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.
d = log [90÷ (60 x 110)] ÷ log [(12 x 20) ÷ (1000 x 8.5)]
d = 1.20