Dynamometer Charts and Well Weighing

Abstract

The purpose of this paper is to present in a convenient form data andexample, necessary in making dynamometer card analyses; also to outline aprocedure of well weighing. Many articles and papers have been written delving into the mathematicalconsiderations relative to the shape and characteristics of dynamometer cards.However, it is recognized that there are too many unknown factors involved insuch calculations to assure a workable degree of accuracy. For this reason theaccepted procedure is to take dynamometer cards on wells in question ratherthan try to calculate the load curve. The polished rod dynamometer is now recognized as a necessary tool formeasuring loads, torque, and horsepower. It is also used to determine pumpaction and trouble-shoot for any seemingly abnormal pumping condition. The apparently infinite variety of dynamometer cards that can be obtained isone reason for the general lack of usage of the dynamometer as a controlinstrument rather than a means for making routine measurements of leads andhorsepower. When it is considered that the dynamometer card is a record of theresultant of all forces acting on the polished rod at any particular instantduring the pumping stroke, the problem is then one of breaking down thisresultant into its various components. As a means of a quick review we shallconsider the examples shown in Figs. 1 to 9, and Tables I and II and thenproceed to the interpretation of variously shaped cards caused by some abnormaloperating condition. In Table II, when we were considering the factors involved in calculating thepeak polished rod load, it can be seen that the factors involved greatlyoversimplify the problem. Certain assumptions are made which mayor maynot be even close to the actual field conditions, such as the specific gravity ofthe fluid generally considered as one; that the crank has constant angularvelocity; that the down-hole friction is zero; and that the fluid lift is fromthe pump. In the following examples we shall see what a variation in fluidweight and friction can do to the general shape and magnitude of thedynamometer card. (Figs. 10 to 22.) T.P. 2854