Numerical Simulation Study of Salt Cavern CO2 Storage in Power-to-Gas System

China’s renewable energy sector is experiencing rapid growth, yet its inherent intermittency is creating significant challenges for balancing power supply and demand. Power-to-gas (PtG) technology, which converts surplus electricity into combustible gas, offers a promising solution. Salt caverns, due to their favorable geological properties, are among the best choices for large-scale underground energy storage in PtG systems. CO2 leakage along the interlayer and salt rock–interlayer interfaces is a key constraint on the CO2 tightness of subsurface salt caverns. This paper focuses on the critical issue of tightness within salt cavern CO2 storage, particularly in the Jintan region. A coupled hydro-mechanics mathematical model is developed to investigate CO2 transportation and leakage in bedded salt caverns, with key variables such as permeability range, pore pressure evolution, and permeability changes being analyzed. The results reveal that permeability plays a decisive role in determining the CO2 transportation rate and leakage extent within the salt rock layer. Notably, the CO2 transportation rate and influence range in the mudstone interlayer are significantly larger than those in the salt rock over the same period. However, with prolonged storage time, the CO2 transportation rate and pressure increase in both salt rock and mudstone interlayer exhibit a decreasing trend, eventually stabilizing as the CO2 pressure front reaches the boundary of the simulation domain. Furthermore, elevated operating pressure markedly expands the permeability range and results in higher cumulative leakage. For a specific salt cavern, the CO2 leakage range can reach up to 142 m, and the leakage volume can reach 522.5 tonnes with the increase in operating pressure during 35 years of operation. Therefore, the setting of operational pressure should fully consider the influence of permeability and mechanical strength of the salt rock and mudstone interlayer. These findings provide valuable insights into optimizing the sealing performance of salt cavern CO2 storage systems under varying conditions.