Reactive transport modelling of fluid-NCG reinjection in Ng? Tamariki geothermal field

Abstract

As global efforts intensify to realise a carbon-neutral future, the reinjection of non-condensable gases (NCG) into geothermal fields offers a promising option to reduce greenhouse gas emissions. The ongoing CO2 reinjection program at Ng? Tamariki, which has been operational since October 2021, reinjects NCG from one of the binary units, reducing annual GHG emissions from the station by 25%(>8,000tCO2e p.a.).
To provide quantitative insights into the geothermal reservoir response at Ng? Tamariki under various NCG reinjection conditions, and to assess mineral reactions and geochemical processes, 2D reactive transport modelling was conducted using TOUGHREACT v.4.13. The models aim to identify major precipitation and dissolution reactions of minerals and their long-term impacts on permeability and the flow paths, elucidate the fate of reinjected NCGs and explore CO2 trapping mechanisms for sequestration. By simulating both injection and production behaviour within the reservoir, the models examine the relationship between these and provide insights into geochemical alteration resulting from changes in the thermodynamic conditions.
Based on the 2D model, an increase of NCG reinjection from 0% to 25% of the total NCG of the plants widened the area of porosity-permeability enhancement, along with increased calcite and anhydrite dissolution, and reduced precipitation of pyrite. However, further increasing NCG reinjection from 25% to 50% was insignificant in terms of changes in porosity-permeability or the amount of anhydrite or pyrite. Only calcite exhibits a more widespread and enhanced dissolution response at the increased NCG reinjection rate.