Experimental study on the cyclic injection-production performance of the underground gas storage with trans-critical carbon dioxide as cushion gas

Underground natural gas storage plays a crucial role in seasonal peak-shaving and strategic reserve management. Using CO2 as cushion gas can reduce natural gas occupancy, enhance storage efficiency, and achieve carbon sequestration. However, the influence mechanism of CO2 trans-critical phase transition behavior on cyclic injection-production performance remains unclear. This study developed a multi-component gas miscible displacement experimental system and conducted systematic cyclic injection-production experiments using sandstone cores across three pressure regimes (3–7 MPa, 6–10 MPa, 8–12 MPa) to quantitatively evaluate the effects of CO2 phase state on working gas production, purity, and sequestration performance. Results demonstrate that optimizing CO2 cushion gas performance requires regulating injection-production pressure relative to the critical point to control mixing zone distribution. The high-pressure injection and low-pressure production mode demonstrates optimal overall performance: liquid CO2 injection forms a high-density cushion layer, with vaporization expansion during depressurization enhancing working gas production by 50–75 % compared to baseline, maintaining purity above 98–99 %, and achieving CO2 sequestration of 93 PV. Stepwise depressurization experiments reveal pressure-dependent mechanisms: under low-pressure injection (<6 MPa), the mixing zone remains distant from the outlet with working gas purity exceeding 98 %; under high-pressure injection (>6 MPa), liquid CO2 pushes the mixing zone toward the outlet, reducing purity below 90 %; the intermediate phase transition zone (6–8 MPa) achieves optimal balance among production, purity, and sequestration. This study systematically elucidates the regulatory mechanism of CO2 trans-critical phase transitions on cyclic injection-production efficiency, providing quantitative experimental evidence for cushion gas optimization in underground gas storage.