GAS MIGRATION CHARACTERISTICS OF COALBEDS 

by 

J. D. Hadden and Albert Sainato 

Bureau of Mines Methane Control Program 

Technical Progress Report - 12 

May 1963 

U . S . DEPARTMENT OF THE INTERIOR 
CONTENTS 

Page 

Abstract .............e...e...ee........e....e.......e....e.....e.e..ee 1 

Introduction . .........................em.e..e.e..e....e....e.......... 1 

Drilling studies in the Pocahontas No . 3 coalbed ........................ 2 

Drilling studies in the Pittsburgh coalbed .............................. 4 

Drilling studies in a western coal.bed ................................... 7 

Proposed methane control techniques ..................................... 7 

Coalbeds with normal pressure gradients ............................ 7 

Coalbeds with abnormal pressure gradients .......................... 8 

Conclusions ..............e.e......e................e...................e 10 

ILLUSTRATIONS 

Fig . 

. Normal gradient near an active face .....................e....e.e.. 3 

. Normal gradient near an inactive face ............................. 4 

. Abnormal gradient .............e...e......m.....e...........e...... 5 

4 . Drill site location in the Pittsburgh coalbed ..................... 6 

5 . Proposed methane control by horizontal drilling into outside ribs . 8 

6 . Proposed methane control by water infusion ........................ 9 

7 . Proposed location of degasification holes for a multiple entry 

system .......................................................... 10 
GAS MIGRATION CHARACTERISTICS OF COALBEDS 

by 

J. D. Hadden and Albert Sainato2 

ABSTRACT 

The Bureau of Mines conducted drilling studies in the Pocahontas No. 3, 

the Pittsburgh, and a western coalbed to establish the gas migration characteristics for each. Gas pressures in the Pittsburgh coalbed were about 

260 lb/in2 and in excess of 550 lb/in2 in the Pocahontas No. 3. Caving in 

drill holes prevented pressure measurements in the western coalbed. The 

Pittsburgh coalbed contained intersecting "clay veins" which formed gas pressure cells. Although these geological features hamper mining, they can be 

used to control the flow of methane into a mine working. Methane control 

techniques are proposed for the Pittsburgh and Pocahontas No. 3 coalbeds. 

INTRODUCTION 

A comprehensive methane control research program is being conducted by 

the Bureau of Mines to establish the geological structural controls that 

govern the migration and retention of methane in coalbeds; determine the physical properties of methane and of the coal and other strata with which methane 

is associated; devise methods by which methane may be removed from, or controlled in, coalbeds prior to and during coal extraction; develop mechanical 

equipment and instrumentation needed to control methane; and conduct field 

trials of the methane control techniques developed. This is one of a series 

of reports on specific aspects of the program. 

The flow of gas through coalbeds conforms with established laws.3 This 

paper is concerned with compressed gas that exists in the fracture system of 

coalbeds and the movement of this gas, which is governed by Darcy's law: 

l~ining engineer, Pittsburgh Mining Research Center, Bureau of Mines, 

Pittsburgh, Pa. 

2~ining engineering technician, Pittsburgh Mining Research Center, Bureau of 

Mines, Pittsburgh, Pa. 

3~ervik, Joseph. An Investigation of the Behavior and Control of Methane Gas. 

Min. Cong. J., v. 53, No. 7, July 1967, pp. 52-57. 
where 

q = volume rate of flow (cm3/sec); 

k = fracture permeability (darcy); 

A = cross section area (cm); 

p = gas viscosity (cp); 

P = gas pressure (atm); 

1 = unit of length (cm). 

This law means that a gas pressure must exist in the coalbed and that the 

amount of gas entering a mine opening is related directly to the pressure gradient (d~/dl) that exists around the opening. 

Coalbeds may contain obstructions to gas flow such as faults, clay veins, 

and want areas. Pressure gradients and, therefore, gas flows into mine workings are affected by these geological structures. 

DRILLING STUDIES IN THE POCAHONTAS NO. 3 COALBED 

The objectives of the studies conducted in the Pocahontas No. 3 coalbed 

were to determine pressure gradients in the coalbed and the magnitude of the 

pressure gradient near an active face and one near an inactive face, and to 

establish the existence of gas pressures in coal pillars. 

To determine pressure gradients, three horizontal holes were drilled into 

virgin coal near an active face to depths of 30, 60, and 150 feet. The holes 

were then sealed and the gas pressures measured as follows: The maximum pressure in the 30-foot hole was 205 lb/in2; in the 60-foot hole 420 lb/in2; and 

in the 150-foot hole 500 lb/in2. Figure 1 shows the curve of the measured 

pressure versus the depth drilled. This curve is typical for a coalbed containing no obstruction, and will be referred to as a normal gradient curve. 

