Drill mud pack concept

Drill bit mud bag means drill bit surface is covered with mud mass. it occurs during drilling and tripping.

After the mud bag of the drill bit occurs, mud bag effect the teeth of the PDC bit .

then reduce the degree of “eaten” into the formation, resulting in the inability to exert its attack force normally, the drilling speed is slowed down, and in severe cases, the drill bit is covered, the water hole is blocked, and the pump pressure is increased. , even without footage, if it occurs during tripping, it may cause annulus blockage.

Common phenomenon of PDC bit mud bag 

• 1. Tools make the footage obviously slow down when drilling, or make it significantly increas when drilling;
• 2. Increasing or decreasing WOB has no obvious effect on ROP;
• 3. The rate of penetration has little effect when the formation changes;
• 4. Under normal circumstances, the pump pressure increases slightly or does not change significantly; but sometimes high pump pressure occurs, even blocking the water hole of the drill bit and blocking the circulation channel;
• 5. The teeth of the drill bit cannot effectively eat into the formatio.
• the torque becomes smaller or make the torque fluctuation range reduce
• 6. The phenomenon of “pulling the piston” occurs during tripping;
• 7. After short trips and trips, the ROP becomes significantly slower.
• Continuing to drill may gradually restore the original ROP ,
• and remove the mud bag, or continue to slow down , cause serious mud bag and make it impossible to drill;
• 8. Drilling tool punctures and short circuits occur in the drilling process, and make the drilling speed obviously slow down after the treatment. 

Causes of PDC bit mud bag  

• 1. Geological factors:
• The stratum to be drilled is the soft mud that not diagenetic in the upper part,
• which is easy to stick to the surface of the drill bit,
• which will cause the drill bit mud bag after compaction.
• The content of internal mud or make solid phase greatly increase,
• and adsorbe on the surface of the drill bit to cause mud bag of the drill bit;
• or the formation contains dispersed gypsum, which causes mud pollution,
• and it is difficult to remove the harmful solid phase in the mud,
• which greatly increases the probability of the drill bit coveredby mud. ; ,

• 2. Mud performance factors: mud inhibition is poor, and the hydration and dispersion of shale cannot control;
• the solid phase content and sticky cutting are too high,
• and the drilled cuttings are difficult to remove and easily adsorbed on the surface of the drill bit. People can not make the drilling fluid and the drill bit mud-pack;
• the specific gravity of the mud is high,
• the water loss is large, and it is easy to form an excessively thick rough mud cake; Relatively easy to adsorb on the drill bit. 

• 3. Engineering technical factors:
• The displacement during drilling is small, which cannot effectively clean the bottom of the hole and the drill bit.
• At the same time, the upward return speed is insufficient, the cuttings stay in the well for a long time,
• and adhere to the well wall to form a thick mud cake, especially in the middle and upper drilling. When the speed is high, it is more serious;
• in soft mudstone formation, the WOB is too large,
• and the formation or drill cuttings form direct contact with the surface of the drill bit,
• resulting in mud bag of the drill bit; long open hole drilling without midway circulation, from the well wall

• 4.The scraped mud cake or cuttings will mud cover the drill bit. 

• Bit selection factors: the design of the water hole cannot meet the chip removal requirements; the chip removal angle of the runner prevents the drill chips from successfully leaving the bottom of the well. 

• 5. Operational level factors:
• the drilling speed is too fast, the drill bit is not sliding down the spiral track,
• but continuously scraping mud cake or drill cuttings on the well wall,
• which is very easy to cause mud bag of the drill bit;
• encounter resistance when drilling down Instead of connecting ,
• the kelly pipe to ream the drill bit,
• and pressed down or flushed down,
• and the mud cake or cuttings scraped from the well wall will cover the drill bit with mud;
• the operation method is wrong when drilling to the bottom, if the turntable start first ,
• and then start the pump, which will also cause the mud bag of the drill bit; when drilling in soft formations, pepple make the drilling will unevenly deliver. 

Prevention of PDC Bit Mud Pack   

• 1. Adjustment of mud performance 

1) Mix oil or increase the amount of lubricant input,

so that the drill cuttings are not easy to adhere to the drill bit;

2) Increase the polymer content in the drilling fluid,

control the water loss, and improve the quality of the mud cake;

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

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

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

6) For the gypsum-bearing strata, we should add in advance to prevent it from polluting the mud. 

7) To maintain and handle the mud performance well,

it is effective to prevent the mud bag of the drill bit before and after the PDC bit inserted into the well;

after we insert the PDC into the well and before the pump is turned on,

a certain amount of detergent (surfactant) is configured to clean the bit. .   

