Experimental investigation of Basalt-CO2 brine interactions at supercritical conditions

Supercritical geothermal systems offer significant potential for enhanced energy production to support the energy transition due to their high enthalpies and the unique properties of supercritical fluids. However, development and utilisation of such promising resources are still in their infancy and face scientific uncertainties, financial and environmental risks, and engineering challenges. Supercritical hydrothermal reservoirs are inherently associated with proximity to magmatic plumbing systems below them. However, it is not yet well understood the extent to which degassed magmatic volatiles may contribute to supercritical hydrothermal reservoir fluid compositions.

CO₂ (and its species) constitutes the largest proportion of gas content from degassing magmas and hydrothermal fluids. Basaltic host rocks are closely associated with many high-temperature mafic magmatic systems around the world and are prime candidates to host supercritical geothermal reservoirs due to their highly reactive mineral compositions and vesicularity. Previous carbon capture and storage (CCS) focused fluid-rock interaction experiments have shown that basalt facilitates carbonation reactions, forming stable carbonate minerals and enabling the sequestration of atmospheric carbon. Here, we extend the scope of experimental investigations of CO₂ brine interaction with basalt to simulate and quantify the effects of magmatic CO₂ infiltration into a supercritical hydrothermal reservoir.

Two supercritical flow-through experiments were performed at 400°C and 500 bar using a high P-T titanium alloy autoclave system at the experimental hydrothermal geochemistry laboratory in GNS Science, New Zealand. In both experiments the same crushed basalt with a fraction size of 355-500 µm was used. In Experiment 1, distilled water and CO₂ brine was injected for 42 days, while in Experiment 2, distilled water, CO₂ and NaCl brine was injected for 49 days. Daily reacted effluent samples were analysed for major cations, anions, and trace elements by ICP-OES, IC and ICP-MS. Results show the development of alteration fronts across the host rock sample along the reactor’s depth in both Experiments 1 and 2. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) on the solid products from Experiment 1 reveal extensive dissolution of primary bytownite and secondary precipitation of chlorite, blocky calcite, and sphene covering the grain surfaces at the fluid entry point. Calcite precipitation was restricted to the fluid entry location whereas chlorite precipitation was observed along the entire reacted sample, decreasing in amount with distance from the fluid entry location. The penetrative dissolution of glass and primary bytownite deep into the basaltic grain alongside precipitation of secondary mineralogy suggests intact basaltic reservoir may experience fluid pathway evolution as CO₂ brines move through them under supercritical conditions.