HGS RESEARCH HIGHLIGHT – Heat Tracing in a Fractured Aquifer with Injection of Hot and Cold Water
Hoffmann, R., Maréchal, J., Selles, A., Dassargues, A., & Goderniaux, P. (2021). Heat Tracing in a Fractured Aquifer with Injection of Hot and Cold Water. In Groundwater (Vol. 60, Issue 2, pp. 192–209). Wiley. https://doi.org/10.1111/gwat.13138
“HGS calculates fluxes coming from the different medium sections into the well, as a function of the implemented hydraulic conditions (Therrien et al. 2010). This allows properly considering the different hydrogeological conditions during the April and August experiments, with the activation or deactivation of the upper fractures in fracture zone 1.”
Fig. 7. Explanation of the model discretization. (a) The mesh with the refinement sections. (b) Schematic cross-section of the vertical plane at the position y = 50 m of the 3D numerical model. Note, “m bgs” stands for meters below ground surface. A discretely fractured porous media is modeled with HydroGeoSphere.
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In this comprehensive study, researchers explore the application of heat as a tracer in fractured porous aquifers, offering new perspectives on groundwater flow and transport dynamics. The research paper investigates the use of hot (50 °C) and cold (10 °C) water injections in a weathered and fractured granite aquifer, where the natural background temperature is 30 °C.
The study demonstrates the potential of temperature as a tracer to delineate fracture-matrix processes, providing valuable insights into groundwater behaviour. By analyzing temperature evolution over time, researchers were able to differentiate between fracture and matrix characteristics, shedding light on thermal diffusion mechanisms within the aquifer.
The breakthrough curves obtained from hot and cold water injections, under varying hydraulic conditions, revealed intriguing tailing behaviours. Notably, the tail slopes of these curves, when plotted on a log-log scale of time versus normalized temperature difference, were close to 1.5, contrasting with traditional solute tracer tests.
Utilizing a process-based numerical model, researchers explored the impact of heat conduction on groundwater heat transport, considering factors such as fracture aperture adjustment and temperature-dependent water density and viscosity. The model successfully reproduced observed breakthrough curve tail slopes, providing valuable insights into thermal transport properties in fractured aquifers.
HydroGeoSphere (HGS), a sophisticated modeling platform known for its ability to simulate coupled surface water-groundwater interactions with unparalleled accuracy, played a crucial role in this study- as researchers leveraged the advanced capabilities of HGS to simulate the injection experiments and analyze the complex thermal processes occurring within the fractured aquifer.
One of the key findings of this study is the distinct characteristics observed between hot and cold water injection experiments. While peak arrival times and initial breakthrough behaviors were similar, differences in thermal recovery rates were attributed to temperature-induced changes in fluid properties and thermal conductivity values.
Fig. 9. Simulated temperature distributions as differences corresponding to the hot (50 °C) and cold (10 °C) water tracer experiments performed in April 2019, at the end of the injection period. Zoom in the vertical plane for (a) the hot water and (b) the cold water injections. The extensions of the impacted areas are similar.
The research highlights the feasibility and utility of cold water tracer tests in warmer fractured aquifers, offering complementary information to traditional solute tracer tests. The findings showcase the importance of considering temperature effects on fluid properties when interpreting thermal tracer data, particularly in aquifers with naturally elevated groundwater temperatures. The study showcases the potential of thermal tracer testing as a valuable tool for characterizing fractured aquifers. Ultimately, this study provides valuable insights into thermal tracer testing as a tool for characterizing fractured aquifers, contributing to ongoing efforts in groundwater management and environmental protection.
Plain Language Summary:
A recent study investigated the use of heat as a tracer to understand how water moves through fractured rocks underground. By injecting hot and cold water into a granite aquifer with a naturally warm temperature, researchers were able to see how heat moved through the rocks and water over time. They found that the way heat traveled was different from traditional methods using chemicals as tracers. The study used HydroGeoSphere (HGS), an advanced computer model, to simulate these heat movements, providing valuable insights into how groundwater behaves in fractured rocks. One important discovery was that cold water injections could be especially useful in aquifers with naturally warm water. Understanding these thermal processes can help scientists better manage groundwater resources and protect the environment. Overall, this research highlights the potential of using heat as a tracer for studying underground water systems.
Abstract:
Heat as a tracer in fractured porous aquifers is more sensitive to fracture-matrix processes than a solute tracer. Temperature evolution as a function of time can be used to differentiate fracture and matrix characteristics. Experimental hot (50 °C) and cold (10 °C) water injections were performed in a weathered and fractured granite aquifer where the natural background temperature is 30 °C. The tailing of the hot and cold breakthrough curves, observed under different hydraulic conditions, was characterized in a log–log plot of time vs. normalized temperature difference, also converted to a residence time distribution (normalized). Dimensionless tail slopes close to 1.5 were observed for hot and cold breakthrough curves, compared to solute tracer tests showing slopes between 2 and 3. This stronger thermal diffusive behavior is explained by heat conduction. Using a process-based numerical model, the impact of heat conduction toward and from the porous rock matrix on groundwater heat transport was explored. Fracture aperture was adjusted depending on the actual hydraulic conditions. Water density and viscosity were considered temperature dependent. The model simulated the increase or reduction of the energy level in the fracture-matrix system and satisfactorily reproduced breakthrough curves tail slopes. This study shows the feasibility and utility of cold water tracer tests in hot fractured aquifers to boost and characterize the thermal matrix diffusion from the matrix toward the flowing groundwater in the fractures. This can be used as complementary information to solute tracer tests that are largely influenced by strong advection in the fractures.