Induced Earthquakes in Oklahoma Dropping, But Not off the Radar
Top US seismic experts say they are keeping a watchful eye on ground shaking in the state as new concerns are raised in neighboring Texas.
In 2015, Oklahoma recorded just over 900 earthquakes of at least a 3.0 magnitude. In 2016, the total fell to 623 and this year’s figure is on track to be about 300.
Since the vast majority of these earthquakes are linked to produced water injections, the trend line is welcome news to the state's oil and gas industry. However, for Jeremy Boak, director of the Oklahoma Geological Survey, 300 of these generally small earthquakes is still too many.
“We are seeing seismicity decline,” he said. “But I’m a little uneasy that it’s flattening out at a rate that I have a hard time believing will be acceptable in Oklahoma.”
Boak offered his thoughts on the topic of induced earthquakes while speaking on an expert panel at the Unconventional Resources Technology Conference (URTeC) held in Austin, Texas this week. Stricter wastewater injection limits and low oil prices are driving the drop in Oklahoma's earthquake frequency, but Boak said it is not known why that drop is “stalling out” instead of continuing, and for that reason he stressed that the issue continues to demand constant monitoring.
Boak’s agency, which is not a regulator but reports its seismic observations and findings to one, determined in 2015 that swarms of daily earthquakes were being caused by disposal well injections of produced water into Oklahoma’s deepest sedimentary formation (known as the Arbuckle) that sits just above a myriad of pre-Cambrian fault lines.
The panel discussion on which he sat revealed how much regulators, the seismology community, and the oil industry have all learned in the past few years about disposal wells and how they can trigger earthquakes. The talk also served to highlight the unfinished business on the matter, and why Texas may become the next hot spot for induced earthquakes.
One of the positive developments to come from this problem is the actionable research that has been produced by people like Mark Zoback, a renowned professor of geophysics at Stanford University in California and one of the leading voices on induced earthquakes.
During his turn behind the lectern, Zoback explained how new models that he has helped create are painting a “robust picture” of the seismic situation in north-central Oklahoma, home to about 95% of the state’s most recent spate of quakes.
“If you know about the stress state and the fault, you can calculate what perturbation of pore pressure might make a fault slip,” Zoback said. “It’s all pretty straightforward, but there’s uncertainty in all the parameters involved.”
One of those uncertainties is the fault lines themselves. Zoback said that the oil industry has handed over data that has helped gain critically important understandings of Oklahoma’s stress state and faults lines—but this data is incomplete because even the best seismic technology may not pick up a fault until it moves.
Although when armed with adequate data sets, the Stanford seismic model has proven to be accurate. Zoback said it found that the fault associated with the second biggest earthquake in Oklahoma history, the 5.7-magnitude Prague quake in 2011, had a 50% chance of slipping. Further history matching showed that other recent earthquakes “were all predictable” had the model and the required data been available.
“The real problem in the analysis is that there’s not a good correlation between the earthquakes and the mapped faults,” Zoback said.
Permian Basin Shaking?
While not all faults can be seen by seismic, many of them have been detected but nevertheless remain invisible to researchers and regulators alike. This is particularly the case in the Permian Basin region of Texas where Zoback said induced earthquakes may be on the rise.
Due to the recent surge in drilling and completion activity there, he said some operators are seeking alternative injection formations that can handle excess volumes of produced water. But because the existing fault maps are “extraordinarily inadequate," Zoback said that modeling the Permian's risk zones for induced-earthquakes cannot move forward until more data is obtained.
And while praising industry partners in Texas for sharing stress-state data, he said oil and gas producers should now hand over the other half of the earthquake equation: the fault maps. “There is a lot of good 3D seismic data out there that is not contributing in a proactive way to the solution to this problem,” Zoback said.
Staying on this point, Boak said that he has found that the biggest obstacle to obtaining the right data sets is usually corporate counsel. “The hardest part of this negotiation, in trying to free up some of these 3D seismic plots, has been trying to figure out how to protect proprietary information and make the lawyers happy,” he said.
Zoback expects that the fault data will see the light of day, but only when the earthquakes in west Texas begin adding up and force the hand of operators to starting sharing more.