Many geothermal fields are naturally fractured systems. In these reservoirs, physical parameters can be heterogeneous and randomly distributed. Classic double porosity models the flow between matrix and fractures, under the hypothesis that petrophysical properties are uniform in each medium. Fractures have the largest permeability and drive the fluid toward the wells. The matrix, with smaller permeability, only acts as a source of fluid for the fractures. Under the concept of multiple porosity- permeability, M continuous porous media interact with each other. Every medium has its own parameters and its own interporosity flow, which can be non-isothermal, stationary, or transitory. Double porosity models can be classified as special cases of this general theoretical concept, applicable to all class reservoirs. In such heterogeneous systems, the numerical simulation of heat and mass coupled flows requires to average physical properties, which sometimes are highly contrasting, independently of the method used in the solution of equations. One crucial problem in multiple porosity reservoirs, is what kind of average should be used to represent parameters in the global transport processes among different media. Irregular spatial distribution of petrophysical parameters affects both mass and energy flows, and the thermodynamic evolution and mechanical behavior of the whole system. Its influence can be as decisive as relative permeabilities or capillary pressure.

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