Research paperPCATS Triaxial: A new geotechnical apparatus for characterizing pressure cores from the Nankai Trough, Japan

- PCATS Triaxial developed for measuring physical properties of pressure core samples.
- Transfer of pressurized core sub-samples into apparatus successfully achieved.
- Stiffness and strength of intact natural gas hydrate-bearing sediments measured.
- Permeability of sediment before and after hydrate dissociation determined.
- PCATS Triaxial can help understand the influence of gas hydrate on sediment behaviour.

Understanding the physical nature and mechanical behaviour of hydrate-bearing sediments is of fundamental importance in assessing the resource potential of methane gas hydrates. Advances in pressure coring techniques and associated processing equipment have enabled intact samples to be recovered under in-situ pressures. However, testing of these samples under the in-situ stress conditions has not been possible. To help address this issue, the PCATS Triaxial apparatus was developed to enable the physical properties of such samples to be measured. The apparatus was deployed for the first time during the JOGMEC funded site investigation of the Eastern Nankai Trough in the summer of 2012, prior to a planned hydrate production test in 2013.A number of pressurized core were recovered and sub-samples successfully tested in PCATS Triaxial to determine a range of geomechanical properties, including small strain stiffness (from resonance testing), stress-strain properties (triaxial shear tests) and permeability. Samples tested included fine-grained soils with no appreciable hydrate, sands with hydrate saturation greater than 20%, and one sample that had a combination of both materials. Testing showed an increase in stiffness and undrained shear strength with increasing grain size, hydrate saturation and applied effective stress. Permeability was significantly reduced for hydrate-bearing sands compared to clayey samples with no hydrate present.