Methane Gas Hydrate Morphology and its Effect on theStiffness and Damping of some Sediments

Gas hydrates are ice–like compounds found in deep sea sediments and permafrosts. Concise detection and quantification of natural methane gas hydrate deposits, will allow for a more robust assessment of gas hydrate as a potential energy resource or natural geohazard. Current seismic methods, used to identify and quantify gas hydrates, have proved to be unreliable in providing accurate information on the extent of natural gas hydrate deposits, due to the lack of understanding on how gas hydrate affects the host sediment. Direct measurement of some hydrate bearing sediment properties has been made possible in recent years through advances in pressure coring techniques, but methods for dynamically testing these samples at in–situ pressures are still unavailable. Laboratory tests on synthetic hydrate bearing sediments have shown that factors such as formation technique, sediment type and use of hydrate former affects the form and structure of hydrate in the pore space and how it interacts with the sediment. The aim of this research was therefore to create methane hydrate in sediments under a variety of conditions, so that the influence of hydrate morphology could be investigated. A number of experiments were conducted using two distinct formation techniques. The first technique formed methane hydrate from the free gas phase in almost fully water saturated conditions. Five sand specimens, with a range of hydrate contents from 10% to 40% were formed and tested in the gas hydrate resonant column (GHRC). Results from these tests were compared with previous results from tests where methane hydrate had been formed from free gas in partially saturated conditions. It was found that formation method had a significant influence on the properties of the hydrate bearing sand, and therefore the morphology of the hydrate in the pore space. The second set of experiments formed methane hydrate from free gas within partially saturated sediments, but where the sediments were made up of coarse granular materials with a variety of particle size and shape. As it had been established that hydrate acts as a cement when formed under partially saturated conditions, the experiments aimed to observe the effect of particle size and shape on hydrate bonding mechanisms. The results showed that the influence of disseminated hydrate on the physical properties of the specimens was affected by both mean particle size and by particle shape, with the surface area of the sediment grains influencing the volume and distribution of hydrate throughout a material and therefore it’s bonding capabilities. In addition to the experiments on synthetic hydrate specimens, five core sections containing naturally occurring gas hydrate in fine grained sedimentsweremade available to the University of Southampton from the Indian National Gas Hydrate Program (NGHP) 01 expedition. High resolution CT imaging of the core sections observed large volumes of methane hydrate as a network of veins throughout the specimens. Due to sample disturbance caused during the depressurisation and subsequent freezing of the samples prior to delivery, dynamic testing in the gas hydrate resonant column apparatus was not feasible. Therefore, the hydrate was dissociated and a number of geotechnical tests were undertaken on the remaining host sediment. Results from these tests suggested that hydrate dissociation could affect host sediment properties, due to a change in water content, salinity and structure.