Interaction between CO2-rich water and hydrated Portland cement and ultramafic rocks

Abstract

CO2 geological storage in deep rock formations is a possible strategy to mitigate the atmospheric CO2 concentration. In storage sites, CO2 is injected through wells. The space between the metal casing of the wellbore and the host rock is filled with Portland cement in which micro-fractures on flow paths between cement and rock may originate. It has been amply shown that the interaction between CO2-rich water, hydrated Portland cement and sedimentary rocks leads to alteration of cement (dissolution of cementitious phases) and rock (dissolution of silicates) and precipitation of secondary minerals. These processes may enhance or seal the existing microfractures through which CO2 may leak. However, the integrity of the cement and (ultra)mafic rocks under acidic conditions has not been investigated. In this study, the geochemical processes associated with the interaction between CO2-rich water and mortar (composed by hydrated Portland cement and quartz aggregates) and two mafic rocks (peridotite and obsidian) were studied to evaluate the stability of wellbores in mafic rocks, i.e., the role of carbonation and dissolution mechanisms during long-term exposures to CO2-rich brines. Three column experiments were performed at 10 bar pressure, room temperature (20 ± 2°C) and two ionic strengths (4.0·10-4 M and 2.8·10-2 M). The chemical composition of the effluents was examined over time and reproduced by 2D reactive transport simulations. Experimental and model results show that dissolution of the cement phases contributed to cement alteration and buffered the solution pH, rendering low rock alteration. Water residence time affected the precipitation of secondary CO2-bearing minerals.