Modeling the Impact of Multiple Physical Processes on the Stimulation and Longevity of Enhanced Geothermal Systems

The evolution of enhanced geothermal systems (EGS) entails spatially and temporally evolving permeability fields. During non-isothermal fluid injection, thermo-elastic stress and fluid pressure changes act upon partially open or hydrothermally altered fracture sets to enhance formation permeability. The physical couplings that drive this behavior are non-linearly dependent upon one another to varying degrees. To explore these interactions we are developing a thermo-hydromechanical (THM) simulator capable of coupling the dominant physics of shear stimulation and allowing flexibility in the use of monolithic or staggered numerical schemes. Permeability is allowed to evolve under several constitutive models tailored to both porous media and fractures, considering the influence of thermo-hydromechanical stress, creep, and elasto-plastic shear and dilation in a ubiquitously fractured medium. From this basis we explore the coupled physical processes that control the evolution of permeability during shear stimulation and long-term evolution of a geothermal reservoir. Previous attempts to model the stimulation and sustainability of EGS are discussed in their relation to the current work and in an attempt to elucidate the dominant mechanisms of permeability alteration, including order-of-magnitude differences in permeability gain during hydraulic stimulation in isothermal versus non-isothermal simulations.