Normal stress‐induced permeability hysteresis of a fracture in a granite cylinder

Abstract

AbstractThe paper examines the influence of axial stress‐induced closure of a fracture on its permeability. The experiments were conducted on a cylinder of Barre Granite measuring 457 mm in diameter and 510 mm in height, containing a central cylindrical cavity of diameter 75 mm. Radial flow hydraulic pulse tests were conducted in a previous research investigation (Selvadurai et al., PAGEOPH, 2005) to determine the permeability characteristics of the intact granite. In the continuation of the research, a fracture was introduced in the cylinder with its nominal plane normal to the axis of the cylinder. Axial compressive stress was applied normal to the plane of the fracture. An increase in the compressive normal stress acting on the fracture caused a reduction in the aperture of the fracture, which resulted in the reduction in its permeability. Steady state radial flow tests were conducted on the fractured axially stressed sample to determine the variation of fracture permeability with axial normal stress. The analytical developments also take into account flow through the matrix region as the normal stress increases. The results of the experimental investigations indicate that the complete stress relief of a fracture previously subjected to a normal stress of 7.5 MPa can result in a permeability increase of approximately three orders of magnitude. These findings are relevant to shallow depth geotechnical construction activities where enhanced fluid flow can be activated by stress relief. As the fracture aperture closes with high normal stress, the flow through the matrix can be appreciable and if this factor is not taken into consideration the interpretation of fracture permeability can be open to error. This factor can be of interest to the interpretation of permeability of fractures in deep crustal settings where the stresses acting normal to the fracture surface can inhibit flow in the fracture.