INITIAL SHUT-IN CONDITIONS CIRCULATION STARTED AT 30SPM 100 400 400 400 780 150 KILL LINE PRESSURE HELD CONSTANT DRILLPILE PRESSURE INCREASED BY SCR PRESSURE CHOKE PRESSURE DROPS BY CHOKE LINE PRESSURE DROP
BOTTOMHOLE PRESSURE STAYS APPROXIMATELY
CONSTANT
SCRS AND CHOKE LINE LOSSES
MINIMUM RATE FOR PUMP
SPM 20 30 40 PSCR 400 680 985 PCL 150 250 370 MUD GAS KEY CIRCULATION STARTED AT MINIMUM RATE, 20SPM 100 100 100 200 600 50
UNABLE TO KEEP THE KILL LINE PRESSURE CONSTANT. EVEN WITH THE CHOKE WIDE OPEN THE KILL LINE PRESSURE INCREASES BY THE SUM OF CHOKE LINE LOSS AND WIDE OPEN CHOKE PRESSURE MINUS THE ORIGINAL SHUT-IN PRESSURE
DRILLPIPE PRESSURE EQUALS THE SUM OF THE ORIGINAL SHUT-IN DRILLPIPE PRESSURE PLUS THE SCR PRESSURE PLUS THE CHOKE LINE LOSS PLUS THE WIDE OPEN CHOKE PRESSURE MINUS THE SHUT-IN CASING PRESSURE
CHOKE PRESSURE WITH CHOKE WIDE OPEN
BOTTOMHOLE PRESSURE INCREASES INFLUX CIRCULATED OUT
WITH ORIGINAL MUD WEIGHT
6-32
March 1995
These pressures as well as the annulus frictional pressure will act at all points in the wellbore and circulating system. The effect of these additional pressures must therefore be analysed at all points in the system and in particular at the openhole weak point.
2 Calculate the initial circulating pressure
The initial circulating pressure is calculated to estimate the standpipe pressure once the pump is up to speed.
For Case A: the initial circulating pressure = Pdp + Pscr
For Case B: the initial circulating pressure = Pdp + Pscr + Pcl + Poc – Pa where: Pscr = show circulating rate pressure (psi)
Pdp = shut-in drillpipe pressure that reflects the kick zone pressure (psi) Pcl = choke line frictional pressure at SCR (psi)
Pa = annulus shut-in pressure (psi)
Poc = choke pressure recorded while circulating at SCR with the choke wide open (psi)
3 Calculate the final circulating pressure
The final circulating pressure, when kill weight mud reaches the bit, for each case is calculated as follows:
For Case A: Final circulating pressure = PscrX MW2 MW1
For Case B: Final circulating pressure = (PscrX MW2) + Pcl + Poc – Pa MW1
where MW2 = weight of the kill mud (SG) MW1 = weight of the original mud (SG)
4 Monitor pressure at the kill line monitor as the pump is brought up to speed
For Case A, the pressure at the kill line monitor is held constant as the pump is brought up to speed. The choke pressure will decrease by an amount equivalent to the choke line friction pressure once the pump is up to speed.
For Case B, the pressure at the kill line monitor will be constant as the pump is brought up to speed. However at some point before the pump is up to the SCR the kill line monitor pressure will start to increase. Once the pump is up to speed the choke will be wide open and the pressure at the kill line monitor will have risen by the proportion of the choke line friction pressure that is not compensated for. (The increase will be equivalent to Pcl + Poc – Pa.)
5 Check the initial circulating pressure once the pump is up to speed
If the initial circulating pressure is significantly different from the calculated value, the pump should be stopped, the well shut in and the cause for the discrepancy determined. If the initial circulating pressure is equal to or reasonably close to the calculated value, the displacement should be continued.
Any marginal difference is likely to be due to the fact that the actual SCR pressure is different from the value used to calculate the initial circulating pressure. The actual SCR pressure can be established from the initial circulating pressure recorded when the pump is up to speed.
