Abstract

The paper provides laboratory test data that was used to select high strength, nickel base alloy, production tubing for Mobil's 823 field. The application is of critical importance because of the large quantity of H2S (0.5 to 5 mole percent at 20,000 psi BHP) present in Mobile Bay wells. A rigorous test present in Mobile Bay wells. A rigorous test and evaluation program was required to ensure that the selected tubulars are immune to stress corrosion cracking and localized corrosion. New test methods consisting of slow strain rate experiments and critical pitting temperature tests were employed to pitting temperature tests were employed to diagnose temperature limitations and alloy class suitability.

The data suggest that some alloy types may exhibit an localized corrosion and stress corrosion cracking at intermediate temperatures (250 - 350 F). The intermediate region of temperature in which cracking is seen for cold worked high alloy tubing is a novel observation for sour gas well environments and is associated with situations in which the normally protective chromium oxide on the metal is transformed to a predominantly sulfide film. Elemental sulfur was found to have a major influence on cracking susceptibility.

This research supports the economic selection of high alloy tubing for critical applications such that the alloy content is matched to the actual field requirements this resulting in significant cost savings. Reliability of production equipment is maximized by a firm understanding of degradation mechanisms and temperatures.

Introduction

A particular materials selection situation is described involving Mobil's 823 field in Mobile Bay. The wells in question classify as moderately sour, having approximately is psia H S partial pressure in equilibrium with produced brine. Material selection decisions produced brine. Material selection decisions based on current technology and laboratory data concluded that a nickel base alloy having the nominal composition 25 Cr and 50 Ni was suitable for the production tubing.

The engineering test program described herein was focused on differentiation of the relative performance capabilities of several proprietary products. The objective was to proprietary products. The objective was to measure capabilities in terms of maximum temperature of safe operation and the effect of environmental variables on corrosion and stress corrosion cracking.

TECHNICAL BACKGROUND

It is now a prevalent practice to complete deep sour gas wells with highly alloyed corrosion resistant materials that contain substantial amounts of chromium, nickel and molybdenum. [1] Production tubing is typically furnished in the cold worked condition at a high strength level (120-170 ksi minimum yield). The use of such materials allows higher production rates, reduces the frequency of workovers, eliminates the necessity for treatment chemicals and maintains a high degree of reliability.

Corrosion resistant alloys (CRA) are expensive (ten to twenty times steel) and their price is a strong function of alloy content. There is, therefore, a need to match the capabilities of the tubing alloy exactly to its requirements.

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