SECTION 8
TIME IS THE ENEMY IN KILLING DEEP WELLS
This section illustrates the problems that occur during a well kick in a very deep well. Usually the major difficulty is that it takes so long to kill the well and displace the mud that either someone gets bored and does not pay attention, or 757e41h some of the machinery breaks down.
Key points. Annular pressure loss is a big factor in the deep well and can cause lost circulation if it is not recognized.
The pressures at the surface get very high with slim holes. In this problem a triplex pump capable of handling 4000 psi (280 kg/cm2) was specified. If a duplex rig pump was used, the pump rate would have had to be reduced to 18 spm during the initial phases of the kill procedures.
When in doubt, or if there are mechanical troubles, the well should be shut in until the problems can be resolved.
PROBLEM 8
Problem 8-0
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Problem facts:
While drilling at 25000 ft (7620 m) a gain in pit volume of 15 bbl (2.4 m3) was noticed. The well was closed in. Do you know how to check to see if the drillpipe pressure is correct?
Mud density = 13 ppg (1.56 SG, 97 lb/ft3)
Casing = 7" at 22000 ft (6705 m)
System pressure loss at 30 spm = 1100 psi (77 kg/cm2)
Annular pressure drop at 30 spm = 400 psi (26 kg/cm2)
There are 1500 bbl (238 m3) of mud in the standby tanks with a density of 15 ppg (1.8 SG, 112 lb/ft3).
The pump is rated at 4000 psi (280 kg/cm2).
It requires 3550 strokes to displace the tapered string of drill pipe.
Fill out the pressure control work sheet.
Do not add a safety margin (trip margin) to the mud weight required.
Problem 8-1
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As you start the pump you notice that at about 25 spm, the annulus pressure is the same as when shut in and the drillpipe pressure is the proper value. You were planning to circulate at 30 spm!
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase pump rate?
4 Decrease pump rate?
5 Everything is OK - continue?
6 Stop the pump and close the well in?
Problem 8-2
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After a few minutes you notice that casing pressure is 2350 psi (165 kg/cm2). Why should the casing pressure come down 400 psi (28 kg/cm2) all of a sudden?
What should you do:
1 Increase choke size?
Decrease choke size?
3 Increase pump rate?
4 Decrease pump rate?
5 Everything is OK - continue?
6 Stop the pump and close the well in?
Problem 8-3
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We are ready to switch the suction and start the heavy mud down the drill pipe. The driller wants to know if he should reciprocate the pipe. You tell him ........ !
What should you know:
1 What is mud density?
2 Number of pump strokes to displace drill pipe?
3 Drill pipe pressure reduction schedule?
4 Final circulating pressure?
Problem 8-4
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After pumping 278 bbls (44 m3) of 15 ppg (1.8 SG, 112 lb/cu ft) mud down the drill pipe you see these pressures.
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase pump rate?
4 Decrease pump rate?
5 Everything is OK - continue?
6 Stop the pump and close the well in?
Problem 8-5
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Everything is going wrong. You have been closing the choke, but it does not seem to raise the drillpipe pressure. On top of that, the kelly hose is trying to jump out of the derrick.
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase pump rate?
4 Decrease pump rate?
5 Everything is OK - continue?
6 Stop the pump and close the well in?
Problem 8-6
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You want to circulate with the #2 pump while they repair the #1 pump. It is not the same size as the #1 pump. At 34 spm you have this condition.
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase the pump rate?
4 Decrease the pump rate?
5 Everything is OK-continue?
6 Stop the pump and close the well in?
Problem 8-7
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Everything was going OK. Then the pressures started to go down. Have you lost circulation?
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase the pump rate?
4 Decrease the pump rate?
5 Everything is OK-continue?
6 Stop the pump and close the well in?
Problem 8-8
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You changed back to #1 pump. From your chart you expect the circulating pressure to be 1270 psi (89 kg/cm2). If you shut the well in (Answer 6), what would you expect the shut in drillpipe pressure to be?
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase the pump rate?
4 Decrease the pump rate?
5 Everything is OK-continue?
6 Stop the pump and close the well in?
Problem 8-9
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The casing pressure is still going down. Why? It looks like the casing pressure should be down to 0 after 4 hours. Why is it taking so long to kill the well?
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase the pump rate?
4 Decrease the pump rate?
5 Everything is OK-continue?
6 Stop the pump and close the well in?
Problem 8-10
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You have pumped 840 bbls (134 m3). 2300 pump strokes more, but the casing pressure only drops to 450 psi (32 kg/cm2). The pit volume is remaining constant. What is wrong?
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase the pump rate?
4 Decrease the pump rate?
5 Everything is OK-continue?
6 Stop the pump and close the well in?
Problem 8-11
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You see salt water at the separator line. Is it time to do something else?
What should you do?
