Problem and treatment of PDC bits

The concept of bit balling

Bit mud coating refers to the surface of the bit is coated with mud, which can occur in the process of drilling and tripping. After the occurrence of bit mud packing, the degree of PDC bit teeth “eating” into the formation is reduced due to the influence of mud packing, resulting in its attack force can not play normally, drilling rate is slowed down, and in severe cases. The bit is blocked and water hole is blocked, resulting in increased pump pressure and even no footage. If it occurs during tripping, it may also cause annular obstruction.

Common phenomena of PDC bit balling

1. The footage is obviously slowed down during drilling, or is obviously increased during drilling;

2. Increasing weight on bit or decreasing weight on bit has no obvious effect on drilling rate;

3. Formation changes have little effect on drilling rate;

4. In general, the pump pressure increases slightly or has no obvious change. But sometimes there will be high pumping pressure, even plugging drill hole, blocking circulation channel;

5. Drill teeth can not effectively eat into the formation, showing reduced torque or torque fluctuation range;

6. The phenomenon of “pulling piston” occurs in tripping;

7. After short trip, the drilling rate slows down significantly. Continued drilling may gradually restore the original drilling rate and release the mud pack. Or continue to slow down resulting in serious mud pack and no drilling;

8. Drilling tool leakage and circulation short circuit occur in drilling, and the drilling rate slows down significantly after processing.

Causes of PDC bit balling

1. Geological factors: The stratum drilled is soft mud which is not rock in the upper part. It is easy to stick to the surface of the pdc bit and cause the bit mud bag after compacting. Although the mud shale in the formation is diagenetic, it is easy to disperse by hydration. Which increases the content of mud or solid phase in the borehole, and then adsorbs on the surface of the bit, resulting in the bit mud bag.

Or the formation contains dispersed gypsum, resulting in mud pollution, the harmful solid phase in the mud is difficult to remove. So that the drill bit is mud bag probability increasing; The formation permeability is high. Under the action of pressure difference, harmful solid phase in wellbore and rock cuttings not carried out in time are absorbing to form thick mud cake. Which accumulates under PDC bit during tripping and causes bit mud bag.

2. Mud performance factors: poor mud inhibition, unable to control the hydration and dispersion of mud shale. If the solid phase content and adhesive cutting are too high, the drilled cuttings are difficult to remove and easy to adsorb on the bit surface. If the solid phase free drilling fluid is using, the bit will never be mud coating. Mud proportion is high, water loss is large, easy to form too thick coarse mud cake. Poor lubrication performance. The surface of the drill can not form an effective protective film, the poor solid phase in drilling fluid is easy to adsorb on the drill.

3. Engineering and technical factors: small drilling displacement, unable to effectively clean the bottom hole and drill bit, at the same time, the upturn speed is insufficient. The cuttings stay in the well for a long time, adhere to the shaft wall to form thick mud cake, especially when the middle and upper drilling rate is high. In the soft mudstone formation, the weight on bit is too high. And the formation or cuttings form direct contact with the bit surface, resulting in bit mud coating. Long open hole trip without halfway circulation, from the wall.

The mud cake or cuttings scraped off the drill will muddy the bit.

• Selection factors of drill bit: the design of water hole can not meet the requirements of chip removal. The cuttings removal Angle of the runner prevents cuttings from escaping the bottom hole.

• Operating level factor: the drilling speed is too fast, the bit is not sliding down the spiral track. But constantly scraping mud cake or drilling cuttings on the shaft wall, which is easy to cause the pdc bit mud bag. When drilling, the resistance is not the circulating tapping of the pipe to wash the bit. But the downward pressure or downward punching. And the mud cake or drilling cuttings scraped from the wall will cover the bit with mud. The operation method is wrong when drilling to the bottom. If the rotary plate is starting first and then the pump is starting, it will also cause the bit mud bag. Uneven feeding occurs when drilling in soft formations.

Prevention of PDC bit balling

• 1, mud performance adjustment

1) Mix oil or increase the amount of lubricant input. So that drilling cuttings are not easy to adhere to the drill bit;

2) Increase the polymer content in drilling fluid to control water loss and improve the quality of mud cake;

3) Improve the inhibition of drilling fluid and reduce the hydration dispersion of mud shale;

4) Reduce the viscosity and shear force of drilling fluid and remove the inferior solid phase in time;

5) For high permeability sand layer, shielding temporary plugging technology can be using to reduce permeability leakage;

6), the paste bearing rock, should be adding in advance alkali, to prevent its pollution of mud.

7) It is effective to maintain and deal with mud performance before and after PDC bit is put into the well to prevent bit mud bag. A certain amount of cleaning agent (surfactant) is also using to clean the bit after PDC is run into the well and before the pump is starting.

• 2, engineering and technical measures oil drilling equipment

1. First, the selection of PDC bit should be done well. And the drill hole and flow channel design should be conducive to chip removal;
2. Before running PDC bit, the mud should be fully circulating to clean the hole and prevent the drilling cuttings remaining in the hole after drilling from continuing to hydrate and disperse;

3) Before PDC is run, make a short trip to scrape and extrude the wall mud cake. And stretch and compacting the thick mud cake to ensure the smooth borehole and eliminate the blockage. In the area of high incidence of pdc bit balling. If all methods can not avoid PDC bit balling, then the use of roller first through the well is necessary;

4) When drilling, the surface of the drill runner is coating with butter to form a protective film. Which reduces the time of direct contact between the drill and the inferior solid phase in the mud, or the drill is wrapping up. So that it has a certain role in deep Wells;

5) When drilling, the bit constantly scrapes the well wall. The mud cake on the well wall or the drilling cuttings in the well will accumulate under the bit. To a certain extent will be pressing on the bit. So it is necessary to carry out the cycle in the middle of drilling, to clean the bit;

6) During drilling, the speed should also be properly controlling to prevent the drill from suddenly punching into the sand bridge and drilling into a pile of mud. Also, at the right speing, the PDC bit will follow the spiral trajectory of the previous bit, rather than pulling mud cakes down the wall.

7) When drilling to the bottom each time, the pump must be starting first. And the displacement should be raising as far as possible to fully wash the bottom hole and the drill bit. After the displacement meets the requirements, the light pressure rotary drilling should be 0.5-1m. Which is also the requirement of PDC bit modeling.

8) Use large displacement drilling as far as possible to ensure full cleaning and cooling of PDC bit;

9) To drill in soft mudstone, low weight on bit.High speed and large displacement should be adopting as far as possible. There is no need to blindly use high weight on bit to pursue the drilling rate of only a little bit higher. Which is just the use of cone bit;

10), the operation should be fine, the drilling pressure must be uniform, can not be large or small.

The treatment of PDC bit balling

• 1. the first principle of dealing with drill mud bag is: do not rush drilling, because the more packaging the more severe;
• 2.It is inevitable to adjust the mud performance, no matter to prevent or treat the bit mud bag. If the bit has signs of mud bag. It should stop drilling immediately and configure cleaning agent to drive into the well to clean the bit at the first time;
• 3.Stop drilling, increase displacement to strengthen the hydraulic washing effect, lift the drill bit from the bottom of the hole, increase the speing and increase the centrifugal force to make the mud easy to be thrown out, and move up and down substantially for several times, and then press down to the bottom of the hole without turning the rotary table for 5-10 minutes, the above process will be repeating; If it doesn’t work within 2 cycles, it’s time to start drilling.