The American Petroleum Institute (API) method for rod pumping system design became available in the mid 1960's. Even after 30 years, questions still arise concerning its utility. This paper examines basic premises of the API method and how these affect accuracy and applicability. A comparison is made with wave equation techniques which are also widely used. It is concluded that the API method is useful and can be applied with confidence as long as underlying assumptions are not violated.
Development of the API method was not instigated by API. Instead it began in the 1950's as a cooperative effort under the auspices of Sucker Rod Pumping Research, Inc. (SRPRI). This non-profit effort was funded by several oil companies and equipment manufacturers. The actual work was done at Midwest Research Institute (MRI) under the direction of the sponsoring companies. An analog computer was constructed to simulate the elastic behavior of the rods as they were driven by a conventional pumping unit. The analog circuits were constructed to simulate anchored tubing and a downhole pump which was filling completely with liquid. The analog computer was capable of creating synthetic dynamometer cards and making predictions of power requirements, unit and rod loadings and pump capacity. Owing to the impracticality of deploying many analog computers for industry use, a graphical method of summarizing the results was developed which became the basis of a hand calculation procedure. At this point, the sponsoring group gave the technology to API (in the early 1960's) and henceforth it has been known as the API method.
A good understanding of the basic assumptions is important. Some of the assumptions are shown in the API literature. Others, less apparent, are included by the writer.
Conventional pumping unit motion is presumed.
Relatively low slip prime-movers are simulated, say equivalent to NEMAD motors and single cylinder gas engines with large flywheel effects.
Steel rod strings are presumed. Tapered strings are simulated as if the rods become smaller with depth. Thus large sinker bars on bottom are not handled correctly.