Bureau of Mines Report of lnvestigations/l984 

Assessment of Methane Hazards 

in an Anomalous Zone 

of a Gulf Coast Salt Dome 

By A. T. lannacchione, R. H. Grau, Ill, A. Sainato, 

T. M. Kohler, and S. J. Schatzel 

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

William P. Clark, Secretary 

BUREAU OF MINES 

Robert C. Horton, Director 
Library of Congress Cataloging in Publication Data: 

Assessment of methane hazards in an anomalous zone of a Gulf Coast 

salt dome, 

(Report of investigations ; 886 1) 

Bibliography: p. 26. 

Supt. of Docs. no.: I 28.23:8861. 

1. Salt mines and mining-Gulf States-Safety measures. 2. Methane-Safety measures. 3. Salt domes-Gulf States. I. Iannacchione, 

Anthony T. 11. Series: Keport of investigations (United States. Bureau of Mines) ; 8861. 

TN23,U43 [TN902.A 11 622s [622'.8] 83-60037 1 

* 
CONTENTS 

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

Introduction 7 ................................................................... Methane in domal salt mines .................................................... 2 

Geologic and engineering considerations ...................................... 2 

Occurrence of anomalous zones ................................................ 4 

The outburst problem ......................................................... 5 

Investigations of anomalous zone at Belle Isle Mine ............................ 4 

Emissions from blasted faces in adjacent normal salt ......................... 5 

Methane emission study through an anomalous zone ............................. 8 

Exploratory drilling into anomalous zone ..................................... 19 

Discussions of potential methane control stl-atr-gies ............................ 23 

Identification. delineation. and orientation of a gassy anomalous zone ....... 23 

Determining the severity of the potential hazard ............................. 23 

Special mine-through procedures and techniques ............................... 24 

Summary and conclusions ........................................................ 25 

References ..................................................................... 26 

ILLUSTRATIONS 

Plan and cross-sectional view of study area within Belle Isle salt mine .. Sequence of events associated with typical outburst triggered by blasting . Pneumatically operated sequential gas sampler designed to collect ambient 

air samples during and after blasting .................................... Pneumatic gravity switch and gas-sampling unit ............................ Location of sequential gas sampler test sites ............................. Gas concentrations from test site 1 ...................................Om.. 

Gas concentrations from test site 2 ....................................... Low-production-grade (wet-argillaceous) salt within anomalous zone of the 

decline .................................................................. Plan view of location of anomalous zone and various positions of decline 

face during emissions study .............................................. Cross-sectional view of advancing face of the decline with ventilation 

pattern and position of monitoring equipment ............................. Methane sensor and sonic anemometer within exhaust duct ................... Vacuum pump and methane sensor for collecting and measuring methane concentration in exhaust duct ............................................... Intake-air methane sensor ................................................. Location and concentration of methane contents from salt samples collected 

in decline area .......................................................... Amount of methane emitted per day for 47 production dates from March 30 to 

July 3 ................................................................... 13 

Salt production rate from decline as a function of salt hauled versus production time ............................................................. 14 

Strip chart recording from methane sensor and sonic anemometer within exhaust duct . Notice the correlation between methane concentration and 

anomalous responses from the sonic anemometer ............................ 14 

Calculated methane emission rates per ton of salt produced from the decline face from March 30 to July 3 ....................................... 15 

Area within the decline where banding is parallel with entry .............. 16 

Plan view of face areas where methane emissions were detected ............. 17 
Page 

Strip chart recordings from entry and exhaust duct sensors showing similar 

methane concentrations.................~~.................................. 18 

Strip chart recording from a nonproduction weekend........ ................ 18 

Strip chart recording of methane concentration measured at relative air 

velocities within exhaust duct ....................................... 19 

Equipment assembly for exploratory drilling in study area................. 20 

Salt core recovered from exploratory drill holes..................... ..... 20 

Location of three exploratory drill holes on 1,400-ft level and their 

position relative to anomalous zone.......................... ............ 20 

Gas flow from hole 2...... ................................................ 21 

Gas-pressure buildup in holes 2 and 3 after encountering anomalous zone... 22 

TABLES 

-h-alysis of gas samples collected from blasted faces... ................... 7 

Average CH4 contents of cores from normal salt and transitional zone, and 

rib samples from anomalous zone.......................................... 21 

Gas analysis from drill hole 2. ........................................... 21 

UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 

cm centimeter m3 cubic meter 

cm3 cubic centimeter m/min meter per minute 

cm3/100g cubiccentimeterper m3/d cubic meter per day 

one hundred grams 

m3 /h cubic meter per hour 

f t foot 

m3/t cubic meter per metric ton 

f t/min foot per minute 

mi mile 

ft3 cubic foot 

min minute 

f t3/d cubic foot per day 

min/d minute per day 

f t3/h cubic foot per hour 

kPa kilopascal 

ft3/ton cubic foot per short ton 

PPm part per million 

g gram 

Pet percent 

h hour 

~sig pound per square inch, 

in inch gage 

. km kilometer s second 

m meter t metric ton 
ASSESSMENT OF METHANE HAZARDS IN AN ANOMALOUS ZONE 

OF A GULF COAST SALT DOME 

By A. T. lannacchione, Re H. Grau A. ~ainato,~ 

T. M. Kohler, and S. J. Schatzel 

ABSTRACT 

This Bureau of Mines research study found gas emission rates from an 

advancing face, and flows and pressure of gas from an exploration drill 

hole were dependent upon the geology of salt encountered in a domal 

salt mine. Normal production-grade "pure" salt adjacent to an anomalous'zone was found to have a methane emission rate of less than 5 ft3/ 

ton (0.1 m3/t) of mined salt from a room advanced by a continuous 

miner. Extremely low gas flows [less the 4 ft3/h (0.1 m3/h)] and lack 

of pressure buildup were also observed from this normal salt in a 154- 

ft (47-m) exploration drill hole. These data indicate that methane 

emissions should not be a problem when mining in this type of salt. 

