Page 200 - Applied Petroleum Geomechanics

In situ stress estimate 195

500
P b Pump-in
450
P prop Shut-in
P i
400 P isip
Surface pressure (psi) 300 P c
350
250
200
150
100
50
0
2.5
3.5
3.0
0.0 0.5 1.0 1.5 2.0 2.0 4.0 6.0
Fluid Pumped (bbls) shut-in time (min.)
Figure 6.4 A typical one-cycle XLOT performed in a borehole showing the relationship
of the fracture initiation and breakdown pressures versus the injection volume.
When the pump is turned off, the pressure begins to decline and the
pressure drops to the instantaneous shut-in pressure (p isip ). As the pressure
declines, the fracture starts to close. If the generated fracture is vertical and
largely in the far field, then the stress acting to close the fracture is equal to
the minimum horizontal stress (s h ). Therefore, the minimum horizontal
stress is equal to the closure pressure (p c ), i.e., the inflection point in the
pressure decline curve (Zhang and Yin, 2017), as shown in Fig. 6.4.

s h ¼ p c (6.15)

Eq. (6.15) is for the case in the normal and strike-slip faulting stress
regimes, and the created fracture is not affected by preexisting fractures. If
natural fractures exist, the hydraulic injection may not create new fractures
but open preexisting arbitrary-oriented fractures. In this condition, the
interpretation of closure pressure from Eq. (6.15) provides an unreliable
estimate of the minimum horizontal stress. However, an inversion type
stress analysis introduced by Cornet and Valette (1984) or Baumgartner and
Rummel (1989) can be used to analyze in situ stresses in preexisting
fractures. The method is based on the shut-in pressure P si as a measure of
the normal stress S n acting across the fracture plane considered:

S n ¼ P si (6.16)
Assuming that the vertical stress S V is overburden stress (a principal
stress) and the stress field linearly varies with depth, the normal stress S n,i