Effects of temperature-dependent viscosity on thermal drawdown-induced fracture flow channeling in enhanced geothermal systems

Harnessing geothermal energy from an enhanced geothermal system (EGS) highly depends on fracture flow and heat transport processes. Thermal drawdown-induced thermal stress has been characterized as a major reason for severe fracture flow channeling (short-circuiting), which further leads to premature thermal breakthrough and impairs long-term thermal performance. In the present study, we quantitatively analyzed a potential flow channeling mitigation mechanism, i.e., the increase of water viscosity with temperature reduction. Through a field-scale single-fracture EGS model that incorporates thermal-hydro-mechanical coupled processes and temperature-dependent water viscosity, we demonstrate that the increase of water viscosity during heat extraction promotes a dispersed fracture flow pattern, which can effectively mitigate thermal drawdown-induced flow channeling and improve long-term thermal performance. The mitigation effect is more noticeable for homogeneous aperture scenarios than for heterogeneous aperture scenarios, especially in the early period of heat production. With a higher in-situ stress and smaller rock Young's modulus, the flow channeling effect of thermal stress becomes weak, and therefore the temperature-dependent viscosity exhibits a more significant flow channeling mitigation effect. The results from the current study provide valuable insights into the optimization of fracture flow and heat transport to achieve more efficient and sustainable energy production from EGSs.