Well control with pipe off the bottom is one of the most serious well control situations because kick below a drillstring can create very complicated situations when compared to a normal well control. This article will teach you about the basic well control when the pipe off bottom.
There are several well control techniques to manage when the kick comes into the well and it is below the pipe;
- Use the Volumetric technique. The volumetric well control to let the gas migrate to surface with the bottom hole pressure nearly constant. This option is applicable when the influx is gas.
- Strip the drillstring back to the bottom. This is applicable with non-migration kicks as water and oil.
- Strip the drillstring utilizing the volumetric well control. This method can be used when you have a gas influx.
- Snub (push) the pipe against the wellbore pressure down to the bottom
- Kill the well off bottom using conventional well control methods as driller’s method and wait and weight. Typically, this is not recommended to use because you won’t get the kick out of the well and you may not be able to determine the kill weight mud correctly.
Since the pipe is off bottom and it is very critical to go back to the bottom to kill the well with the normal well control methods; therefore, there are two special techniques to trip the drillstring back to the bottom which are stripping and snubbing.
Figure 1 – Kick Off Bottom
Snubbing – it is when you push the string through the BOP because the weight of the drillstring is less than the upward force created by wellbore pressure. Typically, the snubbing operation cannot be performed with a normal rig set up. It is required several specialized tools which you don’t normally have on the rig. This operation is normally performed by hydraulic workover units.
Stripping – it is when you trip the string through the BOP, typically annular preventer, when the string weight is more than the upward force pushing up from the well.
The kick is below the bit and this can result in same reading on both drill pipe pressure and casing pressure. You don’t know the correct pressure to determine kill weight fluid because both sides don’t have one single fluid column.
Figure 2 – Same Reading on Both Casing and Drillpipe Pressure
The kick type must be identified in order to determine if the volumetric well control will be applied during the stripping operation. By observing surface pressures, if there is increase in surface pressure, it will most likely be gas influx. On other hands, if there is no change, it is a high possibility to be fluid kick. If you take a gas influx, the volumetric control must be considered for stripping operation.
Stripping drillpipe into the well is required to have an inside blow out preventor in the drillstring. The IBOP will prevent the flow to come up and the forward circulation can be performed. It is very critical that the IBOP is in a good condition prior to using it.
For stripping operation, there are two techniques which are annular stripping and ram combination stripping. The annular stripping is quite simple way to strip into the well but the ram combination is quite complicated and you may have more chance to damage the BOP components. If you have a choice for this kind of operation, we would like to recommend you to use the annular stripping technique.
Before the annular stripping operation, it is recommend reducing the annular operating pressure to allow the drill string to be stripped easily and the annular element does not face excessive pressure when the tool joints are pushed through it. Additionally, it might be helpful to have a surge dampener in the closing line in order to maintain constant closing pressure while the tool joints are being stripped through.
Surface pressure is another factor that can limit the annular stripping. Operating life of the annular element will drastically reduce because of high surface pressure. If the well is shut in with high surface pressure, you are required to reduce surface pressure prior to stripping. There are some options to get the surface pressure down as listed below;
- Bullhead with heavier mud
- Lubricate and bleed if the influx is on surface
- Circulate an influx out if you know that the influx is above the bit
For the next topic, we will discuss into some details of this well control method, the related calculations and procedure.
Reference books: Well Control Books