Abstract
The thermal conductivity of fractured rock masses is an important parameter for the analysis of energy geosystems, yet, its measurement is challenged by specimen size requirements. Fluids within fractures have lower thermal conductivities than rock minerals and heat flow lines constrict through contacting asperities. Together, heat flow constriction and phonon boundary scattering cause an apparent temperature discontinuity across the fracture, typically represented as a thermal contact resistance. We investigate the thermal contact resistance in fractured limestone and its evolution during loading and unloading (sigma '=10 kPa to sigma '=3000 kPa) for clean and gouge-filled fractures, under both air-dry and water-saturated conditions. The fracture thermal contact resistance decreases during loading because of the increase in the true contact area, gouge and asperity crushing, and fracture filling by produced fines that contribute new conduction pathways. These processes convey high stress sensitivity and loading hysteresis to the fracture thermal contact resistance. Water fills the fracture interstices and forms menisci at mineral contacts that significantly improve heat conduction even in partially saturated rock masses. The rock mass effective thermal conductivity can be estimated by combining the intact rock thermal conductivity with measurements of the thermal contact resistance of a single fracture under field boundary conditions.