UNITED STATES DEPARTMENT OF THE INTERIOR 
UNITED STATES DEPARTMENT OF THE INTERIOR 

Cecil D. Andrus, Secretary 

BUREAU OF MINES 
Elder, Curtis H 

Effects of hydraulic stimulation on coalbeds and associated strata / by Curtis H. Elder. [Washington] : U.S. Dept. 

of the Interior, Bureau of Mines, 1977. 

20 p. : ill., maps, diagrams ; 27 cm. (Report of investigations - 

Ilureau of Mines ; 8260) 

Bibliography: p. 20. 

1. Methane, 2. (:oal mines and mining - safety measures. 3. 

Mine gases. I. United States. Hureau of Mines. 11. Title. 111. 

Title: llydraulic stimulation. IV. Series: United States. Ilureau 

of Mines. Report of investigations - Hureau of Mines ; 8260. 

TN23.U7 no. 8260 622.06173 

U.S. Ilept. of the Int. Library 
CONTENTS 

Page 

Abstract ............................................................ 1 

Introduction ............................................................. 1 

Acknowledgments ......................................................... 2 

Pittsburgh coalbed. Washington County. Pa .. Test hole USBM No . 4 ........ 2 

Drilling and hydraulic stimulation .................................. 2 

Observations underground ............................................ 5 

Illinois No . 6 coalbed, Jefferson County, I11 . . Test hole USBM No . 1NE .. 10 

Drilling and hydraulic stimulation .................................. 10 

Observations underground ........................................... 12 

Discussion and conclusions ............................................... 18 

References .............................................................. 20 

ILLUSTRATIONS 

Location of hydraulic stimulation test hole in relation to mine 

development at time of stimulation ................................ Structure map contoured on Pittsburgh coalbed showing location 

of Vesta No . 5 mine and test hole ................................. Pressure and flow rate charts recorded during hydraulic stimulation . Mine development at time the hole was intercepted ................... Induced fracture orientation relative to mine entry development ..... Induced fracture in face cleat of Pittsburgh coalbed ................ Induced sand-filled fractures in butt cleat direction in Pittsburgh 

coalbed ........................................................... Detail of induced fracture in middle of Pittsburgh coalbed at 

maximum width ..................................................... Detail of induced fracture terminating at draw slate or rock 

strata overlying coalbed .......................................... Induced fracture after 2 years of exposure in mine .................. Location of degasification borehole pattern for No . 6 coalbed, 

Inland mine, Jefferson County, I11 ................................ Pressure and flow rate charts recorded during hydraulic stimulation . Induced fracture orientation as related to mine entry development 

at Inland mine .................................................... Geologic structure map of No . 6 coalbed contoured on No . 6 coalbed, 

showing directional trend of induced fracture ..................... Trace of sand-filled induced fracture in roof coal of mine entry 

of Inland mine at point A on figure 13 ............................ Detail of induced fractures in roof coal and termination of fracture at rock strata overlying coalbed at point A on figure 13 ..... Sand-filled induced fracture i n r i b and roof coal at point C on 

figure 13 ..................................................... 16 

18 . Sand-filled induced fracture in rib.and roof coal at point B on 

figure 13 ......................................................... 16 

19 . Location of induced fracture in bleeder entry after the area was 

mined out ...................................................... 17 

20 . Induced fracture after 2-1/2 years of exposure to mine atmosphere 

at point A on figure 19 ........................................... 17 
ILLUSTRATIONS --Continued 

Page 

21. Induced fracture after 2-112 years of exposure to mine atmosphere 

at point B on figure 19.. ......................................... 18 

TABLES 

1. Designs for stimulation of borehole USBM No. 4, ,Washington County, 

Pa........ ...................................................... 3 

2. Designs for stimulation of borehole USBM No. lNE, Jefferson 

County, I11 ....................................................... 11 
EFFECTS OF HYDRAULIC STIMULATION ON COALBEDS 

AND ASSOCIATED STRATA 

by 

Curtis H. Elder 

ABSTRACT 

In studies conducted by the Bureau of Mines, two test areas were hydraulically stimulated and exposed by mining: One in the Pittsburgh coalbed at 

the Vesta No. 5 mine, Washington County, Pa.; and one in the Illinois No. 6 

coalbed at the Inland mine, Jefferson County, Ill. Induced fractures were 

contained totally within the coalbed with no adverse effect on the stability 

of associated rock strata. Induced fractures were vertical and were propagated normal to the direction of least residual tectonic stress. Gas production was increased fivefold to twentyfold by hydraulic stimulation. 

