Response Characteristics of Multipole Sonic Log in Double Casing Strings

Abstract

The method of using multi-layer casing to achieve interlayer isolation in deep wells has become very common, and the demand for cementing quality evaluation in multi-layer casing wells is becoming more and more urgent. However, there is no effective method for evaluating the cementing quality in multi-layer cased wells, especially when the production (inner layer) casing is eccentric. In this paper, the multipole acoustic wavefield is calculated in the double-cased well with the inner casing being centered and eccentric based on the Bessel translation addition theorem, aiming to explore the feasibility of using multipole array acoustic logging for evaluating the cementation quality between the outer casing and the formation in the double-casing wells under complex well conditions. The calculation results show that the non-axisymmetric wave field excited by the multipole acoustic source is very complex, and there are coexistence of multipole modes and mutual coupling of guided waves when the inner casing is eccentric. In addition, the velocity dispersion curves of the outer Stoneley waves and the casing flexural waves are closely associated with the acoustic impedance of the medium filled in the Annulus between the outer casing and formation, which can be used to evaluate the cementation quality of the outer annulus, and this phenomenon is not affected by the position and size of the inner casing. The analysis results of this study verified the feasibility of using multipole acoustic logging for cement evaluation in double-casing wells.