The acoustic field in a cased hole is studied through numerical modeling by combining experiment measurement when the first and scond interfaces are bonded well. The effects of the density of the cement, the diameter and thickness of the steel pipe on the amplitude of casing arrival （ACA） are investigated, and a part of the numerical results are compared with the experimental results. These results show that the ACA decreases with the increasing density of the cement. There exists a large difference between the ACAs for the low- and normal-density cements. Therefore, the different standard should be taken in the bonding evaluation for ce- ments with different densities. As the thickness of the steel pipe increases while its diameter keeps as a constant, the arrival time of the casing wave remains unvaried, while the ACA in- creases. But, when the diameter of the pipe with a constant thickness increases, the arrival time of the easing wave is delayed, and the ACA decreases. As for three kinds of the steel pipe commonly used in oilfields, the relative amplitude of the casing arrival is larger in the big pipe. In addition, the numerical results of the varying trend of the relative amplitude of the casing arrival with the density of cements, on the whole, are in agreement with the experimental ones.
I present an algorithm that uses cross-dipole wireline data only in order to estimate the HTI stiffness tensor for sandstone formations under in-situ asymmetric lateral (azimuthal) stress conditions.The algorithm is based on the generalization of terms "excess compliance" and "fracture weakness" developed within the linear slip interface theory for fractured rocks and is applied here to describe the effect of grain contacts in loose sandstones.I introduce the term "plane of weakness" being oriented (aligned) orthogonal to theminimal horizontal principal stress direction in order to describe the overall effective weakness of sandstone caused by the different principal stresses.For the quantification of this phenomenon I use the anisotropic Gassmann model.As a result I am able to calculate a HTI stiffness tensor for the interval length of a saturated sandstone formation and the respective Thomsen's parameters.The input data required for these calculations have to be provided by wireline logging and will consist of porosity,density,P-wave velocity,fast and slow shear wave velocities and oil-water saturation ratio.The algorithm in its current form is applicable to sandstone reservoirs only.Its limitation is based on two assumptions,which state that all the measured anisotropy is induced by the present stress in sandstone and that the unstressed sandstone would be nearly isotropic.From a technical viewpoint this algorithm can be implemented fairly easily in data acquisition and interpretation software relying on correct estimation of anisotropy parameters.It is also cheap because it does not require any additional measurements apart from the cross-dipole 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.
