The objective of the various kill well method is to circulate out any invading fluid and circulate a satisfactory weight of kill mud into the well without allowing further fluid into the hole. Ideally this should be done with the minimum of damage to the well.
If this can be done, then once the kill mud has been fully circulated around the well, it is possible to open up the well and restart normal operations. Generally, a kill mud which just provides hydrostatic balance for formation pressure is circulated.
This allows approximately constant bottom hole pressure which is slightly greater than formation pressure to be maintained as the kill well circulation proceeds because of the additional small circulating friction pressure loss.
After circulation, the well is opened up again and the mud weight may be further increased to provide a safety or trip margin.
General Kill Well Method
- Balance Of Pressures Kill Well Method
- Constant BHP Kill Well Method
- Driller Kill Well Method
Balance Of Pressures Kill Well Method
Once the well is shut-in, providing nothing has broken down, the pressures in the well will be in balance. What is lacking in hydrostatic head of fluid in the well is now being made up by surface applied pressure on the annulus and on the drillpipe.
This allows us to determine what the formation pressure is and hence what kill mud weight is required to achieve balance.
On the drillpipe side of the U-tube see picture bellow
The mixture of mud and formation fluid in the annulus makes it impossible to determine formation pressure using the casing information. The drillpipe, however, is full of clean mud of known weight and can be used as a ‘barometer’ of what is happening downhole.
PF = Head of Mud in Drillpipe + SIDPP
Once the formation pressure is known, the mud weight required to balance, or ‘kill’, it can be calculated, since
Constant BHP Kill Well Method
There are three ‘constant bottom-hole pressure’ kill well methods in common use today which are:
• Driller’s Method
• Wait & Weight Method (also known as the ‘Engineer’s Method’)
• Concurrent Method.
These three techniques are very similar in principle, and differ only in respect of when kill mud is pumped down. In the Driller’s method, the kill is split into two circulations. During the first, the kick fluid is circulated without changing the mud weight; once the kick is out, the mud is weighted up and pumped around the well on the second circulation. The Wait and Weight method achieves both of these operations simultaneously. Kill mud is prepared before starting the kill, and the kick fluid is circulated out while this mud is circulated into the well. In the Concurrent method, a compromise is adopted between these two methods. The kick fluid is circulated out while the mud being circulated in, is weighted up in stages, towards the kill weight. Of the three methods, the Wait & Weight method is the most popular, and the one recommended for use in most instances. It produces lower well pressures than either of the other methods, with an evident benefit to safety. Each of the three methods is now described in detail.
The Driller’s Kill Well Method
In the Driller’s method, the kick is circulated out of the hole using the existing mud weight. The mud weight is then raised to the required level and circulated around the well. Two complete circulation’s are thus required, as a minimum, for this method. Since it deals separately with the removal of the kick and the addition of kill weight mud, it is generally considered to be the simplest of well control methods, and it requires least arithmetic. However, this results, in the well being circulated under pressure for a relatively long time, possibly the longest of the three methods, with an increased possibility of choke problems. Also, the annular pressures produced during the first circulation are higher than produced with any other method.
CAUTION: Because very high annular pressure may arise when killing a gas kick well with this method, care should be taken. Annular will be at a maximum immedietly before gas arrives at surface, and casing burst pressure limitations may be critical. For this reason, the driller’s method is not generally recommended.
In general, this method is most used on small land rigs where the Driller may have little help and limited equipment. A variant of this method is also used in some workover operations. In addition the simplicity of the Driller’s method makes it useful when only limited information is available about the well conditions.
- FIRST CIRCULATION: Pump the kick out of the well, using existing mud weight
- SECOND CIRCULATION: Pump kill weight mud around the well.
a) Advantages of driller’s method
- Minimum arithmetic
- Minimum waiting around time – can start kill well at once
- Minimum information required.
b) Disadvantages of driller’s method
- Highest annular pressure produced
- Maximum well under pressure time
- Longest one chock time.
c) Procedure for driller’s method
1) The well is closed in and the information recorded.
2) If a slow circulating rate pressure, PSCR, has been taken, then calculate the pressure required on the drillpipe for the first circulation of the well.
Initial Circulation = Slow Circulation Rate + Shut In Drillpipe
ICP = PSCR + SIDPP Pressure
3) Open the choke about one quarter, start the pump and break circulation; then bring the pump up to the Kill Well Rate.
4) While the Driller is bringing the pump up to the Kill Well Rate, the choke operator should operate the choke so as to keep the casing pressure at or near the closed in casing pressure reading.
5) Once the pump is up to the Kill Well Rate, the choke operator should transfer his attention to the drillpipe pressure gauge and adjust the choke to maintain the INITIAL CIRCULATING PRESSURE on the drillpipe pressure gauge.
6) The INITIAL CIRCULATING PRESSURE is held constant on the drillpipe pressure gauge by adjusting the choke throughout the whole of the first circulation, until all of the kick fluid has been circulated out of the well. The pump rate must also be held
constant at the KILL WELL RATE throughout this period.
7) Once the kick is out of the hole, shut the well in and mix up the kill mud weight required.
Kill Weight Mud (ppg) = Old Mud Weight + [SIDPP (psi)÷ TVD (ft)÷ 0.052]
NOTE 1: This is a kill weight mud to balance formation pressure. It is the lowest possible mud weight which will ‘kill’ the well. Once the well is dead, it will be necessary to increase the mud weight further to provide a trip margin.
NOTE 2: Some operators prefer to continue circulating the well while kill weight mud is being mixed. There is no theoretical reason why this should not be done, though it does result in further wear and tear on equipment under pressure – in particular the choke.
8) Once the kill mud is ready, open the choke about one quarter, start the pump and break circulation. Then bring the pump up to the kill rate.
9) While the Driller is bringing the pump up to the kill rate, the choke operator should operate the choke so as to keep the casing pressure steady at the same pressure as when closed in.
10) While the drillpipe is being filled with heavy mud, hold the casing pressure constant. The drillpipe pressure will go down as the drillpipe is being slugged with the heavier mud. In practice, if all the kick was properly removed in the first circulation, the choke should not need to be touched once the pumps are steady at the Kill Well Rate, until kill mud reaches the bit.Once the kill mud reaches the bit, the pressure held on the drillpipe is just that required to circulate the mud around the well. This is the slow circulating rate pressure, increased slightly for the extra mud weight.