Increasing Sucker Rods' Working Capacities

Abstract

Technology Update Artificial lifting systems are being pushed to their operational limits, given the more demanding field operating conditions that require deeper wells and higher production flows. Over the years, the number of secondary-recovery and nonconventional oil projects has increased, and more is being asked of rod pumping (RP) and progressive-cavity pumping (PCP) systems. Both pumping systems have found the sucker-rod string to be one of their weakest links. Stress analysis and failure statistics for different parts of sucker rods have found the weakest point to be located in its coupled connection. Rod-pin and coupling failures have become common problems in fields, making it clear that the fatigue resistance of the connection is inferior to the fatigue resistance of the rod body. Several potential solutions designed to increase rods’ working capacities have been attempted, but none of them attacked the problem genesis: the connection. Recently, however, a premium-connection rod has been developed with a new thread design that has demonstrated very positive results in reducing connection fatigue (Fig. 1). Design Process Finite element analysis (FEA) is a numerical tool used to verify designs before field testing can be performed, and with FEA, the most significant design constraints of conventional rod connections were analyzed as a basis for developing the new premium connection design. The main points addressed in the development of the new design were the following: Loosening tendency of the connection because of a gap between threads. This gap allows movement between coupling and pin threads when rods undergo compression and/or shocks. Permanent deformations of the thread (as verified by FEA) during makeup and well operations. These deformations make crack propagation more likely. They also result in a difficult makeup operation when repeating displacements. Nonuniform stress distribution in thread profile generates regions of high stress concentration and, as a result, makes the pin and coupling more susceptible to the appearance of cracks (also verified with FEA). High reliance on the makeup operation because both overtorque (increase in the stress level) and undertorque (stress concentration during well operation) are harmful for the connection and produce failures in sucker-rod pins and couplings. In conventional design, thread effectiveness is highly dependent on the cleaning operation before the string is run into the well and an inefficient makeup can easily result and later produce failures.