Getting Greater Penetration When Drilling Horizontal

By Scott Ellenbecker | September 28, 2010

The residential construction near Washington, D.C., is booming with new developments. Country fields lined with streets of million-dollar homes are surrounded by pastures spotted with black angus cows. The rolling hills and narrow streams are giving way to urban sprawl.

Washington Gas, and contractors like Bison, Inc. — the family-owned company operated by Bob, Doris and son Robert Ait — are feeding utility services to these monster homes as fast as the equipment allows.

The boom in the directional drilling business waned a few years ago, but quality operations like Bison have come through stronger and wiser.

When a hole needs to be drilled that others are having problems with, Bison gets the call. Robert Ait operates the drill for the company and has a real feel for drilling a hole. Although he has many secrets, a lot of his success comes down to just plain experience.

"One time a guy told me he can finesse a hole, I don't know how you do that," says Robert. For Robert it comes down to psi, rpm, water, and how and when to apply each — and the quality of the equipment they run.

"We fell in love with our little tractor," he said referring to his Ditch Witch 2720. "And as far as I'm concerned that 40 is the leader in the industry," he said, pointing to Atlas Copco's Secoroc SPX 40 tricone bit.

Bison operates mostly the 5-1/2-inch SPX 40 bits, but also uses the 5-1/2-inch SPX 50 depending on the hardness of the formation. "The 50 is really all this machine can handle," says Robert, "but what I don't have in torque I make up for in speed [rpm]." But to protect his bit in harder rock, consistency is better.

Robert says in this area he drills at about 30 feet an hour or about 10 minutes per rod. "You can never go faster than what your water will allow," adding, "you can really mess things up if you go too fast."

The more rock that's cut the thicker the mud gets. Robert puts about 42 gallons a minute in the hole. Bob added, "We like to see the mud a consistency of a heavy milkshake. If it gets slushy it's too thick."

The size of the hole is determined by the product. For instance a 5-1/2-inch hole can handle three, 2-inch fiber conduits. "You can squeeze fiber in a hole, but gas line is another story," says Robert.

When running a gas line, specifications requires half again as much space in the hole. So an 8-inch gas line needs at least a 12-inch hole. But because the hole needs to be really clean Robert likes to go a little bigger and use a 14-inch reamer to expand the hole. The 2720 can bore up to a 20-inch hole using an Atlas Copco reamer bit in hard rock and a 30-inch fly cutter in soft material.

"The gas company is really careful with their lines, and the line can't have gouges or scratches," says Robert.

Opening a Hole

Running large gas lines requires the use of a hole opener if the machine can't drill a large enough hole in one pass. Pilot bits are available in sizes from 4-3/4 inches to 17-1/2 inches. The strategy of sizing a drill for the most common use has worked well for Bison and they can use hole openers to ream the hole when necessary.

Bison has been successful with their 2720 drill and Atlas Copco split set strategy because it gives them the versatility to drill in almost any location, yet the ability to drill the necessary hole size.

Traditionally where tricone bits were used to fabricate hole openers, Atlas Copco's split set products utilize common, random, cutting-structure bit thirds, precisely positioned to assure equal load distribution. What that comes down to is that the modern split set design has an extended the life.

Improvements in carbide shapes and grades also prolong cutting structure integrity. Bearing and seal package technology as well as pressure compensated lubrication systems have added to bit life, but are also giving greater penetration rates.

What sets this type of split set hole opener apart is that it there is better utilization of available force which results in better performance, improving the penetration rate. One rotation of the cone provides complete bottom hole coverage.

For Robert Ait, the Atlas Copco bit is the "only bit I'm interested in using anymore." And as long has he continues to get greater penetrations rates and long bit life, he doesn't see that changing.