ABSTRACT:

In this paper, we report on a scoping study that was prompted by operational issues through an Oligocene smectite-rich shale that involved changes in borehole inclination with respect to the bedding. A core characterization workflow is used to specifically probe geomechanical heterogeneity and anisotropy for static and dynamic elastic properties as well as failure strength. Initial petrophysical scanning of the core surface provides a first indication of existing heterogeneity for properties of interest and assists in devising an efficient sampling strategy. Over the three-foot section analyzed, and despite its apparent homogeneity, the core exhibits a two-fold variation in reduced Young’s modulus between softer and stiffer zones, which is tied to slight changes in carbonate content. Confined elastic and mechanical measurements reveal strength anisotropy of the order of 20% and P-wave and S-wave velocity anisotropies of about 20% and 30%, respectively. Moreover, testing shows that the shale is weakest at oblique angle to the bedding due to weak bed parallel surfaces which activate when favorably oriented. These results suggest that anisotropy and heterogeneity both need to be accounted for in borehole stability models involving smectite-rich material.

1. INTRODUCTION
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