Modeling Thermal Fracturing During Operation of Enhanced Geothermal Systems: Improved Heat-Transfer Area and Reservoir Sustainability

In current concept of an enhanced geothermal system, a pair of horizontal wells are used to circulate fluid through stimulated hydraulic fractures; reservoir sustainability and improvement during operation are critical for long-term power generation. Injecting cold fluid into a high-temperature reservoir will cause significant cooling and thermal stress, resulting in secondary thermal fractures perpendicular to primary hydraulic fractures and forming a well-connected fracture network for heat transfer. First, we derived analytical solutions of dimensionless fracture length L, aperture profile, and spacing D (as well as pattern) as functions of time τ and effective confining stress T using a plane strain model and the displacement discontinuity method (Chen & Zhou, 2022, 10.1029/2021JB022964). It was observed that fracture length L increases nonlinearly with τ1/2 and then transitions to scaling law L=f(T)τ1/2, indicating that late-time fracture length increases linearly with the square root of cooling time. The scaling coefficient f(T) shows the effects of inter-fracture stress interaction and fracture arrest. The solutions and scaling law provide fast predictions for all reservoir and cooling conditions using (single) model parameter T. Application to the Utah FORGE EGS site with demonstrates that thermal fractures reach 0.67, 6.25, and 78.00 m in length, 0.49, 2.30, and 13.00 m in spacing, and 0.43, 2.09, and 12.19 mm in surface aperture at 1, 100 and 10,000 days. Second, numerical modeling was conducted to investigate evolution of the network of thermal and hydraulic fractures and enhancement of heat-transfer area. Modeling results shows significant improvement of heat transfer efficiency.