Optimizing Cluster Spacing in Enhanced Geothermal Systems

Abstract

The economic viability of Enhanced Geothermal Systems (EGS) is fundamentally governed by the balance between thermal energy recovery and the associated costs of subsurface engineering and sustained surface injection. This study conducts a comprehensive numerical investigation to evaluate the thermal performance of a field-scale EGS fluid circulation system. Geologic and well completion data from Fervo’s EGS wells in Milford Valley, Utah is used as an example case. A fully coupled hydro-thermal simulator was employed, integrating reservoir, fracture, and wellbore domains to model long-term fluid circulation involving two injection wells and one production well. Sensitivity analyses were performed to assess the impact of key subsurface and operational parameters, including fracture permeability distribution, proportion and spatial arrangement of active fractures, injection rate, inter-well spacing, and cluster spacing. Simulation results highlight that achieving high energy recovery rates in EGS requires strong inter-well hydraulic connectivity and a uniform distribution of fracture permeability. Furthermore, cluster spacing emerges as a critical design parameter, as it governs the trade-off between effective subsurface thermal sweep and surface injection load. To maximize the heat recovery over time, the cluster spacing, and the injection rate can be optimized for a given well spacing. This relationship between these three operator-controlled variables is explored in detail. It is found that for a given well spacing, the optimum cluster spacing increases with the total injection rate. The overall injection rate must be maintained as high as possible but is limited by the fracture propagation pressure (minimum in-situ stress) and the parasitic energy losses needed to pump the fluid. Under these constraints an optimum cluster spacing can be obtained that will maximize the heat recovery rate. These findings provide valuable insights into optimizing EGS design to maximize net energy output and project profitability.