Mechanical characterization of Laffan and Nahr Umr anisotropic shales

Abstract

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 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 (cohesion and angle of internal friction) different than the strength properties of the intact rock. By minimizing the Root Mean Square Error (RMSE), 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. Therefore, the transverse isotropy assumption is a reasonable model to be used when dealing with these laminated sedimentary rocks.