Dehua Chen

Studies on phase and group velocities from acoustic logging

It is still argued whether we measure phase or group velocities using acoustic logging tools. In this paper, three kinds of models are used to investigate this problem by theoretical analyses and numerical simulations. First, we use the plane-wave superposition model containing two plane waves with different velocities and able to change the values of phase velocity and group velocity. The numerical results show that whether phase velocity is higher or lower than group velocity, using the slowness-time coherence (STC) method we can only get phase velocities. Second, according to the results of the dispersion analysis and branch-cut integration, in a rigid boundary borehole model the results of dispersion curves and the waveforms of the first-order mode show that the velocities obtained by the STC method are phase velocities while group velocities obtained by arrival time picking. Finally, dipole logging in a slow formation model is investigated using dispersion analysis and real-axis integration. The results of dispersion curves and full wave trains show similar conclusions as the borehole model with rigid boundary conditions.

Rough interfaces and ultrasonic imaging logging behind casing

Ultrasonic leaky Lamb waves are sensitive to defects and debonding in multilayer media. In this study, we use the finite-difference method to simulate the response of flexural waves in the presence of defects owing to casing corrosion and rough fluctuations at the cement-formation interface. The ultrasonic obliquely incidence could effectively stimulate the flexural waves. The defects owing to casing corrosion change the amplitude of the earlyarrival flexural wave, which gradually decrease with increasing defect thickness on the exterior walls and is the lowest when the defect length and wavelength were comparable. The scattering at the defects decreases the energy of flexural waves in the casing that leaks directly to fluids. For rough cement-formation interface, the early-arrival flexural waves do not change, whereas the late-arrival flexural waves have reduced amplitude owing to the scattering at rough interface.

Wave propagation in a fluid-filled shell excited by a dipole source

Wave propagation in a fluid-filled cylindrical shell excited by a dipole source is investigated for the design of a new kind of calibration pit for dipole acoustic logging tools. Based on classical elasticity wave equations, phase and group velocity dispersion curves of each mode, excitation spectra and mode contributions to wave field in the shell are presented. The dispersion curves show that the lowest mode exists in the entire frequency range with the phase velocity smaller than shear wave velocity of the shell, higher-order modes have cutoff frequencies, below the cutoff frequencies these modes are non-propagating. Analyses on mode excitation spectra and their contributions suggest that the lowest mode has potential to be used in calibrating the velocity measurement accuracy of dipole acoustic logging tools quantitatively, provided that the contributions of this mode to the wave field dominating the first arrivals gathered by a specific dipole acoustic logging tool.

Axisymmetric wave dispersion in outside traction-free fluid-filled cylindrical shells

In order to develop a kind of standard scale pits, studies on acoustic wavefield in outside traction-free fluid-filled cylindrical borehole are necessary. This article will calculate and analyze the dispersive characteristics of the symmetric mode wave in the cylindrical waveguide structure. Numerical results show that, the dispersive curves of the outside traction-free fluid-filled cylindrical shells are the coupled dispersive curves of the cylindrical shell in vacuum and the fluid cylinder. The factors influencing the dispersive characteristics of the cylindrical shell in vacuum and the fluid cylinder influence the dispersive characteristics of the outside traction-free fluid-filled cylindrical shell in the same way. This work provides some basic theory for the construction of a new kind of standard scale pits.

Wave simulations for cementing quality sonic logging using a finite difference method

Cementing quality evaluation is significant in oil production. We use high order staggered grid finite difference method to simulate borehole acoustic field of different cement cases. First, results from the finite difference method and real axis integration for the open-hole case with fast formation are compared. Then, using the plane wave pulse source, the received waveforms of cases with different media outside the casing are calculated. Numerical results show that the various modes of wave amplitudes are very different when the media is different. Our numerical results show that, as acoustic impedance of cements decreases, the amplitude of S0 Lamb mode increases, whereas amplitude of A0 Lamb mode decreases.

Research on sensitivity of dipole receiving transducers

The dipole receiving transducers play an important part in Cross-dipole array acoustic well logging tools. It is necessary to study the properties of the dipole receiving transducer, especially the sensitivity of it. In this paper, transducers in two ways of wiring, including parallel connection and series connection, were considered numerically and experimentally. The sensitivities obtained by numerical and experimental method respectively were contrasted. The results show that as a receiving transducer, the receiving sensitivity in series connection is higher, which can guide us in the dipole receiving transducer design. Keywords: dipole, receiving sensitivity, receiving transducer

Effects of eccentric acoustic source on the amplitude and arriving time of the first arrival in cased boreholes

The acoustic fields excited by eccentric acoustic source in cased boreholes bonded by cements with different densities are simulated numerically using 2.5-dimension (2.5D) finite-difference method (FDM). The effects of the source eccentricity on the amplitude and arriving time of the first arrival (FA) in the full waveform are investigated. The numerical results show that the amplitude of the FA will decrease quickly and its arriving time will go ahead as the source eccentricity increases. The change of the arriving time can be estimated approximatively using geometrical acoustics theory. As the eccentric distance of the acoustic source reaches 1/4 of the borehole radius, the amplitude of the FA will reduce to below 20% of the centered case. The quantitative varying trends of the amplitude and arriving time of the FA are the same both in the free pipe and in boned pipe. Therefore, during the cementing evaluation, the tools eccentricity should be estimated through the arriving time of the first arrival, a...

RESONANT FREQUENCY SHIFTS OF A FLUID FILLED CAVITY CAUSED BY A BUBBLE

In the previous studies for estimating acoustic wave velocities and attenuations of a rock specimen in a low frequency range using an acoustic resonance spectroscopy method, it was found that bubbles in a fluid filled cavity reduce the resonant frequency of the cavity significantly, which makes the measurement unstable. In this paper, this phenomenon is explained by using a simple model of a spherical fluid filled cavity with a single air bubble. It is pointed out that air bubble effects are caused by the vibration of the bubble coupled with the vibration of the cavity and, therefore, the measurement must be carefully prepared to prevent any air bubbles from entering the cavity.