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Ensuring the integrity of oil and gas cased-wellbores is of crucial importance to operators around the world to maintain safe production and environmental compliance. While sonic and ultrasonic measurements have been developed for evaluating the well integrity of single casing strings, there is currently a gap in characterizing the cement and annular fill behind two or more casing strings due to the lack of sensitivity of the ultrasonic tools and the complexity of the sonic response. This abstract describes an experimental study to characterize the sonic response and help fill this gap in conjunction with a theoretical modeling effort that is reported elsewhere. Scaled laboratory experiments are conducted to acquire reference data used to verify the modeling approach developed to predict the guided modal characteristics of axially-propagating waves in concentric and non-concentric cylindrical structures immersed in fluid. These structures simulate the geometries and environment encountered downhole when conducting acoustical measurements to evaluate the cementation of oil and gas cased wells. Typically, cement is expected to fill the annular space between the outer string and rock formation. Nevertheless, cement damages such as fluid channels, partial cement coverage, and weak bonding interfaces, which can compromise well safety as well as environmental compliance, inevitably occur due to the harsh downhole environment and downhole operations. Measurements are made in a dual-string steel pipe of quarter scale in an immersion tank or in a sandstone formation with concentric as well as a variety of eccentric configurations. A set of samples are prepared with the cement in annulus B (the space beyond the outer casing) with purposefully introduced fluid channels in a controlled manner. A comparison of the experimental and numerical results indicates features in the sonic response that are indicative of channels in annulus B as distinguished from eccentricity effects. These results suggest a new well integrity evaluation work-flow to characterize cement quality and coverage in annulus B taking advantage of the rich acoustic features in the sonic response. [ABSTRACT FROM AUTHOR]

Once an oil and gas wellbore has been drilled, steel casings and cement slurry are placed to ensure structural support, protection from fluid invasion, and most importantly to provide zonal isolation. The actual wellbore and string structure is rarely concentric but rather is often an eccentric one, especially in deviated boreholes. The term “eccentricity” is used to describe how off-center a casing string is within another pipe or the open-hole. In a typical double-string configuration, the inner casing is eccentered with respect to the outer string which itself is also eccentered within the cylindrical hole. The annuli may or may not be filled with solid cement, and the cement may have liquid-filled channels or be disbonded over localized azimuthal ranges. The complexity of wave propagation along axial intervals is significant in that multiple modes can be excited and detected with characteristics that are affected by the various parameters, including eccentering, in a non-linear fashion. A successful diagnosis of cement flaws largely relies on a thorough understanding of the complex acoustic modal information. The present study employs both modeling and experiments to fully understand the acoustic wave propagation in the complex, fluid-solid nested, cylindrically layered structures, with geometric eccentricities. The experimental results show excellent agreement with the theoretical predictions from newly developed, borehole acoustic modeling approaches. As such, it provides the basis for better understanding the operative wave physics and providing the means for effective inspection methodologies to assess well integrity and zonal isolation of oil wells. [ABSTRACT FROM AUTHOR]