Abstract:

Fracture-cavity carbonate reservoirs are highly heterogeneous, presenting complex relationships between reservoir pore space and seepage flow. These complexities pose significant challenges for in-situ stress analysis, selection of injection and production parameters, and evaluations of mechanical integrity. In order to further clarify the variation of in-situ stress during the operation of fracture-cavity carbonate underground gas storage（UGS）, ensure the mechanical integrity during the operation of UGS and increase the upper limit pressure, the model was developed to analyze the stress distribution in fracture-cavity carbonate gas storage and to monitor the variations in four-dimensional in-situ stress. This model also assesses the mechanical integrity across different pore spaces. The findings reveal that: ① Stress concentration is more pronounced in fracture-cavity carbonate reservoirs than in homogeneous ones, with the lowest stress levels often occurring at cavity boundaries. ② Pore pressure and stress fluctuations are more severe in fracture-cavity environments, increasing the likelihood of shear or tensile failures at cavity boundaries during UGS operations. ③ During gas production, shear failure tends to occur along the direction of minimum principal stress, whereas tensile failure is more probable along the direction of maximum principal stress during gas injection. ④ Compared to homogeneous reservoirs, fracture-cavity reservoirs are more prone to tension or shear failures during gas injection but are generally safer during gas production, though shear failures around cavities are more likely. These results provide valuable theoretical and methodological insights for in-situ stress analysis and mechanical integrity assessments of fracture-cavity carbonate UGS.

Key words: fracture-cavity underground gas storage（UGS）, fluid-solid coupling, mechanical integrity, cyclic injection and production, four-dimensional in-situ stress