A Reactive Tracer Analysis Method for Monitoring Thermal Drawdown in Geothermal Reservoirs

Abstract

Tracers provide an important means of interrogating the subsurface to provide critical information about hydraulic characteristics of the reservoir. In a geothermal reservoir, reactive tracers may add information about the thermal state of the reservoir. Tracer test analysis methods vary, but generally involve curve fitting to a simple model of the system or a moment analysis approach. The Idaho National Laboratory is currently developing an analysis tool that provides a variety of commonly used approaches in a single program, and which extends to analysis of reactive tracers. To provide a reactive tracer test analysis method that can be easily implemented without extensive knowledge of the reservoir geometry, we include an approach that relies on a simplified model of the subsurface. The approach involves (1) fitting a simplified reservoir model geometry to a combination of conservative and reactive tracer data (2) using the assumed geometry to interpret changes in reactive tracer concentrations (from tests conducted at different times) and (3) examining the sensitivity of inferences about thermal evolution to the assumptions about geometry. Sensitivity of such an analysis depends on the sensitivity of a reactive tracer to changes in its temperature-time history. We illustrate sensitivity differences between several possible reaction combinations.