BEHAVIOR OF COAL-GAS RESERVOIRS 

by 

Joseph Cervik 

Bureau of Mines Methane Control Program 

Technical Progress Report - 10 

April 1969 

U. S. DEPARTMENT OF THE INTERIOR 
CONTENTS 

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

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

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

Micropore structure of coal .............................................. 2 

Mass transport laws ...................................................... 2 

Classification of coalbeds ............................................... 4 

Degasification by surface boreholes ...................................... 6 

Summary and conclusions .................................................. 8 

Nomenclature ............................................................. 9 

References ............................................................... 10 

ILLUSTRATIONS 

Fig . 

1 . Comparison of methane liberation during a mining and idle period ... 5 

2 . Classification of coalbeds according to mode of mass transport ..... 5 

3 . Comparison of pressure gradients near active and inactive areas 

of a mine ....................................................... 7 

4 . Production decline curve for coal-gas wells ........................ 8 
BEHAVIOR OF COAL-GAS RESERVOIRS 

by 

Joseph Cervik 

ABSTRACT 

Gas occurs in coalbeds in an adsorbed and a free gas state. Adsorbed gas 

is stored in the micropore structure and its transport is governed by Fick's 

law. The free gas occurs in the fracture system and flows according to Darcy's 

law. These two modes of mass transport are interdependent. Production 

decline curves of coal-gas wells are of the constant percentage decline type 

and, thus, show no indications of flow characteristics peculiar to coal-gas 

reservoirs. The effectiveness of surface boreho les as a degas ificat ion scheme 

depends upon both good fracture permeability and a high fracture density. 

Conventional methods of reservoir engineering analysis are not applicable to 

coalbeds. 

INTRODUCTION 

Mining of deep coalbeds (1,500 to 2,000 feet) and production of the associated gas is analogous to a gas well with an expanding well bore radius. Gas 

production of 10 to 15 million standard cubic feet per day is not uncommon 

from deep mines. State and Federal codes require that each unit volume of gas 

be diluted with approximately 100 volumes of air in order to maintain methaneair mixtures below the explosibility range of 5 to 15 percent methane. 

Mining deeper coalbeds requires smaller mine openings and these mines 

tend to be more gassy. Thus, the use of ventilation (dilution) as a means of 

controlling methane concentrations at active face areas and in the returns is 

limited severely. Degasification of coalbeds prior to mining and methods of 

controlling the flow during mining are necessary. Before effective methods of 

control and degasification can be developed, the laws governing mass transport 

through coalbeds must be understood. 

The purpose of this paper is to present the fundamental concepts governing the transport of gases through a coalbed, and to define one of the environmental problems related to rapid, economical, and safe mining of coal 

deposits. 

l Supervisory geophysicist, Pittsburgh Mining Research Center, Bureau of Mines, 

Pittsburgh, Pa. 
ACKNOWLEDGMENTS 

Acknowledgment is made to management of Equitable Gas Company, Pittsburgh, 

Pa., for permission to publish the production decline curve for the coal-gas 

wells. 

MICROPORE STRUCTURE OF COAL 

Coal is formed from plant substances which were preserved in various 

states of degradation in a favorable environment, and later altered by chemical and physical processes. However, there is no universal agreement on what 

chemical and physical changes take place in the transformation of plant substances to coal. The origin, petrography, and classification of coal are 

adequately discussed in the first and supplementary volumes of Chemistry of 

Coal Utilization (2, - 4r? and need not be repeated here. 

Methane is a byproduct of the coalification process. Under proper burial 

conditions, methane cannot escape from the coal forming substance and is dispersed through the bed as a free gas during various stages of coalification. 

One of the outstanding features of coal is its microporous texture, which 

plays an important part inmany physicochemical properties of coal such as its 

gas-retaining capacity, its reactivity towards various chemical reagents, and 

the possibility of transforming coal into highly adsorbing materials such as 

activated carbons. 

