PREDICTION OF EROSION-CORROSION PENETRATION RATE IN A CO_2 ENVIRONMENT WITH SAND

摘要

In oil and gas wells producing CO_2 saturated water, carbon steel piping can corrode rapidly depending on the nature of the hydrocarbon phases produced and on flow and environmental parameters. In some CO_2 environments, a protective iron carbonate scale can form on carbon steel piping walls and reduce corrosion rates to within acceptable design limits. If sand is also being produced, protective scales can be removed or prevented from forming on piping walls at points where sand particles entrained in the flowstream 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 is often referred to as "erosion-corrosion." In some cases, pitting occurs at impingement points resulting in extremely high penetration rates. A laboratory flow loop circulating a CO_2 saturated NaCl solution and sand has been employed 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 CO_2 corrosion are used to apply the results of the research to the computation of "threshold velocities" - flow velocities below which protective scales keep corrosion rates low, but, above which erosion of protective scale enables localized, high, bare-metal corrosion rates. Once it is known whether the threshold velocity is above or below an intended flow velocity, erosion and corrosion penetration rates can be predicted using published procedures.