Production Tubing Integrity Evaluation for Complex Workover Well Operations in Niger Delta

Abstract

Abstract To successfully carry out a workover on a well that was completed more than 20 years ago as a dual string completion with a dual packer and two Permanent Packers is complicated. The presence of a slickline fish of unknown dimensions in hole further complicated the nature of the workover thus making it susceptible to failure during the retrieval of the tubings. A Tubing Integrity Evaluation Methodology (TIEM) was used to properly assess potential failure points and craft out execution strategy to mitigate them. The TIEM adopted in this paper involved carrying out a detailed tubing stress analysis on the completion tubings where axial loads exerted on the tubing; weight, buoyancy, point loads on crossovers, piston loads, ballooning; as well as triaxial stress effect of axial, radial and tangential stresses were all estimated. The yield strength and yield stress completion tubing were de-rated by 30 percent as a precautionary measure due to the age of the completion tubings. The maximum overpulls the tubings could sustain without failing by parting, axially or yielding triaxially were calculated. These overpull forces was compared with the force required to unseat the dual packer as well as snap out from seal assemblies of the permanent packers to decide whether the tubing was to be cut prior to pulling or not during the tubing retrieval operation. Several fishing scenarios of slickline and drill pipe fishing were analysed based on the location of the slickline fish. Based on the results of the detailed tubing stress analysis, the maximum overpull the short string and long string sections could sustain were 58,000 lbs and 23,000 lbs respectively while the maximum force required to unseat the dual packer was 50,000 lbs. Thus, it was decided to pull the short string within the overpull limit while the long string was to be cut above the packer prior to unseating the packer. The Tubing evaluation helped to properly assess potential failure points and craft out execution strategy to mitigate or avoid potential problems associated with the workover operations with unknown dimensions of fish in the wellbore.