Field observations at the Fenton Hill enhanced geothermal system test site support mixed-mechanism stimulation

The Fenton Hill enhanced geothermal system (EGS) test site was the first of its kind, and interpretations of field observations from the project have influenced the past four decades of EGS development. In this study, we hypothesized that stimulation (i.e., permeability enhancement) in the Fenton Hill reservoir occurred through a mixed-mechanism process that involved propagation of hydraulic splay fractures encouraged by the stress changes induced as natural fractures opened and failed in shear. We used a hydromechanical fractured reservoir numerical model to validate the efficacy of the mixed-mechanism stimulation conceptual model. Our modeling results were consistent with the observations recorded during the Fenton Hill field experiments in three distinct ways: (1) a marked increase in injectivity occurred at a threshold injection pressure, (2) the near wellbore injectivity enhancement following each stimulation treatment was reversible, and (3) seismicity propagated in a direction that was inconsistent with the orientation of the maximum principal stress, despite injection having occurred at pressures significantly above the fracturing pressure. The modeling results demonstrate that several independent hydromechanical observations could be replicated by the mixed-mechanism stimulation conceptual model. In contrast, the observations could not be explained by a pure mode-I hydraulic fracture propagation nor by pure shear stimulation. Distinct fracture sets are activated through the mixed-mechanism stimulation process; the natural fractures provide most of the heat transfer surface area, and the tensile splay fractures form the bulk of the fluid storage volume. Future EGS projects could take advantage of mixed-mechanism stimulation to design wellbore completion and reservoir engineering and strategies to increase effective transmissivity, improve heat mining efficiency, and extend useful reservoir lifetime.