Cobble Presents Unusual Complications For Drilling

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

The amount of cobble in a bore path will limit the effective bore length on a directional drill -- the longer the bore, the more the exposure to the risk of bore hole collapse. It is wise to keep the bores to 250 to 300 feet, if possible.

Drilling fluid mixes are important in every directional drill application, but the proper additive mix is absolutely essential when in cobble. Most difficult are cobble formations that have no soil or sand to hold in fluid.

The cost of drilling in cobble can be excessive due to extremely high wear of ground-engaging components. Therefore, it is important to take this into consideration during the bidding process.

When considering a job with cobble in an area where a company has no experience, local drillers and equipment dealers can be sources of information about what’s required to be successful.

Melfred Borzall, Peter Melsheimer, marketing director: When drilling the pilot bore, the main problem is maintaining grade or making changes in direction -- the cobbles bounce around the bit making it very difficult to hold line and grade. During pullback, the main problem coming back through cobble is with hole stabilization. Sometimes it is not possible to make the drilling fluid thick enough to keep the hole entirely open. Special backreamers and pulling heads must be utilized to force the cobbles into the wall of the hole while the product pipe is being pulled in.

Tooling for cobbles is different from those designed for dirt conditions in several ways. First, they must be built incredibly tough. They have to not only wear well in abrasive conditions, but also be able to withstand impact and shock against rocks. Next, they need to be able to attack the rock gradually. A bit should have a taper, gradually increasing in diameter.

Our Steep Taper UltraBit 3 blade comes to a steep point to help “wobble" through the cobbles. The point, combined with the position and type of carbide cutters, allows the operator to rock the bit back and forth and wobble in between the cobble stones. There is a massive amount of carbide granule-impregnated hard facing on all wear edges to help increase the life of the bit.

The Ogre backreamer is unlike any other reamer on the market. There a large number of carbide cutters spiraled around the reamer so that each cutter is only doing a portion of the work. This helps ensure that even in cobble conditions, the reamer runs as smoothly as possible. The gradual taper also aids in forcing any cobbles that cannot pass through the deep flutes to be pressed into the wall of the borehole.