Geothermal power station operators in Aotearoa New Zealand have already achieved significant emissions reductions by reinjecting CO₂ back into geothermal reservoirs, lowering geothermal carbon emissions by approximately 25%.

While this represents a major step toward carbon-neutral geothermal energy, increasing CO₂ and NCG reinjection introduces technical and environmental challenges that vary from field to field and must be carefully evaluated.

At the Ngā Tamariki geothermal field, this research has been carried out through close collaboration with Mercury NZ, which is currently reinjecting CO₂ and planning to scale reinjection from 25% to 80%, alongside expanded power generation. The RCE team developed scenario-based reactive transport models to estimate potential chemical and physical changes within the reservoir, providing guidance for injection strategies and operational decision-making.

A key strength of this work is the deep level of industry engagement. Mercury NZ’s reservoir engineers, geologists, and geochemists actively contribute field data, operational insights, and targeted samples, enabling the development of realistic, field-informed predictive models. Regular technical meetings facilitate continuous feedback, ensuring the modelling reflects real reservoir conditions and ongoing injection experience.

Siahaan and Kaya’s modelling integrated core samples from the NM9 well, mineralogical characterisation, and measured reservoir fluid chemistry to assess both reaction kinetics and equilibrium processes. The resulting framework supports evaluation across a range of mineral and fluid compositions, reactive surface areas, time scales, and temperature–pressure conditions, with relevance to geothermal systems such as Ngā Tamariki and beyond.

PhD student Yeftamikha Siahaan presented results from the RCE research programme at the NZGW 2024, where the work received the Best Student Paper Award, recognising its contribution to advancing reservoir-scale CO₂ reinjection strategies.