An area that had been developed for 6 months was selected to compare the 

pressure gradient at an older section of the mine with the pressure gradient 

that exists near an active face. A hole was drilled into the coal in this 

area and pressure measurements were made at five different depths. This hole 

was drilled 250 feet deep and the gas pressure measured at this distance was 

105 lb/in2. Figure 2 shows the gradient for these five measurements. 

From the data on these holes, the pressure gradient near the active face 

area is 420 lb/in2 per 60 feet, or 7 lb/in2/ft. The gradient from the developed area is 105 lb/in2 per 250 feet, or 0.42 lb/in2/ft, which is substantially lower than the gradient near the active face. According to Darcy's 

law, gas flows will be greater at active face areas than inactive areas. 
DEPTH DRI LLED, f t 

FIGURE 1. - Normal Gradient Near an Active Face. 

To establish if coal pillars contained methane under pressure, two 

165-foot-long, horizontal bleeder holes were drilled to the centers of two 

large (330 by 1,000 feet) coal pillars. The gas issuing from the hole in the 

first pillar contained 95 percent methane at the rate of 20,900 cubic feet per 

day. When the hole was sealed, the shut-in gas pressure reached 31 lb/in2. 

Similarly, gas issuing from the hole in the second panel contained 95 percent 

methane at a rate of 17,900 cubic feet per day. After the hole was sealed, 

the shut-in pressure built up to 105 lb/in2. An additional test was made on 

a normal-sized (85 by 85 feet) coal pillar by drilling to the center of the 

pillar. After sealing the hole, no measurable gas pressure was in it. 
HOLE DEPTH, ft 

FIGURE 2. - Normal Gradient Near an Inactive Face. 

Measurements have shown that roof pressures crush the periphery of large 

blocks of coal as much as 40 to 60 feet deep, creating an area of reduced permeability. Gas desorbing from solid coal within the block cannot escape, and 

its pressure builds up within the block. This explains the gas pressure measurements in the two large pillars. 

DRILLING STUDIES IN THE PITTSBURGH COALBED 

Drilling studies were conducted in the Pittsburgh coalbed in southwestern 

Pennsylvania to determine the influence of clay veins on the behavior of the 

gas. This was demonstrated by the pressure measurements taken from one of the 

horizontal drill holes made in the Pittsburgh bed. Mining in the vicinity of 

the hole stopped 5 days prior to drilling. The hole was started and advanced 

18.5 feet before encountering a clay vein. Before continuing through the clay 

vein, the hole was tested for gas but none was found. After the 3-foot clay 

vein was penetrated, the hole was again tested for gas and a pressure of 

221 lb/in2 was measured. Drilling was continued until a second clay vein was 

encountered at 36 feet. The hole was again sealed to measure the pressure at 

this depth, and the pressure of 221 lb/in2 remained. The second 3-foot clay 

vein was penetrated and sealed with packers. The pressure behind the second 

clay vein was 263 lb/in2--an increase of 42 lb/in2 over the pressure measured 

in front of the second clay vein. The pressure readings also indicate that 

the clay veins isolate sections of the coalbed. Figure 3 is a plot of these 

measurements and this type of curve will be referred to as an abnormal 

gradient . 

Four months later, the area was examined to locate and map the clay veins 

exposed by mining in the vicinity of this drill hole. The location and thickness of the clay veins, the positions of the faces at the time of drilling, 
and the location of the drill hole (No. 3) are shown in figure 4. This figure 

shows that clay veins intersect to form closed cells. The pressure readings 

for these cells indicate that each cell has characteristics independent of 

cells surrounding it. 

The term "clay vein" is a misnomer. These so-called "clay veins" are 

lithified clay fracture fillings and, therefore, sparks can be produced by the 

action of bits during mining. Complete penetration of a clay vein is accompanied by a large flow of methane and presents a high potential for an 

ignition. 
 SCALE,^^^ ] nl rl 

Entry - 00 0 

m 

I - 2 - 3 4 

1-1 m5- 6 

FIGURE 4. - Drill Site Location in the Pittsburgh Coalbed. 
DRILLING STUDIES IN A WESTERN COALBED 

Studies were conducted in a western coalbed to measure reservoir pressure 

and pressure gradients. The studies were hampered because of hole caving 

which was caused by severe overburden pressures. 

