ABSTRACT: This paper discusses the results of an experimental investigation carried out to study the effect of changes in gas pressure, and effective stress, on permeability of coal in a reservoir. Gas pressure-permeability relationship was established experimentally for cylindrical coal specimens held under triaxial stress. The results suggest that coal, unlike most rocks, exhibits a negative Klinkenberg Effect. With decreasing gas pressure, the permeability of coal decreases, suggesting contraction of the cleats and microfractures, thus closing the flow paths. However, for an adsorbing gas like methane, the effect is significantly different. With decreasing gas pressure, the permeability first decreases. But once the gas pressure falls below the desorption pressure, i.e., the gas starts desorbing, the permeability rises dramatically - in spite of increased effective stress. Since most of the methane stored in coal is by sorption, permeability of a coalbed can be expected to increase significantly over the life of a producing well.
1 INTRODUCTION
Recently the U.S. Geological survey reduced by 40% its estimate of undiscovered domestic sources of conventional oil and gas. Although these projections are subject to refinement, there can be little doubt that unconventional resources will play a vital role in meeting the U.S. energy demand in the years to come. One such resource is the coalbed methane, estimated in U.S. coal reserves alone to be in excess of 400 trillion cu ft (Mroz, 1983). When compared with U.S. natural gas reserves of less than 200 Tcf, and annual consumption of 16 Tcf, this resource is of great significance. In this respect, coal - as the origin and storehouse of truly immense quantities of natural gas - is clearly one of the most significant rediscoveries of the last ten years (Petroleum Frontiers, 1986).
Over the last few years, significant advances have been made towards designing systems for recovery of methane from coalbeds. At the present, there are approximately 700 wells producing gas from coal basins in Alabama, Colorado and New Mexico. Methane from other coal reserves, with mining activities, is vented to atmosphere to assist mining operations. Despite limited success with the existing wells, potential uncertainties in production controls and deficiencies in state-of-the-art recovery techniques and their application, particularly for the deeper coals, represent barriers to more effective recovery of this vast resource (Anon, 1988). Although some of these difficulties are due to complexities of the process, they are also caused due to inadequate understanding of some of the phenomena involved in behavior of a coal seam as a reservoir. Coal seams show considerable differences in behavior from normal porous gas reservoirs in both, the mode of gas storage and permeability characteristics.
Gas is primarily stored in coal in adsorbed form, this quantity being dependent on the gas pressure in the pores. Once the flow of gas is initiated, gas pressure in the coal pores starts decreasing, thus lowering the capacity of coal to retain methane in adsorbed form. The process of desorption is thus initiated making more gas available for flow towards the well.
