Experimental evidence of the anisotropy of tracer dispersion in rough fractures with sheared walls

[1] Dispersion experiments are compared for two model fractures with identical complementary rough walls but with a relative shear displacement δ parallel (δ ∥ U) or perpendicular (δ ⊥ U) to the flow velocity U. The flowing fluid is a shear thinning polymer solution with a Newtonian behavior at low shear rates. For δ ⊥ U, the mixing fronts display large structures well reproduced by assuming parallel channels of conductance deduced from the aperture field. This model also explains the amplification of the structures in the shear thinning regime and the distribution of the local transit times (x, y). For δ ∥ U, the front is much flatter. The local thickness of the front is characterized by a dispersivity α(x, y): its distribution is narrow enough to define an effective value α(Pe) only for δ ∥ U, and, in this case, α(Pe) has a Taylor-Aris-like variation with Pe.