A 3D illustration of induced seismicity in an underground mine | Courtesy of CEMI
In underground mines deeper than 1,000 metres, rock mass in situ stress distribution becomes a big problem. Hard, intact rock – undisturbed and undamaged
by nearby mining processes – can hold huge amounts of stress within it, and, when released, that stress can manifest as a violent rock burst, jeopardizing
the safety of people and equipment. Stress management often involves the use of explosives to precondition or diminish the dangerous pre-existing stresses
within the rock mass, but explosives are both dangerous and can only produce a very localized effect. The Centre for Excellence in Mining Innovation (CEMI)
in Sudbury, and its industry and university partners, think hydraulic fracturing can do this a better way. And oil and gas companies are intrigued by the
prospects of additional insights that might be gained by modifying the existing stress fields during hydraulic fracturing.
Starting in October 2012, CEMI began a two- to three-year-long experiment investigating the use of hydraulic fracturing – or fracking – for stress
management purposes, which effectively consists of three case studies, explains CEMI’s COO and R&D director Damien Duff. The first case study began in
June at Newcrest Mining’s Cadia Valley operations in New South Wales, Australia. It involves designing an underground hydraulic fracturing experiment “in
such a way that it will help that company with its stress management issues in the rock mass surrounding the ore zone.” Ultimately, the goal is to
characterize the effects of particular fracking treatments – in terms of the rate of injection, pressure, volume, and type of fluid and additives – in
order to optimize the properties of subsurface fractures for each industry’s purposes.
The study is implementing extensive instrumentation and data gathering both before and during the fracking treatments. It is also using the expertise of
both the oil and gas industry and academia, in order to characterize the individual treatments and their subsequent effects. Microseismic instrumentation
will be complemented by new techniques, potentially including high-precision tiltmeters and fibre optic cables anchored into half kilometre-long boreholes
near the treatment area, to precisely measure the strain changes in the rock mass. The various types of data gathered will be combined and calibrated
against the direct underground observation of the treated areas to deliver a complete picture of the processes.
That direct observation is something no one in the oil and gas industry has ever done before at these depths. Newcrest will be the first company to mine
back through the treated rock at this scale: it is an opportunity that has oil and gas companies anxious to get involved.
Oiling the gears
Calgary-based ConocoPhillips and competitors Nexen and Shell, along with Houston, Texas’ Anadarko, have all ponied up capital along with their modelling,
designing, and instrumenting fracking treatment expertise to facilitate the study and get access to the project’s results.
There has never been an economic justification or, until now, a real opportunity to dig up fracked areas, says University of Waterloo professor Maurice
Dusseault, who is part of the academic consortium behind the research. By driving a tunnel through the hydraulically fractured volumes of rock at Cadia
Valley and the next two trial sites, oil and gas companies will be able to better validate their modelling techniques, calibrate them, and assess important
factors, like what a particular type of frack does.
Larry Matthews, a senior technical geophysicist with ConocoPhillips, says he leapt at the chance to obtain the direct observation data. “We record these
microseismic events with sensitive instrumentation at two- to 2.5-kilometre depths, analyse and draw conclusions about what we’re actually doing to the
stresses and preferential permeability pathways. But we can never actually go down there and see with anything larger than a hole provided by a typical
While the methods employed sound a bit like a kindergarten art project, they are most certainly sophisticated. “We’ll inject tracers and coloured sand into
the fluid, so we know what we injected and when we injected it and under what pressure,” Dusseault explains. “And we’ll be mining through it, so we can map
it, and get an idea of what’s happening. We’ll be able to answer some silly little questions, like if you frack fast, do you have just one [fracture] plane
that opens, or many planes?”
That kind of uncertainty has simply been accepted as a part of the fracking business until now, says Matthews; with little data to work with, it has had to
be. The uncertainty has given rise to conflicting fracking philosophies, with various combinations of pressure, duration, and fracturing fluid touted for
various rock types. By actually observing the physical results of different fracking treatments in these trials, the companies hope to add some hard
science to what has been an art, by correlating their seismic readings with the physical changes in the rock, and then correlating that with recovery.
Meanwhile, mining companies like project sponsors Vale, Rio Tinto and Newcrest are also excited. The prospect of a safe, reliable stress management method
has them eager to learn more. As mines go deeper, says Duff, the rock burst issue becomes increasingly important – and difficult. “We’re having to revisit
just how this stress issue can best be dealt with,” he says. “With hydraulic fracturing, if it works as we hope it will, stress can be moved away from
where it can cause you damage to where it can’t.
“By fracturing the rock, you reduce its capacity to cause you problems due to stress buildup, because it’s no longer capable of holding the stress,” Duff
adds. “In the process, you’ve [also] potentially made the rock easier to mine through.” Better yet, by proactively dealing with the geomechanical stress,
mine performance becomes more reliable, development timelines get shortened, and ore bodies can be accessed faster, he says.
Although the hard rock in deep underground mines is not identical to the type of shales typically being exploited now by horizontal wells and hydraulic
fracture stimulation in oil and gas formations, it is more similar than one might think, says Dusseault. “Those rocks in shale oil and gas fields are very
stiff, strong, dense and impermeable,” he notes.
ConocoPhillips’ Matthews agrees: “It turns out that from a geomechanical point of view, the volcanic rock that we’re talking about in this mineback
experiment has great similarities with the kind of older, very black, very hard, high-quartz shales we’re now dealing with.”
The inclusion or absence of oil and gas probably does not change the mechanics much either, according to Dusseault. And in any case, the consensus is that
any data is better than none at all. “We think we understand the differences in properties, and can factor that into our interpretation and analysis in a
reasonable way,” he says.
Sharing knowledge yields benefits all around
With major players from both the oil and gas and mining sectors putting their own money into the project, CEMI is hopeful that a forthcoming NSERC
Collaborative Research and Development grant application will be approved.
“The backing of [both] industry and academia really gives the effort credibility,” notes Duff. A current fund of around $250,000 was supplied by initial
industry sponsors. But with potential for another $250,000 from NSERC, and new entrants paying $250,000 (for miners) and $165,000 (for oil and gas
companies), the project hopes to garner additional investment over the next three years.
According to Matthews, the investment made perfect sense: “For ConocoPhillips alone, we are spending hundreds of millions of dollars a year in drilling
horizontal wells and hydraulically fracture treating them. It’s what we do now. If we could improve that process five per cent, we would pay for our
involvement in this consortium so many times over that you can’t even imagine. And that’s just ConocoPhillips. The whole industry is doing this. We’re
talking billions of dollars being spent.”
Eventually, CEMI hopes that the knowledge gained from this study will also allow fracking to be adapted for use in smaller stoping operations like those in
Ontario and elsewhere in Canada, increasing both safety and productivity. The first test in Canadian rock could come as soon as next year. And perhaps some
day soon, the consortium partners hope, hydraulic fracturing will become as useful a method for mining as it now is for oil and gas.