Tradeoffs Between Planar and Geologic Discrete Fracture Networks for EGS: Insights from Utah FORGE

Abstract

Discrete Fracture Network (DFN) models are widely used to evaluate fluid flow and heat transfer in Enhanced Geothermal Systems (EGS). However, the choice of fracture representation introduces a fundamental tradeoff between complexity and computational efficiency. In this study, we compare two DFN approaches applied to the Utah FORGE site: a geologic DFN derived from microseismic data, geomechanical characterization, and borehole observations, and a simplified planar fracture DFN in which fractures are represented as idealized planes connecting injection and production wells. The geologic DFN captures the heterogeneity of natural fracture systems, including variable orientations, lengths, and connectivity. This complexity leads to more realistic predictions of fluid circulation pathways and thermal drawdown, but it comes at the cost of high computational demand. By contrast, the planar fracture DFN sacrifices geologic realism for efficiency, allowing rapid parameter exploration and long-term forecasting. While this simplification enables broader scenario testing, it risks overestimating fracture connectivity and sweep efficiency. Simulation results indicate that fracture connectivity, rather than fracture count or geometry alone, exerts the strongest control on long-term heat recovery. The geologic DFN better represents site-specific variability, while the planar DFN is more practical for sensitivity studies and optimization tasks. This comparison highlights the importance of selecting the appropriate DFN representation based on research objectives, balancing the need for geological accuracy with computational tractability in EGS modeling.