What Is a Sour Environment?
Authors: Tuttle R.N. 
Published: December 2007

Summary
The mere presence of H2S in a wet oil or gas system may cause either weightloss corrosion or sulfide stress cracking (SSC) of carbon and low-alloy steels.in addition, H2S may enhance chloride stress corrosion cracking (SCC) ofstainless steels and other higher alloys. The likelihood that these phenomenawill occur depends on the severity of the environment. This paper dealsprimarily with sulfide SSC. primarily with sulfide SSC.

Introduction
The evaluation of a specific oil and gas environment is often difficult butmay be critical to the successful operation of the field in question. The" sour environment" definition depends upon both the metallic-alloycharacteristics and the environmental conditions.
The Natl. Assn. of Corrosion Engineers (NACE) does provide some guidance inmaking this critical analysis. The NACE definition of sour environment has notchanged since 1978 and is still appropriate for most carbon and low-alloysteels. However, it does not appear to be appropriate for high-strength steelsor the many corrosion-resistant alloys (CRA's) now in use.

History
Many new fields in the oil and gas industry were developed in the 1950's and1960's. In some of these fields. particularly in Canada, the H2S content in thegas phase was higher and the environmental conditions were more severe thanthose experienced during the development of sour-oil fields then inoperation.
Metal failures began to occur in the sourgas wells and surface equipment. The industry quickly began research and established committees to findsolutions to the problem. Laboratory and field data provided problem. Laboratory and field data provided clues on which environments caused equipmentfailures and why. Predicting the environmental severity necessary to causefailures was complex and involved evaluating environmental conditions (such asthe concentration of H2S present, pressure, temperature, and the presence orabsence of liquid water) and alloy characteristics (including chemistry, heattreatment, strength, and manufacturing and service history).

NACE Definition of Sour Environment
NACE Standard MR0175-881 provides guidelines for defining a sour environmentfor the general selection of carbon and lowalloy steels and is based onindustry experience and laboratory data available when it was developed. Thisdefinition states that the 0.05-psia [0.34-kPaj H2S partial pressure in the gasphase distinguishes SSC pressure in the gas phase distinguishes SSC failuresfrom no failures in susceptible carbon and low-alloy steels. Standard MR0175-88also includes two figures that show the determination of H2S partial pressurefrom the total system pressure and the pressure from the total system pressureand the H2S concentration. The standard has not been altered but does include acautionary note that states that high-strength carrion and low-alloy steels mayfail at H2S partial pressures less than 0.05 psia [less than 0.35 kPai, pressures less than 0.05 psia [less than 0.35 kPai, particularly at lowtemperatures. particularly at low temperatures. At the time Standard MR0175-88was prepared, industry field experience had prepared, industry field experiencehad demonstrated that the presence of a continuous inhibitive oil phase wasbeneficial at low pressures. it was thought that the oil pressures. it wasthought that the oil inhibited overall corrosion and reduced the tendenency forSSC. Conversely. the absence of hydrocarbon liquids increased the risk of SSCin susceptible carbon and low-alloy steels. Thus, separate figures weredeveloped for gas and multiphase systems. These figures were identical at totalsystem pressures &gt;265 psia [&gt;1.8 MPa]. pressures &gt;265 psia[&gt;1.8 MPa]. Fortunately, most carbon and low-alloy steels do becomeresistant to SSC at some elevated temperature that depends on the steel'schemical composition. hardness (strength), heat treatment, and prior servicehistory. NACE provided guidance or the use of the most common higher-strengthAPI tubulars, such as the recommended minimum temperature for API Specification5-CT grades shown in Table 1.

Comments on NACE Definition
The figures in Standard MR0175-88 may be used as a guide for determiningwhether a particular gas is sour enough to cause SSC particular gas is sourenough to cause SSC of susceptible carbon and low-alloy steels.
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