In the adjacent anomalous zone, methane emission rates ranged from 15 

to 70 ft3/ton (0.4 to 1.8 m3/t) of mined salt from an advancing face. 

Gas flows of approximately 42 ft3/h (1.2 m3/h), and pressures in excess 

of 900 psig (6,200 kPa) were observed in an exploration drill hole when 

the anomalous zone was encountered. This information suggests a greater methane emission hazard from the anomalous zone than from the associated normal salt. If this trend of increasing emissions problems associated with anomalous zones of salt is common with similar anomalous 

zones encountered or observed in other domal salt mines, then special 

control strategies should be employed to minimize this potential mining 

hazard. 

l~eologist. 

 inin in^ engineer. 

3~ining engineering technician. 

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

This Bureau of Mines study is an assessment of the potential methane hazard 

to mining of an anomalous zone in a 

Louisiana domal salt mine. Detailed geologic mapping, emissions studies, physical properties tests related to gas, and 

methane content of salt samples from an 

anomalous zone encountered at the Belle 

Isle Mine are presented and provide a 

basis for discussion of a control strategy for this particular problem area 

(fig. 1). 

Although methane (CH4) has long been 

recognized as a hazard in coal mining, the danger it poses in domal salt 

mines has only recently become apparent. 

Throughout the history of mining salt in 

the Gulf Coast region, hazardous meth-ane 

emissions have occurred at infrequent intervals. The inconsistent nature of the 

methane problem has detracted from the 

seriousness of the hazard and, consequently, hindered the development of control strategies and procedures. Analogies can be drawn with the early coal 

mining experience in the United States. 

Prior to the Federal Coal Mine Health and 

Safety Act of 1969, coal mines could be 

classified as either gassy or nongassy. 

However, history has shown some of the 

worst coal mine methane-induced explosions have occurred in mines originally 

class if ied as "nongassy" (1) .4 

Recently. new blasting techniques, 

deeper mining levels, and higher extraction rates have increased the frequency 

of hazardous methane occurrences. These 

occurrences have sometimes been manifested as violent outbursts of methane 

and salt. Outbursts are generally associated with blasting. but occur only in a 

smaii fraction of the totai number of 

faces. However, these hazardous methane 

emissions have often occurred in groups 

and are sometimes associated within 

regions or specific zones within several 

of the domal salt mines. 

This report shows an association of a 

methane occurrence with certain salt 

characteristics. The recognition and 

prediction of these areas can reduce the 

potential for hazardous methane emission 

problems by developing special control 

procedures and techniques. 

METHANE IN DOMAL SALT FiINES 

The unique geologic and physical characteristics of domal salt mines have inf luenced the occurrence of hazardous 

methane conditions. Previous investigations (2-6) have determined some of the -- characteristics of a violent outburst of 

salt, which often results in hazardous 

methane conditions. Our investigation 

deals with the occurrence and characteristics of some types of gassy salt. The 

sections that follow will examine some of 

the geologic and physical properties associated with gassy salt and propose 

a control strategy for this potential 

hazard. 

GEOLOGIC AND ENGINEERING CONSIDERATIONS 

stocks range in diameter from 1 to 5 

mi (2 to 8 kin). Dome flanks vary from 

vertical to inclined with sharp to gradational contacts with country rock (7). - 

Salt bands with thickness ranging from 

0.5 in (i cm) to 3 ft (1 m), reveal the 

dominant orientations of a diapiric 

structure. The salt within the domes has 

been so intensely folded that the bands 

are nearly vertical (5, 8-9). Therefore, -- as the mine develops 6orizontally, it advances through band after band of nearly 

vertically bedded salt. Multiple-level 

development should penetrate similar layers of salt at the same relative location (depending on the dip of the bands). 

Salt domes of the Gulf Coast region 4~nderlined numbers in parentheses rehave elliptical or circular shapes and fer to items in the list of references at 

taper at depth. Tops of individual salt the end of of this report. 
- 1,100 - 

P czzz3 a u 

Decl~ne J 

W 200 

O- -1,500 - Horizontol scale,ft 

SECTION A-B 

FIGURE 1. - Plan and cross-sectional view of study area within Belle Isle salt mine. 
developed by blasting to a height of 35 domal salt mines operated in southern 

ft (11 m) and a width of 30 to 100 ft (10 Louisiana. He has also indicated that 

to 30 m). Benched areas range from 60 to outbursts often occur along linear zones 

120 ft (20 to 36 m) high. that are associated with clastics, liquids, gases, fluid inclusions, shearing, 

OCCURRENCE OF ANOMALOUS ZONES and abnormally dark salt. 

