Stem

Key Takeaways

• Stem weight calculation for a given slickline operation requires knowing the wellhead pressure, the OD of the tool string at its largest cross-section (typically the largest tool in the string, often the jar or the gauge), and the stuffing box friction force (measured during the rig-up by observing the tension required to move the wire through the stuffing box at the wellhead), with the minimum stem weight calculated as (wellhead pressure times tool OD cross-section area) plus stuffing box friction minus the weight of the wire in the fluid column below the stem: a typical calculation for a well with 1,500 psi wellhead pressure and a 1.5-inch OD tool string gives an upward hydraulic force of 1,500 times (pi times 0.75 squared) = approximately 2,650 pounds of upward force on the tool; if the tool string itself weighs 150 pounds in the fluid, the stem must supply at least 2,500 additional pounds of downward force to ensure the tool descends; in practice, a safety margin of 20 to 50 percent is added to the minimum calculated stem weight to ensure reliable descent against the maximum expected wellhead pressure (which may spike during gas-producing wells as the pressure cycles with the production flow) and against stuffing box friction that increases with seal wear.
• High-density stems using lead, tungsten, or mercury alloy-filled interiors provide the maximum weight per unit length possible within a constrained OD, which is critical in wells with small-diameter production tubing (where the available stem OD may be limited to 1.25 inches or less) and high wellhead pressure that requires more weight than a solid steel stem of the same diameter can provide: tungsten-filled stems can achieve densities of 11 to 15 grams per cubic centimeter versus 7.85 for solid steel, providing 40 to 90 percent more weight per foot in the same diameter envelope; mercury-filled stems (historically common but now restricted in many jurisdictions due to mercury hazard concerns) achieve similar or higher densities; lead-filled stems offer densities of 9 to 10 grams per cubic centimeter and are still widely used where mercury has been phased out; the selection between standard and high-density stems is driven by the ratio of wellhead pressure to the available stem diameter that can be run through the wellhead and production tubing restrictions, with high-density stems reserved for the highest-pressure, smallest-tubing applications where standard steel stems cannot provide adequate weight.
• Stem design and connection integrity are critical safety considerations because stems that disconnect from the tool string during slickline operations leave steel weight bars as junk in the wellbore that must be fished out before normal operations can resume: the standard pin-and-box threaded connection between stems and between the bottom stem and the top of the tool string uses right-hand threads that tighten under the clockwise torque applied to the slickline by the rope socket and toolstring as the wire twists during running and pulling, preventing accidental unthreading from the normal operational torque; reverse-torque connections (left-hand threads or snap-lock designs) are used on the bottom of the stem string (where right-hand torque could unthread the connection during specific operating conditions) and in wells where the wellbore deviation or fluid drag causes the tool string to rotate in a direction that would unthread standard right-hand threads; the stem OD must provide clearance to the production tubing ID not only for descent but also for retrieval past scale deposits, wax accumulations, and tubing deformations that may have reduced the tubing ID below its nominal value since the well was completed.
• Fishing for lost or stuck stems represents one of the most common wireline remedial operations because the same wellhead pressure that the stems are intended to overcome can also cause them to be ejected from the wellbore if the wire parts while the tool string is above the wellhead valve and the valve is opened before the stem string has been safely secured: if the wire parts with the stems and tool string inside the tubing below the wellhead valve, the stems and tools must be recovered using conventional fishing techniques (wireline overshot, magnetic jar, or coiled tubing fishing tools) before normal slickline operations can resume; the cost of fishing a typical four-stem string plus the tool (including the downtime for the fishing operation, typically one to three days of unit time) can easily exceed USD 50,000 to 100,000 in offshore environments, making wire condition monitoring (checking wire for corrosion pits, fatigue breaks, and kinks before each run) and proper stem connection inspection the highest-return preventive maintenance activities in slickline operations; stem diameter measurement at each connection thread before assembly confirms that the threads have not been damaged by previous make-up torque beyond the point where they will securely hold the stem string under the upward hydraulic force of the wellhead pressure.
• Coiled tubing stem applications differ from slickline in that coiled tubing uses a different mechanism to overcome wellhead pressure: the CT itself is large enough in diameter that the wellhead pressure acts on the CT cross-section rather than on a small-diameter wire, and the CT injector provides the mechanical push needed to advance the CT into the well against the wellhead pressure, eliminating the need for a separate stem weight; however, in CT-conveyed wireline or CT-conveyed logging operations where a wireline cable is run inside the CT to convey tools that must extend beyond the CT end, a stem assembly similar to slickline stems may be attached to the tool string below the CT end to add weight for descent in highly deviated sections where the tool cannot descend by gravity alone; the CT sinker bar (which plays the same role as the slickline stem in providing additional downward weight to advance a tool past a deviated or horizontal section) is sized to the same hydraulic force balance as the slickline stem but does not need to overcome wellhead pressure directly, since that is handled by the CT injector.