Numerical Simulation and Borehole Azimuthal Acoustic Imaging of Near-Borehole Caves in Formations with Axially Non-Uniform Wave Velocities

During the exploration and development of oil and gas fields, near-borehole formations exhibit considerable axial heterogeneity in elastic-wave velocity. However, existing borehole azimuthal acoustic imaging methods often ignore the effect of this heterogeneity on imaging performance and thus cannot accurately locate anomalous near-borehole bodies in formations with axially non-uniform wave velocities. Hence, a borehole azimuthal acoustic imaging method based on ray tracing and spatial scanning was developed to resolve this problem. Subsequently, the borehole azimuthal acoustic imaging responses of near-borehole caves in formations with axially uniform and axially non-uniform wave velocities were numerically simulated. Single-shot spatial-scanning imaging and multi-shot stack imaging were then implemented separately for PP (incident P-waves scattered as P-waves) scattered-echo data obtained from forward modelling. The results revealed that in formations with axially uniform velocities, the waveforms of scattered echoes from the near-borehole caves exhibited a typical parabolic variation as a function of depth. Conversely, in formations with axially non-uniform velocities, the waveforms exhibited an asymmetric, curved variation pattern with respect to depth. The conventional downhole three-dimensional spatial-scanning method could not accurately locate the near-borehole anomalies in formations with axially non-uniform wave velocities, producing large imaging errors for the simulated caves. By contrast, the proposed imaging method more precisely determined the radial distances, azimuths and depths of the simulated caves. The proposed method may broaden the application scope of acoustic remote detection logging in the exploration and development of complex heterogeneous reservoirs.