People exploring for mineral riches can do radiometric, electromagnetic or some other kind of geophysical surveying to locate their best chances of hitting pay dirt. But still, they must drill if they want to know for sure.
"Drilling generally represents the largest single cost associated with mineral exploration and the delineation of an ore deposit once it has been discovered," state the authors of Evolutionary and Revolutionary Technologies for Mining, a 2002 U.S. National Research Council book. It's expensive but, to repeat, it's intrinsic to the exploration process.
Various drilling methods have been developed to suit conditions, but the two principal methods in use today are diamond core and reverse circulation drilling.
In diamond core drilling, a diamond drill bit rotates at the end of the drill pipe. An opening in the drill bit provides space for a solid column of rock to move up the pipe, producing a cylindrical core sample. Most drill pipes are 10 or 20 feet long; the depth of drilling (and thus the length of core sample) is increased by fastening a new pipe onto the one already in the ground.
Reverse circulation (RVC) drilling produces samples of rock chips instead of solid columns. The RVC drill pipe is double-walled – a pipe inside a pipe. Compressed air is forced down the annular space between the pipe walls as the drill penetrates. The air carries rock chips up the centre chamber (the "return pipe") to the surface for collection as a sample for assaying.
It’s called “reverse circulation,” by the way, because of the route cuttings take to the surface. In other circulation drilling, compressed air is sent down the centre chamber and the chips come up via the annular space.
Core drilling is definitely the more “senior” of the two methods, as it’s been used for more than a century. RVC drilling dates back to about 60 years ago.
Tim Bremner, a Senior Vice-President of Foraco International, says RVC drilling becomes less efficient as you go deeper. “Reverse circulation drilling is typically done with compressed air as the drilling medium, versus a water-based fluid usually in core drilling or rotary drilling. And you can appreciate that it is a less efficient drilling if you have to compress air, and as the hole gets deeper the energy requirements increase exponentially.
“Core drilling becomes more economical at depth versus reverse circulation drilling,” Bremner continues from his office in North Bay, Ontario. “And generally the equipment for core drilling is much smaller, so there’s an economical cut-off as well as a practical cut-off for reverse circulation drilling equipment, where diamond can go so much deeper.”
He says RVC drilling is “very seldom done exclusively” in Canadian mineral exploration, partly because it produces lower-quality samples. “It’s not an inferior drilling method, but you’re getting rock chips in reverse circulation.
“A core drilling sample is an in situ sample taken from a precise location, totally undisturbed,” he explains. “Reverse circulation drilling can be used as a first pass to get a broad idea of what the opportunity is. When you want to go further with advanced exploration, you generally follow up with core drilling because it gives you much better information.”
With knowledge drawn from decades in mining and exploration, he says: “If you look at the total amount of drilling done around the world for mineral exploration, you’ll see it’s predominantly core drilling and reverse circulation is done as another exploration tool in parallel with it.”
Bremner adds that Foraco, incidentally, was a pioneer in RVC drilling, having developed and patented early reverse-circulation techniques more than 50 years ago.
Deep in Saskatchewan
A project by NUNA Drilling in northern Saskatchewan combined RVC drilling with kelly drilling , a type of rotary drilling which alternates between breaking up rock and raising the chips to the surface. Shore Gold Inc. chose Vancouver-headquartered NUNA to drill dozens of holes for bulk samples in a kimberlite-rich area east of Prince Albert. NUNA accomplished this through use of two sets of mobile Bauer BG 36 rigs, large-diameter drill rigs capable of reaching depths of up to 350 metres (more than 1,100 feet).The project ran over a couple of years ending in 2008 with about 25 personnel on sit at any time, going round-the-clock every day. Bob Huculak of NUNA Training Technologies (a sister company to NUNA Drilling) counts equipment and geological conditions among the great challenges.
“One of the big challenges was getting the machines from Germany to the site in Saskatchewan,” he says from Prince Albert, noting that the drilling machines were specially built for the Shore project to do both RVC and kelly drilling. Kelly drilling was done first, then RVC drilling as greater depths were reached.
Geological conditions were a vexing issue because the kimberlite at the Fort a la Corne site is underneath as much as 120 metres of sand, mudstone, boulders and glacial till. “The soil varied quite a bit,” says Huculak, “so there was a real challenge in adjusting to the different soil conditions as they came along.” Equipment, skilled personnel and patience were critical to overcoming the difficulties.
Foraco has done exploration drilling in many countries. One of its recent Canadian projects is in the Northwest Territories, collecting large-diameter bulk samples for Diavik Diamond Mine.
“What we do there is really simulate the mining operation by drilling a series of large-diameter holes – these are 22 inches in diameter – approximately 350 or 400 metres deep, and we collect all the kimberlite that we rotary drill in large one-tonne sacks, run it through their mill and process it as they would process ore, to verify the grade of the kimberlite pipe as they go down,” Bremner explains.
“That’s a technique that Foraco uses fairly exclusively, and it’s called large-diameter flood reverse. It’s an interesting project, and not too many companies do it.”
Bremner says the Diavik experience illustrates why Foraco is “so focused on being able to provide a number of services for our clients. Diavik had a flood-reverse project for us, but they also have core drilling needs and they have reverse circulation drilling needs as well. Our focus is to try to be able to provide a complete set of drilling techniques and types for our customer depending on what they want – as opposed to being just a diamond driller or just a rotary driller, for example.”
Bremner adds that the folks at Foraco are also very proud of the work they’ve done for Xstrata and Vale in Sudbury, where “the holes are technically challenging. They’re upwards of three kilometres deep, and they’re directionally drilled to very specific targets. There are not too many drilling companies that can put the resources in place to meet the demands of those two customers, and we’re currently doing that.”
Drilling techniques haven’t changed much in recent decades, but Bremner says exploration drilling has become safe and more productive. “It’s evolved into an extremely safe industry, and now are customers have us drilling much deeper than we ever had before, doing directional work and requiring much larger samples.
“Typically the mining industry borrows a lot of technology passed on from the oil and gas business,” he adds. “A lot of it needs to be re-engineered to accommodate conditions.
“Directional techniques, for example, were common to oil and gas for many years, where in our industry it hasn’t been so common except for the last 10 years. Some of those technologies scale down well while others do not and they have to be re-engineered.”
He says the most significant technological advancement may well lie in synthetic diamonds for drill bits. – an innovation that’s a few decades old and has “dramatically extended the life of the bit and produced a very, very consistent drilling tool.
“Before,” says Bremner, “the industry relied on industrial-grade diamonds that were actual natural diamonds, and the bit life was terribly inconsistent and the productivity was much lower. With man-made diamonds, the predictability of performance is there, and the bit life can be tenfold what it was with natural stone.”
Pat Chamberlin of drilling-products supplier Dimatec Inc. agrees. “Synthetic diamonds are more predictable,” he says in Winnipeg. “As we all know, no two natural diamonds are the same and have the same cut – whereas with a synthetic diamond it’s all controlled and all the diamonds are the same.”
He adds that equipment has also improved tremendously over the years to improve efficiency.
Chamberlin and Bremner agree that things are looking up for the business of drilling down. Demand for copper, iron and other metals has been growing, which means mineral exploration should continue apace.
“We don’t see it going to break out and get extremely hot – we don’t see super growth – but at the same time we don’t see why the market should come off,” says Bremner. “There’s lots of opportunity for Foraco.”
He says one limiting factor is that it’s difficult to find skilled operators. It seems drilling companies have to go deep for talent as well as ore.
