Enhanced Geothermal Systems (EGS) are geothermal resources that are developed through hydraulic stimulation. Inadequate permeability and production from natural fractures and pores can be overcome via injection of cold water below fracturing pressure, by conventional hydraulic fracturing, or by some cyclic combination of these processes. At low injection rates or where thermal fracturing is being exploited, shearing of pre-existing weaknesses and potentially developing virgin fractures is envisioned to provide permeable, self-propped pathways. Alternatively, injection at pressures substantially above the minimum principal stress can also hydraulically connect, reopen, or create fractures and also possibly induce shearing (as known from microseismic monitoring). The heat from this artificially fractured reservoir is subsequently transferred to the injected fluid and extracted through a production well. Conventional steam turbines or binary cycle power plant can be employed for electric generation. The technical challenges in developing EGS reservoirs are substantial and include controlling fracture direction and morphology, establishing an adequate heat transfer surface area, and maintaining conductivity. The latter was the focal point of investigation. If tensile fractures are generated, they need to be explicitly held open by proppant (and it needs to be ensured that the proppant is not produced back into the wellbore). The conductivity of bauxite propped fractures over extended periods of time and at elevated temperatures were measured in laboratory tests in order to assess the temporal and thermal dependency of conductivity in a typical surrogate fracture.
Laboratory fracture conductivity tests were performed to evaluate the benefits and longevity of fractures propped by asperity override and whether this supplemental conductivity is preserved at temperature and over time. Conductivities from the asperity propped fractures were compared to the conductivity of proppant filled fractures to determine the need for proppant in EGS systems.