Abstract

Environmental and material limits specified in ANSI/NACE MR0175/ISO15156-3:2015 restrict this use of austenitic stainless steel alloys under sour conditions typically encountered in thermal in-situ oil sand operations. This paper is a follow up study to one presented in 2016 (NACE-2016-7892), where C-ring testing was conducted to evaluate the susceptibility of austenitic stainless steels to conditions simulating a severe downhole environment; i.e. 230 °C, pH of 3.5, and variable chloride concentrations and H2S partial pressures. Prior results indicated that after 720 hours of exposure, material did not show any cracking when tested at 10,000 mg/L Cl- and 1,000 kPa pH2S, and at 50,000 mg/L Cl- and 100 kPa pH2S.

In recent testing, three heats of UNS S30403 were tested under the same environmental conditions above and cracking was observed in some heats, suggesting borderline resistance to cracking under these severe conditions and unsuitability per ANSI/NACE MR0175/ISO15156-3:2015. Additional C-ring testing was performed in a less severe environment consisting of 230 °C, pH of 5, a chloride concentration of 10,000 mg/L and 100 kPa pH2S, where the same three heats of UNS S30403 demonstrated no cracking.

Introduction

While carbon steel is the most commonly used material for in-situ facilities including gathering lines and plant piping; austenitic stainless steels (SS) have commonly, and effectively, been used in Alberta in-situ thermal operations for a variety of applications such as: valve trim, instrumentation, and vessel internals.1 The nature of in-situ thermal processes routinely exposes these components to elevated temperatures (>200 °C), high chloride concentrations and varying H2S levels. The aforementioned environmental conditions are known to promote environmental cracking (EC), such as chloride stress corrosion cracking (CSCC) and sulphide stress cracking (SSC). The potential for these conditions to cause cracking in stainless steel components must be evaluated during the material selection process to ensure the long-term integrity of these systems. Material guidelines such as ANSI(1)/NACE MR0175/ISO(2)15156-3:2015 and EFC(3) publications 16 and 17 offer recommendations for material selection; however, these guidelines cover only a limited range of environmental conditions and often fall short of those encountered in oil sands in-situ operations.2,3,4 For this reason, austenitic stainless steels have often been used by Canadian in-situ oil sand operators beyond the limits established by these standards, often without incident1. Despite the performance of these materials in the field, in many cases it is difficult to establish a solid technical basis to challenge the established operating limits of these alloys due to a lack of well documented operating histories, and relevant laboratory testing. The track record of these materials, as well as experimental results show that some grades of stainless steel perform well beyond these recommended limits; suggesting that the current guidelines are overly conservative, and that these materials can potentially be used under more severe conditions without the need to resort to more expensive corrosion resistant alloys.5

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