Barrier Verification During Plug and Abandonment Using Spectral Noise Logging Technology, Reference Cells Yard Test

Abstract

With the onset of offshore decommissioning resulting in an increasing number of permanent plug and abandonment (P&A) campaigns it is critical to manage these in a safe and cost-effective way. A robust methodology is important to manage the magnitude of wells in Norway to be P&A’d including the need to comply with Norsok Standard D-010 (Norsok, 2013). Many technologies have come to the forefront to aid with safe permanent well abandonment, with different criteria to be met, principally by verifying the integrity of the casing and cement barriers across the zones that are plugged. To inspect the cement barrier interval, spectral noise logging was utilized to investigate noise generated by flow in fluid and gas phases. A practical test in a controlled environment was required to document the sensitivity of the spectral noise logging techniques ability to detect liquid and gas flow for different annular cement quality scenarios. Full-scale barrier reference test cells were constructed to represent a range of cement defects such as gas channels, mud channels and de-bonded cement. These test cells have been utilized to evaluate the performance of well barrier verification technologies. The measurement resolution of spectral noise logs to detect flow behind casing was investigated by flowing water and gas through the different leakage paths. The flowrate range used for this experiment was between 1 and 1300 mL/min for water and between 1 and 24 L/min for gas (N2). The experiment was performed in a low noise environment to avoid ambient noise sources that might compromise the experiment. P&A regulations and recommended practices were considered when developing the test procedure. Controlled annular leakage rates of water and gas were established using barrier reference cells representing good cement, no cement, gas channels and micro-annuli. Spectral noise logging measurements were recorded using a memory based, commercially available logging tool and the results were analyzed. The data showed the spectral noise logging tool was able to detect the noise generated by low flowrates of water and gas through different leakage paths. The yard test results provide a basis for considering noise logging and its spectrum analysis as an alternative means or an addition to currently used cement bond logging technologies for cement barrier verification. The applicability of utilizing spectral noise logging as an additional proof of annular barrier integrity was demonstrated.