Premature failure of tubing used in sweet Extra Arab Light grade crude oil production well

A 30 feet long and 3.5in. diameter joints tubing used in wet sweet Extra Arab Light grade crude oil production well failed after 3years of service. The bottom-hole temperature and pressure were, 82°C and 170atm, respectively. The tubing had a nominal wall thickness of 6.45mm and was located at starting depth of 1465m. The average production rate and water cut of the oil well during the last 3years of operation were 8761 BPD (barrel per day) and 7.72%, respectively. Failure occurred due to corrosion resulting in leaks of three out of 5 consecutive joints of the tubing. On the other hand, rest of the 120 joints did not show any leaks. All failures occurred close to the joints. Various sections of the failed joints were metallurgically evaluated using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The corrosion product was identified using photometric and potentiometric analysis combined with X-ray diffraction. Gas and water samples obtained from the production well were also analyzed. The material of the production tubing was identified as API 5CT grade J-55 carbon steel. Visual inspection revealed plastic deformation of tubing alloy at localized regions and internal corrosion as well as localized pitting at failed locations. Evidence of external corrosion was not found. Predominant mode of failure was sweet CO₂ corrosion in the form of mesa attack combined with chloride corrosion. Operating conditions of low pH, high pressure and high temperature along with presence of organic acid is thought to be responsible for the observed mesa attack. Replacement with a super 3-13% Cr production tubing and application of cathodic protection were suggested as a permanent solution to the corrosion problem whereas a number of short-term measures were also recommended.