Comparison of Three Tracer Tests at the Raft River Geothermal Site

Abstract

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 RRG-4 (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 oC) injection into the 140 oC geothermal reservoir.