Anomalous types of salt, different in THE OUTBURST PROBLEM 

composition from normal salt (99 pct pure 

halite), have been associated with min- Outbursts, violent explosions of salting problems. Zones of anomalous salt rock and gas released under high prescan be identified from the more corn- sure, have been known to occur within 

mon normal salt by several of the follow- domal salt mines for many years. The 

ing characteristics: (1) brine and gas 

seeps, (2) presence of clastic material, (3) shearing, (4) recrystallization, 

(5j expansion joints, and (6) association 

with outbursts (10). The anomalous salt 

zone of this stu* is characterized mainly by fluid flows, predominently in the 

form of methane gas and brine, and the 

increased occurrence of sandstone, shale, 

and clay material, causing the overall 

salt mass to appear dark in color. 

The origin of the anomalous zones may 

be tied to the concept that salt domes 

consist of many stocks or spines of salt 

that have risen separately through time 

at varying rates (10-11). -- The marginal 

areas of these "spines" are defined by 

clastic gouge material, which is included 

into the salt core. It has been suggested that the spines move upward at 

different rates, incorporating the spines 

with their associated anomalous zones 

into the central portion of the dome. 

Because many of these anomalous zones are 

comprised of country rock rich in organic 

and inorganic solids and liquids, they 

are recognized underground as zones that 

differ significantly from those of normal 

salt. Kupfer (11) has identified anomalous zones in four of the five sister 

association between outbursts and large 

emissions of methane and other gases have 

been documented in several reports (2, 

4-6). The hazard resulting from a rzpid -- 

release of large volumes of methane and 

the subsequent inundation of an entire 

mine, which may last several days, has 

induced much discussion about the resultant danger and the proper techniques to 

safely and effectively deal with it. 

It is generally accepted that outbursts 

occur during or shortly after blasting of 

a salt face. Thoms (12) has implied that 

the driving force foran outburst in salt 

is entrapped gas. Other theories attributed high rock stresses as the dominating force. 

A salt outburst creates a cavity in the 

roof or rib area that generally extends 

vertically upward (fig. 2). These openings range from 3 ft (1 m) to 30 ft (10 

m) in diameter and 3 ft (1 m) to greater 

than 70 ft (22 m) 'in height. Outbursts 

observed at the Jefferson Island Mine 

were in the same relative position at 

both the 1,300-ft (400-m) and 1,500-ft 

(460-m) levels. This is attributed to 

the near vertical orientation of a very 

gassy zone of salt. 

INVESTIGATIONS OF ANOMALOUS ZONE AT BELLE ISLE MINE 

An anomalous zone within the Belle and anomalous salt zones and (3) evaluate 

Isle Mine was investigated in detail to the usefulness of this data in predicting 

(1) correlate specific gas emissions with the location of potential problem areas 

particular salt types, (2) compare physi- within the mine. 

cal properties related to gas of normal 
A B 

FIGURE 2. - Sequence of events associated 

EMISSIONS FROM BLASTED FACES IN 

ADJACENT NORMAL SALT 

Before presenting results of the emissions study conducted in the anomalous 

zone, we must understand the relative gas 

emissions experienced during typical production face advancement in the normal 

salt adjacent to the anomalous zone. 

Estimation of gas emissions from large 

blasted faces is complicated by the withdrawal of all personnel from the mine 

during blasting and lack of discernable 

persistent air currents within many face 

areas. Therefore, our results can be 

taken only as an indication of gas concentrations after blasting. Two mineface areas in normal salt adjacent to the 

anomalous zone were sampled immediately 

after a blast to determine the approximate concentrations of gases present. 

Gas sampling was accomplished by using a permissible sequential gas sampler 

(fig. 3). This pneumatically operated 

device designed for this express purpose 

is actuated by the pressure wave from the 

face blast. The blast wave moves an air 

vane connected to a blast-operated switch 

(fig. 4) that initiates a timing sequence 

with typical outburst triggered by blasting. 

within a control box. Once the preset 

time is reached, a pneumatic signal is 

sent to the sampling unit (fig. 4), which 

opens a pneumatically operated valve releasing liquid from the sample chamber 

so that ambient air can replace it. This 

system is designed to collect a series of 

gas samples at different time intervals. 

These samples can be analyzed at a later 

time . 

The sequential gas sampler was first 

tested at a blasted face on the 1,400 ft 

(430 m) level approximately 650 ft (200 

m) from the anomalous zone. Face dimensions were 26 ft (8 m) by 32 ft (10 m). 

Approximately 160 ft/min (49 m/min) of 

air was moving through the entry in which 

the sampler was placed (fig. 5). This 

entry is approximately 50 ft (15 m) from 

the blasted face area. During the face 

blast, the intake fan was deenergized to 

prevent damage by the overpressure wave 

from the blast. Approximately 5 to 10 s 

after the blast, the surface fan was 

turned on again. The blast caused the 

air to move from the face, up the intake 

shaft, then back down the shaft, and 

to recirculate past the sampler (fig. 

6). The point of interest here is that 
y pq

tial gas sampler designed to collect ambient air 

samples during and after blasting. 

maximum concentrations of methane detected in the air samples collected at 

the blasted face were less than 0.01 pct 

(table 1). 

