Prediction of Erosion-Corrosion Penetration Rate in a Carbon Dioxide Environment with Sand

In oil and gas wells producing carbon dioxide (CO₂) with saturated water, carbon steel piping can corrode rapidly depending upon the nature of the hydrocarbon phases produced and on flow and environmental parameters. In some CO₂ environments, a protective iron carbonate (FeCO₃) scale can form on carbon steel piping walls and reduce corrosion rates to within acceptable design limits. If sand also is being produced, protective scales can be removed or prevented from forming on piping walls at points where sand particles entrained in the flow stream impinge the walls. When this happens, bare metal corrosion is enabled at these impingement points, and corrosion rates again reach high levels. This process involves both erosion and corrosion mechanisms and often is referred to as “erosion-corrosion.” In some cases, pitting occurs at impingement points, resulting in extremely high penetration rates. In the present work, a laboratory flow loop circulating a CO₂-saturated sodium chloride (NaCl) solution and sand was used to identify conditions defining the boundary between scale formation (low penetration rates) and loss of protective scale by sand erosion (high penetration rates). Published procedures for predicting erosion and CO₂ corrosion were used to apply the results to the computation of “threshold velocities,” or flow velocities below which protective scales keep corrosion rates low but above which erosion of protective scale enables localized, high, bare-metal corrosion rates. It was shown that erosion and corrosion penetration rates can be predicted using published procedures once it is known whether the threshold velocity is above or below an intended flow velocity.