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

Abstract

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.