Transport and detection of carbon dioxide in dilute aquifers

Carbon storage in deep saline reservoirs has the potential to lower the amount of CO2 emitted to the atmosphere and to mitigate global warming. Leakage back to the atmosphere through abandoned wells and along faults would reduce the efficiency of carbon storage, possibly leading to health and ecological hazards at the ground surface, and possibly impacting water quality of near-surface dilute aquifers. We use static equilibrium and reactive transport simulations to test the hypothesis that perturbations in water chemistry associated with a CO2 gas leak into dilute groundwater are important measures for the potential release of CO2 to the atmosphere. Simulation parameters are constrained by groundwater chemistry, flow, and lithology from the High Plains aquifer. The High Plains aquifer is used to represent a typical sedimentary aquifer overlying a deep CO2 storage reservoir. Specifically, we address the relationships between CO2 flux, groundwater flow, and detection time and distance. The CO2 flux ranges from 10(3) to 2 x 10(6) t/yr to assess chemical perturbations resulting from relatively small leaks that may compromise long-term storage, water quality, and surface ecology, and larger leaks characteristic of short-term well failure. Reactive transport simulations show the CO2 leakage into a dilute groundwater creates a slightly acid plume that can be detected at some distance from the leak source due to groundwater flow and CO2 buoyancy. pH breakthrough curves demonstrate that CO2 leaks can be easily detected for CO2 flux 10(4) t/yr within a 15-month time period. Sustained pumping in a developed aquifer mixes the CO2-affected water with the ambient water and enhances pH signal for small leaks (10(3) t/yr) and reduces pH signal for larger leaks (10(4) t/yr). Detection of CO2 leaks in aquifers by changes in pH and carbonate chemistry is readily available and well understood. Leaks produce a measurable change in pH that is still within range of natural waters, even for high flux rates (2 x 10(6) t/yr). Reactive transport modeling is a critical component to the design and effective performance of measurement, monitoring, and verification plans for carbon storage. (C) 2008 Elsevier Ltd. All rights reserved.