This is another example demonstrating how bottom hole pressure will be when you maintain constant surface pressure and let gas bubble migrating up.
From the previous post, Gas Behavior and Bottom Hole Pressure in a Shut in well, you know that if you take gas kick and you do nothing buy only letting gas migrating up, you will have an increase in casing pressure (surface pressure) and you may end up breaking shoe or surface equipment. You know that this practice is a bad idea.
OK, I need to do something differently. I will maintain constant surface pressure by bleeding some mud in order to compensate pressure increment.
Do you think this is a good idea to do that?
Let’s I go though all detail about it.
You have the same well. The well is shut-in without pipe in hole. 5 bbl of gas kick is taken and initial shut in casing pressure is equal to 400 psi. Hydrostatic head on top of gas is 4000 psi.
I maintain surface pressure constant at 400 psi by bleeding 5 bbl of mud. It means that you allow gas to expand another 5 bbl so the gas influx is 10 bbl in total. According to the Bolye’s Law, you can determine gas influx pressure.
Pressure of gas (P1) is 4400 which equates to the bottom hole pressure.
Volume of gas at beginning (V1) is 5 bbl. V2 is 10 bbl.
P1 x V1 = P2 x V2
4,400 x 5 = P2 x 10
P2 = 2,200 psi ->Gas pressure decreases due to gas expansion.
Let’s take a look how bottom pressure will be.
I apply the hydrostatic pressure concept.
Bottom Hole Pressure = Hydrostatic Pressure + Surface Pressure
Surface pressure: remain constant because you bleed some mud to maintain it.
Hydrostatic pressure: The overall hydrostatic pressure will decrease because gas expansion will displace volume of mud.
Bottom hole pressure: It will decrease because of reducing hydrostatic pressure in the wellbore.
Conclusion: If you have gas influx and you maintain casing pressure constant by bleeding off mud volume, you will reduce the bottom pressure. Therefore, the influx will allow into the wellbore more. This is not good idea.
Reference book: Well Control Books