Bit pressure is a necessary condition for rock breaking at the bottom of the well. The size of the bit pressure determines the method and characteristics of rock breaking, which will directly affect the drilling speed and the damage method of the drill bit. During drilling, the drill bit is influenced by axial pressure and reversal forces. The cutting teeth are worn, dull or damaged in the process of pressing and shearing the rock, which will definitely influence the drilling speed. As the bit pressure increases, the drilling speed will continue to improve. The bearings of the drill bit and components like cutting teeth will also accelerate wear and influence the drilling speed. The relation between bit pressure and drilling speed has three different stages.
1. Surface crushing stage: When the bit pressure is less than the rock pressing hardness, the cutting teeth cannot cut into the rock, but can only break the rock in the form of friction on the rock surface, which makes more wear of cutting teeth. Although the drilling speed increases proportionally with the increase in bit pressure, the drilling speed is very low.
2. Fatigue crushing stage: When the bit pressure is close to the hardness of the rock, many cracks will occur in the rock surface through the cutting teeth are not cut into the rock. The repeated use of cutting teeth also produces volume breakage.
3. Volume crushing stage: When the bit pressure is increased above the hardness of the large rock, the cutting teeth will cut into the rock and produce volume crushing, and the drilling effect is significant, which is normal drilling. Therefore, the bit pressure on the drill bit applied to the roller bit must satisfy the cutting teeth to press into the rock, which causes the rock to produce volumetric crushing.
After doubling the bit pressure, the experimental roller bit drilled rocks of different grades, and the results show that:
Different rocks have different drilling speeds when increasing the bit pressure.
Among them, the growth rate of drilling rate for medium-hard rocks (rock grade 6 to 7) is relatively high, while that for soft rocks (rock grade 4 to 5) and hard rocks (grade 8 to 9) is relatively small. When drilling in cohesive soft rock, it is easy to produce water plugging and paste drilling, so the bit pressure should be smaller. When drilling into rock formations with large abrasiveness, lack of bit pressure is easy to form early wear bits, the bit pressure should be increased appropriately. When drilling into fractured rock formations, it is easy to have bit jumping, and the bit pressure on the bit should be appropriately reduced to prevent chipping and breaking of cutting teeth. Bit pressure is an important parameter of drilling. It is necessary to make full use of the role of cutting teeth in cutting into the rock formation and to minimize the wear of cutting teeth.