An experimental study to characterize changes in physical properties of basalts and basanites during reaction with carbonic acid

Abstract

One of the leading hydrothermal alteration processes in volcanic environments is when rock-forming minerals, with high concentrations of iron, magnesium and calcium, react with CO2 and water to form carbonate minerals. This geologic concept is used to the advantage of geologic sequestration of anthropogenic CO2. Basalts are thought to be one of the promising rock types for mineral sequestration of CO2 by creating such hydrothermal mineral alterations. Rock-fluid interactions occurring during the mineral carbonation process alter the rock structure, resulting in changes in the rock’s storage and fluid transport properties. In order to characterize these changes, CO2-water-rock alteration in young basalts and basanites (less than 0.3 Ma) of the Auckland Volcanic Field was studied over a 140-day reaction period. This study investigates how whole core basalts and basanites with similar chemistry but different porosity, permeability, pore geometry and crystallinity alter due to CO2-water-rock reactions. Rock dissolution mechanisms are found to dominate secondary mineral precipitation, during the CO2-water-rock reactions, increasing the porosity and decreasing the rigidity of all samples. Ankerites and aluminosilicates (clay or zeolite) precipitate as secondary mineral phases. The sample with the highest initial porosity and permeability (high reactive surface area), and low crystallinity (high volcanic glass content) shows maximum secondary mineral precipitation along with an increase in porosity and permeability, and a decrease in density and rigidity, after CO2-water-rock reactions. For all samples, a general linear trend is observed between the logarithm of change in their porosity due to CO2-water-rock reactions, and their volcanic glass content. In addition to secondary mineral precipitation, crystallinity and rock dissolution are found to influence the fluid transport (porosity and permeability) and rock mechanical properties (elasticity) of basalts and basanites. This has important implications for field scale CO2 sequestration projects, with regards to the initial site characterization and safety concerns about loss in the rock’s rigidity. An implication is that changes in density and elasticity of these rocks by dissolution during CO2-water-rock reactions, need to be taken into account during time-lapse seismic monitoring of CO2 sequestration sites.