关键词: 储气库, 多物理场耦合, CO₂注入过程, 垫底气, 孔-渗演化机制

Abstract: As an integral component of China’s energy regulation and strategic resource allocation, underground gas storage (UGS) engineering offers significant potential for application. Utilizing CO₂ as a cushion gas can simultaneously enhance economic viability and contribute to the realization of "dual carbon" goals. However, the CO₂ injection process involves intricate mechanisms characterized by the synergistic effects of seepage, heat transfer, and rock matrix strain. Currently, the majority of domestic and international research prioritizes seepage field analysis, with insufficient attention paid to the coupled impacts of molecular diffusion, thermal conduction, and matrix deformation on reservoir porosity-permeability structures and fluid flow behavior. To further elucidate the coupling mechanisms between the seepage, thermodynamic, and stress fields, as well as the porosity-permeability evolution characteristics, this study establishes a diffusion-inclusive thermo-hydro-mechanical (THM) coupled mathematical model integrated with molecular simulation results and performs finite element numerical simulations. The research results indicate as follows. (1) During the injection of supercritical CO₂, the overall displacement process is synergistically governed by convection, gravitational segregation, and molecular diffusion. Macroscopic transport is dominated by pressure gradient-driven convection, whereas molecular diffusion significantly broadens the displacement front and facilitates the formation of a finite miscible transition zone; concurrently, gravitational segregation causes CO₂ to migrate preferentially along the bottom of reservoir. (2) Upon the injection of low-temperature CO₂, heat exchange occurs with the interstitial natural gas and the surrounding host rock via thermal conduction and convective heat transfer mechanisms, resulting in a maximum temperature decrease of 9 K in the near-wellbore region. Increasing the injection temperature can effectively optimize the thermal distribution in reservoirs. (3) The synergistic effects of pore pressure and thermal stress lead to a reduction in the effective stress of the rock, resulting in maximum increments of 1.6 % and 4.8 % in porosity and permeability, respectively. (4) The initial reservoir porosity exerts the most significant influence on porosity-permeability evolution, followed by initial permeability, while injection temperature has the least impact. (5) Rational injection-production configurations facilitate CO₂ sequestration and optimize the cushion gas efficiency of the storage facility; specifically, under a multi-cycle periodic strategy with an injection rate of 40×10⁴m³/d and a production rate of 17.7×10⁴m³/d, the CO₂ cushion gas efficiency reaches 55.48 % . These findings provide theoretical support and technical references for the rational construction and safe, efficient operation of un derground gas storage facilities.

Key words: underground gas storage, multiphysics coupling, CO₂ injection process, cushion gas, porosity-permeability evolution mechanism