Effect of stress field and mechanical deformation on permeability and fracture self-sealing. Progress report on the Stress Path Permeameter experiment conducted on Callovo-Oxfordian Claystone

Abstract

This report describes in detail the stress-path permeameter (SPP) apparatus and the first test 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) 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 show that the CoX has a very pronounced time-dependent component of deformation. This has implications for the following tests conducted on CoX as part of the FORGE project, but also has implications when comparing tests on CoX 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 sixteenth 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. Test SPP-CoX-1 will be used to determine the stress-path steps of test SPP-CoX-2. This much more detailed test will follow 5 or 6 steps up to ultimate failure and will include a full hydraulic test, a gas injection test to determine the gas entry pressure, and constant pore-pressure flow tests to examine the changes in permeability along the stress-path. At all times throughout the test radial and axial deformation will be monitored