The second test of the sequential gas 

sampler was also conducted on the 1,400- 

ft (430-m) level but at a distance of approximately 750 ft (230 m) from the anomalous zone (fig. 5). The room dimensions 

were similar to those of test 1 [22 by 35 

ft (7 by 11 m)], but the ventilating air 

currents were extremely slow. Smoke tube 

surveys at the sampling locations indicated a velocity of approximately 40 ft/ 

min (12 m/min). However, at other locations within this face area, relative 

magnitudes and directions of air currents varied. Analyses of the samples 

collected from the tests are shown in 

table 1. Figure 7 shows the change in 

FIGURE 5. - Location of sequential gas sampler test sites. 

concentrations of methane after the 

blast. The maximum methane concentration 

at the sampling location was found to be 
0.004 pct. The average methane concentration for 30 min after the blast was 

0.003 pct. 

5 10 15 

TIME, min 

Sulfur hexafluoride (SF6) was used as a 

tracer gas in both tests to check the 

reliability of the sampling system. The 

TABLE 1. - Analysis of gas samples 

collected from blasted faces 

-- Sample I CH4, pct I SF~, pct I Time, s 

TIME, min 

TEST 1, NOV. 17, 1981 

x 10-7 

3.5 x 827 

.0024 lo'9 

FIGURE 6. - Gas concentrations from test site 1. FIGURE 7. - Gas concentrations from test site 2. 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

.0024 

.004 

.002 

.0019 

.0028 

.OO21 

.0027 

. 002 

.0017 

.001 

.0009 

5.57 x 10" 

6.55 x 10'~ 

2.52 x lo'9 

1.43 x 10" 

5.65 x 10'1° 

5.5 x 10"O 

5 x 

1.55 x 10-lo 

1.43 x 10-lo 

1.1 x 10-1° 

3.05~ 10-lo 

156 

275 

460 

657 

832 

996 

3,060 

3,420 

4,020 

4,320 

5,100 
FIGURE 8. - Lo~-~roduction-~rode (wet-argillaceous) salt within anomalous zone of the decline. 
FIGURE 9. - Plan view of location of anomalous zone and various positions of decline face 

during emissions study. 

the face. Also, the face was driven by a 

continuous miner as opposed to a conventional mining system. Conventional mining, with the use of explosives to blast 

production faces, prevents emission-rate 

studies because of the short-term disruption of low-velocity air currents by 

pressure waves from the blast. The use 

of the continuous-mining system did not 

disrupt ventilation patterns. 

A monitoring system capable of recording velocity and methane concentrations 

was placed in the exhaust duct (fig. 11). 

An additional methane-sampling system, 

composed of a vacuum pump and methane 

sensor (fig. 12), withdrew a gas sample 

from the duct for comparison with the 

methane concentration recorded by the 

other sensor located in the duct, Fresh 

air was sampled from a third position to 

determine intake-air quality (fig. 13). 

Periodically, gas samples were collected 

in evacuated tubes for chromatagraphic 

determination of gas concentrations. Also, numerous ventilation surveys were 

conducted to determine average velocities 

and air quantities within the study area. 

The salt within the anomalous zone 

study area is dark, argillaceous, wet and 

gassy. Dissolution analysis conducted by 

Schatzel and Hyman (13) showed the methane content to average approximately 0.33 
FIGURE 11. - Methane sensor and sonic anemometer within exhaust duct. 
FIGURE 13. - Intake-air methane sensor. 
Daily totals of methane emitted during 

production are shown in figure 15. 

\4- 5. F' :;. = \?- 

. -~,:-. , .';' - <.,:':3'.0 ,573, 1, I 0.0173 =$ $'- ,-;,2 ' .< - 

- > 

0.0 I 0 

LEGEND 

0.006 

IZl Anomalous zone 

Location of salt samples used 

~n occluded gas measurements [ (Values In cubic foot per Ion STP); 

b Direction of banding 

0 - 40 80 

Scale. fl 

FIGURE 14 - Location and concentration of methane 

contents from salt samples collected in decline area. 

dust produced during mining causes erratic velocity measurements as compared 

to the consistent velocity measurement 

during nonproductive periods. Therefore, 

emission rates (ft3/ton; m3/t) were estimated by comparing the minimum and maximum values of methane emitted per day 

(fig. 15) with the total estimated tons 

of salt produced per day (fig. 16). Figure 18 shows the emission rates based on 

the estimated tons mined for each of the 

47 production days for which reliable 

data were available. 

Analyses of the data shows (1) a decrease in the emission rate occurred between March 30 to July 3 as the normal 

salt was approached, (2) an increase in 

emission rate when mining perpendicular 

to rather than parallel to the direction 

of banding, (3) and a correlation between 

emission rates with methane content data. 

The emission rate within the anomalous 

zone averaged 27.3 ft3/ton (0.7 m3/t) of 

mined salt, but the rate dropped to less 
Mar. Apr. May June July 

PRODUCTION TIME, days 

FIGURE 15. - Amount of methane emitted per day for 47 production dates from March 30 to July 3. 

than 5 ft3/ton (0.1 m3/t) of mined salt 

for the transition zone to normal salt. 

Figure 9 shows the position of the face 

during the later part of June and beginning of July when the transition to normal salt was mined. 

There was a 44-pct decrease in emission 

rates as the face advanced parallel to 

the banding as opposed to when the face 

advanced at an angle to banding. This 

may be caused by an enhanced drainage of 

gas from the strata due to permeability 

where fractures intersect perpendicular 

rather than parallel to the face. The 

average emission rate from March 30 to 

May 19 was approximately 37.5 ft3/ton 

(1.0 m3/t) of mined salt. The angle between the banding at the salt face and 

the orientation of face advancement for 

this first period was 40 to 80 deg, while 

the orientation of the banding in the 

salt and the face advancement were approximately parallel for the second period (fig. 19). The second period (May 20 

to July 1) had an average emission rate 

of 20.9 ft3/ton (0.5 m3/t) of mined salt. 

