Nonlinear Behaviors within Granite Rough Fracture under Heat-Flow Coupling Effects

In the deep underground, the surrounding environment of the fractured rocks is subjected to coupled thermal–hydrological–mechanical effects. The fluid flow behaviors within the fractured rocks are complicated in such circumstances and require full consideration of these coupled effects. Therefore, we investigate this issue by performing a series of flow tests on artificial fracture granites under the combination of normal stresses (1–10 MPa) and water injection pressure (1–3.5 MPa) at different temperatures. The obtained results show that the flow behaviors within rock fractures are strongly dependent on the thermal effect. The elevated temperature could greatly enhance the permeability of the fractures. The Forchheimer law under the thermal effect is examined, showing a satisfactory fitting result, which indicates that Darcy’s law is not suitable in conditions where the thermal effect must be fully considered. The nonlinear factor is larger for higher temperature cases, implying a stronger nonlinear effect. The onset of nonlinear flow in higher temperature cases appears earlier. These observations related to the thermal effect can be due to two mechanisms: the decreased flow resistance caused by the decrease in water viscosity and the rock temperature-induced fracture closure. Limited to the adopted temperature range, the effect of the former on water flow behaviors dominated over that of the latter in this study. The obtained results are expected to enhance the understanding of water flow in fractured networks and provide helpful guidance for engineering practice.