Measurement resolution of acoustic reflection imaging logging with a cylindrical phased array acoustic receiver station

Abstract

Azimuthal acoustic reflection imaging logging plays an important role in the exploration and development of fracture-cavity reservoirs, because it can image geological structures within tens of meters around the borehole. However, minimal evaluations have been conducted for the detection of small-scale geological anomalies, particularly the simultaneous measurement of multiple small-scale scatterers. In this study, the scanning-reception implementations of phased array acoustic receiver stations in the Borehole Azimuthal Acoustic Reflection Imaging Tool (BAR) were developed based on the beamforming principle. The measurement responses of BAR were simulated and the scanning-reception performances of arcuate phased array (APA) and cylindrical phased array (CPA) acoustic receiver stations were compared using a three-dimensional (3D) finite difference algorithm for one or two small-scale caves in a formation away from a fluid-filled borehole. The azimuth and elevation measurement resolutions of CPA acoustic receiver stations were determined by analyzing the scattered wavefields under the circumstances of two caves on the same horizontal or vertical plane. Numerical simulation results showed that the echo waves scattered from small-scale caves were similar to those reflected by large-scale formation interfaces outside the borehole. The azimuth angle of a single cave could be obtained by calculating the amplitudes of P–P echoes at different azimuths received by the APA acoustic receiver station. Compared with the APA scanning-reception mode, the 3D scanning-reception mode based on the CPA acoustic receiver station was more applicable when two caves were simultaneously located outside the borehole. Under the calculation conditions of this study, the CPA acoustic receiver station had a limit azimuth resolution of ∼60°–90° and a limit elevation resolution of ∼8.8°–17.4° when two scatterers were located on the same horizontal or vertical plane. This work verified the superiority of the CPA scanning-reception mode for the detection of small-scale geological anomalies and provided a theoretical basis for the wider application of BAR.