Shut-in Drill pipe pressure SIDPP

Shut-in drill pipe pressure (sidpp) is the surface pressure on drill string side when an influx has taken place and the well is shut. While Shut-in casing pressure SICP is the surface pressure on casing side.

There is a assumption in well-control calculation, When the kick takes place formation fluid flows into annulus and not inside the drill-pipe. Which is genrally true, as the genral direction of fluid flow is from drill pipe to annulus side.

u-tube-effect-drilling

Initially when the well is under control (no kick), Hydrostatic pressure on drill pipe side (Pdp) is equal to hydrostatic pressure on annulus side (Pannulus) due to U-tube effect. When the formation fluid flows into the wellbore it displaces the heavyier drilling fluid out of the annulus. Due to which pressure on the annulus side (Pannulus) will decrease untill the well is shut-in while the hydroststaic pressure on the drillpipe side (Pdp) remains unchanged. So now:-

Pdp > Pannulus

Once the well is shut, shut-in surface pressure (for both drill pipe and casing side) increases and becomes stable after sometime. At that time time flow into the wellbore stops showing bottom hole pressure is equal to formation pressure.

Formation Pressure (FP) = Pdp + SIDPP = Pannulus+ SICP
And as Pdp> Pannulus , hence SIDPP< SICP
Which means Shut-in casing pressure is higher than Shut-in drill pipe pressure.

How does shut-in pressure arises?

To understand that let us take an example.
shut-in pressure calculation
Befrore wellbore shut-in
While drilling, an overpressured zone is encounter with formation pressure=2100psi Initially the bottom hole pressure is 1800 on both sides, but as the formation fluid enters the annulus BHP on annulus decreases to 1750psi while BHP on drill pipe side is still 1800 psi.
shut-in pressure calculation
After wellbore shut-in

On annulus side 1750psi cancels 1750psi of formation pressure while remaining 350psi of formation pressure is felt through out the annulus towards upward direction and is seen as SICP at surface. And as the well is shut there will be equal and opposite reaction pressure of 350 psi towards downward direction. And the bottom hole pressure becomes
BHPannulus=1750+350=2100psi

On drill pipe side 1800psi cancels 1800psi of formation pressure while remaining 300psi of formation pressure is felt through out the drill string and is seen as SIDPP at surface. And as the well is shut there will be equal and opposite reaction pressure of 300 psi in the downward direction. And the bottom hole pressure becomes
BHPdrillpipe= 1800 + 300=2100psi

Conclusion:- After shut-in BHP on bothsides becomes equal to formation pressure due to which inflow of formation fluid stops. Out of the two pressures, drill pipe pressure (Pdp) is more important and is used to calculate formation pressure and kill mud weight as Pannulus has two unknown variables influx height and density.

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