In the first circulation of driller’s method, driller circulates gas kick with 25 spm and the initial circulating pressure (ICP) is 1600 psi. The initial shut in drill pipe pressure is 450 psi. After shift change, another driller accidentally changes pump rate to 30 spm but he still holds drill pipe pressure constant.
What will happen to bottom hole pressure?
Let’ start with the basic formula
ICP = SIDDP + SCR
When the pump speed is increased, slow circulating rate (SCR), which is caused by friction, will increase in order to maintain constant bottom hole pressure. However, for this case, the drill pipe pressure is maintained constant with while increasing pump rate therefore the bottom pressure will decrease.
How much bottom hole pressure will decrease?
Current SCR = ICP – SIDPP = 1600 – 450 = 1150 psi
New SCR = 1656 psi
With new pump rate at 30 spm, the new SCR should be 1656psi but the driller maintains the old SCR, 1150 psi. Therefore, the bottom hole pressure will decrease by 506 psi (1656 – 1150).
Conclusion: The point that I would like to mention is that when you change your pump rate while performing well control operation. You must ensure that you do proper way to maintain bottom hole pressure. Otherwise, you may accidentally either decrease or increase the bottom hole pressure. If you accidentally decrease the bottom hole pressure, the influx will continue coming into a well and you will be in the big trouble. On the other hand, if you accidentally increase the bottom hole pressure, you may break wellbore and end up with lost circulation issue.
Reference book: Well Control Books