• 2. Engineering technical measures Oil drilling equipment 

1) First, do a good job in the selection of the PDC bit. The design of the bit water hole and flow channel should be conducive to chip removal;    2) Before running the PDC bit, we make mud fully circulated to clean the wellbore to prevent it from staying in the well after tripping The drill cuttings in the eye continue to hydrate and disperse;

3) Before running into the PDC, make short trips, scrape and squeeze the mud cake on the well wall, thin and compact the thick mud cake, try to keep the wellbore unobstructed and eliminate blockage; , if all the methods can not avoid the mud bag of PDC bit, then it is necessary to use the roller to pass the well first; 

4) When drilling, apply butter on the surface of the drill bit flow channel to form a protective film to reduce the direct contact time between the drill bit and the inferior solid phase in the mud, or wrap the drill bit, which is effective even in deep wells. certain effect;

5) When drilling, the drill bit continuously scrapes the well wall, the mud cake on the well wall or the drill cuttings remaining in the well will accumulate under the drill bit, and it will compact on the drill bit to a certain extent. It is also necessary to rinse the drill bit; 

6) During the drilling process, we make the speed properly controlled to prevent the drill bit from suddenly rushing into the sand bridge and drilling into a pile of mud; in addition, if the speed is appropriate, the PDC bit will follow the spiral wellbore track drilled by the previous bit. Travel instead of pulling down massive mudcakes on the well wall. 

7) Peopele make the pump turn on every time the drill drill to the bottom, and make the displacement increas as much as possible, to fully flush the bottom of the well and the bit. After the displacement meets the requirements, we make the rotary drilling lightly press for 0.5-1m, which is also the requirement of the PDC bit shape.

8) Use large displacement drilling as much as possible to ensure adequate cleaning and cooling of PDC bits;

9) When drilling in soft mudstone, we make low WOB, high speed, and large displacement use as much as possible. There is no need to blindly use high WOB to pursue a slightly higher ROP, which is only the use of roller cone bits. Way;

10) The operation should be fine, and the pressure of the drill should be uniform, and it should not be big or small. 

Handling of PDC bit mud bag  

• 1. The first principle of handling the mud bag of the drill bit is: don’t rush to drill, because the more the bag is, the more powerful it is;
• 2. Regardless of the prevention or treatment of the mud bag of the drill bit, it is inevitable to adjust the mud performance. If the we find drill bit to have mud bag signs, Peopel stop the drilling immediately and inject the cleaning agent into the well for the first time to clean the drill bit;
• 3. Stop drilling, increase the displacement to enhance the hydraulic flushing effect, lift the drill bit up from the bottom of the well, increase the rotational speed to increase the centrifugal force to make the mud block easy to throw out, and move up and down a lot for several times, and then press down to the bottom of the well without opening the turntable Cycle for 5-10 minutes and repeat the above process; if it does not work within 2 cycles, you should consider tripping.

summary

1. The problem of the mud bag of the drill bit is often mainly caused by the high solid content in the mud, the small displacement of the drilling fluid cannot effectively clean the drill bit, and the large content of colloidal mudstone in the formation itself is easy to adhere to the drill bit. It usually occurs 2-3 hours after the mud conversion, during the drilling process of the double-stone layer and for about 1 hour after replacing the drill bit. The PDC bit removes all the water holes of the drill bit before entering the well, resulting in the flow rate of the water holes being too low and the water holes being blocked, resulting in Mud-wrapped blade. 

The treatment method is: control the amount of poorly dispersed clay and sulfite asphalt when converting and processing the mud, start the solid control equipment in time to remove the content of tiny harmful solid particles in the mud during drilling, and replace the PDC bit in the middle of drilling. Displacement cycle removes the mud adhering to the bit in a timely manner.

After the end, it is possible to cycle with a large displacement for a few minutes before drilling. Mastering these key links can effectively prevent the PDC bit from being muddy. It is worth noting that technicians should correctly understand that the riser pump pressure is composed of circulating pressure and nozzle pressure drop. As long as the nozzle pressure drop is zero, the vertical pressure will not be affected even if the water eye is installed again. Therefore, by removing the eye, not only the vertical pressure cannot be changed, but also it is often easy to cause the PDC bit mud bag. 

2. Explore and master the control law of directional well trajectory in each block as soon as possible, optimize the drilling tool structure, and minimize the use of PDC caused by midway adjustment of azimuth and well inclination. If it is necessary to use compound drilling in the lower well section with poor drillability , The use of high-end PDC bits with good wear resistance can improve the use effect of PDC bits and increase the drilling speed.