Thermal tracing of flow and transport in fractured media

Flow in fractured media is known to be very heterogeneous, both at the fracture scale where aperture fluctuations can imply significant flow channeling, and at the scale of fracture networks, which are characterized by a large distribution of fracture lengths and transmissivities. The coexistence of fast pathways and stagnation zones, dominated by diffusive transport, implies broad residence time distributions. The characterization of flow and transport patterns from pumping or tracer tests in such system is a strongly under-constrained inverse problem. The coupling of several types of data is a possible way to reduce the associated uncertainty. Temperature is a parameter that may have a good potential for providing new constraints on flow heterogeneity, as discussed in recent reviews (Anderson, 2005, Saar, 2011). Furthermore, recent technical developments, such as distributed temperature sensing, allow measuring temperature with high accuracy and fine spatial resolution. Temperature anomalies can be either naturally induced by flow heterogeneities or created by injection of a hot or cold tracer. In the first case, the natural geothermal gradient is perturbed locally by fracture upflow (positive anomaly) or downflow (negative anomaly). Using a borehole scale model of flow and heat transport, we show that perturbed borehole temperature profiles can be used to estimate vertical borehole flow profiles (Klepikova et al., 2011). Thus, we propose a methodology to inverse temperature profiles under ambient, single borehole and cross borehole pumping conditions for estimating fracture hydraulic and connectivity properties. We then discuss the interest of using thermal tracer tests compared to classical tracer tests. We conducted a series of thermal and solute tracer tests at the fractured crystalline aquifer of Ploemeur, France. Thermal tracer tests were performed by injecting continuously 50 degrees Celsius water in a fracture located at 50 meters depth. The breakthrough curves measured in an adjacent borehole show a significant time lag between the thermal and solute breakthrough due to the large coefficient of heat diffusion compared to molecular diffusion. Combining heat and solute tracer tests allows measuring tracer dispersion with Peclet numbers varying over orders of magnitude, thus providing important constraints on the effective transport behavior. References: - Anderson MP (2005), Heat as a ground water tracer. Ground Water 43(6):951-968 - Saar M. O. (2011), Review: Geothermal heat as a tracer of large-scale groundwater flow and as a means to determine permeability field, Hydrogeology Journal, 19: 31-52 - Klepikova, M., T. Le Borgne., O. Bour, P. Davy (2011), A methodology for using temperature-depth profiles under ambient, single and cross-borehole pumping conditions to estimate fracture hydraulic properties, Journal of Hydrology 407, 145-152