INTRODUCTION 

Hydraulic stimulation is a process developed by the oil industry for 

increasing productivity of a reservoir. Vertical test holes drilled in 

advance of mining have been hydraulically stimulated to degasify the coalbed 

more efficiently by increasing the gas flow. Increases of fivefold to 

twentyfold have been achieved (1-2) - - .2 The procedure consists of inducing 

fractures in a coalbed by applying hydraulic pressure with controlled injection of gelled water and propping sand. The fractures are propagated several 

hundred feet into the coalbed by pumping large volumes of the treatment fluid. 

Flow characteristics of the fluid and size of propping sand were found to 

affect the length and width of the induced fracture. The propped fracture 

forms an open, *permeable path of high-flow capacity to the borehole. 

In spite of its advantages, concern for maintaining the integrity of mine 

roof and floor has deterred coal mine management from applying hydraulic stimulation to increase gas flow from vertical degasification boreholes in advance 

of mining. This concern motivated the Bureau of Mines to investigate the 

effects of the stimulation treatment on coalbeds and the rock strata that form 

the roof and floor of the mine in several areas of the United States. 

'~eolo~ist, Pittsburgh Mining and Safety Research, Bureau of Mines, Pittsburgh, 

Pa. 

2~nderlined numbers in parentheses refer to items in the list of references at 

the end of this report. 
Hydraulically stimulated areas have been exposed by mining at test sites 

in the Pittsburgh and Illinois No. 6 coalbeds, providing the opportunity for 

observation underground. This paper documents the hydraulic stimulation procedures and subsequent underground observations of the effect on these coalbeds and associated roof and floor rock strata. 

ACKNOWLEDGMENTS 

I thank Herbert Steinman, chief engineer, and Bob Thomas, mine superintendent, Vesta No. 5 mine, Jones & Laughlin Steel Corp., California, Pa.; and 

Dick Shockley, engineer, Inland mine, Inland Steel Co., Sesser, Ill., for providing rights-of-way, mine data, and access to the mines, and especially for 

their assistance in achieving the goals of this investigation. Thanks are 

also due to Ray Wenzel, assistant division engineer, and Halliburton Services 

Co., Pittsburgh, Pa., for their help in designing and implementing the stimulation treatment. /-- 

I 1 

Maximum calculated / \ 

stimulation radius / ' Stimulation PITTSBURGH COALBED, WASH- 

\/ Y' INGTON COUNTY, PA. : 

I 

Face 

N 70" W 

0 

1 

200 

Scale, feet 

Diamond drill 

hole No. 12 

FIGURE 1. - Location of hydraulic stimulation test hole 

TEST HOLE USBM NO. 4 

Drilling and Hydraulic 

Stimulation 

To determine the effect 

of hydraulic stimulation on 

the Pittsburgh coalbed and 

the overlying rock strata, a 

test hole was drilled at 

Vesta No. 5 mine in Washington County, Pa. The hole 

was located 500 feet ahead 

of an active developing section of the mine (fig. 1) . 

Drilling began April 10, 

1974, with a rotary drill 

rig. Twenty feet of 7-5/8- 

inch surface casing was set 

for control of shallow 

water -bearing strata. A 

6-1/4-inch hole was then 

drilled to a total depth of 

597 feet. The Pittsburgh 

coalbed was reached at 588 

feet. Six feet of the coalbed was drilled, and the 

hole terminated three feet 

be low the coa lbed . 

in relation to mine development at time of stimulation. 
The test hole penetrated a series of interbedded shales, limestones, 

sandstones, and coalbeds in the Monongahela and Dunkard Group sediments of the 

Pennsylvanian System. The drill site lies near the axis of a small synclinal 

fold on the flank of the Amity anticline (fig. 2). In the vicinity of the 

borehole, the Pittsburgh coalbed is immediately overlain by a thin shale (draw 

slate) and underlain by calcareous claystone. 

A plan was designed to hydraulically stimulate the coalbed within a 500- 

foot radius from the test hole so that the induced fractures would not intercept mine entries (fig. 1). 

Table 1 shows the estimated results expected for the stimulation treatment for volumes of 5,000 and 10,000 gallons of gelled water (3). Design 1 

was chosen in order to contain the fracture within the area of-unmined coal. 

The propping sand was treated with a baked-on fluorescent dye for easier 

recognition in the mine when mining intercepted the fracture. 

TABLE 1. - Designs for stimulation of borehole USBM No. 4, 

Washington Countv. Pa. 

Total volume.. ........................... .Mgal.. 

Pad volume ................................ Mgal.. 

Propped fracture length ................... feet.. 

Propped fracture height .................... do ... Viscosity .......................... centipoises.. 

