ABSTRACT

This paper presents environmental performance data on welded nickel alloy under high temperature, high pressure, ‘ultra-sour’ conditions. Welded coupons of nickel alloy UNS N06625 were subjected to three levels of nominal stress at 200 °C (50%, 75%, 100% AYS) and tested for 30 days in an environment consisting of 3.5 wt% NaCl solution with a partial pressure of 8.8 MPa (88 bar) H2S, 2.65 MPa (26.5) bar N2 and 1.55 MPa (15.5 bar) H2O. Coupons were visually assessed and showed no signs of environmentally assisted cracking (EaC). However, a thin, tenacious scale comprising various sulfides was formed with no significant metal loss observed over the test period. More detailed characterization, including scanning electron microscopy and X-ray diffraction studies, revealed the microstructure and phases in the scales formed on the surface of the coupons. Examination showed regions with two distinct scale morphologies: a fine adherent scale of mixed nickel sulfides and larger, faceted features in regions adjacent to the weld, which were determined to be FeS2 (pyrite).

One may conclude that nickel alloy UNS N06625 performs well in an ‘ultra-sour’ (>5 MPa pH2S) environment at high temperature (200 °C) and high pressure (13 MPa) and shows no evidence of environmentally assisted cracking. However, it must be noted that UNS N06625 is not immune to corrosion in such environments.

INTRODUCTION

While global coal consumption did decline by 1% in 2015, the world set new consumption records for petroleum and natural gas. The net impact was a total increase in the world's fossil fuel consumption of about 0.7% in 2016.1 Although renewables are becoming part of the energy mix, fossil fuel is expected to remain important in the foreseeable future. According to International Energy Agency projections, natural gas consumption is likely to rise by 50% by 2040. Growth in oil demand slows over the projection period, but tops 103 million barrels per day by 2040.2

Reserves of natural gas are abundant, with proven gas reserves in 2016 of 186.6 trillion cubic meters worldwide.3 However, nearly 40% of the fields lying untapped contain concentrations of CO2 and H2S that pose significant engineering challenges to their development.4 The exploitation of oil and gas fields containing high concentrations of acid gases requires materials that can perform in harsh environments without failure. It has often been assumed in the oil and gas industry that corrosion is of primary concern in CO2-containing environments5-8, while sulfide stress cracking is the primary concern in H2S-containing environments.9,10 However, this does not imply that corrosion does not occur in H2S11-13 or cracking has not been reported in CO2.14-18 In general, both are concerns for materials selection in the oil and gas industry.19

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