Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications

Abstract 159

Impact of Sedimentary Mineralogy on Geophysical and Geomechanical Properties of Hydrate-bearing Sediments

Yang, J., Chapoy, A., and Tohidi, B.
6th International Conference on Gas Hydrates, Vancouver, Canada, July 6-10 (2008)

Gas production from gas hydrates and deepwater drilling through hydrate-bearing sediments could severely disturb seafloor stability. Instable seafloor may trigger local subsea landslides, which could lead to the release of large quantities of methane into atmosphere, or wellbore collapse damaging drilling facilities and also causing safety concerns. To have better understanding of the impact of sedimentary mineralogy on the stability of marine sediments containing gas hydrates, a series of experiments were conducted using various mineral compositions of sediments, including silica sand, sand + 7 mass% kaolinite clay, sand + 7 mass% smectite clay, sand + 20 mass% kaolinite clay, sand + 20 mass% smectite clay. Geophysical properties of the sediments were determined after methane hydrate formation, including density, compressibility, acoustic velocities (both VP and VS), and bulk modulus and shear modulus. Results of the experiments show that the sediments containing high concentration of clays have significantly higher compressibility than others tested in this work. Furthermore, under low effective stress, sediments containing kaolinite showed higher compressibility than those containing the same concentration of smectite. The presence of clays in sand packs seemed to have a greater impact on shear elasticity of hydrate-bearing sediments than bulk elasticity. Geomechanical response to methane hydrate decomposition in the sediments was examined through a series of depressurisation experiments. It was observed that the higher the clay concentration in sediments containing methane hydrates, the softer the sediments. Once the pressure decreased to or outside the methane hydrate stability zone, some large spikes of instantaneous strains occurred, which could indicate a potential risk to trigger large subsea landslides under real conditions. Furthermore, it was found that hydrate decomposition in sediments containing smectite may cause higher risks to seabed stability compared with those containing kaolinite.

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