Low-salinity brine injected into crude oil forms nanoscale droplets that help separate oil from rock in reservoirs, according to Rice University engineers. The black ring around the droplets, seen in a cryogenic electron microscope image, is asphaltene. Source: Wenhua Guo/Rice University
It’s always been pretty common knowledge that water and oil don’t mix, but a group of researchers at Rice University’s Brown School of Engineering are proving otherwise.
The team recently found that microscopic saltwater droplets emulsify crude oil when each as the right composition. Furthering the understanding of how they combine could help to enhance oil recovery, they say. The work was published in the open-access Nature Journal Scientific Reports.
Lead by Sibani Lisa Biswal, Rice chemical and biological engineer, the team confirmed that wells are more productive when water with the right salt concentration is matched to both the water and rock, carbonate or sandstone formation from which the crude is being pulled. If the low-salinity brine creates emulsion droplets in a specific crude, the brine appears to also alter the wettability of the rock, which in turn, determines the ease at which the rock will release oil, they report.
Co-lead author and researcher at Shell Jin Song says that although hints of seawater’s effect came from wells in the North Sea, research on the impact has been limited. “Oil companies found that when they injected seawater, which has relatively low salinity, oil recovery was surprisingly good,” he says. “Usually, in the oil and gas industry, when they’re looking into low-salinity water, they tend to focus on the effect of the brine and ignore the effect of the oil.
“So, people haven’t been able to find a good indicator tor any correlation between the effectiveness of low-salinity water and experimental conditions,” he continues. “Our work is the first to identify some of the properties of the oil that indicate how effective this technique can be in a specific field.”
The team tested how injected brine is dispersed and how it affects oils’ interfacial tension and electrostatic interactions with rock.
“How to characterize wettability accurately is a challenge,” Biswal says. “Oftentimes, we assume that reservoir rock underground are under a mixed-wet state, with regions that are oil-wet and regions that are water-wet.
High-salinity brine mixed with crude oil does not appear to emulsify like low-salinity brine does, according to Rice University engineers studying the phenomenon. Their results have implications for enhanced oil recovery. Source: Wenhua Guo/Rice University
“If you can alter your oil-wet sites to water-wet sites, then there’s less of a driving force to hold the oil to the mineral surface. In low-salinity water injection, the brine is able to displace the trapped oil,” she continues. “As you change from oil-wet to water-wet, the oil is released from the mineral surface.”
For their research, the team tested two brines, one high-salinity and one with a quarter of the salinity of seawater, on Indiana limestone cores against six crude oils from the Gulf of Mexico, Southeast Asian and the Middle East, and a seventh oil with added asphaltene. From this, they found that high-salinity brine inhibited water droplets from emulsifying in the crude, unlike the low-salinity samples.
To further their understanding of the emulsion, Rice research scientist Wenhua Guo took cryogenic electron microscope images of 100 oil-water mixtures. The images showed droplets ranging from 70 to 700 nanometers.
“This is the first time anyone has seen these water droplets inside crude oil,” Biswal said. “They spontaneously arise inside the crude oil when you expose it to a low-salinity brine.”
Biswal says that the right brine could be a low-cost alternative to chemical surfactants.
“You can change the salt concentration to modify the composition of the brine and get the same effect as in including the detergent,” she says. “So, it’s basically a low-cost technique trying to achieve the same goal as detergent.”
The research was supported by the Abu Dhabi National Oil Co., and Rice’s Consortium for Process in Porous Media.
