In Enhanced Geothermal Systems (EGS), hydraulic stimulation is used to improve formation permeability, most often in crystalline rock such as granite. The classical concept of hydraulic stimulation is that a continuous, planar fracture initiates and propagates through the formation. However, in EGS projects, a variety of observations suggest that the classical concept does not apply. In the 1980s, a concept emerged that stimulation occurs through induced slip on preexisting fractures. This concept is now widely accepted in the EGS literature, and the propagation of new fractures is typically not considered to be an important process at most EGS projects. A study at the Rosemanowes EGS project in the 1980s was very influential in promoting the shear stimulation concept, and has been widely cited ever since. The study was based on observations from the stimulation of the well RH12 in November 1982. Microseismicity was induced during injection, and the the cloud of microseismicity was observed to propagate downward. Based on calculations related to the trends in stress with depth, the downward propagation of microseismicity was interpreted as indicating shear stimulation had occurred, not the propagation of new fractures. I investigated this interpretation by performing simulations with a numerical simulator that couples fluid flow with transmissivity evolution and the stresses induced by fracture deformation. The simulation results suggest that the trends in stress with depth are unlikely to have been responsible for the downward propagation of stimulation. The trends in stress with depth are very gradual and are likely to have been overwhelmed by the pressure gradient created in the formation due to flow. The direction of propagation may have been controlled by random heterogeneity in the natural fracture network and the in-situ stress state. Historical observations of microseismicity at EGS projects support the idea that the direction of propagation is strongly controlled by the local idiosyncrasies of the natural fracture network. The microseismic observations are probably inconsistent with the interpretation that a single, planar hydraulic fracture formed, but they do not rule out the possibility that hydraulic fractures formed as part of a network of both new and preexisting fractures.

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