In the study of sorption of gases and vapors on solids having fine pores, 

mass transport by diffusion plays an important role. In earlier studies, 

adequate attention was not paid to the contribution of diffusional resistance 

to the rate of establishment of an equilibrium concentration. Consequently, 

discrepancies existed between values of internal surface area of coal obtained 

by low temperature gas adsorption, and those obtained from heat of wetting in 

methanol at about room temperature. Marsh (5) - reviews the various methods of 

determining surface areas of coal and concludes that surface area of coals 

are mostly in the range 200 to 300 square meters per gram (2,150 to 3,230 

square feet per gram), Van der Sornmen and others (7) measured the volume of 

methane adsorbed on the internal surface area of coal to about 7,500 psig. 

They determined a saturation value of 21.34 cubic centimeters per gram (684 

cubic feet per ton) for pressures above 1,500 psig. It is interesting to note 

that the amount of gas adsorbed is approximately 28 times the volume of coal 

at standard conditions. Consequently, an acre-foot of such coal contains 

approximately 1.2 million standard cubic feet of methane. Anderson and 

others (1) - show that the micropore diameter is of the order of5 Angstromunits. 

MASS TRANSPORT LAWS 

Coalbeds in the United States are naturally fractured. The degree of 

fracturing varies from one coalbed to another. For example, the Pittsburgh 

coalbed is a blocky-type coal with prominent cleavage planes at 90 degrees 

"Underlined numbers in parentheses refer to items in the list of references at 

the end of this report. 
and a fracture spacing of the order of 6 inches. The Pocahontas No. 3 coalbed 

is a friable-type deposit with cleavage planes at approximately 90 degrees 

that are not as prominent as the planes in the Pittsburgh coalbed. The fracture spacing in the Pocahontas No. 3 coalbed is of the order of 1/4-inch. 

Coalbeds, therefore, have a fracture porosity and permeability. Mass transport through the fracture system is governed by Darcyls law, namely: 

The driving force for this mode of transport is a pressure gradient. 

Presumably, a coalbed in a late diagenetic stage is one solid mass and 

all gas was contained in the micropore structure. Later, jointing developed 

in the coalbeds and allowed gas to bieed f~-om the sclid coal into the fracture 

system. Mass transport through solid coal (micropore structure) is governed 

by Fickts law of diffusion, namely: 

The driving force for this mode of transport is a concentration gradient, 

Sevenster (6) found that the diffusion coefficient of powdered coal for a 

number of gases is of the order of 10-l3 square centimeters per second at 

atmospheric pressure. 

Although these two modes of transport are separate and distinct phenomena, 

they are interdependent. This interdependency can be expressed over a limited 

range of pressures by the following empirical equation (L): 

In virgin coal, the pressure in the fracture system and the concentration 

of gas in the micropore structure are in equilibrium. If the pressure is 

reduced, the equilibrium is broken and gas begins to bleed from the matrix 

until a new equilibrium is reached that is characteristic of the gas pressure 

in the fracture system. 

Coal-gas reservoirs, thus, consist of two distinct elements; namely, 

fractures and matrix, Each has its characteristic mode of mass transport that 

are interdependent. Such reservoirs are more complex than "conventional" gas 

reservoirs having the distinct elements, fractures, and matrix, each with its 

characteristic permeability and porosity. Therefore, the extension of conventional methods of reservoir engineering analysis to coalbeds thus is not 

justified. 

The desorption of gas from coal is not an instantaneous process, Its 

release can be described qualitatively as "slow bleeding." For example, 1 

pound of 1/4-inch coal and 1 pound of fine coal (275- to 325-mesh) will adsorb 

the same quantity of methane at 15 psig. However, when pressure is reduced to 

atmospheric, the fine coal (275- to 325-mesh) desorbs all its gas in about 
1 hour; the 1/4-inch coal requires about 30 days. For solid coal particles of 

the order of 1/2-inch, the desorption process may take 6 months to 1 year. 

Consequently, the rate of desorption of gas from coal depends upon equilibrated pressure, coal particles size and geometry, and the diffusivity 

coefficient. 

Most coalbeds have an associated gas or reservoir pressure. Reservoir 

pressure in the Pittsburgh coalbed is of the order of 260 psig and in excess of 

550 psig in the Pocahontas No. 3 coalbed. These pressures are far in excess of 

those used in sorption studies in the United States. Most of the studies of 

the physical properties of coal relating to diffusion transport were made on 

European coals, and no comparable studies have been made on United States 

coals. Consequently, very little is known about the sorption process at L 1- Lrlese p~-ess-ures . 

