48 ft to the 

southwest of the borehole and was generally '/x in wide. 

The fracture penetrated 13 in into the coalbed on the rib 

24 ft northeast of the borehole and 7 in into the coalbed on 

the rib 48 ft southwest of the borehole. No penetration of 

the cement-filled roof fracture into the coalbed was 

reported at its maximum extent to the northeast. No 

evidence of horizontal fractures or adverse effects from the 

roof penetration by the vertical fractures was reported. 

It was speculated by Lambert (18) that the bulk of the 

treatment was placed into the floor rock exposed in the 

Scale, f t 7 Uncertain orientation 

Flgure 3.-Mine map, borehole TW-1, with fracture orientatlons (18-19). 

EL Casing 

+ L/// ..................... Floor 

LEGEND 

$(;') Mud-filled frocture 

x x Cement-filled fracture 

0 Sand and gel patch 

? Character of fracture unknown 

Flgure 4.-Schematic cross sectlon parallel to face cleat 

fractures at borehole TW-1 (18). 
OAK GROVE MINE, BOREHOLE TW-2 (20) design selected would theoretically produce two vertical 

sand-filled fracture wings, 1 in wide and 5 ft high, each 

Borehole Completion extending approximately 150 ft out from the borehole. The 

Borehole TW-2 was drilled 6% in. in diameter to a 

depth of 857 ft, approximately 235 ft above the target Blue 

Creek Coalbed. The remainder of the hole, to the total 

depth of 1,150.6 ft, was drilled 6% in. in diameter using an 

air-foam drilling fluid. The borehole was cased to its total 

depth with 4VPin-OD casing and the bottom 500 ft was 

cemented with 13.8-lblgal cement. The tcp of the Blue 

Creek Coalbed was at a depth of 1,093.4 ft and the coalbed 

was 5.0 ft thick. At the time of completion, borehole TW-2 

was located 600 ft in advance of mining. 

Treatment Implementation (October 24, 1976) 

Prestimulation strata characterization tests included 

natural gamma ray and gamma density wireline geophysical logs. Lambert (20) reported several interesting 

observations related to the use of the density logs, 

including the identification of the location of the slots cut 

into the casing and correlation of density changes in the 

coalbed to the type of fractures observed underground. 

These observations are summarized in the "Underground 

Observations" section of this discussion. 

A jetting tool was used to cut slots through the casing 

at the target coalbed. The intent was to cut four vertical 

slots 90" apart, from the base of the coalbed to within 1 ft 

of the top. The tool was positioned using wireline 

geophysical logging tools. 

The stimulation treatment record for borehole TW-2 

is shown in figure 5. The treatment utilized gelled water 

as specified in table 1 and sand proppant. The stimulation 

Table 1 .-Specifications of gelled stimulation fluid, borehole 

TW-2 (20) 

Cone, per 10" 

Base flu~d gal H,O 

Surfactant gal 3 

Fluld loss add~t~ve Ib 50 

Gell~ng agent Ib 66 7 

Breaker Ib 2 

Complexer gal 0 4 

actual treatment was characterized by high injection 

pressures that averaged 2,400 lbflin2(ga) at the surface. 

Maximum pressure recorded was 2,500 lbflinyga). No 

obvious formation breakdown was observed by 1,ambert 

(20). An instantaneous shut-in pressure was recorded at 

2,200 lbflinyga) surface pressure. Fluid injection rates 

averaged approximately 8 bbllmin. Sand concentrations 

were generally 2 lblgal, but were briefly increased to 4 

lblgal prior to completing injection of the planned sand 

weight. A total of 3,500 gal of gelled water and 4,000 lb of 

sand proppant (1,000 lb of 20140-mesh, 3,000 lb of 

10120-mesh) were used in the treatment. 

During uninterrupted water pumping periods, gas 

production averaged about 15,000 ft'ld with maximum 

temporary rates of 80,000 ft.'/d immediately after servicing the downhole water pump. A complete history of the 

production rates and problems can be found in the paper 

by Lambert (20). 

Underground Observations 

Borehole TW-2 was mined through on February 15. 

1977, approximately 4 months after stimulation. The area 

of the borehole intercept is shown in map view on figure 6. 

Figure 7 is a cross-section drawing of the face advanced 

approximately 4 ft beyond the borehole showing the 

induced fractures and the exposed well casing. Observations of the cased borehole revealed a poor cement job, 

with only about half of the annular space between the 

casing and the formation filled with cement in the roof and 

floor. Sand andlor gel filled the remaining portion of the 

annular space. No cement was found adhering to the 

casing within the coalbed interval. The only fully 

penetrating slots through the casing were found in the 

floor rock interval below the target coalbed. The casing 

had expanded outward in this interval where it was 

pierced by four roughly diamond-shaped slots nearly 5Ih 

in long and 2 to 2% in wide. The tops of the slots were 

I I 1 I 1 I 1 I I I I I 1 I I I 1 - 

Start water pad 

1: I - 

Sand,2 lb/gal - 

Sand,4 I b/gal T r - 

Stop sand - 

//-- 

r Shutdown - 

r------ \ 

- 

i- - 

KEY -------_ - 

- I --- Surface pressure 

I . . . . . . . Pumping rate, water - 

-A (and sand) - 

..**** ...*.. .. .. . .... .... L. I I , 0. ..* 1 I I 1 I I 1 0 

0 4 8 I2 16 20 24 28 32 36 

TIME, min 

Flgure 5.-Treatment record, borehole TW-2. (Modlfled from Lambert (20)) 
about 4 in below the base of the coalbed and were oriented 

N 60" E and N 15" W. In addition to the slots, five holes 

(0.3-in maximum diameter) were located in an interval 5 

to 9 in above the slots. 

Lambert (20) reported that before the start of 

production the presence of the slots only in the floor rock 

below the target coalbed could have been revealed by a 

comparison of density logs from before and after slotting 

operations (fig. 8). The logs run after the slotting show 

a significant density decrease along the casing interval 

identified by underground observation to have been 

successfully slotted. 

Factors reported to have possibly contributed to the 

poor slotting included insufficient time allowed to cut 

through the casing; slight rotation or twisting of the tool 

Borehole 

Cleat 

J Coal 

I LEGEND 

Vertical fracture, 

sand filled 

0 Unmined area 

5 10 

Scale, f t I OFigure 6.-Mine map, borehoie TW-2, with fracture orientations. (Modified from Lambert (20)) 

as it was lowered through the prescribed interval; and the 

very soft, friable nature of the coal, which may have 

caused absorption of much of the cutting energy. 

Underground observations supporting these hypotheses 

include the presence of vertically oriented grooves on the 

inner surface of the casing opposite the coalbed interval 

where the casing had not been completely cut through, 

and location of the successfully cut slots and perforations 

adjacent only to the "hard" floor rock and "hard" lower 

portion of the coalbed. 

The location of the slots in the hard floor rock below 

the target coalbed did not prevent the stimulation fluids 

from entering the coalbed, but may have contributed to 

the high initial treatment pressures. The high pressures 

throughout the treatment were attributed by Lambert 

(20) to the high viscosity of the treating fluid and to the 

flow resistance created by the small openings in the 

casing. 

Three well-defined vertical sand-filled fractures were 

present in the hard lower coal interval shown in figure 7. 

The fracture on the right side of the face (location 1, figure 

7) was 4% in wide near the floor. To date this is the widest 

sand-filled coalbed fracture observed underground. The 

fracture narrowed significantly at the interface between 

the lower hard coal and the upper soft friable coal, 

inclined and eventually became horizontal near the top of 

the coalbed. The other two vertical fractures in the lower 

coal section (locations 2 and 3, figure 7) inclined and 

became horizontal at or near the hard-soft coal interface. 

Each of these fractures terminated at the base of the 

coalbed and did not penetrate the roof rock. 

The vertical portions of the three sand-filled fractures 

were oriented as shown on figure 6, and, according to 

Lambert (201, "followed local fracture planes in the coal." 

The fractures to the north of the borehole (locations 2 and 

Shale, dark gray, hard 

I/2- in max width Inclined f facture 

Coal,f r ia ble, 

I-in max width 

- Vertical fracture 

jllL Locations of small 

11 perforations in casing 

L.-4-1S---%5-- -- 

Figure 7.-Cross section of coal face at borehole TW-2, with fracture orientations. (Modified from Lambert (20)) 
- Density increase 

Figure 8.-Comparison of gamma density logs before and after jet slotting, borehole TW-2. (Modified from Lambert (20)) 

3, figure 6) were oriented generally in the face cleat 

direction (N 70" El, and those to the south (location 1, 

figure 6) appeared to be developed along butt cleats (N 23" 

W). 

The two primary fractures observed in the more 

friable upper section of the coalbed appeared to have 

intercepted and propagated along inclined shear planes in 

the coal. No penetration of the roof strata was observed. 

In addition to the common orthogonal vertical cleat 

system found in most coalbeds, the coal section at the Oak 

Grove Mine and other mines in the area displays 

secondary vertical and inclined fracture systems that 

intersect at various angles and with a variety of 

orientations (16). These complicated fracture systems are 

the result of the complex geological history of the area, 

and they can apparently influence the character of the 

induced fractures. Lambert (20) pointed out that the 

vertical fractures were contained in the lower, hard, 

distinctly cleated coal zone (correlated to the denser 

interval on the density log, figure 8) and that horizontal 

and inclined fractures were contained in the upper, 

friable, highly sheared portion of the coalbed (correlatable 

to less dense interval on the density log). 

Examination of the entries developed around borehole TW-2 revealed only one other exposure of sand-filled 

vertical fractures (location 4, figure 6) to the northeast of 

the borehole. The maximum lateral extent of vertical 

sand-filled fractures was about 16 ft from the borehole. 

Horizontal sand-filled fractures extended a maximum of 8 

ft from the borehole. It should be pointed out that the 

maximum 16 ft of observed lateral propagation of the 

vertical fractures is far short of the designed 150-ft 

sand-filled fracture length. The gelled fluids used in the 

treatment included an enzyme-type breaker to reduce the 

fluid viscosity within a few days, but the presence of 

unbroken gel around the casing as observed underground 

4 months after the treatment, as previously described in 

the section on borehole TW-1, indicates that the breaker 

did not perform as intended. If unbroken gel was still 

present in the induced fractures, it is quite likely that 

permeability and gas production would be reduced. 

OAK GROVE MINE, BOREHOLE TW-3 (21) 

Borehole Completion 

Borehole TW-3 was drilled 8 in. in diameter to a depth 

of 1,001.2 ft, approximately 73 ft above the top of the 

prime target of the Blue Creek Coalbed. The remainder of 

the hole was drilled 77/~ in. in diameter, to the total depth 

of 1,113.5 ft, using foam as the drilling fluid. The borehole 

was drilled to a depth 30 ft below the base of the 5-ft-thick 

Blue Creek Coalbed. The borehole was cased with 

4'12-in-OD casing to a depth of 1,066 ft, approximately 2 ft 

into the 3-ft-thick Mary Lee Coalbed. Approximately 7 ft 

of rock separated the Mary Lee and Blue Creek Coalbeds. 

The casing was cemented in place using a cement 

packer shoe, casing centralizers, and cement baskets in an 

effort to keep the cement from reaching the Blue Creek 

Coalbed in the open hole below the bottom of casing. The 

risk associated with this procedure was of cement leakage 

around the packer shoe and the potential of formation 

damage in the coalbed. A latex-based, low-fluid-loss class 

H cement with a 15.3-lblgal weight was used in borehole 

TW-3. At the time of completion, the borehole was 

approximately 590 ft ahead of mining. 
several shale partings. The exact depth of the Mary Lee 

Coalbed in the borehole was determined through the use 

of wireline geophysical logs made prior to perforating the 

casing with shaped explosive charges. 

The treatment was conducted through tubing using 

5,040 gal of 75-pct-quality nitrogen-generated foam, 3,350 

Ib of 801100-mesh sand, and 3,550 Ib of 20/40-mesh size 

glass beads. The glass beads were used to aid in 

identifying any penetration of the stimulation treatment 

801100-mesh sand and the sand in the roof fracture was 

predominantly 20140-mesh size. The glass beads used to 

stimulate the overlying Mary Lee Coalbed were not found 

in the Blue Creek Coalbed. Failure to contain the casing 

cement above the Blue Creek Coalbed was evidenced by 

the presence of cement in the same vertical fracture (coal 

and root) as the sand proppant and as far as 80 ft from the 

borehole to the southwest. The orientation of the vertical 

sand-filled fractures in the coalbed and overlying strata 

I I I I I 

Shutdown for IS! P Shut down Nhstart water flush ShutStart foam pad, 10 bbl /min 

- r 

down 

Start 80/100-mesh sand 

Start 20140-mesh sand 1 

/--~-c-- 

I 7 

I 

KEY 

\ \ 

--- Surface pressure 

I I I I I 

TIME, min 

Figure 9.-Treatment record, borehole TW-3 (18). 
LEGEND - 

- Vert~cal fracture in r~b only, sand 0 100 200 

and cement filled I 1 1 

-..- Vertical fracture in roof and rib. Scale, ft 

sand and cement filled 

I.'I Roof rolls 

Figure 10.-Mine map, boreholes TW-3 and TW-4, with 

fracture orientations. (Modified from Lambert (21)) 

was N 67" E. Compilation of orientation measurements of 

naturally occurring roof joints (total of 57) in the vicinity 

of the borehole TW-3 (and the nearby borehole TW-4, to be 

discussed in the next section) indicated an average roof 

joint orientation of N 66" E. This orientation is essentially 

the same as that observed for the sand-filled roof fracture 

near borehole TW-3, and may be an indication that the 

sand-filled roof fracture was a preexisting roof joint 

entered by the stimulation treatment. The analysis of 357 

cleat orientations in the same area produced a relatively 

broad range of readings (most face cleats were between N 

45" E and N 65" E) with an average of N 53" E reported by 

Lambert (21 1. No horizontal fracture component was 

observed. 

Borehole TW-3 was eventually mined through and a 

sand-filled vertical fracture trending N 68" E was traced 

across three new entries for a total lateral extent of 210 ft. 

Details of this fracture wing exposure are not available. 

Supplementary roof support was installed in the 

vicinity of borehole TW-3. This is the only reported case 

out of 22 Government-sponsored stimulation treatments 

where supplementary support was deemed necessary by 

mine management. The evaluation of the situation and 

description of the additional support was described by 

mine management (21) as follows: 

The roof in the general area of TW-3 was of a 

quality typical of most of the mine developed to date; 

however, in the immediate area of the bottom of 

TW-3 in which both the upper and lower coal seams 

had been fractured, it was necessary to install 

supplementary roof support. This supplemental roof 

support was required when mine management 

observed roof movement along the west rib of the 

entry driven due north of the bottom of the borehole, 

additional draw rock separation, roof becoming 

excessively drummy, and water seeping out of 

cracks in the roof. The supplemental roof support 

consisted of boxing the intersection immediately to 

the south of the borehole with 6-inch H-beams, 

installing 4-inch H-beams supported by timbers for 

distances of 70 ft to the east and 40 ft to the north of 

the borehole, and installing 10-foot-long expansionshell anchored roof bolts between the standard 

4-foot-long resin roof bolts to ensure that the bolts 

anchored in the solid rock above the upper coal 

seam. 

Installation of the supplementary supports required additional time as well as close inspection by 

mine management to ensure its adequacy; however, 

it was possible to mine through this area without 

experiencing any roof fall during mining operations. 

Lambert's (21) comments on the roof conditions in 

this area are as follows: 

The entry nearest TW-3 is also shown (fig. 10) to 

be a location of natural roof disturbances. Geologists 

also noted the presence of many wet roof joints in 

this area. Except for the single sand-filled crack 

leading from TW-3 (fig. lo), no evidence could be 

found to indicate that these wet roof joints had been 

created by stimulation. Unusually large amounts of 

seepage from roof openings near TW-3 could, 

however, be due to the borehole's presence, since it is 

an accumulation point for water within a coal unit 

only 7 ft above the open entry. 

Lambert (21) further commented on possible reasons 

why the stimulation treatment for borehole TW-3 (and 

TW-4, to be discussed in the next section) did not remain 

in the coalbed: 

1. The fine-mesh sand used as a fluid-loss agent 

was the only sand found to be contained in the coal. 

This sand is thought to have caused the propagating 

fracture to clog or screen-out in the coal early in the 

treatment. With continued pumping, it is possible 

that the fracturing fluids entered a preexisting 

plane of weakness in the shale roof rock. It is 

unlikely that this very hard roof rock was actually 

"broken" by the 1,400 psig pressure used during 

stimulation. No evidence could be found at either 

underground test site to indicate that the roof 

offered much resistance to the stimulation. Such 

evidence would include the presence of horizontal 

fractures or short propagations along butt cleat 

directions as was seen in other stimulation treatments. 

2. Cement used to set casing in TW-3 (and TW-4) 

was found in the roof, and up to 280 ft away from 

the wellbores. The roof strata was already broken or 

fractured sometime before casing was cemented in 

the test wells, probably just after the time when the 

holes were drilled. 

OAK GROVE MINE, BOREHOLE TW-4 (21) 

Borehole Completion 

Borehole TW-4 was drilled 510 ft southwest of 

borehole TW-3 (fig. 10). It was drilled 8 in. in diameter to a 

depth of 995.5 ft, approximately 70 ft above the top of the 

target Blue Creek Coalbed. The remainder of the hole was 

drilled to a 77/t3-in diameter to the total depth of 1,086 ft, 

using clear water as the drilling fluid. The borehole was 

drilled to a depth 15 ft below the base of the 5-ft-thick Blue 

Creek Coalbed. The borehole was cased with 4%-in-OD 

casing to a depth of. 1,065.7 ft, approximately 0.7 ft into 

the top of the Blue Creek Coalbed. The casing was 
cemented in place using a cement packer shoe and vertical sand-filled (mostly 80/100-mesh size) fracture, 

associated equipment and cement specifications as used on similar to those observed at the TW-3 borehole location, 

borehole TW-3. At the time of completion, the borehole was found in the vicinity of borehole TW-4 (fig. 10). It 

was approximately 1,010 ft ahead of mining. began as a hairline crack about 3 ft above the base of the 

Blue Creek Coalbed, and gradually widened to approx- Treatment Implementation (July 18, 1977) imatelv 3/16 in at the top of the coalbed. The maximum 

Prestimulation strata characterization tests included 

gamma ray, density, resistivity, and self-potential wireline geophysical logs. A jetting tool was used to cut tvo 

slots, spaced 180" apart, into the coalbed. 

