Resumen de Geochemical kinetics during CO2 sequestration: the reactivity of the Hontomín caprock and the hydration of MgO
A test site for CO2 geological storage is situated in Hontomín (Burgos, northern Spain) with a reservoir rock that is mainly composed of limestone. The reservoir rock is a deep saline aquifer, which contains a NaCl- and sulfate-rich groundwater in equilibrium with calcite and gypsum, and is covered by a very low permeability formation composed of marls, marly limestone and bitominous shales which acts as a caprock. During and after CO2 injection, since the resident groundwater contains sulfate, the resulting CO2-rich acid solution may gives rise to the dissolution and precipitation may occur. These reactions that may imply changes in the porosity, permeability and pore structure of the rock could vary the CO2 seal capacity of the caprock. Therefore, performing reliable experiments and reactive transport modeling to gain knowledge about the overall process of gypsum precipitation at the expense of calcite dissolution in CO2-rich solutions and its implications for the hydrodynamic properties of the caprock is necessary. A first aim of this thesis is to better understand these coupled reactions by assessing the effect that PTotal, pCO2, T, mineralogy, acidity and solution saturation state exert on these reactions. To this end, flow-through experiments with illite powder samples and flow-through experiments and columns filled with crushed marly limestone are conducted under different PTotal-pCO2 conditions (atmospheric: 1-10-3.5 and subcritical: 10-10 bar), T (25 and 60 °C) and input solution compositions (gypsum-undersaturated and gypsum-equilibrated solutions). A second aim of this PhD study is to evaluate the interaction between the Hontomín marl and CO2-rich sulfate solutions under supercritical CO2 conditions (PTotal = 150 bar, pCO2 = 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 min-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. The third aim is to study caustic magnesia (MgO) as an alternative to Portland cement, not only to be used in the space between the well casing and the rock but also to seal rock fractures (grouting). The overall MgO-carbonation process is considered to happen when MgO hydrates rapidly to form brucite (Mg(OH)2). When brucite dissolves in a Ca-rich and CO2-saturated solution, the solution supersaturates with respect to Ca and/or Mg carbonates (e.g., dolomite (CaMg(CO3)2), nesquehonite (MgCO3·3(H2O)), hydromagnesite (Mg5(CO3)4(OH)2·4(H2O)) and magnesite (MgCO3)). Different T and pCO2 conditions will determine the formation of these carbonates. The molar volumes of the implicated minerals (cm3 mol-1) [(Mg(OH)2 (24.63), CaCO3 (36.93), MgCO3 (28.02), CaMg(CO3)2 (64.37), Mg5(CO3)4(OH)2·4(H2O) (208.08), MgCO3·3(H2O) (75.47)], with large molar volumes for the secondary phases, favor a potential decrease in porosity and hence the sealing of cracks in cement structures, preventing CO2 leakage. MgO carbonation has been studied by means of batch experiments under subcritic (pCO2 of 10 and 50 bar and T of 25, 70 and 90 °C) and supercritic (pCO2 of 74 bar and T of 70 and 90 °C) CO2 conditions. In all cases, CrunchFlow numerical code was used to perform 1D, 2D and OD reactive transport simulations of the experiments to evaluate mineral reaction rates in the system and quantify the porosity variation in the columns, percolation and batch experiments respectively.
