Geological controls on coalbed methane reservoir capacity and gas content

The influence of coal composition and rank on coalbed methane reservoir cap acity, gas content and gas saturation have been investigated for a series o f Australian, Canadian and United States coals. Globally there is no or lit tle correlation between coal rank and methane adsorption capacity (as commo nly assumed), although in particular basins there are general trends with r ank and composition. Micropore (1) capacity and surface area, determined fr om low-pressure carbon dioxide adsorption isotherms (0 degrees C) decrease and then increase with rank, passing through a minimum in the high volatile bituminous coal rank. There is no consistent variation in modal micropore size or size distribution with rank. For suites of isorank coals there is a poor (Permian Bulli and Wongawilli seams, Australia) to good positive corr elation (Gates Formation coal seams, Lower Cretaceous, Canada) between vitr inite content and methane adsorption capacity (dry ash free and as received bases). For each isorank coal suite there is a good correlation between mi cropore capacity and abundance of vitrinite and high-pressure methane adsor ption capacity. Comparison of coals of similar composition, but from differ ent isorank suites, indicates however that the coals with the highest metha ne adsorption capacity do not necessarily have the highest micropore capaci ty. Such results are counterintuitive: most pore volume available for metha ne adsorption is generally assumed to be microporosity. It is suspected tha t moisture prevents methane from accessing (some?) microporosity or compete s with methane for adsorption space. Carbon dioxide micropore capacity (whi ch is performed on a dry basis) may thus not reflect the microporosity avai lable for gas adsorption in moisture equilibrated coals. Alternatively beca use of differing physical and chemical properties of carbon dioxide and met hane, apparent microporosity available to carbon dioxide (due to greater en ergy of adsorption) may not be available to methane. Total desorbed gas fro m canister analyses of Sydney Basin Australian coals suggests that most coa ls are under-saturated. For a small data set the degree of saturation is ne gatively correlated with permeability (r = -0.78) suggesting preferential g as leakage from the higher permeability coals which are vitrite rich. Some degree of under-saturation is probably the result of increased reservoir ca pacity as a consequence of uplift of the coal seams and thus lower temperat ures following gas generation. Rates of desorption are correlated with tota l gas content (r = 0.79) but do not correlate with rank or any compositiona l parameter. (C) 1998 Elsevier Science B.V. All rights reserved.