Phase Behavior and Mass Transfer During Oil Reservoir Conversion to Underground Gas Storage: Effects on Storage Capacity and Recovery Efficiency.

Comprehensive research on phase behavior during the staged conversion of producing oil reservoirs into underground natural gas storage remains limited, particularly under cyclic injection-withdrawal conditions where interphase mass transfer continuously evolves. In this study, we performed high-pressure/high-temperature (HP/HT) visualization experiments using a multistage protocol that mimics reservoir-to-UGS conversion, including primary depletion followed by six rolling cycles of gas injection-withdrawal (upper pressure fixed at 29 MPa; lower pressure stepwise decreased to 10 MPa). To quantify the interphase mechanism in a cycle-resolved manner, we introduced the net mass transfer (NMT) and the net mass transfer rate (NMRT), defined as the net interphase transfer magnitude normalized by the pressure change during each injection or withdrawal step; NMRT > 0 indicates net gas-to-oil transfer dominated by condensation/dissolution, whereas NMRT < 0 indicates net oil-to-gas transfer dominated by vaporization/extraction.The results reveal three distinct stages: (1) Depletion Stage, characterized by oil-phase vaporization and initial gas cap formation, with recovery efficiency controlled by depletion pressure; (2) Rolling Storage Construction (Cycles 1-4), which injection shows positive NMRT (0.115 mL/MPa; condensation/dissolution), lightening the oil and improving the phase envelope, whereas withdrawal shows negative NMRT (-0.357 mL/MPa; vaporization/extraction), enriching C7+ and weakening extraction over cycles;(3) Late-Stage Construction (Cycles 5-6), where condensation/dissolution and vaporization/extraction processes approach equilibrium, with net mass transfer rates nearing zero. After depletion and six rolling cycles, the cumulative oil recovery reached 56.02%, and the available gas storage capacity under standard conditions reached 9.98 × 103 cm3, corresponding to 68.6% of the reactor pore volume. These findings clarify how cycle-wise mass transfer governs phase behavior evolution and provide guidance for optimizing operational parameters during reservoir-to-UGS conversion.