Figure 9 shows that on approximately May 

19 the face made a sharp turn to the east 

and essentially followed the salt bands 

until a second turn occurred at July 1. 

The association of anomalous salt with 

measurable methane emissions is shown in 

figure 20. 
@6-29 

240 - - 

220 - KEY - 

6 - 1 0, e tc. Production date 

200 - - 4-28. @4-9 

180 - 6-23 - 

160 - 4- 23 - 

140 - 6-9 - 

120 - - 

100 - - 

- 

- 4-7 

- 

@4-1 - 

111, lllIlIlI11. 

100 200 300 400 500 600 700 800 900 1,000 1,100 1,200 

PRODUCTION, min /d 

FIGURE 16. - Salt production rate from decline as a function of salt hauled versus production time. 

Lunch Shift 

change Lunch 

w1 1 I Production I ( Production I / Production I I ~roductionl Nonproduction I 

z 0.2 

:z 

I- Q 

O 

12 

noon 

10 12 2 

midnight 

FIGURE 17. - Strip chart recording from methane sensor and sonic anemometer within exhaust 

duct. Not ice the correlation between methane concentrat ion and anomalous responses from the 

sonic anemometer. 

Comparison of these emission data with 0.7 m3/t)]. Some of the reasons for this 

the methane content data developed by can be attributed to the differences in 

Schatzel and Hyman (13), - also suggests sample size and physical conditions under 

permeability within this anomalous zone. which the data are estimated. However, 

The difference between the methane con- the large differences between these two 

tent and the average measured emission sets of data indicate that the methane 

rates for the sampled area is 80 times content of salt at the face cannot acgreater I0.33 to 27.3 ft3/ton (0.009 to count for a significant portion of the 
KEY I Range of data 

Mar. Apr. May June July 

PRODUCTION TIME, days 

FIGURE 18. - Calculated methane emission rates per ton of salt produced from the decline face 

from March 30 to July 3. 

methane emitted into the air exhausted 

from the face. Permeability would allow 

for a larger reservoir from which methane 

could drain during advancement of the 

face. This is an important consideration 

since domal salt is generally thought of 

as impermeable. Previous investigations 

by Thoms in an area of normal production 

grade salt at the Jefferson Island (12) 

Mine have shown in situ permeabilityto 

be nearly zero, with only some mininginduced permeability observed near free 

faces of salt pillars. 

Reliable determination of emission 

rates were not possible after July 3 because of the addition of a flexible blowing air duct in the decline. This addition to the ventilation system disrupted 

the continuous, even flow of air down the 

decline, across the face, and back up the 

decline through the rigid exhaust duct. 

When the booster blowing air duct was 

forcing air down the decline, the entry 

itself probably had recirculation. There 

is some evidence that one current of air 

was moving down the decline along the 
FIGURE 19. - Area within the decline where banding is parallel with entry. 

bottom of the entry while simultaneously 

another current of air was moving up the 

entry along the roof. Also. the auxiliary ventilation systems would reduce the 

overall velocity of the current of air 

moving down the entry. 

The intake air currents split at the 

face, with one portion being exhausted 

from the decline through the rigid duct 

and another portion moving away from the 

face along the entry roof. The location 

of the entry sensor is shown in figure 

20. During the several production days 

after July 3 precise analyses of gas samples showed concentrations of methane 

were similar at both the exhaust duct 

sensor and the entry sensor. Strip chart 
Scale, ft 

FIGURE 20. - Plan view of face areas where methane emissions were detected. 

recordings from methane sensors during 

several of the production days show this 

similarity in concentrations (fig. 21). 

This data confirms split air currents in 

the decline entry after July due to the 

addition of auxiliary ventilation. 

During another short period (July 25- 

26) when the face was advancing through 

the normal salt, an interesting methane 

emission phenomenon was observed. Over 

the weekend, when there was no production 

at the face, methane was measured by the 

sensor in the entry and not by the sensor 

in the exhaust duct (fig. 22). Since the 

sensor in the exhaust duct was measuring 

the air directly from the face area, we 

assume the emissions from the face were 

close to zero. However, during this same 

period, the sensor in the entry, which 

was probably analyzing the air moving up 

the decline away from the face, had a 

methane concentration of approximately 

0.05 pct. A possible explanation of this 

phenomenon is that the rib within the 

anomalous zone away from the face area 
I I I I I 1 

Methane in decline entry - 

9 0.2 I I I I I I 

IW Methane in exhaust duct 

.I - - 

FIGURE 21. - Strip chart recordings from entry and exhaust duct sensors showing similar methane 

concentrat ions. 

L 

t-a 

ga 

I I I I I I I I I - 

- Methane in exhaust duct - 

- 

- I I I I 

Sonic anemometer I I 1 1 1 I 

Voltage output 

I I I I I I I- I 

r I 1 I 1 I 1 I I 

L Methane in decline entry 

- 

8 12 4 8 12 4 8 12 10 

a.m. noon midnight noon p.m. 

