In this study we have measured the undrained pore pressure response of Berea sandstone and Indiana limestone in response to changes in mean and deviatoric stresses. In particular, we have performed tests up to a confining pressure of 70 MPa and differential stress raging from 0 to 150 MPa. For Berea SS, the pore pressure responded to confining pressure (also mean stress because there was no differential loading) in the usual manner corresponding to ?p=B?sm, with B ranging from 0.3 to 0.55. Similarly, for Indiana limestone, B was measured to be 0.15 to 0.46. On the other hand, when the mean and deviatoric stresses were both increased during the test, a B in the range of 0.37 to -0.55 was measure for the Berea sandstone. This reduction in pore pressure increase with deviatoric loading, is suggestive of volumetric deformation under deviatoric loading. At high deviatoric stress levels and undrained conditions, the pore pressure response consists of both elastic and inelastic volumetric strain. For Indiana Limestone, the pore pressure was measured before and after failure. Skempton factor A and B values for different confining pressure and differential axial loadings in elastic and post peak regions for Indiana Limestone also have been measured. After yielding, the inelastic response was eliminated by repeated stress cycling, to capture the reversible elastic component. The sensitivity of pore pressure to deviatoric stress measured at constant confining pressure was found to decrease with increasing deviatoric stress level resulting in a smaller value for A.
Knowledge of pore pressure and the pore variation in rock is important in a number of reservoir geomechanics problems including wellbore stability and reservoir compaction. Conventional pore-pressure estimation methods are based on one-dimensional compaction theory and depend on a relationship between porosity and vertical effective stress. However Strike-slip or reverse faulting environments especially require a different way of estimating pore-pressure, since the overburden is not the maximum stress. Rock deformation changes the pore volume and can change the pore pressure. The latter can be very significant under undrained condition.
Usually, the pore pressure variation in response to volumetric deformation is analyzed within the framework of isotropic linear poroelasticity. As a result, a pore pressure change is predicted only by changes in the mean stress. However, as shown by Skempton and more recently by Wang (1997), deviatoric stress also causes the pore pressure to change.
(Equation in full paper)
Where, B is also called the isotropic Skempton coefficient. Wang  studied the reversible pore pressure response of rocks to changes in deviatoric stress for Indiana limestone at a confining pressure of 27.6 MPa. In his study, tests at zero deviatoric stress yielded B = 0.53 when fit by Eq. 2. He then increased both the mean and deviatoric stress by applying axial load. In this case, the best fit by Eq. 2 resulted in B of 0.34 and 0.39 for the two reported tests. This represented a remarkable change in poroelastic response for a rock under reversible loading conditions.