Detection of Microannulus by the use of Acoustic Measurements
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Well integrity is crucial to ensure safe operations over the lifecycle of a well. Poor cement bonding between casing and cement is a common problem, yet crucial for the wells integrity. If neglected, it could function as a possible leak path for reservoir fluid migration. Current techniques for cement bond verification have proven limited when investigating the cement quality in the presence of a microannulus. In many cases, they lack the ability to quantify the thickness of the debonded cement sheath interface, and its significance. Due to these limitations, a new concept for cement-to-casing bond logging tool is being proposed. This thesis is a combined literature study and experimental study of techniques for cement evaluation. The bonding between casing and cement is emphasized, while detection of microannulus is set as the main focus. The existing acoustic techniques for cement evaluation have been highlighted, and both their advantages and disadvantages have been discussed. This thesis also includes description of both the set-up and the main components used during an experimental test of a novel well logging tool, Annulus Verification Tool. The objective of this logging tool is to both detect and quantify the size of a microannulus. This will be achieved by applying an outward linear mechanical force to the casing installed in a test rig, where the resulting displacement will be measured. In addition, this thesis includes theory regarding ultrasonic transducers, combined with an experiment to test its ability to detect debonding between steel and cement. The ultrasonic measurements were conducted on customized samples from the laboratory with pre-installed microannulus. Results from the literature study show that a sonic log is not sufficient for cement evaluation in the presence of a liquid filled microannulus, and that ultrasonic logging provide more accurate measurement over the complete circumference of the cement sheath, as it applies rotational subs. Nevertheless, applying a combination of both sonic and ultrasonic tools has proven to be a suitable solution for cement evaluation. From the sample preparation it appears that obtaining good quality cement without use of any additives to the cement slurry is challenging. The ultrasonic transducer proved to be excellent for thickness measurements of material with fine grains such as steel, while less so for materials with coarse grains such as cement. The transducer also provides the opportunity of identifying the substance behind the material, as long as it differs from the material itself. Results from the experiment also reveal that the transducer requires relatively good bonding between two materials for the emitted signal to transfer through the interface. There is a lack of thorough published research regarding the impact microannulus has on the well integrity. For future work, the author recommends that current procedures applied for zonal isolation verification should be investigated. Additionally, the author recommends that sensors should be specifically designed for the purpose of detecting microannulus, and be tested on cement samples to analyze if ultrasonic transducers are convenient for this purpose. Ultrasonic transducers should be applied on cement samples that have sufficient bonding between the materials involved, for instance by use of epoxy.