Wettability of Kerogen in Shale Reservoirs by CO ₂ /N ₂ Injection: Molecular Simulation of Temperature, Pressure, and Ion Effects

Understanding the wettability transition of shale organic matter in CO₂/N₂ environments is crucial for optimizing geological CO₂ sequestration and gas recovery operations. In this study, molecular dynamics simulations were employed to investigate the wettability behavior of kerogen under CO₂ and N₂ atmospheres across a wide range of temperatures (298.15 to 378.15 K) and pressures (5 to 65 MPa). The effects of brine salt type, concentration, and gas mixture composition were also considered. Results show that increasing the CO₂ pressure significantly increases the water contact angle on kerogen surfaces, indicating reduced water wettability. In contrast, N₂ exhibits only a minor effect. Temperature has a weakening effect on contact angles under the CO₂ environment but a negligible influence in the presence of N₂. Salt cations such as Mg²⁺ and Ca²⁺ increase the contact angle more significantly than Na⁺ at the same concentration. Furthermore, as the N₂ proportion increases in CO₂-N₂ mixtures, water contact angles decrease, revealing a stronger adsorption affinity of CO₂ on shale organic matter. These findings provide molecular-level insights into how geological and compositional factors control the organic surface wettability of shales, contributing to the optimization of CO₂ geological storage and enhanced gas recovery strategies.