CLASSIFICATION OF COALBEDS 

Mass transport in European coalbeds is c o n t r o l l e d predominantly by Fickl s 

law. Methane control measures in European coalbeds are concerned primarily 

with the diffusion coefficient, D. This is supported by the fact that all 

effective methods of degasifying European coalbeds require fracturing of the 

coal. The rate of desorption of gas is dependent upon effective coal particle 

size, with smaller particles releasing gas much faster than larger particles. 

In contrast, coalbeds in the Appalachian area of the United States appear 

to fall into the category of mass transport predominantly by Darcyls law. 

Figure 1 shows the variation of methane content in a return entry of a section 

of a mine where a continuous miner was operating. Air velocity was constant 

during these measurements. During the 78-minute mining period, methane content in the outby air rose from 0.52 to 0.97 percent. The small dips represent intervals of time the miner ceased mining. This part of the curve represents a steepening of the pressure gradient by advancing into the reservoir, 

and according to Darcyls law, gas flows will increase. The second part of the 

curve is a 140-minute idle period and reflects the readjustment of the pressure gradient to lower values. The significant feature of this figure is that 

the continuous-mining machine is able to advance the face at a much faster 

rate than the drainage radius is able to recede into the coalbed. 

Coalbeds can be classified according to modes of mass transport into mine 

openings. This classification is shown in figure 2. No coalbed exhibits mass 

transport by Darcyls law alone. Methane and other gases have an affinity for 

coal and, therefore, diffusion transport exists always. Undoubtedly, there 

are coalbeds where transport by both Fickls and Darcyls laws is important and 

of equal magnitude. Experience in mining United States coalbeds is generally 

limited to shaft depths of less than 1,500 feet. Most of the coal mines of 

Great Britain and Western Europe are much deeper. As deeper coalbeds are 

exploited in the United States, the mode of mass transport may shift gradually 

with depth from the Darcy mode to diffusion transport. Methane control measures and degasification techniques developed for Darcyls mode of transport are 

not applicable to beds where diffusion transport predominates. The converse 

also is true. 
0 40 80 120 160 200 240 

TIME, min 

FIGURE 1. - Comparison of Methane Liberation During a Mining and Idle Period. 

FIGURE 2. - Classification of Coalbeds According to Mode of Mass Transport. 

Classification of coalbeds 

I I 

Predominately 

Darcy's law 

L 

Predominately 

Fick's law 

Corn bination of Fick's 

and Darcy's law 
DEGASIFICATION BY SURFACE BOREHOLES 

For Darcy type flow, degasification and methane control measures will be 

directed towards methods of reducing pressure, reducing fracture permeability, 

or filling of the fracture pore volume with a more viscous fluid. There are 

several methods of controlling gas flows underground during mining; these 

methods include water infusion, use of foam, and underground blocking method. 

These methods have been presented by Cervik (2). The only degasification 

scheme that will be discussed in this paper is surface boreholes. 

Surface boreholes provide a method of degasifying a coalbed before it is 

opened to production or degasifying a section of a coalbed far in advance of 

mining. Coalbeds contain large quantities of gas; however, the production ---& -- ~a~es are general ly small because reservoir pressures may be relat ivcly !GW in 

comparison to other producing horizons, and diffusion transport through the 

matrix is a slow process, The design of an economical degasification scheme 

is dependent strongly upon proper well spacing. The cost of such a program 

must be balanced against cost reduction in ventilation, higher productivity, 

and, possibly, revenue from the sale of produced gas. 

Figure 3 shows measurements of pressure gradients that exist around mine 

openings. Curve 1 was measured in an area where coal was being mined actively. 

Curve 2 was measured in an area that had been inactive 6 months. Note that 

the gradients are much steeper near the active face areas; these are the areas 

where heavy gas flows occur, Figure 3 shows that the drainage radius near an 

active face is about 80 feet, and about 1,300 feet (extrapolated) at an 

exposed rib. These curves were obtained by drilling horizontal holes incrementally and measuring pressure. 

Curve 2 of figure 3 was monitored for a period of approximately 4 months. 

