Comparison of Three Tracer Tests at the Raft River Geothermal Site

Three conservative tracer tests have been conducted through the Bridge Fault fracture zone at the Raft River Geothermal (RRG) site. All three tests were conducted between injection well RRG-5 and production wells RRG-1 (790 m distance) and RRG4 (740 m distance). The injection well is used during the summer months to provide pressure support to the production wells. The first test was conducted in 2008 using 136 kg of fluorescein tracer. Two additional tracers were injected in 2010. The first 2010 tracer injected was 100 kg fluorescein disodium hydrate salt on June, 21. The second tracer (100 kg 2,6-naphthalene disulfonic acid sodium salt) was injected one month later on July 21. Sampling of the two productions wells was conducted over 179 days to obtain the tails of both 2010 tracer tests. Tracer concentrations were measured using HPLC with a fluorescence detector. For the 2008 test, 80% of the tracer was recovered at the two production wells. 85% of the recovered tracer mass was from well RRG-4 indicating a greater flow pathway connection between the injection well and RRG-4 than RRG-1. Despite the differences between the injection rates for the two tests (~950 gpm to 440 gpm) between the 2008 and 2010, fluorescein tracer results appear to be similar between the 2008 and 2010 tests for well RRG-4 with peak concentrations arriving approximately 20 days after injection. The tracer breakthrough results for all three tests suggest that the reservoir is highly fractured and no flow path deviation can be noted during the 30 days of cold water (55 C) injection into the 140 C geothermal reservoir. BACKGROUND Chemical tracer tests in geothermal systems are used to examine the flow pathways in a reservoir. Typically these tests are conducted to determine connectivity between parts of the reservoir, swept volume, fluid velocities, and reservoir geometry often in an attempt to determine the potential of thermal breakthrough of injected water. For this paper, we will discuss two sequential conservative tracer tests that were conducted in 2010 as part of a larger project to evaluate thermally degrading tracers. The purpose of these conservative tracer tests was to help define an appropriate conceptual model of the flow and to be used to interpret the thermally degrading tracer in an attempt to quantify the location of the cooling front before it reaches a production well. The Raft River geothermal reservoir is located in Cassia County Idaho approximately 6 miles north of the Utah/Idaho border near the town of Malta (Figure 1). This site was heavily studied by the U.S. Figure 1: Location of the Raft River geothermal site (from U.S. Geothermal Inc.) Department of Energy from 1975 to 1982 and was the testing site of the first commercial scale binary (isobutene) cycle geothermal power plant in the world. Presently this site is owned and operated by U.S. Geothermal and is producing power from a 13MW (nominal) binary isopentane power system.