Risk-based Area of Review Delineation and Leakage Risk Assessment for San Juan Basin CarbonSAFE Site in the United States

The impact to underground sources of drinking waters (USDW) due to leakage through legacy wells is a big concern for geological carbon sequestration. The area of review (AoR) is defined as the region surrounding the storage project location where USDWs may be endangered by the injection activity. The storage reservoir developed under U.S. Department of Energy (DOE) funded San Juan Basin CarbonSAFE project is over-pressurized relative to the overlaying USDW, per U.S. Environmental Protection Agency (EPA) guidance, more sophisticated methods are required to delineate the AoR comparing to under-pressurized and hydrostatic scenarios. Based on Burton-Kelly’s risk-based AoR approach (Burton-Kelly M.E. et al., 2021), this study applies a new version of Semi-Analytical Leakage Solution for Aquifers (SALSA) model (Cihan A., User Guide for SALSA, Lawrence Berkeley National Laboratory, 2022) to carry out risk-based AoR delineation supporting the San Juan Basin CarbonSAFE storage project. The approach combines semi-analytical solutions for estimating storage formation brine leakage through a hypothetical leaky wellbore with the results of physics-based Computer Modeling Group (CMG) numerical reservoir simulations. A relationship of incremental brine leakage versus pressure buildup during the injection was derived. Risk-based AoR was delineated based on the incremental brine leakage map by applying certain leakage threshold criteria. Separate analyses are specifically designed for the heterogeneously distributed USDW condition at San Juan Basin CarbonSAFE site. <br><br>The study shows that the threshold criteria for incremental brine leakage and the wellbore effective permeability strongly influence the delineated AoR size. Depending on the threshold criteria, the risk-based AoR can be much smaller comparing to the pressure front delineation method at San Juan Basin storage site while complying with EPA Class VI Underground Injection Control (UIC) Rule. Under the scenario where the leaky wellbore is open to a high permeability intermediate aquifer (thief zone) between the cap rock and USDW, the risk-based AoR is no larger than the areal extent of the CO2 plume in the storage reservoir because the pressure buildup in the storage reservoir beyond the CO2 plume is insufficient to drive brine up a hypothetical leaky wellbore into the USDW. However, even under the conservative assumption that the leaky wellbore is not open to a thief zone, the incremental leakage is less than 0.05 m3 over 30 years assuming the wellbore effective permeability 0.1 mD. A quantitative leakage risk assessment was also performed to evaluate the potential CO2 and brine leakage through the existing wells within the risk-based AoR. The leakage risk assessment was performed using the NRAP-Open-IAM tool developed by U.S. DOE’s National Risk Assessment Partnership. The calculations indicated no significant threat to regional groundwater due to leakage through the existing wells within the predicted area of review.<br><br>This work provides helpful facilitation on corrective action strategy for the legacy wells inside AoR and leakage risk assessment for the San Juan Basin CarbonSAFE project. As a site-specific application, the risk-based AoR explores novel method and provide support for the long-term monitoring strategy of geological carbon storage. This study can be a supplementary to U.S. EPA guidance on AoR delineation for over-pressured reservoir system.