Quantitative Tracer Test Interpretation for Reservoir Cooling Assessment in the Hoffell Geothermal System, Iceland

Sammanfattning

Geothermal energy is a renewable energy source, though its sustainable utilisation requires careful management. Reinjection of spent geothermal fluids is commonly practised in hydrothermal systems to counteract the pressure drawdown induced by production. Yet this method is not risk-free, as thermal breakthrough can occur, reducing heat-extraction efficiency and long-term energy output. To assess this risk, understanding subsurface fluid flow between injection and production wells is essential. A tracer test was conducted at the Hoffell low-temperature geothermal field in Iceland to assess the flow paths within the system and the hydraulic connections between the injection well (HF-01) and the production wells (HF-03, HF-04, and HF-05). A quantitative tracer interpretation was performed using the software TRACER, which employs an automatic inversion technique to estimate flow characteristics in one-dimensional channels connecting the injection and production wells. The inversion results were then used to predict the temperature decline during long-term reinjection at varying rates. The findings of this thesis showed that the highest tracer recovery occurred through well HF-04, where cooling is predicted to be greatest. Minimal tracer recovery was observed through HF-05, and the predicted cooling was considered insignificant. Because production from HF-03 was interrupted for most of the test, its results were considered less reliable than those from the other wells. The findings suggest that the current well configuration may lead to a significant thermal breakthrough. Thus, an alternative injection strategy should be adopted, using a new injection well in a location with minimal cooling risk, to ensure long-term, sustainable exploitation of the geothermal resource. Selecting a new injection site will require additional research, including further tracer testing.