Strength and post-peak response of Colorado shale at high pressure and temperature

• Temperature effects on the behavior of Colorado shale were investigated.
• Strength envelopes were nonlinear with material weakening at high temperatures.
• A simple non-isothermal criterion was proposed to describe strength envelopes.
• Constitutive thermo-mechanical features of the material behavior were described.
• Post-peak deformations were shown to be governed by the smooth shear plane.

The paper presents the thermo-mechanical strength characterization of Colorado shale which serves as a cap rock to underlying oil-rich formations in Cold Lake area, east-central Alberta, Canada. Samples from the Westgate and Joli Fou formations were tested at high temperatures along triaxial compression stress paths with effective consolidation pressures and temperatures consistent with in-situ stresses and operative conditions. These lab tests were complemented with indirect tensile strength, unconfined compression and triaxial constant effective mean stress tests at ambient temperature for a more comprehensive knowledge of the material strength, particularly, at low confining pressures. The lab experimental results revealed brittle behavior of the tested shales with considerable strain softening. Both peak and post-peak strength envelopes of the tested material were observed to be nonlinear with a pronounced curvature at low mean stresses and to decrease at high temperatures. A simple thermo-mechanical failure criterion is proposed to capture the aforementioned constitutive features. This provides a modeling framework for determining the material strength close to the zone of tensile failure at different temperatures which is of utmost relevance to cap rock integrity assessments. Finally, measured axial and radial deformations as well as X-ray tomography images were used to verify that the observed constant rate of dilation in the post-peak response of the samples was a reflection of the morphology of the virtually smooth shear plane, and not representative of the material behavior.