Fluid-CO2 injection impact in a geothermal reservoir: Evaluation with 3-D reactive transport modeling

Abstract

Geothermal energy is commonly recognized as an environmentally friendly source of energy. However, geothermal fluids have unusually high CO2 content, particularly, in carbonated geothermal reservoirs. An efficient method to mitigate the CO2 emissions of geothermal power plants is to re-inject the captured CO2 with the effluent fluid to mineralize mainly into calcite under reservoir conditions (e.g., reservoir temperatures ranging from 200 to 220 degrees C). One of the major concerns about the re-injection of the captured CO2 is to predict the geochemical interaction between the injected fluid-CO2 and rock, and the corresponding alterations that occur due to the re-injection in the reservoir parameters. For this study, we have selected one of the largest geothermal fields, which is located in the western part of Turkey. A predictive 3-D reactive transport modeling is carried out using TOUGHREACT v1.2 for potential fluid-CO2 injection into deep metamorphic formation rocks consisting of marble, quartzite, and schist. Each rock type with different mineral constituents is examined in three different scenarios. Moreover, in each scenario, three annual injection rates, 500, 2000, and 4000 tons of CO2 for ten years of continuous injection are tested. The aims of the study are first the evaluation of the dynamic fluid-rock interactions with maximum possible CO2-charged fluid injection and second the identification of the mineralization process. The modeling results indicate that the maximum CO2-charged fluid mixture remains stable as a single-phase in all scenarios. The mineralization process of CO2 in the reservoir is limited due to the different mineral contents of the considered metamorphic rocks and the differences between the CO2 mass fraction of the injected fluid and the reservoir. In addition, the injected fluid temperature, the pH, the convective dispersive fluxes between the wells also affect the chemical process. These results show differences compared to the CarbFix project carried out in basaltic Icelandic geothermal reservoirs where the CO2 mineralization process was successful.