Fracking quake study called ‘extraordinary’
U of C scientists detail mechanisms that create fault lines, seismic events
A University of Calgary study has made a breakthrough toward explaining how hydraulic fracturing caused earthquakes near Fox Creek.
The research, published Thursday in the academic journal Science, discovered that seismic activity occurs through two distinct processes.
“This study has provided extraordinary new details about processes of fault activation by pore pressure increase or stress changes,” said David Eaton, professor of geophysics at the U of C and co-author of the study with a former student, Xuewei Bao.
“We can now begin to address important questions — the ‘how’ and ‘why’ of seismicity induced by hydraulic fracturing — that previously were not discernible.”
An association between fracking, a process that involves injecting high-pressure fluids, sand and chemicals underground to crack open rock layers and release oil and gas, and earthquakes was made more than four years ago.
In the latest research, Eaton and Bao have figured out the detailed mechanism behind that connection.
It could help find new ways to assess risk and mitigate the hazards of earthquakes caused by fracking.
The U of C scientists analyzed data from private and public seismograph stations around Fox Creek that went back to the winter of 2015 when there were two significant earthquakes.
It included a 4.4-magnitude event west of Fox Creek that came after a cluster of seismic events a month earlier.
They started with 90 known small earthquakes in the area and ended up with a catalogue of 905 events that included micro-earthquakes.
“It gave us a much more complete picture of the activity in the area,” said Eaton.
They also looked at information from monitors installed after the 2015 earthquake and a database of hydraulic fracturing data from each well in the area.
All of the information helped to resolve individual geological features.
“What we were able to start to observe are discrete fault planes,” he said, noting they found two strands on a north-south fault system that ran parallel to two horizontally drilled wells.
Eaton compared it to finding a hair-line fracture on an X-ray.
“You can show it to different people — some people might identify it and others not,” he said.
“The faults in this area tend to be very subtle in how they are expressed.
“Now we have a much better idea of where they are and what the geometry of these faults is.”
Eaton said the results suggest there has to be a pre-existing fault that’s capable of producing an earthquake.
“We need better techniques to identify the existence of these faults and to map them so this is a good step in that direction,” he said. “The second thing you need is a source of elevated pressure or stress in the subsurface so induced seismicity.
“Hydraulic fracturing is one type of fluid injection that can induce earthquake activity.”
Eaton said had some surprises during the research.
“The behaviour of these faults differs depending on the proximity to the hydraulic fracturing,” he said, explaining there was an east strand and a west strand.
The east strand saw seismic activity during hydraulic fracturing, but stopped after it was finished.
In the west strand, located between two horizontal wells, it likely got infiltrated with fluids that led to increased core pressure.
“The pressurized fault was persistently active for several months after the hydraulic fracturing was done,” said Eaton, noting the largest event happened two weeks after the work was complete.
It also took place at a depth of about four kilometres in the upper levels of Precambrian basement rocks.
“We didn’t expect that behaviour,” he said.
Eaton said the details will help to inform future research to generate simulations and come up with ways for industry and regulators to mitigate risk.