Reducing liquid layer ambiguity of well integrity measurements through extensional mode analysis

Acoustic non-destructive evaluation is widely used in the oilfield industry to evaluate the state of the material behind the steel pipes, called casings, which are used in the construction of subterranean oil and gas wells to retrieve hydrocarbons. The annular region between a casing and the surrounding geological formation is usually cemented to provide mechanical support for the casing as well as to ensure zonal isolation between different geological layers. The proper placement of the cement is typically evaluated using sonic and ultrasonic techniques. However, these measurements are often perturbed in the presence of a liquid-filled microannuli, a thin layer of liquid between the pipe and the cement, which renders the interpretation prone to ambiguity and error. To reduce uncertainties in the presents of microannuli, we investigate the benefits of supplementing the industry-standard thickness mode resonance technique with a measurement using the zero-order extensional mode. We built a full analytical model of a cemented borehole which included all the layers present and allowed to finely tune the boundary conditions between them. To validate this model, we developed an experimental apparatus and a set of procedures to produce highly controlled cases of liquid-filled microannuli in the lab. Finally, we benchmarked the results of the model against these experimental scenarios. The validity of the analytical model was evaluated through the comparison with several experiments. We showed that the model's predictions were consistent with the experimental results and allowed a comprehensive assessment of the presence of liquid-filled microannuli. Using our modeling approach, we further demonstrated that the presence of a liquid-filled microannulus lead to a much stronger response of the zero-order extensional mode (compared to the other techniques) due to its high sensitivity to shear coupling between the steel and the annular material. As a result, we propose to complement the current pulse-echo measurement with a pitch-catch measurement based on the zero-order extensional mode of the pipe which allows a much easier and less ambiguous detection of liquid-filled microannuli.