Snub Drilling

Snub Drilling Overview If very high formation pressures and uncontrollable loss of circulation are expected during underbalanced drilling, use of a snubbing or a coiled tubing (CT) unit may be necessary. This type of underbalanced drilling is known as snub drilling. Snub drilling should be considered for wells with: Very high annular surface pressureVertical fracturesSevere loss of circulationSurface equipment limitationsPersonnel and rig safety considerations Communication between rig-site personnel may become a problem when drilling with extreme surface pressures in snub drilling. Hearing protection is necessary for everyone on the rig floor and close to the choke manifolds. Coiled tubing equipment isolates the wellbore during the entire drilling operation and avoids killing wells before tripping. Coiled tubing, however, is also limited by higher cost compared to conventional rigs, and cannot run jointed pipe for casing and tubing completion operations. Snubbing offers a versatile and safe solution in underbalanced drilling operations. The advantages of using snubbing units are as follows: Pressure control is handled with the snubbing unit’s BOPs, with its annular preventer as a primary system and its rams as secondary preventers.In the event of "pipe light" conditions, the snubbing unit can provide the necessary control to inject or pull pipe and tools under pressure. Recommended for You Dry Air 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 completionsWorkover and re-completionsFishing and other remedial operationsWell 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. FIGURE 1 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 componentsAuxiliary 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) 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. Recommended for You Foam Drilling | What is Foam Drilling? Assessment 1. In snub drilling, hearing protection is necessary for everyone on the rig floor and close to the choke manifolds. A .True ✔B .False 2. Snubbing was originally developed as A .a stimulation technique.B .a cementing method.C .a blow-out control method. ✔ 3. What is the major disadvantage of using coiled tubing units for snub drilling? A .High costB .Cannot run jointed pipe for casing falseC .Hole size limitationsD .A, B and C ✔ 4. A snubbing unit 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 only under static well conditions. A .TrueB .False ✔ 5. What is the major benefit of using snubbing units as opposed to standard drilling and workover rigs? A .Reduced space requirementsB .Ease of transport falseC .Reduced trip timesD .Improved safetyE .A, B, C and D ✔ 6. Which segment of a typical BOP stack has the stripper bowl and stripper rams? A .Primary control system ✔B .Secondary control systemC .Tertiary control system Recommended for You Dry Air Drilling Nitrogen Drilling Natural Gas Drilling Mist Drilling Foam Drilling | What is Foam Drilling? Gasified Liquids Flow Drilling