The Characteristics of the Acoustic Field Ahead of the Bit in “Look-Ahead” Acoustic Logging While Drilling

As the existing acoustic logging tools cannot meet the requirements of "look-ahead" detection, the research on “look-ahead” acoustic logging-while-drilling (LWD) was performed based on the phased array technology. The directional radiation and scanning reception of acoustic energy were realized by the linear phased array (LPA) acoustic wave radiator and arcuate phased array (APA) acoustic receiver stations. The responses of monopole and phased array acoustic logging with a formation interface ahead of the drill bit are numerically simulated by the finite-difference algorithm. The results showed that the acoustic fields of “look-ahead” acoustic LWD are more complex than those of conventional acoustic reflection logging, because of the wave scattering at the well bottom. Compared with the monopole acoustic radiator, the amplitude of P-P echo can be significantly enhanced by directionally enhancing the acoustic energy radiated into the formation ahead of the drill bit with LPA acoustic wave radiator. Compared with the monopole acoustic receiver, the APA acoustic receiver station can approximately determine the azimuth of the formation interface ahead of the drill bit by analyzing the amplitude distribution of P-P echo waves in the scanning reception waveforms. The research results confirmed that detecting geological anomalies ahead of the drill bit with phased array acoustic logging is feasible, and it can provide a theoretical basis for the development of industrial prototypes.