The Blue Creek Coalbed was to be stimulated through 

23/~-in-diameter tubing with a 75-pct-quality nitrogengenerated foam with sand proppant similar to that of 

borehole TW-3. However, the stimulation treatment had 

to be terminated prematurely when foam containing sand 

was observed flowing from around the outside of the 

casing at the wellhead, indicating a poor cement job. A 

total of 12,200 gal of foam with 12,520 lb of sand (10,000 lb 

of 80/100-mesh and 2,520 lb of 20140-mesh) was used in 

the treatment. An initial formation break was interpreted 

by Lambert (21) to have occurred at a surface pressure of 

700 to 750 1bf/in2(ga), similar to that observed at borehole 

TW-3. The maximum surface treating pressure was 1,800 

lbf/in2(ga), with an average surface pressure of about 

1,500 lbf/in2(ga) for the treatment at injection rates 

averaging 10 bbllmin. The continuous stimulation treatment record for this borehole is shown in figure 11. 

Borehole TW-4 was not equipped with a water 

pumping system, however, it still produced gas at rates as 

high as 183,000 ft3/d. Rates averaged 150,000 ftYd then 

dropped sharply to the final measured rate of 40,000 ft3/d 

prior to plugging. Lambert (21) reported that there was 

some indication when comparing the production histories 

of boreholes TW-3 and TW-4 that the two wells interfered 

with each other. It was also reported that there was 

evidence of nitrogen gas communication to TW-3 when 

TW-4 was stimulated, indicating the movement of 

fracture fluids to greater distances than could be traced by 

the presence of sand-filled fractures underground. 

I Underground Observations 

Borehole TW-4 was mined through on November 16, 

1977, approximately 4 months after stimulation. A single 

lateraiextent of this fracture to the northeast was about 

220 ft on an average azimuth of N 67" E, similar to the 

orientation of naturally occurring roof joints in the area. 

Sand was again found in a roof fracture above the coalbed 

as at borehole TW-3. Mining did not proceed more than a 

few feet to the west of the borehole, therefore the 

maximum lateral extent in this direction is unknown. 

A large quantity of casing cement was found below 

the packer shoe at borehole TW-4. Cement covered most of 

the exposed wellbore in the coalbed, except where the two 

slots, each 5 in wide, were cut 9 to 12 in into the coal. The 

orientation of these slots was found to be approximately 

90" to the orientation of the sand-filled vertical fracture. 

Cement was also found in the sand-filled vertical fracture 

in the coal and roof. 

An evaluation by mine management (21) of the roof 

conditions in the vicinity of borehole TW-4 is as follows: 

Because the roof of the mine in the general 

vicinity of TW-4 was of a quality which required 

installation of more roof support than used in many 

other portions of the mine, it was difficult to assess 

the effect of the fracture on the mine roof at this 

location. However, no further increment of supplemental roof support was required at the base of 

the borehole or along the fracture wings. 

Lambert's (21) comments on the roof conditions in 

this area are as follows: 

Previous geologic inspection of the mine near 

TW-4 showed the well to be nearly surrounded by 

minor deformation and dislocations in the coal 

associated with lenticular shale ridges projecting 

from the roof. Areas of the mine where these 

features, called "rolls", were observed are shown in 

figure 10. Because inherent rock weaknesses develop near such features, these areas generally 

require some degree of additional roof support. The 

necessity for more roof support near TW-4 than used 

I I 1 I I I I I 

Shutdown for lSlP Start water flush -1 r 

Shutdown 

Start foam pad, Start 20/40=mesh 

I0 bbl/min sand, 1.5 Ib/gal 

RW\-O\, .J \ 

I<---/ 

1 'i - --40- LStart 80/100-mesh 

- {I' sand, I lb/gal 'J\ I - 

I \ ?I I KEY \- 

I \ I Surface pressure --- \ -- 

\ I I I I I 1 I I 1 

TIME, min 

Figure I1 .-Treatment record, borehole TW-4 (18). I 
gpyg(pg

ray, and density) and direct observation of the open hole 

Packer interval 11 1 E 

0 

Vertical scale, ft 

Paint-filled 

vertical fractures Blue Creek 

Coalbed 

Figure 12.-Stratigraphic section and near-wellbore underground observations in the Mary Lee-Blue Creek Coalbed 

intervals, borehole TW-5. (Modified from Larnbert (9) and Mahoney 

(22)) 

this point and the packer was removed from the hole. A 

slug (10 bbl) of fresh water containing 3 gal of red 

fluorescent paint and 1 lb of fluorescein dye, was injected 

into the formation for future underground characterization followed by an additional 10 bbl of water. A total of 

2,200 gal of water was injected into the formation prior to 

abandonment of the initial treatment attempt. 

Examination of the packer after removal revealed two 

180" opposed, %s-in-wide impressions along a 6-ft length 

of the rubber element. These impressions were further 

characterized (22) by use of the TV camera as "score 

marks" in the wellbore "extending in a vertical direction 

from the top of the Blue Creek seam through the 

middleman to the base of Mary Lee seam." Mahoney (22) 

indicated that it was possible that the pressure used to 

inflate the packer element was sufficient to create the 

observed cracks before the actual injection of the water. 

The packer was reinstalled at its previous position in 

the wellbore, and the Blue Creek Coalbed was successfully 

treated without any apparent leakage of treatment fluids 

around the packer. The treatment for borehole TW-5 was 

designed to utilize a 75-pct-quality nitrogen-generated 

foam without any sand proppant to investigate the 

production characteristics and the effect on the surrounding strata resulting from a no-proppant stimulation. The 

benefit expected from a no-proppant stimulation was 

confinement of the treatment to the coalbed (minimizing 

the potential for roof penetration and possible roof 

damage) by using low injection rates (9). Cost savings 

from deleting the sand proppant from the stimulation 

treatment were also anticipated, both from the reduced 
initial expenditures and also by decreasing the workover 

costs related to downhole pump failures experienced early 

in the production phase on previous boreholes using sand 

proppant. 

Table 2 summarizes the treatment implemented in 

borehole TW-5. The continuous stimulation treatment 

record for this borehole is shown in figure 13. Foam 

injection began at a rate of 2 bbllmin and was increased in 

1-bbllmin increments at 30-min intervals up to a rate of 5 

bbllmin. The 5-bbllmin rate was maintained for 1.5 h and 

was then increased to 6 bbllmin for an additional 1.5 h. A 

total of 53,000 gal of 75-pct-quality nitrogen-generated 

foam was injected into the formation. Blue fluorescent 

paint and fluorescein dye were also injected with the foam 

to differentiate movement of the foam treatment fluid 

from the initial water injection when the treatment area 

was eventually intercepted by mining. The surface 

pressure gradually increased from about 1,500 Ibflin2(ga) 

to a maximum of 2,000 lbflinyga) as the injection rate 

increased from 2 to 6 bbllmin. Average surface treatment 

pressures were about 1,900 1bfIinYga). 

A total of 6.6 MMft" of gas at an average rate of 49 

Mft"1d was produced during the 4.5-month production life 

of the borehole. A complete production history can be 

found in reference 22. 

Underground Observations 

Borehole TW-5 was mined through on July 12, 1979, 

approximately 7 months after stimulation. The borehole 

Table 2.-Summary of treatment chronology, borehole 

TW-5 (22) 

Foam Wellhead 

Ttme, min Injection Vol- pressure, rate ume, 

bbl Ibf/~n~(ga) bbllmin 

was intercepted very close to the southeast corner of a 

pillar (fig. 14). A large cavity eroded by the jet slotting 

procedure was observed near the base of the borehole. The 

cavity appeared to be conical in shape with its base 

approximately 1 ft above the bottom of the Blue Creek 

Coalbed (fig. 12). Sand from the slotting procedures 

covered the bottom of the cavity. The height of the cavity 

was 23 in and the radius of the base was approximately 36 

in. 

Examination of the exposed wellbore revea.led the two 

180" opposed cracks first observed in the packer element 

and with the TV camera. The two primary fractures were 

developed in the upper part of the Blue Creek Coalbed and 

extended through the middleman into the base of the 

overlying Mary Lee Coalbed. Two additional short 

vertical fractures were also present near the top of the 

Blue Creek Coalbed (fig. 12). A paint-coated hairline 

vertical crack extended approximately 8 ft from the 

borehole and, according to Lambert (9), appeared to be a 

lateral extension of the vertical fracture observed at the 

wellbore. This crack was observed in the roof strata above 

the commonly mined interval. The commonly mined 

height includes the two coal stringers and intervening 

carbonaceous shale directly above the main bench of the 

Blue Creek Coalbed. The roof crack was oriented at N 66" 

E, close to the N 61" E predominant face cleat orientation. 

The only other evidence of penetration of fracture 

fluids into strata above the mined interval was the 

presence of blue fluorescent paint in roof joints approximately parallel to the face cleat orientation at locations 

25,29,30, and 31 at the distal end of the fluid penetration 

to the southwest (fig. 14). No adverse mining conditions or 

roof movement were reported associated with these roof 

penetrations. 

Figure 14 shows the location and orientation of 

vertical paint-filled fractures observed in the vicinity of 

borehole TW-5. If an orientation on the fracture was not 

obtained but was recorded as being in the face cleat 

direction, the fracture trace was plotted as being oriented 

at an azimuth similar to that measured for nearby 

fractures. These fractures are identified by a ? in figure 14. 

I 1 I I 1 

Start foam,2 bbl/min Shutdown 1 ; , 

Shut down N2 

- Foam,4 bbl /min Foam, 6 bbl/min 

Foam,5 bbl /min 

- 

I 

/ --------,---/--- 

-4-- 

1 ./ l 

1 KEY - --- Surface pressure I 

I 

I I I I 1 L 

0 50 1 00 1 50 200 250 300 

TIME, min 

Figure 13.-Treatment record, borehole TW-5 (1 9). 
Cleat 

2/ 

I 

LEGEND 

Vertical fracture, paint coated 

I: Horizontal fracture, paint coated 

? Uncertain orientation or extent 

0 75 1 50 L I J 

Scale, ft 

Figure 14.-Mine map, borehole TW-5, with fracture orientations. (Modified from Mahoney (22) and Boyer (19)) 

Table 3.-Summary of fracture observations (paint-coated surfaces), borehole TW-5 (19) 

Location' 

1 ...... 

2..... 

3 ... 

4..... 

5. .. 

6. .... 

7.. .. 

8 .. 

Fractures 

1 

1 

1 

3 

1 

1 

Zone 

Zone 

Zone Fracture description, Orientawldth, ft vertlcal tion 

NAp Top of BC to 1 st coal stringer N 60" E 

NAp Top of BC to 2d coal strtnger N 63" E 

NAp Top 3 In of BC to 2d coal stringer N 62" E 

1 Tcp 1 In of BC to 2d coal stringer N 61" E 

NAp NR NR 

NAp NR N 61" E 

20 Small NR 

5 NR N 63" E, 

N 65" E 

Paint Fracture descri~tion. 

color horizontal, 

NR ..... NR. 

NR ..... .NR. 

NR . . NR. 

NR ... NR. 

Blue .. .NR. 

Red ... ..NR. 

NR ....... .NR. 

Red, blue .... NR. 

1 

Zone 

NR N 29" W 

Face cleat-top 15 in of BC to 1 st coal stringer, butt N 69" E, 

cleat-aenerallv to^ 8 to 15 In of BC. max to^ 30 In of BC N 26" W 

Zone , . Floor toroof ..... ........ N 54" E. 1 red, On top of 1 st coal stringer. 

rest NR. 

NR ..... .NR. 

NR ..... .On roof and on interface between 

blocky zones in BC. 

NR .... On top of 1 st coal stringer. 

NR ... NR. 

NR .. . .NR. 

Blue On top of 1 st coal stringer. 

.do ...... NR. 

do ...... .On top of 1 st coal stringer. 

..do ...... .NR. 

..do ........ On top of 1 st coal stringer. 

..do ........ .NR. 

.do ....... .NR. 

.do ...... .NR. 

do ......... NR. 

.do ......... .On top of 1 st coal stringer. 

..do ....... .Top of BC. 

do ... .NR. 

.do . . . . NR. 

do ... .NR. 

.do ..... .NR. 

Top 18 in of BC to 1 st coal strlnger . . 

Floor to roof ...... . . . . 

N 59" E 

Butt Cleat 

N 60" E, 

1 

Zone 

Zone 

Zone 

1 

1 

1 

1 

Zone 

1 

2 

2 

1 

1 

1 

1 

1 

1 

1 

1 

Several floor-to-2d-coal stringers, rest NR N 59" E, 

NR N 58" E, 

Top 30 ~n of BC N 60" E, 

Floor to 1 st coal strln er NR 

Top 12 In of BC to beyow 3d coal strlnger N 59" E 

Top of BC to 1 st coal stringer N59"E 

Floor to roof Random 

do NR 

do N 56" E 

Intersect at 60" NE and NW 

M~ddle of BC to roof N 50" E 

Floor to roof NR 

Floor to 1 st coal strlnger NR 

do N 62" E 

M~ddle of coal to top of coal N 67" E 

Mlddle of coal to 2d coal strlnger N 56" E 

do N 56" E 

do N62"E 

BC Blue Creek Coalbed NAp Not applicable. NR Not reported. ' Shown on figure 14. 
Additional information on the actual observations for 

each numbered fracture or fracture zone is given in table 

3. 

The lateral extent of the horizontal paint-coated 

surfaces are also shown in figure 14. Horizontal paintcoated surfaces included the three highly polished 

slickensided surfaces at the top of the Blue Creek Coalbed 

and on top of the two thin coal stringers in the 1-ft interval 

directly above the Blue Creek Coalbed (fig. 12). One or 

more of the three horizontal surfaces we& coated with 

fluorescent paint at any particular location, and without 

the presence of the paint, these interfaces penetrated by 

the stimulation fluids could not be differentiated from 

"normal" surfaces. 

As discussed previously (221, general practice 

throughout the Oak Grove Mine is to mine up to the 

second coal stringer because the "slickensided coal 

surfaces demonstrated very little adhesion to the surrounding strata." Mahoney (22) also indicated that the 

inherent horizontal weakness at the top of the mined 

interval "appeared to have prevented upward growth of 

the induced fractures," and "it was not uncommon to 

observe an abrupt termination of the vertically decorated 

planes wherever they abutted one or the other coal 

stringer." The horizontal paint-coated surfaces as shown 

in figure 14 cover an essentially elliptical area with the 

long axis oriented approximately parallel to the face cleat 

orientation. The maximum extent of paint on a horizontal 

surface was approximately 230 ft to the southwest of the 

borehole. 

Numerous vertical fractures or zones of paint-coated 

cleat surfaces were identified around borehole TW-5. Most 

of these fractures were oriented in the face cleat direction 

and only four locations (9-10,12,23) had butt cleats coated 

with paint (fig. 14). As with the horizontal fractures, the 

vertical fractures were only identifiable with the fluorescent paint and were essentially only the width of the face 

cleat itself. There appears to be a difference in the 

character of the paths of stimulation fluid movement 

observed to the northeast and to the southwest of the 

borehole. There are fewer vertical fractures or zones of 

fractures to the northeast than to the southwest (7 vs 24). 

The fractures to the northeast are generally confined to 

the upper portion of the Blue Creek Coalbed and the 

approximately 1 ft of overlying strata containing the two 

1-in-thick coal stringers. Even the fractures close to the 

borehole at locations 7 and 8 are contained near the top of 

the coal interval. The fractures to the northeast are also 

contained in a narrow band very close to the axis of the 

elliptical pattern of the horizontal paint distribution. 

The vertical fractures to the southwest (fig. 141, in 

addition to being more numerous, commonly extended the 

entire height of the coalbed (locations 11,13-14, 17,20-22, 

25-27) as far as 220 ft from the borehole (location 27). At 

the remaining locations to the southwest, the vertical 

paint-coated fractures were confined to the upper portions 

of the coalbed up to the two overlying coal stringers. Zones 

of multiple paint-coated cleat surfaces appeared to be 

slightly more common to the southwest (locations 11, 

13-14, 20, 23) than to the northeast (locations 3, 7-81. 

Single fractures predominated at locations farthest from 

the borehole. The vertical fractures to the southwest were 

found in a band with a maximum width of approximately 

130 ft (from location 19 to 16) compared to the very narrow 

band of vertical fractures found to the northeast. The 

maximum lateral extent of vertical fractures was 370 ft at 

location 31 to the southwest. 

The reason for the observed differences in the 

distribution of vertical paint-coated cleat surfaces may be 

related to a differential penetration capability along the 

face and/or butt cleat to the southwest and northeast. If 

penetration of cleat was easier to the southwest of the 

borehole, fluids would perhaps initially move along a face 

cleat in that direction until pressures built up sufficiently 

at the distal end to force the fracture along butt cleat. This 

stairstepping along face and butt cleats is a fairly common 

occurrence as noted throughout this report. Multiple 

repetitions of this stairstepping could produce the wider 

band of paint-coated fractures to the southwest of the 

borehole where the only butt cleat fractures were 

observed. The narrow band of fractures observed to the 

northeast of the borehole could have been induced at the 

same time as those to the southwest or near the end of the 

treatment when treatment pressures were not sufficiently 

high to continue fracture propagation to the southwest. 

There is also a wider dispersion of paint on horizontal 

surfaces to the southwest, perhaps indicating an easier 

horizontal penetration perpendicular to the long axis of 

the treatment fluid distribution in this area. The reason 

for the supposed easier penetration of cleat southwest of 

the borehole is unknown. 

OAK GROVE MINE, BOREHOLE DHM-5 (17) 

Borehole Completion 

Borehole DHM-5 was drilled 6% in. in diameter to a 

depth of 1,376.5 ft, approximately 7 ft above the top of the 

target Blue Creek Coalbed at 1,383.5 ft, 4 ft into the 

parting between the Mary Lee and Blue Creek Coalbeds. 

The borehole was cased with 4%-in-OD casing to the total 

depth of the hole and the casing was cemented in place 

with API class A cement with 18 pct NaCl additive. The 

hole was completed 37/s in. in diameter to a total depth of 

1,414 ft, approximately 25 ft below the bottom of the 

5.6-ft-thick Blue Creek Coalbed. 

Treatment Implementation (February 25, 1981) 

A suite of strata characterization geophysical logs 

were run prior to stimulation. A jetting tool was 

positioned approximately 1.5 ft above the bottom of the 

Blue Creek Coalbed and rotated to create a horizontal slot 

in the coalbed exposed in the wellbore. Very little coal was 

seen in the fluid returns, therefore the tool was raised 3 ft 

after 15 min of jetting. A good coal return was observed at 

the new position. Water injection rates were 2.5 bbllmin 

with sand added at a rate of 0.8 lblgal, for a total fluid 

volume of 145 bbl and 2,000 lb of sand. Surface pressures 

during jetting averaged 1,500 lbf/in2(ga). 

