The effects of methane gas hydrate on the dynamic properties of a sand

Priest, Jeffrey A (2004) The effects of methane gas hydrate on the dynamic properties of a sand. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Gas hydrates are solid, ice like, compounds that form in marine sediments and in permafrost regions. The methane gas stored in hydrates is considered by some to be an important future energy resource, whilst others are concerned that it may play a role in global warming, or be a geotechnical hazard during deep offshore oil and gas development. Because methane hydrates occur in deep water and dissociate rapidly when brought to the surface, direct methods of investigation cannot readily be used to assess their global distribution and concentration. Remote seismic geophysical prospecting methods, which depend upon variations in the compressional wave velocity (Vp), the shear wave velocity (Vs), and damping (Q_P^-1 and Q_S^-1), can be of great value in measuring insitu properties. However, a correct interpretation of acquired seismic data requires an understanding of the seismic properties of hydrate-bearing sediments. The aims of this research were therefore to develop a technique for producing synthetic gas hydrates in the laboratory, to construct apparatus allowing measurement of the physical properties of hydrate-bearing Sand specimens, and to determine the impact of hydrate content on dynamic properties of a sand.

Thirteen sand specimens with differing volumes of hydrate within the pore space were tested using a specially designed and constructed resonant column apparatus. This allowed the dynamic properties of hydrate-bearing laboratory specimens to be measured under conditions relevant to current seismic methods. Results shows a bipartite relationship between velocity and hydrate content, with a transition zone between 3 - 5 % hydrate content, for both V_P and V_S. Effective stress stiffness dependency reduces rapidly with increasing but low levels of hydrate cementing, and remains low at higher hydrate contents. These facets of behaviour suggest an initially rapid cementation of sand grain contacts at low methane hydrate contents, followed by infilling of the pore space at higher hydrate contents. It is also shown that damping (Q_P^-1 and Q_S^-1) increases with hydrate content up to the transition zone and reduces thereafter to a value twice that for non-hydrate-bearing specimens These results, for the first time, show m detail how methane hydrate cementation affects seismic and geotechnical properties in typical quartz sand.