Researchers: Fracking spurs bigger quakes at deeper depths
Five small earthquake sequences triggered by fracking in eastern Ohio between 2013 and 2015 were more complex than researchers previously understood, revealing that fracking-linked seismic disturbances originate at different geological depths and that deeper events pose greater risks.
The new study — led by researchers at Miami University in Ohio and published in the Proceedings of the National Academy of Sciences — uncovered interesting dynamics about the rare, but concerning, earthquakes associated with deep natural gas wells targeting the Utica and Point Pleasant shale formations that sit about two miles underground.
The findings offer clues that might eventually allow drilling
companies to adapt their operations to avoid setting off earthquakes. For now, the study suggests, stopping fracking temporarily may help head off more intense quakes once they have started.
But the fact that a bigger quake hasn’t struck the region so far has been partly luck.
More than a dozen cases of man-made, or induced, seismic events have been associated with Utica Shale fracking or gas wastewater disposal wells in recent years in eastern Ohio — a region that had “essentially no documented seismicity before 2010,” the researchers wrote.
In Pennsylvania, regulators linked a series of small earthquakes in 2016 to fracking in Utica Shale wells in Lawrence County — the first, and so far only, quakes linked to fracking in the commonwealth.
A combination of technologies has turned the Appalachian region into one of the richest sources of natural gas on the planet in the past decade. Companies can steer wells horizontally more than three miles along a target shale layer and then crack gas free from the rock with injections of high-pressure fluids and sand.
More than 2,200 Utica Shalewells have been drilled in Ohio and a few hundred more in Pennsylvania. Marcellus Shale wells in Pennsylvania now number about 11,000.
All of the five quake sequences studied by the researchers in Harrison County, Ohio, started in previously unknown faults in rock layers beneath the horizontal paths of the Utica gas wells underground.
Some of the faults were restricted to shallower layers of relatively weaker rocks, while others began in the deep, crystalline basement rock — below the layers of sedimentary rock that built up on top of it over hundreds of millions of years.
The deeper earthquakes were more powerful, apparently reflecting slippage along more mature stressed faults, and the shaking there continued in some cases over a month after the fracking that triggered it stopped.
Smaller, shallower earthquakes tended to stop more quickly.
The researchers also saw two dynamics at play in causing the rocks to slip.
Fluid used to crack the gas-rich shale sometimes apparently escaped its target and followed pathways to deeper geologic layers, where the pressure from the fluid activated stressed faults.
They saw evidence of this in wells where more liquid returned to the surface than would be expected for the amount of gas produced, indicating the fractures had tapped into deep water networks.
In other cases, the fluid apparently stayed within its Utica Shale target but built up so much pressure there that the rocks bowed out, sending a cascading force through lower rock layers until the faults were triggered.
One telltale sign was timing: Earthquakes started too soon after fracking — within two or three hours — for fluids to get to the earthquake’s source a kilometer away in time.
“Most of the studies that we’ve seen at this point are really just arguing for one or the other” of the triggering mechanisms, called pore fluid pressure changes or poroelastic stress transfer, said Michael Brudzinski, a Miami University geology professor, who was one of the authors. “The evidence in our paper supports both of these things.
“We’re suggesting that they are probably operating in tandem to create the earthquakes.”
It is not clear what operators of deep Utica and Point Pleasant wells can do to avoid the problem.
Fracking into layers far above the basement level of rock seems to steeply reduce any seismic risk. That is why Marcellus Shale wells — located about a mile above the Utica Shale — haven’t been implicated in earthquakes.
But researchers have not yet identified any variables that could help predict in advance which Utica wells drilled to similar depths might trigger shallow versus deeper quakes — or not trigger any at all.
The faults are unknown before the drilling companies get there.
The slipping rocks at the source of these earthquakes tend to move along a horizontal break, making them difficult to detect with common imaging technologies designed to pick up faults that rise vertically through rock layers.
The study’s authors also looked for patterns between seismic activity and the pressure and volume of fluid that was forced underground when each segment of the wells was fracked — but they found no clear links.
“My recommendation would be if you are going to drill a Utica well within a kilometer of the basement [level], it makes sense to do some seismic monitoring,” Mr. Brudzinski said,
The point, he said, is to see, “Do you get seismicity, and if you do, is it shallow or is it deep, and if it’s deep, how quickly is the magnitude increasing?”
Gas well operators, obviously, do not want to set off earthquakes, but an added incentive for them to try to avoid it is that it may hinder a well’s performance.
The researchers found that gas production tended to fall off faster in wells that set off bigger, deeper quakes than in nearby wells that didn’t cause earthquakes or only caused small shallower ones.
Regulators in both Ohio and Pennsylvania have adopted protocols that call for drilling companies in seismic hazard areas to monitor for subtle quakes during fracking and to make adjustments or stop their operations if the shaking grows to certain thresholds.
The researchers found some evidence that this helps.
Following the strongest earthquake under one of the well pads they studied, the operator worked with Ohio regulators to stop fracking two wells for over two weeks and later finished fracking using lower pressures and volumes.
“I think in this case they tried several strategies and, it seems, helped prevent them from getting above a magnitude 3 level,” said Mr. Brudzinski.
At that point, the well would have been shut down.
“You could argue that this is the kind of outcome you’d want,” he said.
There is also a measure of luck at work, he noted.
The authors wrote that the fact that earthquakes in their study area had not yet reached magnitude 3 “may be fortuitous.”
Using calculations from other studies, they found the wells they analyzed had the potential to set off seismic events as large as magnitude 4.5 based on how much total fluid was injected, well within the range of what people can feel at the surface.
A magnitude 3.7 earthquake near fracking operations farther south in Ohio in June last year lends support to the idea that magnitude 4 earthquakes can be spurred by fracking in eastern Ohio, they wrote, “raising the likelihood of a damaging event.”
Mr. Brudzinski cautioned that the occurrence of fracking-induced earthquakes is still very rare.
In Ohio, where the Utica Shale is frequently the target for drillers, only about 1 in 500 wells triggers measurable seismic activity, and the vast majority of that has been well below the level that people can feel, he said.
“Yes, this is an issue that we want to pay close attention to,” he said, “but we’re talking about a rather small number of wells that are having this problem.”
Gas well operators, obviously, do not want to set off earthquakes, but an added incentive for them to try to avoid it is that it may hinder a well’s performance.