Wellbore barrier integrity and design optimization: A case study on CO₂ huff and puff well

As the major oil fields in China step into the middle and late stages of development, the wellbore integrity accidents caused by the " secondary oil recovery” of old wells and even the " enhanced oil recovery” happen frequently. The on-site integrity construction of old wells is insufficient, and the harsh working conditions brought by the stimulation measures leads to serious corrosion of pipe strings and threatens the service life of pipe strings and the wellbore integrity, so as to bring about great economic loss to the oilfields while resulting in the hidden troubles to the safe operation of oil fields. The deficiencies of existing wellbore integrity standards and the important research directions were analyzed based on the development status of domestic oil and gas fields. Then, the concept of corrosion integrity management was introduced, and the wellbore barrier optimization method based on the dual action of wellbore corrosion risk and load risk was proposed with CO₂ huff and puff well as the example. Finally, the failure sensitive region and service life of huff and puff well were determined. It is indicated that limit huff and puff cycles of the case well is 7, the tubing failure sensitive region is 800-1 500 m and the casing failure sensitive region is 1 700-2 000 m. It is suggested to carry out important protection in the failure sensitive regions. The research results are of great significance to design the wellbore barrier, prolong the workover interval of old wells and establish the scientific, applicable and beneficial integrity management system according to the idea of " global control, key protection and economy and efficiency” based on the actual situations of each oilfield.