Rig Site Tips: Issues Related to Drilling Bit

There are many issues which have to be considered when running a drilling bit, starting from cutters damage, bearing failure, bit balling to plagued or lost nozzles. All these last issues can affect the overall drilling performance till getting to costly operations like fishing.

Lost Nozzle

Losing a nozzle causes a sudden decrease in stand pipe pressure. The simulation can give an indication of the number of lost nozzles. If the nozzle bore start to be washed out, the pressure continues to decrease gradually.

The drilling can be carried out with lost nozzle with acceptable rate of penetration but the nozzle can damage the cutter structures leading to other problems as bit gauge decrease or water ways damage.

The nozzle can be pushed toward soft formation and drilling can be performed with low damage on bit body.

The loss of nozzle can cause bit balling due the decrease in horse power per square inch HSI parameter and consequently reduced ROP.

Fig 2 - Lost nozzles on PDC

Plugged Nozzle

Bit nozzle can be plugged by many materials such as formation, lost circulation materials and motor rubbers.

The increase of the stand pipe pressures can be the main indication of the plugged nozzle. The drilling in this case can be continued.

If many nozzles are plugged which can be checked by comparing the stand pipe pressure and simulation, the bit has to be unplugged if not it has to be pull out of the hole.

In order to unplug the nozzles, some actions can be applied with precautions:

- Increase the flow rate off bottom to the maximum allowed flow rate for at least 5 minutes.

- Lift the drill string off bottom and drop it without touching the bottom of the hole to avoid damaging the bit cutters or blades. Also, it has to take in consideration the surge effect.

- Due to the reduction in the flow area the bit can be balled in soft formation and get cutters wear in abrasive formations.

Premature Cutters Wear

The cutters structures can be damaged for many reasons, this damage can affect enormously the bit performance.

If the cutters worn out, to keep the same level of rate of penetration, it should be added more weight on bit which accelerate the cutters damage.

If the gauge of drilling bit becomes less than the nominal gauge, the bit generate under-gauge hole. This can generate high bottom torque even with reduced weight on bit, affect the rate of penetration and increase in stand pipe pressure. Under-gauge hole can make stabilizers hang and cause BHA damage.

For the impregnate bit, if the bit face wears this cause stand pipe pressure increase due the reduction of the flow area.

For the PDC bit, if the cutters wear, this results low generated torque because the cutters become less aggressive. Also for the PDC bit, if the bit blades warn down this reduces the water ways surface and leads to SPP increase.

Fig 1- Different cutter wear types

Failed Bearings on Roller Cones Bits

It has to be mentioned that if the bearings have failed, the drilling bit should be pull out of the hole immediately. Ignoring the bearing failure can lead to cone lost in the hole leading to expensive fishing operations.

In order to avoid such situations, the symptoms of bearing failure have to be monitored closely which can be listed as follow:

- Torque spikes which are not related to formation changes

- Due to the skew angle of the failed cone, the ROP can increase temporarily

- Difficulties to control the tool face (While directional drilling)

Bit Balling

Usually this type of problems occurs when drilling soft sticky formations. There are some formations which react with water considerably and become sticky. In other cases, the drilled cuttings generated from shales and clays adhere to the drilling bit and the weight on bit cannot be transmitted to the formation that means the cutters will be isolated to dig in the formation. In this last case the rate of penetration will decrease dramatically.

The water ways on drilling bit can be also plugged and the lifting of the cuttings to the surface will be inefficient which can increase the SPP and causing the mud losses.

The bit balling is characterized by low torque, low rate of penetration ROP and increased stand pipe pressure SPP.

Fig 3 - Balled Drilling Bit

There are some actions which can be applied to solve this problem:

- Increasing the flow rate to maximum

- Increase the rotation off bottom as high as possible

- Pull the sting off bottom and shake it to get rid of this sticky formation

- Pump lost circulation material to get the erosion effect on the balled bit

After performing the last actions, it is recommended to tag bottom with high level of flow rate and rotary speed with reduced weight on bit till getting an acceptable progress.