Procedure and calculation example for Change in Circulating Rate while circulating an influx during a well control incident.
1. Calculate the average mud weight (Wav) in the string at the time pumping is stopped or changed.
2. Calculate shut in drillpipe pressure (PDP) at the time pumping is stopped/changed.
3. Maintain Pchoke constant for the short period it takes to change the circulating rate to the selected pumping speed.
4. Read PST at the new pump rate and PLOT this pressure at time T1 (change in circulation rate).
5. Calcualte new initial circulating pressure (ICP) = PST - PDP.
6. Calculate new final circulating pressure (FCP)
= ICP x new mud weight
average mud weight (Wav).
7. Plot FCP at time T2 (when kill mud reaches the bit).
8. Connect points PST and FCP with a straight line. This line represents the standpipe pressure whilst pumping the new mud to the bit.
Calculation Example
A kick sheet was completed using the following data.
SIDPP = 530 psi
SICP = 720 psi
Dynamic pressure loss at 30 SPM = 650 psi
Initial mud weight = 14.1 ppg
Hole depth = 12,366 ft TVD
Kick sheet details
Kill mud weight = 14.9 ppg
ICP = 1180 psi
FCP = 688 psi
Pressure drop / 100 stks = 24.5 psi
After 600 strokes, it was decided to change the pump strokes from 30 strokes/min to 25 strokes/min.
Wav = Average mud density = 14.2 ppg
PDP = 1180 (ICP) - 600 stks * 0.245 press. drop/stk - 650 (SCR30)= 383 psi
PST = Pressure at 25 stks / min = 850 psi
ICPNEW = PST - PDP = 850 - 383 = 467 psi
FCPNEW = ICPNEW * (New Mud Weight / Average Mud Wt.)
= 467 * (14.9 / 14.2 ) = 490 psi