FIGURE 22. - Strip chart recording from a nonproduction weekend. 

was emitting methane that was being 

transported up the decline along the 

roof. The spike of methane from both 

sensors at approximately 12 p.m. (July 

26, fig. 22) occurred because of a power 

outtage, which cut off the ventilation 

fans and allowed natural ventilation to 

develop. During this period of natural 

ventilation, there was a small build 

up of approximately 0.45 pct methane 

throughout the methane-monitoring area. 

When the fan was turned on again, methane 

concentrations dropped rapidly to zero. 

A similar small build up of methane was 

observed when a fan was not in operation 

on June 2 (fig. 23). The maximum concentration of methane measured during this 

period was approximately 0.17 pct. 

The data collected after July are offered only as observations because of the 

complication to the ventilation system. 
I fan operating I 

hwL 2 f ons operating 

I l am. I p.m. 3 5 7 9 I I 

FIGURE 23. - Strip chart recording of methane concentration measured at relative air velocities 

within exhaust duct. 

However, rib emissions probably occurred 

because of the permeability of the salt 

from this anomalous zone. It should also 

be pointed out that even without forced 

ventilation, methane emitted from the rib 

did not raise the concentrations in the 

entry above one-half of a percent. 

EXPLORATORY DRILLING INTO ANOMALOUS ZONE 

To further investigate some of the 

physical characteristics associated with 

the area of high emissions, an exploratory drill hole was cored horizontally 

characteristics and also to determine 

the methane content associated with the 

change from normal to anomalous salt 

(fig. 25). 

Three exploratory drill holes were 

needed to investigate further the gas 

characteristics of this anomalous zone. 

Hole 1 was cored entirely through normal 

salt (fig. 26). This hole, which represents typical salt from a normal nongassy 

face, was cored approximately 154 ft (47 

m) at a bearing of 293" and an angle of 

0.25" above the horizontal from the face 

into the anomalous zone. The drill hole of room F. Methane flows were zero exyielded data pertaining to the flows, cept when pumping the inner core barrel 

pressure, and composition of the gas from down the hole. Even then, the methane 

the salt rock penetrated (fig. 24). concentration of the gases exiting the 

Twenty-seven NQ size (diameter of 2.75 drill hole reached only 0.1 pct (the rein; 45 cm3) cores from the drill hole maining 99.9 pct consisted of air). No 

were collected to ascertain a descrip- other flows were measurable during the 

tion of the physical and compositional drilling of hole 1. Upon completion of 
LEGEND 

I, Direction of banding 

Intake shall 

FIGURE 25. - Salt core recovered from exploratory drill holes. Cores were physically inspected 

and bagged for methane content test. 

drilling, the hole was shut-in for approximately 20 h. During this time, no 

pressure buildup was observed within the 

hole. Methane contents of the cores were 

also consistently low (table 2). 

FIGURE 26. - Location of three exploratory dril I 

holes on 1,400-ft level and theirposition relative 

to anomalous zone. 

Hole 2 was drilled from an area of 

normal nongassy salt into the edge of 

the anomalous gassy salt. The hole was 

drilled approximately 131 ft (40 m) at a 

bearing of 325' and an angle of 7" above 
TABLE 2. - Average CH4 contents of cores from normal salt and transitional 

zone, and rib samples from the anomalous zone 

TABLE 3. - Gas analysis from drill hole 2, percent 

Salt type or source 

Normal............. 

Transitional zone l . Anomalous zone..... 

Time 

tering 

after 

anomalous 

encoun- (-1 

1 02 + N2 I Ar 

l~ark bands adjacent to anomalous zone. 

Number of 

samples 

23 

4 

5 

CHI 1 CHI I Location of sample 

4 in (10 cm) inside 

end of driii pipe. 

Do. 

Do. 

Exhaust from drill 

hole after pressure 

test. 

Do. 

4 in (10 cm) inside 

end of drill pipe. 

Do. 

Rel. conc. 

of CH4 

1 

4 

220 

l Other hydrocarbons. 

2~low rate dropped to near zero. Air from entry began contaminating drill hole. 

the horizontal plane from the face of 

room C (fig. 26). Methane emissions were 

zero from the first 120 ft (37 m) of the 

drill hole. The core from the first 120 

ft (37 m) was found to be typical of the 

normal salt found in the study area. At 

approximately 125 ft (38 m), a distinctive odor signaled the penetration into a 

gassy zone of salt. This odor was later 

attributed to the presence of a small 

concentration of mercaptans. Drilling 

was interrupted after approximately 5 ft 

(1.5 m) more of penetration so that gas 

concentrations and flows could be measured. Methane concentrations of over 90 

pct were round with a methanometer and 

later confirmed by gas chromatographic 

analysis (table 3). 

Mean (z), ft3/ton 

(cm3/100 g) 

0.0015 (0.0046) 

-0066 (.0206) 

.3304 ( 1.0324) 

only two observations of flow could be 

made (fig. 27). Approximately 500 ft3 

(14.2 m3) of methane was estimated to 

Std. dev. (01, ft3/ton 

(cm3/100 g) 

0.0011 (0.0034) 

.0047 ( .0146) 

.3656 (1.1424) 

