The effect of water vapor on methane adsorption in the nanopores of shale

Abstract

Water plays an essential role in shale gas migration and adsorption, and most studies on the influence of water on shale gas adsorption refer only to moisture-equilibrated shales. To investigate the impact of water vapor on methane adsorption in shales, three experiments were conducted and compared: (1) pure methane adsorption onto dry shale (PMD), (2) pure methane adsorption onto moisture-equilibrated shale (PMMS), and (3) simultaneous adsorption of water vapor and methane (SAWM) onto shale. The experimental results reveal that the detrimental impact of water vapor on methane adsorption is inferior to that of preadsorbed water. Two mechanisms, water blocking and adsorption competition, are responsible for the reduction and difference in the methane adsorption capacity and adsorption rate between PMMS and SAWM. Compared to PMD, the methane adsorption capacity decreases by 81–96% in PMMS and by 20–49% in SAWM. Methane adsorption equilibrium is achieved the fastest in PMD. Before the equilibration degree reaches 95%, methane adsorption during SAWM progresses more rapidly, while the reverse occurs when the equilibration degree exceeds 95%. Water vapor can impact micropores more easily during the premoistening process, which obviously lowers methane adsorption. The adverse effect of water vapor on micro-to mesopores is inhibited by high methane partial pressure in SAWM. In PMMS, adsorbed water mainly occupies micropores (0.3–1.5 nm), and methane is adsorbed in pores larger than 1.5 nm. In SAWM, methane preferentially occupies micropores; the competition between methane and water vapor is mainly concentrated in mesopores (1.5–20 nm).