Field Demonstration of the Combined Use of Thermally-Degrading and Cation-Exchanging Tracers to Predict Thermal Drawdown in a Geothermal Reservoir
This paper presents the results and interpretation of a reactive tracer test in a geothermal reservoir involving a cation-exchanging tracer to interrogate flow pathway surface area to volume ratios and a thermally-degrading tracer to interrogate average flow pathway temperatures. The test interpretations, which are based on comparing the reactive tracer responses to that of a nonreactive tracer, are used to predict when thermal drawdown will occur at the production well due to injecting cooler, spent geothermal water into an injection well. It is shown that the cation- exchanging tracer provides constraints on the timing of thermal drawdown, and the thermally-degrading tracer provides constraints on the magnitude of thermal drawdown. Thus, the combined use of the two reactive tracers provides a better prediction of thermal drawdown than either one alone. Uncertainties associated with the method, including assumptions made about flow pathway geometry, are discussed. It is also shown that interrogating average temperatures along flow pathways using a thermally-degrading tracer at different times can be translated into an estimate of when thermal drawdown will occur without direct knowledge of flow pathway geometry.