Application of TOUGHREACT to performance evaluations of geothermal heat pump systems

Numerical simulations were performed with a non-isothermal multi-phase reactive geochemical transport code, TOUGHREACT for predicting changes in temperature, permeability, porosity, and injectivity resulting from a long term operation of a geothermal heat-pump (GHP) system. A one-dimensional model was developed to simulate a GHP system for a potential geothermal reservoir with down-hole water temperature of 200°C. The GHP system was developed for (1) an open-loop system with a set of extraction and injection wells 600 meter apart, and (2) a closed-loop system with a heat exchanger. The same ground water was either injected into the ground through the injection well of the open-loop system or placed in the heat exchanger in the closed-loop system. Injecting water temperature (65°C or 35°C), pressure (+2MPa or +5MPa), chemical composition, and well casing (stainless stell or PVC) for the closed-loop system were varied. The feedback effects of changes in permeability and porosity resulting from mineral dissolution or precipitation on liquid injectivity and heat extractability were evaluated. The results showed that the impacts of the mineral precipitation on liquid injectivity and heat extractability were significant in the open-loop system than in the closed-loop system. This study also showed the injecting water temperature and well casing material would not be significant to improve the performance of a closed-loop GHP system. For the comprehensive evaluation of a GHP system performance, a system-level modeling considering economic and technical aspects was suggested.