Geological differences from neighboring Ohio and few disposal wells for fracking wastewater keep Pennsylvania from experiencing earthquakes related to deep-shale drilling for natural gas, experts say.
Though the U.S. Geological Survey reports a sharp increase in quakes nationwide since 2009, including those 3.0 magnitude or higher, Pennsylvania hasn’t recorded seismic activity — even with 19,359 shale-gas wells at latest count, according to the state Department of Environmental Resources.
But in Ohio, where drillers also are tapping the Marcellus and Utica shale formations, dozens of small quakes have occurred and, in 2012, a rare 4.0 magnitude tremor struck near Youngstown, about an hour’s drive from Pittsburgh. Ohio has granted permits for about 44 Marcellus wells and 1,932 Utica wells, according to its Department of Natural Resources.
Geologists point to the gas industry’s wastewater wells — reservoirs that hold salty, chemical-laden water called brine, the end product of fracking — as the most frequent cause of quakes related to drilling. With 203 such wells, and 18 being drilled, Ohio far outpaces Pennsylvania, which has seven active brine wells.
“If you drill a well and pump fluids under high pressure close to a basement fault, you run the risk of induced seismicity,” said Scott Gorham, a petroleum geologist and director of exploration at Seneca Resources Corp., headquartered in Houston.
Ohio’s wastewater wells are deeper, too — a maximum depth of 10,000 feet compared to about 4,600 feet in Pennsylvania. Wastewater wells are under pressure, but are not fracked — hydraulic fracturing of rock — the way that gas wells are.
Drillers in Pennsylvania have paid Ohio millions of dollars to accept waste trucked from drill sites for disposal across the border: a total of 13 million barrels, or 546 million gallons, in 2014 alone, Ohio Department of Natural Resources records show. The state runs its own injection well program, but the U.S. Environmental Protection Agency runs Pennsylvania’s and must sign off on any proposed brine well.
Ohio’s geology is better-suited for injection wells than Pennsylvania’s, geologists say.
A crucial distinction is a thick layer of salt, called salina, that insulates drilling activity in Pennsylvania from distributing fault lines at the bottom of the earth’s crust. Ohio’s salina gradually thins moving westward.
“The salina is softer and can act as a bit of a cushion,” David Yoxtheimer, a petroleum geologist with Penn State University’s Marcellus Center for Outreach and Research, said of Pennsylvania’s geology. “The basement rock, in essence, is somewhat deeper in Pennsylvania. … We do have more of a buffer. It’s significant.”
Gaps in rocks at the base of the earth’s crust — called basement faults — can be particularly hazardous if disturbed.
“The deeper you go, in general, the more critically stressed the earth can get,” Yoxtheimer said.
Most gas drillers, whether they’re drilling a brine well or gas well, recognize the risk of drilling or pumping water near a basement fault, said Gorham with Seneca Resources. A company could lose expensive fracking fluid have to re-route the direction of its drilling, delaying production and adding cost.
“There’s not even an incentive on our part to go there,” he said.
Both states permit brine disposal wells below the salina layer, though Ohio has allowed drilling down to less than 1,000 feet above the basement, Gorham said. Wells to tap Utica gas often are just as deep in Ohio, he said. The depth of the average Marcellus well is 5,000 to 9,000 feet in Pennsylvania and Ohio. Utica depths in Pennsylvania reach 14,000 feet and in Ohio, up to 10,000 feet, according to Penn State’s Marcellus Center for Outreach and Research.
“That’s when you get into problems, when you’re really close to the basement and close to a fault,” Gorham said.
Information and images from beneath the Earth’s surface are crucial to gas companies when deciding where to drill. Seneca Resources and other companies spend millions of dollars to capture 3-D images of rock formations using sonar-like technology that records sound waves as they bounce off layers.
These images provide a “road map” to shale spots that are best to tap, or the geological potholes to avoid. Seismic information about specific shale-gas wells often is proprietary.
Despite the effectiveness of 3-D seismic images, some fault lines are harder to see, which can make earthquake prevention a challenge. Vertical fault lines are easier to spot than horizontal ones, said Michael Brudzinski, professor of seismology at Miami University of Ohio, who co-authored a report in January linking fracking to increased seismic activity in Ohio.
“I think part of the issue is that it’s difficult to observe the kinds of faults that seem to have been activated in a couple of … cases,” he said.
“The scanning techniques people use to look for faults, they’re much better at being able to recognize vertical faults than the horizontal ones.”
This article was written by Katelyn Ferral from The Pittsburgh Tribune-Review and was legally licensed through the NewsCred publisher network.