Key factors for determining groundwater impacts due to leakage from geologic carbon sequestration reservoirs

Abstract In this paper we describe potential impacts to groundwater quality due to CO2 and brine leakage, discuss an approach to calculate thresholds under which “no impact” to groundwater occurs, describe the time scale for impact on groundwater, and discuss the probability of detecting a groundwater plume should leakage occur. To facilitate this, multi-phase flow and reactive transport simulations and reduced-order models were developed for two classes of aquifers, considering uncertainty in leakage source terms and aquifer hydrogeology. We targeted an unconfined fractured carbonate aquifer based on the Edwards Aquifer in Texas and a confined alluvium aquifer based on the High Plains Aquifer in Kansas, which share characteristics typical of many drinking water aquifers in the United States. The hypothetical leakage scenarios centered on the notion that wellbores are the most likely conduits for brine and CO2 leaks. Leakage uncertainty was based on hypothetical injection of CO2 for 50 years at a rate of 5 million tons per year into a depleted oil/gas reservoir with high permeability and, one or more wells provided leakage pathways from the storage reservoir to the overlying aquifer. This scenario corresponds to a storage site with historical oil/gas production and some poorly completed legacy wells that went undetected through site evaluation, operations, and post-closure. For the aquifer systems and leakage scenarios studied here, CO2 and brine leakage are likely to drive pH below and increase total dissolved solids (TDS) above the “no-impact thresholds”; and the subsequent plumes, although small, are likely to persist for long periods of time in the absence of remediation. In these scenarios, however, risk to human health may not be significant for two reasons. First, our simulated plume volumes are much smaller than the average inter-well spacing (1–2.6 wells/km2) for these representative aquifers, so the impacted groundwater would be unlikely to be pumped for drinking water. Second, even within the impacted plume volumes little water exceeds the primary maximum contamination levels. These observations point to: • The potential utility of uncertainty quantification methods to evaluate the risk of leakage and inform monitoring and corrective action plans of a potential site for long-term CO2 storage by capturing storage reservoir, leakage pathway, and aquifer heterogeneity. • The importance of establishing baseline groundwater chemistry that captures the pre-injection variability of underground sources of drinking water (USDW) above the reservoir because the EPA has adopted a “no net degradation” policy toward the protection of groundwater resources. • The need to test and develop spatially diverse monitoring techniques capable of detecting leakage early to employ effective mitigation strategies, and more importantly to add confidence to assessments used to evaluate the length of the post-injection site care. In our study, the probability of detecting plumes using existing wells to sample the groundwater chemistry was very low, because the plumes were relatively small in both aquifers. • The need to develop methodologies that prevent and/or directly detect leakage prior to reaching USDWs, because our simulations predict that even small amounts of CO2 and brine, when left unmitigated, can change USDW pH and TDS concentrations for long periods of time.