Pump off controllers have earned a place in the technology of rod pumping. Many methods exist for sensing when a well has pumped off. Most widely used are techniques based on the surface dynamometer card or motor speed or production rate.
This paper describes several methods for sensing pump off using the downhole pump dynamometer card. These include areas inside of the pump card, areas outside of the pump card, set point and liquid fillage, among others. Procedures for calibrating the controllers are described together with provisions for high fluid level recovery. Combining the liquid fillage method with variable frequency drives and eddy current drives is presented as a way of performing variable speed - no stop control.
Pump card monitors (PCMs) hold promise of being useful devices which are easy to apply and comprehend.
Application of pump off controllers (POCs) began about 25 years ago. These controllers have proven useful in saving power and minimizing wear and tear on rod pumping equipment. These savings are realized by stopping the unit when the pump is not filling satisfactorily. The POC has gradually evolved into more than a device to sense pump off and to stop the unit.1 Rod load based controllers can also sense abnormal rod loads and shut the unit down before the pump and/or rods are damaged. Controllers that monitor motor speed can also prevent damage to the prime mover and gearbox. Controllers that monitor liquid rate can also prevent damage to the surface stuffing box packing. POCs sometimes increase production slightly by detecting malfunctions sooner than ordinary surveillance techniques. At this writing, about 35000 wells worldwide have been equipped with POCs.
The purpose of this paper is to share ideas concerning how the downhole pumpcard can be used to sense pump off. While creation of the downhole card is not simple, use of the pump card to sense fluid pound is. The complexity of computing the pump card can be hidden in software and hardware without troubling the user. The result is a pump off control concept that is more easily understood and applied.
The downhole pump card is computed by solving the one dimensional wave equation
Equation (1) Available In Full Paper
subject to measured boundary conditions comprising time histories of surface rod load L(t) and surface rod position P(t). The resulting solutions are used to construct pump dyno cards of utility in well analysis and control.
Oil producers began to be familiar with computed downhole pump dynamometer cards2 about 30 years ago. These cards have the advantage of simplicity. Only two types of cards have to be comprehended in calibrating a PCM. These are full liquid fillage and incomplete liquid fillage (fluid pound or gas interference).
With anchored tubing, pump cards that indicate full liquid fillage (no fluid pound or gas interference) have roughly a rectangular shape (Figure I). At topand bottom of the stroke the pump is virtually stationary as load is transferred from rods to tubing and vice versa.