Biot’s Effective Stress Coefficient Tensor Measurements on Mancos Shale

The poroelastic effect on deep buried shale is of high interest and value for evaluating coupled effects of rock deformation and pore pressure change during oil/gas field operations. The concept of Biot’s effective stress law has been proved to be very useful in describing the effect of pore fluid pressure on the mechanical response of porous earth materials. In this work, a methodology is developed and applied to obtain the stress dependent Biot’s tensor for a transversely isotropic shale (the Mancos shale) using both static and dynamic methods with three plugs (a vertical, a horizontal, and a 45 degree plug) from the same location. The horizontal and vertical plugs yield very close results for the grain bulk modulus, indicating grain bulk modulus can be well accepted as a scalar quantity for a specific shale, which is in agreement with our previous tests on many other mudstones (Zhou et al., 2017). Biot’s coefficients in the vertical and horizontal directions (or cross and along the bedding) all decrease with increasing effective stress, and thus (effective) stress dependent. Furthermore, the different behaviors of Biot’s coefficients along vertical and horizontal directions indicate that Biot’s coefficient is indeed a tensor for shale, and the pore pressure changes not only modify the normal stress component but also induce deviatoric stress changes. As a result, the pore pressure change in shale can always introduce more complicated effective stress change in comparison with an isotropic model. Static measurements are all higher than their dynamic counterparts. Permeability in the horizontal direction are much higher than that along the vertical direction for Mancos shale, indicating the hydro-anisotropy as defined by the permeability tensor is even much more pronounced than the poro-mechanic anisotropy as characterized by the Biot’s tensor.