Performance of a modified particle swarm optimization in determining the thickness of the liquid layer outside a well casing pipe

Abstract

The liquid layer outside a well casing develops from flaws along the cement–casing interface, which have been identified as leakage pathways in wellbores. The thickness of the liquid layer is one of the important parameters for determining cement bond quality. However, it is difficult to accurately determine the thickness of the liquid layer outside a well casing pipe with the traditional ultrasonic pulse echo method. To address this problem, we propose a novel inversion method for determining the thickness of the liquid layer between the casing and the cement sheath. According to test specimens, multi-layered structures are modeled to determine the overall reflected wave at the mud–casing interface. An improved particle swarm optimization algorithm is used to simultaneously inverse the thicknesses of the liquid layer, the casing, and the annular space between the casing and formation. Using synthetic data, this algorithm is more robust and stable when compared with standard particle swarm optimization inversion procedures. In addition, the influences of noise in the reflected wave and deviations of medium parameters on the inversion results of the liquid layer thickness are discussed. Finally, the method was used to determine the thicknesses of the liquid layer in test specimens. Numerical and experimental results demonstrate that the improved particle swarm optimization algorithm provides an effective approach to accurately determine the thickness of the liquid layer outside a well casing pipe.