Bit Rotation

Operational Effects of Rotational Speed for Impregnated Diamond Core Bits and Casing Shoes

Diamond impregnated bits are intended for use in harder, consolidated formations. The primary cutting mechanism by which they operate is known as stress crack propagation. This mechanism occurs when the compressive strength of the formation is high and the static load applied to each diamond crystal is insufficient to cause significant immediate penetration of the rock. Fracturing is induced by stress relaxation in the diamond’s track resulting from the rotation of the bit after the diamond has passed over it. This causes a series of cracks to form in the rock. When a particular diamond track is extensively fractured, the debris is carried away by the rotation of the bit. Each time a diamond passes over a certain track, further subsurface damage is caused in the formation. This allows the bit to advance through the formation.

Diamond impregnated bits have the lowest degree of cutter exposure when compared to the other types of cutting media that are used on rotary drill bits. As such, it is possible to apply relatively high rotational speeds.

A peripheral speed of 2.7 to 4.7 metres/second (9 to 15.5 feet/second) measured on the outside diameter of the bit crown will often provide an acceptable rate of penetration (ROP). A higher peripheral speed will frequently yield a higher ROP. However, higher peripheral speeds often reduce the rate of wear on the bit’s matrix layer – not allowing new layers of synthetic diamond to become exposed thus causing the original layers of diamond to become flat and polished. Conversely, a lower than recommended peripheral speed will cause the bit’s matrix layer to abrade more readily, particularly under higher bit loads. This may result in the premature failure of the bit crown.

Click here to view or download the Recommended Operating Parameters for Rotational Speed for Diamond Impregnated Core Bits in .pdf format. (94kb)

Operational Effects of Rotational Speed for Surface-Set Diamond Core Bits

TSP-set and surface-set diamond bits are used to drill geological formations that are predominantly medium to soft in terms of hardness. When compared with diamond impregnated bits whose cutting mechanism is largely attributed to the creation of local stress cracks in the rock through the point loading of individual diamond crystals, TSP-set and surface-set diamond bits tend to cut with more of a sawing or ploughing action. As a result, the rotational speeds that are recommended for use are lower than those used on similarly sized diamond impregnated bits.

A peripheral speed of 1.2 to 3.3 metres/second (4 to 11 feet/second) measured on the outside diameter of the bit crown will often provide an acceptable rate of penetration (ROP). In general, rotational speeds approaching the MINIMUM recommendation should be applied for bits with a high degree of cutter exposure. Conversely, rotational speeds approaching the MAXIMUM recommendation should be applied for bits with a low degree of cutter exposure.

Click here to view or download the Recommended Operating Parameters for Rotational Speed for TSP-Set and Surface-Set Core Bits in .pdf format. (87kb)

Operational Effects of Rotational Speed for PCD-Set and Tungsten-Carbide-Chip Core Bits

These types of bits are used to drill relatively soft formations. The mechanism by which they cut is to simply shear the formation being drilled. As such, the rotational speeds that are required are substantially lower than those used for the diamond impregnated, surface-set diamond and TSP-set types.

A peripheral speed of 0.8 to 2.4 metres/second (2.7 to 7.8 feet/second) measured on the outside diameter of the bit crown will often provide an acceptable rate of penetration (ROP). In general, rotational speeds approaching the MINIMUM recommendation should be applied for PCD-set bits with a high degree of cutter exposure. Conversely, rotational speeds approaching the MAXIMUM recommendation should be applied for tungsten-carbide-chip bits with a low degree of cutter exposure.

Click here to view or download the Recommended Operating Parameters for Rotational Speed for PCD-Set and Tungsten-Carbide-Chip Core Bits in .pdf format. (81kb)

Operational Effects of Rotational Speed for Saw-Tooth Tungsten-Carbide Core Bits

These types of bits are used to drill very soft formations. Like PCD-set bits, their cutting mechanism is shear and their cutting elements are relatively large in size and small in number. Due to the high degree of cutter exposure, the rotational speeds that are required are very low when compared with the cutting media used on other bits types.

A peripheral speed of 0.5 to 1.2 metres/second (1.7 to 4.0 feet/second) measured on the outside diameter of the bit crown will often provide an acceptable rate of penetration (ROP).

Click here to view or download the Recommended Operating Parameters for Rotational Speed for Saw-Tooth Tungsten-Carbide Core Bits in .pdf format. (79kb)