The University of Southampton
University of Southampton Institutional Repository

Effect of dissociation on the properties of hydrate bearing sediments

Effect of dissociation on the properties of hydrate bearing sediments
Effect of dissociation on the properties of hydrate bearing sediments
Gas hydrates are clathrate hydrates, which are solid, ice-like compounds. Gas hydrates exist where there is an ample supply of gas and water combined with high pressure and/or low temperature conditions. In nature these are found in sediments where permafrost is present, and in deep-marine sediments. The morphology of gas hydrate within a sediment has a large impact on the strength and stiffness properties of hydrate bearing sediments. Gas hydrates are metastable and they dissociate if the temperature and/or pressure conditions are sufficiently altered. The dissociation of gas hydrate and its potential as a submarine geohazard have become of increasing importance as oil and gas exploration activities extend into significant water depths on continental margins and seas where gas hydrates are known to exist. Such activities may lead to dissociation of hydrate, possibly increasing pore pressure, and altering the stiffness and strength of the sediment. Due to difficulty in performing field testing and obtaining undisturbed in-situ samples for testing, at present, hydrate dissociation in the natural environment and its effects are hypothesised on the basis of remote observations. Therefore, a series of well-controlled laboratory tests were conducted on laboratory-prepared methane hydrate bearing sand sediments.

The tests were undertaken with hydrate saturation ranging from 7% to 27% in the Gas Hydrate Resonant Column Apparatus (GHRC). Factors such as effective stress were also assessed with regard to specimen stiffness. Resonant column testing during hydrate formation and dissociation processes carried out for the first time, such that not only final change in specimen properties to be determined as a function of total hydrate saturation but also the change in specimen properties as function of the percentage of hydrate formation and dissociation. Test results showed that a rapid reduction in stiffness occurred for a minor change in hydrate saturation of sand specimens where dissociation was induced by temperature increase, but for specimens that were dissociated using the pressure reduction method a slower reduction occurred. In contrast, during hydrate formation stiffness increased more gradually. In addition, test results showed that the hydrate formation using the excess gas method led to higher increases in the shear stiffness compared to the flexural stiffness of specimens, and the linear stiffness threshold limit of hydrate bearing specimens were lower than the non-hydrate bearing sands.

In addition to laboratory tests, an analytical model was built to predict the increase in pore pressure under undrained conditions within hydrate bearing sediment during dissociation. The results obtained from the laboratory tests were used to compare the predicted results from the model. Analytical model showed that the rise in pore pressure within a sediment was dependent on a number of factors: major factors were initial pore pressure, amount of hydrate dissociation, cage occupancy of gas within hydrate, stiffness of the sediment, and degree of water saturation; Minor factors were methane gas solubility in water, and methane hydrate density.
Sultaniya, Amit Kumar
7eb54122-3ec8-4a87-be5d-0434b817fee8
Sultaniya, Amit Kumar
7eb54122-3ec8-4a87-be5d-0434b817fee8
Priest, J.A.
2c22f55d-ea01-4485-9a16-d5c79f79a972

Sultaniya, Amit Kumar (2011) Effect of dissociation on the properties of hydrate bearing sediments. University of Southampton, School of Civil Engineering and the Environment, Doctoral Thesis, 215pp.

Record type: Thesis (Doctoral)

Abstract

Gas hydrates are clathrate hydrates, which are solid, ice-like compounds. Gas hydrates exist where there is an ample supply of gas and water combined with high pressure and/or low temperature conditions. In nature these are found in sediments where permafrost is present, and in deep-marine sediments. The morphology of gas hydrate within a sediment has a large impact on the strength and stiffness properties of hydrate bearing sediments. Gas hydrates are metastable and they dissociate if the temperature and/or pressure conditions are sufficiently altered. The dissociation of gas hydrate and its potential as a submarine geohazard have become of increasing importance as oil and gas exploration activities extend into significant water depths on continental margins and seas where gas hydrates are known to exist. Such activities may lead to dissociation of hydrate, possibly increasing pore pressure, and altering the stiffness and strength of the sediment. Due to difficulty in performing field testing and obtaining undisturbed in-situ samples for testing, at present, hydrate dissociation in the natural environment and its effects are hypothesised on the basis of remote observations. Therefore, a series of well-controlled laboratory tests were conducted on laboratory-prepared methane hydrate bearing sand sediments.

The tests were undertaken with hydrate saturation ranging from 7% to 27% in the Gas Hydrate Resonant Column Apparatus (GHRC). Factors such as effective stress were also assessed with regard to specimen stiffness. Resonant column testing during hydrate formation and dissociation processes carried out for the first time, such that not only final change in specimen properties to be determined as a function of total hydrate saturation but also the change in specimen properties as function of the percentage of hydrate formation and dissociation. Test results showed that a rapid reduction in stiffness occurred for a minor change in hydrate saturation of sand specimens where dissociation was induced by temperature increase, but for specimens that were dissociated using the pressure reduction method a slower reduction occurred. In contrast, during hydrate formation stiffness increased more gradually. In addition, test results showed that the hydrate formation using the excess gas method led to higher increases in the shear stiffness compared to the flexural stiffness of specimens, and the linear stiffness threshold limit of hydrate bearing specimens were lower than the non-hydrate bearing sands.

In addition to laboratory tests, an analytical model was built to predict the increase in pore pressure under undrained conditions within hydrate bearing sediment during dissociation. The results obtained from the laboratory tests were used to compare the predicted results from the model. Analytical model showed that the rise in pore pressure within a sediment was dependent on a number of factors: major factors were initial pore pressure, amount of hydrate dissociation, cage occupancy of gas within hydrate, stiffness of the sediment, and degree of water saturation; Minor factors were methane gas solubility in water, and methane hydrate density.

Text
AmitKumarSultaniya_PhD_Thesis.pdf - Other
Download (9MB)

More information

Published date: November 2011
Organisations: University of Southampton, Faculty of Engineering and the Environment

Identifiers

Local EPrints ID: 210950
URI: http://eprints.soton.ac.uk/id/eprint/210950
PURE UUID: 83c56a5f-01da-4b97-a86f-4b6df1b16674

Catalogue record

Date deposited: 21 Mar 2012 16:31
Last modified: 29 Jan 2020 14:56

Export record

Contributors

Author: Amit Kumar Sultaniya
Thesis advisor: J.A. Priest

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of http://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×