Conferences - Seminars

  Thursday 6 December 2018 16:15 - 17:30 MED 2 2423

MEchanics GAthering -MEGA- Seminar: Talk1 - A semi-infinite hydraulic fracture driven by a shear-thinning fluid; Talk2 - Evolution of aseismic and seismic slip on a pressurized dilatant fault with frictional weakening properties

By  Fatima-Ezzahra Moukhtari and Federico CiardoGEL, EPFL

A semi-infinite hydraulic fracture driven by a shear-thinning fluid by Fatima-Ezzahra MoukhtariGEL, EPFL
Abstract Complex fluids are often used in hydraulic fracturing practice due to their interesting shear thinning properties that notably minimize pressure losses when pumped down the wellbore while allowing for large fracture width during hydraulic fracture propagation. We focus here on the near-tip region of a hydraulic fracture where most of the solid-fluid non-linearities reside. We solve the coupled problem of a semi-infinite hydraulic fracture propagating at a constant velocity in an impermeable linearly elastic material. We allow for the occurrence of a region without fluid of a-priori unknown length at the fracture tip. We use the Carreau rheological model - which we compare with simpler rheological models - in order to properly account for the shear thinning of fracturing fluid between the low and large shear rates Newtonian limits. The solution exhibits a complex structure with up to four distinct asymptotic regions: a region governed by the classical linear elastic fracture mechanics behaviour near the tip, a high shear rate viscosity asymptotic and power-law asymptotic regions in the intermediate field and a low shear rate viscosity asymptote far away from the fracture tip. The occurrence and order of magnitude of the extent of these different asymptotic regions are obtained analytically via scaling arguments. Our results quantify how shear thinning drastically reduces the size of the fluid lag compared to a Newtonian fluid.
F. E. Moukhtari and B. Lecampion. A semi-infinite hydraulic fracture driven by a shear thinning fluid. Journal of Fluid Mechanics, 838:573–605, 2018.

Evolution of aseismic and seismic slip on a pressurized dilatant fault with frictional weakening properties by Federico CiardoGEL, EPFL
Abstract Aseismic slip associated with shear crack activation on a pressurized fault may or may not turn into seismic slip depending on in situ conditions, frictional properties, value of injection over-pressure and dilatant compliance fault behavior. By focusing on a “young” fault, for which the mechanically weak gouge unit accommodating slip is also the most permeable, we develop a planar bi-dimensional fault hydro-mechanical model accounting for weakening of frictional properties with slip. Specifically, we study the effect of fault dilatancy on the fluid driven shear crack propagation, when the in-situ background shear stress to is both larger and lower than residual frictional strength tr (i.e. unstable and ultimately stable fault, respectively). The undrained pore pressure drop associated with dilatancy induces a shear fault strengthening near the propagating crack tips. An undrained residual fault shear strength tur is thus introduced. Under small scale yielding condition, we show theoretically that an otherwise unstable fault never exhibit transition to seismic slip when tur is larger than to. 
The numerical solutions of the fully coupled hydro-mechanical problem confirm such stabilization. Finally, we show that if the fault longitudinal permeability increases considerably during the dilatant process, the aseismic crack growth accelerates but never diverges. In other words, the undrained fault stabilization due to dilatancy still holds for large increase of permeability.  

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