The impact of surface roughness on heat transport in fractured rocks

Understanding heat transfer in rock fractures is crucial for optimizing geothermal energy extraction, nuclear waste storage, and other subsurface engineering applications. In geothermal systems, the understanding of thermal behaviour in fractured media is still challenging, due to the complexity of fracture geometry, heterogeneous properties of the fractures and the host rock, and varying fluid flow dynamics influenced by temperature-dependent fracture aperture. Considering that the aperture and shape of fractures can promote preferential transport of fluids and heat, several numerical and experimental studies have demonstrated that these preferential paths, or “flow channeling,” significantly impact heat transfer. However, there is no clear consensus on the effects of flow channeling on the thermal exchange between the fluid and the rock matrix, as some authors observed a decrease, due to increased flow velocity and shortened transit times in the channeled regions, while others report an increase, as radial conduction from the channel to the matrix is more efficient for heat transfer than the linear conduction assumed in a parallel plate model. This study explores the relationship between fracture roughness and heat transfer mechanisms, focusing on advective and diffusive processes under saturated conditions. Finite element numerical models are employed to simulate fluid flow and heat transfer in a set of simplified fracture geometries in which the fracture walls are represented through a sinusoidal function. These models include three scenarios: a fully-mated fracture geometry formed by two aligned sinusoidal surfaces, a fully-unmated configuration, and an intermediate geometry that transitions between the two mentioned geometries. Preliminary results indicate that surface roughness influences convective heat transfer by inducing localized flow channeling. This effect is quantified by observing the thermal attenuation and the lag time of the induced cold pulse imposed over the system. Notably, depending on the fracture geometry, distinct temperature peaks and varying heat recovery tailing profiles are observed across different scenarios. Further work is needed to define appropriate model dimensions, select suitable heat and flow parameters, and refine the time discretization. Additional numerical experimentation is required to determine the optimal approach for modelling the fracture, such as choosing between a function-based or fracture-based representation.