The role of interseam strata in the retention of CO2 and CH4 in a coal seam gas system

Abstract

Abstract Clastic sedimentary rocks associated with coal seams affect the retention of gas and the rate of escape of CO 2 and CH 4 from a coal seam gas (CSG) system. Quantifying sealing properties using a parameter based on molecular diffusion and permeation of gases through rock and coal matrices allows the effects to be evaluated. Observations made on a CSG system in the Hunter Coalfield, Sydney Basin, Australia, show that the gas contents vary between ∼0.5 m 3 /t and ∼10 m 3 /t. The CSG consists predominantly (>80%) of CH 4 . Analyses of stable carbon isotopes indicate that the CH 4 is mainly biogenic, with gases in deeper seams showing mixing with thermogenic sources. At depths less than about 500 m, the coal seams containing elevated gas contents are generally overlain by low permeability, fine-grained clastic rocks such as mudstone and carbonaceous shale. Using a purpose-built system to measure matrix permeability and diffusivity of a coal and its roof and floor rocks allows characterisation of long-term permeation and diffusive flow. Measurements indicate that permeation and diffusion take place at different rates depending on gas type. A main finding of the study is that CH 4 permeates more rapidly than CO 2 whereas CO 2 diffuses more rapidly than CH 4 . For the studied samples, gas diffusion in coal is slower than in the clastic rocks, indicating that the minor coal seams are even better seals compared to the interseam clastic rocks for the main coal seam reservoir. Differences in flow regimes vary with lithology and therefore differences in lithology affect the retention of CO 2 versus CH 4 . For example, the permeation of both CO 2 and CH 4 is several times more rapid in the sandstone than for the claystone analysed, but diffusive flow is similar for these two samples. Carrying out analyses such as those done in the present study will assist in evaluating gas containment properties of interseam strata in CSG systems over given periods of time. Quantification of these properties will assist in evaluating the potential for CO 2 injection and long-term storage in coal seams.