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

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

Name dc:type.qualificationname

Ph.D.

Level dc:type.qualificationlevel

doctoral

Grantor dc:publisher.institution

University of Southampton

Year dc:date.issued

2009

Author dc:creator

• Rees, Emily V.L.

Advisors dc:contributor.advisor

• Clayton, Chris
• Priest, Jeff
• Best, Angus