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| EPSRC Reference: |
EP/D052556/1 |
| Title: |
Capillary controls on gas hydrate growth and dissociation in synthetic and natural porous media: PVT, NMR, Neutron Diffraction and SANS |
| Principal Investigator: |
Professor B Tohidi |
| Other Investigators: |
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| Researcher Co-investigator: |
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| Project Partner: |
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| Department: |
Institute Of Petroleum Engineering |
| Organisation: |
Heriot-Watt University |
| Scheme: |
Standard Research |
| Starts: |
01 February 2006 |
Ends: |
31 January 2009 |
Value (£): |
285,731
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| EPSRC Research Topic Classifications: |
| Multiphase Flow |
Oil and Gas Extraction |
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| EPSRC Industrial Sector Classifications: |
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| Related Grants: |
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| Panel History: |
| Panel Date | Panel Name | Outcome |
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17 Nov 2005
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Engineering Science (Flow) Panel
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Announced
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Summary |
Gas hydrates are ice-like solids which form from water and gas molecules at low temperature and high pressure conditions. Within the hydrate structure, water molecules form a network cage-like cavities of varying size within which gas molecules are trapped in a compressed form.
In the 1970's it was recognised that very large quantities of methane gas hydrate occur naturally in sediments of the subsea continental slopes and the subsurface of Arctic permafrost regions. Since this discovery, global interest in methane hydrates has grown steadily, with research expanding particularly rapidly over the past decade. Important issues driving research include the potential for methane hydrates as an energy resource, the possibilities for CO2 disposal as gas hydrates beneath the seafloor, increasing awareness of the relationship between seafloor hydrate destablisation and large subsea landslides, the potential hazard hydrate destabilisation could pose to deepwater oil/gas platforms, pipelines and subsea cables, and long-term considerations with respect to hydrate stability, methane (a potent greenhouse gas) release to the atmosphere, and global climate changes.
In the past, models for the formation and distribution of gas hydrates in marine sediments generally assumed that laboratory measurements on bulk (no sediments present) water-gas systems could be directly applied to the natural environment. Ocean floor drilling has confirmed that the Base of Hydrate Stability Zone (BHSZ) in seafloor sediments commonly lies close to pressure and temperature conditions calculated from bulk laboratory hydrate measurements, however there are a number of sites where the thickness of the Hydrate Stability Zone (HSZ) is much less than predicted, suggesting that host sediments are somehow acting to inhibit hydrate growth and/or stability.
The mechanisms by which sediments may alter hydrate stability are still poorly understood. Variations in gas composition (e.g. the addition of CO2) can promote hydrate stability, while saline pore waters will act to inhibit hydrates. However, where gas and pore water salt concentrations are reasonably well established, alternative mechanisms of inhibition must be considered when predicted and actual BHSZs do not agree. One factor that could potentially alter the stability of gas hydrates and influence their distribution within sediments is pore size and geometry.
It is well-established that, when confined to narrow pores, fluids can be subject to very high internal (capillary) pressures. High capillary pressures can result in changes in the temperature/pressure conditions where phase transitions such as liquid freezing and melting take place. As sediments which host gas hydrates are commonly characterised by fine-grained silts, muds and clays, often with quite narrow mean pore diameters, capillary inhibition has previously been proposed as a mechanism to explain the observed differences between predicted and actual hydrate stability zones.
The aim of this work is to examine the relationship between pore size, geometry, capillary pressures and gas hydrate growth and dissociation conditions in synthetic and natural sediments, and to assess the extent to which capillary inhibition is a factor in seafloor/permafrost hydrate systems.
A variety of experimental approaches will be used to investigate capillary effects on hydrate growth from the micro (pore) to macro (core scales). Novel synthetic pore micromodels will be used to visually study hydrate crystal growth patterns at the pore scale, complimenting and supporting large volume, long-duration, pressure-volume-temperature-composition measurements on sediment cores, while Nuclear Magnetic Resonance (NMR) will be used to probe fluid states (hydrate, water, gas) and distribution within pores. Experimental data will be combined to develop a model capable of predicting hydrate growth and dissociation conditions as a function of sediment pore size distribution.
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| Final Report Summary |
Gas hydrates are ice-like solids formed from water and gas (e.g. methane, CO2) at low temperatures and high pressures. Within the hydrate structure, water molecules form a network of cage-like cavities which trap the gas molecules in a compressed form; 1 m3 of methane hydrate can hold up to 170 m3 of methane gas. Very large quantities of methane hydrate occur naturally in sediments of the subsea continental slopes and Arctic permafrost regions. Since their discovery in the 1970's, global interest in gas hydrates has grown steadily, with research expanding particularly rapidly over the past decade. Important issues driving research include the potential for methane hydrate production as a low carbon energy resource, the possibilities for utilising gas hydrates in subsea/subsurface CO2 disposal, increasing awareness of the link between hydrate destabilisation, large subsea landslides and tsunami generation, the potential hazards hydrates pose to deepwater oil/gas drilling operations, pipelines and subsea cables, and long-term considerations with respect to hydrate stability, methane (a potent greenhouse gas) release to the atmosphere, and global climate change.
The primary aim of this project was to examine the role of host sediment induced capillary effects on hydrate growth/dissociation conditions and what role these play in controlling formation/distribution in natural sediments. Additional factors which might influence this behaviour, including the effect of free gas (bubbles) and confining pressure (i.e. whether sediments were soft/unconsolidated or more rigid/rock-like) were also to be addressed. Two main experimental approaches were employed: (1) traditional Pressure-Volume-Temperature (PVT) studies and (2) advanced Nuclear Magnetic Resonance (NMR) techniques.
Through the combined experimental approach used, the project was successful in achieving its primary aims. Results confirm that capillary effects have a major influence on hydrate growth/dissociation in both synthetic and real sediment samples, even in those with very large pore diameters such as sandstone. Analyses of growth/disassociation hysteresis patterns for a variety of samples with different pore structures/size distributions showed that while dissociation conditions are primarily dependent on pore diameter/pore shape, growth conditions are additionally dependent on how pores are connected to each other; this factor being a major cause of hysteresis effects.
Importantly, contrary to widespread belief, results demonstrate that sediments do not have to be rigid for capillary inhibition to take place. While hydrate segregation (sediment grains displaced by growing hydrate masses) with minimal inhibition occurs in water saturated sediments, when significant free gas is present as bubbles in the pore space, hydrate is forced to grow in the narrow space at sediment grain contacts, 'cementing' hydrate grains, but also reducing growth/dissociation conditions to significantly lower temperatures. Furthermore, experiments also showed that while capillary pressures are even through samples, the distribution of gas/water/hydrates is not; capillary forces can readily result in large areas of the pore space being gas saturated while other regions are hydrate and/or water saturated. In addition, during the hydrate growth/dissociation process, balancing of capillary forces in response can cause significant gas/water/hydrate mobility/redistribution with pores. These findings will significantly aid researchers developing geophysical models for gas hydrates, particularly with respect to seismic identification/quantification, gas production from hydrate deposits, and their link to seafloor stability. In addition to experimental studies, as a core goal of the project, an improved model was also developed for predicting gas hydrate growth/dissociation conditions in natural sediments. This software has been licensed to 4 major international oil and gas companies.
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| Further Information: |
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| Organisation Website: |
http://www.hw.ac.uk |
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