Evaluation of CO2 storage and enhanced gas recovery potential in gas shale using kerogen nanopore systems with mesopores and micropores

With the continuous development of shale gas, CO2 storage in shale gas reservoirs has gained significant attention due to the massive available space in these reservoirs. These shale formations exhibit a complex pore structure with micropores (<2 nm) and mesopores (2-50 nm), which profoundly influence gas sorption behaviors. This work used grand canonical Monte Carlo to study the sorption behavior of CH4, CO2, and their binary mixtures (from 1:9 to 9:1) in kerogen nanopore systems with mesopores and micropores at 327 K and pressures ranging from 1 to 50 MPa. The kerogen nanopore system with mesopores and micropores faithfully describes sorption isotherms. The adsorption profile highlights the adsorption characteristics and relative contribution of mesopores and micropores. The molar ratio of CO2 to CH4 in the replacement process is greater than 1, indicating the possibility of achieving negative emissions by storing CO2 in shale gas reservoirs. The extraction ratio reveals that CH4 in micropores is more easily extracted. As for CO2 storage in shale gas formation, the sorption phase dominates at low pressure, while the free gas phase becomes more significant at higher pressure. At a reservoir pressure of 35 MPa, approximately 125 tons of CO2 can be stored in 1000 m3 of shale gas formation. Using CO2 huff-n-puff technology, the recovery factor more than doubles compared to pressure drawdown. While pressure drawdown primarily extracts CH4 from the free gas phase, CO2 injection extracts CH4 from the sorption phase, particularly micropores. This research provides valuable insights into the competitive sorption behavior of CH4 and CO2 in shale nanopore systems for further development of CO2 storage and enhanced gas recovery technologies.