Mechanical properties of hydrate-bearing turbidite reservoir in the first gas production test site of the Eastern Nankai Trough

Abstract

The study of mechanical properties of marine sediments is essential for the prediction of the occurrence of geohazards (e.g., subsea landslides and seafloor subsidence) and the design of submarine structures for offshore industry. In this study, triaxial compression tests of gas-hydrate-bearing sandy sediments and clayey-silty sediments were conducted. The sediments were recovered by pressure coring in the Eastern Nankai Trough, the area of the first Japanese offshore production test. Soil index properties were measured and revealed porosity of 40%–50%, with porosity decreasing gradually with greater depth below the seafloor. The mean particle size was less than 10 μm for clayey-silty sediments and approximately 100 μm for sandy sediments. Permeability, estimated by a consolidation process of triaxial testing and with X-ray diffraction analysis, depended on the content of fines, which consisted chiefly of mica, kaolinite, and smectite. The results of undrained compression tests for clayey-silty sediments showed positive excess pore pressure under all test conditions. This mechanical behavior indicates that the core samples are normally consolidated sediments. Drained compression tests showed that the strength and stiffness of sandy sediments increase with hydrate saturation. Furthermore, the volumetric strain of hydrate-bearing sediments changed from compression to dilative. This result was obtained for hydrate saturation values (Sh) of more than 70%. The shear strength of hydrate-bearing turbidite sediments of the Eastern Nankai Trough is shown to be a function of the confining pressure.