Removing the ‘cholesterol’ of crude oil and gas pipes
Microfluidic devices have been used to study commercial chemical dispersants which prevent the build-up of asphaltene in oil and gas wells and pipelines.
Asphaltene is a complex of hydrocarbon molecules found in crude oil. Highly valuable, it is the source of asphalt and can also be used to make waterproofing and roofing materials, corrosion inhibitors and other products. When it builds up in a pipeline however, asphaltene can pose an expensive problem. Asphaltenes are dubbed the ‘cholesterol’ of the oil industry, as they can coagulate and then slow, or even halt, the flow of oil and gas in reservoir rock.
Scientists from Rice University (RU) in the US have now looked into the actions of dispersants used to clear pipelines of asphaltene build-up. Their results have been published in the American Chemical Society journal, Energy and Fuels.
Rice engineer Sibani Lisa Biswal and colleagues used unique microfluidic devices, instruments that use a small amount of fluid on a microchip to perform tests, to study four commercial chemical dispersants that curtail the creation of asphaltene. Using the microfluidic devices, they were able to observe how the dispersals reacted with the asphaltenes.
Until now, chemical companies have typically performed static bulk tests on anti-asphaltene products, Biswal said in a statement from RU. The microfluidic discs from Rice lab however, allowed researchers to watch the dynamics of asphaltene deposition in real time, with or without dispersants and at different flow rates.
“Everything in our system is transparent,” Biswal said. “Crude oil hasn’t been very compatible with the microfluidic devices others are using (because the channels and pillars are too wide), and the type of devices we’re making have only been possible with recent materials. We’re one of the early groups to push the idea that we can use these systems to visualise oil flow processes.”
Surprise discovery
Using a microscope, Biswal and graduate student Yu-Jiun Lin were able to watch as asphaltene formed delta-shaped clumps on pillars in the microfluidic devices. Surprisingly, they noted that when chemical dispersants were added, the deposits actually appeared even sooner, but swiftly began to break apart and fall away in the flow. Their observations confirmed that the dispersants make the asphaltene particles smaller.
“The idea is, if you make the crude oil nanoparticles smaller, it’s less likely that they’re going to be able to deposit inside a pipeline of plug porous media,” Biswal said.
“But almost all tests up to now have been done on a bulk scale and very few under flowing conditions. Companies were just seeing if their chemicals make particles smaller. And they do. What they didn’t understand is that the smaller the particle is, the less likely it’s going to follow the fluid stream. In the presence of dispersants, deposits can actually get worse.”
The key, however, is that the dispersants seem to chemically alter the asphaltene, increasing the repulsion between the aggregates, making it more difficult for particles to stick together. “We refer to them as softer asphaltenes,” Biswal said.
“It doesn’t take much force to break up large aggregates.”
Lin said dispersant manufacturers typically use litres of crude oil in each test. “We just need a millilitRE of crude, and we get better resolution than they do,” he said. “When the asphaltene content is very low, traditional methods fail to see a difference in chemicals, or even a deposit.”
