Basalt Reactivity Variability with Reservoir Depth in Supercritical CO2 and Aqueous Phases

Abstract

Abstract Long term storage of CO2 in geologic formations is currently considered the most attractive option to reduce greenhouse gas emissions while continuing to utilize fossil fuels for energy production. Injected CO2 is expected to reside as a buoyant water-saturated supercritical fluid in contact with reservoir rock, the caprock system, and related formation waters. As reported by McGrail et al., experiments with basalts demonstrated surprisingly rapid carbonate mineral formation occurring with samples suspended in the supercritical CO2 (sc CO2) phase. Those experiments were limited to a few temperatures and CO2 pressures representing relatively shallow ( ≈ 1 k m ) reservoir depths. Because continental flood basalts can extend to depths of 5 km or more, in this paper we extend the earlier results across a pressure-temperature range representative of these greater depths. Different basalt samples, including well cuttings from the borehole used in a pilot-scale basalt sequestration project (Eastern Washington, U.S.) and core samples from the Central Atlantic Magmatic Province, were exposed to aqueous solutions in equilibrium with sc CO2 and water-rich sc CO2 at six different pressures and temperatures for select periods of time (30 to 180 days). Conditions corresponding to a shallow injection of CO2 (7.4 MPa, 34 °C) indicate limited reactivity with basalt; surface carbonate precipitates were not easily identified on post-reacted basalt grains. Basalts exposed under identical times appeared increasingly more reacted with simulated depths. Tests conducted at higher pressures (≥12 MPa) and temperatures (≥55 °C), reveal a wide variety of surface precipitates forming in both fluid phases. Under shallow conditions tiny clusters of aragonite needles began forming in the wet sc CO2 fluid, whereas in the CO2 saturated water, cation substituted calcite developed thin radiating coatings. Although these types of coatings were sparse, conditions corresponding to deeper depths showed increasing carbonate precipitation. Basalts exposed to aqueous dissolved CO2 (25.5 MPa, 116 °C) for 30 days were coated in tiny nodules of precipitate (∼100 μm in diameter) that were identified by micro x-ray diffraction as ankerite, [Ca(Fe,Mg)(CO3)2], a variety of dolomite commonly associated with hydrothermal and metamorphic environments. Surface characterization by SEM revealed well-developed round nodules composed of discrete individual platelets. In contrast, reaction products forming on the basalt in the corresponding wet sc CO2 phase had completely different morphology, appearing in an optical microscope as a surface coating instead of discrete nodules. Examination by SEM revealed layers of discrete platelets forming a cover over a few discrete nodules. Longer test durations (180 days) produced severe iron staining along with minerals structures similar to rhodochrosite and kutnohorite. These preliminary experiments show strong evidence of the faster rate of increase in mineralization reactions taking place in the sc CO2 phase, transformation reactions that are just beginning to be explored in detail.