Effect of Field Caprock Shale Exposure to CO₂ on Its Mechanical Properties—A Comparison of Experimental Techniques

Laboratory tests were performed on the Draupne shale formation, which may serve as a seal over CO2 storage sites. Different techniques were used to assess the integrity and mechanical properties of the shale, with the main objective of investigating whether exposure to CO2 would in any manner alter these properties. The laboratory methods used encompass traditional triaxial tests; however, with fluid substitution prior to increasing axial stress to failure. These tests were conducted on smaller cylindrical plugs than standard, taking advantage of the finer grained nature of the shale. Another set of experiments used the low-frequency technique, whereby small amplitude, cyclic axial strains are applied on the specimen, allowing a direct measurement of stiffness. Long exposure, with change of fluid from brine to CO2, allowed for quantifying small changes in stiffness, thanks to the many repeated cycles of non-destructive testing. In a final experimental technique, the punch test, shear strength of the same material was obtained by cutting a central disk from a larger intact shale disk, while measuring the shear force needed to perform the cut.