Page 217 - Applied Petroleum Geomechanics

212 Applied Petroleum Geomechanics

It can be observed that the tensile strength effect is stronger in Eq. (6.50)
than that in Eq. (6.48) proposed by Haimson and Fairhurst (1967). The
equation, Eq. (6.50), predicts a higher tensile strength effect; therefore, it
may be used as the upper bound formation breakdown pressure.
For a vertical borehole and no fluid penetration in the formation before
the fracture is initiated, the upper bound maximum horizontal stress can be
obtained by rearranging Eq. (6.50):

(6.51)
s H ¼ 3s h p b p p þ s T þ kT 0
To obtain reliable and repeatable closure pressure and rock tensile
strength data, a second pressurization cycle needs to be performed from the
initial hydraulic fracture test (Fig. 6.12) to obtain fracture reopening data.
Because a fracture has been created by the first cycle of the XLOT, there
should be no tensile strength in the fracture reopening process. The pressure
at the time of reopening of the fracture created from the previous cycle can
be used to estimate rock tensile strength (T 0 ):
T 0 ¼ p b p r (6.52)
where p r is the fracture reopening pressure (refer to Fig. 6.12).
Fig. 6.13 presents an example of a two-cycle XLOT in an offshore well
of the North Sea. Normally, the recorded pump pressure is the gauge

Figure 6.13 Measured pump pressure and pressure gradient with consideration of
downhole mud pressure versus time (test points) in a two-cycle XLOT at a vertical
3
depth of 1000 m. In the figure the equivalent mud weight (EMW) is in sg (or g/cm ).