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In situ stress estimate 203
Stress/pressure (psi)
0 2000 4000 6000 8000 10000
7200
Pore pressure Sh in sandstones/siltstones
7300 Sh measured
Sh calculated
7400 Hydrostatic Pp
Overburden
7500 Sh measured, shale
Depth (ft) 7600 Sh in shales
Pp interpolated
7700
7800
7900
8000
8100
Figure 6.9 Pore pressure, measured and calculated minimum horizontal stresses (S h ),
and overburden stress versus depth in Well MWX-2 based on the data of Warpinski
et al. (1985).
match the measured results in sandstones and siltstones, where measured
pore pressure data are available. The calculated results do not match the
measured results in shales, and this may be caused by the unreliable pore
pressure estimate in the shales. There were no measured pore pressures in
shales, and the pore pressures provided in shales were interpolated from the
pressures measured in adjacent sandstones (Warpinski et al., 1985). The
mismatch in the minimum horizontal stress calculation may also be caused
by the tectonic stress.
6.3.2 Minimum horizontal stress with tectonic impact
The uniaxial strain model is oversimplified for estimating the minimum
horizontal stress in a tectonic region. Therefore, modification is needed.
If the tectonic stress is considered (as proposed by Daines, 1982), the
minimum horizontal stress can be written as:
n
s h ¼ ðs V ap p Þþ ap p þ s min (6.27)
tect
1 n
min
where s tect is the tectonic stress in the minimum horizontal stress direction.
It is also named as the excess minimum horizontal stress, and
E
s min ¼ ðε h þ nε H Þ (6.28)
tect
1 n 2