Please cite this article in press as: Dávila, G., et al., Interaction between a fractured marl caprock and CO 2 -rich sulfate solution under supercritical CO 2 conditions. Int. J. Greenhouse Gas Control (2015), http://dx.doi.org/10.1016/j.ijggc.2015.11.005 Geological CO 2 sequestration at pilot-plant scale will be developed at Hontomín (Spain). CO 2 will be injected into a limestone reservoir that contains a NaCl-and sulfate-rich groundwater in equilibrium with calcite and gypsum. The caprock site is composed of marl. The present study seeks to evaluate the interaction between the Hontomín marl and CO 2 -rich sulfate solutions under supercritical CO 2 conditions (P Total = 150 bar, pCO 2 = 61 bar and T = 60 • C). Flow-through percolation experiments were performed using artificially fractured cores to elucidate (i) the role of the composition of the injected solutions (S-free and S-rich solutions) and (ii) the effect of the flow rate (0.2, 1 and 60 mL h −1 ) on fracture permeability. Major dissolution of calcite (S-free and S-rich solutions) and precipitation of gypsum (S-rich solution) together with minor dissolution of the silicate minerals contributed to the formation of an altered skeleton-like zone (mainly made up of unreacted clays) along the fracture walls. Dissolution patterns changed from face dissolution to wormhole formation and uniform dissolution with increasing Peclet numbers. ARTICLE IN PRESSIn S-free experiments, fracture permeability did not significantly change regardless of the flow rate despite the fact that a large amount of calcite dissolved. In S-rich solution experiments, fracture permeability decreased under slow flow rates (0.2 and 1 mL h −1 ) because of gypsum precipitation that sealed the fracture. At the highest flow rate (60 mL h −1 ), fracture permeability increased because calcite dissolution predominated over gypsum precipitation.

Injection of CO 2 at depth will cause the acidification of groundwater. As a preliminary study for the potential use of MgO as an alternative to Portland cement in injection wells, MgO carbonation has been studied by means of stirred batch experiments under subcritic (pCO 2 of 10 and 50 bar and T of 25, 70 and 90 °C) and supercritic (pCO 2 of 74 bar and T of 70 and 90 °C) CO 2 conditions. Magnesium oxide reacts with CO 2-containing and Ca-rich water nearly equilibrated with respect to calcite. MgO quickly hydrates to brucite (Mg(OH) 2) which dissolves causing the precipitation of magnesium carbonate phases. Precipitation of these secondary phases (magnesite and/or metastable phases such as nesquehonite (MgCO 3 •3H 2 O) or hydromagnesite (Mg 5 (CO 3) 4 (OH) 2 •4(H 2 O)) depends on pCO 2 , temperature and solid/water content. In a constant solid/water ratio, the precipitation of the non-hydrated Mg carbonate is favored by increasing temperature and pCO 2. MgO efficiency; Dávila et al. (2016) 2 The experimental variation of Mg and Ca concentrations and pH over time at the different temperatures and pCO 2 has been simulated using the CrunchFlow reactive transport code. Simulations reproduce the experimental evolution of the aqueous concentrations and indicate a decrease in porosity when increasing temperature and pCO 2. This decrease in porosity would be beneficial for the sealing properties of the cement. These results have been used in the simulation of an application case with a deep borehole surrounded by MgO cement at 90 °C.