Modeling Shear Stimulation of the Desert Peak EGS Well 27-15 Using a Coupled Thermal-Hydrological-Mechanical Simulator

A numerical model has been developed to describe injectivity improvements at well 27-15 in the Desert Peak geothermal field, Nevada, under shear stimulation conditions. A Mohr-Coulomb failure criterion, coupled with an empirical fracture permeability law based on laboratory experiments, describes permeability evolution of a fractured rock volume under applied pore pressure and stresses, including thermal contributions. The model includes synthetic fracture populations acted on by an ambient stress field based on well-bore image logging and in situ stress measurements from well 27-15. The permeability model is implemented in the software code FEHM (Finite Element Heat and Mass transfer) and applied to the problem of injection at well 27-15 for four pressure steps less than the least principal stress. The calibrated model matches measured injection rates at all four stimulation pressures (1.5, 2.2, 3.1 and 3.7 MPa), capturing the timing and magnitude of injectivity increases observed during the 3.1 MPa step. For the observed extensional stress state and under intermediate injection pressures, the model initially favors greatest permeability enhancement parallel to the maximum horizontal principal stress, a moderate vertical enhancement, and a smaller gain parallel to the minimum horizontal principal stress.