Caprock interaction with CO2: A laboratory study of reactivity of shale with supercritical CO2 and brine

Abstract

Abstract Crushed rock from two caprock samples, a carbonate-rich shale and a clay-rich shale, were reacted with a mixture of brine and supercritical CO 2 (CO 2 –brine) in a laboratory batch reactor, at different temperature and pressure conditions. The samples were cored from a proposed underground CO 2 storage site near the town of Longyearbyen in Svalbard. The reacting fluid was a mixture of 1 M NaCl solution and CO 2 (110 bar) and the water/rock ratio was 20:1. Carbon dioxide was injected into the reactors after the solution had been bubbled with N 2 , in order to mimic O 2 -depleted natural storage conditions. A control reaction was also run on the clay-rich shale sample, where the crushed rock was reacted with brine (CO 2 -free brine) at the same experimental conditions. A total of 8 batch reaction experiments were run at temperatures ranging from 80 to 250 °C and total pressures of 110 bar (∼40 bar for the control experiment). The experiments lasted 1–5 weeks. Fluid analysis showed that the aqueous concentration of major elements (i.e. Ca, Mg, Fe, K, Al) and SiO 2 increased in all experiments. Release rates of Fe and SiO 2 were more pronounced in solutions reacted with CO 2 –brine as compared to those reacted with CO 2 -free brine. For samples reacted with the CO 2 –brine, lower temperature reactions (80 °C) released much more Fe and SiO 2 than higher temperature reactions (150–250 °C). Analysis by SEM and XRD of reacted solids also revealed changes in mineralogical compositions. The carbonate-rich shale was more reactive at 250 °C, as revealed by the dissolution of plagioclase and clay minerals (illite and chlorite), dissolution and re-precipitation of carbonates, and the formation of smectite. Carbon dioxide was also permanently sequestered as calcite in the same sample. The clay-rich shale reacted with CO 2 –brine did not show major mineralogical alteration. However, a significant amount of analcime was formed in the clay-rich shale reacted with CO 2 -free brine; while no trace of analcime was observed in either of the samples reacted with CO 2 –brine.