Abstract

The ability to optimize the use of carbon steel in corrosive service presents many economic advantages, including minimizing the use of expensive corrosion resistant alloys, reducing well count by enabling optimized large bore completions, and eliminating additional offshore pipelines and facilities. An integrated approach to corrosion modeling and testing is employed by ExxonMobil to reliably extend the application of carbon steel.

The integrated approach to predicting corrosion has five primary elements:

  1. Rigorously establish the environmental conditions by conducting thermodynamic and compositional hydraulic analyses, and characterize how these conditions are expected to change over time.

  2. Identify the local environmental conditions and the types of corrosion that are expected to occur (e.g., weight loss, pitting, environmental cracking), including sensitivity and upset cases.

  3. Conduct realistic corrosion tests under the identified field conditions by simulating brine chemistry, dissolved acid gas concentrations, hydrocarbon effects, fluid shear stresses, and flow regime in appropriate laboratory equipment. Specialized laboratory test apparati, such as a large-diameter sour, multiphase flow loop and large-volume high-pressure high temperature autoclave test cells, have been designed and constructed to ensure proper reproduction of field conditions.

  4. Mathematically extrapolate the results of the laboratory tests to the field, enabling calculation of expected tubular life.

  5. Conduct life cycle cost analysis.

This paper will describe how this integrated approach to predicting corrosion is used to evaluate the use of carbon steel in oil and gas production environments. Special emphasis will be placed on the prediction of pitting corrosion in H2S-containing environments.

Introduction

From the material selection perspective, the design decision is generally between CRAs and carbon steel, often with inhibition. The ability to optimize the use of carbon steel in corrosive service often presents economic advantages, primarily through a significant reduction in capital expenditures. The ability to run wet gas and full wellstream pipelines and flowlines can further reduce costs by eliminating offshore dehydration facilities.

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