EDMONTON — Scientists have figured out how hydraulic fracking causes earthquakes in northern Alberta, but they have a way to go before they can use that knowledge to predict if it will cause temblors in other areas.
"Right now, there isn't a good solution for how to do that with induced earthquakes," said Ryan Schultz of the Alberta Geological Survey and lead author of a paper published Thursday on the hundreds of quakes that have occurred around the Duvernay oil and gas field since 2013.
The largest happened in January 2016 when the ground shook with a magnitude measuring between 4.2 and 4.8. Pictures shook on the walls of homes in Fox Creek, a community in the centre of the field.
Previous research had narrowed the cause down to fracking, which involves pumping high-pressure fluids underground to create tiny cracks in rock to release natural gas or oil.
But Schultz and his colleagues were still puzzled as to how the quakes happened. The field had been fracked for three years before tremors began, they noted, and some parts of it hadn't shook at all.
Analyzing data from about 300 wells using a set of complex statistical tools, the scientists concluded that earthquakes occurred when certain operating procedures coincided with the right kind of geology.
"Location matters," Schultz said.
The team found that at least four factors have to be present before human-caused earthquakes are likely to happen. The underlying rock needs a fault line; the faults have to be oriented in a direction that can slip; and there has to be a way for fracking fluid to pressure it.
You could put a nice straight-line relationship between the number of earthquakes you get and the injected volume.Ryan Schultz
"There has to be some sort of pathway that allows fluid flow to happen between the operation and the fault," said Schultz. "It's only when you have all of these geological ingredients that you can have induced earthquakes."
The fourth factor is the volume of fracking fluid. Too much in a susceptible area and a quake results.
"You could put a nice straight-line relationship between the number of earthquakes you get and the injected volume," Schultz said. "The more volume you put into the ground, the more earthquakes you get, the more likely you are to get a big one."
Not all parts of the Duvernay have the right geology, the scientists realized. It wasn't until companies began drilling in an area where conditions were right that the earthquakes began.
Still can't predict when the quakes will happen
Schultz said the analysis won't allow geologists to predict when fluid volumes could cause earthquakes in other fields — at least not yet. It's too hard to know if the right geology is present until after the quakes actually happen.
"Constraining those things in the subsurface ahead of time is notoriously difficult to do."
The research, however, does help operators working in areas where those factors are known to be present, he said. And it does add an important piece to the puzzle of how fracking causes the ground to move.
Alberta's energy regulator has already changed its rules for operators in the Duvernay area to try to mitigate the risk of further quakes.
The next step, said Schultz, is to look for other ways to know if an area may be susceptible to slipping.
"Are there other things you can look for?" he asked. "Are there ways you can start building a forecast approach?
"We want to get a better sense of that spatial relationship."
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