Molecular insights into recovery of shale gas by CO2 injection in kerogen slit nanopores

The unique structures of kerogen greatly complicate the recovery mechanisms of natural gas and challenge the energy industry. In this research, a molecular insight into recovery of shale gas by CO2 in kerogen slit nanostructures are studied using GCGM (Grant Canonical Monte Carlo) and MD (Molecular Dynamics) simulations and compared to that of kerogen matrix. We find that pressure has a positive effect on CH4 and CO2 adsorption capacity, which is opposite to temperature. The isosteric heat of CO2 is larger than CH4, indicating a higher affinity of CO2 to kerogen and lower self-diffusion coefficient. The competitive adsorption of CO2 over CH4 is higher at lower CO2 model fraction, suggesting less amount of CO2 is required to recover the same amount of CH4 at the early stage of CO2 injection. In kerogen slit nanopores, CH4 adsorption state is less stable than in kerogen matrix, which introduces a higher diffusion of CH4. The adsorption selectivities in kerogen matrix are almost over 5, while the selectivities are lower than 3.5 in kerogen slit nanopores. Therefore, the selectivities in kerogen matrix is higher than in slit. It indicates that more CH4 can be displaced out from formation without slits (or fractures) than with fractures when the same amount of CO2 is injected. The displacement efficiency increases with increasing “bulk” pressure, and it can reach over 80% when the “bulk” pressure is 20.0 MPa, suggesting CO2 can be an ideal candidate for CH4 recovery. We also find that a few CH4 are still inside the kerogen matrix, which is difficult to recover by CO2 injection. We hope that our research may serve as a reference for developing shale reservoirs with natural or artificial fractures.