Pump fillage is the percentage of a rod pump's barrel that actually fills with liquid on each stroke, and it is one of the most important numbers for running a rod-lift well well. A pump that fills completely is producing everything it can and treating its rods and pump gently; a pump that only partly fills is wasting strokes, risking damage, and telling you the well cannot keep up with the pumping speed. Because fillage reveals whether the well is being over-pumped, it is the primary signal a pump-off controller uses to decide when to slow or idle the well. This guide defines pump fillage, explains how a controller computes it from the downhole card, and shows why falling fillage is the cue to back off.
Pump Fillage (Rod Pump) in one line: Pump fillage is the fraction of the downhole pump barrel that fills with liquid during each stroke, expressed as a percentage of the full pump volume. A well that supplies the pump as fast as it is being pumped shows fillage near one hundred percent, while a pump being run faster than the reservoir can feed it fills only partway. A pump-off controller computes fillage from the shape of the downhole dynamometer card and uses falling fillage as the primary signal to slow the well down or idle it so the fluid level can recover.
On the upstroke, the traveling valve closes and the plunger lifts the fluid above it while the standing valve opens and lets new liquid enter the barrel from the well below. Pump fillage is simply how much of the barrel that incoming liquid actually fills before the stroke reverses. If the well supplies liquid fast enough, the barrel fills completely and fillage is essentially one hundred percent - every stroke lifts a full charge of liquid. If the well cannot supply liquid that fast, only part of the barrel fills, and the rest of the stroke volume is empty space or gas.
Fillage is the direct measure of how well the pump's speed is matched to the well's ability to feed it. Full fillage means the pump is producing its full potential for the stroke and speed it is running, and it means each stroke is being used efficiently. Low fillage means the pump is outrunning its supply, so strokes are being spent moving little or no liquid - lost production and wasted energy - and it warns that the conditions for fluid pound and gas interference are present, since a partly filled barrel is exactly what causes both.
Because fillage compares actual liquid delivered against the pump's full capacity, it is effectively a measure of pumping efficiency at the downhole pump. Real production from the well is the pump's theoretical displacement multiplied by the fillage, so a well running at fifty percent fillage is producing only about half of what its stroke length and speed could deliver. Bringing fillage back up - usually by slowing the pump to match inflow - is how that lost efficiency is recovered.
Fillage is not measured directly at the pump but is read from the downhole dynamometer card, the plot of load versus position at the pump that a controller computes from surface measurements. When the pump fills completely, the downhole card is a full, roughly rectangular shape: load is high across the whole upstroke because the plunger is lifting a full column of liquid, and it drops cleanly on the downstroke. That full shape corresponds to full fillage.
When the pump under-fills, the card changes in a characteristic way. Near the top of the stroke the plunger runs out of liquid to lift, so the high load ends early and the card's top-right corner collapses inward - the load drops off before the stroke completes because there was no more fluid to carry. A pump-off controller measures how far along the stroke that load drop-off occurs and translates it into a fillage percentage: the earlier in the stroke the load falls, the less of the barrel was filled and the lower the fillage. This card-based calculation is what lets the controller quantify fillage on every stroke without any sensor down in the pump.
Because the calculation comes straight from the card's shape, it depends on having a good downhole card, which the controller derives by processing the surface load and position through the rod string. A clean, well-computed card gives a reliable fillage number; a noisy or poorly computed one makes fillage harder to trust. This is one reason accurate surface measurement and a sound downhole-card computation matter so much - they are the foundation of the fillage number that drives the whole control decision.
Falling fillage is the single clearest sign that a well is being pumped faster than it can be supplied, which makes it the primary trigger for a pump-off controller. As the pump draws the fluid level down, fillage drops; when it falls below a set threshold, the controller acts to protect the well. On a timer-style or on/off controller, that means shutting the unit down for a rest so the fluid level recovers before restarting. On a variable-speed setup, it means slowing the pump so its speed drops to match the well's inflow and fillage climbs back toward full.
Reacting to fillage protects the equipment as much as it optimizes production. A pump running at low fillage is a pump that is pounding fluid or drawing gas on nearly every stroke, and both of those punish the rods, tubing, and pump. By slowing or idling the well the moment fillage drops, the controller stops those damaging strokes rather than letting them accumulate wear. It also stops wasting energy driving a unit that is barely lifting liquid, so cycling on fillage saves both equipment and power.
On a cloud SCADA platform such as Merobix, fillage becomes a field-wide health signal rather than a number buried in a single controller. Trending fillage on every well lets an operator see which wells are holding full fillage and which are slipping, so a well that has started under-filling - and the setting or reservoir change behind it - is caught early. Watching fillage across many wells from one place turns it into a ranking of which wells need their pumping speed or cycling adjusted, concentrating attention where production is being lost and equipment is at risk.
It means the pump barrel fills completely with liquid on each stroke, so every stroke lifts a full charge of fluid. That happens when the reservoir supplies liquid to the pump at least as fast as the pump is cycling. Full fillage is the efficient, gentle condition for a rod pump - the well is producing its full potential for that stroke and speed, and there is no partly filled barrel to cause fluid pound or gas interference.
It reads fillage from the downhole dynamometer card, which it computes from surface load and position. When the pump fills fully, the card is a full shape with high load across the whole upstroke; when it under-fills, the load drops off early and the card's top corner collapses inward. The controller measures how far along the stroke the load falls and converts that into a fillage percentage, so the earlier the drop-off, the lower the fillage.
Falling fillage means the pump is outrunning the reservoir's ability to supply it, so the barrel is only partly filling and strokes are wasted moving little liquid. Those partly filled strokes cause fluid pound and gas interference that damage the rods, tubing, and pump. Slowing or idling the well lets the fluid level recover so fillage climbs back toward full, which restores production and stops the damaging strokes.
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