Reactive Transport Modeling of CO2-Rich Injection Fluids in the Ngawha Geothermal System
Student: Carter Daniel Johnson
Year: 2023
Supervisors: Juliet Ann Newson, Thomas Ratouis
Abstract:
For the geothermal power sector, a growing area of interest is alternative disposal technologies for greenhouse gas (GHG) emissions. One alternative to the usual technique of venting the naturally occurring non-condensable gases (NCG) to the atmosphere is geologic sequestration. Co-injection with the geothermal fluids is an attractive option as it eliminates the need for sophisticated compression equipment and utilizes an already operating injection system.
The current injection system at the Ngawha, New Zealand site passively reinjects between 20 and 50% of extracted NCGs from the reservoir. Impacts of NCG-rich injection fluids on the reservoir can be wide reaching and can cause precipitation and/or dissolution of minerals. This may have a significant effect on the permeabilities, pressures, and production capacity of the reservoir. In the last decade laboratory experiments have been conducted of fluid-rock interaction between greywacke and geothermal brine containing high concentrations of CO2 and other NCGs, but no chemical model specific to conditions at Ngawha have been developed.
This work attempts to further quantify these fluid-rock interactions with TOUGHReact to better understand the geochemical behavior of greywacke, brine, and NCGs under conditions specific to the Ngawha reservoir. A conceptual model of the chemical system was developed and reacted with the mineralogy of the altered greywacke at Ngawha to produce realistic values of reservoir chemistry. Similar steps were taken to “pre-react” single blocks of injection fluid with varying concentrations of CO2 before fully coupled numerical and chemical simulations took place. A grid was developed to model a feed zone of an injection well, then a numerical flow simulation was run to produce stable flow conditions before chemical reactions were introduced. The results of each of these preparatory stages were used in two scenarios of fully coupled transport (20% and 80% CO2 injection).
Results from the simulation demonstrated a large decrease in porosity near the injection well, which provides an interesting counterpoint to the majority of studies on NCG injection which result mainly in the opposite outcome. The study also indicated that the concentration of CO2 in the injection fluid directly affected the precipitation of minerals near the injection well but had very little effect on reactions further into the feed zone.