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Modeling the Onset of Thermal Short-Circuiting in EGSs with a Coupled THM-Wellbore Approach
Qitao ZHANG, Arash DAHI TALEGHANI, Kun JI
[The Pennsylvania State University, USA]
Thermal short-circuiting poses a significant threat to the long-term efficiency and economic viability of Enhanced Geothermal System (EGS) projects. This study investigates the interplay of the primary physical mechanisms that collectively drive this phenomenon, including thermal edge effects, non-isothermal wellbore flow, and thermal destressing. Results from a coupled Thermo-Hydro-Mechanical-Wellbore (THM-W) model demonstrate how an initially homogeneous reservoir develops preferential flow paths through a powerful self-reinforcing feedback loop. Simulation results show that this process is initiated by the wellbore friction loss and then the thermal edge effect, which causes accelerated cooling and contraction of the rock surrounding the central fractures. This thermo-mechanical opening becomes the dominant flow control mechanism after approximately 28 years, causing the central fracture aperture to widen from 5 mm (compact proppant pack) to over 7 mm and its permeability to increase from 5×10⁻¹¹ m² to 4.06×10⁻⁸ m². This widening creates a high-permeability channel that captures a disproportionate share of the injected fluid—peaking at 9.15 kg/s (30% of total flow)—and leads to premature thermal breakthrough. The analysis further reveals that while propped fractures enhance hydraulic efficiency, they also accelerate this thermal drawdown. A comparison with a simplified Thermo-Hydraulic (TH-W) model confirms that neglecting these geomechanical effects leads to a significant overestimation of the final produced fluid temperature by over 60 K after 50 years.
Topic: Enhanced Geothermal Systems