A study on the effect of gas shale composition and pore structure on methane sorption

Ten moist, powdered European shale samples were analyzed for their sorption properties by volumetric method. The adsorption capacities were correlated to the shale organic types and maturity. The pore-size distribution obtained from low-pressure CO2 micropore adsorption was also correlated with the porosity and shale organic types. Furthermore, pore volume and average pore width were taken into consideration to determine the dominant parameters controlling adsorption. To identify the discrepancy between available and actual pore space for adsorption, helium and krypton gases were used for void volume estimation. Methane adsorption isotherms follow Langmuir Type I behavior and, in general, showed a positive trend with Total Organic Content (TOC) and Hg-porosity although some deviations were also observed. Low to moderate level of hysteresis between adsorption and desorption isotherms for some samples was visible, which may be attributed to the experimental uncertainty and existence of heterogeneous pores for shale-methane interaction. The low-pressure micropore adsorption analysis indicated dominance of nanopore and very fine micropores in the shale matrix structure along with associated microporosity of the clay materials. The observed negative adsortion or decline in adsorption isotherm are related to the mismatch of the available pore spaces for helium and methane. In general, He-calibrated isotherms showed higher levels of adsorption than the corresponding Kr-calibrated isotherms although the unit void volume for all samples follow a negative trend with the maximum methane capacity.