Managed Saffman-Taylor instability during overflush in hydraulic fracturing

Based on fracture mechanics, we analyzed how large a portion of the fracture may be left unsupported before it is severely pinched during drawdown. A parametric study was performed on the displacement of the yield-stress slurry by the overflushing fluid. Qualitatively, when fingers of the overflushing fluid can be created at the overflush/slurry interface, large slurry pillars are preserved in the near-wellbore area, which may keep the fracture open. Three main scenarios of fluids distribution are identified and classified in terms of the oveflush fluid-to-slurry viscosity ratio ξ: (i) the slumping-dominated regime, where light clean overflush fluid goes on top of sedimenting heavy proppant-laden suspension (ξ≳0.1), (ii) an intermediate scenario of slumping combined with fingering (ξ∼0.01), and, finally, (iii) a pure fingering-dominated scenario, when the slurry viscosity is high (ξ<10−3). The third scenario minimizes the geomechanical risks of overflushing by providing tiny fingers that are unlikely to be pinched out during fracture closure.