Here, we consider Enhanced Geothermal Systems (EGS), which rely on increased permeability and artificially created fracture networks in the subsurface, thus increasing the efficiency of geothermal power plants. By injecting cold supercritical working fluid (usually H2O or CO2) into the “damaged matrix”, new fractures are created. This dynamically changing fracture system allows the working fluid to flow efficiently through the reservoir and to extract thermal energy at a higher rate.

We present a modeling framework for such geothermal reservoirs, which is based on a hierarchical approach, i.e. a discrete representation is employed to model flow and transport through large fractures and a continuum representation is used for the huge number of small fractures (damaged matrix). In this way, the addition of new fractures during enhancement is computationally inexpensive, since whenever a new fracture is created, a small number of control volumes is added to the existing mesh thus allowing the flow computation for dynamically changing reservoirs. Important criteria are the accurate and efficient coupling between flow and transport calculations, consistent transfer of mass and energy between the continuum and the discrete fracture representations, treatment of heat conduction in the rock and across the interfaces with the fractures, inclusion of the integral effects on the permeability due to geochemistry, and of course a general interface with a geomechanics module. Moreover, the framework has to be suitable for both the creation and production phases.

Here, it is explained how the coupling of flow and transport in the discrete and continuous fracture representations in combination with heat conduction in the rock can be modeled. Various test cases representing scenarios during reservoir creation and production are considered and the resulting pressure and temperature evaluations are discussed.

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