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Oil characterisation is key to preventing hydrate formation

21st February 2013


Research carried out in Norway promises to bring new solutions to the problem of dealing with hydrate formation in pipelines. Sean Ottewell reports.

Operators of subsea fields on the continental shelf spend huge sums on combating insidious ice-like crystals known as hydrates. However, the formation of hydrates is like a big chemical jigsaw puzzle that is only slowly being pieced together.

Now scientists at SINTEF, the largest independent research organisation in Scandinavia, believe that they have made a major breakthough in the puzzle - a move that is already attracting attention from one of the world's biggest oil companies.

"If all the pieces fall into place, they will help to reduce the currently high costs of preventing hydrates from forming in pipelines," says senior scientist Sylvi Hoiland in the Bergen office of SINTEF Petroleum Research.

Financial support

Hoiland leads the Hyades hydrates project which receives financial support from the Research Council of Norway, and in which SINTEF has been joined by the University of Bergen, StatoilHydro's Research Centre in Bergen and the US oil company Chevron.

The project continues a long-lasting cooperation between the Department of Chemistry at the University of Bergen and Norsk Hydro - an effort that began on a small scale but which has gradually increased in scale as the international interest has grown.

The results of the project have recently won praise from Shell, one of the petroleum industry's largest companies.

The backcloth to the project is oil and gas production from subsea fields, from which the well-flow is brought to nearby platforms or ashore by pipeline, nowadays over even longer distances. This is not always an easy matter.

The trouble is that wells seldom produce only pure oil or pure gas. Well-flows are usually a mixture of oil, natural gas and water.

In the pipelines that carry such mixtures across the seabed, the gas and water form hydrates if the temperature is low enough. And it can be cold when pipeline transport takes place over long distances under water, as the cold water at the bottom of the sea cools the mixture inside the pipeline.

Likewise, when maintenance operations or other circumstances mean that production has to be shut down for a while, the temperature inside the pipe may fall so far that it offers ideal conditions for hydrate formation. And some of the hydrate crystals that form are worse than others (Fig. 1).

Snow-like crystals

Certain hydrates have the property of forming sticky, wet snow-like crystals. These can grow into large plugs that completely block the pipeline.

In order to prevent hydrate formation, the oil companies pour large volumes of antifreeze (methanol or glycol) into wells, pipelines and process equipment. Similarly, when planned or unanticipated shutdowns take place, large quantities of antifreeze are added in order to prevent hydrate formation. All of which is extremely expensive.

The problem is not only that antifreeze is expensive; shutdowns involve more production downtime, because of the long time needed to pump large quantities of chemicals down the pipeline. And on the shelf, lost time means lost revenues.

But now, the scientific puzzle in Bergen has led to the hope that the consumption of chemicals can be reduced in the future. This hope is related to the fact that certain characteristics of some types of oil prevent hydrates turn into sticky snowballs. When such types are present in the pipeline, the hydrates remain in the form of a fine powder that is easily carried through the pipeline.

But what are the chemical components of the problem-free oil that makes it so? That is the key question that the Hyades project scientists are studying.

"Our aim is to end up with a situation in which we can bring a sample of oil into a lab so that a simple analysis will tell us the following: that the hydrates that form in the presence of oil A will block the pipeline, while oil B will not present any problem at all," says Hoiland.

Hoiland emphasises that the Hyades project on its own will not solve the problem, but that it should take us a step further towards that goal. The international petroleum industry has already taken note of the project.

"At a recent conference, Shell acknowledged that it regards our method of characterising oils as promising, which of course is nice to hear, for us who have helped to develop the method," says Hoiland.

According to Per Fotland, a specialist in the field development department at StatoilHydro's Research Centre in Bergen, the petroleum industry has a lot to gain if the Hyades project's oil analysis methods meet with success.

"We may be able to predict that in some fields, we can reduce the use of chemicals drastically, compared to current levels of consumption, and perhaps even see that a small dose of antifreeze every couple of weeks will be sufficient. In some cases, such a finding could mean that field development projects that were previously regarded as unprofitable could become profitable after all," he says.

The StatoilHydro specialist says that the oil analysis tool that the Hyades team aims to develop will be an important prerequisite for the prospect of developing such predictive explanations.

"But we also need other bits of the puzzle. For example, we need to know more about the effect of different flow regimes on hydrate formation," he says.

According to Hoiland, as soon as the scientists manage to close in on the components that actually make unproblematic oil unproblematic, an interesting new problem will appear.

The question is whether it will be possible to blend factory-produced copies of these key components into the problem oil as a substitute for the antifreeze. "It's an exciting thought, but we have a long way to go yet," she says.

Environmetal laws

The SINTEF scientist explains that such key components are already commercially available, but that they are toxic and cannot be used on the Norwegian continental shelf, because of the country's strict environmental laws.

"If we manage to identify the natural components in the oil, we will have a good point of departure for designing environmentally friendly alternatives."

The Hyades project is a continuation of research initiated by Hydro in 2000. According to Hoiland, Professor Tanja Barth of the University of Bergen has made important contributions since the start of the project, with her knowledge of the molecular composition of oil.

Chemists in SINTEF's Bergen office, all of whom were educated at the University of Bergen, have contributed their expertise on interactions between hydrates and the oil molecules.

"We have been ploughing new ground by demonstrating that these interactions can be studied in the laboratory," concluded Hoiland.







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