Question and answers: permeability 147US National Committee for Rock Mechanics, 19963), packer testing at
one location in a borehole may encounter a single, highly conductive,
extensive, horizontal fracture. When the hydraulic conductivity value
has been determined from such test results, it does not make sense to
say that a 10 m thickness of the rock stratum is twice as transmissive as
a 5 m thickness, when there is only one major fracture conduit present.
For this reason, the term 'fracture transmissivity' is receiving increasing
usage.
49.5 The horizontal section below shows two sets of rock fractures
in a 10 m square rock block of unit thickness4. On the left-hand
side, there is a hydraulic head of 3 m; on the right-hand side, the
head is 1 m; and along the top and bottom edges the head linearly
decreases from the left to the right.Hydraulic
head of 3 m
on this sideHydraulic
head of 1 m
on this sideaThe (x,y) co-ordinates (in m) of the fracture intersections, Le. the
numbered nodes, relative to an origin at the bottom left of the
diagram, are given in the table below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
0.00 0.00 0.00 2.10 2.74 5.16 6.61 8.06 5.16 6.13 3.06 5.81 6.61 7.42 8.71 10.00 10.00 10.00
6.94 4.19 1.29 5.16 4.52 2.42 1.13 0.00 5.32 2.58 10.00 6.61 5.65 4.84 3.23 1.45 6.94 5.00
Assuming that all the fractures have the same aperture, 0.12 mm,
and that there is no variation in flow throughout the thickness of the
rock block, determine the nodal heads and hence the direction and
magnitude of flow in each fracture segment.
"ommittee on Fracture Characterization and Fluid Flow, US National Committee
for Rock Mechanics, (1996) Rock Fractures and Fluid Ffozu. National Academy Press,
Washington, DC.
4This question is based on Example 11.3 in the book Discontinuity Analysis for Rock
Enginemkg by S. D. Priest, with the numerical values slightly changed.