Geothermal Holes in Under Three Hours
When it comes to drilling through overburden, nothing buzzes through sand, silt and gravel like a sonic. But, on one recent occasion, the drilling speed surprised even the company who manufactures the sonic drill rig. Drilling a test hole for a future geothermal project, a Sonic Drill Corporation rig was able to bore past 300 ft. and complete the hole in two hours and three minutes. No other drill exists that could do the job any faster.
The drilling project, part of a law library extension for the University of B.C., was contracted to Hemmera Energy, a division of Hemmera Environmental Services Consultants in Vancouver, B.C., Canada. In this initial first step, the company was asked to conduct a feasibility study to see if it was practical to install a geothermal field in the proposed extension. “Our role is to do the test holes to see if a larger scale project is feasible,” said Christiaan Iacoe, an environmental scientist and consultant at Hemmera Energy. “If you’re going to drill 200 holes or more, it’s good to know the conditions.”
Located on the campus near the high sand bluffs overlooking Burrard Inlet, the plan was to drill a single 350-foot hole. The initial hole was drilled using a conventional mud rotary rig but, when the drill rig got past the 320-foot mark, it was stopped in its tracks. That’s when the sonic drill was brought in as a “rescue rig.” In typical fashion, the sonic rig buzzed quickly and easily through the same challenging conditions that jammed the conventional rig – the only problem was when they installed the geothermal loop into the hole, it was too buoyant due to salt water intrusion.
“We always add rebar to compensate but, this time, we didn’t have enough,” said Sonic Drilling Ltd. general manager, Bill Fitzgerald. “By the time we got more rebar, our pipe had now become stuck. We overdrilled the stuck pipe, removed it, moved the rig ahead, cleaned up and drilled the next hole,” he said. “I don’t know the time on that one. We didn’t measure it but it must have been pretty close. We installed the geothermal loop and there was no problem.”
When it comes to test holes, Iacoe says any failed attempts are just as useful as ones that are successful. “If it shows it’s not realistic to drill at that site, that’s really important. Our job is to produce a feasibility report, so what we do is drill the test hole and install a geothermal test loop. We have a piece of equipment that runs off a pretty big generator, that applies a constant temperature to the fluid in the loop,” he added. “That gives us a temperature versus time situation to see what the actual heat transfer is.”
Iacoe says, based on the geology of the site as well as moisture conditions and other factors, they get a range of values including thermal conductivity, thermal diffusivity and deep ground temperature. If the decision is made to proceed with a larger field, this information gives mechanical engineers, in the design phase, the ability to use the actual numbers in designing the system, rather than projections. “That’s way, way more accurate,” says Iacoe.
“We also pinpoint challenges at each site for full-scale construction. We have a lot of experience working with Bill and Sonic, and they can drill through things other drills can’t,” adds Iacoe. “Their drill holes are fully encased so there is not as much sloughing and they can grout a borehole and retract the casing, compared to a mud rotary, which just leaves an open hole.”
Sonic Drill Corporation rigs use an award-winning, patented drill head to transmit vibrations and power through a drill string. The energy produced liquefies overburden and bedrock and pushes the material up and away from the drill pipe. This enables a sonic drill to achieve penetration rates 3-5 times greater than conventional drilling systems such as mud rotary, air rotary and auger drilling – all without the use of drilling mud and while drilling through overburden. “The sonic is fast and so we count on getting through those zones before a problem arises,” explains Fitzgerald.
“We do jobs all the time that require us to drill through a lot of overburden and we do it better than anybody else.”