Rock Drilling: The Right Tools for a Solid Bore

Once upon a time, encountering rock formations during a bore could completely stall an horizontal directional drilling (HDD) operation. Back then, if rock was encountered, bores would be abandoned.

Fast forward to today and a time in which advancements in tooling technology and processes have made rock drilling commonplace and opened a vast market for both HDD contractors and utility service providers.
But even today, the right tools and processes can mean the difference between completing the job and completing it profitably — and there’s a lot of homework to be done before tool selection can begin.

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“It’s important to gather as much information as possible about the soil conditions and rock formations along the bore path before you ever start,” says Sharewell HDD president Greg Wilson. He encourages contractors to take core samples, test the hardness of the rock and map formation changes on the bore plan, “so the driller will know when he’ll be entering and leaving different formations,” he says.

Additionally, make sure you have the right rig for the job. For the tooling to work as designed, it must be powered by a drilling rig with enough thrust and torque to turn the tooling properly and cut the hole efficiently. A proper drilling fluid program is also critical, as fluid must be pumped at a higher volume during rock bores in order to both clean the hole and provide the hydraulic power to operate some types of downhole equipment.

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LoTorque hole openerDrill Bits
The drill bit is the frontline soldier of the drill string, and no bit is more widely used on rock bores than the tri-cone. Decades of experience have gone into the design of today’s tri-cone bits and several options are available to optimize drilling in different types of rock.

For softer rock formations — 8,000 psi compressive strength and lower — milled tooth bits offer the fastest penetration rates at a lower cost.

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For rock formations that are harder than 8,000 psi, a tri-cone bit with tungsten carbide inserts is often the go-to bit. In these extreme conditions, the type of insert is determined by the hardness of the rock.

“The harder the rock, the more conical or rounded the TCI inserts should be,” says Wilson.

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Mud Motors
While dedicated rock drills are available from some manufacturers, few contractors drill in rock all the time. For those who don’t, mud motors are a good option. These efficient tools, which use the hydraulic flow of the drilling fluid to drive the drill bit, can be powered by virtually any standard drilling rig with a pump capable of providing the necessary flow required by the downhole motor, giving the contractor immense flexibility.
But choosing the right tool for your job — and maintaining it religiously — is critical for success.

“It’s important to optimize the flow rate of the drill with the mud motor’s capabilities,” says Wilson. “This helps make the best use of the motor’s torque output and rotation at the bit, for the best penetration rates and steerability.”

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Another important consideration is the drill pipe outer diameter (OD). Wilson recommends choosing a motor with an OD similar to that of the drill pipe, “Which should mean similar mechanical limitations,” he says.
Finally, protect your downhole investment with proper operation and regular maintenance. Minimizing motor stalls during operation will extend the life of the tool, and flushing it with fresh water after every bore will keep the motor free from dried mud residue that can cause damage.

“If run within the specifications outlined in the operation manual, the mud motor should perform really well for up to 100 to 125 hours of drilling,” says Wilson. At this interval, the tool should be sent in to the manufacturer for inspection of the power section and transmission, and replacement of the necessary parts.

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Hole Openers
For a successful rock bore, the choice of tools during backreaming is just as important as those you choose during the pilot hole. As with bits, many types of rock hole openers are available to match a variety of job specs and rock formations.

Split bits are single-use hole openers fabricated from tri-cone bit segments welded to a mandrel or shaft in order to achieve a predetermined cutting diameter. Reusable hole openers are another option. They come in a variety of shaft sizes, and each shaft can be fitted in the field with bits of varying sizes and tooth designs.
Once again, careful bore planning and execution will optimize the hole opener’s capabilities and make for a more productive, profitable bore. According to Wilson, this includes first determining how many reaming passes will be necessary based on the hardness of the rock and the final hole size desired. This determines the size of the hole openers you choose.

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“Throughout the hole opening process, it’s vital that the hole openers be operated within the recommended specifications,” says Wilson. This includes closely monitoring the torque, drill string rpm and the amount of pullback force being applied. It also includes removing the hole opener at recommended intervals to check the condition of the cutters and to replace them if necessary.

With today’s tools and technologies, contractors have a lot to gain by entering the rock drilling market, and very little to lose so long as they choose — and properly use — the right tool for the job.

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Dan Sharpe, vice president of product development at Sharewell HDD.

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