Doctors often warn about the dangers of “bad” cholesterol, which can lead to the build-up of plaque in arteries, narrowing the area through which blood flows and often leading to heart attacks and strokes.
In the oil industry, there are substances called asphaltenes that are often described as the oil pipe’s equivalent of cholesterol. They can cause significant and costly problems.
Heavy, viscous and with a tendency to stick together, asphaltenes, which as their name suggest are used to produce asphalt, clog up pipelines and can make transporting and refining oil extremely difficult.
“They can stick to surfaces. They can cause all manner of problems in terms of blockages,” says asphaltene researcher Spencer Taylor, a professor of chemistry at the University of Surrey in England.
“Certain parts of the production well are susceptible to blockages by crude oils. As the rest of the oil travels down, these effectively fur up the pipeline.”
Later on, at refineries, asphaltenes can form thick layers on heat exchangers, meaning more energy is needed to heat the crude. Here, their action can be compared to limescale, which builds up on a kettle’s heating elements.
It is no wonder then that Prof Taylor describes asphaltenes as “bad news in the whole lifetime of crude oil”.
Many companies have paid for research into ways to help deal with them, including the Abu Dhabi National Oil Company.
Among the studies it has funded is one led by Dr Mohammad Tavakkoli, a postdoctoral research associate, and Dr Francisco Vargas, an assistant professor in chemical and biomolecular engineering at Rice University in the US.
Published in the journal Energy and Fuels, it describes a new way of measuring the smallest asphaltene particles – those with a diameter of less than one micron, or one-thousandth of a millimetre.
Current methods, such as microscopy and looking at how the asphaltene absorbs infrared light, are less able to detect particles this small.
Using samples of Middle East crude, the researchers first took out particles larger than 100 nanometres, or one-hundredth of a micron in size, using a centrifuge.
Then they used a hydrocarbon liquid called heptane that caused the asphaltene particles to precipitate or come out of solution, and another hydrocarbon, the solvent toluene, to produce a suspension of the remaining asphaltene particles.
The resulting liquid was tested for the way it absorbed near-infrared light. The suspension’s absorbance was compared to that of a laboratory-made artificial oil without asphaltenes.
This technique is named the “indirect method” because it measures the absorbance of a suspension of particles, rather than of the oil itself.
Aside from being able to detect smaller particles, the researchers say their new technique has advantages, including that it can be used on crude oils with a wide range of asphaltene content, with those tested ranging from 0.1 per cent to 5 per cent.
There are large variations: some crudes are 20 per cent asphaltene, Middle East crudes are typically several per cent asphaltene and waxy crudes have the least.
Prof Neal Skipper of the University College London is another scientist who has researched ways to analyse the smallest asphaltene particles. He has looked at how samples of oil scatter beams of neutrons, the uncharged particles found in the nucleus of atoms.
“You fire a beam of neutrons at the sample and the way they scatter depends on the sort of aggregation. You can look at smaller aggregates that you would with light scattering,” he says.
This shows what causes aggregation and can highlight ways in which it can be prevented – or in some instances encouraged – by adding substances to the oil.
“It’s better to get the stuff out of solution where there isn’t a constriction,” Prof Skipper says.
However, it is not a method that would be used on a day-to-day basis in the field. His co-researcher, Dr Tom Headen, who works at Isis, a neutron source at the Rutherford Appleton Laboratory in Oxfordshire, England, says the method requires “a significant amount of infrastructure” – either a nuclear reactor or a particle accelerator.
“Therefore samples must be taken to the facility for neutron diffraction and, where required, the industrial conditions replicated as closely as possible,” says Dr Headen, who conducted work with Prof Skipper while he was his PhD student in London.
So although neutron scattering can be used to gain a better understanding of the behaviour of asphaltenes, they are not likely to be used regularly to assess particular oilfields to decide on extraction methods.
Dr Headen says the various techniques used to analyse asphaltenes should be seen as complementary, as “many techniques are used to give the fullest possible picture”.
Another line of research centres on understanding better the structure of asphaltenes. This is Prof Taylor’s focus.
“We still don’t know their structure. We know what components are there. We don’t know quite how they’re put together, because they vary so much from one oil to another,” he says.
“That’s one reason why we’re still working on them after something like 50 years. They’ve resisted most of the material analysis techniques trying to get to the heart of their structures.”
Prof Taylor is engaged in breaking apart asphaltenes chemically and trying to work out how they fit together again. Until researchers make further progress, he concedes the oil industry will “have to live with them and know they’re going to cause problems”.
“There’s no magic bullet at the moment to prevent them from precipitating,” he says.
But as more funds are channelled into asphaltene research this may change, and there could be better ways to deal with a group of substances that still cause major headaches to oil companies.