Working Principle of Roller Cone Drill Bits

In August 1908, an American named HowardR.Hughes obtained the first patent for a roller cone drill bit. Initially, the idea was to convert the rotary motion of the drill string into an impact crushing action on the bottom of the well, allowing the rotary drill to break hard formations in a manner similar to percussive drilling. This was followed by the invention of two-cone and three-cone bits. The three-cone bit, having significant advantages over other bits, continues to be used since its invention. This article introduces the principle of the three-cone bit.

Revolution and Rotation of the Roller Cone Drill Bit

When a roller cone drill bit is in operation, the teeth fixed on the cone rotate clockwise around the axis of the bit, which is called revolution; the rotation of the teeth counterclockwise around the axis of the cone is known as rotation. The speed of the cone's rotation is related to the speed of the bit's revolution and the action of the teeth on the bottom of the hole. The rotation of the cone results from the interaction force between the teeth and the rock of the formation during rock breaking.

Impact Crushing Action of the Roller Cone Drill Bit

During operation, the weight on bit exerts force through the teeth on the rock. As the cone rolls, contact between the teeth and the bottom of the well alternates between single-tooth and double-tooth. When a single tooth contacts the bottom, the cone's center is at its highest position; when two teeth contact, the cone's center drops. The rolling of the cone continually shifts the center position up and down, causing the bit to perform reciprocal axial movement, known as axial vibration of the bit.

This axial vibration causes the drill string to compress and stretch continually, creating an impact load. This load translates through the drill bit teeth into an impact force against the formation, aiding in rock breaking along with the static load from the weight on bit. This combined action forms the main mechanism through which the roller cone drill bit crushes rocks. Although beneficial for breaking rock, the impact load can prematurely damage the bit's bearings and cause the teeth, especially those made of carbide, to chip.

Shearing Action of the Roller Cone Drill Bit

In addition to impact crushing, the roller cone drill bit generates a shearing action on formation rocks. This shearing mainly occurs through the sliding motion of the teeth as the cone rolls at the bottom of the well. The sliding results from three structural features of the bit: override, re-entrant cones, and axis dislocation. Sliding caused by override and re-entrant cones not only contributes to rock breaking by impact and compression but also shears the rock between adjacent tooth craters. Axis dislocation produces both axial sliding and cutting actions on the formation and shears rock between tooth rows. While tooth sliding helps to shear the bottom rock, thereby improving efficiency, it also causes increased wear on the teeth. Axial sliding from axis dislocation results in wear on the inner faces of the teeth, while tangential sliding from override and re-entrant cones causes wear on the sides of the teeth.

Self-cleaning Ability of the Roller Cone Drill Bit

During operation, especially in soft formations, rock cuttings tend to accumulate between the teeth, creating a muddied condition that can impede drilling progress. A self-cleaning bit design intermeshes the teeth of different cones, with one cone's teeth cleaning the rock cuttings from another's teeth. This process is known as the roller cone drill bit's self-cleaning capability.