Page 212 - Applied Petroleum Geomechanics

In situ stress estimate 207

0 5 10 15 20 25 30 35
0
100
200
300
400
500
600
700
σ 8
800
900
σ
1000
1100
1200
Figure 6.11 Measured s h and p p data and empirical correlations of the maximum and
minimum horizontal stresses, overburden stress, and pore pressure in coalbed
methane reservoir of the Qinshui basin, China.

Analyzing measured minimum horizontal stress data from mini-frac
tests, Meng et al. (2011) obtained the following relation (with constant
b ¼ 0.035) for the minimum horizontal stress in the Qinshui coalbed
methane basin (Fig. 6.11):

s h ¼ 0:45ðs V p p Þþ p p þ 0:035s V (6.35)
It can be seen from Eqs. (6.33e6.35) that k and b values are different in
different areas. If the measured minimum horizontal stress data (e.g., LOT,
DFIT) are available, k and b values can be back-calculated from Eq. (6.31),
which can be used to estimate the minimum horizontal stress in a similar area.

6.3.3 Minimum horizontal stress in anisotropic rocks
Some sedimentary rocks, particularly shales, are transversely isotropic, and
rock properties (e.g., Young’s modulus and Poisson’s ratio) are very different
in horizontal and vertical directions. For transversely isotropic rocks the
minimum horizontal stress can be expressed as follows, if the uniaxial strain
model is applicable (refer to Chapter 1, Section 1.5.2 for derivations):

E h n V
s h ¼ ðs V a V p p Þþ a h p p (6.36)
E V ð1 n h Þ