PUMP STROKES ICP
KILL MUD AT BIT 0
FCP
Figure 5.4
FIRST CIRCULATION
Profile of Circulating and Annular Pressure
Water Influx
START FINISH
Gas Influx
Annular Pressure
Time or Pump Strokes
Annular Pressure
Pressure Constant
START Circulating Pressure FINISH
Drill Pipe Closed in Pressure Circulating Pressure at Reduced Rate
Time or Pump Strokes
Figure 5.5
Surface to Bit
START FINISH
Constant
Pressure Constant
START FINISH
Circulating Pressure with Kill Mud
Well Dead in Drill Pipe
Surface to Bit
SECOND CIRCULATION
Profile of Circulating and Annular Pressure While Killing by Driller's Method
START
Annular Pressure
Time or Pump Strokes
Annular Pressure Circulating Pressure
Drill Pipe Closed in Pressure Circulating Pressure at Reduced Rate
Time or Pump Strokes
Determination of Initial Circulating Pressure
If no slow circulating rate pressure has been taken, then the initial circulating pressure can be determined using the start-up procedures described in the circulations of the Driller’s Method.
Where the casing pressure has been held constant while the pumps are brought up to a kill rate, the drill pipe pressure reading will be the initial circulating pressure.
WARNING: the existence of a predetermined kill rate gives rig personnel a wrong impression that a kick must be circulated exclusively at this rate.
The procedure consists of:
1. Noting casing pressure reading.
2. Adjusting pumps to new kill rate. Adjusting choke to hold casing pressure constant at the value noted.
3. As soon as the driller has the pumps settled on the new rate, return to the drill pipe pressure gauge. Note this new reading is the circulating pressure for the new pump rate and maintain this.
4. Check choke orifice size, in relation to kill rate
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 preferable to maintain pump rate constant as much as possible. Any decision to change pump rate should be taken early.
5.4 THE 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.
Once the well is shut in and pressures stabilised, the shut in drill pipe 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 drill pipe. At
commencement, enough drill pipe pressure must be held to circulate the mud, plus a reserve equivalent to the original shut in drill pipe 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 drill pipe 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 drill pipe pressure is held at the final circulating pressure by choke adjustment.
Advantages of the Wait and Weight Method
• Lowest wellbore pressures, and lowest surface pressures - this means less equipment stress.
• Minimum ‘on-choke’ circulating time - less chance of washing out the choke.
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.
Procedure for the Wait and Weight Method
The Wait and Weight method uses the same calculations already described for a drill pipe pressure schedule. The calculations are:
Kill Mud Weight = Original Mud Weight + _______(SIDPP)_______
(PPG) (PPG) True Vertical Depth x.052
At the start of the circulation, with kill mud:
Initial Circulating Slow Circulating Rate Shut in Drill pipe
Pressure = Pressure + Pressure
(ICP) (SCRP) (SIDPP)
Once the capacity of the drill string is calculated, it is possible to draw a graph showing how drill pipe pressure varies as kill mud is pumped down to the bit.
(See Figure 5.6)
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 drill pipe pressure gauge.
As the kill mud proceeds down the drill pipe, the drill pipe 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 drill pipe pressure tends to drop of its own accord as the kill mud moves down. Little or no choke adjustment is required.
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 drill pipe pressure 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 drill pipe 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!
Figure 5.6
START
Surface to Bit
Pressure Constant
START FINISH
Circulating Pressure with Kill Mud
Well Dead in Drill Pipe
Surface to Bit