Geochemical detection of carbon dioxide in dilute aquifers

BACKGROUND: Carbon storage in deep saline reservoirs has the potential to lower the amount of CO(2 )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 CO(2 )gas leak into dilute groundwater are important measures for the potential release of CO(2 )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 CO(2 )storage reservoir. Specifically, we address the relationships between CO(2 )flux, groundwater flow, detection time and distance. The CO(2 )flux ranges from 10(3 )to 2 × 10(6 )t/yr (0.63 to 1250 t/m(2)/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. RESULTS: For the scenarios we studied, our simulations show pH and carbonate chemistry are good indicators for leakage of stored CO(2 )into an overlying aquifer because elevated CO(2 )yields a more acid pH than the ambient groundwater. CO(2 )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 CO(2 )buoyancy. pH breakthrough curves demonstrate that CO(2 )leaks can be easily detected for CO(2 )flux ≥ 10(4 )t/yr within a 15-month time period at a monitoring well screened within a permeable layer 500 m downstream from the vertical gas trace. At lower flux rates, the CO(2 )dissolves in the aqueous phase in the lower most permeable unit and does not reach the monitoring well. Sustained pumping in a developed aquifer mixes the CO(2)-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). CONCLUSION: The ability to detect CO(2 )leakage from a storage reservoir to overlying dilute groundwater is dependent on CO(2 )solubility, leak flux, CO(2 )buoyancy, and groundwater flow. Our simulations show that the most likely places to detect CO(2 )are at the base of the confining layer near the water table where CO(2 )gas accumulates and is transported laterally in all directions, and downstream of the vertical gas trace where groundwater flow is great enough to transport dissolved CO(2 )laterally. Our simulations show that CO(2 )may not rise high enough in the aquifer to be detected because aqueous solubility and lateral groundwater transport within the lower aquifer unit exceeds gas pressure build-up and buoyancy needed to drive the CO(2 )gas upwards.