Final report of FORGE WP4.1.1: the stress-path permeameter experiment conducted on Callovo-Oxfordian Claystone

This report describes in detail the stress-path permeameter (SPP) apparatus and the test programme conducted on Callovo-Oxfordian (COx) Claystone from the Bure underground research laboratory (URL) in France. Funding for this study has been provided by the French radioactive waste management operator, Andra, the European Union (FORGE Project, Project number 230357) and the British Geological Survey through its well-founded laboratory programme and the Geosphere Containment project (part of the BGS core strategic programme). The results from the first test conducted using the SPP showed that COx has a very pronounced time-dependent component of deformation. This had implications for the following test conducted on COx and also has implications when comparing tests that have been deformed at a much faster rate. Test SPP_COx-1 was conducted with water as a test permeant at constant pore-pressure along a pre-defined stress-path. Volumetric deformation was observed during 16 steps along the stress-path, with considerable time-dependent deformation and anisotropy seen in radial strain. The 16th stage saw the sample fail through the formation of a fracture after the sample had experienced constant stress conditions for 5.5 days; this emphasises the observed time dependent deformation. The results from test SPP_COx-2 clearly showed that the sample dilated at the onset of gas propagation; dilatancy was observed in three radial and one axial direction. A component of this volumetric deformation was associated with changes in pore-pressure. However, pore-pressure variation cannot account for the full amount of strain recorded and a proportion of the strain observed was the result of gas migration by dilatant pathway formation. Prior to the sample attaining steady-state flow, outflow from the sample slowly reduced and the conductive features experienced self-sealing. Gas injection pressure was raised and back-pressure was carefully lowered; neither course of action re-initiated flow through the sample.