NOTE: This procedure is satisfactory at any time during a kill providing the mud weight in the drill string is not changing during the process. It is however
6.3.2 Wait and Weight Method
The ‘Wait and Weight’ is sometimes referred to as the ‘Engineers Method’ or the ‘One Circulation Method’. It does, at least in theory, kill the well in one circulation.
This is the preferred method used by most operators and recommended by many well killing experts. Its principal advantage is that it provides the lowest annular pressures during the circulation of the kill, making it the safest of the commonly used kill methods.
Once the well is shut in and pressures stabilised, the shut in drillpipe pressure is used to calculate the kill mud weight. Mud of the required weight is made up in the mud pits. When ready, kill mud is pumped down the drillpipe. At commencement enough drillpipe pressure must be held to circulate the mud, plus a reserve equivalent to the original shut in drillpipe pressure. This total steadily decreases as the mud goes down to the bit, until with kill mud at the bit, the required pressure is simply that needed to pump kill mud around the well.
The choke is adjusted to reduce drillpipe pressure while kill mud is pumped down the string.
With kill mud at the bit, the static head of mud in the drill pipe balances formation pressure.
For the remainder of the circulation, as the influx is pumped to the surface, followed by drill pipe contents and the kill mud, the drillpipe pressure is held at the final circulating value by choke adjustment.
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a)
•
Advantages of the wait and weight method
LOWEST WELL BORE PRESSURES, AND LOWEST SURFACE PRESSURES – this means less EQUIPMENT STRESS.
MINIMUM ‘ON-CHOKE’ CIRCULATING TIME.
• b)
•
•
Disadvantages of the wait and weight method
CONSIDERABLE WAITING TIME (while weighting up) – GAS MIGRATION?
IF LARGE INCREASES IN MUD WEIGHT REQUIRED, THIS IS DIFFICULT TO DO UNIFORMLY IN ONE STAGE.
c) The Procedure for the wait and weight method
Wait and Weight method uses the same calculations already described for a drillpipe pressure schedule. The calculations are:
Kill Mud Weight (ppg)
Original Mud Weight
+
[
SIDPP ÷ TVD ÷ 0.052]
= (ppg)
At the start of the circulation, with kill mud:
Initial Circulating
Once the capacity of the drill string is calculated, it is possible to draw a graph showing how drillpipe pressure varies as kill mud is pumped down to the bit.
Once kill mud is ready, the start-up procedure is as previously described.
The choke is cracked open, the pump started to break circulation, and then brought up slowly to the Kill Rate.
While the Driller brings the pump up to the Kill Rate, the choke operator works the choke so as to keep the casing pressure at or as near as possible to the closed in casing pressure reading.
When the pump is up to the Kill Rate, the choke operator transfers to the drillpipe pressure gauge, adjusting the choke if necessary to achieve the INITIAL CIRCULATING PRESSURE on the drillpipe pressure gauge.
As the kill mud proceeds down the drillpipe, the drillpipe pressure is allowed to drop steadily from the Initial Circulating Pressure to the Final Circulating Pressure, by choke adjustment.
Where the kick is a small one, at or near the bottom of the hole, the drillpipe pressure tends to drop of its own accord as the kill mud moves down. Little or no choke adjustment is required.
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Only in cases of diffused gas kicks with gas far up the annulus will significant choke adjustments be needed during this period.
After kill mud has reached the bit, the drillpipe pressures is maintained at the Final Circulating Pressure, until the kill mud returns to surface.
As with the Driller’s method, this Final Circulating pressure is held constant as long as pump rate is held constant at the selected value. If, for any reason, the pump rate is felt to be wrong, it can be changed using the same procedure described previously. However, pump rate changes should be avoided, where possible.
While the pump rate is adjusted, the casing pressure is held steady by adjusting the choke.
Once the pump is stabilised at its new speed, the revised circulating pressure is read from the drillpipe gauge. If a gas influx is very near to the surface, adjusting pump rate by holding a steady casing pressure may significantly increase the bottom hole pressure. This is due to the rapid expansion of gas near the surface. Alterations in pump rate are to be made early on!
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The following two graphs depict pressure variations during the Wait and Weight method.
Graph 3
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Figure 4
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