Approximately 4 h after penetration, a 

gas flow of 42 ft3/h (1.2 m3/h) was mea- A 

V sured. One hour later, the .flows dropped 2 4 6 8 10 12 14 16 18 20 

to near zero [4 ft3/h (0.1 m3/h)]; however, methane concentrations remained TIME, h 

above 90 pct. Conditions were such that FIGURE 27. - Gas flow from hole 2. 
have been emitted from this hole 20 h 

after encountering the gas flow. This 

total flow is significantly less than 

that of another exploration hole drilled 

by the mine operator which had a total 

flow of 39,000 to 49,000 ft3 (1,104 to 

1,388 m3) of methane. The hole was 

drilled from the decline face to a depth 

of approximately 150 ft (46 m) within 

this anomalous zone (14). - 

Close examination of the salt core 

showed a change in the lithology of the 

salt where penetration of the gassy zone 

occurred. Two dark bands of salt, 1 to 2 

ft (0.3 to 0.6 m) thick, were observed to 

occupy the interval from w-hich gas emissions were first encountered. These dark 

bands are similar to those observed directly adjacent to the dark argillaceous 

salt of the anomalous zone approximately 168 ft (51 m) away in the decline 

(fig. 26). The drill hole had just begun 

to encounter the anomalous zone, and the 

bands represent a transitional zone between normal and anomalous salt. Core 

samples from the dark bands adjacent to 

the anomalous zone had an increase in 

methane content by a factor of 4 when 

compared to that of normal salt. Rib 

samples collected from the anomalous zone 

were approximately 220 times higher in 

methane content (table 2). 

Hole 2 was extended from 131 ft (40 m) 

to 153 ft (47 m) 9 months after the initial coring. In addition, a third hole 

was rotary drilled 163 ft (50 m) at a 

bearing of 320" and an inclination of 3" 

(fig. 26). In situ gas pressures were 

May 1983 

May 10 

TIME, days 

FIGURE 28. - Ga~-~ressure buildup in holes 2 and 3 after encountering anomalous zone. 
measured in both holes by packing the 

portions of the hole located in the anomalous zone. Gas pressures in hole 3 rose 

to 903 psig (6,226 kPa) in a 6-day period 

(fig. 28). However, in hole 2, which had 

been partially drilled prior to hole 3, 

pressure did not exceed 10 psig (69 kPa) 

until the pressure in hole 3 exceeded 700 

psig (4,830 kPa). At this point, hole 2 

began to build up pressure over a 4-day 

period, reaching approximately 175 psig 

(1,210 kPa). These two holes were 24 ft 

(7 m) apart at their end points. 

Information from the three drill holes, 

along with the other data previously 

DISCUSSIONS OF POTENTIAL 

A possible methane control strategy 

would include identification, delineation, and orientation of the gassy anomalous zone; determination of the degree of 

severity of this potential hazard; development of special mine-through techniques 

and procedures; and, if it is believed 

the hazard may be too great, how to avoid 

the zone. 

IDENTIFICATION, DELINEATION, AND 

ORIENTATION OF A GASSY ANOMALOUS ZONE 

Before a methane control plan can be 

formulated, the potential problem area 

must be identified. The identification 

of a gassy anomalous zone can be accomplished by geologic in-mine mapping 

investigations and an explorational 

drilling program. 

Geologic in-mine mapping should include 

an inspection and description of salt 

types with emphasis on the presence of 

brine or gas seeps, fluid or gaseous inclusions, grain size, striations, orientation of banding, and location and geometry of outburst. Once the mine has been 

sufficiently mapped, trends of anomalous 

zones of salt can be established for 

the entire mine. This will allow for 

projections of anomalous zones into unmined portions of the salt dome close to 

the working sections. Also, because the 

domal salt is often nearly vertically 

bedded, projections of these trends can 

discussed in this report, indicates that 

the normal salt adjacent to the anomalous 

zone presents a less significant methane 

hazard than does the anomalous zone itself. It appears that higher methane 

emission rates are a result of localized 

permeability, increased methane contents, 

and excessive gas pressure of the salt 

within this anomalous zone. These characteristics will also increase the potential for other methane hazards such as 

outburst. This implies the need for special control technologies for mining 

through a gassy anomalous zone such as 

that found in the study area. 

METHANE CONTROL STRATEGIES 

be made into superjacent and subjacent 

mining levels. 

Exploratory drilling confirms projections made from in-mine mapping near the 

working face areas. Core drilling is 

recommended for providing the most reliable information, but examination and 

testing of cuttings from rotary drilling 

can supply some useful information. Because of the great difficulty and potential hazards associated with drilling 

into salt domes from the surface, this 

exploration program should be conducted 

from within the mine. The length and 

number of exploratory drill holes needed 

to successfully identify an anomalous 

zone cannot yet be established. Some 

mines have established procedures of 

drilling several 100- to 300-ft (30- to 

90-m) holes from every face developing in 

unmined regions of the dome. The number 

and lengths of these holes should be dependent upon both projected conditions 

from in-mine mapping and results of initial drilling from that particular face. 

DETERMINING THE SEVERITY OF THE 

POTENTIAL HAZARD 

Several combined techniques could be 

used to determine the degree of severity to be expected when mining through a 

projected gassy anomalous zone. Accurate 

records of gas emission problems encountered during development of the face 
should be routinely collected. Detailed 

physical property tests related to gas 

of an anticipated anomalous zone with 

exploratory drilling that incorporates 

the dissolution techniques developed by 

Hyman (15) - will aid in evaluating the 

severity of the potential methane hazard. Although conditions within the 

anomalous zone may change rapidly (permeability, pressure, methane content, 

etc.), previous mine-through data could 

be used to confirm results developed from 

these tests conducted during exploratory 

drilling. 

