Tubing Anchors: Why Rod-Pumped Tubing Is Anchored and How to Set One
Short answer: a tubing anchor grips the casing at the bottom of the tubing string so the tubing can be landed in tension. Without one, the fluid load shifts from the tubing to the rods and back every stroke, so the bottom of the tubing moves up and down ("breathing") and buckles around the rods on the upstroke. That costs pump stroke and wears rods, tubing and casing. An anchor-catcher also catches the tubing if it parts. 12
What does a tubing anchor do?
A tubing anchor holds the bottom of the tubing to the casing so the tubing can be pulled into tension and kept there. Tension keeps the tubing straight and stops it changing length during the stroke, so the pump barrel stays put while the plunger moves, and the lower tubing doesn't coil around the rods. 12
The anchor sits at the bottom of the tubing string. Slips on the tool bite into the casing wall; with the tubing then stretched from surface, the string hangs straight between the wellhead and the anchor. 2 The anchor-catcher, used in most rod-pumping wells where tubing tension is needed, has a second job: if the tubing parts, it keeps the string from falling to bottom and saves a fishing job. 124
Two things are lost when the tubing hangs free: stroke and run life. The sections below take them in turn. See tubing anchor in the glossary.
Why unanchored tubing moves: breathing and buckling
The fluid load changes hands every stroke. On the downstroke the standing valve holds it and the tubing stretches; on the upstroke the rods pick it up and the tubing shortens. That cycle, called breathing, moves the pump barrel with the plunger. On the upstroke the lower tubing also buckles helically around the rods. 12
- Breathing. On the downstroke the tubing carries the fluid load and elongates while the rods shorten. On the upstroke the rods carry it, the rods elongate and the tubing shortens. 2 Once the well reaches pumping equilibrium, the bottom of free tubing goes to the same point on every upstroke and back to the same point on every downstroke. 1
- Buckling. Internal pressure tends to buckle the tubing. On the downstroke that tendency is balanced by the fluid load acting down on the closed standing valve. On the upstroke part of that load moves to the rods, and the lower tubing coils helically around them even though it may still be in tension. It straightens again as the rods start down. 1 See helical buckling.
The rods then finish every upstroke through buckled tubing. 1 This is tubing buckling. Buckling of the rod string, which happens on the downstroke, is a separate problem with a separate fix; see sucker rod buckling.
What unanchored tubing costs
Free tubing costs production, because part of every stroke moves the barrel instead of displacing fluid. It costs equipment too: buckled tubing wears the rods, the tubing and the pump, couplings rub the casing, and the cycling load fatigues the tubing until it parts. More friction also means more power. 12
- Lost stroke. In one worked example from the literature, a 6,000-ft well pumping at 12 strokes per minute through a 2-in. pump loses 8.04 in. of stroke to cyclical tubing movement, or 44.96 bbl/d of pump displacement. 1
- Wear. Rods rub the inside of the coiled tubing, the couplings wear against the casing, and the barrel can cock against the plunger every stroke. Corrosive fluids and sand make all of it worse. 1 See rod-on-tubing wear.
