Mechanical Stability Analysis of Fracture-Cavity Carbonate Gas Storage

The Chinese Laoweng Chang gas storage is developed from a typical depleted fractured-cavity carbonate gas reservoir. Due to the fast injection and production rates in gas storage period, the reservoir rocks, the caprock, the faults and the base may all suffer the ricks of failure. In this paper, the importance of cavities on stress distribution and failure in reservoir rock is emphasized. First, the influence of cavities on stress and failure risks is analyzed with numerical models. The analytical solutions for stress distribution around cavity is derived to be used for reservoir scale problems. The practical geological and geomechanical models of Laoweng Chang gas storage are used to analyze the mechanical risks of different regions. The stress evolution of each grid during production and injection processes is calculated with finite element method by one-way fluid-solid coupling. The calculated stresses of each grid in reservoir rock are considered as boundary conditions to obtain the stress distribution around cavities. Mohr-Coulomb criterion and tensile failure criterion are then used to judge the shearing and tensile failures. The results indicate that the low permeable caprock and base are less susceptible to failure. The existence of cavities increases the risks of both tensile and shearing failure in the reservoir rock. However, the failure of fault is the still dominate the mechanical stability of the gas storage. This work provides an effective method for considering the influence of cavities on reservoir rock failure in practical analysis. 1. INTRODUCTION The underground gas storage, as a type of "artificial gas reservoir," serves as an effective means to ensure gas supply security and balance natural gas supply and demand. These storage facilities operate through multi-cycle high-speed injection and production processes, featuring injection and production rates that surpass those of natural gas reservoirs by 20-30 times, thereby exacerbating the risks associated with gas leakage and water intrusion. The occurrence of a leakage accident could result in irreversible economic losses. Therefore, studying the changes in in-situ stress during the operation of gas storage and conducting mechanical integrity assessments are particularly important (Jia et al., 2020; Verdon et al., 2013; Li et al., 2023). The Laoweng Chang gas storage in southwest China is a typical fracture-cavity carbonate gas storage. The stress distributions are significantly influenced by cavities and other medium spaces (natural fractures and rock matrix), posing greater challenges in characterization compared to conventional porous gas storage (Zhang et al., 2019).