Test Results of a Corrosion Logging Technique Using Electromagnetic Thickness and Pipe Analysis Logging Tools

Abstract

Summary Recent innovations in subsurface corrosion practices of the Arabian American Oil Co. (ARAMCO) have reduced logging and workover costs substantially and have permitted the detection of corrosion in the outer string of two concentric casing strings. At the request of ARAMCO, Schlumberger conducted test under both simulated and field conditions. Results showed that the data required to evaluate casing corrosion in a 7-in. × 9 5/8 -in. completion can be obtained during a single logging run using a 21.6-in. coil spacing electromagnetic thickness tool (ETT-A TM) sonde (as opposed to two runs with 17.6-in. and 21.6-in. sondes previously used). In addition, corrosion of the outer string of 9 5/8-in. or 13 3/8-in. casing can be detected by using the results of the ETT-A logs and pipe-analysis tool (PAT) logs or caliper logs. To date, the application of this technique has been very successful in ARAMCO's operations. Introduction For several years ARAMCO has used the ETT-A in conjunction with the multifingered caliper (MFC) tool for monitoring casing corrosion in single strings of both 7-in. and 9 5/8-in. casing. The results of these logs were often the basis for workover recommendations in injection wells and to a greater extent, when run in conjunction with workover operations on producing wells, the basis for additional operations to ensure good wellbore integrity. In the past, two separate logging runs were made to evaluate corrosion in each of the 7-in. and 9 5/8-in. casing strings. The 17.6-in. coil spacing ETT-A sonde was used in 7-in. casing while the 21.6-in. coil spacing ETT-A sonde was employed in 9 5/8-in. casing. The obvious disadvantage of this technique is the additional cost incurred in making two separate logging runs. An additional disadvantage is the added workover rig downtime. As corrosion in the near-surface outer string of dual casing strings increased, it became imperative to develop a technique by which corrosion in the outer string could be detected and corrected before complete breakdown of the casing resulted in underground communication or surface leaks. To this end ARAMCO approached Schlumberger to determine whether any of the corrosion monitoring tools (specifically the ETT-A) then in use could be adapted to detect corrosion in the outer string of two strings of pipe and to determine whether an optimal coil spacing existed that could be used in both 7-in. and 9 5/8-in. casing strings. Tests were undertaken by Schlumberger in their AlKhobar. Saudi Arabia, shops to determine the optimal coil spacing of the ETT-A and to confirm that this spacing could respond to the total metal of dual casing strings. Once this optimal spacing was determined, it would then be possible to use the ETT-A to check the total metal present, and when run in conjunction with the PAT or MFC, it would be possible to differentiate the corrosion affecting the inner and outer strings. JPT P. 801^