During this interval no pressure changes occurred, indicating that the drainage radius was stationary. These measurements substantiate the observation 

that gassy mines left idle for a number of years do not degasify. When the 

mine is reopened and entries are driven for approximately 1,000 feet, the mine 

is as gassy as before closure. Based on the data presented in figure 3, a 

reasonable spacing program for the Pocahontas No. 3 coalbed is of the order 

of 0.5 mile or one well per 160 acres. 

Figure 4 shows a production decline curve for a group of 20 wells drilled 

into the Pittsburgh coalbed. These wells are on a 1,500-foot spacing. The 

curve is a typical constant percentage decline, and there are no indications 

of flow characteristics peculiar to coal-gas reservoirs. Initial production 

rates were of the order of 15 to 20 million standard cubic feet per day and 

the economic life is of the order of 35 to 40 years. Adequate technical data 

are not available such as shut-in pressures, drawdown tests, and in situ 

permeability tests. 

The successful use of surface boreholes as a degasification scheme in 

coalbeds will depend upon two necessary conditions; namely, good fracture 

permeability and high fracture density. Both conditions must be met. A bed 

with good fracture premeability and low fracture density is in the same 
DISTANCE, ft 

-- FIGURE 3. - Comparison of Pressure Gradients Near Active and Inactive Areas of -- 

a Mine. 
FIGURE 4. - Production Decline Curve for Coal-Gas Wells. 

1 I I I I 

- . . . 

category as a bed with poor fracture permeability and high fracture density. 

Neither is susceptible to surface borehole degasification. Both can be 

classified as beds where diffusion transport predominates. As mass transport 

shifts from the Darcy mode toward diffusion transport, the effectiveness of 

surface boreholes as a means of degasification decreases. 

3 

g 9- 

oz 

gs a 

5 

SUMMARY AND CONCLUSIONS 

Mass transport through coalbeds is governed by Darcy's law and Fick's law 

of diffusion. Thus, the extension of conventional methods of reservoir 

engineering analysis to coalbeds is not justified. 

1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 

YEARS 

4 

6- - 

I I I 

Internal surface area of coal can be as high as 1.5 million square feet 

per pound of coal and 0.34 standard cubic feet of gas per pound of coal can be 

adsorbed on the internal surface area at the saturation pressure of 1,500 psig. 

1 

The effectiveness of surface boreholes as a degasification scheme will 

depend upon two necessary conditions--good fracture permeability and high 

fracture density. Both conditions must be met. 

Production decline curves show no indications of flow characteristics 

peculiar to coalbeds. Economic life of coal-gas wells is of the order of 35 

to 40 years. 
NOMENCLATURE 

q = volume flow rate (cm3/sec 

k = fracture permeability (Darcy) 

A = cross-sectional area (cn? ) 

IJ. = gas viscosity (centipoise) 

P = gas pressure (atm) 

L = length (cm) 

q1 = volume flow rate (cm3/sec) 

D = diffusivity coefficient (cnf? /set) 

C = concentration of CH4 in solid coal (cm3 of CH,/C~~ of coal) 

Co = equilibrated concentration of CH4 in solid coal 

(cm3 of CH*/C~~ of coal) 

b and n = constants 
REFERENCES 

1. Anderson, R. B., J. Bayer, and L. J. E. Hofer. Equilibrium Sorption 

Studies of Methane on Pittsburgh Seam and Pocahontas No. 3 Seam Coal. 

Coal Science. Advances in Chemistry Series 55. Am. Chem. Soc., 

Washington, D.C., 1966, pp. 386-399. 

2. Cervik, J. An Investigation of the Behavior and Control of Methane Gas. 

Min. Cong. J., v. 53, July 1967, p. 52. 

3. Hendricks, T. A. Chemistry of Coal Utilization. John Wiley & Sons, 

New York, v. 1, 1945, pp. 1-24. 

4. Lowry, H. H. Chemistry of Coal Utilization. John Wiley L Sons, New York, 

suppl. v., 1963, pp. 1-34. 

5. Marsh, H. The Determination of Surface Areas of Coals--Some Physicochemical Considerations. Fuel, v. 33, 1965, p. 253. 

6. Sevenster, P. G. Diffusion of Gases Through Coal. Fuel, v. 38, 1959, 

p. 403. 

7. Van der Sommen, J., and others. Chemical Structure and Properties of 

Coal, XII--Sorption Capacity for Methane. Fuel, v. 34, 1955, p. 444.