Study of Sweet Corrosion at Low Water Cut in Multiphase Pipelines

Abstract Large diameter, long distance multiphase flow lines are now the predominant trend in oil and gas production. Economics demand the use of carbon steel as the material of construction. However, carbon steel is prone to corrosion from the flowing mixture of oil/water and gas. The prediction of the onset of corrosion with increasing water cuts is very important in the design and operation of the flow lines. A study of corrosion for high pressure, large diameter pipelines under horizontal and 2 degrees oil/water/gas multiphase flow conditions was carried out. The water cuts studied were 10%, 20% and 30% ASTM saltwater. A light oil with viscosity of 2 cP was used as the oil phase. Carbon dioxide gas was used in the gas phase. Temperature was maintained at 40 °C. The superficial gas velocities ranged from 1 m/s to 9 m/s under pressure of 0.27 MPa, 0.45 MPa and 0.79 MPa. The predominant flow was found to be slug flow and all corrosion experiments were carried out in this regime. The results show that measurable corrosion still occurs even at these low water cuts. For example, at a carbon dioxide partial pressure of 0.79 MPa for 30% water, the corrosion rate increases from 2.2 mm/year to 3.3 mm/year with an increase in Froude number from 6 to 12. The corrosion rate increases with an increase in slug frequency at the same Froude number. At a pressure of 0.27 MPa, Froude number of 6 for 30% water cut, the corrosion rate increases from 0.8 mm/year to 1.4 mm/year with increase in slug frequency from 6 slugs/minute to 38 slugs/minute. At a Froude number of 12, the corrosion rate increases from 1.2 mm/year to 2.3 mm/year with increase in slug frequency from in slug frequency from 6 slugs/minute to 43 slugs/minute. At a pressure of 0.27 MPa, Froude number of 6 for 10% water cut, no corrosion was observed in horizontal pipes.