Correction of gas adsorption capacity in quartz nanoslit and its application in recovering shale gas resources by CO2 injection: A molecular simulation

Accurate molecular simulations of the competitive adsorption behaviors of CH4 and CO2 in shale reservoirs are crucial for understanding the fundamental mechanisms of fluids storage and recovery under actual burial depth. In this work, the shale thickness of 79.82 nm (143 layers) at each side of quartz wall in theoretical model was advanced for gas adsorption capacity correction. In this case, the adsorption mechanism of shale gas and its recovery mechanism by CO2 injection were systematically pursued at varied burial depth, pore size, moisture content, and gas mole fraction by Grand Canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. Adsorbed and free capacities of CH4 gas were determined individually. Gas loss rate and recovery efficiency were put forward to evaluate the recovery of shale gas by H2O and CO2 molecules. The obtained results show that the enrichment region of CH4 is ∼2400 m, which is predicted to be the economically feasible mining depth of CH4. Shallow burial depth (600–800 m) should be favorable for the sequestration of CO2. The results obtained are expected to provide significant theoretical guidance for the reliable evaluation and economic exploitation of shale gas as well as the sequestration of CO2.