Experimental and Numerical Analysis of Fracture Conductivity in Granite Cores Using Newtonian and Non-Newtonian Fluids

ABSTRACT: Enhanced Geothermal Systems (EGS) offer promising potential for renewable energy across the globe. Challenges to heat extraction include the optimal design of working fluids to mitigate early thermal breakthrough or flow channeling. In this work, we investigate the flow of non-Newtonian fluids in rock fractures using a tri-axial Hoek cell to assess their applicability in managing fracture conductivity and mitigating short circuiting. Fracture flow rates are recorded and presented. In this work, we investigate the behavior of WBF-2 (water-based fluid) and OBF-1 (oil-based fluid) in a roughly fractured Sierra white granite core at 20 and 80 °C under varying injection pressures. The experimental results indicate a decrease in flow rate when non-Newtonian fluids are injected, with a more significant reduction observed for OBF-1 than for WBF-2. Both fluids exhibit higher flow rates with increasing temperature and injection pressure. WBF-2, being miscible with DI water, flowed easily under both temperature conditions and recovered faster, while OBF-1 only flowed at high injection pressures for both temperatures and showed a more dramatic increase in flow rate with increasing temperature and injection pressure. Additionally, OBF-1 showed a sharp decrease and subsequently, an increase in flow rate when switching from water to OBF-1 and vice versa. The post-cycle flow rate recovered faster and closer to the pre-cycle flow rate for WBF-2. Results suggest that OBF-1 can block flow in the cooler fractures more effectively than WBF-2, however, WBF-2 restores fracture conductivity more efficiently and quickly. Therefore, within an EGS fracture network, WBF-2 may be an effective fluid when there is no significant difference between the temperature of flow paths and when a low impact in production volume is expected to reach market demand. On the other hand, OBF-1 may be a suitable alternative when the impact of the flow channel is significant, or a longer recovery time is expected while production is expected to be relatively low. Numerical results show that WBF-2 can recover heat as effectively as water under EGS conditions only requiring more pressure drop for the same flow rate. However, further analysis is needed to understand the behavior of WBF-2. The results presented herein provide insights into the potential of employing non-Newtonian fluids as alternative working fluid in EGS to mitigate short circuiting and improve heat recovery over the EGS life cycle.