Enhanced Geothermal Reservoirs with two Fluid Cavities and Unequal Solid and Fluid Temperatures

Thermo-hydro-mechanical (THM) constitutive equations and generalized diffusion and transfer constitutive relations are developed in a comprehensive, coupled and unified framework, assuming a deformable rock formation. Particular attention is laid on both mass and energy exchanges between the cavities which are controlled by the out-of-balances of the chemical potentials and by the out-of-balances of the coldnesses, respectively. Emphasis is laid a) on the mass exchanges between the pore system and the fracture network, which are endowed each with their own pressure, and mainly; b) on the energy exchanges between the rock, the pore network and the fracture network, which are endowed each with their own temperature. Thermo-hydro-mechanical (THM) constitutive equations and generalized diffusion and transfer constitutive relations are developed in a comprehensive, coupled and unified framework, assuming a deformable rock formation. Particular attention is laid on both mass and energy exchanges between the cavities which are controlled by the out-of-balances of the chemical potentials and by the out-of-balances of the coldnesses, respectively. The model is applied to simulate circulation tests using a domestic finite element code. The parameters are calibrated from the thermal outputs of the Fenton Hill and Rosemanowes reservoirs. At variance with a double porosity model with Local Thermal Equilibrium (LTE), the Local thermal Non Equilibrium model (LTNE) displays the characteristic two step time profile that is reported for these two reservoirs. In agreement with field data, fluid loss is observed to be high initially and decreases with time. A sensitivity analysis is performed to determine the influences of the internal length scales, namely fracture spacing and crack aperture, in the complete framework of the dual porosity (2 pressures 2P) and local thermal non equilibrium (3 temperatures 3T) . The fine description of the effective stress, pore and fracture pressures, and solid, pore and fracture temperatures of the most general format (2P-3T) is essentially unchanged when the model is specialized to (2P-2T) with equal pore and solid temperatures. At variance, the quality of the description is degraded for the (1P-2T) model that omits the permeability contribution of the pores, and for the (1P-1T) standard single porosity LTE model. The progressive transition is quantified during circulation tests at the Fenton Hill HDR.