Optimizing Hydraulic-Fracture Networks for Enhanced Thermal Recovery in Single-Well Geothermal Systems: A Coupled Thermo-Hydro-Mechanical (THM) Modeling Study

Enhanced Geothermal Systems (EGS) in hot dry rock (HDR) often require hydraulic stimulation to create permeable flow paths. In naturally fractured reservoirs, hydraulic fractures are strongly guided by pre-existing fractures, shaping connectivity and controlling post-stimulation fluid flow and heat transfer. This study investigates the influence of hydraulic-fracturing parameters on thermal recovery in Single-Well Enhanced Geothermal Systems (SEGS) with natural fractures. A two-dimensional hydraulic-fracturing model is developed using the finite element method (FEM) with zero-thickness cohesive elements to simulate fracture initiation and propagation under the influence of natural fractures. The resulting fracture geometries are embedded into a coupled thermo-hydro-mechanical (THM) model to simulate long-term heat extraction. Parametric analyses include fracturing cluster spacing, number of clusters, injection rate, and natural fracture orientation. Results show that cluster spacing affects both fracture geometry and thermal-front evolution: larger spacing promotes longer fractures but may reduce cold-front coverage near the production zone. Natural fracture orientation dominates stimulation effectiveness: larger intersection angles enhance cross-connected networks, increase fluid-rock contact, and improve both heat extraction rate and cumulative production. Increasing the number of clusters shortens individual fractures and intensifies preferential flow along pre-existing fractures, reducing long-term recovery. Higher injection rates enhance fracture propagation and network complexity, enlarging the effective heat-exchange area and strengthening convective heat transfer. By explicitly linking stimulation-induced fracture geometry to subsequent THM heat extraction, this study provides quantitative guidance for optimizing hydraulic-fracturing design in SEGS development.