Page 197 - Fundamentals of Gas Shale Reservoirs

WELLBORE INSTABILITY IN GAS SHALE RESERVOIRS 177
various rock types, for example, Donath (1964), Chenevert can vary significantly as the angle between the direction of
and Gatline (1965), McLamore and Gray (1967), and Hoek the axial load and bedding planes varies. Aadony (1988)
(1968) on shale. As a result of these studies, it was noted developed a simulator to study the wellbore instability of
that the maximum strength observed was at angles β = 0° or highly inclined borehole in VTI rock formations. Ong and
β = 90°, whereas the minimum strength was found to be Roegiers (1993) studied the influence of anisotropic stress
happening at an approximate angle of β = 30°. Here, β is the on borehole stability using an anisotropic strength criterion
angle between plane of weakness (i.e., foliation or bedding for assessing compressive failure. They indicated that well­
planes) and the direction of the maximum load applied to bore stability was significantly influenced by the mechanical
the sample. anisotropy of the rock. Gazaniol et al. (1994) found that rock
The applications of plane of weakness in oil and gas dril­ strength anisotropy has a great influence on wellbore insta­
ling were introduced by Aadnoy (1988). In modeling highly bility and that a wellbore could fail along the bedding planes
inclined boreholes, he investigated the effects of wellbore if the trajectory of the well is not properly selected. Last and
inclination, anisotropic elastic rock properties, anisotropic McLean (1995) discussed that drilling perpendicular to bed­
stresses, and anisotropic rock strength. As a result of this ding planes was beneficial as it improved wellbore stability
study, it was seen that if the borehole wall is in the same in Cusiana Field, where they performed their studies. They
plane as the borehole axis and the normal axis to the bedding pointed out that wellbore stability was affected by the
plane, one Mohr–Coulomb envelope applies for all borehole relative angle between the wellbore and the bedding planes.
angles. This is the least serious and therefore the preferred Similar conclusions were reached by Skelton et al. (1995)
case. On the other hand, if the least in situ stress is normal on where they showed that the tangent section inclination and
the plane of the borehole axis and the axis is normal on the azimuth of the wellbore should be perpendicular to the bed­
bedding plane, the directional‐shear‐strength properties ding dip and strike of the formations, respectively, in order to
come into play. Now, the borehole has a potential collapse avoid formation layers from sliding along their bedding
problem in the inclination range 15° < γ < 35°. It should be planes. Niandou et al. (1997) observed that during lab test­
noted that this applies only to sedimentary rocks with a plane ing of the Tournemire shale, failure was caused by extension
of weakness. and sliding of bedding planes or shear band development
Because of the specific geomechanical properties of within the shale matrix. Okland and Cook (1998) noted that
shale (high pore pressure, alignment of phyllosilicates due for the Draupne Formation, the “angle of attack” between
to overburden diagenesis), slip surfaces may exhibit signif­ the wellbore and the bedding planes should always exceed
icantly more potential to fail as compared to stronger rock 20° so as to improve wellbore stability. Willson et al. (1999)
units, such as limestone and sandstone. For this reason, noted that bedding plane slippage could result from unfavor­
shale instability is an important design factor in drilling able interaction between in situ stresses, well trajectory, and
practices. bedding planes. They also pointed out that the reduced
strength (friction or cohesion) acting on the bedding plane
could also result in greater and sometimes catastrophic insta­
8.4.1 Structurally Controlled Instability
bility. Russel et al. (2003) found that it is important to deter­
Structurally controlled instability due to slippage of plane of mine the relative angle between the well trajectory and the
weakness is likely to happen when drilling in shales, similar rock structure because this angle dictates the stability of the
to other layered formations. In drilling into shale formations, formation when drilling close to the bedding dips or at unfa­
borehole orientation, with respect to the direction of in situ vorable angles to fracture planes.
stresses, in addition to the magnitude of the in situ stresses According to Aadnoy et al. (2009), orientation of plane of
and the location of failure on the borehole wall with respect weakness with certain wellbore plane inclinations can cause
to the bedding plane orientation should be determined. the borehole to become unstable. To show this, let us assume
According to Al‐Ajmi and Zimmerman (2009), failure crite­ that the in situ principal stresses (i.e., σ , σ , and σ ) are asso­
h
H
v
rion does not significantly influence the optimal drilling ciated with the coordinate system (x′, y′, z′), as shown in
trajectory. This conclusion has also been reported in a Figure 8.5. The z′‐axis is parallel to σ , x′‐axis is parallel to
1
number of publications (Chen et al., 1996; Djurhuus and σ , and y′‐axis is parallel to σ . These virgin formation
2
3
Aadnoy, 2003; Kårstad and Aadnoy, 2005; Moos et al., 1998; stresses should be transformed to another coordinate system
Zhou et al., 1996). Many other studies have also attempted to (x, y, z), to conveniently determine the stress distribution
assess those wells drilled through laminated (anisotropic) around a borehole. Figure 8.5 shows the (x, y, z) coordinate
rocks (Aadony, 1988; Fjaer et al., 1992; Niandou et al., system, where the z‐axis is parallel to the borehole axis, the
1997; Ong and Roegiers, 1993; Singh et al., 1989; Yang and x‐axis is parallel to the lowermost radial direction of the
Gray, 1970). Chenevert and Gatlin (1965) studied the borehole, and the y‐axis is horizontal (after Al‐Ajmi and
mechanical anisotropy of laminated sedimentary rocks and Zimmerman, 2009). This transformation can be obtained by
determined that formation compressive strength and stiffness a rotation of α around the z′‐axis, and then a rotation of angle