Scientists are crafting a new cheap ingredient that can make carbon dioxide work harder and smarter in extracting oil.
With a recent $1.3 million grant from the National Energy Technology Laboratory for unconventional gas and oil technologies, a pair of researchers at the University of Pittsburgh is tackling a problem that has dogged the oil industry for decades. Their solution may provide a market for captured greenhouse gas emissions in the future.
Eric Beckman, a professor of chemical engineering at the University of Pittsburgh, explained that oil deposits are not large pools filling vast, underground caverns waiting to be sucked up; instead, oil is embedded in the small chambers of porous rocks and usually needs something to squeeze it out.
"When you do an [oil] extraction, the bulk of it stays in the ground when it's done," Beckman said. "When we talk about a field being exhausted in Texas or Oklahoma, it doesn't mean that it's empty."
One well-used strategy to eke out more crude is to pump water into oil wells, but as the saying goes, oil and water do not mix. Drillers need millions of gallons of water for the process, which ends up contaminating the water and still leaves a great deal of oil behind.
Oil companies now alternate well injections between water and liquid carbon dioxide in some wells. Carbon dioxide forms a liquid under high pressure and weakens oil's grip on rocks better than water. However, its liquid form tends to be too thin to sweep through reservoirs pushing oil; instead, it pokes through it like fingers. Liquid carbon dioxide is roughly 20 times less viscous than water and between five and 15 times less viscous than varieties of crude oil.
Switching back and forth lets drillers dissolve and then push the oil out from its captivity, but it is a tedious process, and it still will not get all the crude out. "The amount of oil [drillers] are getting is not what they'd like, but better than nothing," Beckman said.
This is not a new problem. Scientists, engineers and developers have been looking for a way to make liquid carbon dioxide more water-like in its viscosity since the 1980s. "To make a long story short, everybody failed," said Bob Enick, a faculty member at NETL's Regional University Alliance and a professor of chemical and petroleum engineering at the University of Pittsburgh. "The fundamental reason was, everybody was trying off-the-shelf chemicals."
Though the other players dropped out, Enick and Beckman continued looking for a viable way to make liquid carbon dioxide more viscous. "Some people think we're nuts, like Don Quixote," Enick said.
The long search for a thickener
The researchers did eventually develop a compound that would thicken the liquid carbon dioxide. "It was only a technical success, which means it worked in the lab, but the thickener was so expensive, it could never be used in the field," Enick said. The compound used fluorine, which made it costly and hazardous to the environment.
The researchers are now investigating three new compound classes that can accomplish the same goals, albeit at lower costs and with minimal environmental risks. One approach uses small hydrocarbon chains. Another uses amine groups that actually react chemically with the carbon dioxide. The third strategy involves mixing two precursors separately into the liquid that interact when they see each other in the solution.
These substances typically have a portion that has a high affinity for the solvent, in this case carbon dioxide, and a section that has an aversion to the solvent. At a molecular level, this creates interactions that tends to thicken the liquid.
But it is a bit more complicated than making a sauce or gravy. The challenge is to formulate the compound just right so it will spread through the solvent evenly instead of forming chunks or clusters that fall out of the solution. "[The thickeners] tend to self-assemble, and what we want them to do is to self-assemble into a very long line," said Enick, adding that thickeners also need to integrate into the carbon dioxide without stirring or heating to keep costs down. "If it takes too much effort, it isn't worth it."