Monday, 20 November 2017

The Bit Selection

THE BIT SELECTION

Bit Selection PDC


Unfortunately, the selection of the best bit to drill a given formation is made by trial and error. The criterion used for selecting a bit is based on the drilling cost per unit interval.

The Cost Per Foot Equation

The best bit for drilling a given formation is the bit that would give the least drilling cost.
The drilling cost per foot. C is,

bit selection equation cost/ft

Initial selection of bit type

The initial selection of bit type can be made on the basis of formation drillability and abrasiveness.

  • Drillability is a measure of how easy the formation is drilled and it is inversely related to the compression strength of the rock. 
  • The abrasiveness of the formation is a measure of how rapidly the teeth of a milled tooth bit wear when drilling the formation. 
  • Normally, abrasiveness increases as the drillability decreases. 

The drilling cost per foot is the final criterion that should be used for selecting a bit.

Rule Of Thumb

Some of the rules of thumb used for bit selection are:

  • High-cost bits tend to be more applicable when the daily rig operation cost is high.
  • Rolling cone bits are the most versatile bits and are good initial choice.
  • Diamond drag bits perform best in nonbrittle formations having a plastic mode of failure, especially in the bottom portion of a deep well, where the high cost of tripping operations favors a long bit life, and a small hole size favors the simplicity of a drag bit design.
  • PDC drag bits perform best in uniform sections of carbonates or evaporites that are not broken up with hard shale stringers or other brittle rock types.
  • PDC drag bits should not be used in gummy formations, which have a strong tendency to stick to the bit cutters.


When using a rolling cone bit:


  • Use the longest tooth suitable for the application.
  • When the rate of tooth wear is much less than the rate of bearing wear, select a longer tooth size, a better bearing design, or apply more bit weight.
  • When the rate of bearing wear is much less than the rate of tooth wear, select a shorter tooth size, a more economical bearing design, or apply less bit weight.


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