THM Modelling of Hydrothermal Circulation in Deep Geothermal Reservoirs

Abstract

Numerous models have been developed for describing deep geothermal reservoirs. Using the open source finite element software ASTER developed by EDF R&D, we carried out 2D simulations of the hydrothermal circulation in the deep geothermal reservoir of Soultz-sous-Forêts. The model is based on the effective description of Thermo-Hydro-Mechanical (THM) coupling at large scale. Such a model has a fourfold interest: a) the physical integration of laboratory measurements (rock physics), well logging, well head parameters, geological description, and geophysics field measurements; b) the construction of a direct model mechanically based for geophysical inversion: fluid flow, fluid pressure, temperature profile, seismicity monitoring, deformation of the ground surface (INSAR/GPS) related to reservoir modification, gravity or electromagnetic geophysical measurements; c) the sensitivity analysis of the parameters involved in the hydrothermal circulation and identification of the dominant ones; d) the development of a decision tool for drilling planning, stimulation and exploitation. In our model, we introduced extended Thermo-Hydro-Mechanical coupling including not only poro-elastic behavior but also the sensitivity of the fluid density, viscosity, and heat capacity to temperature and pressure. The behavior of solid rock grains is assumed to be thermo-elastic and linear. Hydraulic and thermal phenomena are governed by Darcy and Fourier laws respectively, and most rock properties (like the specific heat at constant stress Cs (T), or the thermal conductivity λ(T,φ)) are assumed to depend on the temperature T and/or porosity φ. The radioactivity of the rocks is taken into account through a heat source term appearing in the balance equation of enthalpy. To characterize as precisely as possible the convective movement of water and the associated heat flow, water properties (specific mass ρw (T,pw ), specific enthalpy hmw (T,pw ) dynamic viscosity µw (T), thermal dilation αw (T), and specific heat Cwp (T)) are assumed to depend on pressure and/or temperature. The entire set of material properties is extracted from references dealing with investigations at Soultz-sous-Forêts when existing. The reservoir is described at large scale (about 10 km in width and 5 km in height) and it is assumed that the medium is homogenous, porous, and saturated with single-phase fluid (considering homogenized effective porous and/or fractured layers, neglecting the details of the fracture networks). We performed a feasibility study and show that a large scale convection regime is possible using realistic parameters. The size of the convection cells (2.8km) is shown to be compatible with field observations. An on-going work addresses the stability of the natural convection cells with respect to major local faults, dip of the sedimentary layers and fluid injections.