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In situ stress estimate 201

Fig. 6.7 presents pore pressure and measured minimum horizontal stress
profiles with depth. The pore pressure is highly overpressured starting from
5200 ft probably because of gas generation. The minimum horizontal stress
in the shale or mudstone at any given depth is consistently greater than that
in the sandstone. This might be caused by the sandstone, which has a lower
Poisson’s ratio than that in the shale or by the lithology-dependent
phenomenon as described by Zhang and Zhang (2017) (refer to Section
5.3 of Chapter 5).

6.3 Minimum horizontal stress calculation
The LOT, FPIT, and DFIT are useful methods to determine the mini-
mum horizontal stress in some sections of a borehole. However, measured
data normally are not available at the location and depth of interest.
Furthermore, most LOT tests only provide fracture initiation pressures or
formation breakdown pressures and do not indicate fracture closure
pressures (i.e., the minimum stress). Micro-frac tests can be performed
while drilling, which provide real-time measurements of horizontal
stresses. However, for a very tight rock the micro-frac test may not be able
to break the formation. Therefore, challenges still exist for efficiently,
routinely, and accurately determining and predicting the minimum
horizontal stress (e.g., Schmitt et al., 2012; Zhang, 2013). In the following
sections, the conventional uniaxial strain model and its improved forms are
examined for estimating the minimum horizontal stress.
6.3.1 Minimum horizontal stress without tectonic impact

In the normal faulting stress regime, the minimum horizontal stress is the
minimum principal stress and can be calculated by assuming the uniaxial
strain condition. For the isotropic rocks and without tectonic stress effect,
the minimum horizontal stress can be expressed in the following:
n
s h ¼ ðs V ap p Þþ ap p (6.26)
1 n
where s h is the minimum horizontal stress; s V is the overburden stress; n is
Poisson’s ratio; p p is the pore pressure; a is Biot’s coefficient. This equation
indicates that the minimum horizontal stress is strongly dependent on
vertical stress and pore pressure. Notice that Eq. (6.26) is the lower bound
minimum horizontal stress (Zhang and Zhang, 2017).