A shear-dependent fracture-zone transmissivity model

Abstract

Shear displacements caused by excavation through a fracture zone can decrease the overall fracture-zone transmissivity. The complex arrangement of flow channels found in fracture zones creates a situation where imposed shear displacements will reduce the aperture of critically oriented fractures. This paper presents a simple flow model based on the en echelon structure found in many fracture zones from which a phenomenological relationship between shear displacement and transmissivity change is established. This model is then used to demonstrate the effect of shear displacement around a circular opening. The effects of shear-induced decreases in transmissivity are most dramatic when a continuous low-transmissivity zone is created around the opening. In this case, the steady-state heads can be relatively high and the resulting hydraulic gradient into the excavation can be extremely large. If the low-transmissivity zone around the opening is discontinuous because, for example, the shear displacements are nonaxisymmetric, then the inflow becomes channelled and the steady-state heads and hydraulic gradients near the excavation substantially decrease. The concept of shear-induced transmissivity reduction in fracture zones provides an alternative mechanism to shear-induced dilation and normal stress induced fracture closure for interpreting and explaining the observed hydraulic response in fracture zones. Key words : transmissivity, shear displacement, fluid flow, en echelon fractures, fracture zone, excavation.