Biot’s coefficient, a parameter connecting the effect of pore pressure on the effective stress in porous media, has been widely used to obtain the effective stress in isotropic formations. For anisotropic formations, recent experimental data and modeling results indicate that there is a difference between the horizontal and vertical Biot’s coefficients. Therefore, Biot coefficient is not only stress dependent, but also shows direction dependence. Hence, the complex stress and direction dependence imply that Biot’s coefficient must be described in a tensor form. The tensor representation of Biot’s coefficient helps to solve many mathematical difficulties and controversies surrounding the effective stress concept that is essential for realistic geomechanics measurements and modeling. Moreover, it provides a more precise and flexible tool in modeling the effect of pore fluids on the effective stress.

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