A series of numerical simulations using a multiphase thermo-hydro-chemical (THC) numerical model is performed to analyze groundwater and carbon dioxide (CO2) flow and hydrogeochemical reactive transport due to geologic storage of CO2 in a deep sandstone aquifer and to evaluate impacts of its mineralogical compositions on efficiency and safety of geologic storage of CO2. The results of the numerical simulations show that the mineralogical compositions of the sandstone aquifer have significant impacts on hydro-chemical behavior of injected CO2 and thus its trapping mechanisms and efficiency. The free fluid phase CO2, which is injected into the sandstone aquifer, initially moves radially from the injection well and accumulates in the pore spaces (i.e., hydrodynamic trapping). Over a long period of time, some of injected CO2 is dissolved into groundwater as bicarbonate and carbonate anions (i.e., solubility trapping) and is finally precipitated as carbonate minerals (i.e., mineral trapping) through hydrogeochemical reactions between groundwater and primary minerals in the sandstone aquifer. Most of hydrogeochemical reactions due to CO2 injection occur in the mixed zone where groundwater and free fluid phase CO2 coexist. Mineral trapping of injected CO2 takes places as precipitation of the secondary carbonate minerals such as ankerite, magnesite, siderite, and dawsonite. Ankerite is the most contributive mineral for the mineral trapping of injected CO2. The trapping mechanisms and efficiency of injected CO2 are significantly influenced by the mineralogical compositions of the sandstone aquifer and are most sensitive to the volume fraction of chlorite. These results are because that the Mg2+ and Fe2+, which are essential for precipitation of the secondary carbonate minerals, are mainly supplied by dissolution of chlorite. As a results, the efficiency of mineral trapping and thus the safety of geologic storage of CO2 increase significantly as the volume fraction of chlorite in the sandstone aquifer increases.
