Numerical simulation of CO2 disposal by mineral trapping in deep aquifers

Carbon dioxide disposal into deep aquifers is a potential means whereby atmospheric emissions of greenhouse gases may be reduced. However, our knowledge of the geohydrology, geochemistry, geophysics, and geomechanics of CO2 disposal must be refined if this technology is to be implemented safely, efficiently, and predictably. As a prelude to a fully coupled treatment of physical and chemical effects of CO2 injection, the authors have analyzed the impact of CO2 immobilization through carbonate mineral precipitation. Batch reaction modeling of the geochemical evolution of 3 different aquifer mineral compositions in the presence of CO2 at high pressure were performed. The modeling considered the following important factors affecting CO2 sequestration: (1) the kinetics of chemical interactions between the host rock minerals and the aqueous phase, (2) CO2 solubility dependence on pressure, temperature and salinity of the system, and (3) redox processes that could be important in deep subsurface environments. The geochemical evolution under CO2 injection conditions was evaluated. In addition, changes in porosity were monitored during the simulations. Results indicate that CO2 sequestration by matrix minerals varies considerably with rock type. Under favorable conditions the amount of CO2 that may be sequestered by precipitation of secondary carbonates is comparable with and can be larger than the effect of CO2 dissolution in pore waters. The precipitation of ankerite and siderite is sensitive to the rate of reduction of Fe(III) mineral precursors such as goethite or glauconite. The accumulation of carbonates in the rock matrix leads to a considerable decrease in porosity. This in turn adversely affects permeability and fluid flow in the aquifer. The numerical experiments described here provide useful insight into sequestration mechanisms, and their controlling geochemical conditions and parameters.