Compressive strength anisotropy of six different claystones was measured, using confined triaxial compression tests. Samples were oriented with the bedding perpendicular to the axial stress, and with the bedding at an acute angle to the axial stress in order to cause slip on bedding. Measured strengths were resolved into values of normal stress and shear stress, and these were fit with linear regression to determine values of cohesion and friction angle. Slip on bedding was found to reduce the cohesion by 10% to 70%, and reduce the friction angle by 7% to 17%, with higher reduction generally observed for the lower-porosity rocks. Loading parallel to bedding resulted in the same strength as loading perpendicular to bedding.
Shales make up the vast majority of formations that must be drilled through to reach oil and gas reservoirs. In some cases, especially with recent shale gas developments, the shales themselves are the reservoir. Shale mechanical behavior impacts drilling and wellbore stability. It also impacts casing damage that can occur after a field is on production. Many instances of shale bedding-related instabilities, caused by strength anisotropy, have been reported in the oilfield literature. Highly-inclined wellbores, drilled nearly parallel to bedding, can be very unstable due to bedding-related failure. Shale mechanical behavior is also important in mining, especially coal mining, and in civil engineering. In these areas as well, shale strength anisotropy can have a large impact. The goal of this study was to quantify strength anisotropy for a cross-section of different shales. The shales were selected to represent a variety of different in-situ compaction states, ranging from low-porosity shales to those with fairly high porosity. All samples are clay-rich, containing 65% or greater clay. Preserved downhole shale cores were used for all samples. The methodology employed was to quantify cohesion and friction angle for failure not influenced by bedding, and compare this to the cohesion and friction angle measured directly from slip on bedding. This was accomplished using confined triaxial compression tests at different levels of confining stress, and using two different sample orientations relative to bedding. For one orientation, axial loading was perpendicular to bedding, while for the other orientation axial loading was at an acute angle to bedding. For one of the shales, a third orientation was used with axial loading parallel to bedding.
Six different claystones (referred to as 'shales' in this paper) were used for this study. According to most rock classification schemes, claystone covers all clay-rich rocks with or without fissility, whereas the term shale is reserved for claystones that exhibit fissility. All six of our studied rocks are referred to loosely as 'shales' even though most of them are not very fissile. Claystones that do not exhibit fissility are usually classified as mudstones. Preserved oilwell cores were used for this study. These shale cores come from different locations around the world and from depths ranging from 5000 to 13000 feet. Increasing burial depth causes greater compaction.
