Effect of\nWater Content and Salinity on CH<sub>4</sub>/CO<sub>2</sub> Competitive\nAdsorption in Organic and Clay Nanopores:\nA Molecular Perspective

CO<sub>2</sub> injection into shale gas reservoirs has\nbeen identified\nas a promising technique for enhancing shale gas productivity and\nachieving permanent CO<sub>2</sub> sequestration. The vast nanopores\npresent in shale offer considerable space for CO<sub>2</sub> storage.\nHowever, it is often observed that shale nanopores can be filled with\nwater, which inevitably affects the storage potential for CO<sub>2</sub>. In this work, molecular dynamics simulations are employed to investigate\nthe influence of water and salinity on CO<sub>2</sub> adsorption behavior\nand storage capacity in both organic and clay nanopores. Simulation\nresults show that the presence of water occupies the accessible adsorption\nspace, resulting in a lower storage capacity of CO<sub>2</sub>. At\nthe water content of 0.06, 0.12, and 0.18 g/cm<sup>3</sup>, the reduction\nin CO<sub>2</sub> adsorption reaches 9.9, 17.1, and 22.2% in kerogen,\nrespectively, greater than 3.4, 12.1, and 19.6% in K-illite. An enhancement\nin pore size can alleviate the CO<sub>2</sub> loss caused by water.\nThe additional NaCl ions result in a further reduction in the adsorption\ncapacity of CO<sub>2</sub>. The van der Waals interaction dominates\nthe fluid–surface interaction. A higher interaction energy\ncan be observed in kerogen for CO<sub>2</sub> with reduced mobility,\nindicating the potential for CO<sub>2</sub> geological storage. Subsequently,\nthe CO<sub>2</sub> storage capacity in the shale pores is evaluated.\nThe kerogen displays a higher storage amount for CO<sub>2</sub> than\nthat for K-illite in any case. The presence of water significantly\nreduces the CO<sub>2</sub> storage capacity by 46.4 and 40.6% in kerogen\nand K-illite at 0.18 g/cm<sup>3</sup>, respectively. This work provides\nan insight into the CO<sub>2</sub> adsorption behavior and storage\ncapacity in shale nanopores under water and salinity environment.