- Tubing parts. Free tubing can part near bottom where it wears, and also higher up, in the straight section, from fatigue as the load cycles on and off. 1
- Power. The added friction takes more lifting power. 2
Published field results from 1964 point the same way: pump efficiency up 20% in one 4,600-ft West Texas well after an anchor-catcher was run, and a Mississippi well that had needed a workover about every 90 days for coupling wear ran more than nine months without one. 1
Tubing anchor types
Anchors differ in which direction they hold the tubing. Tension anchors and anchor-catchers hold it in tension and stop breathing and buckling; compensating and hydraulic anchors set themselves as the tubing stretches; compression anchors stop the movement but leave the tubing buckled, so they are rarely the right choice. 13
| Type | How it holds | Stroke loss | Tubing buckling | Notes |
|---|---|---|---|---|
| Tension anchor | Set by rotation; the tubing can't move up | Stopped | Stopped, if landed in enough tension | Needs a stretch calculation 1 |
| Compensating anchor | Walks down and resets until the tubing is fully stretched | Stopped | Stopped | No pre-stretch or calculation needed 1 |
| Hydraulic anchor | Set by the same hydrostatic pressure that stretches the tubing | Stopped | Lower tubing still buckles 1 | Sets at the deepest point of tubing elongation 3 |
| Compression anchor | Set by rotation; the tubing can't move down | Stopped | Made worse, on both strokes | Catches parted tubing, but wear is usually serious 1 |
| Anchor-catcher | Holds the tubing in tension and stops it falling if it parts | Stopped | Stopped | Recommended where corrosion parts tubing 14 |
Where only production matters, any of these recovers the lost stroke. Where wear is a problem, use one that holds the tubing in enough tension to stop buckling: a tension anchor, compensating anchor or anchor-catcher. If corrosion is also parting tubing, use the anchor-catcher. 1
Where to set a tubing anchor
Set the anchor immediately below the pump, with the seating nipple screwed into the anchor's top sub. Anything between the anchor and the pump is unanchored tubing that can still breathe. Don't set an anchor in bad casing. 23
- Below the pump. That is the standard position. If the pump has to go below the anchor, the pump must pass through the anchor's mandrel, so the mandrel bore and its tensile strength need checking. 2
- Above the pump. The tubing between the anchor and the seating nipple is free to stretch, and on a pump card it behaves like unanchored tubing over that length. 3 The tubing weight and fluid load hanging below the anchor must stay within the tool's rating. 2
- Casing condition. Anchors shouldn't go into bad casing, which causes trouble in wells that make sand or scale unless the casing is redressed. 2
- Deep wells. At 8,000 ft and deeper, two or more drag springs are used, one above the other. 2
How to set tubing anchor tension
Calculate the tension the tubing needs to stay straight once the well is pumped down, convert it to inches of stretch, and pull that stretch at surface. Always measure stretch, not pounds on the weight indicator, because tubing-to-casing friction and indicator error make the load reading unreliable. 23
Running and setting an anchor-catcher, in outline: 2
- Run in with a right-hand turn put into the tubing every 5 or 10 stands, so the slips aren't dulled on the way down.
- Set at depth by turning the tubing to the left, about 5 to 8 turns, until the slips contact the casing. Hold left-hand torque while pulling strain and setting down a few times to work the play out of the tool.
- Release the torque, then apply the calculated tension as inches of stretch.
- To release later, put the tubing in slight compression and turn it to the right, again about 5 to 8 turns at the anchor. If that fails, an overpull shears the emergency pins. 24
The tension has to cover every effect that would otherwise shorten or lengthen the tubing once the well is on production: the fluid load that moves onto the tubing (the piston effect), the heating of the tubing by produced fluid, ballooning from the pressure difference across the tubing wall, and buoyancy. 2 The fluid load and heating lengthen the tubing; the drop in annulus fluid level and casing pressure shorten it. 3 Because some of these change over a well's life, the calculation is run for several cases (at installation, pumped off) and the largest stretch is used. 2
The pull is converted with a stretch constant: stretch (in.) = pull (1,000 lb) × length (1,000 ft) × stretch constant. For 2 3/8-in., 4.6–4.7 lb/ft tubing the constant is 0.30675 in. per 1,000 lb per 1,000 ft. 2 One published example for that tubing, with pump and anchor at 6,000 ft, works out to about 20.6 in. of stretch. 2
The wellhead matters too. A screw-type tubing head needs the tubing overstretched 18 in. or more to get slips under the top collar; a slip-type head needs straight tubing so the slips bite evenly. 2 A compensating or hydraulic anchor avoids the calculation altogether. 1
How a pump card shows an unset tubing anchor
On the downhole pump card, unanchored tubing makes the sides lean: load is picked up and released over part of the stroke while the tubing stretches. A lean past the line for fully unanchored tubing points to an unset anchor; a smaller lean is often just pump slippage. 3
- U1 Full pump
Load on at the bottom, off at the top: the reference rectangle.
- U2 Unanchored tubing
Pickup and release spread over the stroke as the tubing stretches: the sides lean.
The method in outline: 3
- Plot two reference lines on the left side of the pump card: the tubing stretch line for the anchored case, and the line for completely unanchored tubing.
