Chemical Impacts of Potential Leakage into Overlying Groundwater Aquifers from a Geologic CO2 Sequestration Reservoir

Abstract:

Leakage of reservoir fluids is a critical concern for geologic CO₂ sequestration (GCS), due to their chemical impacts on overlying groundwater quality by reducing pH and triggering geochemical reactions that might release toxic trace metals. An effective risk assessment of potential leakage flux, impacted area, and early detection criteria may provide useful information for GCS projects to guide activities for protecting underground sources of drinking water (USDWs). In this study, our goal is to quantify potential chemical risks to USDWs of CO₂ and brine leakage. We present a risk assessment case study of the Farnsworth unit (FWU), an active commercial-scale CO₂-enhanced oil recovery field, overlain by the Ogallala aquifer. Over one million metric tons of net CO₂ has been successfully stored since 2010. It is the study site of the Southwest Regional Partnership on Carbon Sequestration (SWP) Phase III, sponsored by the U.S. Department of Energy (DOE) and the National Energy Technology Laboratory (NETL). To improve the reliability of this assessment, the ranges of uncertain parameters were carefully selected based on abundant shallow groundwater monitoring data. A response surface methodology (RSM) based on a geochemical model was developed to quantify risks associated with groundwater chemistry changes following reservoir fluids intrusion. Salient results suggest that trace metals are likely to maintain low concentrations due to adsorption onto clay minerals; no-impact thresholds based on site monitoring data could be a preferable reference for groundwater quality evaluations; and pH is suggested as an indicator for early detection of a leakage. This study provides a quantitative insight for monitoring strategies, which may enhance the safety of geologic CO₂ sequestration.