Investigation of pore size effects on adsorption behavior of shale gas

Understanding the effects of pore size on shale gas adsorption behavior is necessary for accurate evaluation of adsorbed gas content under geological conditions. Shale is a porous medium, and the pore structure of the shale reservoir is complicated, with a wide distribution of aperture sizes. Critical parameters for investigating pore size effects on shale gas adsorption behavior were determined, using Grand Canonical Monte Carlo (GCMC) simulations, and the shale gas occurrence state in varying sized kerogen pores was documented, by linking GCMC simulations to the experimental pore size distribution. It was found that using the excess adsorption estimation, in terms of per unit surface area (PUSA), which was obtained from the free gas density calculated by using the GCMC method in a bulk simulation cell, and then derived from the free volume probed by the methane, was a reasonable way of demonstrating pore size effects on shale gas adsorption behavior. The distribution profiles of both the gas density and the interaction energy, rather than their average values, could be used to reflect this pore size effect objectively. Gas density in the adsorption phase rose non-monotonically with reducing pore size, under the combined influence of the interactions' overlapping effects and the limited pore space, and the overlapping threshold was determined to be 1.24 nm for the experiments. The gas in the pores that were smaller than the overlapping threshold, which was difficult to desorb under geological pressures, accounted for approximately 40.53% of the total adsorbed gas in the kerogen. The adsorbed gas in the kerogen lay mainly (84.97%) in smaller pores ( < 5 nm), while the free gas was mainly located (77.70%) in larger pores ( > 5 nm).