Average Downstroke Load (Sucker-Rod Pump)

Average downstroke load in a sucker-rod pump system depends on several factors, including the pump's design, the fluid being pumped, the depth of the well, and the rod string dynamics. In sucker-rod pumping (often referred to as beam pumping), the downstroke load is typically lower than the upstroke load because, during the downstroke, the pump plunger is moving downward, assisted by gravity and the weight of the rod string, while the upstroke lifts the fluid column and rods against gravity.

 

The total weight of the rods decreases during downstroke due to buoyancy effects in the fluid.  The hydrostatic pressure of the fluid in the tubing reduces the effective weight.  Friction between rods, tubing, and any deviations in the wellbore adds to the downstroke load.  A deeper or more inefficient pump will affect the load calculations.

 

During the downstroke, the traveling valve in the pump is open, and the fluid inside the pump is displaced deally, the load on the rods during the downstroke should be minimal, as the pressure inside the pump barrel is close to the discharge pressure.  Average downstroke load is primarily influenced by the weight of the rod string in the fluid (buoyed weight) and the frictional forces in the system, minus the fluid load that was lifted during the upstroke.  It can be calculated using the polished rod load data, which is often measured with a dynamometer in the field.  The formula typically involves subtracting the fluid load (transferred to the tubing during the downstroke) from the total rod weight.  However, factors like fluid viscosity and friction between the plunger and barrel can introduce some load.