Cobble Presents Unusual Complications For Drilling

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

When estimating costs for jobs that contain cobble, contractors must be careful not to bid too low. Expect production to go slow and expect more than normal wear on tools. Be proactive and get the right tools to provide an edge from the start. Be patient, and ask for help from tooling, rig and drill mud suppliers.

Vermeer, Chris Fontana, cutting edge/attachment parts sales manager: Cobble can define many different conditions. It may be a loose conglomerate of glacial stones or glacial till sized from pea sized to basketball sized, mixed with sand and soils to create the condition, or packed tightly from thousands of years of compression which we term “cemented cobble.” Or it can be a conglomerate of broken or fractured sandstone, limestone or other rock that has been reclaimed, replaced or used for fill in construction or other similar application.

Maintaining the pilot bore plan in cobble is difficult as the bit deflects off oversized cobble, thus changing the bore path. Great care and patience is needed as to not deflect and oversteer drill pipe, thus damaging, breaking and potentially loosing pilot bore tooling. Vermeer offers several varying sizes and styles of pilot bore bits designed not only to provide maximum performance in cobble, but added wearability. In addition, a higher-end product specifically designed for cobble pilot bores is effective and designed to minimize deflection in the hole. It gives the operator greater control of the bore path requiring fewer corrections.

The challenges during backreaming and product installation are dependent on product size of the majority of the cobble. Most of the time in cobble, the hole will not stay open during backreaming. No matter how much drill fluid and other additives are used, the viscosity will not be stout enough to hold large rocks and stones to the bore wall and bring them out of the hole. Also, it must be determined that the product being installed can withstand the weight and pressures the cobble presents. A bore may have to be broken up in several shorter segments as to minimize the damage to the product installed as it is “muscled” through the ground as the cobble temporarily is displaced.