Measurement of hydraulically ineffective area on a fracture under normal stress condition

We measured the hydraulically ineffective area, which is defined as the aperture area that would not contribute significantly to fluid flow in a fracture, of a single tensile fracture with offset displacement of up to 5 mm under normal stress condition of up to 100 MPa using a granite rock sample. Our objective is to confirm the mechanics of confining pressure and offset displacement dependencies in permeability of a single tensile fracture evaluated by Watanabe et al. (2004). The pressure sensitive sheet approach, which enables simple and direct measurement of the hydraulically ineffective area, is adopted in our experiment. Our results show similar stress dependency to that of the permeability of a fracture, which indicates a close relationship between the hydraulically ineffective area and permeability. On the other hand, the hydraulically ineffective area decreases with increasing offset displacement. Our measurements strongly support the results of the permeability tests reported by Watanabe et al. (2004). The perpendicular distribution in the hydraulically ineffective area with increasing offset displacement suggests that the anisotropy in fracture permeability increases because the permeability could depend on fracture aperture that could be major flow area in a fracture.