Experimental and Numerical Investigation on Gas–Liquid Two‐Phase Breakthrough Behavior of Wellbore Micro‐Annuli in CO 2 Geological Storage

ABSTRACT In carbon capture, utilization, and storage (CCUS) projects, interfacial micro‐annuli caused by the sealing failure of wellbore cement sheaths serve as the dominant pathways for CO 2 leakage. Current evaluation methods for cement sheath sealing performance are mostly limited to simplistic single‐phase fluid tests, which can hardly reflect the complex gas–liquid environment and dynamic fluid migration laws in downhole micro‐annuli. To address this issue, this study systematically evaluated the sealing performance of cement sheaths under single‐phase and two‐phase conditions for four cement systems by combining full‐scale physical experiments and established finite element models. The results indicate that quantitative analysis based on the Brooks–Corey model shows that the pore‐size distribution index(λ) increased from 1.26 (System 1) to 4.82 (System 4). This dense structure of System 4 can induce strong additional capillary resistance, significantly enhancing the gas–liquid breakthrough pressure and reducing leakage rates in simulated downhole environments. Furthermore, the finite element seepage model visually reproduced the spatial distribution of preferential fluid channeling and revealed the leakage mechanism of the cement sheath. The calculated leakage rates are highly consistent with the retardation phenomena observed in physical experiments, verifying the reliability of the model. This work provides a solid theoretical framework and technical support for optimizing cement design in CCUS gas injection wells and ensuring the safety of long‐term geological sequestration.