Scientists have linked a significant increase in earthquakes in Oklahoma to oil and gas drilling in the region.
A team of researchers demonstrated the spike in earthquakes closely coincided with the disposal of salty wastewater into the Arbuckle formation, which is a 7,000-foot-deep, sedimentary formation in Oklahoma, Stanford's School of Earth, Energy, and Environmental Science reported. The researchers also showed the primary trigger of these quakes was not "flow back water" from hydraulic fracturing, but is rather "produced water" that naturally co-exists with the oil and gas found within the Earth. This produced water is generally separated from oil and gas and injected back into disposal wells.
"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 Mark Zoback, who worked on the study with PhD student Rall Walsh.
The study contributed to the decision by the Oklahoma Geological Survey (OGS) issue a statement that it was "very likely" most of the recent earthquakes had been caused by the injection of produced water into disposal wells. Before 2008, the state of Oklahoma experienced one or two magnitude 4 earthquakes per decade, but since 2014 there have been a whopping 24 of these seismic events.
"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 percent of the wastewater disposal is produced water, not hydraulic fracturing flowback water," Zoback said.
The model used in the study suggests wastewater disposal is increasing the pore pressure in the Arbuckle formation, which sits directly above the rock layer where earthquake faults lie. When this pressure builds up along the fault lines and eventually overcomes the frictional strength that holds the sides of the fault together, the fault slips and releases earthquake-causing energy. The pressure from the wastewater injections is also believed to be spreading throughout the Arbuckle formation, and could soon affect fault lines located far from well sites.
"You can easily imagine that if a fault wasn't located directly beneath a well, but several miles away, it would take time for the fluid pressure to propagate," Walsh said.
One possible solution to this problem could be to cease injection of produced wastewater into the Arbuckle formation entirely, and instead inject it back into producing formations like the Mississippian Lime.
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."
The findings were published in a recent edition of the journal Science Advances.
