Drag bit – Drilling Equipment – Readyzone

Drag bit


Drag bit

Drag bit has no independently moving parts, and the term is applied specifically to bits of the blade type. The simplest of the drag bits are the fishtail bits with two blades spaced 180 degrees from each other. Bits having three or four blades are also used, but such bits are usually fingered so that the total length of the cutting edges does not exceed the length of the hole diameter by more than about 20 per cent. The blades on modern bits are furnished with tungsten carbide inserts or are otherwise hard surfaced to reduce wear. Short blades are preferred, for their use permits the mud-discharge nozzels to be positional a short distance above the bottom of the hole so that maximum jet energy can be utilized in the drilling. The mud streams flowing out of the discharge nozzels are directed to the bottom a short distance ahead of each cutting edge. Drag bits are used in drilling soft formation, and under ideal condition the drilling action probably resembles the turning of earth by a plow. The mud streams directed to the bottom of the hole break up material loosened by the bit and carry it upward to the surface. In many soft formations, hole can be made by the jetting action of the drilling fluid. In most cases, however, the chief function of the mud appears to be removing cutting and keeping the bit and bottom of the hole clean. 

Disk bits

These bits are interesting from a historical standpoint. They are a form of drag bit in which the cutting edges are mounted on disks. The disks are mounted off-center with respect to the axis of the drill string, so that as the drill string is rotated, the scraping action on the bottom causes the disks to rotate slowly. In this manner the total cutting edges available for drilling are increased by comparison with the stationary blades of the drag bit. Two or four disks are mounted in the bits. The bottom of the drilled hole is rounded, a form adapted to the flushing of cutting by the mud stream. The disk bit inherently does not have the weight-bearing capacity of the drag bit and does not provide as much clearance at the bottom of the hole for removal of cutting. It is related to the drag bit somewhat in the same manner that the Zublin differential bit is related to ordinary rolling cutter bits in that both have cutting elements which rotate and present successive surfaces against the bottom of the hole.

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Keywords – Drag bit, drag bits, drag bit drilling, drag drill bit, drag drilling bit

Reference : Mc Cray & Cole, Oil Well drilling Technology, New India Publication.

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