ABSTRACT

In the past few years, a new family of materials has entered the downhole tubular market, known as the alloyed 13%Cr steels. These are a development of the conventional API 5CT grade 13%Cr steels to which alloying elements of Ni, MO and Cu have been added, hence offering a number of advantages. In the present paper, domains of application for the 95ksi grade alloyed 13%Cr steels with respect to sour service conditions have been defined on pH vs. H$ partial pressure plots. Corrosion properties have also been determined for two other aspects; chloride stress corrosion cracking (ClSCC) and high temperature corrosion resistance. The results have reiterated the importance of solution chemistry, pH and H$ partial pressure in defining the corrosion properties of steels.

INTRODUCTION

The choice of appropriate materials remains a major challenge in delivering cost effective options in the exploitation of oil and gas. A number of parameters are important in making the final choice, amongst which are mechanical and corrosion properties, availability and cost. Corrosion resistance in general, and resistance to CO, and H$ corrosion in particular, constitute a major factor in this decision.

Over the last few years, 13%Cr steel tubulars have provided good service in well completion applications, giving high CO, corrosion resistance together with satisfactory tolerance to H$ damage[l-4]. However, this generation of alloys shows a poor resistance to high levels of H$, high chloride concentrations and high temperatures. In recent years, a new family of alloys has entered the market known as alloyed 13%Cr steels. These are a development of the conventional 13%Cr steel (i.e. API 5CT LSO 13%Cr) to which alloying elements of nickel, molybdenum and copper have been added. These alloys offer a number of advantages compared to API 5CT grade 13%Cr steel, principally: Higher strength and toughness - available in strengths up to 12Oksi Much better corrosion resistance at high temperatures (>I2000 Slightly improved resistance to sour conditions (at the same strength level) These alloys are also more economical than conventional grades of duplex stainless steels, which are considered as one of the main alternatives to API 5CT grade 13%Cr steel where higher strength or corrosion resistance is required. This paper establishes the application regimes for the alloyed 13%Cr steels for downhole duties in sour service conditions. The study covered two principal aspects of corrosion performance :- * Resistance to sulphide stress cracking (SSC) High temperature corrosion conditions (namely; resistance to general and localized pitting corrosion). In addition, a limited reference to the resistance of these alloys to chloride stress corrosion cracking (ClSCC) is made. As there are a large number of alloyed 13%Cr steels on the market, it was not possible to evaluate all the alloys available. Therefore, a few selected alloys were fully evaluated. In addition, past test data generated for specific applications has been incorporated into the final domain diagrams.

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