Page 172 - Rock Mechanics For Underground Mining

PRE-MINING STATE OF STRESS

Figure 5.7 Principles of stress mea-
surement by hydraulic fracturing: (a)
packed-off test section; (b) cross sec-
tion of hole, and fracture orientation
relative to plane principal stresses.

A record of borehole pressure during an hydraulic fracturing experiment, in which
typically several cycles of pressure application and decline are examined, is shown
in Figure 5.8. Two key parameters defined on the borehole pressure record are the
instantaneous shut-in pressure p s and the crack re-opening pressure p r . The shut-in
pressure or fracture closure pressure defines the field principal stress component per-
pendicular to the plane of the fracture. As suggested by Figure 5.7b, this corresponds
to the minor principal stress p 2 acting in the plane of section. The crack re-opening
pressure is the borehole pressure sufficient to separate the fracture surfaces under
the state of stress existing at the hole boundary. The crack re-opening pressure, the
shut-in pressure and the pore pressure, u, at the test horizon may be used to estimate
the major principal stress in the following way.
The minimum boundary stress, min , around a circular hole in rock subject to biaxial
stress, with field stresses of magnitudes p 1 and p 2 ,isgiven by

(5.11)
min = 3p 2 − p 1
When a pressure p 0 is applied to the interior of the borehole, the induced tangential
stress

at the hole wall is

=−p 0

Figure 5.8 Pressure vs. time record
for a hydraulic fracturing experiment
(after Enever and Chopra, 1986).
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