While man-made earthquakes in the central United States have been linked to disposal of drilling wastewater, a new paper links a growing pattern of quakes in western Canada to the specific practice of hydraulic fracturing.
A team of scientists from Canadian universities and government agencies compared earthquakes in a broad swath of western Canada to "fracked" oil and gas wells and found a strong correlation.
"In western Canada," the study said, "most recent cases of induced seismicity are highly correlated in time and space with hydraulic fracturing."
Their paper, being published today online in the journal Seismological Research Letters, linked quakes to 39 fracked wells in the foothills region of the Western Canada Sedimentary Basin.
They also correlated seismicity linked to disposal operations, but that trend has remained steady in recent years. The number of quakes linked to fracking has shot up significantly.
The earthquakes in the United States linked to fracturing have been relatively weak, usually barely strong enough to be felt. But quakes linked to fracking in Canada have been as strong as magnitude 4.6.
Scientists have known for decades that injection of any industrial wastewater deep underground can cause earthquakes. Oil and gas production accounts for the largest volume of waste fluid disposed of in this way.
In the United States, scientists have documented a startling rise in earthquakes in oil and gas production areas, particularly Oklahoma (EnergyWire, March 8). But they've linked most of the quakes large enough to be felt or cause damage to disposal operations. They have frequently been frustrated that many people insist on saying that those quakes are associated with hydraulic fracturing (EnergyWire, June 10, 2015).
But there have been documented cases of hydraulic fracturing itself causing earthquakes, particularly in Ohio and Oklahoma (EnergyWire, Oct. 31, 2014).
The study was done by scientists from Western University in Canada, the University of Calgary, the British Columbia Oil and Gas Commission and the Alberta Geological Survey.
They looked at a 175,000-square-mile area in western Canada with 12,289 fracked wells. The area has had about 240 earthquakes in the last 30 years. But the rate has shot upward in the past five years as oil development has accelerated, driven by hydraulic fracturing and horizontal drilling.
The team screened for quakes that happened within 12 miles of a fracked well, within three months of the frack job. They correlated 39 fracked wells with earthquakes.
From 1985 to 2015, there were 258 quakes in the study area greater than magnitude 3. The researchers determined that 21 were natural. The rest are believed to have been triggered by disposal or hydraulic fracturing.
"In the second part of the period -- 2010-2015 -- the fractions shift increasingly to [hydraulic fracturing]," said lead author Gail Atkinson of Western University.
Fracking, the paper says, can trigger an earthquake in two ways. The fracture created by the process can reach a fault, causing it to slip. Or, if the fracture doesn't reach a fault, it could still change underground pressures on a fault enough to trigger it.
While the proportion of fracked wells linked to earthquakes is small -- less than 1 percent -- the paper says there's still a hazard because thousands of wells are being fracked each year. And buildings in the area haven't been built to withstand earthquakes, because it hasn't been considered a quake-prone area.
"The likelihood of damaging earthquakes and their potential consequences needs to be carefully assessed when planning HF [hydraulic fracturing] operations in this area," the study says.
The scientists note that their findings clash with numerous studies in the United States that have linked seismic swarms to disposal. But they say the studies finding the hazard from fracking to be negligible might need to be re-examined.
It's possible that more quakes are linked to hydraulic fracturing than they realize, but the evidence has been masked by the flood of disposal-linked quakes.
The study says that scientists may have been underestimating the potential strength of the earthquakes, by saying the relatively low fluid volumes keep them small. But the study found that some fracking-related quakes have exceeded the models used to make that point. Instead, the study says, the size of the quake might be determined by the size of the fault it ruptures.
"We expect that more earthquakes will occur, at least in some areas," the study said, "and their maximum magnitudes may exceed the values observed to date."
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