Common Phenomena and Causes Analysis of PDC Drill Bit Mud Inclusion

Common phenomena of mud caking on PDC drill bit

• Obvious slowdown in footage or rise in drilling during drilling;

• Increasing or decreasing the drilling pressure has no significant effect on the drilling speed;

• Little impact on drilling speed when the formation changes;

• In general, the pump pressure slightly increases or remains unchanged. However, sometimes high pump pressure may occur, even blocking the circulation channel by clogging the water hole of PDC drill bit;

• The teeth of the drill bit cannot effectively penetrate into the formation, resulting in a decrease in torque or a reduction in the fluctuation range of torque;

• "Pulling the piston" occurs during lift and lowering drilling;

• After short lift and lowering drilling, the drilling speed slows down significantly, and continuing to drill may gradually restore the original drilling speed and relieve mud caking, or continue to slow down, resulting in severe mud caking and inability to drill further;

• Drilling tool puncture or leakage occurs during drilling, and after processing, the drilling speed slows down significantly.

Causes of mud caking on PDC drill bit

Geology factors

The drilled formation is unconsolidated soft mud in the upper part, which is easy to adhere to the surface of the drill bit and become compacted, thus causing mud caking on the drill bit. The mud shale in the formation, although consolidated, is easily hydrated and dispersed, increasing the mud or solid phase content in the wellbore and adsorbing onto the surface of the drill bit, causing mud caking. Alternatively, if there is dispersed gypsum in the formation, it can cause mud pollution. The harmful solid phase is difficult to remove from the mud, greatly increasing the chance of mud caking. In formations with high permeability, under the effect of pressure differential, harmful solid phases and rock debris that are not carried out in a timely manner can be adsorbed, forming a thick mud cake underneath the PDC drill bit, resulting in mud caking during lift and lowering drilling.

Mud performance factors

Poor mud inhibition, unable to control the hydration and dispersion of mud shale, high solid phase content and shear stress, and drilling debris is difficult to remove, easily adsorbing on the surface of the drill bit. If an oil-based drilling fluid with no solid phase is used, the drill bit will not get clogged. The mud density is too high, causing excessive thick and rough mud cakes due to high water losses. Poor lubrication performance, unable to form an effective protective film on the surface of the drill bit, and poor solid phase in the drilling fluid can easily adsorb on the drill bit.

Engineering and technical factors

The flow rate during drilling is small, which cannot effectively clean the bottom of the wellbore and PDC drill bits for sale. At the same time, the up return speed is not sufficient, causing rock debris to stay in the wellbore for a long time, adhering to the wellbore wall and forming a thick mud cake, especially when the drilling speed is high in the middle and upper parts. If the drilling pressure is too high in the soft mudstone formation, the formation or drill debris directly contacts the surface of the drill bit, causing mud caking on the drill bit. During long bare-eye down drilling, mid-way circulation was not performed, and the mud cakes or debris scraped off the wellbore wall would wrap around the drill bit.

PDC bit selection factors

The water hole design cannot meet the requirements of debris removal. The outlet angle of the flow channel obstructs the smooth removal of the drill debris from the bottom of the wellbore.

Operational level factors

Drilling too fast causes the drill bit to scrape the mud cake or debris off the wellbore wall instead of following the spiral trajectory downwards, which may easily cause mud caking on the drill bit. When encountering resistance during drilling, instead of circulating and flushing the drill bit with a square drill rod, downward pressure or downward impact may scrape off the mud cake or drill debris from the wellbore wall, causing mud caking on the drill bit. When drilling to the bottom, incorrect operating methods, such as starting the turntable before starting the pump, can also cause mud caking on the drill bit. During drilling in soft formations, uneven feeding may occur.

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