Abstract

The effect of H2S on the aqueous corrosion behavior of mild steel was evaluated at HPHT conditions (supercritical CO2 pressure) at a total pressure of 12 MPa and a temperature of 160°C. The corrosion rate of steel samples was determined by electrochemical and weight loss measurements. The surface/cross-sectional morphology and the composition of the corrosion product layers were analyzed by using surface analytical techniques (SEM, EDS, and XRD). Results showed that the corrosion rate decreased with time and no significant difference was observed in the presence of 1000 and 2000 ppm of H2S at HPHT CO2 conditions. Surface and cross-sectional analyses revealed that the corrosion process is governed by the formation of FeCO3 regardless of the presence of H2S. Furthermore, the corrosion behavior of mild steel in these conditions did not depend significantly on flow velocity.

Introduction

The presence of large quantities of CO2 (from 25% to 89%) in gas fields have imposed increased challenges on the economics of project development and subsequent operations.1 A major issue in developing those fields is corrosion and materials technology because of aggressive environments (i.e., high CO2 contents and/or presence of H2S) with high pressures and high temperatures (HPHT).2 Although corrosion resistance alloys (CRAs) have been available as a materials selection option for these severe environments, carbon and low alloy steels are still widely used as tubing materials due to their strength, availability and cost.3-7

The increment of temperatures and CO2 pressure in production wells may lead to CO2 in supercritical state if the temperature and the pressure are over 31.1°C and 7.38 MPa, respectively.8 Corrosion issues in supercritical CO2 environments relating to carbon capture and storage (CCS), enhanced oil recovery (EOR), and deepwater oil and gas production applications have recently been investigated at relatively low temperatures (< 90°C),9-13 and it has been understood that aqueous corrosion mechanisms in high pressure CO2 are similar to those in low pressure CO2 conditions.14

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