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Abstract

The present paper has for purpose to describe the experience with the use of AISI 4320 steel sucker rods in the corrosive environments of Unidad Económica Rincón de los Sauces (Neuquén - Argentina) of Repsol-YPF and show the results obtained until now.

Introduction

The Unidad Económica Rincón de los Sauces (Neuquén, Argentina) of Repsol-YPF operates oil producing wells with fluids of high corrosivity in the Chihuído de la Sierra Negra and Lomitas oil fields.

These oil producing wells, lifted mainly with beam pumping systems, of Chihuído de la Sierra Negra and Lomitas fields that are using these type of sucker rods have depths of 1000-1200 m, gross productions rates between 100-150 m3/day and water cuts between 70-80%.

The high loads level that these wells have make almost impossible the use of API Grade K sucker rods. On the other hand, the completions use 2 7/8" diameter tubing that, due to the high flow production rate, difficult the action of the corrosion inhibitors (high flow velocity).

These fluids have a high content of gaseous CO2 (50-70% molar), with work pressures oscillating between 70-100 psi, resulting in partial CO2 pressures higher than 30 psi. According to NACE criteria and to the observations made at fields, these are very corrosive wells from the carbonic corrosion point of view.

These oil fields have also had, during the last years, a great influence of secondary recovery process that occasioned an increment in the gaseous SH2 concentration, produced principally for the bacterial contamination of the reservoir.

The presence of sour gas synergies the corrosive process, for that, the use of materials with greater corrosion resistance is a priority in order to extend their useful life.

Field tests with bronze-aluminum coated sucker rods were carried out in 1998.

That experience took 2 years and had good results, but the solution aimed mainly to a reduced group wells with high production rates and high failures rates, that are capable to match the economic balance. Even so, nowadays there are strings made with this material with more than 4 years of use.

To achieve a general solution, it was necessary to look for a different alternative to the steels used up to the moment so, from year 2000, began the utilization of steel AISI 4320 sucker rods with the purpose of analyzing their performance.

Development

1 - Characteristics of the steel

The AISI 4320 sucker rod has the API Grade D rod mechanical properties and a chemical composition similar to the API Grade K rod.

After the thermal treatment of normalized and quenched, the microstructure is bainitic.

Its mechanical properties are the following:

1. Yield Strength (Ksi): 85 min

2. Failure Stress (Ksi): 115/140

Usually grade D sucker rods (AISI 4142 Steel) are used in these oil fields.