The stimulation treatment for borehole DHM-5 

consisted of 75-pct-quality nitrogen foam without sand 

proppant and was conducted through tubing with a packer 

set in the casing. Formation break was reported by Boyer 

(1 7) at 800 lbf/in2(ga) surface and 1,400 lbf/in2(ga) bottom 

hole pressures. A continuous stimulation treatment 

record for this hole is not available. A 50,600-gal 

treatment was designed for this borehole, but only 40,866 

gal of foam was used because of the excessive amount of 
LEGEND - Vertical fracture In coal 

Vertical fracture in roof 

ond rtb 

0 100 203 

N 

I 

- 

Scale, ft 

DETAIL .4 

Cleat 

I 

DETAIL B 0 300 600 

Stall, n 0 

I 

100 200 

Scale,fl 

Figure 15.-Mine map, boreholes DHM-5 and DHM-6, with 

fracture orientations. (Modified from Boyer (17)) 

Blue Creek Coalbeds so that the Blue Creek Coalbed could 

be completed open hole. However, the driller did not 

recognize drilling into the Mary Lee Coalbed and he 

continued drilling, eventually drilling through the Blue 

Creek Coalbed and approximately another 1.6 ft below 

before drilling was stopped. The hole was cased to the total 

depth with 43'-in-OD casing, which was cemented in place 

with API class A cement with 18 pct NaCl additive. An 

additional 40 ft of 3%-in-diameter hole was drilled below 

the casing before wireline geophysical logs were run to 

evaluate the hole and locate the target Blue Creek 

Coalbed. The Blue Creek Coalbed was identified at 1,238.6 

to 1,244.4 ft for a total of 5.8 ft of coal, all inadvertently 

behind casing. The parting between the Mary Lee and 

Blue Creek Coalbeds was 10.8 ft and the Mary Lee 

Coalbed was only 1 ft thick at this location. 

Treatment Implementation (December 20, 1980) 

A jetting tool was positioned at 1,244 ft, near the 

bottom of the Blue Creek Coalbed interval to cut two 

vertical slots through the casing and cement to expose the 

coalbed to the wellbore. A good return of coal was obtained 

in the water circulated to surface during the jetting, 

indicating a successful penetration into the coalbed. 

Pressures recorded at the surface during the slotting 

varied between 1,800 to 2,000 lbf/in2(ga). The water 

injection rate was 3 bbllmin with a sand rate of 2 Iblgal for 

a total fluid volume of 80 bbl of water containing 1,500 lb 

of sand. 
The stimulation treatment began with a 75-pctquality nitrogen-generated foam pad at an injection rate 

of 3 bbllmin and was increased in 1-bbllmin increments 

until a rate of 7 bbllmin was reached (table 4). A surface 

breakdown pressure of 700 lbf/in2(ga) was reported (1 7). A 

total of 40,320 gal of foam pad was injected into the 

coalbed before the addition of 20140-mesh sand proppant. 

Sand was added to the treatment at a rate of 1 lblgal of 

foam while the foam rate stayed constant at 7 bbllmin. A 

total of 10,000 lb of sand was injected with 10,248 gal of 

foam during the final 35 min of pumping. Fluorescent 

paint was included with the treatment fluids. Surface 

pressures ranged from 800 to 900 lbf/in2(ga) during the 

treatment. 

Bottom hole pressures were continuously monitored 

during the stimulation treatment and are presented in 

figure 16. The bottom hole pressure increased steadily as 

the injection rate was increased, up to a rate of 4 bbllmin. 

At the 4-bbllmin rate the bottom hole pressure began to 

decline, even though the injection rate was progressively 

increased to 7 bbllmin. The bottom hole pressure 

increased only slightly when sand was first added to the 

treatment, but as sand injection continued, the pressure 

gradually dropped. The bottom hole pressure never 

returned to the maximum observed at the 4-bbllmin 

injection rate. 

Table 4.-Summary of treatment chronology, borehole 

DHM-6 (1 7) 

Time, min 

Injection Volume, 

rate, 

bbllmin gal 

Foam 

Av bottom hole 

pressure, 

Ibf/in2(ga) 

1,050 

1,100 

1,090 

1,080 

1,090 

1.080 

'With sand added. 

At the time of the stimulation, the borehole was 

located approximately 1,800 ft from active mining. Gas 

production from this borehole quickly reached 80,000 to 

90,000 ft3/d and remained at this rate until March 16, 

1981 (approximately 3 months after stimulation) when 

the east wing of the fracture was intercepted by mining at 

a location 160 ft northeast ,of the borehole (fig. 15). 

Production rates declined and were somewhat erratic 

after the mine-through of the east fracture .wing, but 

generally ranged between 40,000 to 50,000 ft3/d until 

mining came very close and finally intercepted the 

borehole in early 1982. A more complete production 

history can be found in reference 17. 

Underground Observations 

Inspection of the well casing exposed by mining of the 

Blue Creek Coalbed revealed two vertical slots, 180" 

apart, cut in a northeast-southwest direction. A vertical 

fracture zone, 1 to 2 in. in width and confined to the upper 

3 ft of the coalbed, was observed on the rib 2 ft to the 

northeast of the borehole. The fracture zone consisted of 

multiple vertical planes, a maximum of '/16 in wide, 

propped with sand and coated with fluorescent paint. A 

single vertical sand-and-paint-coated fracture was also 

observed in the 2 ft of exposed roof rock at this location. 

Boyer (2 7) commented that it was "difficult to ascertain if 

the roof fracture was produced by the hydraulic stimulation or if the hydraulic stimulation opened a pre-existing 

roof joint." The northeast wing of the fracture was next 

observed on the rib around the corner from the previous 

exposure, and 180 ft from the borehole (fig. 15). The 

fracture was still confined to the upper 3 ft of the coalbed, 

but consisted of a 10- to 12-in-wide zone of multiple 

vertical paint-coated face cleat surfaces with no sand 

proppant. The fracture terminated at the characteristically smooth, mirrorlike slickensided surface at the top of the 

coalbed. No penetration of the roof was observed at this 

I I 1 1 1 1 I 

Start foam, 3 bbl /min 

Start sand -L 

- 

KEY - Bottom-hole pressure 

I 

A I I 1 I I I 

0 25 50 75 1 00 1 25 1 50 1 75 200 

TIME, min 

Flgure 16.-Treatment record, borehole DHM-6 (1 7). 
location or any further along this fracture wing. The 

fracture could be traced an additional 450 ft across the five 

entries to the northeast, finally entering the barrier pillar 

630 ft from the borehole, the longest vertical fracture 

traced underground to date. The fracture was generally 

uniform in character across the five entries, consisting of 

multiple paint-coated vertical cleat surfaces in a narrow 

zone at the top of the coalbed. At the last observation point 

the fracture consisted of a 1- to 2-in-wide zone in the upper 

1 ft of the coalbed. The northeast fracture wing varied 

slightly in orientation through its 630-ft traceable length, 

but generally followed an orientation of N 67" E, 

essentially the same orientation measured for face cleat at 

the mine site. No horizontal component was observed at 

any location along or in the vicinity of this fracture wing. 

The vertical fracture wing to the southwest was 

observed on the rib 95 ft from the borehole (fig. 15). The 

fracture consisted of multiple vertical planes propped with 

sand and contained fluorescent paint in a 1- to 2-in-wide 

zone confined generally in the upper 3 ft of the coalbed. 

Small amounts of sand and paint were also seen in the 

lower portion of the coalbed. The fractures were oriented 

similarly to those in the northeast wing and appeared to 

be face cleats. No horizontal component was observed at 

this location. A single sand-and-paint-coated fracture, '/4 

in wide, was present in the mine roof at the wellbore and 

was traced to the mine face 95 ft away. Small amounts of 

gas and water were flowing from the roof fracture near the 

wellbore, and from the fracture at the coal face. Boyer (1 7) 

concluded after detailed observations of the mine roof that 

this roof fracture was probably a preexisting roof joint 

that had been penetrated by the stimulation fluid and 

proppant. He also stated that "no roof or coal stability 

problems were experienced in this area of the mine." 

PITTSBURGH COALBED, PENNSYLVANIA 

VESTA NO. 5 MINE, BOREHOLE USBM-4 (8) 

Borehole Completion 

Borehole USBM-4 was drilled 6% in. in diameter to a 

depth of 597 ft. The only casing in the hole was 20 ft of 

7%-in-diameter surface casing at the top of the hole. The 

hole was not cased to the coalbed because of the short time 

before anticipated mine-through of the borehole, which 

would preclude gas production; therefore, the expense of 

casing the hole did not seem justified. The Pittsburgh 

Coalbed was located at a depth of 588 to 594 ft for a total 

coal thickness of 6 ft. The total depth of the hole was 3 ft 

below the base of the Pittsburgh Coalbed. The immediate 

roof rock at the borehole was shale, and the floor rock was 

a calcareous shale. At the time of completion the borehole 

was 500 ft in advance of mining. 

treatment design to be implemented was to use gelled 

water and sand to create two fracture wings, each 387 ft 

long, of which 219 ft would be propped by a sand pack, 5 ft 

high and 0.22 in wide. A 2,000-gal gelled water pad was 

first injected into the coalbed to control leakoff. Formation 

break was reported at a surface pressure of 500 lbf/in2(ga). 

A pumping rate of 10.5 bbllmin was maintained 

throughout the treatment. Following the pad, an additional 5,300 gal of gelled water and 3,500 lb of 10120-mesh 

sand (3,000 lb coated with fluorescent dye followed by 500 

lb of uncoated sand) was injected into the coalbed 

according to the schedule shown in table 5. It should be 

noted that sand injection rates reported by Elder (8) in 

table 5 when totaled (2,637 lb) do not match the 3,500 lb 

Table 5.-Summary of treatment chronology, borehole 

USBM-4 (8) 

Fluid volume, gal Sand rate, Iblgal 

Treatment Implementation (April 17, 1974) 2,00O(pad) ................................. 

1,300 

NAP ....................................... 0.25 

Borehole USBM-4 was treated through 2%-in- 650 .50 1,750 ....................................... .75 diameter tubing with an open hole packer set at 585 ft, 3 ft 1,350 ....................................... .50 

above the Pittsburgh Coalbed. The stimulation treatment 250 (flush) .................................. NAP 

record for this borehole is shown on figure 17. The NAP ~ot applicable. 

I 1 I I I I 1 I I I 1 - 

KEY 

- -I- Surface pressure ....... Pumping rate, water 

(and sand) - - -,-------- d-- 

- g :\ - 

\ ............................... 

il 

I I I 

: '- y-t--f--. I I 1 -, I 

TIME, min 

Figure 17.-Treatment record, borehole USBMB. (Modified from Elder (8)) 
terminated before reaching the floor. 

There was no apparent butt cleat fracture observed on Figure 19.-Cross section of face cleat fracture in entry at 

the rib south of the borehole nor was there any face cleat location 1 near borehole USBM-4. (Modified from Elder (8)) 

fracture reported to the east of the borehole. The 

maximum extent of the butt cleat fracture into the 

northern rib is not known, but the maximum extent of 20 

ft reported for the face cleat fracture is far short of the 

designed 219 ft of sand-propped fracture length. There 

were no roof penetrations or adverse mining conditions 

'/ LEGEND 

4 Vertical fracture, sand filled 

0 Unmined area 

0 10 20 

Sand in face cleat 

u --- - - 

- --- - - Floor - - - - - - Scaie,ft - 

Flgure 18.-Mine map, borehole USBM-4, with fracture Figure 20.-Cross section of butt cleat fractures on rib at 

orientations. (Modified from Elder (8)) locatlon 2 near borehole USBM4. (Modified from Elder (8)) 
reported in the vicinity of borehole USBM-4. The site was 

revisited 2 yr after the initial mine-through, with no 

apparent roof or rib deterioration noted. 

EMERALD MINE, BOREHOLE EM-5 (9-10) 

Borehole Completion 

Borehole EM-5 was drilled 6l/4 in. in diameter to a 

depth of approximately 810 ft (40 ft below the Pittsburgh 

Coalbed) and cased with 4%-in-OD casing. The bottom of 

the casing was set in the shale above the target Pittsburgh 

Coalbed, and a formation packer shoe on the end of the 

casing was used to keep the cement from reaching the 

coalbed below. The coalbed was 6 ft thick and was located 

from 764 to 770 ft in the borehole. Prior to stimulation and 

production, the formation packer shoe was drilled out to 

provide communications with the coalbed below. At the 

time of completion, mining had not yet been initiated at 

the Emerald Mine. The borehole was located approximately 500 ft from the access shaft shown on figure 21. 

Treatment Implementation (September 1, 1976) 

Prestimulation strata characterization tests included 

gamma ray and neutron geophysical logs. A continuous 

stimulation treatment record is not available for this 

borehole. The treatment was initiated with a 900-gal 

water pad followed by 600 gal of 75-pct-quality nitrogengenerated foam. A formation breakdown pressure of 800 

lbf/in2(ga) was reported (10). A total of 31,500 gal of foam 

with 10,000 lb of sand (10/20- or 20140-mesh) was injected 

into the coalbed. The average foam injection rate was 11.6 

bbllmin, with the actual rate gradually increased as the 

sand rate was increased. The average surface treatment 

pressure was 1,375 lbf/in2(ga). Gas production began 

relatively low but increased to a maximum of 117,000 ft3/d 

after 7 months of production. The rate declined to 60,000 

to 75,000 ftVd over the next several months. Details of the 

production history can be found in reference 10. 

Underground Observations 

A fracture wing from borehole EM-5 was intercepted 

by mining on July 21, 1978, approximately 2 yr after 

stimulation. The borehole location itself was not exposed 

by mining. The area of the fracture interception is shown 

in map view on figure 22. At the rib closest to the borehole, 

a single vertical sand-filled fracture up to 1% in wide was 

observed extending from the floor to the roof (fig. 23). At 

the interface between the top of the coalbed and the 

overlying hard shale, the vertical fracture became 

horizontal. The sand-filled horizontal fracture was to Y" 

in wide, and at its maximum lateral extent was observed 

50 ft from the presumed borehole location. The sand-filled 

vertical fracture could be traced across two entries of the 

mine, and at its maximum observed extent, it was 

approximately 140 ft from the presumed borehole location. The vertical fractures were propagated along face 

cleats, which averaged N 68" W in orientation. The width 

of the vertical sand-filled fracture varied from 1 l/2 in at the 

exposure nearest the borehole to only a hairline at its 

maximum extent, where it was only partially filled with 

sand. No evidence of penetration of the roof strata was 

observed associated with the stimulation treatment at 

borehole EM-5. 

0 

EM-I I 

0 0 

EM-5 EM-6 

0 

EM-7 

Cleat 0 

EM-8 

Scale, f t 

Figure 21.-Map of Emerald Mlne area with locations of 

boreholes EM-5, EM-6, EM-7, EM-8, and EM-11. (Modified from 

Lambert (9)) 

- Vertlcal fracture, sand fllled 

= Horizontal fracture, sand filled 

0 - 50 100 

Scale, ft 

Flgure 22.-Mine map, borehole EM-5, with fracture orientations. (Modified from Lamber! (9)) 

Shale 

T 

Sand filled 

I k ln wide iL 

(Not to scale) 

Figure 23.-Cross section of fractures on rib closest to 

borehole EM-5. (Modified from Lambert (9)) 
EMERALD MINE, BOREHOLE EM-6 (9-10) 

Borehole Completion 

Borehole EM-6 was drilled 6% in. in diameter to a 

depth of approximately 628 ft (40 ft below the base of the 

Pittsburgh Coalbed) and cased with 4%-in-OD casing. The 

bottom of casing was set in the shale above the target 

Pittsburgh Coalbed, and a formation packer shoe on the 

end of the casing was used to keep the cement from 

reaching the coalbed below. The coalbed was 6 ft thick and 

was located from 582 to 588 ft in the borehole. Prior to 

stimulation and production, the formation packer shoe 

was drilled out to provide communication with the coalbed 

below. At the time of completion, mining had not yet been 

initiated at the Emerald Mine. The borehole was located 

approximately 900 ft from the access shaft shown in figure 

21. 

Treatment Implementation (April 15, 1976) 

Prestimulation strata characterization tests included 

gamma ray and neutron wireline geophysical logs. The 

continuous stimulation treatment record for this borehole 

is shown in figure 24. The treatment was initiated with a 

900-gal water pad followed by 600 gal of 75-pct-quality 

nitrogen-generated foam. A formation break pressure of 

950 lbf/in2(ga) was reported. A total of 29,200 gal of foam 

with 14,000 lb of sand (10120- or 20140-mesh) was injected 

into the coalbed. 

The average foam injection rate was 17.7 bbllmin, 

with the actual rate gradually increasing as the sand rate 

increased. The average surface treating pressure was 

1,500 lbflinyga). It was reported by Steidl (10) that a 

screenout occurred near the planned end of the treatment 

causing only a slightly premature termination of the 

stimulation. Maximum surface pressure of about 2,150 

lbflinyga) was reached at the start of sand injection and 

again at the end of the treatment when the screenout was 

reported. 

Gas production was generally erratic in the beginning, however, a maximum rate of 110,000 ftYd was 

reported 1 yr after production began. The rate gradually 

declined to near 25,000 ft3/d at the termination of 

production prior to mine-through. Details of the production history can be found in references 9 and 10. 

Underground Observations 

Borehole EM-6 was mined through on November 9, 

1978, approximately 29'2 yr after stimulation. The area of 

the borehole interception is shown in map view in figure 

25. A single vertical sand-filled fracture, approximately 

2% in wide and extending from the base of the coalbed to a 

point about 3 ft above the base was observed on the west 

rib, 10 ft away from the intercepted borehole (fig. 26). The 

vertical fracture was propagated along face cleats and 

several discontinuous sand-filled fractures extended to the 

top of the main coal bench. Several additional thin 

discontinuous sand-filled vertical fractures paralleled the 

main fracture and were also developed in face cleats. The 

average orientation was N 68" W. One of the short vertical - 

fractures penetrated the thin roof shale above the main 

coal bench and terminated at the base of the rider coal. 

An inclined fracture split off from the main vertical 

fracture and stairstepped its way up to the top of the 

coalbed where it became horizontal for a short distance 

before inclining through the thin (8-10 in) roof shale and 

finally becoming horizontal at the base of the rider coal. 

