Increased production through the existing facilities together with life extension of ageing assets are the essential integral part of a strategy to address the current low margins. These require suitable materials and corrosion control guidelines to enable improved safety and economy. In this context, optimisation and selection of appropriate materials, which can tolerate H2S , are critical issues. This paper describes the approach necessary to optimise materials for sour service duties and provides respective guidelines focusing on the selection and utilisation of carbon and low alloy steels. It combines laboratory assessment with field experience hence offering effective means of corrosion design for sour service production.
Corrosion has a wide-ranging implication on the integrity of materials used in the petroleum industry. It manifests itself in several forms in which CO 2 corrosion (sweet corrosion) and H2S corrosion (sour corrosion) are by far the most prevalent forms of attack encountered in oil and gas production. The implication of corrosion can be viewed in terms of its effect on capital and operational expenditures (CAPEX and OPEX) and health, safety and the environment (HSE). Resistance to sulphide stress cracking (SSC) is often the principal factor affecting the choice of materials for H2S-containing environments, since its occurrence can result in a catastrophic and potentially hazardous failure ~-3.
The presence of wet H2S promotes and exacerbates many types of environmental cracking involving a range of mechanisms 16. The service conditions within which these types of cracking may become integrity concern and hence require metallurgical design or operational precautions are known as "sour service". This is in contrast to "sweet service" where no metallurgical design or operational precautions are normally required in order to avoid environmental cracking. The presence of H2S and brine in produced fluids not only give rise to increased corrosion rates, but also can lead to environmental fracture associated with enhanced uptake of hydrogen atoms into the steel. The risk of SSC and lack of a realistic knowledge on the corrosivity of produced fluids often leads to the selection of highly alloyed, highly priced materials with abundant cost penalty 2'4'6.
This paper covers the current understanding of H2S damage applicable to upstream production by demonstrating different types of damage and their possible implications.
The paper includes cost implication, test approach to enable correct and cost effective qualification of candidate materials. A distinction has been made between low acidity environments characteristics of exploration and production regimes and damage caused at neutral to high pH conditions experienced in downstream and process industries. The paper focuses on the selection and utilisation of carbon and low alloy steels and is divided into five complementary stages:
1. Types of H2S damage
2. Cost implication of sour service materials
3. The development of application domains to enable safe and economic materials choice
4. The approach to evaluation of materials for sour service applications
5. Industrial implications of the approach.
All stages will be interrelated through a combined objective of optimising materials for oil and gas production systems. The outcome has led to the development of the limits of sour service for a wide range of steels enabling the selection, confident and reliable utilisation of the most suitable and cost effective material for sour service duties.
