Cobble Presents Unusual Complications For Drilling

By Jeff Griffin, Senior Editor | December 2010 Vol. 65 No. 12

Cobble -- usually rounded pieces of rock that can range in size from a marble to a basketball -- is considered one of the most difficult and challenging soil conditions for making a directional drilling installation.

“Cobble is not fun,” said Boyd Simon, P.E., field services division manager for Ranger Directional Drilling.

“It’s hard to work through both for the pilot hole and during pullback -- cobble lodges in the bit or reamer. It’s hard to keep the hole open because loose pieces keep falling in. It’s also hard to get mud where it needs to be, and cobble is very difficult to bring out of the hole.”

Special downhole tools are available for cobble projects, and using the right fluid additives properly mixed is more important when drilling in cobble than in “dirt” formations.

Representatives of three manufacturers discuss cobble and products available to drill in cobble conditions.

Ditch Witch, Richard Levings, senior product manager: Glacial till -- popularly known as cobble -- was created by rocks rolling in glaciers which formed a rounded and hardened rock. It is usually in layers and can be quite difficult to penetrate and hold open when directional drilling. These formations sometimes will have soil, sand or loam in between them. They are present all around the world, but are most prevalent in the northwest and southeast U.S., southwest and southeast Canada and Alaska, and in some mountain regions of the U.S.

Cobble is extremely hard and very difficult to cut, making directional drilling difficult. During drilling, cobble pieces move and therefore rock bits cannot penetrate them because of the constant movement; instead of being cut, pieces of cobble roll away and do not stay in place. Ditch Witch offers both drilling bits and backreamers designed to be effective in cobble, but most effective, we believe is the All Terrain drilling system.

The cutting structure of the reamers are designed to have low contact points to be more of a displacement and wear mechanism rather than an actual cutting surface on the reamer. As stated before, cobble is displaced, not cut; therefore, trying to use dirt reamers in cobble is futile.

The most effective way we know for drilling in cobble is the All Terrain (AT) drilling system. With this two-shaft system, the power of the inner shaft, combined with slow rotation, helps maintain a straight bore path in cobble with the outer pipe controlling steering. In addition, we offer a cobble pipe that is close to a flush pipe which reduces the drag when the cobble falls back in on the drill pipe.