Evaluating the Risk to Groundwater by Unintended Migration of CO2 and Brine from Geologic Carbon Storage Projects

Abstract:

Under the Environmental Protection Agency’s Underground Injection Control Class VI Rule, the area surrounding CO₂ injection wells must be monitored to ensure Underground Sources of Drinking Water are not endangered due to CO₂ or brine movement through faults and legacy wells. The U.S. Department of Energy’s National Risk Assessment Program (NRAP) has developed an open source integrated assessment model (NRAP-Open-IAM) that can be used to evaluate the risk to groundwater from unintended migration of CO₂ and brine from the storage reservoir. By incorporating uncertainty in reservoir, wellbore and aquifer characteristics into a model of the geologic storage system, the potential impacts to groundwater from leakage through an open wellbore can be used to determine a risk-based Area of Review (AoR). Within the AoR, the probability of containment of CO₂ within the reservoir can be estimated based on probability distributions for the effective permeability of potentially leaking wells that penetrate the reservoir and verified by geophysical monitoring. This estimation can be used to support claims of the integrity of a geologic storage complex in order to take credit for long-term storage. The results of these simulations can also be used to design an optimized monitoring network, resulting in cost savings for a carbon storage project while protecting groundwater by targeting the highest leakage risk locations. Further, the results of these simulations can guide the determination of a Post-Injection Site Care (PISC) period. This approach has been applied at a proposed GCS site to demonstrate that the risk of groundwater impacts decreases sharply after the end of injection. The characterization and modeling data from the first GCS site to obtain a Class VI permit, FutureGen 2.0, has been used to demonstrate each of these applications and has been made publicly available for further research.