In this paper, the measurement and variation of physical and mechanical properties such as strength, anisotropic elastic stiffness coefficients, Cij, and Biot?s pore pressure coefficients, aij, of Woodford shale when in contact with fluids and under different stress modes were studied using the Inclined Direct Shear Testing Device (IDSTD?) method. In addition, strength variability with sample orientation was also investigated by comparing the IDSTD? strength of samples cored perpendicular to the apparent bedding planes to samples obtained from the same depth but cored parallel to the bedding planes. The results show that despite a relatively high content of non-clay and low-swelling clay minerals, Woodford shale exhibits clear transversely isotropic characteristic in terms of dynamic moduli as well as a high sensitivity of compressive strength to fluid exposure. On the other hand, with a limited number of samples tested, the conclusion about Woodford shale strength anisotropy is not yet clear.
1. INTRODUCTION
1.1. Motivation
Knowing the effects of fluid exposure and the directional dependency of shale properties such as strength, elastic stiffness moduli and poroelastic mechanical coefficients is crucial for well-bore stability analysis, hydraulic fracturing design, reservoir compaction, and many other field applications in the oil and gas industry. However, to date, existing rock mechanics testing procedures and studies attempting to quantify these effects rely on conventional methods and techniques which generally require the use of relatively large samples, with diameter and length dimensions of 5.08 cm by 10.16 cm (2 inch by 4 inch) or 2.54 cm by 5.08 cm (1 inch by 2 inch), resulting in a very long fluid circulation time due to the low intrinsic permeability and solute diffusivity of shale rocks. Moreover, conventional-size samples for traditional rock testing methods are not always available, for example, naturally fractured rocks may splinter into relatively small chips and pieces upon retrieval [1]. The sensitivity and complexity of obtaining large shale samples in lab environments also often presents significant challenges. In addition, local inhomogeneity in sedimentary rocks such as layering or concretion of evaporites, in some instances, can make the use of standard-size samples misleading or unrepresentative for mechanical characterization of the main matrix.
Fig. 1. Schematic of IDSTD? setup(available in full paper)
The Inclined Direct Shear Testing Device (IDSTD?) method, as illustrated in Figure 1, has been designed to mechanically characterize small size samples, 2.01 cm in diameter by 0.72 cm in thickness (~ 0.8 inch by 0.3 inch) not only to overcome the aforementioned challenges but also to increase the statistics of sample testing from one core retrieval while maintaining a reasonable testing time. This equipment and method allow the testing of penny shape samples subjected to different applied stress states and fluid circulation for any desired time of exposure while the dynamic elastic moduli can be simultaneously acquired and monitored.
1.2. Application on Woodford Shale
Woodford shale formation, deposited during the lower Mississippian and upper Devonian period in anaerobic marine environment, is found throughout the central part of US Midwest and is one of the major gas plays of the region [2, 3].
