Numerical investigation of natural fracture activation and leakage risk during CO ₂ sequestration in depleted sandstone reservoirs

Abstract

Geological CO ₂ storage (CO ₂ S) is a critical strategy for mitigating greenhouse gas emissions and addressing global climate change. To overcome the challenges associated with complex coupled processes and the difficulty of accurately assessing leakage risks in depleted sandstone gas reservoirs, this study develops fully coupled hydro‐mechanical numerical models based on the discrete element particle‐flow method under two distinct injection scenarios. The effects of natural fracture (NF) cementation strength, in‐situ stress difference, and injection rate on the evolution of the CO ₂ seepage field and the induced stress field are systematically investigated. Four key characterization parameters—including leakage flux and average fracture aperture—are employed to quantitatively evaluate leakage risks associated with NF activation. When the cementation strength ratio ( CSR ) exceeds approximately 0.5, the probability of leakage path formation tends to decrease to below about 2%, accompanied by a near‐zero CO ₂ leakage flux (CLF). The results show that: (1) Higher in‐situ stress differences and injection rates accelerate fracture propagation and increase the probability of forming connected leakage paths, with the injection rate exerting the more significant influence. In contrast, under high injection rates, the CLF can reach the order of 10 ⁻² –10 ⁻¹  kg/(m ² ·s). (2) Weakly cemented NFs are more susceptible to activation, leading to larger fracture apertures and elevated CO ₂ flux, whereas CSR  > 0.5 markedly suppresses fracture connectivity and leakage flux. (3) Hydraulic‐fracture‐assisted injection enlarges the CO ₂ coverage area and inhibits NF activation near primary fractures, but excessive injection rates may trigger secondary fracture propagation and elevate leakage risks. Optimizing reservoir selection and injection parameters effectively mitigates leakage, providing a mechanistic and quantitative foundation for safe and efficient CO ₂ S.