Energy Reports (Nov 2022)
Influence of stress redistribution and fracture orientation on fracture permeability under consideration of surrounding rock in underground gas storage
Abstract
UGS (underground gas storage) is of great significance for ensuring emergent supply and CCUS (Carbon Capture, Utilization, and Storage). However, very different from gas reservoir development, some stress changes in reservoir, caprock, and bottom support rock may affect its operation. Sichuan Basin is rich in natural-gas resources and geological targets for the UGS rebuilt from fractured carbonate reservoirs. Moreover, gas storage is featured by rapid injection and production, and the change of in-situ stress affects intensively the permeability of reservoir fracture. However, most existing models on the fracture permeability assume that the surrounding-rock stress is constant. So, a new model has been established for the fracture permeability based on the Eshelby theory, considering the effect of both direct caprock and bottom support rock (also called surrounding rock) and the stress redistribution mechanism of the entire geological body during UGS injection and production. Results show that for the fracture, the permeability is related to not only its compressibility and orientation, but also reservoir shape and elastic property of the entire geological body including reservoir and surrounding rock. In most disk-shaped or elliptical reservoirs, vertical fracture is less susceptible to pore pressure than horizontal one. The surrounding-rock elastic property is also an important factor in the fracture-permeability model. Hard surrounding rock can reduce the stress sensitivity in fracture. When the elastic modulus ratio (elastic modulus of reservoir vs. that of surrounding rock) is 0.1, the fracture permeability decreases by only 15%. This model is also applicable to multiple fractures. Using the superposition principle, it is found that the main direction of permeability and the degree of permeability anisotropy may change when there are multiple fractures in the reservoir in spite of the same fracture compressibility.
