Heat transport in deforming fractured rocks: the effects of fracture slip and opening

Heat transfer in fractured rocks is a key process for deep geothermal energy exploitation. Fractures represent the main pathways for fluid flow and advective heat transport, while diffusive thermal exchange occurs between the fluid within the fractures and the surrounding host rock. These two processes occur over very different spatial and temporal scales, and their variability is strongly influenced by fracture–rock heterogeneity, which ultimately controls geothermal performance.

In this work, we discuss two transient mechanical processes that can alter the geometry of fractured rocks during fluid circulation, thereby affecting heat transport and, consequently, the efficiency of geothermal plants. The first process involves flow channeling induced by shear slip activation in critically stressed fractures. We analyze this phenomenon at the single-fracture scale. Using analytical solutions and numerical simulations, we investigate the thermal response to the injection of a cold temperature pulse into a rough fracture, considering both synthetic and real heterogeneous aperture fields. The results reveal that fracture roughness has a significant influence on heat transport, with post-peak tailings of the breakthrough curves showing an anomalous transient decay rate in time before evolving toward the asymptotic regime with a -3/2 decay rate, which is characteristic of fracture-matrix diffusive heat exchange. This behavior is sensitive to variations in the fracture aperture field caused by the activation of relative sliding between fracture surfaces, with larger slips leading to earlier temperature peaks and delayed transitions to the asymptotic diffusive regime.

The second process focuses on cooling-induced thermal contraction of the rock surrounding the fractures, which tends to increase fracture aperture and directly affects fluid flow and advective heat transport. We analyze this phenomenon at the scale of the fractured rock mass. By means of a hybrid methodology that combines an analytical model with a particle tracking approach applied to Discrete Fracture Networks (DFNs), we numerically investigate the impact of cold fluid circulation in systems of fractures with different characteristics. Results show that rock contraction accelerates the advective transport resulting in a faster recovery of cold fluid at the outlet.

These analyses allow identifying the characteristics of fractured rocks that are most critical for heat transport under the occurrence of fracture slip and opening. This understanding is crucial to control the performance and lifetime of geothermal exploitations.