Results of the First Application of Perfluorocarbons and Alcohols in a Multi-Well Vapor and Two-Phase Tracer Test at the Darajat Geothermal Field, Indonesia, and Implications for Injection Management

摘要

Long-term sustainability of a vapor-dominated geothermal resource is largely determined by the injection of fluids back into the reservoir. Properly managed, injection generates "new steam" as the water boils when it comes into contact with hot dry rock in the reservoir. Without injection the field would dry out and production would go into decline. For the development of a long term injection strategy, it was decided to undertake a multiwell vapor and two-phase tracer test. By using both types of tracers, it was expected we could quantify the volume of injected water that was being boiled in the reservoir and produced by the surrounding steam production wells and characterize the boiling processes in terms of steam fractions. For time and cost efficiency, it was desirable to inject tracers into the three existing injection wells simultaneously. As multiple high-temperature vapor tracers for geothermal were not available, Chevron Geothermal Indonesia, Ltd., collaborated with Thermochem, Inc., to develop and test new vapor-phase tracers and to run field tests to confirm the tracers effectiveness. Freon compounds, such as chlorofluorocarbons (CFC) and hydrofluorocarbons (HFC), have been used routinely as vapor tracers in geothermal. However, CFC's are no longer used due to ozone-depletion issues, and the only HFC remaining for practical use is R-134a. Sulfur hexafluoride (SF6) is not used due to reaction with silicate rocks in the reservoir. A new class of geothermal vapor-phase tracers for geothermal applications, perfluorocarbons (PFC), have been tested in the lab by Thermochem for a preliminary evaluation of thermal and chemical stability. Two PFC's were found to be stable to at least 280 C in the presence of reservoir rock and water for one week. These compounds are very insoluble in water, so they had to be emulsified for application as water to steam tracers. Once the water boils in the reservoir they partition to the vapor phase and follow the steam path. The two PFC's were tested along with R-134a and three light alcohols in the first multi-well, vapor and two-phase tracer test for a geothermal reservoir. The results and interpretation presented in this paper are preliminary in that the test is on-going and some late return curves have not yet fully characterized. However, some interesting observations can already be made qualitatively about the relative response of the vapor- and two-phase tracers and implications for boiling processes occurring in the reservoir, characterization of fluid flow paths within the reservoir and the reservoir structures that are controlling fluid flow movement. The observation was confirmed by integration of tracer tests results with geochemistry, geophysics, geology and production well performance data. The tracer results also confirm semi-permeable barriers within the reservoir, which will help design injection distribution strategies for improved injection management. The results will be used to develop alternative injection strategies to best manage reservoir performance and maximize heat recovery from the reservoir rock.