An Integrated Approach to Accurate Corrosion Prediction

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 10675, "Accurate Corrosion Prediction Through an Integrated Approach," by R.V. Reddy, J.L. Nelson, and J.L. Pacheco, ExxonMobil, prepared for the 2005 International Petroleum Technology Conference, Doha, Qatar, 21–23 November. The ability to optimize carbon-steel use in corrosive service presents many economic advantages including minimizing use of expensive corrosion-resistant alloys (CRAs) and reducing well count. The full-length paper details how an integrated approach to corrosion modeling and testing is used to evaluate the use of carbon steel in oil and gas production environments. Introduction The ability to use carbon steel in corrosive service presents significant economic advantages, primarily in a reduction in capital expenditures (CAPEXs). Another advantage of carbon steel is its availability in a large variety of product sizes and forms. The methods by which carbon steel and other material options are evaluated are critical to ensure long-term reliability with minimal life-cycle cost (LCC). An integrated approach to materials and corrosion engineering is necessary to identify opportunities and optimize CAPEXs and operating expenditures (OPEXs) over the life of the asset. Material and corrosion engineers and chemists work together to design and execute care-fully designed laboratory test programs. Both design and operations personnel participate in the final material selection decision. Corrosion prediction presents many technical challenges, including identification and characterization of the corrosion mechanisms that are relevant to the application and extrapolation of relatively short-term laboratory-test results to long-term material performance. Corrosion prediction must account for changes in production conditions that occur over the full life cycle of the asset, such as pressure depletion, introduction of formation water, reservoir souring, field changes such as addition of new wells, process upsets, shutdowns, and infrequent planned events such as acidizing treatments. Integrated Approach Establish Environmental Conditions. The first step in the integrated approach to materials and corrosion engineering is to establish accurate environmental conditions by conducting thermodynamic and compositional hydraulic analyses and to characterize how these conditions are expected to change as the field matures. Reservoir-modeling and preliminary process-modeling results usually are available early in the design process. The results of these studies are used in thermodynamic and compositional hydraulic models to calculate the parameters of interest for corrosion modeling, including water-dropout locations and flow regimes. Such modeling is performed for the life cycle of the well using the best available information to identify locations of environmental conditions expected to cause the worst corrosion over the tubular life cycle. Identify Local Environmental Conditions and Corrosion Types. Expected Corrosion Modes. While general weight-loss corrosion is considered for all production environments, pitting also must be considered, especially in environments containing hydrogen sulfide (H2S). The relative importance of general weight-loss and pitting corrosion varies. For some H2S-containing environments, carbon-steel materials are considered to be suitable for sour service. However, for highly sour service, confirmatory tests are run to ensure that the selected carbon-steel grade is resistant to sulfide stress cracking (SSC) and/or hydrogen-induced cracking in the environments of interest.