The sand-filled horizontal fracture on top of the main coal 

bench was about '/z in thick and the horizontal fracture at 

the base of the rider coal was l/4 to Yz in thick. No 

additional exposures were observed for the fractures seen 

on the ribs adjacent to the borehole. The extent of the 

fractures into the ribs is unknown. No adverse mining 

conditions were encountered due to the penetration of the 

0 5 10 15 20 25 30 35 40 45 

TIME, min 

Figure 24.-Treatment record, borehole EM-6. 
- Vertical fracture, sand filled 

- - Horizontal fracture, sand filled 

0 - 50 100 

Scale,ft 

Flgure 25.-Mine map, borehole EM-6, with fracture orientations. (Modified from Lambert (9)) 

Treatment Implementation (September 8, 1976) 

Prestimulation strata characterization tests included 

gamma ray and neutron wireline geophysical logs. A 

continuous stimulation treatment record is not available 

for this borehole. The treatment was initiated with a 

900-gal water pad followed by 600 gal of 75-pct-quality 

nitrogen-generated foam. A formation break pressure was 

not reported. A total of 29,000 gal of foam with 7,400 lb of 

sand (10120- or 20140-mesh) were injected into the coalbed. 

The average foam injection rate was 10.8 bbllmin, with 

the actual rate gradually increasing as the sand rate 

increased. The average surface treatment pressure was 

1,400 lbf/in2(ga). It was reported by Steidl (10) that a 

screenout occurred near the planned end of the treatment 

causing only a slightly premature termination of the 

stimulation. Gas production from the borehole began very 

high (120,000 ftYd) reaching a maximum of 140,000 ft31d 

after only 1 month of production. The rate eventually 

declined to about 6,000 ft3/d at the termination of gas 

production prior to mine-through. Details of the production history can be found in references 9 and 10. 

(Not to scale) 

Figure 26.-Cross section of fractures on rib west of 

borehole EM-6. (Modified from Lambert (9)) 

strata overlying the main bench of the Pittsburgh 

Coalbed. In fact, common mining practice at the mine has 

been to extract the thin roof shale and part of the rider 

coal leaving a thin interval of the rider coal as the actual 

mine roof. 

EMERALD MINE, BOREHOLE EM-7 (9-10) 

Borehole Completion 

Borehole EM-7 was drilled 6% in. in diameter to a 

depth of approximately 774 ft (40 ft below the base of the 

Pittsburgh Coalbed) and cased with 4%-in-OD casing. The 

bottom of casing was set in the shale above the target 

Pittsburgh Coalbed, and a formation packer shoe on the 

bottom of the casing was used to keep the cement from 

reaching the coalbed below. The coalbed was 6 ft thick and 

was located from 728 to 734 ft in the borehole. Prior to 

stimulation and production, the formation packer shoe 

was drilled out to provide communication with the coalbed 

below. At the time of completion, mining had not yet been 

initiated at the Emerald Mine. The borehole was located 

approximately 1,100 ft from the access shaft shown in 

figure 21. 

Underground Observations 

Borehole EM-7 was mined through on December 1, 

1978, approximately 2% yr after stimulation. Vertical 

sand-filled fractures, to in wide, were reported a 

maximum of 18 ft laterally from the borehole. No 

horizontal fractures were present nor was there any 

penetration of strata above the main bench of the 

Pittsburgh Coalbed. 

EMERALD MINE, BOREHOLE EM-8 

Borehole Completion (10) 

Borehole EM-8 was drilled 6% in. in diameter to a 

depth of approximately 692 ft (40 ft below the base of the 

Pittsburgh Coalbed) and cased with 4lh-in-OD casing. The 

bottom of the casing was set in the shale above the target 

Pittsburgh Coalbed, and a formation packer shoe on the 

end of the casing was used to keep the cement from 

reaching the coalbed below. The coalbed was 6 ft thick and 

was located from 646 to 652 ft in the borehole. Prior to 

stimulation and production, the formation packer shoe 

was drilled out to provide communication with the coalbed 

below. At the time of completion, mining had not yet been 

initiated at the Emerald Mine. The borehole was located 

approximately 1,600 ft from the access shaft shown on 

figure 21. 

Treatment lmplementation (September 15, 1976) 

(10) 

Prestimulation strata characterization tests included 

gamma ray and neutron wireline geophysical logs. The 

continuous stimulation treatment record for this borehole 

is shown in figure 27. The treatment was initiated with a 

900-gal water pad followed by 600 gal of 75-pct-quality 

nitrogen-generated foam. A formation break was not 

reported. A total of 42,000 gal of foam with 12,800 lb of 

sand (10120- or 20140-mesh) were injected into the coalbed. 

The average foam injection rate was 10.8 bbllmin, with 
TIME, min 

Figure 27.-Treatment record, borehole EM-8. 

8 

4 

1,600- 

1,400-: 

1,200 

1,000 

800 

the actual rate gradually increasing as the sand rate 

increased. The average surface treatment pressure was 

1,050 lbf/in2(ga). Gas production from borehole EM-8 was 

never very high, with the maximum rate of 21,000 ft"/d 

being achieved approximately 1 month after stimulation. 

The rate eventually declined to only 2,000 ft3/d prior to the 

termination of gas production just before mine-through. 

Details of the production history can be found in reference 

10. 

...; I I I I I I I I I I 1 I . . 1 I I 1 1 1 . . --Water pad : - . 

. Start foam . . 

-: : ., r Start sand.O.25 lblgal 

. . .* 

. I I '/\-- 0--c---- 

Shut- 

-: : \,crd --.--.----

I 3 - 

: 4 I 3 I -i/ t- ? , I 1 *- I 

Underground Observations 

600 

400-1 

200 

The area around borehole EM-8 was mined through 

in the spring of 1979, approximately 2Y2 yr after 

stimulation. A detailed underground investigation was 

not conducted until the summer of 1985 when the authors 

of this report visited the site. Vertical sand-filled fractures 

were observed at four locations on the eastern rib of the 

pillar presumed to contain the borehole (fig. 28). At 

location 1, the sand-filled fracture extended approximately 3.8 ft into the main bench of the coalbed from the top 

(fig. 29). At the top of the coalbed the fracture was l/ls in 

wide and tapered to only the width of a single sand grain 

at the bottom. The fracture was propagated along a face 

cleat with an orientation of N 65" W. The fracture 

penetrated up into the shale above the main bench where 

it intercepted and propagated along an inclined fracture 

which was about '/l6 in wide. A second inclined fracture 

close to and nearly parallel to the first was also filled with 

sand. Both of these fractures terminated at the top of the 

shale, at the interface with the overlying rider coal. The 

two inclined fractures in the thin (0.9 ft) shale were very 

similar in appearance to other non-sand-filled naturally 

occurring fractures in the area, and are assumed to have 

existed prior to and been invaded by the stimulation 

treatment. A second short vertical sand-filled fracture, 

one sand grain thick, extended down from the top of the 

coalbed approximately 1 ft into the coalbed at location 1. 

-1 I .............. ................ .. ........................... ............... .**. : . I :I 1;' ::-**a. I 

:\ t : KEY i-2 

I :I t --- Surface pressure I :\ 1 ....... -1 : ' 1 Pumping rate, water - : '1 (and sand) I :: II 01.1 

. I I I 1 I I I I I 1 I I I I I 1; 0 

No adverse mining conditions were reported nor were 

deteriorated roof conditions observed at this location, even 

after exposure of the fracture for 6 yr. As noted earlier, the 

common mining height at this mine includes the shale 

unit that was penetrated by the stimulation treatment. 

The vertical sand-filled fracture at location 2 (fig. 30) 

was contained in the lower 2 ft of the coalbed and turned 

horizontal above a distinctive bony coal band. The vertical 

fracture also stairstepped short distances along bedding 

planes in the coalbed before turning completely horizontal. The vertical fracture was '/16 in wide and was the same 

thickness but discontinuous after turning horizontal. The 

vertical fracture was contained in a face cleat with an 

orientation of N 72" W. There also seemed to be a 

sand-filled butt cleat fracture at this location, but the 

exposure was such that it could not be conclusively 

delineated. 

At location 3, two short (1%-ft) en echelon sand-filled 

vertical fractures extended from the roof approximately 3 

ft down into the coalbed (fig. 31). The upper fracture was 

1/16 in wide near the top and widened to % in at the bottom. 

The second fracture was '/a in wide at the top and 

narrowed to Yl6 in at the bottom. Both fractures were 

propagated along face cleats with an orientation of N 66" 

W. No roof penetration or horizontal fractures were 

observed at this location. 

At location 4, a sand-filled fracture was essentially 

vertical but did stairstep short distances on horizontal 

bedding planes in the coal (fig. 32). Above the distinct 

shale bands the fracture was about 3/~6 in wide, and 

narrowed slightly to '/a in below the shale bands. The 

fracture extended from the floor to the top of the coalbed, 

and penetrated 0.15 ft into the overlying shale. No 

adverse mining conditions were experienced as a result of 

this minor penetration of the thin shale bed. The fracture 

at this location was predominately propagated along face 

cleats with an orientation of N 67" W. Short sand-filled butt 

cleat fractures were also observed. This location, at 55 
ft, was the maximum observed extent of a vertical fracture 

from the presumed location of the borehole. No horizontal 

fracture was present at this location. 

An extensive investigation of the pillars surrounding 

the one presumed to contain the borehole did not reveal 

any additional vertical fractures. Even though only minor 

exposures of butt cleat fractures were observed, a 

substantial portion of the fracture must have stairstepped 

along the butt cleat because it is otherwise impossible to 

extend the four nearly parallel face cleat oriented 

fractures back to the presumed location of the borehole. 

Horizontal fractures were observed at several locations, generally to the south of the borehole (fig. 28). The 

fractures were found on horizontal bedding planes in the 

thin shale below the rider (above the main bench), and on 

the shale-coal interface above and below the rider coal. At 

some locations, horizontal sand-filled fractures could be 

found on more than one of the described horizontal 

surfaces. The fractures ranged from 3/16 in to essentially 

only a single sand grain in thickness. The maximum 

distance a horizontal sand-filled fracture was observed 

from the presumed location of the borehole was 115 ft. 

LEGEND 

I \vertical fracture, sand filled 

Horizontal fracture, sand filled : 

-x-x-x- Bottom of rider 

-x-%- Bottom and top of rider 

-sss- In shale below rider 

Figure 28.-Mine map, borehole EM-8, with fracture orientations. 
- - 

CU 0.1 Shale 

Shale 

Figure 29.-Cross sectlon of fractures on rlb at locatlon 1 Figure 30.-Cross sectlon of fractures on rlb at locatlon 2 

near borehole EM-8. near borehole EM-8. 

CU - $1 =Shale 9 I - Shale 

-1 -. o! 

0. Shale shale 1 

Figure 31.-Cross sectlon of fractures on rlb at locatlon 3 Flgure 32.-Cross sectlon of fracture on rib at locatlon 4 near 

near borehole EM-8. borehole EM-8. 
EMERALD MINE, BOREHOLE EM-1 1 (9) 

Borehole Completion 

N \ LEGEND 1 --Horizontal fractures, 

sand filled 

Borehole EM-11 was drilled to a depth of 725 ft, 6 ft 

below the base of the 6-ft-thick Pittsburgh Coalbed. The 

top of the coalbed was at a depth of 713 ft. The hole was O Bore hole EM 

cased and cemented to the top of the coalbed. At the time 

of completion, little mining had been completed at the 

Emerald Mine. The borehole was located approximately 

1,000 ft from the bottom of the slope shown in figure 21. 0 

u 

50 68" \ f'" 

Treatment Implementation (May 14, 1977) Scale, ft '/ -LJ-.- 

Cleat 

Figure 33.-Mine map, borehoie EM-1 1, with fracture orientaBorehole EM-11 was stimulated with a patented tions. (Modified from Lambert (9)) 

treatment called a Kiel Frac (23). This treatment technique 

utilized multiple pressurization-depressurization cycles - - 

that would theoretically create long vertical fractures along 

transverse directions, then prop the fractures with reservoir 

rock material that sloughs off during the treatment. The 

treatment fluid was water with 801100- and 20140-mesh 

sand. The sand was added primarily to control fluid loss. In 

addition, the patent (23) claims that "the fracture formed in 

the producing formation may be confined to the upper 

portion of the producing formation where lithology 

(overlying rock) will limit upward growth ofthe fracture and 

sand properly scheduled will limit the downward growth of 

the fracture." 

The treatment on borehole EM-11 used over 54,600 

gal of water, injected at a rate of about 19 bbllmin, and 

10,000 lb of sand proppant. Surface pressures during the 

pressurization cycles averaged about 1,200 lbf/in2(ga). A 

continuous treatment record is not available for this 

borehole. Gas production from this borehole began 

relatively high (over 40,000 ft3/d) and reached a maximum 

of almost 87,000 ft3/d within a month after production 

began. The production rate gradually declined to 3,000 

ft3/d prior to termination of production before minethrough. 

Underground Observations 

Sand-filled fractures from borehole EM-11 were 

intercepted in June of 1978, approximately 1 yr after 

stimulation. The borehole was not mined through. The 

area of the fracture interception is shown in map view in 

figure 33. The only observed fractures were horizontal to 

slightly inclined and were located in the rider coal above 

the main bench of the Pittsburgh Coalbed (fig. 34). One of 

the fractures curved up into the shale overlying the rider 

coal. 

The penetration of the upper shale bed at this location 

may have been influenced by the presence of a clay vein. 

Clay veins are commonly characterized by planes of 

weakness at their interface with surrounding strata. The 

location of the exposures of horizontal fractures was 

approximately 110 ft from the presumed location of the 

borehole. No other exposure of sand-filled fractures was 

observed, and no adverse mining conditions were reported 

related to the presence of the horizontal fractures in the 

rider coal, which is commonly mined along with the main 

bench. Since borehole EM-11 was not completely mined 

around, additional sand-filled fractures may have been 

created, but not mined through. Vertical fractures that 

would have extended from the borehole in the face cleat 

direction could have been intercepted by the mine 

Coal bed 

25 - -_C - -- _C - - - - Floor -- - - - 

(Not to scale) 

Figure 34.-Cross section of fractures on rib near borehole 

EM-1 1. (Modified from Lambert (9)) 

development available for inspection, unless significant 

stairstepping (as the Kiel Frac was specifically supposed 

to accomplish) along the butt cleats had occurred. 

CUMBERLAND MINE, BOREHOLE CNG-1034 

Borehole Completion (24) 

Borehole CNG-1034 was drilled 77/s in. in diameter to 

a depth of 825 ft, a point 63 ft below the base of the 

Pittsburgh Coalbed. The target Pittsburgh Coalbed was 8 

ft thick and located from 754 to 762 ft in the borehole. The 

hole was cased with 5%-in-OD casing to a depth of 755 ft, 

1 ft into the top of the coalbed. A retrievable packer was 

set to prevent cement infiltration into the coalbed below 

casing. The distance from the borehole to active mining is 

not known. 

Treatment Implementation (May 5, 1977) (24) 

Prestimulation strata characterization tests included 

natural gamma ray, density, neutron, temperature, and 

caliper wireline geophysical logs along with a driller's log 

and drilling time log. The hole was stimulated with 

75-pct-quality nitrogen-generated foam with sand proppant. Prior to the stimulation treatment a mixture of 

Cal-sealbnd 3/4-in limestone chips was used to fill the 

4Reference to specific products does not imply endorsement by the 

Bureau of Mines. 
bottom of the hole below the Pittsburgh Coalbed to keep 

the treating fluids from invading the strata below. The 

treatment was initiated by pumping a 2,000-gal foam pad 

at a rate of 10 bbllmin. A formation break at 800 

lbf/in2(ga) surface was reported for this treatment. A 

continuous stimulation treatment record is not available 

for this borehole, but a summary of the general treatment 

chronology is presented in table 6. 

Table 6.-Summary of treatment chronology, borehole 

CNG-1034 (24) 

Foam vol, ' Sand 

gal Rate, Size Weight, 

Ibiqal Ib 

2,000 (pad) .......... NAp ........ NAP 

2.000 .............. N8 Minus 100 mesh . 1.000 

2,000 ................. 1.0 .. do ........... 2,000 

4,000 ................. 1.0 20140 mesh .... 4,000 

10,000 ................. 1.5 .. do ......... 15,000 

1.000 .................. 1.5 .. do ........... '1 500 

840 (flush) ........... NAp NAp ........... NAP 

NAp Not applicable. Figure 35.-Mine map, borehole CNG-1034, with fracture 

Constant injection rate of 10 bblimin. orientations (19). 

2Dye coated. 

Foam was injected at a constant rate of 10 bbllmin 

throughout the treatment, with sand added at an initial 

rate of 0.5 lb/gal and gradually increased in 0.5-lblgal 

increments to a maximum rate of 1.5 lblgal. Minus 

100-mesh sand was added at the beginning of the 

treatment as a fluid loss agent before the 20140-mesh sand 

was added as a proppant. Near the end of the treatment, 

1,500 lb of dye-coated sand was added to the stimulation 

fluid. A total of 21,840 gal of foam, 640 gal of water (flush 

at end), and 23,500 lb of sand were used in the stimulation 

treatment. The average surface treatment pressure was 

reported to be 1,200 lbf/in2(ga). The computer-designed 

treatment was to propagate a vertical fracture with a 

length of 2,764 ft. Gas production from this hole was low, 

with the maximum rate of 5,400 ft3/d reported approximately 1 month after stimulation. 

Underground Observations (1 9) 

Fractures from borehole CNG-1034 were intercepted 

in 1981, approximately 4 yr after stimulation. A vertical 

fracture filled with sand was observed 85 ft to the east of 

the presumed location of the borehole (fig. 35). The 

fracture extended from the floor to the roof along face cleat 

and varied in width from '/s to Y4 in. The fracture could be 

traced in the roof coal across the entry in the face cleat 

direction to where it was exposed as a vertical fracture on 

the corner of the opposite rib. The fracture continued 

through the corner of the rib, exiting on the opposite side. 

The fracture could be traced an additional 10 ft across the 

entry in the roof coal, approximately 130 ft from the 

presumed location of the borehole. No penetration of the 

strata above the coalbed was reported along the 

exposed length of the fracture. 

The entry adjacent to the pillar that contained the 

borehole would also be expected to contain an exposure of 

the fracture since it is closer to the borehole than the 

exposures previously described, however, a roof fall in this 

entry prevented direct observation of the area. Investigation of the roof fall revealed that it appeared to be related 

to a "roll" in the roof, which was characterized by highly 

disturbed bedding and abundant slickensided surfaces 

that characteristically have a lack of cohesiveness. Roof 

falls associated with similar geologic conditions are 

common at the mine site, therefore, the mine company 

personnel did not attribute the roof fall to the stimulation 

treatment. No evidence of a vertical fracture wing was 

seen to the west-northwest of the borehole, nor were any 

horizontal fractures reported at any location. 

