Evaluation of the impact of CO2, aqueous fluid, and reservoir rock interactions on the geologic sequestration of CO2, with special emphasis on economic implications

Lowering the costs of front-end processes in the geologic sequestration of CO2 can dramatically lower the overall costs. One approach is to sequester less-pure CO2 waste streams that are less expensive or require less energy to separate from flue gas, a coal gasification process, etc. The objective of this research is to evaluate the impacts of an impure CO2 waste stream on geologic sequestration using both reaction progress and reactive transport simulators. The simulators serve as numerical laboratories within which a series of computational experiments can be designed, carried out, and analyzed to quantify sensitivity of the overall injection/sequestration process to specific compositional, hydrologic, structural, thermodynamic, and kinetic parameters associated with the injection fluid and subsurface environment. Introduction The costs of separation, capture and compression of CO2 from point sources, e.g., coal-fired power plants, have been estimated to account for 75% of the total cost of a geologic sequestration process. Obviously, lowering the costs of the front end processes can dramatically lower the overall costs. One approach to lower cost is to permit a waste stream to be less than pure CO2. The evaluation of the impacts of this impure CO2 waste stream on geologic sequestration is the goal of this project. The 3 primary candidate formation/reservoir types suggested for geologic sequestration (oil and gas reservoirs, coal beds and deep saline formations) all contain associated formation aqueous phases. Oil fields under active EOR (enhanced oil recovery) may have additional (foreign) water used in flooding. Although CO2 injected as part of a geologic sequestration effort may initially behave as an immiscible phase, it can chemically react with water to form carbonic acid and then further react with or form mineral phases once it has dissociated into bicarbonate or carbonate aqueous species. This interaction of CO2 with water is the ultimate basis for the geologic sequestration processes of solubility trapping (as carbonate aqueous species) and mineral trapping (as carbonate minerals). It is essential, therefore, to have a good understanding of the effect of CO2 injection on aqueous and mineral phase chemistries, as well as the physics of flow involving immiscible fluids. Only a handful of directly relevant experiments and supporting geochemical modeling have been done to date (e.g. (Czernichowski-Lauriol et al., 1996; Gunter et al., 1997; Sass et al., 2000)). These preliminary studies keyed on the impact of elevated CO2 fugacity on mineral dissolution and precipitation. They drew some general inferences from their results relating to dissolution of carbonate cements and the potential for both carbonate and silicate mineral formation with the accompanying impact on porosity/permeability. These inferences relate directly to injectivity issues, as well as to the long term performance of geologic sequestration owing to solubility and mineral trapping. However, these early studies did not begin to assess the direct or indirect impact of waste stream contaminants (NOx, SOx, H2S, etc.) on injectivity and long term performance. More recently, modeling efforts have begun to investigate the impact of waste stream contaminants (Gunter et al., 2000). These impacts must be evaluated in order to optimize front end processes (separation, capture and compression) and lower costs.