Snubbing Jack

Key Takeaways

• The snubbing force required to push a pipe string into a pressurized wellbore is the hydraulic buoyancy force acting on the pipe cross-sectional area: snubbing force (lbf) = wellbore pressure (psi) x pipe cross-sectional area (in2), where the pipe cross-sectional area is the full OD area of the pipe (not the annular area) because the wellbore fluid pressure acts on the entire bottom face of the pipe string below the lower pipe rams of the BOP; for 2-3/8-inch drillpipe (OD 2.375 inches) at 5,000 psi wellbore pressure, the snubbing force is 5,000 x pi/4 x (2.375)^2 = 22,200 lbf (about 10 metric tons) of upward force that the snubbing jacks must overcome to push the pipe into the hole; as the pipe string is run deeper and its weight increases, the string eventually becomes "overbalanced" (the string weight exceeds the hydraulic upward force), at which point the string wants to go into the hole under its own weight and the snubbing jacks transition from pushing the pipe in to controlling its descent rate (holding back the string on the pulling slips rather than pushing on the snubbing slips); the critical neutral point (the depth at which the string weight equals the hydraulic upward force) is calculated before the job to determine the string length at which the transition from snubbing (pushing) to pulling mode (holding) occurs.
• The snubbing jack assembly includes two sets of hydraulically actuated pipe slip assemblies mounted on the hydraulic cylinder: the snubbing slips (which grip the pipe to push it into the hole by grabbing on the downstroke of the cylinder) and the holding slips (which grip the pipe during the return stroke of the cylinder to hold the pipe in place while the snubbing slips release and re-engage above the current pipe position for the next stroke); the two slip sets work in alternating sequence like a hydraulic rope-climbing mechanism, with one set always gripping the pipe so that the string never falls freely; the slip dies (the serrated gripping inserts in the slip bodies) must provide adequate gripping force to transfer the full axial load without damaging the pipe surface (which would create stress concentration sites for fatigue or corrosion cracking), while being designed for the specific pipe OD being run; the slip design for pipe running in compression (snubbing into the hole against pressure) differs from slip design for pipe running in tension (pulling out of the hole under weight) because the axial force direction reverses and the pipe must not be deformed by the compressive side load from the slip wedge action.
• Live well intervention using a snubbing unit allows operations (drilling, milling, fishing, logging, perforating, plug setting) to be performed in a live wellbore without killing the well, reducing or eliminating the formation damage associated with kill fluid invasion, avoiding the environmental and safety risks of flaring or venting gas killed from the well, and in some cases significantly reducing the total intervention time and cost compared to a kill-and-workover sequence; snubbing units are used for tubing-conveyed perforating (TCP) operations in high-pressure wells where underbalanced perforating in a live well is the preferred completion technique, for pipe fishing and milling operations in wells that cannot be safely killed (because the kill fluid would damage a highly permeable formation or because the formation pressure is too uncertain to safely maintain kill weight), for re-completion operations (adding or changing perforations, installing new packers, running new completion string) in producing gas wells where killing would defer months of production, and for emergency operations (fishing for the BOP rams, removing stuck downhole tools) in wells where the risk of a blowout during the intervention must be actively managed.
• Dual-ram BOP stack management is critical during snubbing operations: the lower pipe rams of the BOP grip the pipe and provide the primary wellbore seal during each slip change (when neither the snubbing nor holding slips are gripping the pipe), the upper pipe rams provide a secondary seal in case the lower rams fail during a slip change, and the blind-shear rams (capable of cutting through the pipe and sealing the wellbore in an emergency) provide the last-resort well control capability; during snubbing operations, the BOP must be cycled (opening one set of rams while the slips grip the pipe, running the pipe through the open rams, then closing the rams before releasing the slips) in a carefully choreographed sequence that ensures the wellbore is always sealed either by the rams or by the slips, with no gap in pressure integrity during any step of the sequence; the BOP hydraulic control system must be capable of closing the rams faster than the pipe can move upward under maximum wellbore pressure (to prevent a runaway pipe from pushing through the open BOP), and the BOP must be pressure-tested to its rated pressure before beginning snubbing operations.
• Coiled tubing versus jointed pipe snubbing units differ in their slip and BOP requirements: coiled tubing (CT) snubbing units use a continuous injector head (a chain-and-gripper mechanism that continuously grips and drives the coiled tubing by chain traction rather than alternating slip grabs), which provides smoother, continuous pipe movement into and out of the hole without the stop-start slip change cycle of jointed pipe snubbing; CT snubbing eliminates the need for BOP ram cycling during pipe advancement (because the CT passes continuously through the BOP stuffing box and injector head rather than being run joint-by-joint), reducing the well control risk during each pipe advancement step; however, CT snubbing is limited by the maximum surface pressure rating of the injector head and BOP stripper system (typically 5,000 to 10,000 psi), the coil fatigue life (CT undergoes cyclic bending through the injector head rollers that limits the total job length before the coil must be inspected for fatigue cracks), and the inability to rotate the CT (rotation requires jointed pipe or a downhole motor), making jointed pipe snubbing preferable for operations requiring rotation such as drilling, milling, or rotary fishing.