COUPLED MULTIFLUX-TOUGH2-TOUGHREACT T-H-M-C MODEL FOR EGS STUDIES

We have developed a coupled numerical model for simulating flows of coolant mass, as well as transport of heat and chemical species, in Enhanced Geothermal System (EGS) reservoirs with discrete fractures. A Computational Fluid Dynamics (CFD) submodel in MULTIFLUX solves for laminar or turbulent flows in any planar-, penny-, or lens-shaped fracture. The CFD submodel includes advective, convective, and diffusive transport of heat and mass in a multicomponent mixture within a fracture. Discrete fractures may form a network of planar fractures and connecting conduits. The fracture aperture of each planar configuration is variable with space and time, due to (a) thermal dilatation of the strata, (b) elastic deformation by hydraulic pressure in the fracture system, and (c) geochemical precipitation and dissolution. The new concept is used in the formulation of a mechanical model for aperture variation in a selfpropped fracture with hydrodynamic pressure and temperature. The CFD submodel is coupled to the submodel for the host geothermal formation, a porous, fractured, and jointed rock mass. Multiphase, multicomponent heat and fluid flow simulations are provided by TOUGH2 and/or TOUGHREACT in the geologic submodel. Coupling of the overlain fracture network system in the CFD model-element to the host geothermal formation involves the Numerical Transport Code Functionalization (NTCF) technique, a modeling accelerator of the iterations in MULTIFLUX. Our new model can be used to interpret short-term injectivity test results and help in evaluating fracture aperture and planar extension. An industrial application example (and validation) of the model is provided for the Desert Peak EGS site in Nevada, operated by ORMAT. The model is also applicable to long-term thermal drawdown simulations and life-cycle studies.