Drilling Engineers mounts core drilling equipment on all of its rotary drilling rigs: three CME-55s, three CME-75s and a Geoprobe 78DT track rig. For core drilling applications, the company has available NX diameter Diamantina Christensen core barrels. The company’s drilling rigs have angle drilling capacity.
Core drilling is a system for down-hole sampling in consolidated sediments. The drilling rig rotates a drill string—composed of hollow drill rods with a core-drilling bit at the bottom of the drill string—to advance the bore hole.
The core-drilling bit is hollow at its center, just as the drill rods are. The outer rim of the drill bit consists of a cutting material, typically industrial-grade diamonds, embedded in a matrix. Variations in the hardness of the cutting material and matrix allow drilling crews to adapt their drill stem to the unique drilling condition of their site.
The grinding of the drill bit, coupled with downward pressure from the drill string, cuts into the subsurface, while pushing a solid core sample into the hollow core barrel. As the drilling progresses, the drill rig injects a flushing medium into the drill string to cool and lubricate the bit. The flushing medium also removes loose cuttings and helps to stabilize the borehole. Water and drilling mud are both commonly used flushing mediums.
Drill crews can extract this sample from drill stem by retracting the complete drill string from the bore hole. But removing the sample that way brings drilling to a halt as the drilling crew raises the drill string to the surface.
Wireline drilling allows field crews to remove core samples from the borehole without displacing the drill string. In wireline drilling, the drill crew attaches a double-tube core barrel just behind the drill bit in the drill string.
The inner tube of this assembly is removable. When the crew is ready to remove a core sample, it lowers an overshot latching device through the hollow drill string. The overshot grabs the retractable inner tube of the core barrel and pulls it to the surface. Once at the surface, the drill crew removes the sample and lowers the inner tube back to the bottom of the drill string. Drill crews carefully arrange and label core samples so that they can be taken back to the laboratory for testing and storage.
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