• Siggins, Anthony
• Raven, Mark
• Nordgard-Bolas, Hege
• Sarout, Joel

Abstract

Multiple stage triaxial tests were performed upon horizontal core plugs of a shale from the Norwegian Sea with a view to evaluating rock strength and the evolution of ultrasonic response during rock deformation. In addition, standard rock physical properties were characterised as well as composition (kaolinite and quartz dominant). The shale has low friction coefficient and cohesive strength and shows moderate strength anisotropy. The main aim of the work was to evaluate the dynamic elastic response of shales to the application of anisotropic stress fields and specifically the impact on velocity and its anisotropy. The latter two are directly related to both the preferred orientation of minerals in the shale and microcracks parallel to this compaction fabric. The orientation of the maximum principal stress parallel to the intrinsic fabric and microcracks was seen to significantly impact on velocity normal to the fabric as stress parallel to the fabric increased. S-wave anisotropy was significantly affected by the increasing stress anisotropy. Stress orientation with respect to fabric orientation was therefore shown to be an important control on the degree of anisotropy of dynamic elastic properties in this shale.

Topics

• mineral
• strength
• anisotropic
• ultrasonic