Rotation speed indicates how fast a drill bit with a certain diameter rotates, and is the target used to measure the reversal speed during the drilling process. When drilling, the rock breaking conditions of rocks with different hardness are different, and the weight on bit affects them differently. Therefore, the influence of the bit rotation speed on the rock breaking process and the mechanical drilling speed requires consideration of lithology and the factors of rock breaking time.
(1) Rotation speed during drilling in soft formation
When drilling in soft, plastic, and low-abrasive rock formations (such as clay-like rock formations), the thickness of the cuttings cut by the cutting teeth is equal to the depth of the cutting teeth into the rock, and the wear of the cutting teeth is small during drilling. Therefore, when drilling in soft formations, when the bit pressure is constant, the rotation speed increases in proportion to the mechanical rotation speed.
(2) Rotation speed during drilling in medium-hard and hard formations
Medium-hard and hard layers are laminated with higher hardness and higher abrasiveness. The cutting teeth are constantly being blunt during drilling, and the contact area between the teeth and the rock is also increasing, which increases the deformation and cracks when the rock is broken, and the difficulty increases. Speed slows down, and more weight on bit is required. With the improvement of stratum hardness, the drill bit breaks the rock at all times, increasing the drilling speed will make the rock crushing process incomplete, and the cutting teeth will be separated from the rock before the rock is fully broken, resulting in the reduction of the rock breaking depth. Therefore, in order to prevent faster wear of the cutting teeth, the rotation speed cannot be increased excessively when drilling in medium-hard and hard formations.
(3) Rotation speed of drilling in different rocks
For different rocks, the drilling speed has a certain change curve and limit speed with the increase of speed. For drilling in clay-like rocks, the drilling speed increases in proportion to the rotational speed; for drilling in solid, highly abrasive rocks, the drilling speed increases with the increase in speed relatively slowly, because the rock breaking time is prolonged and the limit speed is higher than other types of rocks. If the speed exceeds the limit speed, the drilling speed will decrease.
After doubling the speed, the experimental roller bit drilled into rocks of different grades. The results showed that:
For marble with rock grade 4, the drilling rate increase rate is 93%. For porphyry granite with rock grade 9, the drilling rate increase rate is only 28%. From grade 4 to 9, the drilling rate increase rate shows a decreasing curve. Therefore, it is beneficial to increase the rotational speed for softer abrasive formations, but it has little meaning for strong and highly abrasive formations.
In practice, the improved speed is restricted by the strength, length of the drill string, the function of the drill bit and the ability of drilling equipment. In the current period, with the development of slim hole drilling skills, as well as the improvement of drill string in the well, and the selection of leading lubricants to reduce the reverse friction resistance, high speed will be fully used.