Original AimsGas 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.
Additional tests not in the original proposal were also performed on ice-water systems, specifically in relation to undersaturated (with respect to water) systems. This was required to aid understanding of methane systems by eliminating the effect of gas consumption by the hydrate. These experiments turned out to be very worthwhile; NMR results in particular revealing that water near pore walls, rather than being an ‘unfrozen liquid’ as commonly believed, water molecules can in fact be in a state of predominantly rotational motion, thus are better described as “plastic ice”.
In the 1970's it was recognised that very large quantities of methane hydrate occur naturally in sediments of the subsea continental slopes and Arctic permafrost regions. Since this discovery, 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 tsunamis 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.
While our knowledge of gas hydrates has grown considerably in recent years, there are still many basic, fundamental processes concerning their formation and distribution that are poorly understood. This makes identification, quantification and prediction of hydrate behaviour difficult. In the past, models for the formation and distribution of gas hydrates in sediments generally assumed that laboratory measurements on bulk (no sediment) systems could be directly applied to the natural sedimentary environment. While ocean floor drilling has confirmed that the Base of Hydrate Stability Zone (BHSZ) commonly lies close to pressure and temperature conditions calculated from bulk laboratory hydrate measurements, there are a number of sites where the thickness of the Hydrate Stability Zone (HSZ) is less than predicted, suggesting that host sediments are somehow acting to inhibit hydrate growth and/or stability.
The mechanisms by which sediments themselves may affect hydrate stability and distribution are not fully appreciated. Variations in gas composition (e.g. the addition of CO2) can promote hydrate stability, while saline pore waters will act to inhibit hydrates as they do ice. 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, crystals and fluids (gases, liquids) can be subject to very high additional internal 'capillary' pressures. High capillary pressures can result in changes in the temperature/pressure conditions where transitions such as liquid freezing and melting take place. As sediments hosting gas hydrates are commonly fine-grained silts, muds and clays, often with 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. Likewise capillary phenomena could also help explain the differences in hydrate growth patterns according to sediment type; in coarse sandy sediments with large pores, hydrates typically grow between grains, filling the available pore space, whilst in fine sediments they generally segregate into distinct layers, pellets or sheets.
Previous studies have confirmed that gas hydrate dissociation (melting) takes place at lower temperatures in narrow pores, confirming capillary effects could play an important role. However, work has primarily focussed on synthetic materials such as porous silicas, rather than real sediments. Furthermore, investigations only considered hydrate dissociation conditions, not growth, which established capillary theory predicts can be significantly different. Preliminary work in this laboratory suggested this was the case, at least for synthetic silicas, with hydrate growth/dissociation patterns showing a clear 'hysteresis'; growth apparently inhibited to even lower temperatures than dissociation.
Thus the primary aim of the project was to determine the origins of this hysteresis and whether it also occurred 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.
|PVT studies are a very reliable, tried and tested method which allow monitoring of hydrate growth and dissociation in sediment samples as a function of temperature through detection of pressure changes. As gas is compressed into hydrates when they form and released on dissociation/melting, precise measurements of pressure allow the volume of hydrate in sediment pores to be determined in relation to temperature, pore water salinity and gas composition. Over the course of the project, established PVT methods for bulk phase equilibria were extended to the determination of hydrate formation/dissociation conditions in porous materials, including the development of novel means to circumvent the problem of supercooling commonly associated with the hydrate nucleation process, allowing accurate measurement of hydrate growth conditions. While developed PVT methods were found to be very reliable and repeatable, yielding high-quality data, issues were apparent when applying these to some systems. For example, when free gas bubbles are present in pores, the gas itself is subjected to an additional capillary pressure which could potentially overprint changes caused by hydrate growth/melting. This is where NMR studies were very advantageous; NMR is not subject to the same problem as it measures the properties of gas and water molecules directly.|
In NMR, powerful static and radio frequency magnetic fields are applied to samples. These are used to change the properties (polarisation) of the nuclei of hydrogen molecules in water and methane away from their normal equilibrium state. The resultant tiny radio frequency signal from the nuclei may be detected and used to measure the time-scales of the return to equilibrium following a pulse, which is determined by the physical interactions between nuclei. Thus the physical and dynamic state of the pore water, the hydrate cage and the methane molecules can be determined, i.e. whether the pore water is liquid, gas hydrate or ice, and/or whether methane is free gas or trapped in hydrate cages. Over the course of the project, a 'Cryoporometry' technique previously developed for characterising pore structures using ice-water melting behaviour was successfully extended to high-pressure gas-water-hydrate systems. Data generated using this new method were used to support PVT studies, providing vital physical information which the latter alone could not provide.
