Assessing the Influence of Ground Water Inflow on Thermal Conductive Heating in Fractured Rock

Thermal conductive heating (TCH) is an innovative remediation technology developed for aggressive in-situ non-aqueous phase liquid (NAPL) source zone treatment. Electrical heater wells are used to conductively transfer heat into the subsurface, thereby elevating temperatures and boiling both ground water and NAPLs. The design of a TCH remediation system needs to consider the cooling effect of incoming ground water. Previous studies, which have focused only on porous media, have used numerical modeling to determine the influence of this cooling. Results from these studies show that in some cases, inflowing ground water may delay or prevent the treatment zone from reaching the target temperature. Proposed solutions for mitigating the impact of influent cooling include the installation of impermeable barriers, steam injection wells, or pumping wells at the periphery of the treatment zone. To date, no study has examined the cooling influence of incoming ground water in fractured rock environments in the context of TCH. In the present study, a new semianalytical solution to the heat equation is derived and utilized to study the influence of fracture properties and ground water flow rates on heat transfer in fractured rock. The semianalytical solution is more computationally efficient than a fully numerical model, permitting a detailed evaluation of parameter sensitivity. In this study, the relative importance of rock type, fracture spacing, fracture aperture, and hydraulic gradient are investigated. Rock properties were concluded to be far less significant than the hydrogeological parameters. In contrast, the subsurface temperature distribution may be entirely governed by the presence of high-aperture fractures, a large hydraulic gradient, or close fracture spacing. In addition to investigating the influence of both rock properties and hydrogeological properties on temperature distribution, two potential methods for mitigating the effect of inflow cooling were investigated. Although the installation of upgradient preheating wells can help to offset the heat loss from the fracture, it appears to be far more effective to simply increase the power of the thermal wells in the treatment area.