ABSTRACT

Historically, the testing of sour corrosion and environmental-assisted cracking (EAC) in the oil and gas industry has focused on oil country tubular goods (OCTG) with design lives of tens of thousands of hours, while wireline and wireline tools are typically exposed to sour conditions for a few hundred hours per year at most. The existing corrosion/EAC information related to wireline materials is either overly conservative or inadequate, which significantly reduces the work scope of a great number of engineering materials that could potentially be used in wireline applications. To overcome this difficulty, the H2S application limits of wireline materials need to be realistically defined.

In this work, the corrosion and EAC resistance of 14 heats of wireline materials, including a cold-rolled carbon steel, low-alloy steels, stainless steels, titanium alloys, nickel-based alloys, and copper-based alloys, were evaluated in two extreme sour environments: 180,000-mg/L chloride brine at 350°F (176.7°C) with partial pressure of H2S (PH2s) 17.2 MPa and partial pressure of CO2 (Pco2) 3.5 MPa, with and without elemental sulfur. The weight-loss corrosion rates, pitting, and cracking behavior of the wireline material were obtained, the result of which could be used as first step to guide the selection of wireline materials in sour service.

INTRODUCTION

H2S and CO2 gas in combination with chloride containing brine water are the main causes of internal corrosion and EAC of materials used in oil and gas production. Wet H2S is corrosive and electrochemically react with alloys to produce atomic hydrogen, which diffuses into the crystal lattice of alloys to induce hydrogen embrittlement. High pressure CO2 results in lower pH, enhances general corrosion, and accelerates the activity of H2S on metal. The chloride ion at low pH breaks down passivity on metal and generates localized pitting sites, which could be taken as stress riser to induce chloride-stress corrosion cracking (Cl-SCC).

As increasing activities of exploration and production of oil and gas in challenging locations, the selection of materials exhibiting high strength, low weight, and good tolerance to high temperature and hostile chemicals environment, is more and more critical. NACE MR 0175-20151 is used as a guideline for selecting materials for sour service. The standard is good for materials used for long-term exposure (e.g. OCTG) but not to those for short-term applications, such as wireline equipment. To overcome this difficulty, an application limit need to be developed to cover the materials used for short-term sour service. However, due to lack of reliable data to correlate the corrosion/EAC results with the failure condition found in practice, the application limit is challenging to be established.

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