A recent string of earthquakes in Oklahoma, the USA, was most probably caused by injections of wastewater into rock formations as part of oil drilling procedures, and not due to fracking, earth scientists have found.
In a paper published in the latest issue of the journal Science Advances, Stanford University researchers Mark Zoback and Rall Walsh stated the rise in seismic activity in the region coincided with the increase of disposal of salty water into the Arbuckle rock formation under Oklahoma and not with hydraulic fracturing activity as previously thought.
"What we've learned in this study is that the fluid injection responsible for most of the recent quakes in Oklahoma is due to production and subsequent injection of massive amounts of wastewater, and is unrelated to hydraulic fracturing," said Professor Zoback from the School of Earth, Energy & Environmental Sciences of Stanford University.
The researchers found that the actual trigger was the injection of what companies call 'produced water' into deep disposal wells. This produced water naturally coexists with oil and gas within the rocks. During the oil extraction process, it is separated from the oil and subsequently disposed of into the rock.
The researchers analysed three areas that have experienced suspiciously many earthquakes in recent years and found that those exact same three areas lay near places were wastewater was being frequently deposited.
By comparing data about the wastewater injections with those about fracking activity, the researchers were able to conclude that the bulk of the injected water was produced as a result of conventional oil extraction techniques, not during hydraulic fracturing.
"We know that some of the produced water came from wells that were hydraulically fractured, but in the three areas of most seismicity, over 95 per cent of the wastewater disposal is produced water, not hydraulic fracturing flowback water," said Zoback.
Zoback and his colleague even tried to find an explanation for the time delay between the actual water disposal and the observed seismic activity. They developed a model that shows the increase in pore pressure inside the Arbuckle formation, which sits atop a basement layer in which geological faults lie.
The increased pore pressure caused by the water injections speeds up natural processes that take place within those geological faults.
Under normal circumstances, active faults in Oklahoma my trigger an earthquake every few thousands of years. Human contribution can considerably accelerate this process.
"The earthquakes in Oklahoma would have happened eventually," Walsh said. "But by injecting water into the faults and pressurising them, we've advanced the clock and made them occur today."
Before 2008, Oklahoma experienced one or two magnitude 4 earthquakes per decade, but in 2014 alone, the state experienced 24 such seismic events.
Since 1997, billions of barrels of water have been pumped into the Arbuckle formation some 2000 metres deep under the surface.
Although the earthquakes are felt throughout much of the state, they pose little danger to the public, but scientists say that the possibility of triggering larger, potentially damaging earthquakes cannot be discounted.
The researchers recommended companies to stop injecting waste water into the Arbuckle formation in order to mitigate the risk of earthquakes. Instead, the water could be disposed of in producing formations such as the Mississippian Lime, an oil-rich limestone layer where much of the produced water in Oklahoma comes from in the first place.
Some companies already re-inject water back into reservoirs in order to displace remaining oil and make it easier to recover. The Stanford study found that this technique, called enhanced oil recovery, does not result in increased earthquakes.
The researchers warned that even if companies opt to use producing formations to store wastewater, the quakes won't cease immediately. "They've already injected so much water that the pressure is still spreading throughout the Arbuckle formation," Zoback said. "The earthquakes won't stop overnight, but they should subside over time."