Geophysical properties and dynamic response of methane-hydrate-bearing sediments to hydrate formation and decomposition

Formation and decomposition of gas hydrates could have a major impact on geophysical and geomechanical properties of hydrate-bearing sediments, therefore, they play a crucial role in seafloor and wellbore stability. Subsea landslides and wellbore collapses are related to dynamic response of sedimentary formations to any geodynamic disturbances.

In this study, dynamic responses of methane hydrate-bearing sediments to hydrate formation and dissociation have been investigated using ultrasonic wave techniques. The results show that ultrasonic waves can be used to investigate the dynamic process of gas-hydrate formation and dissociation in sediments. Geophysical properties of methane-hydrate-bearing sediments composed of silica sand, and silica sand with kaolinite or montmorillonite were determined. The results demonstrate that the presence of the clays significantly increases the compressibility and shear modulus of the sediments. In depressurization tests, geomechanical responses show that methane-hydrate-bearing sediments suddenly become highly compressible at the hydrate dissociation pressure. Furthermore, the results show that the sediments with the clays, especially with montmorillonite, will deform more severely than pure silica sand when the system pressure reaches the dissociation point of methane hydrates. Large instantaneous strains at the dissociation pressure of methane hydrate might be a great risk to trigger large seafloor landslides and wellbore collapse.