Risk evaluation of CO2 leakage through fracture zone in geological storage reservoir

CO2 storage in saline aquifers is an effective way to reduce the emission of CO2 into the atmosphere. The po-tential leakage problems are the main challenge for CO2 storage projects. However, the quantitative evaluation and analysis of the driving forces for the potential CO2 leakage problems are still lacking. In this paper, a stratified model embedded with a highly-permeable fracture zone was constructed to simulate CO2 leakage in the post-injection period. The acting forces and dimensionless numbers were systematically analyzed, and four stages were defined based on the leakage characteristics. Ultimately, a novel regression model was developed to predict the percentage of secure storage for CO2 at any dimensionless numbers. Results showed four distinct CO2 leakage stages: fast CO2 leakage stage (T1); decreasing CO2 leakage stage (T2); fast water sink stage (T3); and stable countercurrent flow stage (T4). Viscous force, gravity force, and capillary force were the dominant driving forces in turn in the first three stages. A stable countercurrent flow and CO2 leakage were maintained in the last period. In addition, the turning point of the flow pattern was delayed, and Gravity number (Gr) and Bond number (Bo) decreased obviously with the increase of initial pressure gradient. More importantly, most of the leaked CO2 was trapped in riskier forms (i.e. structural and residual trapping), and heavy acidification occurred in the shallow aquifer. The novel regression model showed that the percentage of secure storage could be enhanced by increasing Gr and/or decreasing the Ca. It is suggested that the pressure gradient at the end of injection should be optimized to store more CO2 and decrease the leakage risk simultaneously.