See Figure 1 to Figure 7 1.2.1 General
Stripping is an emergency well control procedure. It requires good planning, proper training of personnel and careful execution.
The primary objective of the stripping operation shall be to maintain a constant bottom hole pressure, thus preventing a build up of excessive borehole pressures or influx from exposed permeable zones.
The following are guidelines for carrying out a successful stripping operation:
- Pressure control is based on a volume balance. This means that for every barrel of pipe stripped in the hole, a barrel of mud must be bled off. Since it is necessary to install an inside BOP before stripping, total displacement must be considered, including both pipe displacement and internal capacity.
- Mud bled from the annulus must be accurately measured in order to maintain the correct volume balance. The mud should be bled through a manually operated choke since a hydraulically operated choke has too much delay in operation. This delay makes it impossible to bleed accurate volumes and thereby maintain proper well pressure.
- Annulus pressure should not be constant while stripping pipe into the hole. It should gradually increase as the pipe is stripped into the lower density kick fluid. This is due to the increased length or height of the influx fluid in the annulus and the resultant loss of hydrostatic pressure.
- When stripping with the annular preventer, the closing pressure on this preventer must be adjusted to allow a small amount of leakage to lubricate and reduce wear on the sealing element. The mud which is allowed to leak past the annular preventer should be measured along with the mud bled through the adjustable choke.
- Drill pipe with rubbers should never be stripped through the annular preventer, because excess friction and wear would be generated due to the rubber to rubber contact.
- If stripping is to be carried out with two sets of pipe rams, then a side outlet is required between the rams. This is necessary to enable the pressure to be equalised, before opening the rams. Opening rams without equalising the pressure, will shorten the life of the sealing element. Pressure from the well must not be used to equalise the pressure across the rams.
Figure 1 shows a typical stripping rig-up. The rig-up can vary depending on the particular choke manifold and BOP stack on the well, at the time stripping is required.
- Figure 1 - Typical Stripping Rig Up
The objective in all stripping operations is the same as the objective when killing a well, i.e. to maintain a constant bottomhole pressure, slightly greater than the formation pressure, throughout the entire operation.
Borehole pressure relationships, during the stripping operation, are explained in Figure 2 to Figure 7. These show the changes in well conditions which occur during a typical stripping operation.
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