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INPUT   DATA

Title

Drilling rate		ft/hr
Rotary speed		rpm
Weight on bit		1000 lb
Bit diameter		inches
Normal pressure gradient		lb/gal
Mud weight in use		lb/gal
Compactibility coefficient

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OUTPUT   VARIABLES   &   GRAPHS

	dimensionless
♦  d-Exponent

d-exponent

This is a factor for evaluating drilling rate and predicting/detecting abnormal pore pressure zones. All things being equal, the d-exponent should increase with depth when drilling in a normal pressure section. A reversal of this trend is an indication of drilling into potential overpressures.

The relevant equation developed by Jorden & Shirley is:

Where:

   R = penetration rate, ft/hr.
   N = rotary speed, rev/minute
   W = weight on bit, 1000 lb.
   D = drillbit diameter, inches
   ρ_n = normal pressure gradient, mud-weight equivalent, lb/gal.
   ρ_m = mud-weight used, lb/gallon
   c = shale compactibility coefficient, dimensionless

BIBLIOGRAPHY

• Moore P.L.; Drilling Practices Manual; Pennwell Books, Tulsa, Oklahoma, 1986, chap. 13.
• Jorden J.R. & Shirley O.J.; Application of drilling performance data to overpressure detection; J Petroleum Tech, Nov., 1966, p. 1387-1394.
• Lapeyrouse N.J.; Formulae And Calculations For Drilling And Workover; Gulf Publ. Co., Houston, 2002, chap. 5.