Page 211 - Applied Petroleum Geomechanics

206 Applied Petroleum Geomechanics

with the main lithologic units and rock types. The overburden stress and
depth (Z) have the following relation: s V ¼ 0.0257Z. The data at shal-
lower depths (< 400 m) are also plotted in Fig. 6.10 for comparison, which
were measured using hydraulic fracturing method in the Yingxiu area (Wu
et al., 2009), near the WFSD-1 site. It can be observed from Fig. 6.10 that
two horizontal stresses are mostly higher than the overburden stress, so that
this area is in the thrust faulting stress regime.
When the excess strains are not available, empirical correlations can be
used to estimate tectonic stresses or strains. Calibrating the minimum stress
data obtained from downhole tests, the tectonic stress can be approximately
estimated from the following relation:
s min ¼ bs V (6.30)
tect
where b is the ratio of the tectonic to overburden stresses or tectonic stress
constant, and b 0.
Therefore, the minimum horizontal stress Eq. (6.27) can be rewritten as
follows:
s h ¼ kðs V ap p Þþ ap p þ bs V (6.31)
where k ¼ n/1 n.
Several examples are given in the following to illustrate tectonic stress
constants in different basins. Tingay et al. (2009) presented in situ stress
measurements in the Champion field, offshore Brunei. This field is in
normal faulting stress regime, and average in situ stress gradients are
s V ¼ 21.8 MPa/km, s h ¼16.5 MPa/km, and s H ¼18 21 MPa/km. Based
on the measured stress and pore pressure data, they obtained the following
pore pressure and in situ stress relation:

s h =s V ¼ 0:58ð p p s V Þþ 0:54 (6.32)
where s h , s V , and p p are in MPa, the same unit used in the following
equations.
Compared this equation to Eq. (6.31) and assuming a ¼ 1, it can be
found that b ¼ 0.12, hence Eq. (6.32) can be rewritten as follows:
s h ¼ 0:42ðs V p p Þþ p p þ 0:12s V (6.33)

The LOT data in deepwater Gulf of Mexico wells show that the
tectonic stress constant is much lower, but with a much higher n/(1-n), e.g.,
in the Mississippi Canyon b ¼ 0.04, and

s h ¼ 0:68ðs V p p Þþ p p þ 0:04s V (6.34)