In view of most wells are completed by casing pipe in oil & gas exploration and development process, it is imperative to extend the application of borehole acoustic reflection imaging (BARI) from open to cased boreholes. To understand the characteristics of signals in borehole acoustic imaging, we study the radiated and reflected elastic waves in an open and cased borehole through numerical simulations. The results show that, for a broadband dipole source, the radiated SH-waves have the highest amplitudes and broadest radiation angles in the cased borehole, which is similar to the observation in an open borehole. It is believed that the cement and casing pipe would prevent to a certain extent the energy excited by the inner borehole sources from propagating into the outside formation. However, our modeling results clearly show the existence of the cement and casing will not influence the amplitude of the radiated SH-waves significantly. In fact, the decrease of the borehole radius, which is caused by cased borehole completion here, will increase the amplitude of the radiated SH-waves. As a result, the signal-to-noise ratio (SNR) of reflected waves relative to direct waves in the cased borehole is higher than that in the open borehole. Meanwhile, the cement bond quality does not heavily affect the SNR of the reflected waves and the SNR increases with the decrease of source frequency. Interestingly, as the cutoff frequency of the dipole flexural-wave mode in the cased borehole increases rapidly with the decrease of the formation S-wave velocity, the amplitude of the direct wave also decreases significantly. This results in an increase of SNR of the reflected waves, which is beneficial for extracting the reflected waves. The field data example certifies our theoretical analysis. Our results provide a theoretical foundation for the through-casing BARI with a dipole source.

鉴于油气勘探开发过程中大多数井都是用套管完成的，因此将钻孔声反射成像（BARI）的应用从开放钻孔扩展到套管钻孔势在必行。为了解钻孔声成像中信号的特征，我们通过数值模拟研究了开放和套管钻孔中的辐射和反射弹性波。结果表明，对于宽带偶极子源，辐射的 SH 波在套管钻孔中具有最高的振幅和最宽的辐射角度，这与在开放钻孔中的观察相似。相信水泥和套管将在一定程度上阻止由内部钻孔源激发的能量传播到外部地层中。然而，我们的建模结果清楚地表明，水泥和套管的存在不会显着影响辐射 SH 波的幅度。事实上，这里的套管钻孔完井导致钻孔半径减小，会增加辐射 SH 波的幅度。因此，套管钻孔中反射波相对于直接波的信噪比 (SNR) 高于开放钻孔中的信噪比。同时，水泥粘结质量对反射波的信噪比影响不大，信噪比随着源频率的降低而增加。有趣的是，作为套管井中偶极弯曲波模的截止频率随着地层横波速度的减小而迅速增加，直波波幅也显着减小。这导致反射波的信噪比增加，有利于提取反射波。现场数据示例证明了我们的理论分析。我们的研究结果为具有偶极子源的穿套管 BARI 提供了理论基础。