Introduction and Historical Perspective:

Natural gas hydrates are crystalline solids composed of water and gas. The gas molecules (guests) are trapped in water cavities (host) that are composed of hydrogen-bonded water molecules. That's a first definition on Natural Gas Hydrates (or NGH), we could also provide the following one : Natural gas hydrates are non-stoichiometric, solid substances that consist of a low amount of gas molecules captured in a mesh cage system made up of water molecules. Those two definitions generally mean the same thing, in a different manners.

Historically, sir Humphry Davy witnessed the first the cristallizing of chlorine hydrates in 1811. However, in 1778, Joseph Priestley performed cold experiments in his Birmingham laboratory by leaving the window open before departing on winter evenings, returning the next morning to observe the results. He observed that vitriolic air (SO2) would impregnate water and cause it to freeze and refreeze, whereas marine acid air (HCl) and fluor acid air (SiF4) would not.With such experiments, Makogon and Gordejev (1992, unpublished data) suggest that Priestley might have discovered clathrate hydrates more than 30 years before Davy’s discovery. But in the facts, Natural gas hydrates were first documented by Sir Humphry Davy (1811).

After that, NGH started being a mere chemical curiosity in which gas and water are transformed into a solid. And approximately from 1934, with the contribution of two french researchers; Villard and De Forcrand, NGH has begun to play an important role in energy business. The problem of hydrate induced blockage in “wet gas” flow systems became a major flow assurance issue for petroleum engineers. The importance of pipeline blockage increased in the 70’s when plugging of even the largest diameter pipelines from offshore, arctic fields or the wells from high-pressure underground storage facilities were reported. Studies showed that large gas hydrate plugs form most often after shut-in pipelines or wells begin to flow.

Hydrates Formation and Structure:

When the constituents of hydrates come into contact under high pressure and low temperature conditions a solid structure at different types of crystals with higher densities than typical fluid hydrocarbons is formed. Hydrates are solid metastable compounds and their properties and stability depend upon temperature and pressure. Hydrates can easily form in pipelines and producing gas wells before the gas has been dehydrated.

Natural gas hydrates (NGH) form in raw multiphase flow as a result of crystallization occurring around the guest molecules at certain operating temperature and pressure conditions. The most widely observed guest molecules in natural gas mixture are methane, ethane, propane, i-butane, n-butane, nitrogen, carbon dioxide and hydrogen sulfide. However, among those, methane based NGH occurs the most naturally. NGH are composed of approximately 85-mol% guest molecule; therefore they have physical properties very close to ice (specially cristalline structure). NGH are part of a larger family of compounds called “Clathrates”, which are inorganic container compounds. Although there are many container-compounds and hydrate formers in crystal structure, the focus in this article is NGH formers and NGH structures.

In general, hydrates are classified by the arrangement of the water molecules in the crystal structure. All common natural gas hydrates belong to the three crystal structures: cubic
structure I (sI), cubic structure II (sII), hexagonal structure (sH) as shown in the Figure below. Structure I is formed with guest molecules having diameters between 4.2 and 6 Å, such as methane, ethane, carbon dioxide, and hydrogen sulfide. Larger (6 Å < d < 7 Å) single guest molecules such as propane or iso-butane will form structure II. Still larger molecules (typically 7Å<d<9Å) such as iso-pentane or neohexane can form structure H when accompanied by smaller molecules such as methane, hydrogen sulfide, or nitrogen.

                                                                                                  *Hydrate cristals (a) sI type, (b) sII type, (c) sH type

The crystal structures of NGH consisting of water molecules are hydrogen-bonded in a solid lattice. The interaction or degree of bonding between individual water molecules and the
guests is very weak, but the overall interaction of the guests with the host structure can be quite strong. In the literatures, more than 130 compounds that are known to form clathrate hydrates with water molecules are mentioned and more emphasis observed to be given to sI and sII hydrates since these are by far the most common NGH structures.

The Effect of NGH on Flow Assurance:

Flow assurance can be defined as an operation that provides a reliable and controlled flow of fluids from the reservoir to the sales point. Flow assurance operation deals with
formation, depositions and blockages of gas hydrates, paraffin, asphaltenes, and scales that can reduce flow efficiency of oil and gas pipelines. Due to significant technical difficulties and challenges, providing safe and efficient flow assurance needs interdisciplinary focus on the issue and joined efforts of scientists, engineers and operation engineers.

It was mentioned by Guo et al. that as a rule of thumb, methane caged NGH will form if the temperature is as high as 4.5 degrees Celsius and pressures are as low as 11.7 bars [1]. As seen in Figure below, mild conditions are required for NGH formations. NGH predictions can be determined by using simulation software and computational methods.

*Calculated pressure – temperature diagram for hydrate formation for the a typical
lean multicomponent gas mixture which composition is indicated within the figure. Gray
area shows the hydrate free region and white are the region where hydrate formation is
possible. Calculation according to the Baillie and Wichert Method. [1]

However, predicting hydrate formation requires more detailed experimental studies for each reservoir fluid since the operating conditions and compositions vary vastly. As a result of both theoretical and experimental investigations, five different NGH prevention methods have been implemented to provide flow assurance. These are:

• Dehydration of wet gas.
• Avoid working under hydrates formation conditions during operations (in terms of pressure or temperature)
• Injection of Thermodynamic Inhibitors (TI) such as glycol or methanol, to decrease the hydrate formation temperature retard NGH crystal formation.
• Injection of Kinetic Inhibitors (KI) to prevent aggregation of hydrate crystals (typically polymer solutions such as polyvinylcaprolactam PVCap)

Above options are applied separately or in matching combinations. Selection of above options depends on fixed and operating cost restrictions, technology availability and knowhow, system characteristics and operation/process flexibility.

NGH as an energy source:

It was previously indicated that hydrates are one of the major problems in the oil and gas industry. Since the discovery of the hydrates, majority of researchs has been spent on the determination of their cristal structure and formation process, and this, to avoid plug formations in the pipelines. However, NGH are also called ''white coal'' due to its capacity to store huge amounts of methane in his cage structure.On average, NGH volume gains are estimated almost as 155 times smaller than the equivalent amount of natural gas at standard conditions, the gain in volume changes depend on the crystal structure of the NGH.

Another interesting aspect of NGH which attracted the scientific community to conduct research on is the large sediments and deposits of NGHs located on the ocean floor. Yet,
there are estimations and speculations on the NGH depositions on the ocean floor, which is greater than the combined total energy resources that we have currently in all means (Figure below)

                                                                *Fossil energy resources[1]

But the biggest problem standing against production of energy from ocean bed deposits (besides technical issues) is the possibility that removal of the methane hydrates may trigger the uncontrolled release of methane gas to the ocean. This uncontrolled incident may result in global climate change since methane in the atmosphere is known to be the biggest contributor to the global warming.

Conclusion:

Whilst one acknowleges the benefits of techniques such as injection of inhibitors such as TI and KI to mitigate the problem, the associated cost and negative environmental impact clearly signal to additional fundamental research in the area. Such work may include the use of more effective and environmentally friendly additives.

The potential of NGH as alternative state of natural gas during transportation still remains untapped even though it has been acknowleged given the abundance of natural gas in the form of hydrate in deep ocean. This area of work would necessarily involve NGH stability studies.

References:

1. M. Atilhan, S. Aparicio, F. Benyahia, E. Deniz ''Natural Gas Hydrates''

2. E. Dendy Sloan, C.A Koh ''Clathrates Hydrates of Natural Gases'' Third Edition

3. J. Caroll ''Natural Gas Hydrates : A Guide for Engineers'' Second Edition