Efficiency of magnesium hydroxide as engineering seal in the geological sequestration of CO2

• MgO carbonation is relevant for MgO-based cements as an alternative to Portland cement in a context of CO2 injection.
• Autoclaved experiments are used to study the MgO carbonation process from subcritical to supercritical pCO2 conditions.
• Reactive transport simulations reproduce the experimental variation of the MgO carbonation.
• Simulations of an application case show that MgO carbonation increases the sealing capacity of the MgO-based cement.

Injection of CO2 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 (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.Magnesium oxide reacts with CO2-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 (MgCO3·3H2O) or hydromagnesite (Mg5(CO3)4(OH)2·4(H2O)) depends on pCO2, 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 pCO2.The experimental variation of Mg and Ca concentrations and pH over time at the different temperatures and pCO2 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 pCO2. 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.