Abandoning a well after it reaches the end of its life cycle requires a barrier evaluation operation across multiple strings. The traditional method for barrier evaluation requires exposing the outer casing by a cut and pull or section mill of the inner casing so that the outer casing can be logged, to verify the barrier seal. The cut and pull and subsequent inner casing/tubing recovery or section milling operations are costly and time consuming, often lasting for several days, thus increasing the overall plug and abandonment (P&A) cost and carbon emissions. This paper will describe a novel logging technology, allowing barrier evaluation of two strings simultaneously without the need to remove the inner string. The dual-string barrier evaluation technology is based on combining advanced ultrasonic multimodality physics with a multimodal deep-probing array sonic measurement providing the capability to map the axial and azimuthal material coverage in the first and second annuli simultaneously. The technology is applicable to evaluate tubing-casing dual-string or casing-casing dual-string environments.

Based on the new technology, operators are no longer required to remove the inner string to evaluate the outer string. This approach revolutionizes conventional operations by reducing P&A rig days to minimize costs and overall carbon emissions. Furthermore, when proactively performing dual-string barrier evaluation offline or rigless in a number of wells marked for P&A, operators can use the log data to optimize future rig-based operations, minimize contingencies, and possibly prioritize the rig schedule for specific wells only.

This paper discusses an offshore case study where the technology was used in two wells in the southern North Sea during the P&A phase. The measurements and observations with dual-string logging technology will be presented in addition to a validation exercise after pulling the inner string. These dual-string log measurements were subsequently used to streamline the abandonment program in real time resulting in minimized rig time and scope, subsequently reducing overall carbon emissions.

Even though the case study covers wells from a P&A operation, the dual-string technology is equally useful in other well scenarios such as sidetracks or even monitoring barrier integrity in production wells in long-term time-lapse well integrity monitoring.

The technology can evaluate different types of barrier material such as cement or formation (shale/salt) squeeze or barite sag in both the first and second strings. Formation squeeze from specific North Sea shale (Williams et al. 2009) or salt members has been tested to act as a competent barrier.