Fracture width .......................... inches.. 

Propping sand .................. 100-pound sacks.. 

Production. ...................... fold increase.. 

On April 17, 1974, the well was treated through 2-318-inch-diameter highpressure tubing. An open hole packer in the tubing string was set at 585 feet 

in a hard formation 3 feet above the coalbed. The treatment used 7,300 gallons of water containing 550 pounds of guar gum, 3,000 pounds of 10- to 20- 

mesh fluorescent-dye-treated sand followed by 500 pounds of 10- to 20-mesh 

regular sand. 

Design 1 

10.7 

Created length1.. ....................... ;.feet.. I 387 

Two thousand gallons of gelled fluid was injected into the coalbed to 

fill all spaces in the coalbed around the borehole and initiate the fracture. 

The formation broke at 500 psig. The induced fracture was propagated into the 

coalbed with an injection rate of 10-112 bpm. The pad volume was followed 

with successive injections of 1,300 gallons of sand-laden treating fluid at 

114 ppg (pounds per gallon) sand concentration, 650 gallons at 112 ppg, 1,750 

gallons at 314 ppg, and 1,350 gallons at 112 ppg sand concentration. The tubing was cleared of sand with 250 gallons of:treat.ing fluid. An injection rate 

of 10-112 bpm was maintained during treatment at an average injection pressure 

of 1,586 psig. A maximum injection pressure of 1,700 psig was reached 

(fig. 3). 

Design 2 

16.9 

662 

=Maximum length of induced fracture created with and without sand 
A KEY MAP 

PENNSYLVANIA 

2 Hydraulic fracture direction 

--A- Cool contours, 20-ft intervols 

0 Test hole 

Exploration dr~ l l hole 

+- Surface roads 

0 

, 

2 000 

Stole, feet 

FIGURE 2. - Structure mop contoured on Pittsburgh coolbed showing location of Vesta No, 5 mine and test hole, 
recorded during hydraul ic 

stimulation, 

TIME, rnin 

Observations Underground 

Mine entries closest to the borehole were monitored by Bureau and mining 

company engineers during the hydraulic fracture treatment. No evidence of gas 

or water emission was observed in the mine entries during the treatment. This 

indicates that the treatment remained within the design limits and did not 

create hazardous underground conditions. Ventilation air samples taken at 

regular intervals during and following treatment showed no increase in gas 

emission into the mine. 

On June 12, 1974, the induced fractures were intercepted by mining. 

Twenty feet of vertical sand-propped fracture was mapped along the face cleat 

direction west of the borehole. The sand-filled fracture ranged from oneeighth to one-half inch in width, extending from the draw slate at the top of 

the coalbed to the mine floor. There was no evidence that the fracture 

extended into the roof or floor rock (figs. 4-6). Wide vertical fractures 

were propagated to the north of the borehole along the butt cleat direction. 

These fractures were 112 to 2-112 inches in width and were packed with the 

propping sand (figs. 7-8). They extended from the base of the draw slate to 

the floor. Again, no evidence was found to indicate that the induced fractures along the butt cleat direction penetrated the roof (fig. 9) or floor 

rock. 

Roof stability was not affected by the induced fracture, and mining 

advanced normally through the treated zone. The dyed propping sand fluoresced 

under ultraviolet radiation, making very thin sand-filled fractures more 

apparent; however, the use of dyed propping sand was not needed to locate the 

fractures. It is estimated that the fractures extended 35 feet or more into 

the coalbed in the butt cleat direction. The fractures were wider than 

expected, averaging 2 inches in width. The greater width of the fractures was 

caused by the use of a heavily gelled treating fluid (75 pounds of guar gum 

per 1,000 gallons); a less viscous fracture fluid (20 pounds of guar gum per 

1,000 gallons) would have created a thinner fracture, propagating deeper into 

the coalbed. Approximately 35 cu ft of fracture volume was created, which was 

filled with 35 cu ft of propping sand. 
//-- - 

0 \ 

Maximum calculated / \ \ stimulation radius ,/ Stimulation I . , tact bole 

0 

w 

200 

Scole, feet 

FIGURE 4. - Mine development at time the hole was intercepted. 
Fractures in butt c

~lll/,,,

////I//////// ////////I/ .P /,r/ z///l"/"/'///'///'//////////h////////////// 

\\ 20 in Fracturein - 

face cleat TA 17 

B 

11-32 f t 

Room t- -------- - 

I -+- 

155 I I I 

I I 

I First cut 

I 

No. 7 entry 

d 

Fa ce 

cleat 

LEGEND 

--- Fracture mined out 

A, 6-Photographs token 

0 

u 

Scale, fe

FIGURE 5; - Induced fracture orientation relative to mine entry development. 
FIGURE 6. - Induced fracture in face cleat of Pittsburgh 

coalbed. Note termination of fracture at 

draw slate; 
FIGURE 8. - Detail of induced fracture in middle of Pitts- 

burgh coalbed at maximum width. 
g 

inch-diameter holes were 

drilled with a rotary drill and cased with 7-inch-outside-diameter steel casing to within a few feet of the top of the coalbed. The northeast corner 

hole, No. 1NE of the pattern, was selected for hydraulic stimulation (fig. 11). 

