Drill mud pack concept
Drill bit mud bag means that the drill bit surface is covered with mud mass, which occurs during drilling and tripping. After the mud bag of the drill bit occurs, the teeth of the PDC bit are affected by the mud bag to 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.
Causes of PDC bit mud bag 3. Engineering technical factors: The displacement during drilling is small, which cannot effectively clean the bottom of the well 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 Handling of PDC bit mud bag
1. Geological factors: The stratum to be drilled is the soft mud that is 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 solid phase is greatly increased, and it is adsorbed 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 being covered by mud. ; High formation permeability, under the action of pressure difference, the harmful solid phase in the wellbore and the cuttings not carried out in time will be adsorbed to form a thick mud cake.
2. Mud performance factors: mud inhibition is poor, and the hydration and dispersion of shale cannot be controlled; 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. The drilling fluid and the drill bit will never be mud-packed; 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.
The scraped mud cake or cuttings will mud cover the drill bit.
4. 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 factor: 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 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, it is 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 is started first , and then start the pump, which will also cause the mud bag of the drill bit; when drilling in soft formations, the drilling will be unevenly delivered.
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 drill bit is found to have mud bag signs, the drilling should be stopped immediately and the cleaning agent should be injected 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 and increase the centrifugal force to make the mud block easy to throw out, and move up and down a few 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. 
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