Erosion-corrosion for carbon steel in sweet production with sand: Modeling and experiments

In the oil and gas production industry, carbon steel tubing and piping are susceptible to erosion-corrosion damage due to the erosive and corrosive nature of the flow. The combined effect of sand erosion and corrosion can be very significant. One form of erosion-corrosion of carbon steels occurs when impinging sand particles remove part or all of a protective iron carbonate (FeCO3) scale allowing corrosion rates to increase to bare metal rates. The role of a FeCO3 layer in reducing corrosion rates in sand–free environments has been studied by many investigators. However, the protection offered by FeCO3 scale when sand is produced is not well defined. A mechanistic approach for predicting metal loss due to sand erosion and CO2 corrosion of carbon steel was developed in the research presented in this thesis. The main contributions of the research were to develop: (1) a mechanistic model of the competition between FeCO 3 scale growth by precipitation and scale removal by erosion; (2) a procedure for predicting erosion-corrosion rates in oil and gas production and transportation systems; and, (3) a computer program to facilitate the prediction of the erosion-corrosion rates. Models from the literature for quantifying iron carbonate scale precipitation and growth rates, and diffusion rates of cathodic reactants and corrosion product species through iron carbonate scale were adapted to this purpose. The solid particle erosion resistance of FeCO3 scale produced under a range of environmental and flow conditions was characterized by direct impingement experiments. Dry and wet FeCO3 scales were subjected to direct impingement by sand at various impingement angles. Scales were pre-formed in a flow loop at 150-200°F (65-93°C), from 6.1-6.5 pH, and 2.4 bar CO2 pressure and then removed from the flow loop for direct impingement testing. The erosion pattern of the scale was characterized by scanning electron microscopy (SEM). Specimens with iron carbonate scale were partially eroded with sand at an impact angle of 60 degrees and then examined using SEM to study erosion mechanisms. Literature and laboratory results, along with a CO2 corrosion rate prediction model and a sand erosion rate prediction model developed by the Erosion/Corrosion Research Center (E/CRC) at The University of Tulsa, were integrated into a single mechanistic model for predicting erosion-corrosion in multiphase flow under steady-state conditions. A research version computer program named SPPS:E-C was developed for this purpose. Single phase erosion-corrosion measurements were compared to predictions obtained by SPPS:E-C. The comparison showed encouraging results.