Numerical assessment of carbon sequestration in a 3D rhyolite hosted reservoir

Abstract

Carbon dioxide produced from geothermal operations may be captured and reinjected for safe and long-term storage to reduce emissions. However, this comes with key process challenges and potential reservoir management risks, requiring careful assessments during the technology testing phase and when gauging long-term resource performance.
The reaction of injected carbon dioxide with minerals in the reservoir formation to form carbonate minerals has the greatest capacity for storage permanence. A critical determinant for carbon mineralisation success is the reservoir rock; for example, the mineralisation of CO2 in basaltic rocks rich in divalent cations, e.g. Ca2+ and Mg2+, has been proven in Iceland. This work investigates the carbon sequestration potential in hydrothermal systems hosted in silicic rocks with lower divalent cation content, e.g., geothermal systems in New Zealand.
Numerical models were used in this study to assess carbon storage via mineralisation in a 3-dimensional hypothetical liquid-dominated geothermal reservoir hosted in rhyolite tuff, the corresponding sequestration potential, and the possible risks, e.g., breakthrough and surface leakage. Model results show that though mineralisation is expected during 40 years of concurrent mass extraction and CO2 reinjection, <1% of the injected CO2 was converted to carbonates during that period. After operations were discontinued, carbon dioxide
mineralisation increased further to >12% in the subsequent 60 years of modelled time. Models to assess the effectivity of methods aimed at promoting mineralisation, i.e., adding divalent cations to the reinjected fluids or pH neutralisation, suggest short-term improvements in mineralisation rates but no significant effect in the long term.