Page 186 - Reservoir Geomechanics

169 Compressive and tensile failures in vertical wells

S hmin

S 1

S 3

S Hmax S Hmax

S hmin

Figure 6.1. Principal stress trajectories around a cylindrical opening in a bi-axial stress field based
on the Kirsch equations (Kirsch 1898). Note that as the wellbore wall is a free surface, the principal
stress trajectories are parallel and perpendicular to it. Where the trajectories of maximum
compressive stress converge, stresses are more compressive (at the azimuth of S hmin in case of a
vertical well). Where the trajectories diverge, the stresses are less compressive (at the azimuth of
S Hmax ).

failure beyond geomechanics and consider briefly wellbore failure that is the result of
the chemical imbalance between drilling mud and the pore waters in shales that contain
reactive clays.

Stress concentration around a cylindrical hole and wellbore failure

The stress concentration around a vertical well drilled parallel to the vertical principal
stress, S v ,inan isotropic, elastic medium is described by the Kirsch equations (Kirsch
1898); see also Jaeger and Cook (1979). As illustrated in Figure 6.1 (taken from Kirsch’s
original paper), the creation of a cylindrical opening (like a wellbore) causes the stress