Heat Transfer in Fractures

Hydro-thermal reservoirs can experience local thermal non-equilibrium situations when the flowing fluid and the surrounding host rock locally diverge in temperatures. As an example, such cases can occur around boreholes when cold water is injected into a warm system, and it takes a finite amount of time and flow distance until thermal equilibrium between phases is obtained. Describing heat transfer explicitly is a challenging task, especially in fractured reservoirs, because the heat transfer coefficient of fractures depends on various parameters, such as flow velocity and aperture. The heat transfer coefficient must be derived from laboratory measurements of the outflow temperature requiring analytical or numerical solutions to account for heat flow inside the rock matrix and the flowing fluid. Based on a newly developed analytical solution to determine the heat transfer coefficient of single fractures, in this work the effect and relevance of fracture network topology and the combination of fractures in general has been investigated. Through multiple flow paths within a reservoir, heat transfer capabilities of individual fractures are less relevant and correspond well to uncertainties in the heat transfer coefficient and its transfer from laboratory to field scale. Heat transfer processes at various scales require different set of parameterizations preferably consistent across scales. This work will present first approaches to transfer findings on single fractures at laboratory scale to field scale.