The best way to get oil out of the ground may be to pump in foam.

Scientists pumped foam into an experimental rig that mimicked the flow paths deep underground and found the foam was more effective than more commonly used materials, such as water and gas.

Oil rarely sits in a pool underground waiting to be pumped out to energy-hungry surface dwellers. Often, it lives in formations of rock and sand and hides in small cracks and crevices that have proved devilishly difficult to tap. Drillers pump various substances downhole to loosen and either push or carry oil to the surface.

Sibani Lisa Biswal, associate professor of chemical and biomolecular engineering at Rice University, created the experimental formations—they look something like children’s ant farms—to see how well foam stacks up against other materials in removing as much oil as possible.

The formations are not much bigger than a postage stamp and include wide channels, and large and small cracks. By pushing various fluids, including foam, into test formations, the researchers can visualize the ways by which foam is able to remove oil from hard-to-reach places.

They can also measure the fluid’s pressure gradient to see how it changes as it navigates the landscape.

Paths of less resistance

The findings are strongly in foam’s favor. Foam dislodged all but 25.1 percent of oil from low-permeability regions after four minutes of pushing it through a test rig, versus 53 percent for water and gas and 98.3 percent for water flooding. This demonstrated efficient use of injected fluid with foam to recover oil.

The less-viscous fluids appear to displace oil in high-permeability regions while blowing right by the smaller cracks that retain their treasure. But foam offers mobility control, which means a higher resistance to flow near large pores.

“The foam’s lamellae (the borders between individual bubbles) add extra resistance to the flow,” Biswal says. “Water and gas don’t have that ability, so it’s easy for them to find paths of least resistance and move straight through. Because foam acts like a more viscous fluid, it’s better able to plug high-permeable regions and penetrate into less-permeable regions.”

Foam tends to dry out as it progresses through the model, says graduate student Charles Conn, lead author of the paper that is published in the journal Lab on a Chip. “The bubbles don’t actually break. It’s more that the liquid drains away and leaves them behind.”

Drying has two effects: It slows the progress of the foam even further and allows surfactant from the lamellae to drain into low-permeability zones, where it forces oil out. Foam may also cut the sheer amount of material that may have to be sent downhole.

One of the challenges will always be to get the foam to the underground formation intact. “It’s nice to know that foam can do these things, but if you can’t generate foam in the reservoir, then it’s not going to be useful,” Conn says. “If you lose the foam, it collapses into slugs of gas and liquid. You really want foam that can regenerate as it moves through the pores.”

Biswal says her lab plans to test foam on core samples that more closely mimic the environment underground.

George Hirasaki, professor emeritus of chemical and biomolecular engineering and Kun Ma, a Rice alumnus, are coauthors of the paper.

The Department of Energy, the Abu Dhabi National Oil Co., the Abu Dhabi Oil R&D Sub-Committee, the Abu Dhabi Co. for Onshore Oil Operations, the Zakum Development Co., the Abu Dhabi Marine Operating Co. and the Petroleum Institute of the United Arab Emirates supported the research.

By. Mike Williams-Rice for Futurity