You have learned from 2 previous topics, gas behavior in shut in well and gas behavior with constant surface pressure. Both ways are not how you control bottom hole pressure. This post I would like to use the same scenario but what it is going to be different is that we will control bottom hole pressure constant.
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.
We control bottom hole pressure by allowing gas to expand, bleeding off amount of mud, and letting surface pressure increasing to compensate hydrostatic loss due to bleeding mud off.
For this example, we will let gas migrate and expand to 10 bbl. It means that you have 5 bbl of gas expansion.
Assume that 5 bbl of mud equates to 50 psi of hydrostatic pressure. We maintain surface pressure at 450 psi while bleeding off 5 bbl of mud.
Let’s apply the hydrostatic pressure concept.
Bottom Hole Pressure = Hydrostatic Pressure + Surface Pressure
Hydrostatic pressure decreases 50 psi due to bleeding off.
Surface pressure increases 50 psi because we would like to compensate what we lose from the hydrostatic.
Therefore, the bottom hole pressure will be constant.
Conclusion: This is the best way to control bottom hole pressure. This concept is used in volumetric control which I will cover it later.
Reference book: Well Control Books