Abstract
The paper presents the application of a holistic approach to corrosion prediction that overcomes classical pitfalls in corrosion testing and modelling at high pressure, high temperature and high CO2 conditions. Thermodynamic modelling of field and lab conditions allows for more accurate predictions by a novel CO2/H2S general corrosion model validated by laboratory tests.
In the proposed workflow, autoclave tests at high pressure and temperature are designed after modeling corrosion in a rigorous thermodynamic framework including fluid-dynamic modelling; the modeled steps include preparation, gas loading and heating of fluid samples at high CO2 concentration, and high flow velocities. An autoclave setup is proposed and validated to simultaneously test different conditions. Corrosion rates are extrapolated to compute service life of the materials and guide material selection.
The results from the model and tests extend the application of selected stainless steel grade beyond the threshold conditions calculated by simplistic models and guidelines. Consideration of fugacities and true aqueous compositions allows for accurate thermodynamic representation of field conditions. Computation by rigorous fluid dynamics of shear stress, multiphase flow and heat transfer effects inside completion geometry lead to a proper interpretation of corrosion mechanisms and models to apply. In the case study used to showcase the workflow, conventional stainless steel is validated for most of the tubing. It is observed that some sections of the system in static condition are not exposed to liquid water, allowing for safe use of carbon steel, while as for other critical parts, more noble materials are deemed necessary.
Harsh environments pose a challenge to the application of conventional steel materials. The workflow applied to the case study allows accurate representation and application of materials in its application limit region, allowing for safe use of carbon steel or less noble stainless steels in those areas of the completion where corrosion is limited by multiphase fluid-dynamics, heat transfer or the both. The approximation is validated for real case study under high CO2 content, and is considered also valid in the transportation of higher amounts of CO2, for example, in CCUS activities.
Fundamental aspects of the corrosion of N80 steel in a formation water system under high CO2 partial pressure at 100 °C