PCATS triaxial testing: Geomechanical properties of sediments from pressure cores recovered from the Bay of Bengal during expedition NGHP-02

Abstract The Indian National Gas Hydrate Program Expedition 02 (NGHP-02) was the most comprehensive gas hydrate drilling expedition carried out to date. From 74 pressure cores that were successfully recovered from various site locations offshore India, 19 subsamples were obtained for further detailed geotechnical analysis using the PCATS Triaxial apparatus to investigate the geomechanical behaviour of the cores, including small-strain stiffness, G max , undrained shear strength, S u , as well as direct-flow measurements of permeability, k. Samples tested included fine-grained muds with no appreciable hydrate through to coarse-grained sands that contained extensive hydrate within the pore space. Although the subsampling and transfer of pressure cores into the PCATS Triaxial under zero effective confining stress, σ′ , remains challenging, 12 samples were successfully transferred and underwent detailed geotechnical characterization. During consolidation, after reapplication of in-situ σ′ , all samples experienced axial and radial consolidation, with variations between samples seemingly dependent on the degree of sample disturbance, stress relaxation between coring and testing, hydrate saturation, S h and grain size, D 50 . The largest values of G max were obtained from samples with high S h in the pore space, with values much higher than that expected for similar sediments without hydrate under comparable σ’. Samples with high permeability had both high S h and larger grain size, suggesting that in coarse-grained sediments sufficient pathways are available for reasonable fluid flow even with high hydrate saturations. Triaxial testing and unconfined compression tests highlighted the influence of hydrate on increasing sediment strength, although the size of the sediment grains appeared to have an important control on the magnitude of the strength increase and the overall stress-strain behaviour. The results from the detailed geotechnical characterization of the recovered pressure cores will be invaluable in helping India identify potential locations for future gas production wells, and assess the long-term performance of such reservoirs.