Triaxial compression and extension test accompanied by ultrasonic and permeability measurements
The highly porous and permeable Castlegate Sandstone (k: 350 mD, φ: 25%) was exposed to a confining pressure of 34 MPa and subjected to axial extension and also axial compression up to failure at 150 MPa (see green stress-strain curve in the graph below.). The porous Castlegate Sandstone compacts strongly during compression. Thus the loss of porosity leads to a decrease in permeability (blue curve) and superimposes the effect of microcrack formation which is recorded by the deceleration of both compression wave velocity (pink triangles) and shear wave velocity (red crosses). Micro crack orientation is parallel to the largest stress component. In the extension test (left part of the diagram) the behaviour of the sonic velocities are exactly opposite to the compression test: here the velocity reduction begins immediately, whereby the effect is stronger on the P wave (for details see (Braun & Jahns 1998).