Impacts and Mechanisms of Water on CH4 Adsorption on Shale Minerals

Formation water (FW) and fracturing fluid (FF) significantly affect the adsorption rules of CH4 on a shale surface. To clarify the impact rules and micromechanisms of FW and FF on CH4 adsorption, isothermal adsorption experiments of CH4 on different shale minerals with different equilibrium water were conducted. Then, the corresponding adsorption models were constructed by Grand Canonical Monte Carlo (GCMC) simulation to perform adsorption simulation after matching experiments. Finally, molecular dynamic (MD) simulation was carried out to study the micromechanisms of FW and FF affecting CH4 adsorption on different shale minerals. The results show that the adsorption capacity of organic matter to CH4 is much stronger than that of other minerals. Compared to dry conditions, the adsorption capacity of organic matter, smectite, illite, and total shale with FW (Sw 15%) decreases to 65, 45, 70, and 55%, respectively. The impact of FF on CH4 adsorption capacity is more significant than FW. The adsorption capacity decreases to 45, 30, 50, and 45% for organic matter, smectite, illite, and total shale with FF (Sw 15%), respectively. FW molecules inhibit CH4 adsorption by occupying adsorption sites on mineral surfaces. However, the HPAM in FF completely covers the mineral surface to compress adsorption space and hinder CH4 adsorption. Although water molecules in both FW and FF occupy part of the adsorption sites on organic matter, the left sites can still absorb large amounts of CH4. It provides theoretical guidance for the efficient development of shale gas.