Geomechanics-related wellbore instability has become a major source of non-productive time in highly deviated wells drilled in oil and gas fields in offshore Abu Dhabi.
These wells are drilled through two highly anisotropic shale formations, namely the Laffan
shale formation and the Nahr Umr shale formation. Most of the models used in the oil and gas
industry do not take account for the strength and elastic anisotropy of shale. Therefore, a
laboratory study was conducted to examine the strength and elastic anisotropy of shales using
uniaxial compression tests and triaxial compression tests.
Jaeger’s Plane of Weakness (JPW) model was used to understand the anisotropic failure
behavior of highly laminated shales of the Laffan and Nahr Umr formations. This model
assumes that for an anisotropic rock, there exists a plane of weakness that has strength
properties different than the strength properties of the intact rock. By minimizing the Root
Mean Square Error, the experimental strength values of the samples, as measured at different
orientations, were fitted to the JPW model. Elastic moduli were also measured on these shales,
as a function of orientation angle. The results showed that the moduli vary with angle according
to the expected tensor transformation law.
The evaluation of the stress concentration around boreholes is an essential step toward
a fully defined geomechanical model. Using the outcome of the experiments conducted on
Laffan and Nahr Umr shales, the stresses around an arbitrary well drilled through the two
formations are calculated. The isotropic stress concentration was calculated using Kirsch’s
equations. The anisotropic stresses were calculated using the closed-form solution of Amadei.
A comparison between the mud weight calculated by Kirsch’s equations and Amadei’s
equations shows that anisotropy has a significant effect on the magnitude of stresses around
boreholes, and therefore, has an effect on the required mud weight to prevent collapse.