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EGS Thermal Recovery Factor from Downhole Engineered Heat Exchangers with Variable Fracture-Driven Interaction Flow Rates
Christine EHLIG-ECONOMIDES, Nelson BARROS-GALVIS
[University of Houston, USA]
The injection well profile from the Fervo Energy commercial doublet well configuration shows considerable flow rate variability among multiple hydraulically fractured stages. Even with uniform flow among the fracture-driven interactions (FDIs), recent work suggests that the heal FDI flow rate will be about 16% greater than the rate at the toe of the downhole engineered heat exchanger (DEHE) under the reported circulation rate. This study assesses what fraction of the maximum recoverable heat the DEHE produces as a thermal recovery factor for DEHE configurations of interest. We define the remaining recoverable heat in a stimulated volume as the energy that can be transferred by reducing its average rock temperature to the final working temperature of the circulating fluid. The model for this study couples models for the flow circulation and heat transfer through the DEHE and the vertical injection and production wells, subject to boundary conditions required for the surface power generation. We define the thermal recovery factor as one minus the time dependent ratio between the original and remaining recoverable heat values, or simply the ratio between the rock temperature change from the original average rock temperature divided by the temperature difference between the original average rock temperature and the lowest plant inlet working fluid temperature for continued plant operation. Sensitivity studies quantify the variation in thermal recovery factors for a DEHE having identical FDI conductivities with variation between maximum and minimum FDI flow rates from less than 1% to 30%. We then quantify the expected thermal recovery factor for the published Fervo Energy injection well profile. This study illustrates the importance of the DEHE design and flow conformance on the thermal recovery efficiency of a DEHE. The thermal recovery factor may be improved by production well or plant workovers that raise the temperature of the produced fluid reaching the power plant inlet and/or enable a lower plant inlet fluid temperature.
Topic: Enhanced Geothermal Systems