Factors Affecting the Roller Bit Drilling

Ⅰ. The influence of drilling pressure on the roller bit drilling

WOB is a necessary condition for rock breaking at the bottom of the well. The size of the WOB determines the method and characteristics of rock breaking, which directly affects the drilling speed and the damage of the drill bit.

During drilling, the rock bit is subjected to axial pressure and reverse force, and the cutting teeth are worn, dulled or damaged during the process of pressing and shearing the rock, which will definitely affect the drilling speed.

With the increase of WOB, the ROP will continue to increase, and the parts such as the bearings and cutting teeth of the drill bit will also wear faster, which will affect the ROP. There are three different periods when the relationship between WOB and ROP changes.

During surface crushing: when the WOB is less than the hardness of the rock, the cutting teeth of the rock bit cannot cut into the rock, and can only break the rock by friction on the surface of the rock, which will cause great wear to the cutting teeth, although the drilling speed also increases with the WOB. Large and proportional to increase, but the drilling speed is very low;

During fatigue and crushing: when the WOB is close to the rock and pressed into the hardness, although the cutting teeth do not cut into the rock, many cracks appear on the rock surface.

During the volume crushing period: when the WOB is added to more than the hardness of the rock, the cutting teeth of the roller cone bit cut into the rock and the volume is crushed, and the drilling effect is displayed, which is normal drilling. Therefore, the weight-on-bit applied on the rock bit must meet the requirement that the cutting teeth can press into the rock, causing the rock to be crushed in volume.

Ⅱ. The influence of rotational speed on the roller bit drilling

The rotational speed indicates how fast the diameter drill bit rotates, and is a sign used to measure the reversal speed during the drilling process. During drilling, the rock breaking conditions of different hardness rocks are different, and the impact of drilling pressure on it is also different. Therefore, the influence of the speed of the rock bit on the rock breaking process and the ROP should be considered. The factors of lithology and rock breaking time.

1. Speed in soft formation drilling

When drilling in soft, plastic, and low abrasive rock formations (such as clay 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 during the roller bit drilling is very little. Therefore, when drilling in soft formations, when the WOB is constant, the rotational speed increases in direct proportion to the ROP.

2. Rotation speed during drilling in medium and hard formations

The medium-hard and hard formations have high hardness and high abrasiveness. The cutting teeth are constantly blunted during drilling, and the contact area between the teeth and the rock is also increasing, which makes the deformation of the rock and the development of cracks extend all the time, which increases the difficulty. , the ROP becomes slower, and a larger WOB is required.

With the increase of formation hardness, the rock-breaking time of the rock bit is extended, and the increase of the drilling speed will cause the rock to be broken and the development will not be complete.

Therefore, limited by the time of rock breaking, in order to prevent the cutting teeth from wearing quickly, the rotational speed should not be increased excessively when drilling in medium-hard and hard formations.

3. The speed of drilling in different rocks

For different rocks, the ROP has a corresponding change curve and limit speed with the increase of the speed. When the roller cone bit is drilled in the clay rock, the drilling speed is proportional to the rotation speed;

Drilling in solid, highly abrasive rock, the ROP increases relatively slowly with the increase of the rotation speed, because the rock breaking time extends, and the limit rotation speed is lower than that of other types of rocks. When the rotation speed exceeds the limit rotation speed, the ROP will decrease.