Page 242 - Reservoir Geomechanics

223 Determination of S 3 from mini-fracs

0

1000
HYDRAULIC
FRACTURING

2000
BREAKOUTS
Depth (meters) 3000 S Hmax

TENSILE
FRACTURES

4000
S hmin

S v
5000
COMBINED
ANALYSIS

6000
0 100 200 300 400
Stress (MPa)

Figure 7.7. Summary of stress measurements made in the KTB scientific research borehole in
Germany obtained using the integrated stress measurement strategy described in the text (Zoback,
Apel et al. 1993).

of S Hmax when the rock strength is known utilizing observations of breakout width.
Because the stress concentration at the edge of a breakout is in equilibrium with the
rock strength, they derived the following:
(C 0 + 2P p +
P + σ
T ) − S hmin (1 + 2 cos 2θ b )
S Hmax = (7.7)
1 − 2 cos 2θ b
where 2θ b ≡ π−w bo .
As illustrated in Chapter 6, ultrasonic televiewer data allow for the accurate deter-
mination of breakout width at any depth in a well. Figure 7.8 shows a compilation of
breakout width and azimuth data from a geothermal research well in Fenton Hill, New