Dynamics of permeability evolution in stimulated geothermal reservoirs

Spatially and temporally evolving permeability fields are fundamentally associated with the operation of enhanced geothermal systems. This chapter explores the resulting magnitude and patterns of permeability alteration as well as the coupled physical processes that control the evolution of permeability during shear stimulation and long-term evolution of a geothermal reservoir. It also explores mechanisms involved in the enhancement of fracture permeability, targeting injection pressures that encourage self-propping shear stimulation alongside elastic pressure and thermal dilatation of fracture sets. Simulation of a short-term, high-injection-rate enhanced geothermal system stimulation explored the importance of hydraulic versus thermal effects and the impact of a temperature differential between fluid in the fractures and the adjoining rock matrix. The effects of variations in fracture spacing on the evolution of permeability is at least as important as the magnitude of initial permeability, and quantifying the driving mechanisms is critical to understanding reservoir response.