A Study on the Utilization of CO2/N2 Injection for Enhanced Recovery and Carbon Sequestration in Shale Gas Reservoirs

During the mid to late stages of shale gas reservoir development, gas production tends to decline, with a substantial amount of shale gas adsorbed within the pores. In competitive adsorption situations, CO2 has an advantage over CH4, promoting CH4 desorption and enabling CO2 adsorption. However, in practical engineering, balancing the cost and source-sink dynamics of pure CO2 presents significant challenges. As a result, mixing N2 is seen as a strategy to reduce costs. Low-field online nuclear magnetic resonance detection experiments were conducted alongside multi-component gas huff-and-puff trials in the Changning area of the Sichuan Basin to investigate the effects of multi-component gas huff-and-puff on enhanced shale gas recovery. The results indicate that: (1) For adsorbed CH4 within shale formations, its adsorption capacity decreases with increasing temperature while it increases under higher pressure conditions; notably, this state significantly influences the gas recovery factor—especially at lower pressures; (2) As temperature rises, molecular thermal motion intensifies, leading to increased utilization compared to pre- and post-huffing phases; conversely, higher pressures enhance shale's adsorption capacity but diminish effectiveness before and after treatment cycles; (3) Three key factors influencing huff-and-puff operations—cycle duration, soaking time, and injection volume—exhibit positive correlations with the gas recovery factors; however, excessive increases in these parameters can lead to diminishing returns on efficiency improvements; (4) Concurrently enhancing CO2 recovery also yields effective storage results: under 0.2 PV injected for soaking three days period achieves 49.79%, while nearly 0.08 PV of storage capacity. In conclusion, CO2-based multi-component gases demonstrate efficacy in improving shale gas recovery through huff-and-puff techniques while enabling effective CO2 storage; optimizing operational parameters and maintaining appropriate pressure control is critical for successful implementation.