Dispersion measurement of acoustic field excited by dipole sources in fluid-filled borehole

Abstract

The flexural wave in fluid-filled borehole excited by dipole sources is dispersive wave. To obtain the phase slowness of dispersive waves, firstly we made several small-scaled model wells, and then measured the acoustical field excited by dipole sources. We researched three kinds of formation, including hard formation (aluminum well models), soft formation (nylon well models) and anisotropy formation (bakelite well models). For hard formation, the flexural mode is the main propagation wave mode in the measured waveforms, and the extracted dispersion curves are well agreed with theory curves. For soft formation, the measured dispersion curves do not well agree with theory curves due to the existence of stoneley wave mode. We drilled seven holes in the anisotropic media with different dip angles. This paper only studied the 7th borehole, whose axis is perpendicular to the symmetry's in this media, which is also called HTI formation model. For the HTI formation model, we observed that the dispersion curve of the slow flexural wave is obviously higher than that of fast flexural wave at the frequency plotted, which is consistent with the theory that the dispersion curves of slow and fast flexural wave arrays for intrinsic anisotropy formation are uncrossed.