Why Failed Floats And Cement Plugs Act As A One Way Back Pressure Valve
Also: Exactly What the Blips on the Macondo Pressure Buildup Curve Are
Consider the closed chamber that is full of about ¾ of moderately compressible 14.17 ppg synthetic oil based drilling fluid and the other ¼ with relatively incompressible seawater. The initial pressure is zero psi yet the reservoir pressure (red arrow) is ~1400 psi higher than the weight of the fluid above the cement plug pushing down (the black arrow). Reservoir inflow is gradual and approximated by the Darcy Equation. The Big Red Arrow is Higher Is Constantly Getting Bigger than the big black arrow and once the red arrow is ~40 psi higher than the black arrow a small amount of influx squeezes by the cement plugs compressing the fluid in the closed chamber and increasing the chamber pressure by ~20 psi. The way this works is that the surface area above the plug is larger than the surface area on the bottom of the plug blocking the flow upward. The pressures above are smaller by 1400 psi initially and yet working over a larger surface area of the plug. Similiarly, the pressures below are larger by 1400 psi and yet working over a smaller surface area of the plug. Once the plug moves only a small distance upward a small amount of influx at higher pressure moves around the plug held down now by less rubber that are only fins of the plug and angled in such a way as to allow fluid to slip by and escape upward. Of course the downward force on the plug increases slightly as this happens because the small increase in volume of influx into the closed chamber causes the internal pressure to increase ~20 psi which is enough to force the plug down to once again restrict upward slippage of any influx by the plug. The reservoir quickly replaces the lost volume of influx that slipped by the cement plug and the pressure builds again under the cement plug until another volume of fluid enters the closed chamber. This process repeats itself approximately 23 times as the pressure in the closed chamber (the drill pipe pressure and kill line pressure yet it is clogged and not reading true pressures). This pressure climbs from 0-1400 psi in this way. The pressure climb from 200 psi to 1200 psi takes 20 blips. Each blip then is 50 psi. This is clearly seen on the NPT chart. The blips from 0-200 psi and from 1200 psi – 1400 psi are clearly more violent than the increase from 200 psi to 1200 psi and this is because the closed chamber is either just being closed (0-200 psi) as the drill pipe is shut in or just being opened slightly (1200 psi – 1400 psi) as the clog in the kill line moves slightly creating a slight opening of the chamber as the size of the chamber increases ever so slightly as the clog (that was originally formed at 1200 psi) is moved in the kill line as the drill pipe pressure reaches, and climbs above, the clog forming pressure of 1200 psi.