Swab

Key Takeaways

• Swab cup design and selection governs the effectiveness of the swab operation: standard rubber swab cups (made from natural rubber, neoprene, nitrile, or Viton depending on the fluid and temperature compatibility requirements) must expand against the tubing inner diameter to form a seal while still sliding downward freely when the swab is run in the hole under its own weight; the cup OD in the relaxed position is slightly smaller than the tubing ID to allow free running in, and the cup expands to fill the tubing when the swab is pulled upward against the fluid load; the differential pressure rating of the cup (the maximum pressure difference across the seal before cup extrusion or bypass occurs) determines the maximum column height of fluid that can be lifted per swab run, typically 500-2,000 feet of fluid column depending on cup design and fluid density; swab cups deteriorate through abrasion (against tubing imperfections, scale deposits, and sand), chemical attack (from H2S, CO2, acids, or solvents in the well fluid), and mechanical fatigue (from repeated compression and expansion cycles), and worn or damaged cups result in fluid bypass around the seal that reduces or eliminates the lifting efficiency; cup inspection before each swab run and replacement at defined intervals is essential for efficient swabbing operations; specialty swab designs include spear-type swabs (for use in tubing with paraffin or scale deposits that would prevent standard cup expansion), fluted swabs (for use in perforated tubing), and tandem swab assemblies (multiple cups in series to increase the pressure differential capacity per run).
• Swabbing to unload a well after hydraulic fracturing is a common post-stimulation procedure in both conventional and unconventional wells when the formation pressure is insufficient to flow back the fracturing fluid to surface unaided: during hydraulic fracturing, 10,000-100,000 barrels of slickwater or gel fracturing fluid at surface conditions are pumped into the formation, of which 20-40% typically returns to surface as flowback during the subsequent production period; when formation pressure is below the hydrostatic pressure of the fluid column in the tubing (which occurs in depleted reservoirs, low-pressure formations, or wells where the initial completion fluid was heavier than the formation fluid), swabbing is used to initiate flowback by pulling the fluid level down in the tubing until the hydrostatic pressure of the remaining column is less than the formation pressure, at which point the well begins flowing naturally; the swabbing procedure must account for the well control risk of initiating natural flow before the surface wellhead and flowback equipment are rigged up and ready to receive the produced fluid, which requires a field safety plan that specifies the maximum acceptable fluid level reduction per swab run and the shut-in procedure if the well begins flowing before flowback equipment is connected.
• Swabbing of liquid-loaded gas wells removes the water column that accumulates when the producing gas velocity falls below the Turner critical velocity (the minimum gas velocity required to continuously lift water droplets to surface): in a water-producing gas well (from a water-bearing zone, from formation water condensation, or from produced water intrusion), the gas velocity required to lift water droplets ranges from 10-25 feet per second depending on tubing size, gas density, water density, and surface tension; when the reservoir pressure declines to the point that gas velocity falls below the Turner critical velocity, water begins to accumulate in the tubing, increasing the backpressure on the formation, further reducing the gas velocity in a positive feedback cycle that eventually causes the well to "load up" and kill itself with the accumulated water column; periodic swabbing removes the accumulated water column and temporarily restores the gas velocity above the Turner threshold, allowing the well to produce again until the next loading event; swab-assisted production is an interim measure used between the failure of the well to lift water naturally and the installation of a permanent artificial lift system (plunger lift, gas lift, or an electric submersible pump), or in wells where the low remaining gas production volumes do not economically justify the capital cost of a permanent lift system but swab visits every few weeks can extend the well's productive life at acceptable operating cost.
• Wireline equipment for swabbing operations consists of a swab unit (a truck or skid-mounted wireline spool with sufficient line capacity and tensile strength for the target well depth), a swab valve (a high-pressure wellhead valve with a stuffing box that seals around the wireline as it exits the wellhead, preventing flow from the well while running the swab in and out of the tubing), and the swab assembly (the mechanical swab cups, jar assembly, and sinker bars required to make the swab run under controlled conditions); the sinker bars (heavy weights above the swab) provide the downward force needed to push the swab through the fluid column against the upward drag of the viscous and dense kill fluid; the jar assembly allows the swab to be mechanically freed if it becomes stuck in the tubing by applying an upward impact load (the jar fires when a predetermined tension is reached, transmitting a high-energy impact to the swab that dislodges it from obstructions such as scale deposits or collapsed tubing); well control during swabbing requires that the swab valve be closed whenever the wireline is not moving and whenever the swab is being rigged up or laid down, and that the annular space (if the well has tubing in the hole) is monitored for pressure buildup that would indicate the well is flowing; swabbing operations in H2S-containing wells require H2S safety precautions (personnel H2S monitors, breathing apparatus, muster points, and emergency response procedures) because the produced well fluid during swabbing may contain toxic concentrations of H2S gas.
• Economic analysis of swab versus alternative artificial lift methods determines whether swabbing is the appropriate long-term production strategy or only an interim measure before a more efficient lift system is installed: swabbing has relatively low capital cost (the swab unit is typically rented, not purchased), no downhole equipment other than the swab cups (which are low-cost consumables), and flexible scheduling (swab runs can be conducted as frequently or infrequently as the production rate justifies); however, swabbing provides discontinuous production (the well produces only when the swab is in the well and for a period afterward before loading up again), requires a skilled wireline crew and truck at the wellsite for each swab run (operating cost per visit typically $2,000-10,000 for a standard gas well swab), and cannot maintain continuous production in wells that load up within hours or days of a swab run; plunger lift (an automatic swab that cycles without wireline intervention using the well's own gas pressure to lift a steel plunger through the liquid column) is the preferred alternative to periodic wireline swabbing in gas wells with sufficient gas-liquid ratio, because it provides continuous automated liquid unloading at much lower operating cost per mcf of gas produced than wireline swabbing requires.