It was drilled to a total depth of 743 feet and penetrated 9 feet of the 

No. 6 coalbed. 

The test hole was drilled through a series of interbedded shales, sandstone, limestones, and coals in the Bond, Modesta, and Carbondale formations 

of the Missourian and Des Moines series of the Pennsylvanian System. The 

drill site is on the southwest flank of the Illinois coal basin. The No. 6 

coalbed is overlain by 18 feet of sandy shale and underlain by 1 foot of fire 

clay. 

A hydraulic stimulation was designed that would induce a fracture within 

a 450-foot radius from the borehole, and thus totally contained within the 

coalbed. Table 2 shows the calculated results expected from three stimulation 

treatment designs for fluid volumes of 5,000, 12,000, and 15,000 gallons (3). 

Design 2 was selected for the stimulation treatment. The coalbed was st&- 

lated through a 2-318-inch-diameter high-pressure tubing. A packer in the 

tubing string was set in the casing at a depth of 720 feet, with a tailpipe 

extending to the middle of the coalbed to a depth of 733 feet. Twelve thousand gallons of water was gelled with 240 pounds of guar gum to form a light 
FIGURE 11. - Location of degasification borehole pattern for No. 6 coalbed, 

In land mine, Jefferson County, 

I I I. 

0 * 1,000 

Scale, feet 

gel fluid. The gel fluid and 6,400 pounds of 10- to 20-mesh propping sand 

were used in the treatment. 

TABLE 2. - Designs for stimulation of borehole USBM No. lNE, 

Jefferson County, 111. 

Production ....................... fold increase.. 

Total volume.. ........................... .Mgal.. 

Pad volume ................................ Mgal.. 

Propped fracture length ................... feet.. 

Propped fracture height .................... do ... 

Design 3 

8.0 

Design 1 

5.5 

- - 

Viscosity .......................... centipoises.. 

Fracture width .......................... inches.. 

Propping sand .................. 100-pound sacks.. 

Created length1 ........................... feet. . 

Two thousand gallons of gel fluid was injected into the coalbed to fill 

all spaces around the borehole and initiate the fracture. The formation broke 

at 1,000 psig. The fracture was propagated into the coalbed with an injection 

rate of 10 bpm. The following volumes of sand-laden gel fluid were successively injected into the coalbed: 3,000 gallons at 114 ppg sand concentration; 

Design 2 

7.5 

5.0 

2.0 

14 0 

6.9 

12.0 

2.0 

305 

6.9 

l~aximum length of induced fracture created with and without sand pro 

6 

0.17 

26 

222 

6 

0.20 

64 

43 1 
2,000 gallons at 112 ppg; 1,800 gallons at 314 ppg; and the final 3,200 gallons at 1 ppg. The average injection pressure was 900 psig, but a maximum 

injection pressure of 1,000 psig was reached (fig. 12). After treatment, the 

test hole was put back on gas and water production. The hole was not producing a significant amount of gas, perhaps owing to formation damage during 

drilling. Gas flow increased from 10 to 4,300 cfd. Testing of the No. 6 

coalbed before stimulation had shown it to have a low gas content and low permeability, so the flow increase was substantial. 

Observations Underground 

The induced vertical fracture packed with propping sand was intercepted 

by mining in May 1974. The induced fracture was propagated in a direction of 

N 76" E from the borehole. It was mapped across four entries of the No. 11 

Right entries off the West main entries of the mine (fig. 13). The induced 

vertical fracture did not follow the directions of the face or butt cleat; it 

followed a path subparallel to the axis of a small anticlinal fold on the 

flank of the coal basin (fig. 14). The stimulation created a single vertical 

fracture in the coalbed. A small amount of water drainage from the fracture 

alerted the operator of the continuous miner that the sand-filled fracture had 

been mined through. The fracture was mapped for 416 feet from the borehole. 

