Numerical studies of CO₂ and brine leakage into a shallow aquifer through an open wellbore

Industrial-scale geological storage of CO₂ in saline aquifers may cause CO₂ and brine leakage from abandoned wells into shallow fresh aquifers. This leakage problem involves the flow dynamics in both the wellbore and the storage reservoir. T2Well/ECO2N, a coupled wellbore-reservoir flow simulator, was used to analyze CO₂ and brine leakage under different conditions with a hypothetical simulation model in water-CO₂-brine systems. Parametric studies on CO₂ and brine leakage, including the salinity, excess pore pressure (EPP) and initially dissolved CO₂ mass fraction, are conducted to understand the mechanism of CO₂ migration. The results show that brine leakage rates increase proportionally with EPP and inversely with the salinity when EPP varies from 0.5 to 1.5 MPa; however, there is no CO₂ leakage into the shallow freshwater aquifer if EPP is less than 0.5 MPa. The dissolved CO₂ mass fraction shows an important influence on the CO₂ plume, as part of the dissolved CO₂ becomes a free phase. Scenario simulation shows that the gas lifting effect will significantly increase the brine leakage rate into the shallow freshwater aquifer under the scenario of 3.89% dissolved CO₂ mass fraction. The equivalent porous media (EPM) approach used to model the wellbore flow has been evaluated and results show that the EPM approach could either under- or over-estimate brine leakage rates under most scenarios. The discrepancies become more significant if a free CO₂ phase evolves. Therefore, a model that can correctly describe the complex flow dynamics in the wellbore is necessary for investigating the leakage problems.