Field Experience: Identification of the Potential Mechanism of Flow Assisted Corrosion Due to a Large Viscosity Contrast in Biphasic Flow of Heavy Crude Oil

Unexpected very fast line piercing was discovered in an onshore field producing heavy oil with low GOR in Africa. Corrosion started seven months after the first evidence of water production (500 mpy) with the piercings affecting the lines at the 6 O’clock position. The carbon dioxide content of the associated gas (15%) was first suspected to be the major cause of corrosion inspite of the existing corrosion mitigation program. Similar leaks continued to develop in other lines although the corrosion rates measured in the inhibited produced water with ER probes were low. Cut sections of the lines revealed local piercings with smooth shapes or grooves at 6 O’clock. Through continuous trial and error, it was discovered that the corrosion rate was accelerated by unforeseen high velocity of the free water as a result of the large viscosity contrast between the oil and the water.
This paper describes the history of the field, the corrosion damage and the subsequent corrective measures adopted. This hypothesis of flow assisted corrosion due to high water flow rate occurring at the bottom of crude oil transportation pipes with the absence of a gas phase is proposed.