Brittle–ductile transition, shear failure and leakage in shales and mudrocks

This paper presents an experimental study on brittle–ductile transition, shear behaviour, the formation of shear fractures in shales and mudrocks, and the hydraulic properties of shales and mudrocks with shear induced fractures. The experimental study is based on triaxial tests on different mudrocks and shales from North Sea reservoirs and adjacent areas sheared at different effective confining stresses. It is shown that during burial, mudrocks behave as normally consolidated materials and exhibit ductile response to increased load. However, several processes like uplift, chemical diagenesis and overpressure build-up turn mudrocks into overconsolidated materials which exhibit brittle behaviour during loading. Triaxial tests with different loading conditions have been performed to establish the brittle-to-ductile transition of various mudrocks. Based on the results of the tests, it is shown that the brittle-to-ductile transition can be related to the overconsolidation ratio. A relationship between normalized undrained shear strength and overconsolidation ratio is established. In combination with this relationship, a correlation between compressional wave velocity and apparent pre-consolidation stress, which accounts for both mechanical and chemical diagenesis, may be used as a tool to evaluate possible leakage of hydrocarbon seals.