The sand-propped fracture extended vertically from the roof shale to a hard 

shale parting approximately 2 feet from the bottom of the coalbed. The fracture was 7 feet in height. It is normal practice in this mine to leave 1 foot 

of roof coal during mining to protect the shale overlying the coalbed from air, 

moisture, and weathering. 

TIME, min 

FIGURE 12. - Pressure and flow rate charts recorded during hydraulic stimulation. 
mately one -quarter inch (figs . 15 -18). The sand -propped fracture did not 

extend into the roof or floor. The roof coal and rock remained stable in all 

the entries intersecting the fracture. Mining developed normally through the 

stimulated areas. Mine engineers noted a reduction in gas emission into the 

mine in the test area resulting from a long period of degasification prior to 

mining. Approximately 61 cu ft of fracture volume was created which was 

filled with sand. Sixty-four cubic feet of sand was used during the treatment. The 3-cu-ft difference can be attributed to sand that settled in the 

borehole. The excess sand was removed from the borehole immediately after 

treatment. 

The site of the induced fracture exposed by mining was inspected 2-1/2 

years after stimulation to assess the long-term effects of weathering on the 

roof strata along the trace of the fracture. No deterioration of roof or 

floor strata was evident. 

The roof along the east-west bleeder entry at point A (fig. 19) had 

remained stable. There had been no spalling of roof or coal rib along the 

trace of the induced fracture (fig. 20). 

At points B and C (fig. 19) in the north-south entries where the induced 

fracture was in close proximity to retreat mining, the roof remained stable 

until the coal in the adjacent pillars was mined out (fig. 21). Roof fall 

resulting from pillar extraction did not reflect any weakness in the roof 
FIGURE 14. - Geologic structure map of No. 6 coalbed contoured on No. 6 coalbed, showing dire

induced fracture. 
FIGURE 15. - Trace of sand-filled induced fracture in roo

coal of mine entry of Inland mine at point A 

on figure 13. 
FIGURE 17. - Sand-filled induced fracture in rib and roof 

coal at point C on figure 13. 
Sand-filled induced 

fracture7 

I 

FIGURE 19. - Location of induced fracture in bleeder 

entry after the area was mined out, 
FIGURE 21. - Induced fracture after 2-1/2 years of exposure to mine atmosphere at point B 

on figure 19. 

strata that could be related to the induced fracture. There was some evidence 

of fluid and sand intruding a few inches into a rock joint that crossed the 

induced fracture at point B (fig. 19). There was no evidence that this 

intrusion of sand and fluid in the rock joint impaired the roof or affected 

its stability. 

DISCUSSION AND CONCLUSIONS 

Observations at two hydraulically stimulated sites in coalbeds indicate 

that the resulting fractures are limited to, and contained within, the coalbed. There was no adverse effect on the stability of the overlying and underlying rock strata, or on mining operations. 

Gas production from hydraulically stimulated degasification boreholes in 

coalbeds show a fivefold to twentyfold increase in sustained gas flow (2). - 

The direction in which the induced fracture may be expected to propagate 

into the coalbed is related to the tectonic stress pattern in the mine area. 
The fracture may follow joint or cleat directions, or be oriented perpendicularly to the least horizontal force or along the path of least resistance (4). - 

The induced fractures observed were vertical. Widths ranged from oneeighth to one-half inch. Highly viscous fracture fluids tend to form wider 

fractures of up to 2-1/2 inches in width. These fractures tend to have a 

smaller radial extent. Less viscous fracture fluids were found to create 

longer fractures, thus a more efficient stimulation. 

Periodic observations of exposed fractures produced by hydraulic stimulation have shown that no deterioration of roof and floor strata, or loss of 

coal rib stability occurred in the areas of induced fractures. The mine 

entries observed remained structurally safe more than 2 years after mine 

interception of induced fractures. 
REFERENCES 

1. Cervik, J., and C. H. Elder. Removing Methane From Coalbeds in Advance of 

Mining by Surface Vertical Boreholes. Proc. Conf. on the Underground 

Mining Environment, Univ. of Mo., Rolla, Mo., Oct. 27-29, 1971, 

pp. 229-240. 

2. Elder, C. H., and M. Deul. Hydraulic Stimulation Increases Degasification 

Rate of Coalbeds. BuMines RI 8047, 1975, 17 pp. 

3. Halliburton Services. The Fracbook Designl~ata Manual for Hydraulic 

Fracturing. Halliburton Services, Duncan, Okla., 1971, pp. 1-111 

4. Hubbert, M. K., and D. G. Willis. Mechanics of Hydraulic Fracturing. 

Trans. AIME, v. 210, 1957, pp. 153-166. 

frU.S. GOVERNMENT PRINTING OFFICE: 1977-703-0011 12