Predicted vs. Measured Loads in a Sucker Rod Pumping System

In order to apply an oilfield pumping unit on the most economical basis, itis often necessary to predict its torsional and structural loading in advanceof well completion. Frequently, a pumping unit is misapplied because thedetermination of these values by simple prediction formulae does not conform toeventually measured loads. An investigation of the simplified predictionformulae most commonly used in applying a sucker rod pumping unit illustrateswhy discrepancies between predicted and measured loads may exist. Introduction One of the numerous problems faced by the pumping unit manufacturer is topredict the torsional and structural loading that will be imposed on thesurface equipment for any given set of well conditions. As a precise determination of these loads involves, complex formulationemploying many variables (some of which are unknown prior to well completion), it becomes necessary to develop a set of prediction equations with solutionswhich are fast and simple and require knowledge of only a few known quantitiessuch as rod and fluid weight and polished rod acceleration. In most cases, these simple and convenient formulae, employing only a smallportion of the items which actually control torsional and structural loading, are entirely adequate to apply the surface pumping unit on a most economicalbasis – but the results obtained are admittedly only approximations and maydiffer considerably from measured loads. In a recent study of two different types of sucker rod pumping units over anumber of applications chosen at random, the predicted peak torque determined by these simplified methods was compared to the measured peak torque. The results showed that, in both types of units, approximately one-third of the applications measured greater than predicted, one-third measured less than predicted and one-third measured about the same as predicted. (See Table I).