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March 1995
For Case A, the actual SCR pressure can be determined from the initial circulating pressure as follows:
Pscr = Pic – Pdp
For Case B, the actual SCR pressure can be determined as follows: Pscr = Pic – Pdp – Pcl – Poc + Pa
For the Wait and Weight Method the final circulating pressure must be recalculated as follows:
For Case A, the final circulating pressure can be determined as follows: Pfc = PscrX MW2
MW1
For Case B, the final circulating pressure is determined as follows: Pfc = (PscrX MW2)+ Pcl – Pa + Poc
MW1
The standpipe pressure should therefore be redrawn to take into account these adjusted figures.
6 Assess the effect of choke line losses at the latter stages of kick
displacement
For Case A: In the latter stages of the displacement the choke pressure required to maintain constant bottomhole pressure will drop. This drop will be most significant once the original mud behind the influx is at the choke. If the required choke pressure drops below the sum of the choke line loss and the wide open choke pressure, it will no longer be possible to completely compensate for the choke line losses.
The resultant increase in wellbore pressure at this stage will be given by: Increase in pressure = Pcl + Poc – Pa
In practice, the choke will be wide open at this stage and the standpipe pressure will rise above final circulating pressure.
When the hole has been circulated to kill weight mud, the circulating pressure will have increased by the sum of the choke line losses and the wide open choke pressure.
For Case B: As the influx expands the choke pressure required at surface will increase. As the required choke pressure increases it will be possible to compensate for a greater proportion of the choke line losses.
If the required choke pressure increases to a value equal to the sum of the choke line loss and the wide open choke pressure it will be possible to compensate for the complete amount of the choke line losses. It should be noted that the most critical period in terms of downhole pressures is likely to occur at early stages in the displacement. In this respect the change in choke line loss compensation at latter stages in the displacement is unlikely to be a critical factor.
6-35 March 1995
6.2
SPECIAL TECHNIQUES
Subsection
Page
2.1 VOLUMETRIC METHOD
6-37
2.2 STRIPPING
6-51
2.3 BULLHEADING
6-71
2.4 SNUBBING
6-79
2.5 BARYTE PLUGS
6-89
2.6 EMERGENCY PROCEDURE
6-97
6-35/366-37
March 1995
6.2
SPECIAL TECHNIQUES
Subsection 2.1
VOLUMETRIC METHOD
Paragraph
Page
1 General 6-38
2 Static Volumetric Method
(Drillpipe pressure used to monitor
bottomhole pressure) 6-38
3 Static Volumetric Method
(Choke pressure used to monitor
bottomhole pressure) 6-40
4 Lubrication 6-46
5 Dynamic Volumetric Control 6-47
Illustrations
6.12 Static Volumetric Method – an example of control
of bottomhole pressure at the choke 6-42
6.13 Static Volumetric Control – illustrating the
consequences of improper procedure 6-43
6.14 Volumetric Control Worksheet – an example for a land rig 6-44
6.15 Static Volumetric Method – choke pressure used
to monitor bottomhole pressure 6-45
6.16 Dynamic Volumetric Method – used to remove
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March 1995
1 General
The Volumetric Method can be used to control the expansion of an influx that is migrating during shut-in periods. It can therefore only be used if significant migration is
occurring. This may occur only in the case of gas kicks.
This method can be used during shut-in periods prior to displacement, or as a means of safely venting an influx from a well in which circumstances prevent the implementation of normal well control techniques.
Situations in which the Volumetric Method may be applicable therefore include: • During any shut-in period after the well has kicked.
• If the pumps are inoperable.
• If there is a washout in the drillstring that prevents displacement of the kick. • If the pipe is a considerable distance off bottom, out of the hole or stuck off bottom. • If the bit is plugged.
• If the pipe has been dropped.
There are four techniques that may be required to deal with an influx that is migrating up the hole. These are as follows:
• Static Volumetric Control: When the drillpipe is on or near bottom and can be used to measure bottomhole pressure.
• Static Volumetric Control: When the drillpipe cannot be used to measure bottomhole pressure.
• Lubrication: When the influx has migrated to the stack this technique is used to replace the influx with mud as the influx is bled at the choke.
• Dynamic Volumetric Control: This technique may be used as an alternative to the above but is most applicable as an alternative to lubrication on a floating rig.
The following Paragraphs can be used as guidelines for the implementation of the above mentioned procedures.