In simulation of fluid injection in fractured geothermal reservoirs, thecharacteristics of the physical processes are severely affected by the localoccurence of connected fractures. To resolve these structurally dominatedprocesses, there is a need to develop discretization strategies that also limitcomputational effort. In this paper we present an upscaling methodology forgeothermal heat transport with fractures represented explicitly in thecomputational grid. The heat transport is modeled by an advection-conductionequation for the temperature, and solved on a highly irregular coarse grid thatpreserves the fracture heterogeneity. The upscaling is based on differentstrategies for the advective term and the conductive term, respectively. Thecoarse scale advective term is constructed from sums of fine scale fluxes,whereas the coarse scale conductive term is constructed based on numericallycomputed basis functions. The method naturally incorporates a coupling betweenthe matrix and the fractures via the discretization, so that explicit transferterms that couple solution variables in the fractures and the matrix areavoided. Numerical results show that the upscaling methodology performs well,in particular for large upscaling ratios, and that it is applicable also tohighly complex fracture networks.
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