Determination of shear properties and evaluation of fracture reactivation for a clay-rich shale: a case study from Svalbard, Arctic Norway

摘要

This paper presents results of the direct shear tests on pre-fractured specimens from Rurikfjellet Formation, Svalbard, Arctic Norway. It also evaluates the risk of shear failure along pre-existing fractures due to possible over pressure. Rurikfjellet Formation is composed of clay-rich shale and is part of the cap rock for the proposed CO2 storage pilot at Svalbard. Shear properties of the cap rock system are of great importance for the integrity of the reservoir beneath. Three specimens from Rurikfjellet Formation were subjected to direct shear test. Results of the experiments showed that the peak friction coefficient was slightly higher than the residual coefficient. Shear stiffness of the specimens varied between 19 and 21 MPa/mm determined from secant method and between 26 and 32 MPa/mm determined from tangent method. The specimens showed slight dilation in the very initial phase of shearing but exhibited strong compression with increased displacement (and normal stress). This indicates that a slip along fractures in Rurikfjellet shale may be governed by compression which in turn implies self-sealing of fractures that may occur during a shear displacement. Evaluation of fracture reactivation under stress conditions close to that of the current situation at Svalbard showed that there is a safety margin of about 4 to 5 MPa overpressure before a failure may occur along the critically oriented fractures. Overall, the measured compression behaviour and the estimated safety margin for Rurikfjellet shale suggest that this unit may act as a good seal for the proposed CO2 storage reservoir.