D exponent is an extrapolation of drilling parameters to get a trend while drilling into over-pressured zones. Usually, mud logger will correct all data, calculate d-exponent and plot the d exponent valve on the curve. The d-exponent can be utilized to detect transition from normal pressure regime to abnormal formation pressure. While drilling, if the change of trend is observed, rig supervisors must be cautious about this situation because this is one of the possible well control indications.
The “d” exponent described from the equation below:
d = log (R ÷ 60N) ÷ log (12W ÷ 1000D)
Where; R = penetration rate in feet per hour
d = exponent in drilling equation, dimensionless
N = rotary speed in rpm
W = weight on bit in kilo pound
D = bit size in inch
** Note: this equation is is valid for constant drilling fluid weight.
Example: Determine the d-exponent from following information.
Rate of penetration (R) = 90 ft/hr
Rotary drilling speed (N) = 110 rpm
Weight on bit (W) = 20 klb
Bit Diameter (D) = 8.5 in.
Solution:
d = log [90÷ (60 x 110)] ÷ log [(12 x 20) ÷ (1000 x 8.5)]
d = 1.20
Please find the Excel Sheet for calculating d-exponent.
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
Note: If you don't want to miss any post from drillingformulas.com, please consider subscribing to our mail list. So you will not miss our posts.
June 4th, 2010 at 9:48 pm
[...] rig to notify the high pressure transition zones. Read and understand about d-Exponent and learn how to calculate d-Exponent and normalized d-Exponent (corrected [...]