Full coupling of flow, thermal and mechanical effects in COMSOL for simulation of EGS

The effective modeling of geothermal reservoirs requires the coupling of geomechanics, fluid flow and thermal processes. Understanding of the complete system with these coupled processes is vital for reservoir stimulation targeted at enhancing reservoir performance and for the understanding, prediction and prevention of induced seismicity. The injection of cold water leads to alterations in strain and in situ stress in the reservoir, which can facilitate fracture initiation, opening, and activation of faults and joints which can lead to induced seismicity. Many commercial EGS sites have reported significant seismicity upon injection and production. However, thermal effects tend to be neglected in models for reservoir stimulation, although there is strong evidence that they can play an important role. We developed a model for EGS in COMSOL with a full coupling of flow, heat transfer and mechanics. Poroelasticity describes the influence of pore pressure on stress and strain, and in turn, the changes in stress and strain will change permeability and porosity, which influences fluid flow. Heat-convecting fluid flow (cold water injected in hot rock) will influence the temperature distribution, which in turn will influence the fluid viscosity (and density), altering again the flow itself. The temperature change will also create thermal stresses, effecting the geomechanics. In the geomechanical model we used Mohr-Coulomb failure and associated shear dilation for fault reactivation. The model is verified and benchmarked against existing models and is currently being related to actual EGS field operations in France and in Iceland. We have especially investigated the role of temperature changes on the stimulation and production stage of geothermal operations. The first step is to estimate temperature profiles around the injection well and the thermal stress effects. In the next step we compare a model that is fully coupled to a model where thermal effects are neglected and its consequences to prediction of possible induced seismicity. Many authors assume that heat convection can be neglected because of the extremely low fluid flow velocity in rocks such as granite. This means that temperature and heat flux can be calculated separately without the contribution of pore pressure and stresses. We found that the validity of this approach is critically dependent on the subsurface properties (permeability, fault density) and on the operational parameters of the stimulation treatment (flow rates, duration). Coupling of Fluid Flow, Heat Transport and Geomechanics.