1 Increase choke size?
2 Decrease choke size?
3 Increase the pump rate?
4 Decrease the pump rate?
5 Everything is OK-continue?
6 Stop the pump and close the well in?
Problem 8-12
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The derrickman tells you that the mud density at the suction is 15 ppg (1.8 SG, 112 lb/ft3), and 13 ppg (1.56 SG, 97 lb/ft3) at the separator.
The gland packing is leaking on a pump rod. Is it OK to shut down for about 15 min to fix this leak?
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase the pump rate?
4 Decrease the pump rate?
5 Continue circulating?
6 Stop the pump and close the well in?
Problem 8-13
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Heavy mud is now returning at the flow line.
What do you do next?
What should you do:
1 Increase choke size?
2 Decrease choke size?
3 Increase the pump rate?
4 Decrease the pump rate?
5 Everything is OK-continue?
6 Stop the pump and close the well in?
ANSWERS
Problem 8-0: If you are in doubt about the drillpipe pressure, open the choke slightly and pump for several minutes. Then turn off the pump and close the choke. The shut in drillpipe pressure will be the same as before if the original pressure was OK.
Problem 8-1: Increase the choke size and increase the pump rate. Stay with your original plan once you start to kill a well unless there is some compelling reason to change.
Problem 8-2: Everything is OK - continue. In this case you are seeing the effect of the annular pressure drop or circulating density on the casing pressure. It is 400 psi (28 kg/cm2) in this deep, slim hole.
Problem 8-3: The casing pressure is too high to risk reciprocating the drill pipe. Also it will take a long time to kill this kick and if you reciprocated the pipe with these kinds of pressures for a long period you are very likely to have trouble.
You should be aware of the following numbers so that you will not get confused or lost during the operation:
1 Mud density in the reserve tank is 15 ppg (1.8 SG, 112lb/ft3).
2 Pump strokes to displace the drill pipe is 3550.
3 Drill-pipe reduction schedule on Figure 8-1.
4 Final circulating pressure = 1270 psi (89 kg/cm2)
Problem 8-4: Increase choke size. The drillpipe pressure is not following the chart. It should be about 1500 psi (105 kg/cm2). At this time the top of the kick has been pumped up to about 12700ft (3870m).
Casing pressure decline is caused by a shortening of the annular column of intruding fluid as it gets into a larger part of the hole, or lost returns. Check the pit volume to see if it is going down. This would indicate lost returns.
Problem 8-5: Stop the pump and close the well in. You can shut any time you have a problem. These conditions with a jumping kelly hose are sign of a pump problem; probably a bad valve.
Problem 8-6: Everything is OK - continue. Now switch back to the graph of drillpipe pressure, but keep the pump at 34 spm. The graph is not accurate now because the pump size is changed, but it is all we have to work with. Switch back to the #1 pump as soon as practical. Notice that the casing pressure from 8-4 (before the pump problem) was used to establish a new circulating pressure at a new pump rate.
Problem 8-7: Increase the pump rate. Using the #2 pump, you must maintain 34 spm. In this case, no one closed the choke when the pump slowed so drillpipe pressure as well as casing went down.
Problem 8-8: Everything is OK - continue. If you stopped the pump and closed the well in at this point, the drillpipe pressure would be 0 because the drill pipe is now full of 15 ppg (1.80 SG, 112 lb/ft3) mud that is heavy enough to dominate the reservoir. Shutin casing pressure (SICP) would be 2250 + 400 = 2650 psi (158 + 28 = 186 kg/cm2). This is the effect of the annular pressure drop in a deep, slim hole. This is normally not too noticeable in a larger hole at shallower depth.
Problem 8-9: Everything is OK. This is a deep hole and takes a long time to circulate out. Check the pits! If the mud level is holding up, you have a saltwater kick. If you are losing mud volume you have lost circulation.
Problem 8-10: Everything is OK - continue. You have a saltwater flow. That is why the casing pressure is not changing very much. If there was a gas kick, the casing pressure would have changed a great deal more. This is a deep hole and it takes a long time circulate.
Problem 8-11: Everything is OK - continue. You should have only 15 bbls (2.4 m3) of salt water to contend with. Dump the salt water and continue circulating. The top of the heavy mud is now at 6300ft (1920 m). Don't give up - keep circulating.
Problem 8-12: Continue circulating. You still have 3000ft (914 m) of light mud in the annulus. Or: Stop the pump and close the well in. Then fix the pump if you think it is important.
Problem 8-13: Now you must add the trip margin to the mud density. It calculates to be 0.3 ppg (0.04 SG, 2 lb/ft3). Do you know how this number was obtained? You can either: Stop the pump and close the well in, then raise the mud density in the pits, Or: Continue-everything is OK - and increase the mud density while circulating.
In any case, the mud density must be increased so that the mud column exerts an additional 400psi (28 kg/cm2).
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