Paper: A COMPARISON OF THE PERFORMANCE OF LINEAR ACTUATOR VERSUS WALKING BEAM ROD PUMPING

Rod pumping units historically used a crank driven walking beam to provide a reciprocating motion for oil and gas production. Several geometries have evolved over the years to produce desirable polished rod motion profiles and gearbox torque loads. These mechanical systems are limited in their ability to manipulate the motion profile and the profile is forever fixed by the selected geometry. Hydraulic cylinder linear actuators became available a number of years ago for reciprocating rod pumping of oil and gas wells. Electrically driven rack and pinion linear actuators have also been recently developed for rod pumping applications. These hydraulic and electric linear pumping units share some common advantages over mechanical pumping units. Linear pumping units are generally less massive than comparable walking beam units and can be mounted directly to the well head. The cost savings on site preparation, transportation logistics, and equipment installation can be substantial. Linear actuators also have an important advantage in their ability to provide programmable control of polished rod motion profile and closed loop control of polished rod load. The relatively low mass of linear actuator mechanisms allows nearly instantaneous adjustment of polished rod velocity and load. Linear pumping units provide independent control of up stroke and down stroke peak velocities as well as the shape of the velocity profile. Controlling the velocity profile allows increased pump cycle rate and associated production without exceeding rod fall velocity limits. Velocity profile control can also be used to reduce gas interference and fluid pound. Rod stroke position can be controlled to provide period pump tapping to overcome down-hole pump problems. Polished rod load control can be used limit minimum rod load, eliminate bridle separation, and damp rod load oscillation.