Mechanical Anisotropy of Gas Shales and Claystones

This abstract/presentation explores mechanical anisotropy of gas shales and claystones and compares the two. The types of anisotropy addressed are 1) static Young’s Modulus anisotropy, 2) static Poisson’s Ratio anisotropy and 3) compressive strength anisotropy. Claystones are found in many ways to be an ‘end member’ of the gas shale universe, and preliminary data suggest that carbonates might represent another logical end member. The dataset consists of mechanical property measurements on fifty different core intervals spread across six different shale gas plays, plus several different clay-rich mudstones. Gas shales mostly have Young’s Moduli values much higher than claystones, and often more anisotropic. Most gas shales have horizontal:vertical modulus ratios of ∼1.4:1 to 2.5:1 or higher. While modulus correlates with bulk density, anisotropy does not. Poisson’s Ratio is also anisotropic, with the vertical Poisson’s Ratio being less than the horizontal. Gas shales also have significant strength anisotropy, more so than most mudstones, due to a low relative shear strength of the bedding planes. Implications of the observed mechanical anisotropy for horizontal stress estimation and for wellbore stability are discussed.