UPPER FREEPORT COALBED, PENNSYLVANIA 

LUCERNE NO. 6 MINE, BOREHOLE RP-1 (12) 

Borehole Completion 

Borehole RP-1 was drilled with air, 7 in. in diameter 

to a depth of 675 ft, approximately 42 ft below the base of 

the target Upper Freeport Coalbed. The coalbed was 6.75 

ft thick with a 5-in-thick shale parting near the top of the 

coal interval, and was located from 626.5 to 633.25 ft in 

the borehole. Prior to casing the borehole, sand was used 

to fill the hole to the top of the coalbed as an extra 

precaution against cement penetrating the coal interval. 

The borehole was cased with 4%-in-OD casing with a 

cement basket shoe on the bottom as the primary means of 

keeping cement from reaching the coalbed below the 

casing. The bottom of casing was set at 622 ft, 4.5 ft above 

the top of the coalbed. The bottom of casing was to be set 

closer to the top of the coalbed, but debris from running 

the casing fell on top of the sand pack preventing the 

casing from reaching the desired depth. The basket shoe 

and sand pack were drilled out to a depth of about 2 ft 

below the base of the coalbed in preparation for 

stimulation. 
Treatment Implementation (January 21, 1982) 

Prestimulation strata characterization tests on this 

borehole were quite extensive. Gamma ray and density 

wireline geophysical logs were obtained prior to casing the 

borehole. A series of in situ state of stress (ISSOS) tests 

were made in the borehole prior to setting casing. The 

ISSOS test is essentially a "ministimulation" treatment 

that introduces a small amount of fluid into isolated zones 

in the wellbore to determine a "breakdown" pressure and 

instantaneous shut-in pressure. 

An ISSOS test in the roof rock above the coalbed (616 

to 624 ft) produced two formation breaks, one at slightly 

less than 800 lbf/in2(ga) bottom hole and a second at about 

1,600 lbf/in2(ga) bottom hole. The first break was 

interpreted to be a vertical fracture and the second a 

horizontal fracture (25). An instantaneous shut-in pressure (ISIP) of 750 lbf/in2(ga) bottom hole was recorded. No 

definitive formation break was recorded for the coalbed 

interval tested (626 to 634 ft); the ISIP was only 490 

lbf/in2(ga) bottom hole, which was also the maximum 

pressure recorded during the test. 

A series of mechanical property tests including 

triaxial compressive strength, Young's modulus, and 

residual friction angles and coefficients were conducted on 

coal and surrounding rocks from the mine site (25). In 

general, the rock was found to have a higher strength 

than the coal. A stimulation treatment would be expected 

to stay in the coalbed if higher stress andlor higher 

strength rocks surrounded it, and the treatment pressures 

are only allowed to barely exceed the pressure necessary 

to "break" the coalbed. In this case, a high enough 

differential between the ISSOS test values and the 

laboratory derived strength data would appear to be 

present to keep the stimulation treatment in the coalbed if 

the treatment pressure was kept near the 490-lbf/inz(ga) 

bottom hole ISIP value of the coalbed. 

The treatment for borehole RP-1 was to be very small, 

using only 2,775 gal of 75-pct-quality nitrogen-generated 

foam (injected at a rate of 5 bbllmin) and 1,360 lb of 

20150-mesh sand proppant. The treatment was small 

because the hole was located only 250 ft from mining. The 

computer-designed treatment would have theoretically 

produced two sand-filled fracture wings each extending for 

a distance of 209 ft from the borehole. The coalbed was not 

jet slotted prior to the stimulation since it was presumed 

to have already been fractured during the ISSOS test. 

During the initial stage of the treatment, while using 

clear water to initiate a fracture in the coal, treatment 

pressures steadily climbed and remained high. The 

continuous stimulation treatment record is shown on 

figure 36. Several cycles of pressure buildup and release 

- I 1 1 1 I I I 1 

KEY 

- - -- Surface pressure - - A Bottom-hole pressure 

. . . . . . . . Pumping rate, water 

----- -- - --A- Upstream density 

--L?--- - 

I I 1 I I 1 1 

5 10 15 20 25 30 35 40 

TIME, min 

Figure 36.-Treatment record, borehole RP-1. 
were made in an attempt to initiate a fracture, without 

success. A total of 1,600 gal of clear water containing 

green fluorescent paint was injected into the coalbed at 

rates ranging from 0.4 to 7.4 bbllmin. No sand proppant 

was included in the injected fluid. Maximum treatment 

pressures of 1,450 lbf/in2(ga) bottom hole and 1,260 

lbf/in2(ga) surface were reached. Table 7 summarizes 

parameters for the stimulation. 

Table 7.-Summary of treatment chronology, boreholes RP-1 

through RP-3 (12) 

Borehole and Treatment 

pigment color Stage Vol, gal 

RP-1: Green ........... Water pad .................... 1,600 

RP-2: 

Blue ................. Foam pad ..................... 10,000 

Orange .............. Foam and 0.5 Iblgal sand ...... 4,400 

Yellow ............... Foam and 0.5 to 1.0 Iblgal sand . 5,400 

RP-3: 

Red ................. Foam pad ..................... 20,000 

Blue ................. Foam and 0.5 Iblgal sand ....... 2,500 

Orange .............. Foam and 1.0 Iblgal sand ....... 1,600 

Yellow .............. .Foam and 1.5 Iblgal sand ..... 6,100 

Underground Observations 

Borehole RP-1 was mined through on February 12, 

1982, less than 1 month after stimulation. The borehole 

was found approximately 25 ft from the expected location 

determined by a deviation survey. Extensive mapping of 

existing geologic conditions was conducted near this 

borehole and the two companion boreholes (RP-2 and 

RP-3) prior to the stimulation and mine-through, to aid in 

interpreting the effect of the treatment on the coalbed and 

surrounding strata. A complete geologic interpretation of 

the area surrounding the three boreholes completed at 

this mine site can be found in the contract final report 

(12). Many visits were also made to the area expected to be 

influenced by the treatments as mining progressed 

towards the boreholes, so that any exposures on the 

advancing faces that would be lost by the continual 

advance would be documented. These multiple site visits, 

the use of fluorescent paint in the treatments, and the 

extensive mining around the three boreholes at the 

Lucerne No. 6 Mine make the underground characterizations (along with those of borehole TW-5 at the Oak Grove 

Mine) the most complete of the stimulation treatments 

The borehole (plus boreholes RP-2, RP-3, KE-2, and Vertical fractures (25)5 from borehole RP-1 were to be discussed later) was not for found in the southeast rib adjacent to the intercepted production since the project goals (BuMines contract 

50333908) were only to stimulate and evaluate the effect borehole (fig. 37). Nine vertical fractures in a zone 8Y2 in 

of the stimulation treatment on the coalbed and surround- 5Boldface numbers in parentheses refer to labeled fracture traces in 

ing strata. figure 37. 

Figure 37.-Mine map, borehoies RP-1 through RP-3, with fracture orientations (12). 
wide were present in the lower bench of coal below the 

shale parting (fig. 38). Two fractures contained sand that 

is assumed to have come from the sand placed at the 

bottom of the hole to protect the coalbed from cement 

invasion during the casing and cementing operation, since 

no sand was injected as part of the treatment. 

Several of the vertical fractures extended through the 

shale parting and into the upper bench. Pink-paint-tagged 

grout that had been used to plug the hole for abandonment 

was found in the upper portion of fractures whose lower 

portion contained sand. All of the fractures were aligned 

parallel to the face cleat (NJ76"iW) and were coated with 

green paint. Fractures that contained only paint were 

visible using the ultraviolet light. They appeared to be no 

wider than the face cleat itself. The sand-filled fractures 

were approximately SF, in wide. The most prominent 

vertical fracture could be traced with the ultraviolet light 

laterally across a small patch of roof coal remaining 

around the borehole. The fracture in the roof coal 

extended laterally from the borehole approximately 6 in to 

the nearby rib and 19 in into the entry in the opposite 

direction. The fluids followed a stairstep path alternating 

along the face and butt cleat in the roof coal, and did not 

travel along a single face cleat for more than 1 or 2 in. A 

summary of the orientation and fracture observations is 

given in table 8. 

No evidence of horizontal fracturing was found close 

to borehole RP-1, but traces of the green paint were found 

on the roof shale at several points in the entry. The 

inferred horizontal extent of the paint pigment is shown in 

figure 39. 

The vertical fractures either terminated at the roof or 

before reaching the roof. No evidence of roof penetration 

by the stimulation was observed. A small roof fall did 

occur at an intersection approximately 25 ft southwest of 

the borehole several days after being mined. The roof fall 

was attributed to the presence of compactional slips in the 

black shale roof. Examinations of the area with the 

ultraviolet light failed to detect any paint on any surface 

associated with the fall or around the fall. Similar roof 

falls related to slips are common in this area of the mine. 

Shale 

- I "=t n = Shale 

-LIryry- 

-Y -- -- -- Underclay - - --- - 

Figure 38.-Cross section of fractures at location 25 on the 

southeast rib near borehole RP-1. (Modified from Murrie (12)) 

Table 8.-Orientation and description of vertical fractures, boreholes RP-1 through RP-3 (12) 

Locatan, y~~~P,'~r;f ","e;4$t Fracture fllllng rnaterlals2 LOcatlonl iY;a;~r;f Que",4;,"t Fracture fill~ng materials2 

1 1 N 70 E R. Y 

2 1 N 46' W 0 Y 

3 1 N 69" E 0, y 

4 1 N 59 E 0, Y. S 

5 1 N54"W O,Y,S,C 

6 4 N 69" W 0 Y, S 

7 1 N 54" W 0, Y. S, C 

8 1 N 73" W 0 

9 1 N 33' E 0. y 

1 N 34" W 0. Y 

10 1 N 70" E 0. y 

1 N 26" E 0, Y 

11 1 N 9" E R. Y 

1 N 19' E R, Y 

1 N86-W R,Y 

12 1 N 27" E Y 

13 1 N 27 E Y 

14 1 N 60 W 0, Y 

15 1 N73W OY.SC 

1 N66 W 0 Y,S C 

16 1 N 62 W 0. Y, S 

17 1 N66W R 

18 1 N71W R 

3 N66W R 

19 1 N 24 E R, Y 

20 1 N 19 E R, Y 

2 1 9 N66W Y 

22 1 N 21 E R 

23 1 N66W R 

24 1 N71W R 

25 9 N 76 W G, S. C 

26 1 N 59 E 0. Y. S 

27 2 N 22 E R 

28 1 N69W Y 

'Shown on f~gure 37 

2Types of fracture-ftll~ng rnater~als-R, red paint, 0, orange palnt G, green pa~nt S sand C cement grout 
(EZZEl Green RP31 

Figure 39.-Mine map, boreholes RP-1 through RP-3, with horizontal distribution of paint on roof (12). 

LUCERNE NO. 6 MINE, BOREHOLE RP-2 (12) 

Borehole Completion 

Borehole RP-2 was drilled with air, 6 in, in diameter, 

to a depth of 660 ft, approximately 24 ft below the base of 

the target Upper Freeport Coalbed. The coalbed was 5.75 

ft thick with a 7-in-thick shale parting near the top of the 

coal interval, and was located from 630.4 to 636.15 ft in 

the borehole. Prior to casing the borehole, sand was used 

to fill the hole to the top of the coalbed as an extra 

precaution against cement penetration of the coal interval. The borehole was cased with 4%-in-OD casing with a 

cement basket shoe on the bottom as the primary means of 

keeping cement from reaching the coalbed below the 

casing. The bottom of casing was set at 627 ft, 3.4 ft above 

the top of the coalbed. Debris falling on top of the sand 

pack during the running of the casing prevented placing 

the bottom of the casing closer to the desired top of the 

coalbed. The cement basket shoe and sand pack were 

drilled out to a depth of about 2 ft below the base of the 

coalbed in preparation for stimulation. 

Treatment Implementation (January 27, 1982) 

Prestimulation strata characterization tests on this 

borehole were limited to gamma ray and density wireline 

geophysical logs and laboratory mechanical property tests 

on general mine samples as described in the discussion of 

borehole RP-1. Prior to initiating the stimulation treatment, a jetting tool was positioned approximately 1.5 ft 

above the base of the coalbed and rotated to erode a 

horizontal slot in the coalbed below the shale parting. The 

stimulation treatment for borehole RP-2 was designed to 

consist of 30,000 gal of 75-pct-quality nitrogen-generated 

foam and 15,000 lb of 20150-mesh sand proppant. 

Breakdown pressures in borehole RP-2 were 1,170 

lbflin2(ga) bottom hole and 945 lbflin2(ga) surface, using 

clear water. The continuous stimulation treatment record 

is shown in figure 40. Blue fluorescent paint was added to 

the foam pad. A summary of the paint colors, sand rates, 

and fluid volumes used in the treatment is given in 

table 7. Pump problems and time restrictions placed on 

the stimulation by the mine operator precluded pumping 

the scheduled 20,000-gal foam pad, which was reduced to 

10,000 gal. 

Treatment pressures declined from 1,210 to 1,105 

lbfiin2(ga) bottom hole and 1,150 to 1,060 lbf/in2(ga) 

surface before problems with the fracturing pumps began. 

The initial sand concentration was 0.5 Ib/gal of foam with 

orange fluorescent paint. After 13 min the sand concentration was increased to 1.0 lblgal of foam with yellow 

fluorescent paint. The treatment pressure began a gradual 

increase almost immediately after increasing the sand 
Figure 40.-Treatment record, borehole RP-2. 

0 241 I I I I I I I + 

concentration, indicating either the inability of the 

induced fractures to accept, or inability of the foam to 

carry, the higher sand load. If the pressure had continued 

to increase unchecked, a point might have been reached 

where the foam could not have carried the suspended sand 

load, and a screenout or dropping of the sand in the 

wellbore would have occurred, prematurely terminating 

the treatment. 

When it appeared that the bottom hole pressure 

would not stabilize or decline, the sand concentration was 

reduced to 0.5 lblgal of foam for the remainder of the 

treatment. Maximum pressures during stage 2 were 1,300 

Ibf/in2(ga) bottom hole and 1,250 lbf/in2(ga) surface. The 

average surface treatment pressure was about 1,150 

lbf/in2(ga). A total of 9,800 gal of foam and 7,000 lb of sand 

were injected into the coalbed during the second stage of 

the treatment. 

m 

\ 

a 

o 16- 

Underground Observations 

.-- Surface pressure 

I KEY - I Bottom-hole pressure 

-e-Downstream(belownitrogen)oeeee~ Pumpingrate,water(and sand) - 

Borehole RP-2 was mined through on March 25,1982, 

approximately 2 months after stimulation. The location of 

the borehole was approximately 25 ft from the expected 

location plotted from a deviation survey of the borehole. 

Numerous vertical fractures were observed surrounding 

the borehole. As mining approached the borehole from the 

northeast (subparallel to the butt cleat), several fractures 

in the butt cleat direction (N 31" E) were intercepted head 

on and traced to the borehole (4) (fig. 37). One of the butt 

cleat fractures (26) was sand filled to the point where it 

entered a pillar, 30 ft from the borehole. The trace of this 

fracture where it emerged on the opposite side of the pillar 

(3)) 67 ft from the borehole, contained only paint. These 

>* - - Upstream (above nitrogen) ---- 

V) 

Z 

0 0 

10 20 30 40 50 60 70 80 90 100 

TIME, min 

fractures generally terminated at the floor and extended 

upward to the top of the shale parting. A horizontal 

component was also observed on the interface between the 

top of the shale parting and the upper coal bench. No 

extensions of the butt cleat fractures were observed 

beyond the borehole to the southwest. 

Vertical fractures in the face cleat direction were 

observed as the borehole was mined through. One fracture 

(7) on the northwest rib (fig. 41) and one (5) to the 

southeast contained sand. The fracture on the northwest 

rib was closest to the borehole and was approximately l/2 

in wide and filled with sand proppant to the bottom of the 

parting. A second vertical fracture split off the main 

fracture approximately midway in the main bench of coal 

and rejoined near the base. Yellow and orange paint were 

found in the sand. The upper section of the fracture 

through the shale parting was filled with grout tagged 

with pink paint used to plug the hole for abandonment. 

The grout also extended a few inches horizontally along 

the interface between the shale parting and the upper 

bench of coal (fig. 41). Sand was found beyond the grout on 

the same horizontal plane in the rib for a maximum 

distance of 17 ft from the borehole to the northeast (fig 

42). The vertical sand-filled portion of the fracture 

narrowed to approximately Yls to l/s in where it extended 

to the southeast rib (5). Grout from the plugging operation 

was again found in the vertical fracture through the shale 

parting. Sand was present for a short distance horizontally at the interface between the top of the shale parting and 

the upper coal bench. A summary of the vertical fracture 

observations is given in table 8. 

The extent of paint on the horizontal interface 

between the top of the shale parting and the upper coal 
"iZ3'0 Cleat 

LEGEND 

EZ4 Yellow RP-I I 0 100 200 

B Red RP-2 Boreholes - Scale, ft 

Figure 42.-Mine map, boreholes RP-1 through UP-3, with horizontal distribution of palnt and sand on top of shale parting (12). 
coal interval and was located between 634.6 and 640.1 ft 

in the borehole. Prior to casing the borehole, sand was 

used to fill the hole to the top of the coalbed as an extra 

precaution against cement penetrating the coal interval. 

The borehole was cased with 4lh-in-OD casing with a 

cement basket shoe on the bottom as the primary means of 

keeping cement from reaching the coalbed below the 

casing. The bottom of casing was set at 632 ft, 2.6 ft above 

the top of the coalbed. As was the experience at the other 

two boreholes at this mine site, debris on top of the sand 

pack prevented placing the bottom of casing closer to the 

desired top of the coalbed. The cement basket shoe and 

sand pack were drilled out to a depth of about 2 ft below 

the base of the coalbed in preparation for stimulation. 

Treatment Implementation (January 30, 1982) 

Prestimulation strata characterization tests on this 

Lurehole were limited to natural gamma ray and density 

wireline geophysical logs and laboratory mechanical 

property tests on general mine samples as described in the 

discussion of borehole RP-1. Prior to initiating the 

stimulation treatment, a jetting tool was positioned 

approximately 1.5 ft above the base of the coalbed and 

rotated to erode a horizontal slot in the coalbed below the 

shale parting. The stimulation treatment for borehole 

RP-3 was designed to consist of 30,000 gal of 75-pctquality nitrogen-generated foam and 15,000 lb of the 

20150-mesh sand proppant. 

