Response characteristics of monopole sonic log in double casing strings

In the well plug-and-abandonment processes, an evaluation of the well's hydraulic barriers is one of the key technologies at the time of operation and the logging data can provide this important information. The existing logging tools for cement bonding quality evaluation are mainly designed for single casing geometries and they may not be able to distinguish the bonding condition between the out casing and formation. In this paper, the response characteristics of monopole array acoustic logging in double casing strings are simulated by three-dimensional finite difference method, and the feasibility of identifying whether the annulus between outer casing and formation is cemented is discussed by using the dispersion characteristics of the casing waves and the Stoneley waves. The numerical results show that while the Annulus between inner and outer casing is filled with cement, the amplitude of the first arriving casing wave is closely associated with the acoustic impedance of the medium filled in the Annulus between the outer casing and formation. If the annulus between inner casing and outer casing is filled with mud, the first arriving casing wave cannot distinguish mud from cement in the Annulus between outer casing and formation. But the amplitude and slowness of the Stoneley waves (denoted by inner Stoneley wave and outer Stoneley wave) show significant difference caused by acoustic impedence contrast of the annulus between outer casing and formation. Compared with mud in the annulus between out casing and formation, both the inner Stoneley-wave amplitude and the outer Stoneley-wave slowness increase substantially with cement in the annulus. The dispersion curves of inner and outer Stoneley waves extracted from simulated waveforms agree with the analytical results. The numerical modeling results provide a theoretical basis for using the casing-mud interaction modes to evaluate the cement bonding in double casing strings and the sonic full-wave logging has great potential in cement bonding evaluation.