| Title | Injectivity Changes at Ngatamariki |
|---|---|
| Authors | J. Potter, A. Calibugan, A. Ferguson, and G. Allan |
| Year | 2020 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Ngatamariki, Taupo Volcanic Zone, injection, injectivity index, temperature, microseismicity |
| Abstract | Ngatamariki geothermal system in the Taupo Volcanic Zone of New Zealand generates power from an 82 MWe binary power station. All the produced geothermal fluid is reinjected into four injection wells minus minor losses of around 2%. Wellhead pressures, flow rates, and injectivity of Ngatamariki reinjection wells have varied over the 7 years of operation. The injectivity index of a well is determined from the amount of fluid injected over the pressure required to inject it. Injectivity is known to vary with temperature of injection fluid and rock formation. Two of the injection wells at Ngatamariki are presented in this work: NM08 and NM09. The injectivity indexes for these wells are determined downhole and incorporates fluid density changes, friction losses along the wellbore, and changes in reservoir pressure. In the first few years of operation after drilling, a large increase in injectivity index has been observed in both wells. In recent years the injectivity has declined overall. Additionally, and investigated here, a varying rate of decline in injectivity index has been observed recently. The temperature of the reinjection brine varies up to 10°C. The correlation between the difference in injectivity index in NM08 and NM09 and change in injection temperature is examined. The correlation between the difference in injectivity index and microseismicity is also investigated. Uncertainties in determining injectivity index are explored. The reasons for injection variation are not fully understood and warrant further investigation. |