Development of methane recovery systems from deep coal beds, to date, has been limited, primarily due to low reservoir permeability resulting from high in situ stresses. This paper discusses the results of a part of an experimental study carried out to investigate the variation in permeability of coal in a reservoir. To simulate in situ conditions, permeability experiments were carried out with specimens held under triaxial stress. Different depths were simulated by different stress conditions. The results suggest that as the flow of methane is initiated, permeability of most coals first decreases. However, once the gas pressure falls below a certain value, the permeability starts increasing. At high stresses (deep coal beds), however, the initial permeability reduction is significantly reduced. Also, the increase in permeability, once the gas pressure falls below 450 psi, is much more dramatic, suggesting that high flow rates can be expected during the later part of the life of producing wells in deep coal beds
Methane and some other gaseous hydrocarbons are generated during the geologic process of coal formation. As a result, methane explosions in underground coal mines has always threatened the safe operation of mines for many years. However, much is being done to convert this liability to a valuable asset (Schwoebel, 1987). Not only has the safety and economics of coal mining been significantly improved, the methane has now become an abundant source of clean energy that lends itself to development with today's technology at today's prices. In the United States alone, estimates indicate that the total coal bed methane resource is between 400 and 850 trillion cu ft - at least two times the total domestic conventional gas resource (Rightmire, 1984). When compared with the annual consumption of 16 Tcf, this resource is of great significance. At the present, there are hundreds of wells producing gas from coal basins in Alabama, Colorado and New Mexico, and it is anticipated that by the end of 1990, this number will exceed 2000. Despite success with the existing wells, potential uncertainties in production controls and lack of state-of-the-art recovery techniques, particularly for the deeper and unmineable coals, represent barriers to effective recovery of this resource (Anon, 1988). Significant advances are required to economically produce gas from coal seams deeper than 3000 feet, which form a major portion of the resource base. The primary problem associated with degasification of deep coal seams is the high in situ stresses, and therefore, very low permeability resulting in poor flowrates. An experimental investigation was carried out to study the effect of stress conditions, and hence, that of the depth, on gas flow behavior of coal and how the flow characteristics vary with time. This paper presents a discussion of the experimental procedure and the preliminary results obtained for the study.
The gas content of coal is made up of two components - free gas compressed in pore spaces, and gas adsorbed on the internal surface of the pores.
