Adsorption Behavior and Phase Equilibrium Modeling of Shale Gas–CO ₂ System in Shale Nanopores

CO2 injection process holds significant potential for reducing carbon emission and enhancing shale gas recovery. On account of the widespread presence of nanoscale pores in shale rocks, after a CO2 injection operation, a competitive adsorption behavior between shale gas and CO2 in nanopores can be induced, and simultaneously, a phase transition phenomenon on the shale gas is observed. In this study, we employ the method of molecular simulation to address the competitive adsorption behavior and phase equilibrium of the shale gas-CO2 system in shale nanopores. First, the nanopore models and molecular models of typical shale fluids are constructed. Moreover, a Grand Canonical Monte Carlo (GCMC) simulation method is applied to simulate the phase behavior of pure CH4 and the competitive adsorption behavior of CH4/CO2 mixture. Subsequently, considering the complicated composition of shale gas and the limitation of the GCMC method, an NPT-Gibbs ensemble Monte Carlo (NPT-GEMC) method is employed to simulate the phase behavior of the shale gas-CO2 system, which can represent the actual fluid state after a CO2 injection process. Thus, after model validation, the effects of pore size, pore-wall properties, and CO2 concentration are discussed. Simultaneously, the influence mechanism of CO2 on the phase envelope of shale gas is also addressed. Results indicate that the presence of nanopores can further enhance the effect of fluid–solid interaction on the confined behavior of fluids, which is the key factor for the critical deviation of CH4 from the bulk phase. In comparison, CO2 can exhibit a stronger adsorption affinity than CH4, particularly under the conditions of a small nanopore size and low pressure. For the phase envelope, it is observed that an increase on the CO2 concentration can significantly alter the entire phase diagram to the right side, which indicates that the duration time of the single-phase flow stage in formation is increased. Furthermore, both the nanopore size and pore-wall type can have a larger influence on the dew-point pressure than bubble-point pressure. This investigation provides some new insights into the phase transition behavior of shale gas after CO2 injection, which is of great significance for the expansion application of CO2 injection in shale gas resources.