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Ecoulements mixtes de granulaires denses et fluides : liquéfaction, sédimentation et fluidofracturation
When fluid pressure gradients rises to sufficiently high levels in porous media, seepage forces can lead to irreversible flow of the solid matrix. This process is unstable, and leads to high permeability channel formation. We will present here experimental and numerical results on this channel formation and the characterization of their dynamics.
Mechanical instability (hydroplanning) in saturated sheared media : We study the behavior of confined gouges when overpressured fluids are mobilized in seismic sources. Indeed, the formation of preferential paths in this situation can severely affect the fluid and heat transport properties in these situations, and thus affect the pore pressurization effects. We will present some results on the influence of intersticial fluids on sheared granular layers, and show that pore pressurization and liquefaction can also occur in drained soils initially compacted.
Channel formation in overpressurized media : We present systems where we inject a fluid at high pressure in a porous material saturated with the same fluid, or where gravity overpressurizes the fluid in interticial pore space during granular deformation. This fluid is either a highly compressible gas (air), or an almost incompressible and viscous fluid (oil or water). We compare both situations. These porous materials are designed as analogs to real rocks, but their cohesion are lowered so that the hydrofracture process can be followed optically in the lab. The fluid is injected on the side of the material. At high overpressures, the formation of hydrofracture fingering patterns and transition to thin branching fractures is followed and analyzed quantitatively. The two situations with air or oil are compared together. Many similarities are observed about shape selections and dynamics, when time is rescaled with the viscosity of the fluid. The adaptation of hybrid granular fluid modelling algorithms to tackle the different types of flows will be presented.
Channel formation in depressurized media : We also study the pattern formation in the decompaction process starting from free boundaries during fluid extraction. The geometry of these preferential paths, their fractal dimension and other characteristics, are extracted from experiments and simulations. In practice, these problems are relevant for important aspects in the formation of increased permeability networks as seen in nature and industry : e.g., in active hydrofracture in boreholes, piping/internal erosion in soils and dams, sand production in oil or water wells, and wormholes in oil sands.
References
[1] Vinningland, J.L., Ø. Johnsen, E.G. Flekkøy, R. Toussaint and K.J. Måløy ; Influence of particle size in Rayleigh Taylor granular flow instability, Phys. Rev. E 81, 041308 (2010).
[2] M.J. Niebling, E.G. Flekkøy, K.J. Måløy, R. Toussaint ; ``The effect of the fluid viscosity and compressibility on a layer of grains falling in a fluid ’’, Phys. Rev. E, (2010).
[3] Goren, L., E. Aharonov, D. Sparks and R. Toussaint ; ``Pore pressure evolution in deforming granular material : A general formulation and the infinitely stiff approximation, J. Geophys. Res., 115, B09216, (2010).
[4] Goren, L., E. Aharonov, D. Sparks and R. Toussaint ; ``The mechanical coupling of fluid-filled granular material under shear’’, P.A.Geoph. (2011).
[5] M.J. Niebling, E.G. Flekkøy, K.J. Måløy, R. Toussaint ; ``Sedimentation instabilities : impact of the fluid compressibility and viscosity’’, Phys. Rev. E, (2010).