The stimulation of borehole RP-3 began with a small 

volume of clear water to break down the coalbed. The 

continuous stimulation treatment record is shown in 

figure 43. Breakdown pressures were 1,320 lbf/in2(ga) 

bottom hole and 1,080 lbf/in2(ga) surface. A 20,000-gal 

foam pad with red fluorescent paint was injected at a rate 

of 6 bbllmin. At the end of the pad, sand proppant was 

added to the treatment in concentrations that were 

gradually increased from 0.5 to 1.5 lb/gal of foam in 

increments of 0.5 lbtgal. At each increase in sand 

concentration a different paint color was introduced as 

shown in table 7. A total of 11,500 lb of sand was injected 

with 10,200 gal of foam during stage 2. Pressures declined 

from 1,365 lbf/in2(ga) bottom hole and 1,110 lbf/in2(ga) 

surface at the beginning of the foam pad to 815 lbf/in2(ga) 

bottom hole and 750 lbf/inYga) surface at the end of the 

treatment with no apparent increase in pressures as sand 

was introduced or its concentration increased. Average 

surface treatment pressure was about. 850 lbf/in2!ga). 

During the latter portion of the stimulation treatment of borehole RP-3, communication was observed with 

borehole RP-2, where small amounts of foam were 

detected bubbling from the casing. The two boreholes are 

approximately 225 ft apart and on a line that is oriented 

25" from the face cleat direction. It is possible that the 

decrease in treatment pressure resulted in part from the 

fractures created in the stimulation of borehole RP-3 

entering the area previously affected by the treatment of 

borehole RP-2. 

I I I 1 

Formation breakdown ShutdownStart water flush1 - 

Stop nitrogen 

Stop sand 

Sand, 1.5 l b/gal 

Sand,l.O lb/gall 1 1 

Start sand, 0.5 I b/gal 1 - 

\ 

3 

. I(- .... . .,*....-.......... .......... *...-**-**-• =* *=**== .. t I 

I I I I . 

I I 1 I I I 1 

--. Surface pressure - Bottom-hole pressure 

K EY 

- --- Downstream (below nitrogen) e-oo-o- Pumping rate, water (and sand) - 

-- Upstream (above nitrogen) 

- ---I--- ---/ . I .C)'L.-.-.-.-.- .-.-• -*-*-a 

I I I I I 

TIME, min 

Figure 43.-Treatment record, borehole RP-3. 
Underground Observations 

Borehole RP-3 was mined through on May 18, 1982, 

approximately 4 months after stimulation. The location of 

the borehole was approximately 25 ft from the expected 

location plotted from a deviation survey of the borehole. 

The greatest number of vertical fractures at the Lucerne 

No. 6 Mine site were found associated with borehole RP-3 

(fig. 37), which also had the largest stimulation treatment. 

Most of the vertical fractures seen near the wellbore were 

parallel to the face cleat. One butt cleat fracture was 

traced from the borehole (19) to the rib (22) of the pillar 

south of the hole and several others (12-13, 20-21) were 

observed at greater distances away from the borehole. The 

only sand-filled fractures (15-16) were in the face cleat 

direction and were oriented in a direction that would 

intercept the borehole. Representative cross sections of 

the northwest (16) and southeast (15) wings of this 

dominant fracture are shown in figure 44. The other 

vertical fractures associated with this borehole contained 

various combinations of red, yellow, and orange fluorescent paint on cleat surfaces in the lower bench of coal. 

The vertical fracture on the northwest rib (16) was 

filled with sand, had a maximum width of approximately 1 

in, and tapered to the width of a cleat at the base. This 

sand-filled fracture extended upward through the shale 

parting and became horizontal on top of the parting. 

Orange and yellow paint were found in the fracture (16) 

nearest the hole in the northwest direction and only red 

was found further out (23) in this direction. The 

sand-filled wing of this major fracture (15) on the 

southeast rib (fig. 44) was slightly more complex than that 

on the northwest rib. Two nearly parallel vertical sandand grout-filled (from the plugging operation) fractures 

were present. The sand filling extended from the floor to 

about halfway up the main coal bench, and the grout filled 

Coal bed 

and 

! d 

the remainder of each fracture up to its maximum vertical 

extent. Maximum width was 3/i in. The shorter vertical 

fracture terminated approximately 2 in below the parting. 

The other fracture terminated at the bottom of the shale 

parting, but a horizontal sand-filled fracture split off from 

it 2 in below the parting. This split intercepted the parting 

at a shallow angle and then cut through it at approximately 45" and became horizontal on top of the parting. 

Orange and yellow fluorescent paint were found in the 

fractures (15) closest to the borehole in this direction, red 

only slightly further away (24), and finally yellow only at 

the furthest extent of this fracture on the next rib (28). A 

summary of the vertical fracture observations is given in 

table 8. 

Sand on the interface between the top of the shale 

parting and the upper bench of coal was found a maximum 

of 200 ft from the borehole (fig. 42). Yellow was the most 

widespread paint on top of the binder, extending a 

maximum of 200 ft, however, there was an apparent 

overlap with the yellow paint injected in borehole RP-2. 

Red paint was found over a wide area, and was distributed 

in a pattern that consisted of several lobes elongated 

parallel to the face and butt cleat directions. This again 

suggests some control of the horizontal fluid placement by 

the cleat system in the coalbed. Very little orange paint 

was found on the top of the shale parting, perhaps because 

it was masked by the red and yellow paints. No blue paint 

was seen in any of the fractures at the Lucerne No. 6 

Mine, probably because of the inability to distinguish it 

from the glow of the ultraviolet light being used and 

because of large areas of blue fluorescent hydraulic fluid 

(from leaking hoses on mining equipment) on the ribs. No 

sand was found on the interface between the roof and the 

top of the coalbed, but red and yellow fluorescent paint 

were mapped as shown in figure 39. 

... :. .., .:.- '. ."..." #-. 

. . i.. .-. . .'!: . -' ' 

- " "' . ..,. . ''.,'.''4. I..'.: -- .,: ,:. .. .. . ,. :..',.. : Sand stone;j::;;:(l.;::. 

Coal bed 

and 

d 

nd 

Figure 44.-Cross section of fractures at location 15 on southeast rib (right) and location 16 on northwest rib (left) near borehole 

RP-3. (Modified from Murrie (12)) 
LOWER KlITANNlNG COALBED, WEST VIRGINIA 

KlTT NO. 1 MINE, BOREHOLE KE-2 (12) 

Borehole Completion 

Borehole KE-2 was drilled with air, 7 in. in diameter, 

to a depth of 685 ft, approximately 20 ft below the base of 

the target Lower Kittanning Coalbed. The coalbed was 5.2 

ft thick, and was located at a depth of 660 to 665.2 ft in the 

borehole. Prior to casing the borehole, sand was used to fill 

the hole to the top of the coalbed as an extra precaution 

against cement penetration of the coal interval. The 

borehole was cased with 4lh-in-OD casing with a cement 

basket shoe on the bottom as the primary means of 

keeping cement from reaching the coalbed below the 

casing. The bottom of casing was set at 659.5 ft, 0.5 ft 

above the top of the coalbed. 

Treatment Implementation (March 19, 1982) 

Prestimulation strata characterization tests on this 

borehole included caliper, gamma ray, and density 

wireline geophysical logs. A series of mechanical property 

tests to measure triaxial compressive strength, Young's 

modulus, and residual friction angles and coefficients 

were conducted on coal and surrounding rock samples 

collected at the mine. Analysis of the test results indicated 

that the roof shale tested was abnormally weak and soft 

for the rock type (25). This weak roof material is probably 

at least partially responsible for the severe roof control 

problems generally experienced at the mine. Extensive 

mapping of existing geologic conditions was conducted at 

the mine to aid in interpreting the effect of the treatment 

on the coalbed and surrounding strata. The details of the 

geologic characterization can be found in reference 12. 

Borehole KE-2 was stimulated with 75-pct-quality 

nitrogen-generated foam and sand proppant. The continuous stimulation treatment record is shown in figure 

45. The coalbed was broken down using a small volume of 

water. Pressures were 1,050 lbf/in2(ga) bottom hole and 

1,000 lbf/in2(ga) surface. The 20,000-gal foam pad with red 

fluorescent paint was injected into the coalbed at a rate of 

8 bbllmin. Treatment pressures gradually increased from 

1,050 to 1,130 lbf/in2(ga) bottom hole and 1,000 to 1,090 

lbf/in2(ga) surface at the end of the foam pad. Stage 2 

began with the introduction of sand proppant at a 

concentration of 1.5 lblgal along with the yellow fluorescent paint. Treatment pressure increased to 1,340 

lbf/in2(ga) bottom hole and 1,290 lbf/in2(ga) surface, 

leveled off, then rapidly rose to 2,300 lbf/in2(ga) bottom 

hole and 2,000 lbf/in2(ga) surface. The rise in treatment 

pressure was so rapid that the sand injection could not be 

stopped quickly enough to prevent a screenout and 

I 1 I 1 1 I I I I 

-Formation breakdown 

with water - 

Start sand,l.S Ib/gal 

-p--- ---- 

KEY 

Surface pressure - Bot tom-hole pressure 1 - / -.- Downstream (below nitrogen) .e.eee Pumping rate, water I 

Upstream (above nitrogen) (and sand) I s .. . .......*.*. . *.**..***. e* .** I - . 

. I . I I I I = I 1, A 

0 10 20 30 40 50 60 

TIME, min 

Figure 45.-Treatment record, borehole KE-2. 
LEGEND 

Distribution of red paint 

on roof 

Gray shale roof 

Sandstone roof - Vertical fracture, paint 

4 0 - 60 I20 

Scale, ft 

Figure 46.-Mine map, borehole KE-2, with fracture orientation. (Modified from Murrie (12)) 

Ibf/in2(ga). Sand proppant was introduced at the end of the 

foam pad in concentrations that were gradually increased 

from 0.25 to 1.5 lblgal in increments of 0.25 lbigal. 

Because of mechanical problems with the methane 

engines used to power the pumping unit on this borehole, 

continuous water and gas production were never achieved. 

Underground Observations 

Sand-filled fractures from borehole DGBH-5 were 

intercepted by mining in August 1984, approximately 2% 

yr after stimulation. A sand-filled vertical fracture was 

intercepted in the entry closest to the borehole (fig. 48). 

The fracture was approximately '/z in wide on the east rib 

closest to the borehole. The fracture paralleled the face 

cleat, but followed a sinuous path as shown in figure 49. 

The fracture extended from the floor to the rocf, but did 

not penetrate the roof. The mine operator reported a flow 

of water and gas from the fracture for some period of time 

after interception. The flow of water and gas was probably 

due to the inability to produce the borehole prior to 

interception. 
TIME, min 

Figure 47.-Treatment record, borehole DGBH-5. 

The fracture became horizontal at the interface 

between the top of the coalbed and the roof where it was 

generally only a few sand grains thick. Sand could be 

traced horizontally along the east rib a maximum of 29 ft 

from the vertical fracture. 

The vertical fracture was also found on the opposite 

side of the entry on the west rib. The fracture was 94 in 

wide and extended from floor to roof (fig. 49). The fracture 

was parallel to the face cleat along a sinuous path similar 

to that on the east rib. There were several thin (several 

sand grains wide) vertical fractures in the lower section of 

the coalbed parallel to the primary fracture. The fracture 

became horizontal at the interface between the top of the 

coalbed and the roof, and sand could be traced a maximum Borehole 

of 51 ft from the position of the vertical fracture on the 

west rib. The distance from the presumed location of the 

borehole to the vertical trace on the west rib was 72 ft and 

the maximum distance that sand on the horizontal surface 

was found from the presumed location of the borehole was 

105 ft. No penetration of the roof was observed at the west - Vertical fracture, sand f~lled 

rib or across the roof of the entry between the two vertical o 30 60 ---- Horizontal fracture, sand filled 

exposures. A thorough inspection of the coalbed on both f=DS Stopping with door 

Scale,ft cc Stopping 

sides of the entry on the opposite side of the west pillar 

failed to reveal any sand-filled vertical or horizontal Figure 48.-Mine map, borehoie DGBH-5, with fracture 

extension of the fracture. orientations. 
Shale 

-- - - - Underclay - - - - - - 

-- - 

Shale 

Coal bed 

Figure 49.-Cross section of fractures on east (right) and west (left) ribs near borehole DGBH-5. 

ILLINOIS NO. 6 COALBED, ILLINOIS 

INLAND STEEL MINE, BOREHOLE 1-NE (8) short production life of the borehole reached only 4,300 

ft3/d, due in part to the low gas content of the coalbed. 

Borehole Completion Underground Observations 

Borehole 1-NE was drilled 9 in. in diameter to a depth 

of 743 ft. The hole was cased to within a few feet of the top 

of the coalbed with 7-in-OD casing. The target Illinois No. 

6 Coalbed was 9 ft thick and was located from 

approximately 729 to 738 ft in the borehole. The coalbed 

was overlain by a thick sandy shale. 

Treatment Implementation (February 19, 1974) 

Borehole 1-NE was treated through 23/8-in-diameter 

tubing with a packer set in the casing at a depth of 720 ft, 

and the open end of the tubing extending to the middle of 

the coalbed at a depth of 733 ft. The treatment design to be 

implemented was to use gelled water (20 lb of guar gum 

per 1,000 gal of water) and sand to create two fracture 

wings, each 431 ft long, of which 305 ft would be propped 

by sand, 6.9 ft high and 0.2 in wide. A 2,000-gal gelled 

water pad was first injected into the coalbed to control 

leakoff. The continuous stimulation treatment record is 

shown on figure 50. Formation break was reported at 

1,050 lbf/in2(ga) surface. A pumping rate of 10 bbllmin was 

maintained throughout the treatment. Following the pad, 

an additional 10,000 gal of gelled water and 6,400 lb of 

10120-mesh sand was injected into the coalbed according 

to the schedule shown in table 9. The average injection 

pressure was 850 lbf/in2(ga) surface. Gas flows during the 

Fractures from the treatment were intercepted by 

mining in May 1974, approximately 3 months after 

stimulation. A single sand-filled vertical fracture was 

observed in four entries northeast of the presumed 

location of the borehole (fig. 51). The fracture extended 

from the roof to a hard shale parting approximately 2 ft 

from the bottom of the 9-ft-thick coalbed. The width of the 

fracture ranged from '/A to 3% in, averaging approximately 

'/4 in. A small amount of water drained from the fracture 

at the time of the mine-through. The sand-filled fracture 

could be traced across several entries in the 1 ft of roof coal 

generally left to protect the overlying shale from 

deterioration. The fracture in the roof coal was essentially 

the same width as the vertical exposures on the rib. 

The orientation of the vertical fracture (N 76" E) did 

not coincide with either the face (N 34" W) or butt (N 45" E) 

cleat, but was closest to the butt cleat orientation (fig. 51). 

It is possible that the induced fracture orientation was 

influenced by the horizontal stress field in the area. The 

major horizontal stress axis in south-central Illinois has 

generally been reported (26-27) to be east to eastnortheast, which would correspond quite well to the N 76" 

E fracture orientation. The reported fracture orientation 

could also be the result of unobserved stairstepping along 

face and butt cleat, which could give an apparent 

orientation for the fracture(s) oblique to the cleat 

orientation. 
* 1 J I 1 I r 1 1 1 I I I 1 1 I I I 1 

- --- KEY 

Surface pressure ....... Pumping rate, water - - /', (and sand) - , Coo-3-,-PoIIoC----3------~ 

I I 

,. *......; ;.****** ..................................... ................. .........I - 

4: :I 1: i !.' : -. 4. . ' . . .' . I I I I 1 I 1 1 1 I I 1 I I 

TIME, min 

Figure 50.-Treatment record, borehole 1-NE. (Modified from Elder (8)) 

Table 9.-Summary of treatment chronology, 

borehole 1-NE (8) 

Fluid volume, gal Sand rate, Ibigal 

2,000 (pad) ........................ 

3,000 

NAP .................................. 0.25 

2,000 ..................................... .50 

1,800 ...................................... .75 

3,200 .................................. 1 

NAp Not applicable 

At its maximum lateral extent, the vertical fracture 

was observed 416 ft from the presumed location of the 

borehole. This is the greatest length observed to date for a 

sand-filled vertical fracture wing. There was essentially 

no penetration of the roof rock except at one point where 

Elder (8) reported "there was some evidence of fluid and 

sand intruding a few inches into a rock joint that crossed 

the induced fracture." No adverse mining conditions were 

reported as a result of the stimulation treatment, and 

even at a follow-up visit 2.5 yr after the stimulation there 

was still no deterioration. No horizontal fracture component was noted at this mine site. 

r 

LEGEND 

-- Vertical fracture in 

ribandroofcoal, I 

sand filled 

0 

Flgure 51 .-Mine map, borehole 1 -NE, with fracture orientations. (Modified from Elder (8)) 

JAWBONE COALBED, VIRGINIA 

McCLURE NO. 1 MINE, BOREHOLE DG-1 A (9) 

Borehole Completion 

Borehole DG-1A was drilled to a depth of approximately 455 ft. The hole was cased with 4%-in-OD casing to 

within about 2 ft of the top of the target Jawbone Coalbed. 

A packer shoe on the bottom of casing was used to keep 

cement from reaching the coalbed. The coalbed was 5 ft 

thick and was located from 425 to 430 ft in the borehole. 

primary treatment consisted of 35,300 gal of 75-pctquality nitrogen-generated foam and 28,000 lb of 201 

40-mesh sand proppant. The average pumping rate was 16 

bbllmin. During a continuous production period over the 

first half of 1979, the borehole reached a maximum gas 

production rate of 45,000 ftVd, then gradually declined to 

slightly less than 30,000 ft3/d. A detailed production 

history of the borehole can be found in the paper by 

Lambert (9). Chronic mechanical problems with the 

methane engine used to power the water pumping unit 

severely limited the production potential of the borehole. 

Treatment Implementation (July 29, 1978) 

Underground Observations 

The borehole was treated down the casing with a 

packer installed at the surface. A continuous treatment Borehole DG-1A was mined through on May 12,1980, 

record is not available for this borehole. The treatment approximately 2 yr after stimulation. Both vertical and 

was initiated with a 4,200-gal water pad. A formation horizontal sand-filled fractures were observed, with the 

break was reported at 1,000 lbf/in2(ga) surface. The horizontal fractures predominating at the shallow depth 
of the coalbed. Vertical sand-filled fractures were found a observed approximately 2 to 5% in below the top of the 

maximum of 20 ft from the borehole and were oriented coalbed and ranged in thickness from 3/~ in to a hairline. 

parallel to the face cleat. The vertical fractures were The sand-filled horizontal fractures were observed over a 

completely contained in the coalbed, and ranged from 5/s in large, generally elliptical, area around the borehole. The 

to a hairline in width. No evidence of roof penetration was sand-filled horizontal fractures were observed a maximum 

observed. The horizontal fractures were generally of about 250 ft from the borehole. 

