Engineering Rock Mechanics

146 Permeability

Stl

Td;

Dimlacement

higher-aperture channels formed between the fracture surfaces. Thus, an
equivalent hydraulic fracture aperture will be required: the value will
be less than the mean geometrical aperture in 2-D, but could be greater
in 3-D.

(b) When the fracture is subjected to a compressive normal stress, its
aperture reduces. An increasing normal stress is required to continue
reducing the aperture, as indicated in the graph above. The mechanical
aperture is the amount by which the fracture opens when it is unloaded
from a normal stress value equal to the intact rock compressive strength
to zero. Thus, the hydraulic aperture and the mechanical aperture are
governed by different aspects of the geometry in 3-D: the hydraulic
aperture is governed by the channels of aperture spaces, and the mech-
anical aperture by the regions where there is connecting rock, i.e. where
the aperture has zero values. (In 2-D, there can be no fracture flow if
the fracture sides are transmitting normal stress, because the aperture
will be zero in that region and hence the flow channel will be blocked.)
Empirical and back analysis approaches are the only practical methods
of establishing equivalent fracture apertures.

49.4 What is meant by the term 'transmissivity'?

A9.4 The term 'transmissivity', T, is used for the product of hydraulic
conductivity and stratum thickness, t: T = Kt. This has a meaning for
a continuum, but there are difficulties when the rock mass hydraulic
conductivity is governed by fracture flow. As pointed out in the US
National Committee for Rock Mechanics publication on rock fractures
and fluid flow (Committee on Fracture Characterization and Fluid Flow,