PDC Bit Drilling Parameters
- When the prior bit is removed, it should be inspected for any damage. If junk was left in the hole, do not run a PDC bit until the hole is cleaned.
- When picking-up a PDC bit, take all the precautions normally taken when handling a diamond bit, and some additional ones:
a. When removing the bit from its box, handle it carefully. Do not roll it out on the rig floor. If the bit is dumped on the floor and some of the cutters are chipped, the bits life will be reduced.
b. The interior of the bit should be inspected to make sure no debris is left inside.
c. The proper bit breaker should be used to make up the bit.
- The bit is one solid piece and does not have the limited flexibility of roller cone bits. Hitting ledges or running through tight spots can damage the gauge cutters.
- If it is necessary to ream when going into the hole, pick up the kelly and run the maximum flow rate. The rotary speed should be about 60, and go through the tight spot slowly.
- When near bottom, the last joint should be washed down slowly at full flow and 40-50 rpm, to avoid plugging the bit with any fill.
a. To locate the bottom of the hole, observe the torque and weight indicators. Because of the type of cutting structure on PDC bits, it is common that the first on-bottom indication is a sudden increase in torque.
b. After the bottom of the hole has been reached, the bit should be lifted a foot or two off bottom, then circulate and rotate slowly for about five minutes to make certain the bottom of the hole is cleaned.
- When ready to start drilling, bring the rotary speed up to 60 and approach bottom. Light weight should be used in order to cut a new hole pattern.
a. At least 1 foot of new hole should be cut in this manner before looking for optimum weight and rotary speed for drilling.
b. In soft formations, the bit will drill quickly with light weight, and the rotary speed should be increased until the bit is drilling at its fastest rate (usually between 100-150 rpm).
c. In hard formations, it will take much longer to drill the one foot. Adding weight too quickly will damage the cutters. Once the bottom hole pattern is established and weight is added, watch the torque indicator for possible problems.
- There is no limit to rotary speed, use as much as possible without damaging the rest of the drillstring.
- The on-bottom torque should approach what is experienced with roller cone bits. If there is no torque buildup, or the penetration rate does not increase with added weight, the formation may not be suitable for PDC bits.
- After making a connection, the bit should be washed back down to bottom. Dropping and then stopping the drillstring suddenly can cause the bit to hit bottom and be damaged due to pipe stretch.
- PDC bits respond dramatically to changing formations, if the rate of penetration suddenly decreases or the bit starts torquing, a change in the weight-on-bit and rotary speed should help.
- When the cutters wear to a point where they will not drill, the bit should be pulled.
If the wear is primarily on the outside, there will be a sudden decrease in the penetration rate and torque, and an increase in standpipe pressure.
If the wear is on the gauge portion, there will be very high on-bottom torque with little weight-on-bit and a decrease in the penetration rate.