Dissolved CO2 effect on the reactivity of the Hontomín reservoir rocks (limestone and sandstone)

A test site for CO2 geological storage is situated in Hontomin (Burgos, northern Spain) with a reservoir rock that is mainly composed of limestone (80-85%) and sandstone (15-20%). The reservoir rock is a deep saline aquifer that is covered by a very low permeability formation which acts as a cap rock. During and after CO2 injection, since the resident groundwater contains sulfate, the resulting CO2-rich acid solution gives rise to the dissolution of carbonate minerals (calcite and dolomite) and secondary sulfate-rich mineral precipitation (gypsum or anhydrite) may occur. These reactions that may imply changes in the porosity, permeability and pore structure of the repository could vary the CO2 storage capacity and injectivity of the reservoir rock. Therefore, better knowledge about the overall process of gypsum precipitation at the expense of carbonate mineral dissolution in CO2-rich solutions and its implications for the hydrodynamic properties of the reservoir rocks is necessary. A first aim of this thesis is to better understand these coupled reactions by assessing the effect that P, pCO2, T, mineralogy, acidity and solution saturation state exert on these reactions. To this end, experiments using columns filled with crushed limestone or dolostone are conducted under different P-pCO2 conditions (atmospheric:1-10-3.5 bar; subcritical: 10-10 bar; and supercritical: 150-34 bar), T (25, 40 and 60 °C) and input solution compositions (gypsum-undersaturated and gypsum-equilibrated solutions). The CrunchFlow and PhreeqC (v.3) numerical codes are used to perform 1D reactive transport simulations of the experiments to evaluate mineral reaction rates in the system and quantify the porosity variation along the column. Within the range of P-pCO2 and T of this study only gypsum precipitation takes place and this only occurs when the injected solution is equilibrated with gypsum. Under the P-pCO2-T conditions, the volume of precipitated gypsum is smaller than the volume of dissolved carbonate minerals, yielding always an increase in porosity (? up to ? 4%). A decrease in T favors limestone dissolution regardless of pCO2 owing to increasing undersaturation with decreasing temperature. However, gypsum precipitation is favored at high T and under atmospheric pCO2 conditions but not at high T and under 10 bar of pCO2 conditions. The increase in limestone dissolution with pCO2 is directly attributed to pH, which is more acidic at higher pCO2. A decrease in T favors limestone dissolution regardless of pCO2 owing to increasing undersaturation with decreasing temperature. However, gypsum precipitation is favored at high T and under atmospheric pCO2 conditions but not at high T and under 10 bar of pCO2 conditions. The increase in limestone dissolution with pCO2 is directly attributed to pH, which is more acidic at higher pCO2. Limestone dissolution induces late gypsum precipitation (long induction time) in contrast to dolostone dissolution, which promotes rapid gypsum precipitation. Moreover, owing to the slow kinetics of dolomite dissolution with respect to that of calcite, both the volume of dissolved mineral and the increase in porosity are larger in the limestone experiments than in the dolostone ones under all pCO2 conditions (10-3.5 and 10 bar). Limestone dissolution induces late gypsum precipitation (long induction time) in contrast to dolostone dissolution, which promotes rapid gypsum precipitation. Moreover, owing to the slow kinetics of dolomite dissolution with respect to that of calcite, both the volume of dissolved mineral and the increase in porosity are larger in the limestone experiments than in the dolostone ones under all pCO2 conditions (10-3.5 and 10 bar).