Thermal Effect Promotes Non-Darcian Flow in Heated Rock Fractures

Water flow through fractured rocks in high-temperature settings is relevant to many subsurface geo-energy engineering projects including geothermal energy extraction, hydrocarbon production, and nuclear waste disposal. However, existing studies have either focused only on flow process within fractures at room temperature or on the thermal effect on fracture permeability. As a result, how and to what extent rock temperature affects the flow behavior in rock fractures remains poorly understood. Herein, we report an experimental study of the heated flow behavior in rock fractures at steady-state elevated temperatures under sequentially increasing flow rates. We find that rock temperature greatly affects the flow behavior within the fracture via two mechanisms: water properties variations and fracture closure. The former reduces the flow resistance, while the latter both reduces the effective hydraulic aperture and enhances the inertial effect. The more heterogeneous the initial aperture distribution is, the more significant the effect of the temperature-induced fracture closure on the flow nonlinearity will be. By combining the Forchheimer equation with water temperature measured at the fracture inlet and outlet, we quantitatively evaluated the effect of these two mechanisms on rock fracture permeability and flow nonlinearity. Finally, we develop an empirical expression for the critical hydraulic gradient of the fracture at different temperatures based on the nonlinear flow model for room-temperature fractures to quantify the emergence of non-Darcian flow. The findings from the present study are expected to shed insight into the prediction and evaluation of fluid flow within deep subsurface fractured reservoirs.