Servicing oil and gas wells requires their integrity assessment both during operation and before abandonment. One of the main objectives in well integrity analysis is the location of metal losses in tubing and/or casing caused by corrosion, erosion or other types of pipe damage. The first three metal barriers (normally tubing, production casing and intermediate casing) are of the most interest to the industry. Dual string completions is an additional complication to through-tubing assessment of the second and third barriers.
Magnetic Imaging Defectoscope (MID) is an electromagnetic scanning tool that recor
ds magnetisation decays induced by high-power electromagnetic pulses. Metal pipe barriers contribute to magnetisation decays at different times depending on their diameters, which makes it possible to differentiate each of them and determine their individual thicknesses. Thickness determination requires the numerical finite-element modelling of each recorded magnetisation decay and iterative fitting of the properties and thickness of every metal barrier to the actual tool readings. A data array can store hundreds of thousands decays, and the data processing optimisation loop therefore requires parallel computing with multi-core processors to process data within a reasonable time frame. The MID hardware and interpretation algorithm have been tested on multiple laboratory stands simulating various downhole multi-barrier completions from 2–7/8″ up to 13 3/8″ pipes with artificial defects of various shapes and sizes ranging from 7 mm to 140 mm.
This paper presents laboratory results and three selected field cases demonstrating the application of Magnetic Imaging Defectoscopy (MID) in single-string and dual-string completions for thickness evaluation of three barriers independently by a memory through-tubing survey.
Well W-01 with metal losses found at the same depth in the second and third barriers, i.e. 9 5/8″ and 13 3/8″ casings
Well W-02 with metal loss found in the second barrier, i.e. 9 5/8″ casing, in an interval containing two strings. This corrosion has been confirmed by a repeated MID survey after pulling out the completion
Well W-03 with through-hole metal loss found in the second and third barriers, i.e. large-diameter 9 5/8″ and 13 3/8″ casings, through a 7″ liner. This through-hole corrosion has been confirmed independently by High-Precision Temperature Logging and Spectral Noise Logging
The above metal losses have been located by through-tubing memory surveys in offshore wells that were to be abandoned. The results of the MID surveys were then used to design environmentally safe abandonment procedures.