Geological controls on coalbed methane reservoir capacity and gas content

Abstract

The influence of coal composition and rank on coalbed methane reservoir capacity, gas content and gas saturation have been investigated for a series of Australian, Canadian and United States coals. Globally there is no or little correlation between coal rank and methane adsorption capacity (as commonly assumed), although in particular basins there are general trends with rank and composition. Micropore 1 Micropores are less than 2 nm in diameter, mesopores 2 to 50 nm in diameter and macropores greater than 50 nm in diameter following current IUPAC classification [International Union of Pure and Applied Chemistry (IUPAC), 1994. Pure Applied Chemistry, 66, 1739]. 1 capacity and surface area, determined from low-pressure carbon dioxide adsorption isotherms (0°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 correlation (Gates Formation coal seams, Lower Cretaceous, Canada) between vitrinite content and methane adsorption capacity (dry ash free and as received bases). For each isorank coal suite there is a good correlation between micropore capacity and abundance of vitrinite and high-pressure methane adsorption capacity. Comparison of coals of similar composition, but from different isorank suites, indicates however that the coals with the highest methane adsorption capacity do not necessarily have the highest micropore capacity. Such results are counterintuitive: most pore volume available for methane adsorption is generally assumed to be microporosity. It is suspected that moisture prevents methane from accessing (some?) microporosity or competes with methane for adsorption space. Carbon dioxide micropore capacity (which is performed on a dry basis) may thus not reflect the microporosity available for gas adsorption in moisture equilibrated coals. Alternatively because of differing physical and chemical properties of carbon dioxide and methane, apparent microporosity available to carbon dioxide (due to greater energy of adsorption) may not be available to methane. Total desorbed gas from canister analyses of Sydney Basin Australian coals suggests that most coals are under-saturated. For a small data set the degree of saturation is negatively correlated with permeability ( r=−0.78) suggesting preferential gas leakage from the higher permeability coals which are vitrite rich. Some degree of under-saturation is probably the result of increased reservoir capacity as a consequence of uplift of the coal seams and thus lower temperatures following gas generation. Rates of desorption are correlated with total gas content ( r=0.79) but do not correlate with rank or any compositional parameter.