Using a fully coupled, open-source THM simulator to examine the role of thermal stresses in shear stimulation of enhanced geothermal systems

摘要

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 simulator capable of coupling the dominant physics of shear stimulation using a variety of methods, allowing flexibility in the use of monolithic or staggered numerical schemes. The new simulator uses standard Galerkin and control-volume finite elements to balance fluid mass, mechanical deformation, and thermal energy with consideration of local thermal non-equilibrium and/or dual-porosity heat exchange between fluids and solids or fractures and intact rock. Similarly, changes in mechanical stress and fluid pressure can be rigorously coupled in single or multiple continua. 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.