Influence of Thermal Properties Heterogeneity on Geothermal Production and Fracture Spacing for Enhanced Geothermal System

Abstract

Enhanced Geothermal Systems (EGS) rely on efficient heat transfer between circulating fluids and hot rock through engineered fracture networks. Reservoir thermal properties—particularly thermal conductivity and specific heat capacity—play a critical role in production temperature, thermal power output, and fracture spacing. These thermal properties are commonly assumed to be homogeneous, yet laboratory measurements from Utah FORGE core and cuttings reveal substantial spatial and temperature-dependent heterogeneity. Core and cutting analyses show that thermal conductivity can vary by up to 67% along depth, largely driven by mineralogical differences, especially quartz content. In addition, thermal conductivity decreases while specific heat capacity increases with increasing temperature, leading to reduced thermal diffusivity at reservoir conditions. This study integrates laboratory measurements with numerical simulations to quantify the influence of thermal property heterogeneity on EGS performance. Numerical simulation results of both single-fracture and multi-fracture doublet systems demonstrate that a 50% increase in thermal conductivity combined with a 20% increase in heat capacity can increase produced thermal power by approximately 11%. In heterogeneous reservoirs, the presence of a 130 m high thermal property layer enhances thermal power output by 3.6%. Furthermore, reduced thermal diffusivity at elevated temperatures decreases the fracture spacing required to maintain thermal independence, lowering fracture spacing from approximately 100 m to 85 m for a 30-year project lifespan. These results highlight the importance of incorporating realistic thermal property heterogeneity into EGS design and performance assessments.