Optimal Weight On Bit And Rotary Speeds
The purpose of selecting bit weight and rotary speeds are to
 produce the lowest drilling cost per foot
 control the direction of the drill hole
 find overpressured zones
 satisfy equipment limitations (solids removal, bha, etc.)
 develop a satisfactory cuttings description log
Optimum Weight and Speed
CONTOUR METHOD
The following example illustrates the contour method of selecting optimal weight on bit and optimal rotary speed. The method consists of constructing the above figure in a two dimensional presentation. This is accomplished by plotting drilling cost per foot contours with known drill bit data on a set of axes. The weight of bit is set as the abscissa and the rotary speed as the ordinate. Most likely no two bits will have the same drilling cost. Thus, one draws the contour lines to either include a value or exclude it within the contour. Those bits which are nearest the optimal weight on bit and rotary speed will be the most cost effective bits.
Six insert bits have been run and their performance and total cost per foot data are listed in the table.
An analysis of the figure showing bit weights and rotary speeds versus cost per foot for each of the bits indicates that the optimal weight and speed is 40,000 pounds and 50 rpm.
The resulting cost per foot is about $35.00/foot.
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