Removal Of Gas Trapped In Bops

In order to displace a gas kick completely from the wellbore several circulations of the well might be needed. During this time some of the gas may have become trapped under closed rams in the BOP stack as shown in Fig 5.28. This has the potential to cause a serious problem if the gas is not removed in a controlled manner. If the rams were opened without removing the trapped gas, the gas would be released into the riser. As the gas migrated, it would expand rapidly and cause the riser to unload mud onto the rig floor.

The most thorough method of gas removal is to leave the well shut on the lower rams whilst displacing the choke and kill lines to water. By closing the kill line valves, pressure can be bled off up the choke line and "U-tubed" up the choke line by opening the pipe rams. This sequence is shown in Fig 5.29 and 5.30.

The surface diverter should be closed during the operations so that any residual gas from the riser can be safely dealt with. Once the riser has been displaced to kill weight mud the lower rams can be opened and the well flow-checked. Calculate any new riser margin or trip margin that might have to be added to the mud weight.

Figure 5.28 TRAPPED GAS IN BOP STACK

Figure 5.28 TRAPPED GAS IN BOP STACK

Kill And Choke Lines Pictures

SECTION 5 : METHODS OF WELL CONTROL

Figure 5.29 REMOVING TRAPPED GAS FROM BOP STACK

KILL LINE

CHOKE LINE

KILL LINE

Kill Choke Line Bop

CHOKE LINE

KILL LINE

CHOKE LINE

Isolate the well from the BOP stack by closing the lower pipe rams.

KILL LINE

CHOKE LINE

KILL LINE

Choke Line

CHOKE LINE

KILL LINE

Trapped Gas

CHOKE LINE

Slowly displace kill line to salt water. As the kill line is displaced to water, increase the kill line circulating pressure by an amount equal to the difference in hydrostatic pressure between kill mud and salt water at stack depth. This will maintain the gas at original pressure with clean salt water returns at surface stop pumping close choke.

Displace riser to kill mud using upper kill line.

SECTION 5 : METHODS OF WELL CONTROL

Figure 5.30 REMOVING TRAPPED GAS FROM BOP STACK

Bop Stacks

At this point the pressure is still trapped in the gas bubble.

Bleed off pressure through the choke to allow the gas to displace water from the choke line.

The gas bubble should now be at close to atmospheric pressure.

Bop Kill Line

Open the pipe rams and allow the riser to U-tube taking returns up the choke line.

Fill the riser as necessary.

Open the lower pipe rams and diverter element.

Flow check the well.

SECTION 5 : METHODS OF WELL CONTROL

Bubble Expansion Example:-

Choke Line Length =

Choke Line Volume =

Kill Mud =

Sea Water Grad =

Pi Vi P2

Example:

Pi Vi

1000' 8 bbls

15 PPg 0.445

780 X 3

15 x 1000 x 0.052 3 bbls

  1. 445 x 1000
  2. 3 bbls = 780 psi

445 psi

This example gives some idea of the large volumes of gas that could be released to atmosphere if the annular is opened without sweeping the stack.

Lets say that we are drilling in 1800' of water and the well has been killed to surface, via the choke line with 16.5 ppg mud.

The hydrostatic head compressing the gas under the bag would be

If the volume of gas trapped below the BOP = 5.46 bbls then:

= 579 bbls

P2 i4.7

573 bbls of gas released at surface.

SECTION 5 : METHODS OF WELL CONTROL

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Responses

  • j gottlieb
    How do you calculate riser margin after the well is killed?
    9 years ago
  • vincent sato
    What are choke and kill lines?
    9 years ago
  • adonay
    How to remver gas from under rams?
    9 years ago
  • Karri
    Why pump kill weight mud down the kill line?
    8 years ago
  • eduardo
    Can trapped gas choke?
    7 years ago
  • Belba
    How do you get choke line friction on bops?
    7 years ago
  • amanda proudfoot
    How to calculate new riser margin after the well has been displaced to kill mud?
    5 years ago
  • jere
    How to calculate gas volume trap below BOP?
    4 years ago

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