Initial tests indicate geothermal CO2 co-injection a “win-win-win” scenario

Graphic of a geothermal production well and the production, re-injection and scaling cycle. (source: GNS Science)

Carlo Cariaga 28 Aug 2024

Lab-scale tests done by New Zealand-based research institute GNS Science have indicated that re-injecting the natural CO2 of geothermal fluids results in a win-win scenario, reducing emissions while also mitigating silica scaling in the reservoir.

Under current practices in most parts of the world, geothermal power stations emit naturally occurring CO2 that is initially dissolved in the geothermal fluid as part of the power generation process. Although a minuscule amount, the CO2 emitted means that geothermal electricity generation is not completely free from carbon emissions and the cost of carbon tax.

There are already several efforts worldwide to re-inject this CO2 into the geothermal reservoir. This has the potential to turn geothermal generators from CO2 emitters to CO2 neutral. A notable example of this is the Ngawha Geothermal Power Station by Top Energy, also in New Zealand, which has achieved 100% reinjection of non-condensible gases as part of the Ngawha Carbon Zero Project. However, there is still a knowledge gap on how a geothermal reservoir would respond to long-term CO2 re-injection.

Results of lab tests

Tests were done using the novel dual-stage autoclave method by GNS Science. Aside from the goal of re-injecting CO2, the theory is that CO2 forms carbonic acid upon dissolution in the fluid. This makes the brine more acidic and helps prevent silica scaling in the reservoir. Scaling in the formation is a particularly big problem in re-injection wells, as it may reduce injectivity and eventually necessitate the drilling of new wells.

The experiment was done using greywacke, a major reservoir rock type found in the Taupo Volcanic Zone. The control experiment showed that moderate silica scaling occurs with standard geothermal brine, and that this scaling is enhanced with the introduction of calcite. However, using brine charged with 2000 mg of CO2/kg brine resulted in nearly zero scaling.

CO2 as a natural cleaning agent

The test results showed that the accelerated removal of calcite from the rock is an important side-effect of co-injection CO2 with geothermal brine. As a “win-win-win” solution, the co-injection of captured CO2 with the geothermal brine can:

• lower the emissions, therefore the carbon tax, for geothermal operators
• make the expended geothermal brine that flows into the subsurface retain its acidity, which slows silica scaling in the rock
• at high concentrations, accelerate the removal of calcite from the rock, meaning that the CO2 is acting as a “cleaning agent” because it removes the cause of silica scaling.

Co-injection can thus eliminate carbon emissions while improving the efficiency of production operations and the sustainability of the geothermal reservoir.

Next steps

Site-specific studies were funded by both domestic and international geothermal operators, and have led to the formulation of the novel concept of CO2 as a silica anti-scalant. The research done by GNS Science was supported by the Strategic Science Investment Fund (SSIF) Programmes Contract C05X1702 granted by the Ministry of Business, Innovation and Employment (MBIE).

Encouraged by the promising results, two New Zealand geothermal power generators have requested further experimentation. The goal is to further explore the effects of CO2 co-injection, potentially leading to more efficient and sustainable geothermal power production.

Source: GNS Science