Thermo-Hydro-Mechanical Modeling of Enhanced Geothermal Systems Using COMSOL Multiphysics

Abstract

The operation of Enhanced Geothermal Systems (EGS) is intended to stimulate the permeability of a network of natural fractures. High pressures and low temperatures of the injected water cause the fractures to slip and to open, thus providing additional injectivity. The modeling of EGS requires the coupling of geomechanics, fluid flow and thermal processes. Understanding of the complete system with these coupled processes is vital, not just for reservoir stimulation targeted at enhancing reservoir performance, but also for the understanding, prediction and prevention of induced seismicity. The injection of cold water and extraction of hot water and steam leads to alterations in in-situ stresses and strains in the reservoir, which can result in fracture initiation, opening, and activation of faults and joints leading to induced seismicity. Indeed, many EGS sites (e.g. Basel, Soultz and the Geysers) have shown significant seismicity upon injection and production. It is generally understood that poromechanical and thermomechanical processes cause a fracture opening and slip and that induced seismicity is also related to water flow and enhanced permeability. However, thermal effects tend to be neglected in models for reservoir stimulation, although there is strong evidence that they can play an important role. In this work we investigated the coupling of all processes, and especially the influence of temperature changes on the full system. Our coupled model was built on basic physical laws and from that physical standpoint we aimed to evaluate induced seismicity. The platform which we used to build the coupled model was COMSOL Multiphysics. Full coupling of flow, heat transfer and mechanics was implemented using three COMSOL built in modules: Darcy’s Law, Solid Mechanics and Heat Transfer in Porous Media. For demonstration, we set up a model with a fracture zone in which the flow was concentrated. This zone was given reduced strength properties when compared to the surrounding matrix, to be able to simulate fault activation. Both the effective normal pressures in the fault zone and shear failure were taken into account to calculate an effective fracture opening – which on its turn impacted the flow by changing storage capacity and the permeability in the fault. Frictional shear weakening and healing was implemented to induce seismic events of larger magnitudes, globally staying in line with fault rupture models and current dynamic seismicity models. The thermal part of the modelling included both heat flow and convection. The induced seismicity is directly proportional to the seismic moment which depends on failing area, shear displacement and shear modulus, and can be calculated using COMSOL. This way, geomechanical coupled modeling can be related to seismic hazard assessment which leads to understanding, predicting and preventing undesired seismicity. The model was used to perform a sensitivity analysis on parameters like temperature of injected fluid, in-situ stress regime (or depth), heterogeneity of the fracture zone and its strength, allowing us to evaluate the trends of their impact on fracture reactivation, seismic moment and moment magnitudes.