Sensitivity of a Reactive-Tracer Based Estimate of Thermal Breakthrough in an EGS to Properties of the Reservoir and Tracer

Abstract

Reactive tracers provide a potential method for measuring thermal drawdown in a geothermal system before significant cooling occurs at the production well. Because reaction rates generally have strong temperature dependence, the conversion of a reactive tracer carried through a reservoir is essentially an indicator of reservoir temperature between the injection well and the production well (Tester et al., 1987; Chrysikopoulos, 1993). With repeated tests, the rate of cooling may be estimated, which may provide critical planning data. While several recent studies have suggested that the sensitivity of this approach to reservoir cooling is low, those analyses were based largely on cooling behavior of isolated fractures. In this study, we examine the sensitivity of successive reactive tracer tests to reservoir cooling under a variety of conditions, and illustrate how that sensitivity relates to the kinetic parameters of a range of compounds suitable for this application. Notably, while the maximum change in the relative concentration between successive reactive tracer tests in a system cooling as an isolated fracture is approximately 35%, a 100% difference could be measured in a system where cooling proceeds as it would for closely spaced fractures, although the early time sensitivity is actually greater for the former system. To illustrate how this approach may be used in an actual system, we present results of our field test of two reactive tracers at U.S. Geothermal Inc.’s Raft River, Idaho site.