Undrained plane strain compression of shale

Pierre shale was tested under an undrained condition within the University of Minnesota Plane-Strain Apparatus, which was modified to allow application and measurement of pore pressure. A rectangular, prismatic specimen was carefully machined and assembled with porous stones between the upper and lower steel platens; the specimen, porous stones, and platens were held together with a custom jig and a thin layer of polyurethane was applied to prevent confining fluid from penetrating the shale. Once air was removed from the system, equal back pressure was applied to the top and bottom of the specimen. The back pressure was incrementally (1 MPa) increased to 8 MPa, and held constant for ten days, with confining pressure 0.35 MPa higher than back pressure. The maximum B-value achieved was 0.63. The undrained, plane strain compression test was performed with an initial confining stress of 4 MPa, and an average lateral displacement rate of 10^-4 mm/s; peak stress was reached in about one hour. Measurements of axial and lateral displacements, axial load, confining and pore pressures provided the necessary information to calculate the undrained and drained elastic parameters of the rock within the framework of poroelasticity.