Fluid Pound: What Causes It and How to Stop It
What is fluid pound?
Fluid pound is the shock that happens when a rod pump's plunger, falling on the downstroke, slams into liquid partway down the barrel instead of moving through a full barrel. It happens when the pump displaces more fluid than the formation delivers, so the barrel only partly fills on each upstroke. 12
On a healthy stroke the barrel fills with liquid on the upstroke. On the downstroke the traveling valve opens almost at once and the plunger moves down through liquid, so load comes off the rods smoothly. On a pounding stroke the top of the barrel holds low-pressure gas instead. The traveling valve stays shut, the rods carry the full fluid load while the plunger falls through gas, and then the plunger hits the liquid. The traveling valve opens and the load drops off the rods all at once. The SLB Energy Glossary describes rod-string load falling by thousands of pounds in a fraction of a second. 1
That repeated shock is why fluid pound is one of the conditions operators work hardest to avoid on rod-pumped wells, from Permian and Midcontinent strippers to heavy-oil wells in Alberta and Oman.
What causes fluid pound?
The main cause is pump-off: the pump's displacement is larger than the well's inflow, so the fluid level falls to the pump intake and the barrel can't fill. A secondary cause, low-pressure gas trapped between the valves, is still debated in the technical literature. 123
Pump-off
Every rod-pumped well has an inflow rate set by the reservoir. Once the fluid level over the pump is drawn down, the pump can only move what flows in. If the unit keeps stroking at a rate built for more fluid, the barrel fills only partly on each upstroke. An older patent on pump-off detection puts it plainly: fluid pound occurs when the well is pumped off. 2
Pump-off creeps in. Inflow declines as a field matures, while the surface unit, pump size and strokes per minute stay where they were set years ago. A well that was correctly sized at completion can be pounding today without anyone changing a setting.
Gas between the valves
The glossary also lists low-pressure gas collecting between the standing and traveling valves as a cause. 1 The traditional explanation is that the plunger hits the fluid level while the pump is partly full of gas.
A study presented at the Southwestern Petroleum Short Course modelled fluid pound and gas interference with a wave-equation program and challenged part of that picture. The authors concluded that gas in the pump is always compressed to a pressure high enough to open the traveling valve before the plunger reaches the liquid. They also noted that most downhole dynamometer cards don't show compression when fluid pound happens. 3
For the field engineer, the practical point is the same either way: a partly filled barrel on a pumped-off well is the condition to remove. Gas interference is a related but separate problem. See the gas-locked rod pump guide for how to tell them apart.
What fluid pound does to the well
Each pounding stroke sends a shock load through the pump, the rod string, the tubing and the surface unit. Over thousands of strokes a day, that shock shortens equipment life and causes failures that end in a workover. 12
The damage shows up in four places:
- The pump. The traveling assembly and valve seats take the impact directly, on every pounding stroke.
- The rod string. The sudden load change sends a stress wave up the rods. At the bottom of the string, the impact can push rods into compression, which adds to the downstroke loads that cause sucker rod buckling.
- The tubing. Rods that flex under the shock rub the tubing wall, and unanchored tubing moves with every impact.
- The surface unit. The FMC pump-off patent notes that the jar of the plunger striking fluid damages both the rod string and the surface equipment. 2 Gearboxes and structures see the same reversing load on every pounding stroke.
The SLB glossary sums it up: fluid pound causes extreme stresses that can lead to premature equipment failure. 1
How to recognize fluid pound
The dynamometer card is the standard way to diagnose fluid pound. A pounding well shows a delayed, sudden load drop on the downstroke. A fluid-level reading close to the pump intake confirms that the well is pumped off. 24
- Dynamometer card. On a full-pump card, load comes off the rods at the very start of the downstroke. On a pounding card, the rods carry full load for part of the downstroke, then the load drops sharply where the plunger hits liquid. Plotted as load against position, the bottom-right corner of the card is cut off. The deeper the cut, the emptier the barrel.
- Pump-off controllers watch the same signal. A classic pump-off controller measures rod load with a load cell and rod position with a position transducer. When the load curve on successive downstrokes crosses a set point, it shuts the well down. 2
- Fluid level. A fluid-level survey measures pump submergence, the height of liquid over the pump intake. 4 Little or no submergence together with a pounding card points to pump-off. A high fluid level with an incomplete card points to gas interference instead.
- Surface signs. Pounding can often be heard and felt at the wellhead: a knock in the rods or the unit on each downstroke. It's a late sign; the card shows the problem first.
- P1 Full pump
Load on at the bottom, off at the top: the reference rectangle.
- P2 Fluid pound
Full load held into the downstroke, then a sharp drop where the plunger hits liquid.
- P3 Gas interference
The bottom-right corner rounds off: gas compresses until the traveling valve opens.
