Molecular insights into CO2 sequestration and enhanced gas recovery in water-bearing shale nanocomposites

CO2 sequestration and enhanced shale gas recovery (CO2-EGR) is a highly promising method for improving shale gas exploitation while simultaneously mitigating greenhouse effect. However, the microscopic mechanisms of CO2-EGR in water-bearing shale nanopores remain to be clarified. In this work, a shale nanopore model, composed of kerogen and quartz, was constructed to investigate water distribution characteristics by molecular dynamics simulation. The water-bearing shale composite model was used to study the competitive adsorption behavior of CH4/CO2 by grand canonical Monte Carlo simulation. A molecular simulation scenario was proposed to reproduce the process of shale gas recovery by CO2 huff-n-puff. The recovery mechanisms during pressure depletion and CO2 huff-n-puff were revealed. The results show that depletion extraction primarily mobilizes water on the kerogen surface, while CO2 huff-n-puff displaces water on both the kerogen and quartz surfaces. The mobilization rate of water molecules can reach 80% after two rounds of CO2 huff-n-puff. Increasing moisture content sequentially affects the adsorbed CH4 on the quartz surface, the adsorbed CH4 on the kerogen surface and the adjacent free CH4. The water-bearing shale nanopores have a higher CO2 sequestration rate, reaching up to 41.1%, due to the dissolution of CO2 into the water phase on the quartz surface and the adsorption of CO2 on the kerogen surface and the quartz water film. Depletion extraction empties substantial void pore spaces, resulting in a reservoir storage effect during the early stage of CO2 huff-n-puff in dry shale reservoirs. The injection volume of CO2 is three times that of the CH4 recovery due to the reservoir storage effect, which inhibits the efficiency of CO2-EGR. This study gains a profound understanding of CO2-EGR in water-bearing shale reservoirs.