ROCK CANYON COALBED, UTAH 

SOLDIER CANYON MINE, BOREHOLE SC-1 in the fluid returns indicated that the tool had been placed 

in the coal interval. The stimulation treatment utilized 

Borehole Completion (12) 75-pct-quality nitrogen-generated foam, sand proppant, 

and green fluorescent paint. A 30,000-gal foam pad was 

Borehole SC-1 was drilled 6 in. in diameter to a depth 

of 740 ft, approximately 42 ft below the base of the target 

Rock Canyon Coalbed. The borehole was cased with 

4%-in-OD casing to the top of the coalbed. A cement 

basket shoe on the bottom of casing was used in addition 

to filling the hole with sand to the top of the coalbed to 

keep the cement from reaching the coalbed. The Rock 

Canyon Coalbed was 13.8 ft thick with 1.7 ft of shale 

partings in the upper 3.2 ft of the coalbed. The coalbed was 

located at a depth of 684 to 697.8 ft in the borehole. 

injected first to control leakoff, followed by 20,000 gal of 

foam containing 15,000 lb of 20140-mesh sand proppant. 

There was some evidence that the foam quality was lower 

than the specified 75 pct, therefore the actual foam 

volumes may be significantly less than reported by the 

service company. Foam treatment rates were maintained 

at approximately 16 bbllmin. Surface pressures during the 

injection of the foam pad quickly reached 1,000 lbf/in2(ga) 

then gradually increased to about 1,250 lbf/in2(ga). No 

obvious formation break was observed. The surface 

treatment pressure during sand injection reached a 

maximum of 1,500 lbf/in2(ga). The average surface 

Treatment Implementation (August 5, 1982) (12) treatment pressure was about 1,350 lbf/in7(ga). The 

continuous treatment record for this stimulation is shown 

Prestimulation strata characterization included gam- in figure 52. The borehole was not completed for 

ma ray, density, and caliper wireline geophysical logs. A production since the project goals were to only stimulate 

jetting tool was positioned about 7 ft above the base of the and evaluate the effect of the stimulation treatment on 

coalbed and rotated to cut a horizontal slot. Coal particles the coalbed and surrounding strata. 

I r Start Sand,O. 75 I b/gaI 

foam pad Sand,l .O I b /gal 1 Start water Shutdown flush, 1 

Sandp.75 l b/gal 

Start sand,0.5 lb/gaI 

dm--- 

/-- 

KEY 

Surface pressure 

Pumping rate, water 

(and sand) 

TI ME, min 

Figure 52.-Treatment record, borehole SC-1. 
Underground Observations 

The area around the presumed location of the 

borehole was mined through in April 1985. A directional 

survey run on the borehole was used to plot the coalbed 

intercept point, which was calculated to be approximately 

35 ft north-northeast of the surface location and near the 

center of an entry (fig. 53). A thorough examination of the 

roof and ribs at the expected underground borehole 

location and all accessible entries in the area failed to 

reveal the borehole or any of the sand proppant used in the 

stimulation. 

Examination of the area with the ultraviolet light 

revealed a substantial amount of green fluorescent 

material on the coal faces, and initially it was thought to 

be the green fluorescent paint used in the treatment. The 

fluorescence was generally restricted to short (less than 

2-in) horizontal patches or bands in the coal. Closer 

examination revealed that the fluorescing material was 

actually naturally occurring resins that are common in 

the Cretaceous age coalbeds of the western United States. 

Blacklight surveys were then conducted at points as far as 

several thousand feet from the presumed location of the 

borehole, where there was essentially no possibility of the 

presence of paint from the stimulation. Green fluorescing 

material of the same character as was observed near the 

presumed borehole location was also seen at these remote 

locations. 

There seemed to be more green fluorescence in the 

area of the presumed borehole location, so it is possible 

that some paint was actually present, but if so, it could not 

be distinguished from the naturally occurring fluorescent 

materials. It is also possible that there actually was more 

Expected coalbed 

intercept location, 

borehole SC- I 

Scale. ft 

Figure 53.-Mine map, borehole SC-1. 

naturally occurring fluorescent material at this location, 

or that it was more noticeable because of the fresh, 

unweathered exposures. In any case, it would be advisable 

to check for any naturally occurring fluorescence before 

selecting a paint color for use in a stimulation treatment. 

If the borehole was located within the general area 

expected, and if the stimulation generally followed the 

face cleat orientation (N 65" W) as would be assumed, then 

a wing of the fracture would have a high probability of 

being intercepted by the available entries at the time of 

observation (fig. 53). The obvious incorrect presumed 

location of borehole SC-1 points out the need for accurate 

surveys (surface, underground, and borehole deviation) 

and the advantages of actually mining through the 

borehole in interpreting underground observations. 

SUMMARY OF OBSERVATIONS 

The areas affected by 22 Government-sponsored 

stimulation treatments in coalbeds have been mined 

through and observed underground. The underground 

observations have revealed a variety of conditions ranging 

from extensive vertical and horizontal sand-filled and/or 

paint-coated fractures to no discernible fractures. A 

summary of the treatment parameters and underground 

observations is given in tables 10 and 11. Eight of the 

treatments included the addition of fluorescent paint as an 

aid in locating and mapping the paths of fluid movement 

in the exposed coalbed and surrounding strata. With the 

paint, it was generally possible to trace the paths of fluid 

movements farther than the presence of the more obvious 

sand-filled fractures. It was also possible to identify 

additional zones of multiple fluid pathways. A good 

example of the additional traceable fracture length was 

observed at borehole DHM-6 (Oak Grove Mine, Alabama), 

where sand-filled fractures could be traced a maximum of 

95 ft compared to paint-coated cleat observed 630 ft from 

the borehole. 

When paint-coated fractures were identified using the 

ultraviolet light, the fracture width was essentially only 

the width of the naturally occurring cleat that had been 

penetrated. The cleat would not have been identified as a 

pathway for stimulation fluid without the presence of the 

paint. It is not known if the paint-coated cleat in fact 

represents a path of increased permeability that will 

enhance the flow of water and gas to the wellbore. 

Most of the observed sand-filled and paint-coated 

vertical fractures penetrated or paralleled face cleat and 

occasionally butt cleat in the coalbed. It is interesting to 

note that the longest sand-filled vertical fracture (416 ft, 

borehole 1-NE, Illinois No. 6 Coalbed) did not parallel a 

cleat direction, but was approximately 30" from the butt 

cleat orientation. This may be the result of the influence of 

local horizontal stress fields or unobserved stairstepping, 

which gave an apparent orientation oblique to the cleat 

orientation. At several locations, fractures that paralleled 

the face cleat did not directly intercept the borehole if 

extended back from their point of observation. Apparently 

these treatment fluids followed a stairstep pathway 

through the coalbed, alternating along face and butt cleat, 

with only the face cleat fractures being observed on the 

exposed ribs and pillars. 

Vertical sand-filled fractures were generally wider 

nearest the borehole and narrowed rapidly away from the 

borehole. The maximum lateral extent of sand-filled 

vertical fracture wings was generally short. Nine boreholes had sand-filled fracture wings with a maximum 

length of 30 ft or less, three boreholes had fracture wings 

70 to 100 ft in length, and only four had wings over 100 ft 

in length. Zones of multiple, thin parallel sand-filled 

and/or paint-coated fractures are common near the 

boreholes. At increasing distances from the boreholes, the 

number of multiple vertical fractures usually decreases 

and only a single fracture remains. Vertical fractures also 
Max observed 

Max observed vertical Zones of horizontal fracture Multiple 

Hole location Fluorescent wing length. ft Max width multiple length, ft horizontal Roof 

Borehole exposed paint used Sand vertical vert~cal Sand Paint fracture penetration 

filled Paint fracture, fractures filled coated coated planes In 

TW-1 ....... Yes ..... No. ........ 5 NAP % Yes ......... NA8p NAp NAp ......... Yes. 

TW-2 ...... Yes ..... No ........ 16 NAP 4'12 Yes ...... NAp Yes ........ No. 

TW-3 ....... Yes ..... No ........ 210 NAP %s No .......... NAP NAp NAp ......... Yes. 

TW-4 ....... Yes ..... No ...... 220 NAP %s NO ......... NAP NAp NAp.. ..... Yes. 

TW-5 ....... Yes Yes ..... ....... NAp 370 Cleat Yes ......... NAp 230 Yes ......... Yes. 

DHM-5 ...... No ...... Yes ...... 340 Cleat Yes ......... NAp NAp NAp ......... Yes. 

DHM-6 .... Yes ... Yes ...... 630 %s Yes ......... NAp NAp NAp ......... Yes. 

USBM-4 ..... Yes .... No ........ 20 NAP 2lh Yes ........ NAp NAp ......... No. 

EM-5 ........ No ...... No ....... 140 NAP ......... N:g .......... llh No NAp No No. 

EM-6 ....... Yes ... No .... 10 NAP 2'h Yes ......... 10 NAp Yes ........ Yes. 

EM-7 ........ Yes .... No ....... 18 NAP lh No ......... NAp NAp ......... No. 

EM-8 ........ No ...... No ........ 55 NAP Yes ......... 7% NAP Yes ......... Yes. 

EM-1 1 ..... No ...... No ...... NAP NAp ......... 110 NAp No .......... Yes. 

CNG-1034 . No . No ....... ytg NAg N? No ... NAp N NAP ......... NO. 

RP-1 ........ Yes ..... Yes ...... 2 '116 Yes ........ .......... N;? No No. 

RP-2 ........ Yes ..... Yes ....... 30 67 lh Yes ......... 265 Yes ......... No. 

RP-3 ........ Yes ..... Yes ....... 20 130 1 Yes ........ 200 200 Yes ......... No. 

KE-2 ........ No ...... Yes ....... 8.5 Cleat No .......... 100 No .......... No. 

DGBH-5 ..... No ..... No ........ N% NAP 2 Yes ........ %g NAp No ......... No. 

1-NE ...... NO ...... No ........ 416 NAP .......... 34 No NAp NAp ......... Yes. 

DG-1A ...... Yes ..... No ........ 20 NAP ......... ......... 5 NR Y% NAp Yes No. 

SC-1 ........ No ...... Yes ....... NAp NAP NAp NAp ......... NAp NAp NAp ......... NAP. 

NAp Not applicable. NR Not reported. 'Sand from fill at bottom of hole, not from treatment. 

commonly do not extend the entire height of the coalbed, 

but are found in only part of the coalbed. In the Blue 

Creek Coalbed, when fractures did not extend the entire 

height of the coalbed they were usually present in the 

upper part, and less commonly, only in the lower part of 

the coalbed. This may be related to variations in the 

physical character of the Blue Creek Coalbed, which is 

commonly more friable and "soft" in the upper part. 

It has been suggested (9, 28-29) that gelled water 

stimulation treatments with sand proppant would theoretically produce relatively short, wide, sand-filled vertical 

fractures and foam treatments would produce longer, 

narrow, sand-filled fractures. Only four of the treatments 

observed underground used gelled water as the treatment 

fluid and therefore a definitive statement relating the 

relative differences between the character of fractures 
resulting from the different fluids is not possible. 

However, the widest sand-filled fracture observed (4% in, 

borehole TW-2) was from a gelled water treatment. A 

2%-in-wide sand-filled fracture was also observed for the 

gelled water treatment on borehole USBM-4, but a 

2%-in-wide sand-filled fracture was also observed for the 

foam treatment on borehole EM-6. The longest sand-filled 

vertical fracture observed (416 ft, borehole 1-NE) was also 

from a gelled water treatment where short fractures 

would be expected. 

The type of stimulation fluid has some influence on 

the character of the induced fractures, as do other factors 

such as treatment volume, injection rate, and depth and 

physical characteristics of the coalbed. Larger volume 

treatments tended to have more observable fractures as 

was seen in the three progressively larger treatments in 

the Upper Freeport Coalbed (boreholes RP-1, RP-2, and 

RP-3). Boreholes TW-5 and DHM-6, which had two of the 

largest volume treatments also had some of the most 

extensive networks of fractures. 

The physical properties of the coalbed seemed to have 

some influence on the propagation of fractures. Many of 

the vertical fractures in the Blue Creek Coalbed were 

present only in the more friable upper part. The vertical 

sand-filled fractures at borehole TW-2 were located in the 

bottom "hard" part of the Blue Creek Coalbed and inclined 

and became horizontal at or near the interface between 

the lower "hard" part and the upper "soft" friable part. 

The Upper Freeport Coalbed (boreholes RP-1, RP-2, and 

RP-3) had a distinct shale parting near the top of the 

coalbed. In some cases the parting acted as a barrier to the 

upward growth of fractures, but it was more commonly 

penetrated by the vertical fractures, which then became 

horizontal at the interface at the top of the parting and 

base of the upper coal bench. A distinct "shaley" band in 

the Pittsburgh Coalbed at borehole EM-8 was generally 

penetrated by the vertical fractures that continued 

vertical above, but in one instance the fracture became 

horizontal above the band. It was also common to see 

small horizontal offsets in the vertical propagation of the 

fractures as different layers or bedding planes in the 

coalbed were encountered. These offsets occasionally 

resulted in an inclined or sinuous appearance to the 

"vertical" fracture. While the physical properties of the 

coalbed seemed to have some influence on the character of 

the fractures, that influence was variable and could not be 

considered predictable. 

A horizontal fracture was discernable in approximately half of the mined through stimulation treatments. 

Five of eight treatments that used fluorescent paint and 

seven additional treatments that did not use paint had 

identifiable horizontal fractures. It is generally thought 

that at increasing depths (overburden pressure) the 

incidence of horizontal fracturing decreases. A horizontal 

fracture was found in stimulated coalbeds as deep as 1,145 

ft in the Blue Creek Coalbed (borehole TW-5). This 

fracture was only identifiable as paint coatings since no 

sand was used in the treatment on borehole TW-5. The 

two treatments with fluorescent paint in the Blue Creek 

Coalbed that did not have identifiable horizontal fractures 

were, at depths of 1,384 ft (borehole DHM-5) and 1,239 ft 

(borehole DHM-6), the deepest stimulations mined 

through. The deepest stimulation with a sand-filled 

horizontal fracture was 1,093 ft in the Blue Creek Coalbed 

at borehole TW-2, however, the rest of the sand-filled 

horizontal fractures were found at depths of 400 to 800 ft. 

It seems likely that additional horizontal fractures (or 

fluid pathways) would have been observed, especially in 

the shallow coalbeds, if fluorescent paint had been used in 

more treatments. 

The most common location for the horizontal fractures is at the top of the coalbed at the interface with the 

roof rock. Occasionally horizontal fractures are found 

along other distinct interfaces such as shale partings, 

shaley or hard, dense bands, or rider coals in the 

immediate roof rock. The distribution of sand or paint on 

horizontal surfaces was geilerally erratic (partially because of the difficulty in mapping their entire area), but in 

two cases (boreholes RP-2 and RP-3) there was some 

suggestion that there was a correlation between cleat 

orientation and horizontal lobes of paint. Also at borehole 

TW-5 the horizontal (and vertical) fractures covered an 

area with a general elliptical shape with the long axis 

parallel to the face cleat orientation. Multiple horizontal 

fractures on different planes have been observed in 

several cases. 

The sand.-filled horizontal fractures are generally 

thickest near the wellbore (maximum of 1 in), usually are 

thin (less than YZ in) but quite variable in thickness along 

short lateral distances, and may be discontinuous along 

an exposed rib. The maximum lateral extents for 

sand-filled horizontal fractures were 250 ft in the Jawbone 

Coalbed (borehole DG-1A) and 200 ft in the Upper 

Freeport Coalbed (borehole RP-3). The maximum lateral 

extents for paint-coated horizontal fractures were 265 and 

200 ft in the Upper Freeport Coalbed (boreholes RP-2 and 

RP-3) and 230 ft in the Blue Creek Coalbed (borehole 

TW-5). 

Penetration by fluids and/or sand proppant into strata 

directly overlying the main bench of coal has been 

observed in nearly half of the treatments intercepted 

underground. In addressing the significance of roof 

penetration it is necessary to point out that roof 

penetration does not necessarily equate to roof damage 

or an adverse effect on mining. There have been no roof 

falls attributed to the penetration of stimulation fluids 

into roof strata from Government-spopsored treatments. 

Lambert (9) did report that supplementary roof support 

was installed as a precautionary measure in the vicinity of 

borehole TW-3 at the Oak Grove Mine, where both the 

mined Blue Creek Coalbed and the overlying Mary Lee 

Coalbed, 5% ft above, were stimulated and where mine 

management observed "roof movement" along a nearby 

rib. According to Lambert (9), the area was mined through 

"without experiencing any roof fall during mining 

operations." 

Except for the Blue Creek Coalbed, most of the 

penetrations of strata above the main coal bench have 

been fairly limited in vertical andlor horizontal extent. In 

several cases, penetration of strata above the Blue Creek 

Coalbed was observed at locations a hundred feet or more 

from a borehole. The reason for the more extensive roof 

penetrations is not conclusively known, but may be 

related to the complex structural history of the area. 

Recent in situ state of stress (ISSOS) tests conducted near 

the Oak Grove Mine indicated lower in situ stress values 

for the rocks surrounding the Mary Lee-Blue Creek 

Coalbeds than measured in the coal (30). In the absence of 

a stress barrier or mechanical property barrier, upward 

fracture breakout is more likely. Upward fracture 

breakout from this coal section was reported during the 

ISSOS testing near the Oak Grove Mine. The presence of 
naturally occurring roof joints coupled with lower or 

similar in situ stresses above the coal probably influenced 

the extent of roof penetration at the Oak Grove Mine. 

In several cases the strata penetrated above coalbeds 

have been weak, thin shales below rider coals, and the 

fractures have been contained in strata that was generally 

mined along with the main bench of coal throughout the 

mine to prevent it from deteriorating and falling at a later 

time. Thus such strata penetration should not really be 

considered roof penetration. 

