Page 218 - Applied Petroleum Geomechanics

In situ stress estimate 213

pressure without considering mud weight (mud pressure). Therefore,
the mud pressure should be added to obtain the bottomhole pressure (see
Eq. 6.25). Based on the pressure data in Fig. 6.13, the calculation of the
maximum horizontal stress is shown in the following:
• The minimum horizontal stress is equal to the closure pressure
s h ¼ p c ¼ 18 MPa.
• Using Eq. (6.52), the tensile strength equals the difference of the
formation breakdown pressure and reopening fracture (p b2 ¼ p r ):
T 0 ¼ p b p b2 ¼ 9.4 MPa.
• The maximum horizontal stress can be calculated from Eq. (6.51) with
p b ¼ 28.2 MPa,p p ¼ 11 MPa, s V ¼ 18.7 MPa, k ¼ O2. Therefore,
S H ¼ 28.1 MPa.
• The results show s H > s V > s h ; this well is in strike-slip faulting stress
regime.

6.4.1.2 For permeable fractures
Eqs. (6.48) and (6.51) specially assume that fluid pressure does not penetrate
into the fracture from the borehole before the fracture begins to open.
However, there are good grounds to believe that the fracture remains
permeable to a significant degree even when closed (Ito et al., 1999). If the
fracture is slightly conductive, the fluid pressure at the wellbore wall may
only partially penetrate, and the pressure inside the fracture at the wellbore
wall is equal to the well pressure (Ito et al., 1999), i.e., p p ¼ p r . Substituting
p p ¼ p r and Eq. (6.52) into Eq. (6.51) with assumption of no thermal effect
and a ¼ 1, the maximum horizontal stress can be obtained by:

s H ¼ 3s h þðk 1Þp b ðk þ 1Þp r (6.53)

In the extreme case with a more conductive fracture, the fluid pressure
may completely penetrate to the crack tip. This implies that the reopening
is dominated by total force formed by the fluid pressure on the fracture
surfaces, and reopening pressure would be equal to the minimum
horizontal stress (Rutqvist et al., 2000):

s h ¼ p r (6.54)
Detournay and Cheng (1988) considered the effect of flow from
the hydraulic fracture into the formation. A poroelastic solution of the
breakdown pressure (Eq. 6.55) was proposed to take into account this
effect for permeable rocks (Haimson and Fairhurst, 1967; Detournay and