The Investigation Of Finite-difference Method Modelling Acoustic Logging In Cartesian Coordinates

Acoustic well logging is one of the important well logging methods in oil well logging.It can be used to explore the site and the character of oil and gas by continuing record and instant acoustic source pulse which is excitated in the well and affected by formation.Its theoretic basement is the excitation and propagation of the elastic wave in cylindrical multi-layer medium and what the most interrelated is the acoustic wave. The mechanic of excitation and propagation of borehole acoustic wave is closely interrelated to the characteristics of the formation,which is one of the important aspect in the theory research of the borehole acoustic wave. Which involves different types of medium :for example ,quasielastic medium and anisotropic medium,different types of acoustic sources:for example,monopole and multiple,different ingredient wave:for example.head wave,transverse wave and all sorts of modes.The theory and the explaining method of acoustic wave logging become more and more important in complicated borehole environment since the developing demand of complicated oil reservoir,gas prospecting and particular explaining. As the complicated of the practice formation and the borehole environment,to apply the pure numerical method-finite-difference method to solve the borehole acoustic wave,is one of the important and effective means in the forward research of acoustic well logging.In this paper, the research work is developed around the finite-difference technique about borehole acoustic wave field in Cartesian coordinates. Utilizing the horizontal well logging technique to improve the output of oil have been extensive taken in the worldwide range.However the well logging interpretation and evalutated technique of horizontal well and incline well are shortage that have blocked the development of this new technique in oil industry. Aiming at that there are interface outside the horizontal well paralleling the axis of the well,circular borehole at cross section,and plane interface,taking the finite-difference method numerical modeling acoustic field inside and outside the well in cartesin coordinates to deal with the complicated practical well logging conditions. At first investigating the nonsplitting perfectly matched layer NPML method for the finite-difference simulation of elastic wave propagation.In the two-dimension cylindrical coordinates using velocity-stress finite-difference method to model the full waveform of the homogeneous water inside and outside the well. The result is right by comparing with the analytical method(real axis intergration,or rai). Then change the parameter and simulate the open well. The result is right too by comparing with the rai and prove the NPML method. The NPML requires practically the same amount of computer storage as for SPML,assuming that the SPML is used only for the boundary region. In this paper all absorbing boundary layers are nonsplitting perfectly matched layers. Then using a three-dimensional vs-fd method to simulate the acoustic fall waveform of the horizontal well. Investigating the homogeneous water and homogeneous solid inside and outside the well,open well and borehole with a nearly and parallel bed boundary in x-axis direction. The result of homogeneous water , homogeneous solid and open well testifying the method is right by comparing with the rai. The parallel bed boundary is soft formation. Chosing the different center frequency to model the full waveform field of the horizontal well. The bed boundary compressional headwave can't be masured because of the limit of computer storage and only receive the furthest waveform at 1.2m receiver offset the source. Using staggered grid and high-order finite differene operator(eighth-order accurate space,second-order accurate time). The higher-order finite difference operator can reduce the error and staggered grid can make equation automatic accord with the interface of the fluid and the formation. However the practical calculation is not so. Some shuld be done to promise the stability of the equation. Monopole source is used in the Cartesian coordinates and the source pulse is introduced into the vs-fd calculation...