Part of the problem of acceptance of the use of 

stimulated vertical boreholes to remove gas from coalbeds 

prior to mining may be related to the use of the terms 

"fracture," "fracturing," and "breakdown," which perhaps 

suggest a catastrophic breakup of the strata treated. The 

evidence from direct underground observation and many 

of the treatment records (including those from boreholes 

not mined through) suggest that new fractures are seldom 

created, but rather naturally occurring planes of weakness (cleat, joints, or bed boundaries) are entered and 

widened to varying degrees. In most cases the penetration 

of strata overlying the main coal bench has been 

attributed to the invasion of preexisting joints, as 

evidenced by similar joints of the same general character 

and orientation occurring throughout a mine. 

CONCLUSIONS 

It is impossible to guarantee that a stimulation of prestimulation strata characterization tests and intreatment in a coalbed will not adversely affect mining in formed treatment design and controlled implementation 

some way. However, the underground evidence and (primarily injection rates and therefore treatment pressexperience to date suggests that the probability of ure) can probably further minimize the chance of adverse 

adversely affecting mining conditions is minimal. The use mining conditions. 

REFERENCES 

1. Finfinger, G. L., and J. Cervik. Review of Horizontal 

Drilling Technology for Methane Drainage From U.S. Coalbeds. 

BuMines IC 8829, 1980, 20 pp. 

2. Hagood, D. W., R. C. Pate, and J. W. Stevenson. Methane 

Control in an Advancing Section of an Underground Coal Mine 

(contract S0395033, Jim Walters Resour, Inc.). BuMines OFR 

94-84, 1983, 39 pp.; NTIS PB 84-185057. 

3. Perry, J. H., G. N. Aul, and J. Cewik. Methane Drainage 

Study in the Sunnyside Coalbed, Utah. BuMines RI 8323, 1978, 

11 PP. 

4. Prosser, L. J., Jr., G. L. Finfinger, and J. Cervik. Methane 

Drainage Study Using an Underground Pipeline, Marianna Mine 

58. BuMines RI 8577, 1981, 29 pp. 

5. Thakur, P. C., and W. N. Poundstone. Horizontal Drilling 

Technology for Advance Degasification. Min. Eng. (Littleton, 

CO), V. 32, NO. 6, 1980, pp. 676-680. 

6. Von Schonfeldt, H., D. R. Pothini, G. N. Aul, and R. L. 

Henderson. Production and Utilization of Coalbed Methane Gas 

in Island Creek Coal Company Mines. Paper in Proceedings of the 

Unconventional Gas Recovery Symposium (May 16-18, 1982, 

Pittsburgh, PA). SPEIDOE 18817, 1982, pp. 235-244. 

7. Dunn, B. W. Coal as a Conventional Source of Methane: A 

Review and Analysis of 50 Wells in Two Production Areas in the 

Black Warrior Basin of Alabama. Paper in Proceedings of the 

Unconventional Gas Recovery Symposium (May 13-15, 1984, 

Pittsburgh, PA). SPEIDOEIGRI 12875, 1984, pp. 407-416. 

8. Elder, C. H. Effects of Hydraulic Stimulation on Coalbeds 

and Associated Strata. BuMines RI 8260, 1977, 20 pp. 

9. Lambert, S. W., M. A. Trevits, and P. F. Steidl. Vertical 

Borehole Design and Completion Practices To Remove Methane 

Gas From Mineable Coalbeds. U.S. Dep. Energy CMTCITR-8012, 

1980, 163 pp. 

10. Steidl, P. F. Foam Stimulation To Enhance Production 

From Degasification Wells in the Pittsburgh Coalbed. BuMines 

RI 8286, 1978, 10 pp. 

11. Stubbs, P. B., F. X. Dobscha, and J. V. Mahoney. 

Degasification of the Blue Creek Coal Seam at Oak Grove Mine. 

Paper in Proceedings of the 2d Annual Symposium on MethaneRecovery-From Coalbeds (Apr. 18-20, 1979, Pittsburgh, PA). ed. 

by R. L. Wise. U.S. Dep. Energy, METCISP-7919, 1979, pp. 

96-113. 

12. Murrie, G. W., J. T. Popp, C. M. McCulloch, and D. J. 

Steele. Development and Evaluation of Near Mine Stimulation 

Techniques for Coalbed Degasification (contract 50333908, 

Dames and Moore). BuMines OFR 135-84, 1984, 127 pp.; NTIS 

PB 84-220664. 

13. McCulloch, C. M., S. W. Lambert, and J. R. White. 

Determining Cleat Orientation of Deeper Coalbeds From 

Overlying Coals. BuMines RI 8116, 1976, 19 pp. 

14. Diamond, W. P., C. M. McCulloch, and B. M. Bench. Use of 

Surface Joint and Photolinear Data for Predicting Subsurface 

Coal Cleat Orientation. BuMines RI 8120, 1976, 13 pp. 

15. Diamond, W. P., G. W. Murrie, and C. M. McCulloch. 

Methane Gas Content of the Mary Lee Group of Coalbeds, 

Jefferson, Tuscaloosa, and Walker Counties, Ala. BuMines RI 

8117, 1976, 9 pp. 

16. Murrie, G. W., W. P. Diamond, and S. W. Lambert. Geology 

of the Mary Lee Group of Coalbeds, Black Warrior Basin, 

Alabama. BuMines RI 8189, 1976, 49 pp. 

17. Boyer, C. M., and P. B. Stubbs. Measurement of Dynamic 

Coalbed Resewoir Conditions Required To Design Near-Mine 

Methane Control Activities in the Warrior Coal Basin. U.S. Dep. 

Energy DOE/ET/14204-1477, Feb. 1983, 133 pp. 

18. Lambert, S. W. (Taurus Exploration, Inc.). Private communication, 1985; available upon request from S. W. Lambert, 

Taurus Exploration, Inc., Tuscaloosa, AL. 

19. Boyer, C. M. (US. Steel Corp.). Private communication, 

1985; available upon request from C. M. Boyer, U.S. Steel Corp., 

Monroeville, PA. 

20. Lambert, S. W., and M. A. Trevits. Methane Drainage: 

Experience With Hydraulic Stimulation Through Slotted Casing. 

BuMines RI 8295, 1978, 16 pp. 

21. . Methane Drainage Ahead of Mining Using Foam 

Stimulation-Mary Lee Coalbed, AL. U.S. Dep. Energy RI-PMTC3(79), Jan. 1979, 22 pp. 

22. Mahoney, J. V., P. B. Stubbs, F. C. Schwerer, and F. X. 

Dobscha. Effects of a No-Proppant Foam Stimulation Treatment 

on a Coal Seam Degasification Borehole. J. Pet. Technol., v. 33, 

No. 11, 1981, pp. 2227-2235. 

23. Kiel, 0. M. Hydraulic Fracturing Process Using Reverse 

Flow. U.S. Pat. 3933205, Jan. 20, 1976. 
24. Frantz, R. L., J. F. Keeney, and R. Stefanko. Feasibility 

Studies and Demonstration of Methane Recovery From Pennsylvania Coal Seams (contract 50155168, PA State Univ.). BuMines 

OFR 4-79, 1978, 187 pp. 

25. Towse, D. F., M. E. Hanson, M. S. Costantino, A. Kusubov, 

B. J. Qualheim, and L. D. Thorson. Coalbed Methane Research 

Annual Report-FY 82. U.S. Dep. Energy UCRL-53421, Mar. 

1983, 36 pp. 

26. Engelder, T. Is there a Genetic Relationship Between 

Selected Regional Joints and Temporary Stress Within the 

Lithosphere of North America? Tectonics, v. 1, No. 2, Apr. 1982. 

pp. 161-177. 

27. Blevins, T. C. Coping With High Lateral Stresses in an 

Underground Illinois Coal Mine. Soc. Min. Eng. AIME preprint 

82-156, 1982, 7 pp. 

28. Halliburton Services. The Fracbook Design Manual for 

Hydraulic Fracturing. Halliburton Services, Duncan, OK, 1971, 

111 pp. 

29. Perkins, T. K., and L. R. Kern. Widths of Hydraulic 

Fractures, Paper in Proceedings of the 36th Annual Fall Meeting 

(Soc. Petrol. Engr., AIME, Oct. 8-11, 1961). SPE-AIME, 1961, pp. 

265-277; J. Pet. Technol., v. 13, No. 9, 1961, pp. 937-949. 

30. Gas Research Institute Multiple Coal Seam Project. Art. in 

Quarterly Review of Methane From Coal Seams Technology. V. 3, 

No. 2, Sept. 1985, pp. 39-48. 
APPENDIX.ACQUlSlTl0N AND LIMITATIONS OF STIMULATION TREATMENT DATA 

There are large measurement uncertainties associated with the recording of any hydraulic stimulation 

treatment. The reader should keep this fact firmly in mind 

when reading the treatment records reproduced in this 

report. In most cases where two or more independent 

records of treatment parameters were kept, these records 

seldom agreed, although supposedly all of the information 

in the records came from the same instruments at the 

same time. The records available from the treatments 

include the recorded pressure and water flow data (and 

any other parameters that may have been recorded 

continuously), the notes usually made by the engineer in 

charge of the treatment, and in some cases notes made by 

Government or other personnel during the treatment. 

Two measurements commonly are recorded during 

every hydraulic stimulation treatment. These are the 

water pumping rate at the pump truck and the surface 

pressure. In addition, during some treatments, additional 

information can be, but typically is not, recorded. This 

additional information includes the density of the fracturing fluid at the pump truck, the density of the fluid as it 

goes into the wellhead, the bottom hole pressure, and the 

fracturing fluid flow rate at the wellhead. 

The liquid pumping rates are determined by counting 

pump strokes and multiplying by a factor based upon the 

cylinder volume and the pump efficiency. The accuracy of 

the measurements is probably about 5 pct. Nitrogen for 

foam fracturing treatments is carried as a liquid, so liquid 

flowmeters are used to measure the nitrogen flow rate 

before it is heated and vaporized. The accuracy of the 

meters is not commonly given, but is probably no better 

than 5 pct. Surface pressures may be measured using a 

variety of types of instruments. Pressure should be the 

most accurately determined quantity of any measured 

during a treatment. It should be possible to determine 

treating pressures to an accuracy of 1 pct (or within 10 

lbf/in2 per 1,000 lbflinL(ga)). In practice an accuracy of 

-t 100 lbf/in2 is probably very good. 

The fluid density is determined by using a densitometer employing a gamma ray source and a detector that 

counts the gamma rays returning through the fluid. Most 

pump trucks carry one to allow measurement and control 

of cement density while cementing casing in oil and gas 

wells. The instruments are calibrated for higher densities 

than those of fracturing fluids, but they should still be 

usable, and they were used to measure sand concentrations during the treatments of some of the wells described 

in this report. 

The exact method of measuring the sand injection 

rate is unclear, but probably it is estimated from the 

density of the sand and water mixture at the truck. The 

method of determining the total weight of sand injected at 

the end of the treatment appears to be to take the 

difference between the original sand volume carried to the 

site and the remaining sand volume after completion of 

the treatment. If this is the case, then the sand injection 

rates given during the course of the treatments (and 

shown on some of the treatment records in this report) are 

estimates only and do not necessarily give accurate sand 

injection rates. 

Bottom hole pressure may be measured in two 

different ways. The most accurate method, but for 

practical reasons the most difficult and expensive to 

employ, is to place a remote-reading pressure gauge at the 

bottom of the hole. A simpler method is to run tubing in 

the hole to the depth of the coalbed and fill the tubing with 

water. At the wellhead a pressure gauge is then installed 

on the top of the tubing. The pressure gauge on top of the 

tubing then reads the bottom hole pressure less the 

hydrostatic pressure of the water in the tubing. The 

hydrostatic pressure can then be added back in to obtain 

the bottom hole pressure. The treatment is conducted by 

pumping fluids down the annulus between the casing and 

the tubing. The method suffers from the drawback that 

the tubing can be sealed off by coal or sand, or gas can 

enter the tubing, causing incorrect pressure readings. 

This method was used in the wells described in this report 

where bottom hole pressures are given. It should be kept 

in mind that bottom hole pressures can differ from surface 

pressures because of the hydrostatic pressure of the 

treating fluid in the borehole and because of friction losses 

due to pumping. These two sources of pressure differences 

have opposite effects, so the bottom hole pressure can be 

less than, greater than, or the same as the surface 

pressure, depending upon the conditions of the treatment. 

During gelled water treatments, all measurements at 

the wellhead will be the same as those at the truck and are 

redundant. However when foam is being pumped, it is 

necessary to determine the density and flow rate of both 

the water and sand mixture leaving the pump truck and of 

the foam mixture after the nitrogen has been added 

(downstream of the pump truck) in order to determine the 

true volumetric pumping rate of the foam (a water, 

nitrogen, and sand mixture). This additional information 

has been obtained for some of the wells described in this 

report, but in general is not available for most foamstimulated wells. 

Because the foam mixture is a compressible fluid, its 

actual (volumetric) pumping rate is controlled by the 

pressure. During a typical foam stimulation treatment the 

liquid and gas flow rates are measured separately and the 

volumes are then added, with the nitrogen flow rate being 

determined at the surface pressure. An attempt is 

generally made to maintain a foam quality of 75 pct (or 75 

pct nitrogen by volume and 25 pct sand and water by 

volume) at the surface for optimum sand carrying 

capacity. This is done by estimating the average treating 

pressure at the beginning of the treatment and calculating the nitrogen required to give a gas volume of 75 pct of 

the total foam volume. 

Occasionally during a treatment as pressures change, 

the nitrogen rate is corrected, but difficulties in adjusting 

the rate of the nitrogen fiow make service company 

personnel reluctant to continually adjust the nitrogen flow 

rate to maintain the proper foam quality. Because of this, 

large errors in the estimated volume of the treatment can 

occur. The estimated treating rate is obtained by 

multiplying the water rate by a factor of 4. This is only 

correct as long as the foam quality is 75 pct. However if 

the pressure deviates from the initial planned treating 

pressure (or indeed if the estimate was incorrect tc~ begin 

with) then the foam quality may never be 75 pct. 

Foam qualities as low as 50 pct have been observed 

and 60 pct is common. If a treatment planned for a 75-pct 

foam quality is actually run at 50-pct quality, then the 

true foam volume pumped will only be one half of that 
initially planned. A planned 50,000-gal foam treatment 

could then become a 25,000-gal treatment with the same 

material masses pumped into the formation. In addition, 

all of these measurements are made at surface conditions 

and the foam quality is adjusted for surface conditions. 

However it is the bottom hole foam quality that really 

should be maintained at 75 pct and without knowing the 

bottom hole pressure there is no guarantee that the 

bottom hole foam quality is correct. 

As an example, consider a treat~nent planned for a 

foam injection rate of 10 bbllmin at a pressure of 500 

lbf/in2(abs). The required nitrogen rate to maintain a 

75-pct quality at this pressure is 185 std ft3/min. If' the 

pressure increases to 1,000 lbf/in2(abs) during the treatment, the required nitrogen volume is doubled to 370 std 

ft3/min. If the nitrogen rate is not increased, which is 

commonly the case, then the foam quality is reduced to 50 

pct and the actual pumping rate becomes 5 bbllmin. This 

effect is more pronounced at the lower pressures used in 

shallow coalbed wells than in deeper holes, which require 

higher treating pressures. For instance it would require a 

pressure increase from 1,000 to 2,100 lbf/in2(abs) to 

necessitate another doubling of the nitrogen rate. Pressure changes of 500 lbf/in2 are common during the course 

of a foam stimulation, but changes of 1,000 1bf/in2 occur 

much less frequently. 

The fact that the measurement errors can be much 

larger at low pressures is a problem because the service 

company personnel, being familiar with oil and gas wells 

treated at much higher pressures are less likely to 

consider it necessary to adjust nitrogen rates during the 

course of a treatment. Their reluctance to adjust the 

nitrogen rate is also increased because it is difficult to 

adjust the nitrogen rate and because the very act of 

increasing the nitrogen rate leads to changes in 

volumetric flow rate, which can increase the pressure 

leading to a need for further increases in nitrogen flow. It 

may take some time to reach an equilibrium flow rate. 

Starting with too much nitrogen is not the answer because 

at high qualities (possibly 85 pct and certainly 90 pct) the 

foam begins to lose its sand carrying properties. 

The measurements of pumping rates, pressures, 

densities, and sand weights are not always highly 

accurate. In the few cases observed by the Bureau where 

two or more independent measurements of the same 

quantity were available, the measurements disagreed by 

a large amount. In the case of a treatment at the Soldier 

Canyon Mine, Utah, at one point there were three surface 

pressure readings available ranging from about 1,500 to 

2,000 lbflin2(ga). None of the three gauges agreed and two 

showed differences of 300 IbfIinYga) (a 20-pct difference). 

Pressure should be the easiest treatment parameter to 

measure accurately. Pumping rates, densities, and nitrogen rates are more difficult to measure accilrately and 

it is therefore reasonable to have even greater suspicion of 

treatment volumes computed from measurements of these 

parameters. 

One way to potentially improve the reliability of the 

treatment records is for customers to discuss the planned 

treatment and the measuring techniques in detail with the 

service company sales engineers well in advance of the 

treatment and with the field engineer just prior to the 

treatment. The actual effectiveness of this scenario is 

probably limited, however. The most important individual 

to discuss the treatment with prior to its start is usually 

the busiest and most difficult to arrange time to talk to. In 

addition, this individual may in some cases misinterpret 

an interest in the details of a treatment as suggesting a 

iack of trust on the customer's part. Requests for 

information on the accuracy of gauges and meters may be 

met with comments that the measurements are exactly 

correct, and that there are no measurement errors, or the 

engineer may simply not know the accuracy of his or her 

equipment. The same is true of a request for information 

on the exact method of operation of the equipment and the 

mode of measurement of the treatment parameters. These 

parameters can vary with the equipment used and can 

only be determined with certainty at the job site by 

discussion with the field engineers. 

One important area where improvement is perhaps 

more likely is in recordkeeping. On many of the records 

studied for this report it was found that service company 

personnel tended to leave out scale information on the 

pressure and flow rate charts. The scale information most 

often left off was the time scale. Customers should be 

careful to request that this information be placed on these 

records or to note the scales themselves and add them to 

the records when they obtain them. It should also be made 

clear to the service company personnel by the customers 

that they want copies of these records, since copies are 

always made, but not necessarily given to the customer 

unless requested. It is also important that the customers 

keep their own notes as a check on the accuracy of the 

service company field engineer's field notes. The job ticket 

used for computing the treatment cost should not be 

trusted to give an accurate history of the treatment, since 

this record may be altered in the field for various reasons, 

such as giving unadvertised job discounts. 

I NT.-BU.OF MI NES ,PGH. ,PA. 28482