ALBERT

Description: 〈span〉〈div〉SUMMARY〈/div〉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 (〈span〉S〈/span〉-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 radiant〈span〉SH, SV〈/span〉 and 〈span〉P〈/span〉 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 〈span〉S〈/span〉-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, 〈span〉SH〈/span〉 wave is the best candidate for ALWD reflection survey due to its highest radiation efficiency. In contrast, the dominant excitation-frequency band of 〈span〉SH〈/span〉 wave gets wider in a slow formation. Besides, the 〈span〉SV〈/span〉- and 〈span〉P〈/span〉-wave radiation efficiencies are also remarkable, implying that both waves can also be used for ALWD reflection survey in slow formations. We expounded the 〈span〉SH-, SV-〈/span〉 and 〈span〉P〈/span〉-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 〈span〉S〈/span〉-wave reflection survey tool, which could have extensive application prospect for geo-steering.〈/span〉