Carbonate precipitation associated with CO ₂ sequestration: An experimental and modeling approach to upscaling from the pore to continuum scale

Subsurface sequestration of CO ₂ in carbonate minerals is possible if an adequate supply of cations is available and if the solution pH increases due to alkalinity increases sufficiently to supersaturate the solution. Since the availability of cations and alkalinity in the aqueous phase is limited, the rate of carbonate mineral sequestration depends strongly on the rate(s) of dissolution of primary cation-bearing phases under CO ₂ injection conditions in the subsurface. Precipitation in turn can affect the rate of carbonate mineral sequestration by modifying permeability and/or reactive surface area (both primary and secondary). Here we present an integrated approach that combines experimental reactive flow columns in which supersaturated, carbonate-rich solutions are injected into calcite packs. Bulk rates of precipitation based on the change in chemistry over the length of the column are compared with spatially resolved determinations of carbonate precipitation using X-ray synchrotron imaging at the micron scale. These data are supplemented by well-stirred reactor experiments to evaluate the rate of precipitation in the absence of transport or "porous medium" effects. Results indicate good agreement between rates determined with fluid chemistry and with microtomography. Using the rates of precipitation determined in the well-stirred flowthrough reactors, it is possible to match the spatially-resolved microtomographic and aqueous data with a continuum model if the generation of new reactive surface area is accounted for. The experimentally-determined value of 0.90 m ²/g for the specific surface area of the neoformed calcite results in reasonable agreement with the continuum model. In addition, pore scale modeling based on coupling of carbonate reactions and Stokes flow calculated with Direct Numerical Simulation, and using pore geometries determined by micro-XCT, are compared with continuum reactive transport modeling of the system that matches bulk effluent chemistry and the spatially-resolved reaction rates. The combined experimental and modeling approach yields an analysis of the upscaling of reaction rates from the pore to continuum scale, while providing a verification of both pore and continuum scale reactive transport modeling of carbonate precipitation.