ABSTRACT

The sulphide stress cracking (SSC) resistance of 32 casing and drill pipe materials corresponding to API 5D/5CT grades C-95 to S-135 has been evaluated using NACE TM0177 tensile testing. The standard method has been extended to include a range of temperature ambient to 107ºC, and H2S contents in the gas phase in the range 100 to 0.1 mol. By defining an acceptable material as one that can bear a minimum load corresponding to 80% specified minimum yield strength for 720 hours in the given combination of H2S content and temperature, the results so derived have been used as the basis for operational limits for each material grade. In most cases these limits extend the useful application of the casing materials compared to NACE MRO175, particularly in marginally sour environments. The results showed that materials of the same API grade sourced from different manufactures had variable performance, particularly when the aggressivity of the environment was increased. It was also found that the SSC resistance of specially modified C-steels that are marketed as “sulphide resistant” was poorer than anticipated. These inconsistencies emphasise that some kind of qualification testing and quality control is necessary when selecting such materials.

A thorough analysis of the mechanical properties, chemical composition, and metallography of the materials facilitated correlation with the SSC results and hence derivation of guidelines for procurement/specification of materials for use in H2S containing environments. These are intended as a practical guide for the selection of casing and drill pipe materials for H2S-semice in formation waters of pH 3.5 or higher.

INTRODUCTION

High strength low alloy casing and drill pipe steels may suffer sulphide stress corrosion cracking (SSC) when used in hydrogen sulphide (H2S) containing environments Generally, such cracking is avoided by following the materials selection guidance given in the current issue of NACE MR0175. Somewhat simplified MR0175 is applicable to ‘sour’ gas or multiphase environments where the mol fraction of H2S gas is 0,0035 or more. The guidelines are reasonably clear for C-steel casing and drill pipe materials, and their application has resulted in very few incidences of failure in practice, indicating that MRO175 has a good “track-record. However, in the Norwegian sector of the North Sea, H2S contents are more typically “marginally sour”, i.e. just within the MRO175 definition of ‘sour’. Adherence to the materials selection guidelines of MR0175 can be too conservative in these fields, and particularly discourages the use of higher strength materials, permitting their application at elevated temperatures only, It is well known that there is a general tendency for higher strength materials to be more susceptible to SSC and this forces a trade-off between strength and SSC resistance that limits the maximum strength of casing and drill pipe that can be confidently deployed in H2S containing wells. This dilemma becomes particularly acute as wells are bored deeper and the demand for higher strength materials increases.

In the absence of more discriminating guidelines for materials selection for H2S-containing environments, more costly high-alloyed steels may be stipulated in conditions where standard

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