Variation of adsorption effects in coals with different particle sizes induced by differences in microscopic adhesion

Abstract

The swelling strain caused by methane adsorption in coal affects reservoir permeability, further complicating the propagation of fluid pressure. The impact of different scales of pores in coal on the adsorption behavior was investigated by combining the reaction heat apparatus C80 and atomic force microscopy (AFM), followed by characterization of adsorption effect variation in coals with different particle sizes due to adhesion differences. The results show that the absolute adsorption volume (AAV) of the high-rank coal sample Chengzhuang (CZ) of 0.85–2.00 mm is 1.06 times higher than that of the medium-rank coal sample Qiyi (QY) under the pressure of 4.2 MPa. With the increase of pressure, more desorption occurs at different sites of adsorbed phase, resulting in the increase in the density of the free phase methane increasing the gap between AAV and excess adsorption volume (EAV). The larger the particle size of coal, the smaller the temperature variation during adsorption. In the AFM measurements, CZ containing 2026 adsorption pores exhibits stronger heterogeneity than QY, leading to the formation of more capillary condensation of methane. For the adsorption heat, methane tends to occupy sites with high adsorption potential at the beginning of adsorption to release large amounts of heat. Moreover, the adhesion force distribution of QY is more uniform than that of CZ, mainly concentrated between 1.5 and 3.0 nN. Subject to microscopic adhesion force, methane molecules in the effective adhesion region can be stably adsorbed to pores. Temperature affects the heterogeneity of adhesion, mainly the higher the temperature, the more concentrated the distribution of adhesion in coal. The findings contribute to the clarification of the adsorption kinetics of nanoscale carriers in coal reservoirs.