- Read the load pickup. With the tubing anchored, the pickup segment plots to the right of the anchored line but to the left of the unanchored line. If it plots to the right of the unanchored line, the tubing is likely unanchored, unset or slipping.
- Rule out slippage. A leaking pump, open clearances, a deep well with low intake pressure or slow pumping speed can all make a card lean like unanchored tubing. A travelling-valve load test identifies a badly leaking pump.
Unanchored tubing also shortens effective plunger travel, lowers peak polished rod load and raises minimum load. The narrower load range might look easier on the rods, but the tubing movement soon raises rod-on-tubing wear and with it rod and tubing failures. 3 The dynamometer card atlas shows the shape next to the others.
Where SRABS fits
A tubing anchor keeps the tubing straight. It does nothing for the rod string inside it, whose bottom goes into compression on the downstroke. As the how it works page puts it, "Rods are built to pull, not to push." SRABS is "a drop-in anti-buckling rod pump that keeps the string in tension where it would otherwise buckle," and on the platform, "Brain reads every dynamometer card, motor current and Echo fluid level, and checks them against the physics of your rod string." Anchored tubing and a string in tension work toward the same goal: a stroke that moves fluid, not steel. See the system.
To estimate what fewer failures and pulls would mean on your own fleet, run the ROI calculator. Its default 100-well scenario is modelled.
The bottom line
Anchor the tubing, land it in tension, and the pump strokes against a fixed barrel. Set an anchor-catcher below the pump, pull the calculated stretch in inches, recheck it when the well's conditions change, and use the pump card to catch an anchor that has let go.
If your rod strings keep failing after the tubing is anchored, look at the bottom of the rod string next, and test a pump that keeps it in tension on one well.
FAQ
What does a tubing anchor do?
It grips the casing at the bottom of the tubing so the tubing can be held in tension. That stops the tubing breathing with each stroke and buckling around the rods on the upstroke, which recovers lost pump stroke and cuts rod, tubing and casing wear.
Does a tubing anchor increase production?
Usually, yes. On free tubing, part of every stroke moves the pump barrel instead of displacing fluid. In one published example, a 6,000-ft well lost 8.04 in. of stroke, about 45 bbl/d of pump displacement, to tubing movement.
What is the difference between a tubing anchor and an anchor-catcher?
A tension anchor holds the tubing in tension. An anchor-catcher does that too, and also stops the tubing falling to bottom if it parts, which saves a fishing job. Anchor-catchers are recommended where corrosion parts tubing.
How much tension should a tubing anchor be set with?
Enough to keep the tubing straight once the well is pumped down, calculated from the fluid load, temperature, ballooning and buoyancy effects for that well. Pull it as inches of stretch, not pounds on the weight indicator.
Where should a tubing anchor be set?
Immediately below the pump, with the seating nipple screwed into the anchor's top sub. Tubing between an anchor set higher and the pump can still stretch and shows on the pump card as unanchored tubing.
How do you know if a tubing anchor is not set?
On the downhole pump card, the load pickup on the upstroke leans to the right past the line for fully unanchored tubing. Rule out pump slippage first, because a leaking or slow-running pump can make the card lean the same way.
Sources
- Logan, J. L., "Tubing Movement and Tubing Anchor Payout in Pumping Wells," Southwestern Petroleum Short Course, 1964. https://www.swpshortcourse.org/sites/default/files/papers/1964017.pdf
- Samayamantula, J. S., Roderick, R., "Tubing Anchor Catcher Applications and Operation," Southwestern Petroleum Short Course, 2013. https://www.swpshortcourse.org/paper/tubing-anchor-catcher-applications-and-operation
- Rowlan, O. L., Fernandez, G., Taylor, C. A., Wang, S., "How to Determine if Tubing Is Unanchored in Rod Pump Wells by Analyzing Dynamometer Data," Southwestern Petroleum Short Course, 2023. https://www.swpshortcourse.org/paper/2023010-how-determine-if-tubing-unanchored
- US Patent 3,077,933, "Tubing anchor and catcher apparatus" (H. L. Bigelow, 1963). https://patents.google.com/patent/US3077933A