Snub Drilling
Snubbing Units
Snubbing is the process of tripping jointed pipe or other downhole tools in a well that has enough back-pressure to eject a drillpipe without any restraining force. Most of the operations that are typically carried out by standard drilling and workover rigs can also be accomplished by snubbing units.
Snubbing is an economical alternative to conventional rigs or coiled tubing units in various drilling, completion and workover operations. Its benefits include reduced space requirements during transport and operation, reduced trip times and improved safety. The use of snubbing technique also avoids the potential of permanent well damage caused by the application of kill fluids during well intervention.
Snubbing units have been used in the petroleum industry since the 1950’s. The technique was originally developed as a blow-out control method. Therefore, the term snubbing has historically been associated with problematic wells and situations, and considered as a “last-resort” option. Nowadays, however, snubbing units are employed in a variety of other applications including:
- Underbalanced drilling (UBD)
- Well completions
- Workover and re-completions
- Fishing and other remedial operations
- Well stimulation
A snubbing unit (Figure 1) is composed of hydraulic rams and opposing pipe slips that can hold the pipe and either pull or push them in or out of the well under live well conditions.
For UBD operations, the snubbing unit is mounted above the rotating control and BOP assembly in order to safely handle the drillpipe as it is tripped back to surface or lowered back in the well.
Snubbing units provide an ideal well intervention solution for environments where available surface space is limited. Since they are mounted on skid beams, they can easily be moved from one location to another without time consuming rig-up and rig-down operations.
A typical snubbing consists of three major sections:
- Base unit (jack)
- Well control components
- Auxiliary equipment
The base unit (jack) is the main mechanical/hydraulic part, which generates and transfers various forces to the work-string. Figure 1 shows a typical jack frame assembly and its critical components. The two sets of slips mounted on the jack frame are mainly responsible for “jacking” the tubing in and out of the wellbore. An outer tube system is also used to guide and direct the tubing into the unit in order to avoid excessive buckling. The work window area below the jack assembly provides enough working clearance to add or remove larger diameter work-string components such as packers, plugs or hangers.
For well control, additional components are added to a conventional BOP that would allow operation under live well conditions. A typical BOP stack (Figure 2)
underneath a snubbing unit used in underbalanced drilling operations has three levels of pressure control:
- Primary control system: stripper bowl, annular BOP and stripper rams.
- Secondary control system: slip rams, wireline rams and kill & choke connections to be used if primary control system fails or is under maintenance.
- Tertiary control system: single blind shear to be used if primary and secondary control systems fail or are under maintenance.
The main control panel of the primary BOP system is located in the workbasket section of basic snubbing unit. The secondary and tertiary system control panels are designed as dedicated devices and normally located along an escape route behind a firewall.
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