In order to get more understanding about driller’s method, there are some quizzes that will help you get clear picture of it.
1st example –
Before start circulating, you have the following information
Original Shut in Drill Pipe Pressure = 500 psi
Initial Circulating Pressure (ICP) = 1500 psi
After end of 1st circulation of Driller’s Method, the drillpipe pressure gauge and the casing pressure gauge shows that the circulating drill pipe pressure = 1500 psi and casing pressure = 650 psi (see the figure below).
Do you think what is happening with the well?
Answer: There is influx (kick) remaining in well because casing pressure is higher than the original shut in drill pipe pressure. Additional circulation is required for this case.
Driller’s method concept is to hold bottom hole pressure constant therefore with neglecting frictional pressure, once influx in out of hole, you should see that casing pressure must be equal to initial shut in drill pipe pressure
Drill pipe side: BHP = Hydrostatic Pressure + Shut in Drill pipe pressure —> equation 1
Casing side: BHP = Hydrostatic pressure + Casing Pressure + Frictional Pressure —> equation 2
Neglect frictional pressure so we can write the formula like this.
Casing side: BHP = Hydrostatic pressure + Casing Pressure —> equation 3
With the assumption stated above, you will see that
equation 1 = equation 3
Then you will know that
Shut in Drill pipe pressure = Casing Pressure
Reference book: Well Control Books