The influence of irreducible water for enhancing CH4 recovery in combination of CO2 storage with CO2 injection in gas reservoirs

CO2 flooding to enhance gas recovery (CO2-EGR) with CO2 storage offers the prospects of significant environ-mental and economic benefits by increasing CH4 recovery while simultaneously storing CO2. Most studies ignored the interaction of water with CO2 and the formation rocks and the influence of irreducible water to enhance CH4 recovery combined with CO2 storage were poorly understood by core-flooding experiments. In this research, a numerical model of a five-spot well pattern was developed by benchmarking against the existing literature, and the model was further modified to investigate the effects of irreducible water on CO2-EGR and CO2 storage in gas reservoirs. The injected CO2 preferentially moved horizontally and gradually flowed vertically to flood CH4, making the displacement front an inclined CO2-CH4 mixing zone. The model with irreducible water changed from 0% to 40%, the CO2 sweep efficiency improved and the CH4 recovery increased by 4.37%. This could be attributed to the narrowing of the CO2-CH4 mixing zone and the change in displacement front from an inclined horizontal plane to a near arc plane with better stability. The cumulatively stored CO2 decreased by 21.24%, due to the early CO2 breakthrough and shut-in times. However, because more CO2 was dissolved in water and trapped in residual gas, the contribution of CO2 solubility trapping and CO2 residual trapping increased by 12.01% and 10.34% respectively, while CO2 structural trapping decreased by 22.86%, resulting in a significant increase in long-term storage security. Mineral reactions had a negligible influence on CO2-EGR and CO2 sequestration in the CO2 flooding process. The higher the salinity, the lower the amount of dissolved CO2, and the lower the CH4 recovery and cumulatively stored CO2, with decreased security of CO2 storage with limited impact. The higher the irreducible water saturation (Swi), the more CO2 dissolved, and the CH4 recovery and long-term storage security of CO2 improved with a reduction of cumulatively stored CO2. Changing the gas relative permeability (Krg) has a small effect for CH4 recovery and CO2 storage. This research may provide new insights into analyzing the mechanism of irreducible water on the influence of CO2-EGR and CO2 storage, laying a foundation for the optimization of CO2-EGR schemes and the implementation of field applications in water -bearing gas reservoirs with CO2 injection in the future.