Calculate d-Exponent to Predict Pore Pressure Trends

This Excel spreadsheet calculates the d-exponent, a parameter used by drilling engineers to investigate ppore pressure trends when drilling into over-pressurized zones.

Normally, formation density increases with the drilling depth. But if the formation contains sand below the surface, then the drilling rate may increases with drilling depth. The d-exponent is used in several calculations to investigate these effects.

Calculate D-Exponent in Excel

Jorden and Shirley (1966) suggested the d-exponent method, basing it on the Bingham equation. A later adjustment to the equation by Rehm and Mcledon (1971) included the effect of mud weight

The modified equation to calculate the d-exponent is

d exponent
  • c is the shale compactibility coefficient
  • ρn is the mud weight equivalent (lb gal-1)
  • ρm is the mud weight used (lb gal-1)
  • D is drill bit diameter (in)
  • W is the weight on the bit (x 1000 lb)
  • R is the penetration rate (rpm)
  • N is the rotary speed.
The d-exponent is often used in Eaton's equation

Eaton's equation
  • P is the formation pore pressure (psi)
  • Pob is the overburden pressure (ft)
  • Z is the vertical depth (ft)
  • d is the d exponent
  • dn is the normal compaction trend of the d exponent
  • Pp is the formation pore pressure (psi)
  • Pp/Z is the normal pore pressure gradient (psi ft-1)


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