Effects of Moisture Contents on Shale Gas Recovery and CO₂ Sequestration

Enhanced recovery of shale gas with CO₂ injection has attracted extensive attention as it combines the advantages of improved efficiency of shale gas recovery and reduced greenhouse gas emissions via CO₂ geological sequestration. On the other hand, the microscopic mechanism of enhanced shale gas recovery with CO₂ injection and the influence of the subsurface water confined in the shale nanopores remain ambiguous. Here, we use grand canonical Monte Carlo (GCMC) simulations to investigate the effect of moisture on the shale gas recovery and CO₂ sequestration by calculating the adsorption of CH₄ and CO₂ in dry and moist kerogen slit pores. Simulation results indicate that water accumulates in the form of clusters in the middle of the kerogen slit pore. Formation of water clusters in kerogen slit pores reduces pore filling by methane molecules, resulting in a decrease in the methane sorption capacity. For the sorption of CH₄/CO₂ binary mixtures in kerogen slit pores, the CH₄ sorption capacity decreases as the moisture content increases, whereas the effect of moisture on CO₂ sorption capacity is related to its mole fraction in the CH₄/CO₂ binary mixture. Furthermore, we propose a reference route for shale gas recovery and find that the pressure drawdown and CO₂ injection exhibit different mechanisms for gas recovery. Pressure drawdown mainly extracts the CH₄ molecules distributed in the middle of kerogen slit pores, while CO₂ injection recovers CH₄ molecules from the adsorption layer. When the water content increases, the recovery ratio of the pressure drawdown declines, while that of CO₂ injection increases, especially in the first stage of CO₂ injection. The CO₂ sequestration efficiency is higher under higher water content. These findings provide the theoretical foundation for optimization of the shale gas recovery process, as well as effective CO₂ sequestration in depleted gas reservoirs.