Experts have known since the 1960s that when oilfield wastewater is pumped into the ground with deep injection wells, earthquakes can occur. The rise of hydraulic fracturing 10 years ago only made this worse. Once hydraulic fracturing, or fracking, hit its peak in 2015, earthquake occurrence started to decrease.
That, of course, doesn’t explain why Oklahoma uncharacteristically experienced several magnitude 5+ earthquakes in 2016. Aided by supercomputers, Ryan Pollyea, a professor at Virginia Tech, and his team think they have solved the mystery—it’s not about the injection itself, it’s about the components of what is being injected.
In a study published in Energy & Environmental Science, Pollyea and his team highlight a secondary process that occurs during injection of oilfield wastewater, or brine, that previous research has not addressed.
When fluids are pumped into deep injection wells, they alter the naturally occurring pressure in geologic formations. It’s these fluid pressure changes that destabilize faults and lead to earthquakes. What Pollyea’s research revealed is that oilfield wastewater, which has a lot of dissolved solid material, is much heavier than naturally occurring fluid. The altered density and viscosity properties are what makes the difference.
“The dense wastewater sinks, increases fluid pressure, and causes deeper earthquakes than would be predicted if the [oilfield brine and naturally occurring fluids] had the same material properties,” said Richard Jayne, a co-author of the study and former Ph.D. student at Virginia Tech, who is now a research hydrogeologist at Sandia National Laboratory.
Using supercomputers at Virginia Tech's Advanced Research Computing division, Pollyea and the team tested their idea by producing more than 100 models of oilfield wastewater disposal using various combinations of geologic properties, wastewater temperature and wastewater density. With this computational approach, the team was able to isolate both the conditions and physical processes that alter fluid pressure in geologic formations.
Since the oilfield wastewater is so heavy, once injected, it will continue to sink further underground—with or without pumping. This means, even if all pumping stopped tomorrow, that’s nothing that can be done about oilfield brine already injected into geologic formations.
“In fact, they'll continue sinking under their own weight for decades after injections cease, and our study shows that the wastewater doesn't have to be much heavier for this to occur,” Pollyea explained.
While the research paper urges caution, the team acknowledges more should be studies before it’s put into practice, including site-specific data and increased sampling. To that end, Pollyea said his research team will continue to work on specific aspects of their new theory, with a focus on how their ideas regarding fluid chemistry affect large injection operations in areas like Oklahoma and Texas.