Borehole azimuthal acoustic imaging using 3D spatial scanning: Application in acoustic detection of nearby wells

At present, it is difficult to image small-scale abnormal geo-bodies near wells using borehole acoustic imaging, as information from scattered waves is neglected and that from reflected waves is mainly used, thus resulting in low imaging resolution. Imaging methods based on scattered waves can help obtain higher-resolution measurements. Herein, a 3D scattered wave spatial scanning imaging method is proposed to make full use of multi-mode scattered waves for imaging small-scale abnormal geo-bodies near the well. The 3D finite-difference time-domain method was used to simulate the acoustic fields for borehole azimuthal acoustic imaging with a fluid-filled target borehole near a measurement well, and the imaging method was applied for the acoustic detection of nearby wells based on forward modeling and field measurement waveform data. The results show that for the fluid-filled target borehole that is approximately parallel to and distant from the measurement well, the amplitude of the scattered PP-wave is the largest followed by the mode converted PS- and SP-wave; and the amplitude of the scattered SS-wave is the smallest, which is related to the radiation, scattering, and reception of the monopole acoustic field. In this case, the scattered wave from the target borehole can be regarded as a plane-wave. The spatial scanning imaging method based on single-mode scattered acoustic waves and the 3D plane-wave imaging method provide similar imaging results. The comprehensive use of multi-mode scattered acoustic waves can further improve the imaging signal-to-noise ratio and resolution. A field example was used to verify the correctness of the forward analysis and the effectiveness of the imaging method. This study will help improve the imaging resolution for the acoustic detection of nearby wells and the evaluation of downhole hydraulic fracturing.