Hydrogeochemical numerical simulation of impacts of mineralogical compositions and convective fluid flow on trapping mechanisms and efficiency of carbon dioxide injected into deep saline sandstone aquifers

A series of numerical simulations, which consider density-dependent (convective) groundwater and carbon dioxide (CO2) flow, is performed using a multiphase hydrogeochemical reactive transport numerical model to evaluate impacts of mineralogical compositions on the trapping mechanisms and efficiency of CO2 injected into a deep saline sandstone aquifer (reservoir rock). The results of the numerical simulations show that the mineralogical compositions of the sandstone aquifer have significant impacts on hydrogeochemical behavior of injected CO2 and thus its trapping mechanisms and efficiency. Injected CO2 is accumulated as a free fluid phase beneath the caprock (i.e., hydrodynamic trapping), then dissolved as aqueous phases such as bicarbonate and carbonate anions into groundwater (i.e., solubility trapping), and finally precipitated as carbonate minerals (i.e., mineral trapping). Mineral trapping of injected CO2 takes places as precipitation of a primary carbonate mineral such as calcite and secondary carbonate minerals such as dawsonite, siderite, ankerite, and magnesite. The patterns of hydrogeochemical reactions depend significantly on the initial presence and absence of chlorite in the sandstone aquifer. For mineral trapping of injected CO2, ankerite is the most dominant mineral when chlorite is initially present, whereas dawsonite is the most dominant mineral when chlorite is initially absent in the sandstone aquifer. Mg2+ and Fe2+, which are essential chemical components of such secondary carbonate minerals (i.e., siderite, ankerite, and magnesite) for mineral trapping of injected CO2, are mainly supplied by dissolution of chlorite. As a result, the precipitation amounts of the secondary carbonate minerals and thus the efficiency of mineral trapping of injected CO2 increase significantly as the volume fraction of chlorite increases in the sandstone aquifer. A series of additional numerical simulations, which consider density-independent (non-convective) multiphase fluid flow, is also performed using the same numerical model, and then its results are compared with those of the above-mentioned numerical simulations, which consider density-dependent (convective) multiphase fluid flow, to evaluate impacts of convective fluid flow on the trapping mechanisms and efficiency of injected CO2. The comparison of the results of both numerical simulations shows that convective fluid flow also has significant impacts on hydrogeochemical behavior of injected CO2 and thus its trapping mechanisms and efficiency. Convective fluid flow reduces the free fluid phase of CO2 (i.e., hydrodynamic trapping) and thus enhances the aqueous and solid phases of CO2 (i.e., initially solubility trapping and then mineral trapping).