Physical properties tests from explorational drill holes provide generalized 

gas flow and gas pressure data of projected anomalous zone so that a comparison 

could be made with other anomalous zones 

and with normal production-grade nongassy 

salt. Additional test concerned with the 

permeability of the zone could be made 

to determine its effect on the flow and 

pressure of the gas. Also, the dominant 

gas type to be expected when mine-through 

occurs could be easily determined by gas 

sampling during drilling. 

Methane content analysis of exploration 

cores will also provide information for 

comparison with other anomalous zones as 

well as the normal salt within the mine. 

To quantify the severity of the hazard, 

it is hoped that evaluation of a projected anomalous zone can be made with reliable results so that comparisons can be 

made with other anomalous zones that have 

been mined-through previously. 

SPECIAL MINE-THROUGH PROCEDURES 

AND TECHNIQUES 

After an anomalous zone has been identif ied in an unmined portion of the salt 

dome and the degree of the emissions 

hazard has been.estimated, special minethrough procedures and techniques should 

be planned. Mining systems to be affected by a control strategy would include (1) selection of equipment (2) mine 

layout, (3) blasting techniques, (4) area 

methane monitoring systems, and (5) methane drainage. When selecting equipment 

for potential methane hazard areas, special attention should be placed on availability and maintenance of permissible 

equipment at the face areas. A continuous mining system would tend to emit 

gas steadily into the ventilation system instead of a rapid release like that 

which may accompany a blasted face. Mine 

layout should consider the location of 

these gassy zones so as to minimize their 

effect on high production areas and maximize the potential for larger quantities 

of ventilating air currents. Many working faces near a gassy zone would enable 

the blasting of a potentially hazardous 

face at a predetermined time, such as the 

last shift of a work week. If larger 

emissions did occur, the mine ventilation 

system would have an entire weekend to 

render the methane harmless, minimizing 

production stoppages. Investigations of 

blasting geometries and techniques may 

enable the mine operator to advance 

through a gassy zone with minimal danger 

of outburst. A sophisticated sensing 

system could be used to continuously monitor emissions during cutting or blasting. The system would consider and monitor every face as a potential emission 

problem. If the gassy zone is permeable, 

drainage holes drilled into high gas 

pressure areas will reduce the gas 

pressure locally, lowering the flow rates 

of methane from the salt into the face 

area. And finally, if conditions are serious enough, total avoidance of the 

gassy zone may be desired. 
SLIMMARY AND CONCLUSIONS 

The important characteristics associated with the methane hazard assessment 

study are as follows: 

1. Normal salt adjacent to the anomalous zone studied in this report is 

considered to present minimal emission 

problems (<SO0 ft3/d, 14 m3/d). 

2. Compared with normal salt, the 

anomalous zone in the study area presented a much greater gas emission problem (averaging 2,500 ft3/d; 71 m3/d). 

3. Methane emission rates from the 

normal salt in the decline area were estimated to contain 0 to 5 ft3/ton (0.1 

m3/t) of methane. 

4. Calculations of methane emission 

rates from the anomalous zone in the 

decline ranged from 10 to 70 ft3/ton (0.3 

to 1.8 m3/t) and averaged approximately 

27.3 ft3/ton (0.7 m3/t) over a span of 47 

production days. 

5. Emissions occurred mainly during 

or shortly after production from advancing faces. 

6. Rib emissions did occur in the 

anomalous zone, but they were generally 

far less significant than face emissions. 

7. Emission rates can vary by as much 

as a factor of 2, depending upon the angle of face advancement to banding in an 

anomalous zone. 

8. Comparison of emission rates with 

methane contents from the decline anomalous zone show the methane contents of 

the salt to be 83 times smaller than the 

emission data. 

9. Pressure and flow of gas within 

the normal salt adjacent to the anomalous 

zone encountered in the explorational 

drilling were extremely low. 

10. Pressures and flow properties of 

gas increased dramatically within the 

anomalous zone. Gas flow of approximately 42 ft3/h (1.2 m3/h) and gas pressures 

of approximately 900 psig (6,200 kPa) 

were measured when exploratory drilling 

encountered the anomalous salt. 

Assumptions developed from this investigation are as follows: 

1. Compared with normal salt, anomalous zones may represent a higher potential methane emission hazard. This is 

substantiated by previous information 

from various studies of anomalous features of Gulf Coast salt mines and by 

this study at the Belle Isle Mine. 

2. The anomalous zone is permeable 

when compared with the generally impermeable nature of the adjacent normal salt. 

3. Permeability variations may depend 

on the orientation of banding and fracture planes within an anomalous zone. 

If the characteristics of the anomalous 

zone investigated in this study are similar to those of other anomalous zones 

within the Gulf Coast Salt Domes, and if 

the ideas developed herein are valid, the 

following conclusions can be drawn: 

1. Anomalous zones can be observed, 

mapped, and projected within limited regions around working sections of underground domal salt mines. 

2. Explorational drilling will aid in 

further identification of these anomalous 

zones away from the working sections. 

Orientation and projection of anomalous 

zones can be defined from trends of banding observed in salt cores. 

3. Data from past mine-throughs and 

physical property tests related to gas of 

an approaching anomalous zone will aid in 

determining the severity of the potential 

hazard and will assist in developing an 

effective strategy for controlling methane hazards in Gulf Coast salt mines. 
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1786. 

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134. 

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