Numerical Simulation Study on the Mechanisms of Enhanced Gas Recovery and CO2 Storage Performance in Tight Gas Reservoirs Considering Time-Dependent Reservoir Effects

Tight sandstone gas reservoirs are characterized by low single-well productivity, rapid natural decline, and generally low recovery rates, which limit the effectiveness of conventional development strategies. The injection of CO2 into tight gas reservoirs offers broad prospects for both enhanced gas recovery (EGR) and integrated CO2 capture, utilization, and storage (CCUS). However, during the depletion and CO2 injection processes, fracture conductivity, reservoir fluid distribution, and gas–water relative permeability evolve continuously, exhibiting pronounced time-varying behavior. Conventional models have certain limitations in characterizing these time-varying mechanisms, often resulting in significant discrepancies between predicted performance and actual conditions. To address this, we develop a robust and generic model that explicitly incorporates the time-varying mechanisms and stress sensitivity. This model also accounts for the non-equilibrium flash (NEF) behavior of the gas–liquid mixture during CO2 injection into tight gas reservoirs. The successive substitution iteration method is adopted for non-equilibrium flash calculation, and the Newton–Raphson iteration (NRI) method is used for obtaining a fully implicit solution of the reservoir flow equations. The accuracy and reliability of the proposed model are verified through comparisons with commercial simulator results, and a quantitative comparison with traditional models highlights the key discrepancies caused by neglecting time-varying mechanisms. Parametric analyses further clarify the impacts of time-varying behavior and stress sensitivity on CO2-EGR performance and CO2 sequestration efficiency. The results show that the displacement process of CO2 in tight gas reservoirs can be divided into four characteristic stages. The results also indicate that if the time-varying effects and the stress sensitivity of the reservoir permeability are ignored, the calculations tend to overestimate the true gas production capacity of the reservoir and underestimate the CO2 sequestration performance.