Structural and Dissolution Trapping of Carbon in Geothermal Reservoirs: A Numerical Assessment

Abstract

As global policy undergoes a transformative shift towards a net-zero carbon future, there is growing interest in reinjecting non-condensable gases, including carbon dioxide, hydrogen sulfide, and methane, from geothermal power plants back into reservoirs for safe and long-term storage. Although these gases are released during power generation, the emissions are significantly lower (e.g. over 85% less in New Zealand) compared to fossil fuel power plants. Reinjection of these gases by compression and dissolution into reinjected fluids from geothermal plants (i.e. brine and condensate) presents an opportunity to ensure net-zero operations. In New Zealand, 100% gas reinjection tests have been underway in the NgāTamariki Power Station in one of its four OEC units since 2021. Similarly, Ngawha Generation has been trialling reinjection for one of its OEC units since 2022, with plans to expand across its stations. Trials are also ongoing in the Te Huka Power Station since late 2022. Carbon dioxide capture and reinjection come with key technological and process challenges and potential reservoir management risks. These factors need careful assessment during the technology testing phase and in terms of long-term resource performance. In this study, we utilise numerical models to assess the carbon storage potential in a hypothetical geothermal reservoir and the possible risks, e.g. breakthrough and surface leakage.