Study Of Sweet Corrosion In Horizontal Multiphase, Carbon Steel Pipelines

Abstract

ABSTRACT Sweet corrosion in carbon steel pipelines carrying oil-water mixtures has long been a problem in the oil industry. A study to determine the corrosion rates and the type of deposits formed on the pipe surface under such multiphase flow conditions is described. Experiment are performed in a 10.16cm ID flow loop. Full pipe flow and slug flow conditions and the effect of a few inhibitors is studied. Temperatures upto 60C and several pressures, flow rates, oil-water fractions are studied. It is observed that non protective iron carbonate scales are formed below 60C. Iron carbide skeleton which emerges from the carbon steel is detected. Corrosion rates increase with an increase in temperature and carbon dioxide partial pressure and liquid flow rates. With an increase in oil/water fraction, corrosion rates increase upto 60% oil, for the conditions studied. Iron carbonate crystal deposition increases with temperature and pressure. Evidence of bubble collapse and localized corrosion is observed under slug flow conditions. INTRODUCTION Corrosion related problems in oil-gas production and processing operations result in millions of dollars each year in down time, lost production and damaged pipelines. With the use of enhanced oil recovery techniques, carbon dioxide corrosion in oil-water pipelines has become common causing much concern in transportation of these multiphase fluids over long distances from remote wells to separation sites. The use of corrosion inhibitors, surfactants and drag reducing agents to reduce the corrosion has met only with partial success. The effectiveness of these corrosion reducing agents depends on the flow regimes existing in the pipeline. Several workers have studied carbon dioxide corrosion in carbon steel pipelines. However most of the studies were carried out in single phase systems using deionized water, or brine solutions saturated with carbon dioxide. Almost all the previous studies were conducted in rotating cylinder electrode equipment and in autoclaves. Recently some research was conducted in small diameter, single and two phase horizontal pipelines. However the flow mechanisms in small diameter pipelines, i.e. less than 5 cm can be different from those existing in 10 cm and larger diameter pipes. The effects of different flow regimes such as stratified, plug flow and slug flow have not been addressed. Hence, any extrapolation of results from single/two phase small diameter pipelines to multiphase larger diameter ones can lead to grave errors in prediction of corrosion rates. Green, Johnson and Choi1 have shown the significant effect of the different flow regimes on corrosion rates. In slug flow, the instantaneous corrosion values obtained when a slug passes were at least two orders of magnitude larger than those existing under stratified flow conditions. Many investigators have studied the chemistry of carbon dioxide corrosion in water and have determined iron carbonate as the main corrosion product.