On the Border Between Brittle and Ductile Behavior of Shale

Experience from North Sea wells shows that shale may be able to form sealing barriers around cased boreholes. Laboratory experiments with hollow cylindrical shale cores with a centralized tube in the borehole demonstrate the same phenomenon. These tests identify under which conditions and with what kind of shales barriers are formed. The focus of the present study is shale characterization, primarily through consolidated undrained (CU) triaxial tests, to assess attributes of barrier forming shales and to better understand the mechanisms. The pore pressure evolution in these tests seem to play a key role in deciding to what extent shale fails in a ductile manner, leading to sealing shale barriers, or in a more brittle manner, where possibly formed barriers are not sealing. In the analysis, Skempton parameters have been determined at different stages in the tests, demonstrating how the failure mode can be driven by the pore pressure changes. Soil mechanics concepts of normally consolidated vs overconsolidated behavior are valid for distinguishing between brittle and ductile shales.