Tracers for Characterizing Enhanced Geothermal Systems

Information about the times of thermal breakthrough and subsequent rates of thermal drawdown in enhanced geothermal systems (EGS) is necessary for reservoir management, designing fracture stimulation and well drilling programs, and forecasting economic return. Thermal breakthrough in heterogeneous porous media can be estimated using conservative tracers and assumptions about heat transfer rates; however, tracers that undergo temperature-dependent changes can provide more detailed information about the thermal profile along the flow path through the reservoir. To be effectively applied, the thermal reaction rates of such temperature sensitive traces must be well characterized for the range of conditions that exist in geothermal systems. Reactive tracers proposed in the literature include benzoic and carboxylic acids (Adams, Moore et al. 1992) and organic esters and amides (Robinson, Tester et al. 1984); however, the practical temperature range over which these tracers can be applied (100-275°C) is somewhat limited. Further, for organic esters and amides, little is known about their sorption to the reservoir matrix and how such reactions impact data interpretation. An alternative approach is to use of tracers where the reference condition is internal to the tracer itself. Two examples are: 1) mineral thermoluminescence, and 2) racemization in polymers of compounds such as amino acids. In these cases internal ratios of states are measured rather than extents of degradation and mass loss. Racemization of a polymeric amino acid is temperature sensitive and therefore can be used to infer temperature history depending on the rate of racemization and stability of the particular amino acid. Heat-induced quenching of thermoluminescence of pre-irradiated LiF can also be used. To protect the tracers from alterations (extraneous reactions, dissolution) in geothermal environments we are encapsulating the tracers in core-shell colloidal structures that will subsequently be tested for their ability to be transported in geologic media and to protect the tracers from incidental reactions. In this report we review the criteria for practical reactive tracers, which serve as the basis for experimental testing and characterization and can be used to identify other potential candidate tracers. We will also discuss the information obtainable from individual tracers, and the possibilities for using multiple tracers to obtain improved information about the thermal history of a reservoir. Finally, we will provide an update on our progress for conducting proof-of-principle tests for reactive tracers in the Raft River geothermal system.