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Abstract: One idea for mitigating the increase in fossil-fuel generated CO2 in the atmosphere is to inject CO2 into subsurface saline sandstone reservoirs. To decide whether to try such sequestration at a globally significant scale will require the ability to predict the fate of injected CO2. Thus, models are needed to predict the rates and extents of subsurface rock–water–gas interactions. Several reactive transport models for CO2 sequestration created in the last decade predicted sequestration in sandstone reservoirs of ∼17 to ∼90kg CO2 m−3. To build confidence in such models, a baseline problem including rock+water chemistry is proposed as the basis for future modeling so that both the models and the parameterizations can be compared systematically. In addition, a reactive diffusion model is used to investigate the fate of injected supercritical CO2 fluid in the proposed baseline reservoir+brine system. In the baseline problem, injected CO2 is redistributed from the supercritical (SC) free phase by dissolution into pore brine and by formation of carbonates in the sandstone. The numerical transport model incorporates a full kinetic description of mineral–water reactions under the assumption that transport is by diffusion only. Sensitivity tests were also run to understand which mineral kinetics reactions are important for CO2 trapping. The diffusion transport model shows that for the first ∼20years (20a) after CO2 diffusion initiates, CO2 is mostly consumed by dissolution into the brine to form CO2,aq (solubility trapping). From 20 to 200a, both solubility and mineral trapping are important as calcite precipitation is driven by dissolution of oligoclase. From 200 to 1000a, mineral trapping is the most important sequestration mechanism, as smectite dissolves and calcite precipitates. Beyond 2000a most trapping is due to formation of aqueous . Ninety-seven percent of the maximum CO2 sequestration, 34.5kg CO2 per m3 of sandstone, is attained by 4000a even though the system does not achieve chemical equilibrium until ∼25,000a. This maximum represents about 20% CO2 dissolved as CO2,aq, 50% dissolved as , and 30% precipitated as calcite. The extent of sequestration as at equilibrium can be calculated from equilibrium thermodynamics and is roughly equivalent to the amount of Na+ in the initial sandstone in a soluble mineral (here, oligoclase). Similarly, the extent of trapping in calcite is determined by the amount of Ca2+ in the initial oligoclase and smectite. Sensitivity analyses show that the rate of CO2 sequestration is sensitive to the mineral–water reaction kinetic constants between approximately 10 and 4000a. The sensitivity of CO2 sequestration to the rate constants decreases in magnitude respectively from oligoclase to albite to smectite. [Copyright &y& Elsevier]

Abstract: It is challenging to predict the degree to which shallow groundwater might be affected by leaks from a CO2 sequestration reservoir, particularly over long time scales and large spatial scales. In this study observations at a CO2 enriched shallow aquifer natural analog were used to develop a predictive model which is then used to simulate leakage scenarios. This natural analog provides the opportunity to make direct field observations of groundwater chemistry in the presence of elevated CO2, to collect aquifer samples and expose them to CO2 under controlled conditions in the laboratory, and to test the ability of multi-phase reactive transport models to reproduce measured geochemical trends at the field-scale. The field observations suggest that brackish water entrained with the upwelling CO2 are a more significant source of trace metals than in situ mobilization of metals due to exposure to CO2. The study focuses on a single trace metal of concern at this site: U. Experimental results indicate that cation exchange/adsorption and dissolution/precipitation of calcite containing trace amounts of U are important reactions controlling U in groundwater at this site, and that the amount of U associated with calcite is fairly well constrained. Simulations incorporating these results into a 3-D multi-phase reactive transport model are able to reproduce the measured ranges and trends between pH, pCO2, Ca, total C, U and Cl− at the field site. Although the true fluxes at the natural analog site are unknown, the cumulative CO2 flux inferred from these simulations are approximately equivalent to 37.8E−3MT, approximately corresponding to a .001% leak rate for injection at a large (750MW) power plant. The leakage scenario simulations suggest that if the leak only persists for a short time the volume of aquifer contaminated by CO2-induced mobilization of U will be relatively small, yet persistent over 100 a. [Copyright &y& Elsevier]