Produce More, Cut Failure: Why Controlling Using Pump Fillage Is Wrong.

Pump fillage is the standard control variable for sucker rod pumps, and pump-off controllers routinely cycle or slow wells on a fillage setpoint. Yet fillage measures how much liquid is lifted per stroke, not the stress in the system. For example two wells at the same fillage can experience very different mechanical damage, depending on the rod string design and the gas pressure. A 40% fillage with smooth gas compression can be gentler on the rod string than one at 70–80% fillage with low pressure gas and a sharp liquid impact. Controlling on fillage alone therefore fails in both directions: it shuts in wells that could keep producing safely, and it keeps pumping wells that are quietly destroying their rods.
We introduce a wear index computed from a full rod string dynamic model quantifies the severity of fluid pound directly. A rapid deceleration on liquid contact launches a stress wave up the rod string that drives compressive loading, buckling exposure, and fatigue. The index is derived from the plunger's deceleration behavior in the load-transfer region of the downstroke. The result is a simple metric that separates gentle compression from damaging impact independently of fillage.
Applied across a fleet of over a thousand producing wells, the index shows that fillage and damage are only loosely coupled. This reframes pump-off control and explains why many operators see low failure rate and maximal production when pumping at low fillage, and why certain wells, especially with fiberglass rod string, see high failure rate with a 80% fillage setpoint. Where the wear index stays low, the smart controller can pump through low fillage and recover production that a fillage setpoint would defer. Where the index is high, the same controller can reduce SPM or shut in even at fillage levels conventionally considered safe. We present the formulation, field examples contrasting fillage with wear, and the resulting control strategy for maximizing runtime without accelerating rod failure.