Geochemical modeling of NCG injection in a geothermal well using doublet well model

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2021-7-29
İlgin, Buğrahan
Geothermal energy is regarded as an environmentally friendly source of energy. However, the amount of non-condensable gases (NCG), which are co-produced with brine, are significantly high in the geothermal fields of Turkey. To overcome the NCG emission produced by power plants, which is mainly CO2, one of the efficient methods is re-injecting the captured NCG into the reservoir. The ultimate aim of the reinjection is to mineralize CO2 in the reservoir as carbonate minerals. In this thesis, a reactive transport modeling study is conducted with TOUGHREACT to scrutinize a potential operation of brine-CO2 injection in a deep fractured metamorphic reservoir formation. A comprehensive geologic model is constructed and utilized in TOUGHREACT with Petrasim interface to simulate reactive transport phenomenon during NCG injection. The developed doublet well model in which a re-injection and a production well are included is calibrated with field measurements. PHREEQC is used to evalaute the reaction-paths between minerals and aqueous species. These evaluations help to determine possible secondary minerals to select in reactive transport model developed with TOUGHREACT. Three rock types with various mineral compositions are introduced to inspect the interaction between different mineral contents and CO2 charged brine injection in high temperature geothermal reservoirs. Three different injection scenarios are examined as the brine-CO2 mixture, supercritical CO2 injection and brine-CO2 mixture injection at lower temperature to observe amorphous silica precipitation. The results are compared to those obtained with injection of brine-only. Composition results demonstrate that the reactions of aqueous species and reservoir minerals are highly dependent on the pH and temperature. Results show that Calcite formation is only observed in a scenario where brine and CO2 is injected as a mixture, whereas in the first two scenario of the study which represent injection of brine-only and injection of CO2-only, Calcite dissolution is observed. Limited mineralization due to CO2 occurs and CO2 is mostly trapped in solution. Moreover, the impact of CO2 injection is examined on Silica reactions. The results demonstrate that lower injection temperature at 85 °C should be selected for CO2 injection. For lower injection temperatures at 85 °C, amorpheous silica precipitation around the injection well may be trigerred. This study may guide innovative injection strategies, which will be conducted in Turkish geothermal fields in the near future.
Citation Formats
B. İlgin, “Geochemical modeling of NCG injection in a geothermal well using doublet well model,” M.S. - Master of Science, Middle East Technical University, 2021.