Quantitative Determination of Rod-Pump Leakage With Dynamometer Techniques

Abstract

Summary This paper describes new methods for quantifying pump leakage that extend the effectiveness of dynamometer analysis and can lessen the dependence on well tests in determining the degree of pump leakage. Actual examples are included to illustrate the techniques. pump leakage. Actual examples are included to illustrate the techniques. Traveling- and standing-valve leakage checks are also described. Introduction The polished-rod dynamometer has many important uses in the determination of surface equipment loads, calculation of downhole pump dynamometer cards, and visual diagnosis of downhole pump conditions. It is also useful in determining the mechanical condition of the downhole pump from simple, inexpensive measurements taken at the surface. All downhole rod pumps include barrels (cylinders), plungers (pistons), and traveling and standing valves (normally balls and seats). As the names suggest, the traveling valve "travels" up and down with the rods and the standing valve "stands still" with respect to the tubing. Fig. 1 illustrates the primary parts of the two types of API pumps, the rod and tubing pumps. In the tubing pump, the barrel is an integral part of the tubing string, allowing the use of a larger plunger than rod pumps do. The entire mechanism (including the barrel) in the rod pump is run inside the tubing on the rods. The three basic rod-pump configurations are (1) top anchor, (2) bottom anchor with traveling barrel, and (3) bottom anchor with stationary barrel. Regardless of the type or configuration, basic pump operation is the same. This paper presents the theory of traveling- and standing-valve checks with a dynamometer and documents three quantitative methods for deriving pump slippage rates. pump slippage rates. Traveling-Valve Check The traveling-valve check tests the integrity of the plunger/barrel fit and traveling-valve ball and seat. According to Stearns, leakage past a plunger can be calculated and depends on the pump's physical dimensions, plunger can be calculated and depends on the pump's physical dimensions, the fluid viscosity and density, and the differential pressure across the plunger. Because the fit is known only when the pump is installed, the plunger. Because the fit is known only when the pump is installed, the leakage rate is unknown as the barrel and plunger wear and the fit deteriorates. Also, leakage can occur through the valves as the balls and seats wear. Thus, as wear progresses, quantifying pump leakage becomes important. Well tests can indicate leakage but are often inaccurate and infrequent. Changes in well productivity can also cloud the issue.