Numerical simulation of reservoir stimulation with reference to the Newberry EGS

Heat production from an enhanced geothermal system (EGS) requires a successful stimulation. Cold water is injected into hot rocks to enhance the heat reservoir permeability. During injection, pore pressure, temperature and the stress field in the reservoir change significantly. Most hot dry rock masses are to some extent naturally fractured at various scales. Fractures could dilate and slip in shear possibly and propagate as a result of stress changes, increasing reservoir permeability. The objective of this work is to analyze the response of a naturally fractured heat reservoir to water injection. A fully coupled thermo-poroelastic finite element model is developed and used to describe the interaction between fluid flow, rock deformation, and heat transfer within the fractured rock. A fracture network model is generated based on field data and implemented into the coupled model. The model is used to simulate and analyze reservoir stimulation in the Newberry EGS Demonstration. In particular, the Phase 2.2 stimulation is modeled using field data on the fracture network, in-situ stress and laboratory data on rock and fracture properties. The simulated injection profile, the evolution of permeability and induced micro-seismic events agree with field observations. Simulation results also show that injection induced stress state changes occur around the injection well explaining the noted differences between the stress regimes obtained prior and subsequent to stimulation.