The lubricate and bleed method involves alternately pumping a kill fluid into the tubing or into the casing if there is no tubing in the well, allowing the kill fluid to fall, then bleeding off a volume of gas until kill fluid reaches the choke. As each volume of kill fluid is pumped into the tubing, the SITP should decrease by a calculated value until the well is eventually killed.
This method is often used for two reasons: 1) shut-in pressures approach the rated working pressure of the wellhead or tubing and dynamic pumping pressure may exceed the limits, as in the case of bullheading, and 2) either to completely kill the well or lower the SITP to a value where other kill methods can be safely employed without exceeding rated limits.
This method can also be applied when the wellbore or perforations are lugged, rendering bullheading useless. In this case, the well can be killed without necessitating the use of tubing or snubbing small diameter tubing.
Users should be aware that the lubricate and bleed method is often a very time consuming process, whereas another method may kill the well more quickly. The following is an example of a typical lubricate and bleed kill procedure.
Example: A workover is planned for a well where the SITP approaches the working pressure of the wellhead equipment. To minimise the possibility of equipment failure, the lubricate and bleed method will be used to reduce the SITP to a level at which bullheading can be safely conducted. The data below will be used to describe this procedure:
TVD = 6500 ft Depth of perforations = 6450 ft
Kill fluid density = 9.0 ppg Wellhead working pressure = 3000 psi
Tubing internal yield = 10,570 psi Tubing capacity = 0.00579 bbl/ft (172.76 ft/bbl)
Calculations: Calculate the expected pressure reduction for each barrel of kill fluid pumped:
psi/bbl = tubing capacity, ft/bbl x 0.052 x kill weight fluid, ppg psi/bbl = 172.76 ft/bbl x 0.052 x 9.0 ppg psi/bbl = 80.85
For each one barrel pumped, the SITP will be reduced by 80.85 psi. Calculate tubing capacity, bbl, to the perforations:
bbl = tubing capacity, bbl/ft x depth to perforations, ft bbl = 0.00579 bbl/ft x 6450 ft bbl = 37.3 bbl
A certain amount of time is required for the kill fluid to fall down the tubing after the pumping stops. The actual waiting time is not to allow fluid to fall, but rather, for gas to migrate up through the kill fluid. Gas migrates at rates of 1000 to 2000 ft/hr. Therefore considerable time is required for fluid to fall or migrate to 6500 ft. Therefore, after pumping, it is important to wait several minutes before bleeding gas to prevent bleeding off kill fluid through the choke.
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