Re-examination of methane-water interaction in response to adsorption interface changes in shale kerogen: Unifying experiment and simulation

Gas-water interaction is pivotal in assessing in-situ shale gas resources and designing the fracturing technology. The mechanism underlying gas/water adsorption, particularly the influence of nanopore structures on methane/ water adsorption behavior, has garnered significant interest within the research community. However, there is currently a lack of comprehensive understanding in applying existing knowledge to rationalize how the gas-water interaction changes with the variation of the adsorption interface. In this study, the methane-water interactions in response to adsorption interface changes of six water-bearing samples were discussed by both experiments and molecular simulations. The results indicated that water loading in nanopores greatly reduced the total methane adsorption capacity, and the negative effect of water on the pore-filling adsorption behavior was greater than that of surface adsorption behavior. The competitive adsorption between gas and water mainly occurs on the nitrogen-containing structure of the secondary adsorption site. Water drives methane from the secondary adsorption sites to sulfur-containing structures by bonding with hydrogen atoms in nitrogencontaining structures. However, due to differences in preferential adsorption sites and micropore accessibility, the role of competitive adsorption in the methane-water interaction process has been overestimated. Experimental and simulation results found that the influence of water on methane adsorption during the transition of the adsorption interface results from two different mechanisms. Water reduces methane adsorption capacity by occupying micropore space. On the other hand, the micropore accessibility of methane and water in over-mature samples cannot be ignored. Due to water's greater polarity and smaller kinetic diameter, water can access some micropores that are inaccessible to methane. Consequently, water affects the adsorption pathway of methane by blocking some micropores that are only accessible to water, resulting in a decrease in methane adsorption capacity.