Metallurgy Selection of Tubing in Sucker Rod Pump Wells Plagued with Pitting and Abrasion Issues

Abstract

Analysis of tubing failure of SRP wells with respect to uniform corrosion, pitting and mechanical abrasion has been carried out. The primary objective includes the identification of root cause of failure and suggesting alternate metallurgy. Many wells in an onshore field in ONGC were facing the acute problem of general corrosion, pitting and rod-tubing wear. The methodology for carrying out the study consists of a Failure Analysis of a retrieved sample of the failed tubing from one of the affected wells. This included a thorough visual inspection, Scanning Electron Microscope analysis and X-Ray Diffraction analysis. The results of these tests were backed up by software simulation in Honeywell Predict. Metallurgy selection involved multiple exhaustive simulation runs in Honeywell Software Socrates which was corroborated by relevant oilfield standards as well as literature available on the subject matter. Based on the failure analysis and simulation runs, it was concluded that besides the issue of uniform corrosion and pitting, many of the affected wells are also facing the problem of tubing failures due to abrasion and mechanical wear. It is pertinent to note that the major contributor of the frequent tubing failures in the candidate wells selected for the study were pitting and corrosion. Nevertheless, Abrasion always remains a key threat to the tubing string integrity in rod-pump wells. Therefore, the existing tubing metallurgy of N-80 grade Carbon Steel was deemed inadequate in the absence of reliable corrosion inhibitor continuous dosing facilities. A tubing metallurgy that takes care of both pitting corrosion as well as abrasion and mechanical wear was sought. UNS 41426/41427 or the modified version of 13 Chrome, commercia lly known as Super Martensitic 13 Chrome, are available in 95 ksi and 110 ksi grades. These grades have a maximum hardness of 28-32 HRC which is substantially high compared to L-80 13 Cr (maximum 23 HRC). Also, as this alloy has 4-6% Nickel, it provides added protection against uniform corrosion as well as pitting and hence was recommended. The paper specifically analyses tubing failure in Sucker rod-pump wells due to corrosion, pitting and abrasion. After exploring various viable options, adequate tubing metallurgy has been recommended that should take care of corrosion, pitting as well as mechanical wear problems.