The first attempts at drilling in the coalbed were made using flushjointed drill casing. Various 2-inch bits were tried with EX casing 

(1-13/16-inch outside diameter) and special E casing (1-3/8-inch outside 

diameter). None of the combinations was successful in drilling deep holes 

consistently. Holes would cave and seize the drill casing. Flushing with 

water would not remove the caved material. 

Using hollow extensible auger steel (1-3/4-inch-diameter), a hole was 

drilled 165 feet deep in virgin coal before striking bottom rock. The circulation of flushing water, combined with action of the auger flights, removed 

caved debris from the hole. 

Gas emission from this hole was 27,000 cubic feet per day but after 4 

days was reduced to 5,000 cubic feet, indicating that the coal around the hole 

has a low permeability. More work must be done to determine why the coal has 

a low permeability and how to degasify the coalbed. 

Pressure measurements could not be made in the oversize hole using inflatable packers. Before more drilling is done in this coalbed, a method will be 

developed for sealing oversized holes. 

PROPOSED METHANE CONTROL TECHNIQUES 

Coalbeds With Normal Pressure Gradients 

A method of degasification conducted at Christopher Coal Company's 

Humphrey No. 7 Mine in 1959 and 19604 required drilling long horizontal holes 

into ribs from the outside entries (fig. 5). The holes were drilled in 

advance of the face at an angle of 15" from the centerline of the outside 

entry. The depth of the hole was determined by the rate of development and 

time required to degasify the area in advance of the face. As an example, if 

a section advanced 100 feet per week and control holes degasified the virgin 

area in a few days, then holes drilled 100 feet in advance of the section 

would be sufficient. On the other hand, if the section advanced more rapidly 

and more time was needed to degasify the virgin area, deeper holes would be 

drilled. This method required development by driving the outside entries 

first, drilling the control holes from these entries, and allowing the holes 

to degasify the virgin area while the inside entries were advanced. Split 

ventilation was required to permit the gas emitted from the control holes to 

drain directly into the outside entries that were the return airways for the 

section. 

4Merritts) William M., W. N. Poundstone, and B. A. Light. Removing Methane 

(Degasification) From the Pittsburgh Coalbed in Northern West Virginia. 

BuMines Rept. of Inv. 5977, 1962, 39 pp. 
LEGEND - Return air __C lntoke air 

Z Permanent - Curtain 

FIGURE 5. - Proposed Methane Control by Horizontal Drilling Into Outside Ri bs. 

Another control technique was to infuse the coalbed with water to form a 

barrier which isolated the area to be mined from the gas reservoir. To form 

the barrier, horizontal holes were drilled across the face of an advancing 

coal section and the coal was infused through these holes. The holes were 

spaced to permit the area infused by one hole to overlap the infused areas of 

the holes on either side. In this manner, a water barrier was formed in the 

coal ahead of the working face. Control holes were angled from the outside 

ribs into the coal on each side of the barrier. These holes were the escape 

routes for the gas displaced from the area during water infusion. The idealized method is shown in figure 6. 

Coalbeds With Abnormal Pressure Gradients 

Clay veins forming closed cells provide an ideal condition for controlling the flow of gas during mining. The clay veins form natural barriers 
FIGURE 6. - Proposed Methane Control by Water Infusion. 

that stop the flow of gas into the face areas from the main gas reservoir of 

the coalbed. Figure 7 shows a proposed method of methane control where clay 

veins are present. This method requires a split ventilation system that uses 

the outside entries for the return airways. This permits the gas from the 

bleeder or control holes to enter directly into the return airways without 

passing over other working places at the face. 

The sequence of mining would be as follows: 

1. Advance outside entries. 

2. Retreat and develop the inside entries. 

3. Fan out degasification holes from the outside entries while mining 

the inside entries. 

Holes at the outside entries may be drilled in a 45" pattern and deep 

enough to penetrate clay veins. These holes will have time to bleed down 

while mining the inside entries. A 45" pattern was chosen to permit a maximum 

coverage with a minimum number of holes. 
FIGURE 7. - Proposed Location of Degasification Holes for a Multiple Entry System. 

CONCLUSIONS 

The flow of gas in U.S. coalbeds is governed predominantly by Darcy's law. 

Therefore, control measures and degasification techniques are aimed primarily 

at reducing pressure, reducing permeability, or filling of the fracture pore 

volume with a more viscous liquid. 

Where the mining of deeper coalbeds requires smaller mine openings, this 

will place practical limitations on the quantity of air supplied to active 

areas of the mine. Since deeper coalbeds tend to be more gassy, the use of 

ventilation as a means of controlling methane concentrations at active face 

areas and in the returns is severely limited. Consequently, methane control 

techniques as described may become mandatory.