SECTION 2
DRILLER'S METHOD
This series of problems in pressure control illustrates the procedure for pumping a kick out of the hole, and then increasing the mud density by using a preplanned pressure control worksheet (Fig 2-1).
The same circulating system is used as was used in the problems in section 1.
Key points This is the procedure generally described as the driller's method. The kick is first circulated out of the hole, then the mud density is increased.
The use of extra mud density as a margin for tripping or as a safety margin is shown in these problems. The annular pressure loss is converted to mud density as the proper margin.
If margin values are chosen arbitrarily, they may induce extra pressures and cause lost circulation. Alternatively, they may not be adequate for pulling the drill pipe. So it is usually best to calculate the annular pressure loss as a mud density term, or use some value related to the mud flow properties and hole geometry.
Notice the relationship between pump rate and system pressure loss is 23423p1519x
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If you compare the casing pressures in the problems in the preceding section with those in this group you will see the relative difference that occurs with increasing gas volumes (compare kick intrusion volumes). Maximum casing pressure tends to increase as the square root of the kick size.
PROBLEM 2
Problem 2-0
Problem facts:
1. During crew change the flow sensor and pit volume totalizer showed a kick, but it was 20 min before the well was shut in. Kick size is 50 bbls (8 m3).
2. Company policy is to pump the gas or water out, shut down and increase the mud density in the pits, then circulate the new mud.
3. Policy requires that you use the pressure control work sheet using pump strokes on graphical analysis.
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Depth = 10000 ft (3050 m)
Hole = 8½"
Drill pipe = 4½"
Mud density is 9.6 ppg (1.15 SG, or 72 lb/ft3)
Pump rate to be 40 spm and system pressure loss at 40 spm is 1420 psi (100 kg/cm2)
800 pump strokes to fill drill pipe
2800 pump strokes to displace annulus
3600 pump strokes to circulate the hole
Annular pressure loss = 0.3 pg (0.04 SG or 72 lb/ft3)
Complete the pressure control work sheet.
Problem 2-1
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While you were filling out the pressure control work sheet the driller got nervous because the casing pressure was rising and started to kill the well. You heard the pumps start and came up to the floor in a panic.
What should you do:
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK - continue?
Stop the pump and close the well in?
Problem 2-2
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Casing pressure is starting to rise.
What should you do:
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK-continue?
Stop the pump and close the well in?
Problem 2-3
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Watch what is happening. All the gauges are moving in different directions.
What should you do?
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK-continue?
Stop the pump and close the well in?
Problem 2-4
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Now what has happened? According to the procedure in 2-0, what is next?
What should you do?
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK-continue?
Stop the pump and close the well in?
Problem 2-5
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It took 35 min to increase the mud density in the pits. You reset the pump strokes on the stroke counter, started the pump and brought the initial circulating pressure to 1990 psi (140 kg/cm²) at 40 spm.
Now look what is happening.
Check the graph on the pressure control work sheet.
What should you do?
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK-continue?
Stop the pump and close the well in?
Problem 2-6
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After awhile, you see this. The drillpipe pressure is a familiar number. Should you feel better now?
What should you do:
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK - continue?
Stop the pump and close the well in?
Problem 2-7
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There are two things that you could do now. If you are confident of what you are doing and your graphical analysis looks good, you should .......
If you are confused and not sure, you should......
What should you do:
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK-continue?
Stop the pump and close the well in?
Problem 2-8
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Panic! The vice-president for drilling and exploration is on the phone. He thinks you are all wrong. To prove the point, you stop the pump and close the well in. This is what you see.
What should you do:
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK - continue?
Stop the pump and close the well in?
Problem 2-9
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It looks as if we might have lost circulation-or have we? The casing pressure has been coming down.
What should you do:
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK - continue?
Stop the pump and close the well in?
Problem 2-10
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The choke is wide open and the casing pressure is zero. Should we put a little pressure on the choke to be sure?
What should you do:
Increase choke size?
Decrease choke size?
Increase pump rate?
Decrease pump rate?
Everything is OK - continue?
Stop the pump and close the well in?
ANSWERS
Problem 2-0: See completed pressure control work sheet (Fig 2-1).
Problem 2-1: Decrease choke size. Initial circulating pressure should be 1990 psi (140 kg/cm²).
Problem 2-2: Everything is OK - continue. Remember, someone made a mistake and let the kick get big. You can expect high casing pressures. If the casing string and surface equipment is properly designed, you should have no problem.
Problem 2-3: Increase pump rate. Do not worry about casing pressure. You cannot control casing pressure if you are going to keep BHP constant. Keep pump strokes constant, and maintain a constant drillpipe pressure using the choke.
Problem 2-4: Stop the pump and close the well in. The procedure was that the well would be shut in and mud density increased after the bubble was pumped out .
Problem 2-5: Everything is OK - continue. The ICP is coming down according to the graph on your pressure control work sheet. Notice the casing pressure.
Problem 2-6: Increase choke size. Check the chart; drillpipe pressure should be 1850 psi (130 kg/cm²). Notice the casing pressure.
Problem 2-7: If you are confident that everything is OK - continue. If in doubt, stop the pump and close the well in. The new mud weight (11.0 ppg) is at the bit. The SIDPP should be zero.
Problem 2-8: Everything is OK - continue at 1620 psi (144 kg/cm²) and 40 spm. The drill pipe is full of mud heavy enough to dominate the reservoir so the SIDPP is zero. The casing pressure is starting to go down as the heavier mud starts up the annulus. Be tactful with the vice-president.
Problems 2-9: Everything is OK - continue. The drillpipe pressure is OK and the casing pressure is continuing to come down as the heavy mud is pumped up the annulus. The clue is that the casing pressure is moving steadily. With lost circulation, it would stop moving or possibly start to rise rapidly. Proof of lost circulation would be reduction in pit volume.
Problem 2-10: Either 5 or 6 is a proper answer. Everything is OK and operations can continue, or the pump can be stopped. The well should be dead. If it doesn't flow when the pump is shut off, it is dead. If there is flow from the choke line, pump some more.
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