Solving a Mysterious Casings Failure Problem in ESP Fitted Production Field

摘要

The Petroleum Development Oman (PDO) operates assets worth US$18 billion in South of Oman. The majority of the producing wells, nearly 60% of them, are highly oil producer when fitted with ESP completion. The intending desired operating life of these wells is greater than 25-years. Field histories showed that 5 casings failure per year in Field X were mainly directly attributed due to onset of severe electrochemical corrosion. A field campaign of downhole logs recorded throughout the field indicated a potentially more severe risk of failures prevailed for outermost barriers within its initial 3 to 5 years of the production life. This sleeping threat was mainly due to setup of long line corrosion cells activity on well casings due to their direct contact with aquifers. The associated corrosion rate found to be abnormally high. The concentric casings set across these aquifers showed generalized wall thinning; which was not confined to any single aquifer; rather entire well structure failure saw across several consecutive aquifers. This observation was in contrary to mostly prevailing phenomena of Middle Eastern historical fields where usually a single aquifer acts as anode. A few wells in Field X failed at much deeper, across unexpected formation zones, creating a mystery about the corrosion mechanism. High rate of casing failures in closely spaced wells left no room for infill drilling. Since the asset was contributing a prime role in Oman’s economy such high number of well failures was serious concern for the operator. Therefore, there was a strong need to put an end to casing failures after reaching out to the root cause of mechanism triggering these failures. In an attempt to solve the problem acquired well integrity logs were uploaded into a dynamic multiwell data base to study; wall loss correlation with formations, affect of completion on downhole corrosion. Moreover, it was intended to find any well to well correlation if any; and to study aerial and lateral trend of corrosion and rate. The outcome of this exercise indicated strong correlation between external corrosion and wells fitted with downhole ESP motors and where water cut was high. Various ultrasonic imaging logs acquired for production casing indicated a direct relation seen between power cable insulation degradation; resulting in leakage/stray currents to flow on casing; and severe damage to well structures. To explore further in that direction downhole investigation trials were conducted with potential profile logs on several wells to depicate the axial current profile changes when electrical submersible pump motors of nearby offset wells were turned ON and OFF. Trials confirmed ESP role and a very striking correlation seen between observed axial current profiles against recorded external corrosion on well integrity surveys. The study suggested that under ideal condition ESP motors were not contributing to well integrity issues. However, a substantial increase in stray current leakage from ESP power cable and motors was seen those depicated a phase to ground power imbalance. Therefore, it can be that the ESPs have the capability to produce the driving force, necessary to cause a significant amount of metal loss on the motor housings and well casings. The finding of this study has strong binding not only to integrity assurance of thousands of wells in Oman and also aboard. This study will help other operators to safely maintain the integrity of their large assets when wells are fitted with ESP completion in Middle East.