Characteristics of the interface acoustoelectric conversion waveform in fluid-filled boreholes

When acoustic waves meet the side wall media in a fluid-filled borehole, the electromagnetic field at the borehole interface between the formation and borehole fluid called the interface acoustoelectric conversion wave signal is converted. The propagation of the refracted P wave, refracted S wave, Stoneley wave, and other borehole-mode waves in the fluid-filled borehole generates accompanying electromagnetic wave signals corresponding to various borehole-mode waves, called accompanying acoustoelectric conversion wave signals. In this study, we examined the interface acoustoelectric conversion wave. First, we numerically simulated the acoustoelectric wave fields in fluid-filled boreholes using a method of real axis integration and analyzed the effects of the borehole radius, porosity, and pore fluid salinity on the interface acoustoelectric conversion waves. The arrival time of the interface acoustoelectric conversion wave increased as the borehole radius increased, and the amplitude of the interface acoustoelectric conversion wave increased as the porosity increased and decreased as the pore fluid salinity increased. Using the self-developed acoustoelectric effect logging tool, we conducted an actual downhole test in a sand shale well in northern China and observed the interface and accompanying acoustoelectric conversion waves. The observations followed the acoustoelectric effect logging theory. Further, to characterize the interface acoustoelectric conversion wave, we calculated its arrival time using a method of long-short time window energy ratio and calculated its energy by taking the summation of squares of the waveform amplitude in a window, respectively. The arrival time approximated to the time in which the acoustic wave propagated from the radiating surface of the acoustic radiator to the side wall interface, and the energy was consistent with the energy ratio of the accompanying conversion Stoneley wave to the acoustic Stoneley wave. The energy curve of the interface acoustoelectric conversion wave and conventional logging curves were highly correlated. The interface acoustoelectric conversion wave in acoustoelectric logging signals can provide a new reference for evaluating the formation parameters associated with open-hole wells.