Miscible transfer of solute in different model fractures: From random to multiscale wall roughness

AURADOU H, ; BOSCHAN A, CHERTCOFF R, D'ANGELO MV, ; HULIN JP; IPPOLITO I

COMPTES RENDUS GEOSCIENCE

ELSEVIER FRANCE-EDITIONS SCIENTIFIQUES MEDICALES ELSEVIER

Año: 2010 vol. 342 p. 644 - 652

1631-0713

Miscible tracer dispersion measurements in transparent model fractures with differenttypes of wall roughness are reported. The nature (Fickian or not) of dispersion isdetermined by studying variations of the mixing front as a function of the distancetravelled but also as a function of the lateral scale over which the tracer concentration isaveraged. The dominant hydrodynamic dispersion mechanisms (velocity profile in thegap, velocity variations in the fracture plane) are established by comparing measurementsusing Newtonian and shear thinning fluids. For small monodisperse rugosities, frontspreading is diffusive with a dominant geometrical dispersion (dispersion coefficientD / Pe or constant dispersivity ld = D/U) at low Pe´ clet numbers Pe; at higher Pe values, onehas either ld / Pe (i.e. Taylor dispersion) for obstacles of height smaller than the gap, orld / Pe0.35 for obstacles bridging the gap. For a self-affine multiscale roughness like inactual rocks and a relative shear displacement~d of complementary walls, the aperture fieldis channelized in the direction perpendicular to ~d. For a mean velocity ~U parallel to thechannels, the global front geometry reflects the velocity contrast between them and ispredicted from the aperture field. For ~U perpendicular to the channels, global frontspreading is much reduced. Local spreading of the front thickness remains mostlycontrolled by Taylor dispersion except in the case of a very strong channelization parallelto ~U.