Rigonomics Supplement: Tips From A Rock Drilling Specialist

By Jeff Griffin, Senior Editor | June 2011 Vol. 66 No. 6

“Many factors must be considered when determining the type of rock drilling equipment to utilize,” Hockran said. “They include length of crossing, diameter of product, geological conditions (whether rock is consolidate or unconsolidated), drilling rig, environmental considerations for fluid/air use, experience of the drill crew and type of steering tool.”

When drilling with a mud motor, one of the critical components is mud pumping capacity, which is determined by the drill string’s inside diameter and mud pump and mud recycling system combination.

“For this reason when drilling with a mud motor typically the smallest drill rig used is a 60,000-pound machine, Hockran said. “This size drill typically uses 3 1/2-inch drill pipe which will provide the necessary mud pumping capacity with a minimum 200 gpm pump and mud recycling system. The larger the diameter of the drill pipe, the larger the mud motor, pump and mud recycling system required.”

H&H uses tri-cone sealed bearing bits for pilot holes and low-torque hole openers to complete the hole opening process in rock.

“In our experience,” said Hockran, “the mud motor and traditional hole openers have proven very successful and consistent in the northeast region of the U.S. Typically, we own the mud motors we use. For a company that drills in rock the majority of the time, it is best to purchase mud motors, but at times we will rent them as needed.”

For rock drilling without a mud motor, Hockran considers two primary options:

• Conventional heads with TCI inserts for cutting efficiency and wear; and
• Air hammers.

“The conventional TCI drill head is effective and efficient in rock formations less than roughly 15,000 psi,” said Hockran. “Harder rock decreases the ability to steer effectively and reduces production. These drill heads are excellent for gravelly and or unconsolidated rock formations as they will not wear out as quickly as a standard ‘dirt’ drill head.

“Air hammers are increasingly becoming a more popular alternative in the HDD rock drilling market. They are very effective for drilling plus-or-minus 6-inch pilot holes in hard rock more than 15,000 psi, and in certain very hard rock exceeding 25,000 psi rock, an air hammer will be more effective and efficient than a mud motor. A limitation with air hammers is that steering is not very effective in unconsolidated formations, and it is sometimes very tricky to keep the drilled hole clean.