Influence of Fluid Rheology on Proppant Transport and Fracture Geometry in Enhanced Geothermal Systems: A Field Comparative Study in Deep Granitic Rock.

Enhanced geothermal systems (EGS) rely on hydraulic fracturing to create permeable pathways in hot dry rock formations, yet the influence of fracturing fluid rheology on proppant transport and fracture geometry remains poorly constrained under field conditions. This study presents a field comparative analysis of cross-linked gel versus slickwater fracturing fluids in deep granitic rock at the Utah FORGE site. Two consecutive stages in well 16A(78)-32 Stage 8 (cross-linked gel, ∼200 cP) and Stage 9 (slickwater, ∼5 cP) were pumped at equivalent high rates (∼80 bpm), with fluid rheology as the primary design difference between stages. Integrated diagnostics including real-time treating pressure analysis, poststimulation production logging (PLT), and distributed fiber-optic strain sensing (DSS) on an offset well were employed. Results show that the cross-linked gel produced a fracture span, defined here as the cross-well fiber-optic response depth range (i.e., the interwell disturbance depth range inferred from fiber-optic response), of 575 ft compared to 145 ft for slickwater, suggesting approximately 4-fold greater vertical extent of fracture-driven interactions. The slickwater treatment exhibited pronounced near-wellbore pressure escalation indicative of screen-out risk, coupled with nonuniform cluster injection efficiency (Gini coefficient = 0.27) including complete exclusion of one perforation cluster. Fiber-optic strain profiles revealed distributed, multipeak fracture signatures for the gel treatment versus concentrated near-wellbore responses for slickwater. While confounding factors including sequential stress shadowing effects cannot be fully excluded, the observed performance differences are consistent with the expected role of fluid rheology in governing proppant transport and fracture geometry. These findings provide field evidence that high-viscosity fracturing fluids may offer advantages for proppant transport and fracture network development in EGS applications in crystalline basement formations.