A long-term reservoir tracer test using conservative naphthalene-di-sulfonate (NDS) type tracers was performed in early 2022 at a known geothermal resource area (KGRA) and operating geothermal field in the Basin and Range Province of the Western United. Unique tracers were injected into four separate injection wells (I-1, I-2, I-3, I-4). Production wells (P-1, P-2, P-3, P-4, P-5) were monitored for tracer returns for a period of ~130 days. Results of this tracer test confirmed strong permeable connections between all production wells and injection wells I-1 and I-4 in the central geothermal reservoir area and established the connection of northern injection wells I-2 and I-3 to the main reservoir via returns in P-2, P-3, and P-4. Tracer return data of sufficient quality was used to estimate mean residence times of injected fluid and to characterize swept pore volume, flow geometry (heterogeneity), sweep efficiency, and thermal front migration times for production-injection pairs. Interpretation of the tracer test results has informed the current understanding of flow pathways and connections in the KGRA and is constrained with recent and historic operational data from the wellfield. Additionally, comparisons of estimated thermal front migration times to observed thermal decline from downhole temperature logs has confirmed the reliability of these calculations and their usefulness in predicting thermal impacts of production and injection regimes.

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