This topic will explain you how wait and weight well control controls bottom pressure constant. There are 2 steps that must be took into consideration separately. The first one is when kill weight mud is displaced to the bit and the second one is when the kill weight mud from bit to surface.
1) Displace kill weight mud to the bit
Once kill weight mud is mixed and displaced into drill pipe, drill pipe pressure will be decreased due to increasing in hydrostatic pressure. In order to control bottom hole pressure, a drill pipe pressure schedule must be developed and followed until kill mud weight to the bit.
Take a look at the equation:
Pressure in drillpipe side = BHP = Pressure in casing side
DPP + HP – Frictional P in drillstring = BHP = HP + CP + Frictional P in annulus
DPP is drill pipe pressure.
HP is hydrostatic pressure.
Frictional P is frictional pressure.
CP is casing pressure.
At drillpipe side:
DPP will drop when kill mud weight is displaced.
HP will increase due to kill mud weight in the drillstring.
Friction P in drillstring will not change so much.
At casing side:
HP will decrease due to gas expansion in the annulus.
CP will increase in order to compensate hydrostatic pressure loss.
Frictional P in annulus had minimal effect on bottom hole pressure due to slow flow rate while killing the well.
2) Displace kill weight mud from bit to surface
Once the kill weight mud comes out of the bit, there is one single column of hydrostatic in the drillstring. Therefore, in order to maintain bottom hole pressure constant, drill pipe pressure must be maintained while displacing.
• Use the drill pipe pressure schedule to control constant bottom hole pressure while displacing kill weight mud to the bit.
• After kill mud out of the bit, maintain drill pipe pressure until circulation complete.