How to stop fluid pound
Fluid pound stops when the pump's displacement matches the well's inflow. The options run from mechanical changes (slower, shorter strokes, a smaller pump) to control (timers, pump-off controllers, variable-speed drives) to closed-loop control on a measured fluid level. 125
| Fix | How it helps | Limits |
|---|---|---|
| Slow the unit | Fewer strokes per minute lower displacement to match inflow 1 | A fixed new speed is wrong again once inflow changes |
| Shorten the stroke | Less displacement per stroke 1 | Needs a field change at the unit; same drift problem |
| Smaller downhole pump | Less displacement per stroke 1 | Needs a pulling job; fixes today's inflow, not next year's |
| Timer (on/off cycles) | Lets the fluid level build up while the well is idle | Set by guesswork; wells still pound when the timer is wrong |
| Pump-off controller | Detects pump-off from the load curve and shuts the well down for an idle period 2 | The well has to pound before it stops, and extra idle time also cuts production 5 |
| Variable-speed drive | Slows the stroke instead of stopping the well | Only as good as the signal and setpoints driving it |
Why on/off control is a trade-off
Pump-off controllers were a big step forward over timers, but they react to pounding rather than prevent it. A Shell patent on controlling rod-pumped wells describes the trade-off: earlier controllers needed the well to actually pump off, which meant repeated fluid pound before the unit stopped. Lengthening idle time cuts the number of pounding strokes, but it also cuts the well's production. 5
Closing the loop on fluid level
The alternative is to hold the pump at the right submergence continuously instead of cycling it on and off. That takes two things: a fluid-level reading that is always current, and a controller that adjusts pump speed as inflow changes.
That is how the SRABS system approaches it. SRABS Echo is "an automated acoustic edge device" that reads fluid level, fully remote. SRABS Brain "reads every dynamometer card, motor current and Echo fluid level," and on autopilot wells "retunes the VSD before the failure happens." Put together, in the words of the solutions page: "Echo sees the gas and the fluid level, and Brain adjusts speed and idle time before the pump starts to pound." See the platform, the system, and the modelled fluid-pound well scenario.
To estimate what fewer failures would mean on your own fleet, run the ROI calculator. Its default 100-well scenario is modelled, not a field result.
The bottom line
Fluid pound is a mismatch between what the pump moves and what the well delivers, and every pounding stroke costs equipment life. Slowing the unit or downsizing the pump fixes the mismatch for today's inflow. Pump-off control limits the damage but trades it for idle time. Keeping the pump at the right submergence all the time removes the cause.
If you have wells pounding on a timer or cycling on a pump-off controller, start with the dyno cards and a fluid-level reading, then test closed-loop control on one well.
FAQ
What is fluid pound in a rod pump?
Fluid pound is the shock when the plunger, moving down through a partly filled barrel, hits the liquid. Load comes off the rods suddenly instead of smoothly, which stresses the pump, rods, tubing and surface unit.
What causes fluid pound?
The main cause is pump-off: the pump displaces more fluid than the formation delivers, so the barrel doesn't fill on the upstroke. Low-pressure gas between the valves is also cited as a cause, though some modelling work disputes that mechanism.
How do you know if a well is fluid pounding?
Look at the dynamometer card. A pounding well carries full load into the downstroke and then drops it sharply, cutting off the bottom-right corner of the card. A fluid level close to the pump intake confirms the well is pumped off.
Is fluid pound the same as gas interference?
No. Fluid pound comes from an empty barrel on a pumped-off well. Gas interference comes from free gas entering the pump, often with plenty of fluid over the intake. The cards look similar, so check the fluid level to tell them apart.
How does a pump-off controller stop fluid pound?
It measures rod load and position on each stroke. When the downstroke load curve crosses a set point that indicates pump-off, it shuts the unit down for an idle period so the fluid level can recover.
Can you prevent fluid pound without shutting the well in?
Yes. Matching pump speed to inflow, with a variable-speed drive controlled from a continuous fluid-level reading, keeps the barrel full without on/off cycles.
Sources
- SLB Energy Glossary, "fluid pound." https://glossary.slb.com/en/terms/f/fluid_pound
- US Patent 4,487,061, "Method and apparatus for detecting well pump-off" (FMC Corporation, 1984). https://patents.google.com/patent/US4487061
- Yavuz, F., Lea, J., Adisoemarta, P., Oetama, T., Cox, J. C., "Effects of Fluid Pound on Beam System," Southwestern Petroleum Short Course. https://www.swpshortcourse.org/paper/f-yavuz-j-lea-p-adisoemarta-t-oetama-and-jc-cox-texas-tech-university
- Acoustic Fluid Level Measurements, Chapter 8, "Applications of Fluid Level Measurements to Pumping Wells," Petroleum Extension Service (PETEX). https://utw10286.utweb.utexas.edu/images/flippingbooks/acoustic_fluid_measure/files/basic-html/page15.html
- US Patent 5,314,016, "Method for controlling rod-pumped wells" (Shell Oil Company, 1994). https://patents.google.com/patent/US5314016