Reactive transport modelling of NCG-reinjection at Ngawha geothermal field

Abstract

Geothermal energy stands as a well-established, dependable, and sustainable source for generating electricity and providing district heating at lower temperatures. In striving to meet New Zealand's ambitious climate targets, geothermal power producers have taken bold steps to drastically reduce the country's carbon emissions. One noteworthy example is the Ng?wh? Geothermal plant operated by Top Energy, which initiated Non-Condensable Gases (NCGs) reinjection directly on site starting in May 2022 with OEC1. This innovative
approach has since been expanded to other units as of early 2023, marking significant progress towards achieving carbon neutrality. This presents an interesting research question: exploring the effects of additional NCG reinjection on key reservoir parameters through a comprehensive modelling study for Ng?wh? field.
Reservoir modelling is crucial in comprehending how fluids move within geothermal reservoirs. However, very few studies are performed that incorporate the chemical interaction occurring between fluid and rock insitu. To understand what happens to injected NCGs and how it chemically alters the reservoir, conducting reactive transport modelling becomes essential. This approach provides detailed insights into how these changes evolve spatially and over time. Such information is crucial for effectively managing resources and gaining confidence in our understanding of the complex thermal, hydraulic, and chemical processes occurring underground.
In this study, 1D numerical models were set up to address the above research aims. One of the primary reinjection wells, NG11, was chosen for detailed analysis. A 1D MINC radial model was generated in TOUGHREACT4™ using publicly accessible information on Ng?wh? field. The MINC approach was selected to represent the highly fractured nature of the Ng?wh? field more accurately. The results from our study provide insights into the principal minerals expected to dissolve and/or precipitate, the subsequent changes on porosity and permeability, and effect of injected CO2 for various scenarios. Three different reinjection scenarios were modelled, and the results suggest that calcite dissolution increases with higher NCG reinjection content. Out of the others minerals considered, pyrite, illite, and albite showed strong tendency for precipitation. Due to the high dissolution rate of calcite compared to other minerals, its effect on overall change in porosity was profound, which led to an increase in porosity. Significantly, the injected CO2 stayed in the aqueous phase due to non-precipitation of calcite and other carbonate minerals.