Acoustic radiation and reflection of a logging-while-drilling dipole source

Abstract

SUMMARY With the comparison to the resistivity ultra-deep measurement, the single-well reflection survey in acoustic logging-while-drilling (ALWD) measurement lags far behind, especially ALWD dipole measurement has long been thought to be little added value. In this paper, we extended the dipole shear-wave (S-wave) reflection survey technology in wireline logging into ALWD and demonstrated the theoretical feasibility of adopting a dipole source–receiver system to perform ALWD reflection survey. For this purpose, we investigated the radiation patterns of radiantSH, SV and P waves, the energy fluxes of guided and radiant waves and their acoustical radiation efficiencies from an LWD dipole acoustic source by comparisons with the wireline results. The analysis results reveal that a dominant excitation-frequency band does exist in ALWD dipole S-wave reflection. Consequently, the expected excitation frequency should be located in the band of the signal with high radiation efficiency, guaranteeing the best radiation performance. In fast formations, SH wave is the best candidate for ALWD reflection survey due to its highest radiation efficiency. In contrast, the dominant excitation-frequency band of SH wave gets wider in a slow formation. Besides, the SV- and P-wave radiation efficiencies are also remarkable, implying that both waves can also be used for ALWD reflection survey in slow formations. We expounded the SH-, SV- and P-reflection behaviours at three typical excitation frequencies by our 3-D finite difference. Simulations to single-well reflection validate the key role of dominant excitation-frequency band and demonstrate the theoretical feasibility of applying the technology to ALWD. Our results can guide the design and measurement methods of ALWD dipole S-wave reflection survey tool, which could have extensive application prospect for geo-steering.