Interaction between CO_2-rich sulfate solutions and carbonate reservoir rocks from atmospheric to supercritical CO_2 conditions: Experiments and modeling

摘要

A test site for CO_2 geological storage is situated in Hontomín (Burgos, northern Spain)with a reservoir rock that is mainly composed of limestone. During and after CO_2 injection, the resulting CO_2-rich acid brine gives rise to the dissolution of carbonate minerals (calcite and dolomite) and gypsum (or anhydrite at depth) may precipitate since the reservoir brine contains sulfate. Experiments using columns filledwith crushed limestone or dolostone were conducted under different P-pCO_2 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). We evaluated the effect of these parameters on the coupled reactions of calcite/dolomite dissolution and gypsum/anhydrite precipitation. The CrunchFlow and PhreeqC (v.3) numerical codes were used to perform reactive transport simulations of the experiments. Within the range of P-pCO_2 and T of this study only gypsum precipitation took place (no anhydrite was detected) and this only occurred when the injected solution was equilibrated with gypsum. Under the P-pCO_2-T conditions, the volume of precipitated gypsum was smaller than the volume of dissolved carbonate minerals, yielding an increase in porosity (Δ? up to ≈4%). A decrease in T favored limestone dissolution regardless of pCO_2 owing to increasing undersaturation with decreasing temperature. However, gypsumprecipitationwas favored at high T and under atmospheric pCO_2 conditions but not at high T and under 10 bar of pCO_2 conditions. The increase in limestone dissolution with pCO_2 was directly attributed to pH, which was more acidic at higher pCO_2. Limestone dissolution induced late gypsum precipitation (long induction time) in contrast to dolostone dissolution, which promoted 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 porositywere larger in the limestone experiments than in the dolostone ones under all pCO_2 conditions (10~(?3.5) and 10 bar). By increasing pCO_2, carbonate dissolution occurred along the column whereas it was localized in the very inlet under atmospheric conditions. This was due to the buffer capacity of the carbonic acid, which maintains pH at around 5 and keeps the solution undersaturated with respect to calcite and dolomite along the column. 1D reactive transport simulations reproduced the experimental data (carbonate dissolution and gypsum precipitation for different P-pCO_2-T conditions). Drawing on reaction rate laws in the literature, we used the reactive surface area to fit the models to the experimental data. The values of the reactive surface area were much smaller than those calculated from the geometric areas.