This invention relates to a system for determining the operating characteristics of a rod-pumped oil well and making automatic control decisions based on those determinations. Previous methods for detection and control of a condition known as "pump off" have evaluated data on a dynagraph, which displays measured polished rod load and measured or calculated polished rod position. Some prior systems, such as U.S. Pat. No. 4,286,925 to Standish, test for pump-off by determining whether the load at a particular point in the downstroke exceeds a preset or user-adjustable limit. Other systems have measured the area within the dynagraph for one full stroke (called a card), which represents work done by the pump, and compared that area against a limit or a "test card." U.S. Pat. No. 3,951,209 to Gibbs, for example, discloses a method of integrating the entire area within a dynagraph. U.S. Pat. No. 4,015,469 to Womack discloses a method of integrating equal portions of the upstroke and downstroke. End Devices, Inc., in its device known as the Model 107DC, disclosed a method of integrating the lower half of the dynagraph, i.e., the downstroke. Our U.S. Pat. No. 4,583,915 discloses a method of integrating a portion of the area below the dynagraph, i.e., the space between measured load and minimum load during a selected time period.
Each of those methods share the shortcomings that they are difficult to adjust and sometimes falsely detect pump-off when the well is in fact full. For example, when a well is shut down for a long period of time, such as for service work, the fluid level may rise in the annuls. That rise in fluid level reduces the hydrostatic head required to lift the fluid to the surface, just as if the well were shallower. When the pump is restarted, therefore, it needs to do less work, and the area inside the dynagraph may be reduced to the point that pump-off will be detected, even though the pump is full. U.S. Pat. No. 4,302,157, issued to Welton, et al., teaches a method for preventing false pump-off detection in which pump-off is recognized only after the pump first operates normally with a full pump. However, the method in that patent is complex and uses a simplistic "rule of thumb," rather than a highly discriminating test. All of the patents cited above are incorporated herein by reference.
The present invention provides an improved system for controlling a well and detecting pump-off.