Predictive modeling of CO₂ sequestration in deep saline sandstone reservoirs: Impacts of geochemical kinetics

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 CO_2,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 HCO3-. 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 CO_2,aq, 50% dissolved as HCO3,aq-, and 30% precipitated as calcite. The extent of sequestration as HCO3- 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.