To better characterise samples, a number of successful applications were made for Neutron Diffraction (ND), Small Angle Neutron Scattering (SANS) and Quasi-Elastic Neutron Scattering (QENS) studies. These techniques involve passing a high-intensity beam of neutrons through sample materials; as the neutrons encounter crystal structural planes and pore walls they are diffracted and scattered. Analyses of diffraction/scattering patterns yielded detailed information on pore structures which were used to better understand gas hydrate growth/dissociation and gas/water/hydrate distribution patterns within them.
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; the hysteresis patterns previously observed in synthetic silicas were confirmed and found to also occur in real sediment samples, even 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. The thermodynamic model uses an advanced 'Cubic Plus Association Equation of State' (CPA EoS), which has significant advantages over traditional models in that hydrogen bonding between water and other molecules (the 'association' component) is accounted for, yielding more reliable predictions. The model was developed with a fully-functioning GUI (graphical user interface) for Microsoft Windows. The user can input gas composition, salinity, and specify pore size distribution parameters, including geometric information. Based on this, gas hydrate growth and/or dissociation conditions can be predicted as a function of temperature (at constant pressure) or pressure (at constant temperature). A more advanced, specifically tailored commercial version of the software has subsequently been developed for industrial use as part of a follow-on project sponsored by industry. This software was licensed to 4 major international oil and gas companies for predicting natural gas hydrate stability near oil/gas wells; the goal being to avoid hydrate dissociation which could cause sediments surrounding wells to destabilise, potentially resulting in well casing collapse, dangerous uncontrolled gas/oil release (i.e. a 'blow-out') and marine pollution.
With respect to gas hydrates, an improved understanding of hydrate growth and dissociation mechanisms in natural and synthetic porous media seafloor should benefit scientists/engineers working in the fields of:
Gas hydrates in general: The proposed project involves measurements of fundamental hydrate phase equilibria, kinetics, and physical properties (e.g. gas solubility, solid-liquid-gas interfacial properties) of systems. This data should be useful to the gas hydrate research community in general. Ocean margin processes, subsea slope stability and global climate change: Gas hydrates play an important role in ocean margin processes and the close relationship between hydrate destabilisation and mass seafloor slope failure is well established (e.g. the Storegga Slide, Norwegian Atlantic margin). Massive methane release from hydrates is believed responsible for periods of rapid global warming in the geological past. The project will provide researchers with basic data for the tuning/validation of models for hydrate growth, accumulation and dissociation in seafloor and permafrost sediments, facilitating improved models for hydrate response and greenhouse gas (methane) release due to environmental pressure/temperature changes.
Improved oil/gas recovery (IOR): Gas hydrate formation in reservoir pore space during IOR operations such as water-alternating-gas (WAG) injection operations is a recognised problem. Detailed information on the mechanisms of hydrate growth in pores should help in the development of techniques to mitigate such problems.
CO2 disposal: Storage of CO2 as hydrates beneath the seafloor has been proposed as a means for CO2 disposal. There is also the possibility that hydrates could act as a secondary seal to sequestered CO2 escaping to the seafloor from storage sites such as saline aquifers or redundant oil/gas reservoirs. Knowledge of fundamental factors controlling hydrates in the seafloor should help in assessing such schemes and the likelihood of gas leakage back to the atmosphere.
In addition to benefiting researchers and engineers working on the above issues, the results of the project should be of interest to workers in a variety of fields. The behaviour of fluids confined to narrow pores is important to many processes including industrial catalysis, gas storage and transportation, soil chemistry and frost heave, biological processes such as protein folding or ionic transport in membranes and the enzymatic activity of proteins.