Recycling of saline produced water may be effective for managing microbes at hydraulic fracturing sites in Alberta, according to a new study by University of Alberta scientists. And the findings may help inform water management strategies at other sites around the world.
The study examined the microbiology of two hydraulic fracturing sites in the Duvernay Formation in Alberta, Canada. It was led by PhD candidate in the Department of Earth and Atmospheric Sciences Cheng Zhong under the supervision of Daniel Alessi, associate professor and the Encana Chair in Water Resources and Brian Lanoil, associate professor in the Department of Biological Sciences.
“Our goal is to understand how the microbial communities change as a function of time in the subsurface after the fracturing process is complete,” said Zhong. “We want to understand how microbial community composition changes during this time and—based on that—build recommendations for industry to better manage microbes in the subsurface.”
The researchers compared two sites that were virtually identical—drilled at the same time, to the same depth, in the same formation—explained Alessi. “The only difference is that one well used regular freshwater as hydraulic fracturing base-fluid, made up of 99 per cent freshwater, while in the other well, the base-fluid was made up of about 10 to 20 per cent recycled produced water, and the rest of the base-fluid was freshwater.”
Produced water is a byproduct of the hydraulic fracturing process that appears when water from the subsurface flows back into the drill site. The substance is highly saline—at the studied site, approximately seven times that of the ocean.
“In the well that used recycled produced water, the microbial communities became undetectable much faster than in the well that used freshwater alone,” explained Alessi. “Salinity was one of the major factors—there are only a few microbial species that can survive in this kind of environment. That salt in the water became a biocide in itself, which appeared to control microbial growth at this site.”
This discovery may inform the way that fracturing fluid is made and the chemicals that are used.
“There are many well locations around the world that have similar conditions as this one, so it is possible that these results can apply there, as well,” added Zhong. “The more site-to-site knowledge we have, the better we can understand the roles that microbes may play in the hydraulic fracturing process.”
The paper, “Temporal Changes in Microbial Community Composition and Geochemistry in Flowback and Produced Water from the Duvernay Formation,” was published in ACS Earth and Space Chemistry (doi: 10.1021/acsearthspacechem.9b00037).