Effect of Water Content and Salinity on CH4/CO2 Competitive Adsorption in Organic and Clay Nanopores: A Molecular Perspective

CO2 injection into shale gas reservoirs has been identified as a promising technique for enhancing shale gas productivity and achieving permanent CO2 sequestration. The vast nanopores present in shale offer considerable space for CO2 storage. However, it is often observed that shale nanopores can be filled with water, which inevitably affects the storage potential for CO2. In this work, molecular dynamics simulations are employed to investigate the influence of water and salinity on CO2 adsorption behavior and storage capacity in both organic and clay nanopores. Simulation results show that the presence of water occupies the accessible adsorption space, resulting in a lower storage capacity of CO2. At the water content of 0.06, 0.12, and 0.18 g/cm3, the reduction in CO2 adsorption reaches 9.9, 17.1, and 22.2% in kerogen, respectively, greater than 3.4, 12.1, and 19.6% in K-illite. An enhancement in pore size can alleviate the CO2 loss caused by water. The additional NaCl ions result in a further reduction in the adsorption capacity of CO2. The van der Waals interaction dominates the fluid–surface interaction. A higher interaction energy can be observed in kerogen for CO2 with reduced mobility, indicating the potential for CO2 geological storage. Subsequently, the CO2 storage capacity in the shale pores is evaluated. The kerogen displays a higher storage amount for CO2 than that for K-illite in any case. The presence of water significantly reduces the CO2 storage capacity by 46.4 and 40.6% in kerogen and K-illite at 0.18 g/cm3, respectively. This work provides an insight into the CO2 adsorption behavior and storage capacity in shale nanopores under water and salinity environment.