PDC Bit Operating Parameters

PDC bits do not have the benefit of the self fluid-cleaning action between rows of teeth like roller cone bits, so they must rely on the bit's hydraulics to flush the cuttings from under the bit to prevent balling. This is accomplished with strategically positioned converging-diverging nozzles which maximize cleaning while minimizing erosion of the body near the nozzle area. Optimum hydraulic range is between 2.0 to 4.0 hydraulic horsepower per square inch. The interchangeable jet nozzles come in standard sizes from 8/32's to 14/32's.

Bit life is controlled by the cutting structure. As stated earlier, the PDC cutting elements provide a self-sharpening edge with the wear resistance of diamonds. This combination is very effective in soft to medium formations such as shale, chalks, limestones, clays, salts and anhydrite. These formations have been drilled at excellent penetration rates with weights between 1000 and 2500 pounds per inch of bit diameter and rotary speeds of 85 to 140 rpm. Economic performance has also been achieved with rotary speeds of 750 rpm and weights of 1000 pounds per inch of bit diameter using downhole motors.

High rotary speeds provide better drill rates and reduce the chances of deviation. Optimum rotary speed varies with formation hardness. A soft, plastic formation would require higher rpm; a hard formation, lower rpm. Most applications require rotary speeds less than 120 rpm.

Lighter weight-on-bit means lower stress on the drill string, with increased string life as a result. There's less drag in directional holes because fewer drill collars are required, reducing the potential for stuck pipe.

These bits have made economical runs in both oil and water base muds. Oil base muds and the addition of lubricants to water base muds will enhance PDC bit performance in formations that tend to be somewhat plastic and sticky.

Formations which should be avoided with PDC bits are soft sticky shales and clays, abrasive sands, and those formations which are very hard. In sticky formations, PDC bits have a tendency to ball up; in abrasive formations or hard formations, cutter wear and breakage occur rapidly. PDC bits cannot drill as broad a range of formations as roller cone bits, but have shown to be competitive with diamond bits. Thus, care must be taken when selecting bits for various applications. When properly applied, most PDC bits can be run in more than one well.

PDC bits cost between $10,000 to $25,000, and savings are measured in terms of trip time saved, longer bit life, improved rates of penetration, and fewer rig hours required to drill a well.

Even though PDC bits have achieved recognition as a viable tool for improved drilling, certain precautions and drilling parameters should be met in order that the bit run be as efficient and economical as possible.

  1. When the prior bit is removed, it should be inspected for any damage. If junk was left in the hole, do not run a PDC bit until the hole is cleaned.
  2. When picking-up a PDC bit, take all the precautions normally taken when handling a diamond bit, and some additional ones:
  3. When removing the bit from its box, handle it carefully. Do not roll it out on the rig floor. If the bit is dumped on the floor and some of the cutters are chipped, the bits life will be reduced.
  4. The interior of the bit should be inspected to make sure no debris is left inside.
  5. The proper bit breaker should be used to make up the bit.
  6. The bit is one solid piece and does not have the limited flexibility of roller cone bits. Hitting ledges or running through tight spots can damage the gauge cutters.
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Responses

  • BILCUZAL
    What precautions should be taken for using pdc bit?
    7 years ago
  • tomas
    What means PDC bit drilling?
    6 years ago

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