Experimental Evaluation of Predicted Undrained Pore Pressure Generation as a Function of Stress Path and Material Orientation in the Draupne Shale

Abstract This study experimentally investigates the effects of various stress paths and orientations on induced undrained pore pressure in the Draupne shale and their predictability using experimentally determined pore pressure parameters and elements of anisotropic poroelasticity. A test procedure considering multiple cycles of various stress paths in a triaxial test on a sample of known orientation is suggested to measure the anisotropic pore pressure parameters needed to predict pore pressure generation. Furthermore, a definition of principal stress increments which detaches them from the principal stress directions is used for consistency and to enable easier access to stress paths rarely examined experimentally. The undrained pore pressure from the multi-cycle stress path triaxial tests showed a significant stress-path dependency in which the normalized pore pressure was linearly correlated with the total stress path. The calculated tensorial pore pressure parameters ( B v and B h ) were used to predict undrained pore pressure as a function of sample axis orientation relative to material symmetry. The predicted changes in pore pressure were found to be very similar to those measured in triaxial tests on samples with various orientations. The experimental results from a multi-cycle stress path triaxial test on a sample with a different orientation demonstrated that also the combined effects of variations in stress orientation and stress path could be successfully predicted. These findings confirm that the tensorial pore pressure parameters can be efficiently used to forecast pore pressure changes in samples of various orientations, even when subjected to stress changes along any stress path. The observations herein suggest that measurements from a single triaxial test can be combined with the proposed definition of principal stress increments to predict undrained pore pressure generation in a North Sea shale subjected to complex changes in stress path and orientation. This approach, therefore, contributes significantly to the understanding of induced pore pressure in low-permeable materials and provides valuable input to geomechanical modelling of various field operations.