Influence of CO2–brine–kerogen wettability on CO2 sequestration in shale: Implications from molecular dynamics simulation

As the main factor influencing the flow and preservation of underground fluids, wettability has a profound impact on CO₂ sequestration (CS). However, the influencing factors and internal interaction mechanisms of shale kerogen wettability remain unclear. In this study, we used molecular dynamics to simulate the influence of temperature, pressure, and salinity on wettability. Furthermore, the results were validated through various methods such as mean square displacement, interaction energy, electrostatic potential energy, hydrogen bonding, van der Waals forces, and electrostatic forces, thereby confirming the reliability of our findings. As temperature increases, water wettability on the surface of kerogen increases. At CO₂ pressures of 10 and 20 MPa, as the temperature increases, the kerogen wettability changes from CO₂ wetting to neutral wetting. As the CO₂ pressure increases, the water wettability on the surface of kerogen weakens. When the pressure is below 7.375 MPa and the temperature is 298 or 313 K, kerogen undergoes a wettability reversal from neutral wetting to CO₂ wetting. As salinity increases, water wettability weakens. Divalent cations (Mg²⁺ and Ca²⁺) have a greater impact on wettability than monovalent cations (Na⁺). Water preferentially adsorbs on N atom positions in kerogen. CO₂ is more likely to form hydrogen bonds and adsorb on the surface of kerogen than H₂O. As the temperature increases, the number of hydrogen bonds between H₂O and kerogen gradually increases, while the increase in pressure reduces the number of hydrogen bonds. Although high pressure helps to increase an amount of CS, it increases the permeability of a cap rock, which is not conducive to CS. Therefore, when determining CO₂ pressure, not only a storage amount but also the storage safety should be considered. This research method and results help optimize the design of CS technology, and have important significance for achieving sustainable development.