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 ∼90 kg 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. ffusion transport model shows that for the first ∼20 years (20 a) after CO2 diffusion initiates, CO2 is mostly consumed by dissolution into the brine to form CO2,aq (solubility trapping). From 20 to 200 a, both solubility and mineral trapping are important as calcite precipitation is driven by dissolution of oligoclase. From 200 to 1000 a, mineral trapping is the most important sequestration mechanism, as smectite dissolves and calcite precipitates. Beyond 2000 a most trapping is due to formation of aqueous HCO 3 - . Ninety-seven percent of the maximum CO2 sequestration, 34.5 kg CO2 per m3 of sandstone, is attained by 4000 a even though the system does not achieve chemical equilibrium until ∼25,000 a. This maximum represents about 20% CO2 dissolved as CO2,aq, 50% dissolved as HCO 3 ,aq - , and 30% precipitated as calcite. The extent of sequestration as HCO 3 - 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 4000 a. The sensitivity of CO2 sequestration to the rate constants decreases in magnitude respectively from oligoclase to albite to smectite.