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.
Accurate wellbore stability prediction in geomechanically unstable formations requires thorough understanding of the drilled rock properties. This includes the ability to predict failure in deviated wells associated with bedding heterogeneity or to better assess the relationship between intrinsic elastic properties and stress/strain boundary conditions for e.g. in situ stress computations and log-based geomechanical forecasting.
This paper presents a geomechanical core analysis workflow that includes petrophysical core scanning for heterogeneity assessment and sample picking, as well as geomechanical testing for anisotropic static/dynamic elastic properties and strength. In particular, the petrophysical scanning includes a mechanical probe called the Impulse Hammer which functions by analyzing the force-time function of a hardened steel sphere mounted on an accelerometer dropped on the surface of the rock. This analysis produces a reduced Young’s modulus at a resolution on the order of the millimeter revealing fine scale heterogeneity. Using the profiles obtained during petrophysical scanning, locations of interest can be chosen for further geomechanical evaluation on plugs.