Effects of Moisture Contents on Shale Gas Recovery and CO Sequestration.

Enhanced recovery of shale gas with CO injection has attracted extensive attention as it combines the advantages of improved efficiency of shale gas recovery and reduced greenhouse gas emissions via CO geological sequestration. On the other hand, the microscopic mechanism of enhanced shale gas recovery with CO 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 CO sequestration by calculating the adsorption of CH and CO 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 CH/CO binary mixtures in kerogen slit pores, the CH sorption capacity decreases as the moisture content increases, whereas the effect of moisture on CO sorption capacity is related to its mole fraction in the CH/CO binary mixture. Furthermore, we propose a reference route for shale gas recovery and find that the pressure drawdown and CO injection exhibit different mechanisms for gas recovery. Pressure drawdown mainly extracts the CH molecules distributed in the middle of kerogen slit pores, while CO injection recovers CH molecules from the adsorption layer. When the water content increases, the recovery ratio of the pressure drawdown declines, while that of CO injection increases, especially in the first stage of CO injection. The CO 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 CO sequestration in depleted gas reservoirs.