Solute transport in a single fracture: Influence of the aperture

H. AURADOU, A. BOSCHAN, I.IPPOLITO, R. CHERTCOFF AND J-P. HULIN

Soultz sous Forets (France)

Congreso; Phase II Scientific Meeting, 2007 Annual International Meeting of the Cooperative European Program SES6-CT-2003-502706 STREP-Pilot Plant (Soultz sous Forets Geothermal Facility) by the European Hot Dry Rock Association; 2007

European Hot Dry Rock Association

Solute transport experiments using fluids which are either Newtonian or non newtonian shear fluids are realized in two types of transparent model fractures: these models represent respectively a random aperture fractures and fractures with complementary rough walls of self affine geometry sheared perpendicular to the flow direction. The structure of the aperture field of the two models has different properties: the random fracture has an aperture field characterized by a finite correlation length while the selfaffine fracture has a heterogeneous aperture field; there are regions spanning over the full length of the fracture where the aperture overcome the mean aperture. These structures have a typical width of the order of the lateral displacement(Auradou et al, 2006). An optical absorption technique is used to determine the concentration distribution and the displacement front geometry during the experiments. In both cases the width of the displacement front increases with the shear thinning properties of the fluids. For random aperture fracture, whatsoever is the width of the region of interest considered, the front spreading is dispersive. A different  picture arises when an open fracture with self affine walls is considered. In this situation, the front spreading is dispersive only for width of the region of interest below the width of the channels. If the width of the region of interest overcome the typical size of the channels the front spreading is no more dispersive and its mean width σ of parallel to the flow increases linearly with distance from the initial injection line. In this latter case, front spreading is shown to be strongly influenced by the spatial structure of the flow field in channels with a long correlation length in the direction perpendicular to the relative shear displacement of the two wall surfaces (and parallel to the mean flow).