Crossed-dipole sonic logging tools measure shear-wave anisotropy in the formation. The anisotropic nature of the formation yields important new information for drilling and completion engineering as well as geophysics and petrophysics.

The Cuitlahuac field is located in the North of Mexico, Burgos Basin. The 200 km2 field has been producing gas from Oligocene sands since 1972. In the last few years it has been reactivated and many more new wells have been drilled. The field is composed of about 20 sand packages, which have predominantly NNE-SSW normal faulting. The beds dip towards the east at a relatively low angle, between 5 and 10°. The compressional slownesses are in the range of 60?100 μs/ft, while the shear slownesses range from 120 to 240 μs/ft over the 800-m logged interval.

In this case study in the Cuitlahuac field, we have used a new modular sonic tool that incorporates improved monopole and cross-dipole transmitter technology plus an extensive receiver array incorporating 13 axial levels with 8 azimuthal sensors at each level. Each receiver is individually digitized, resulting in 104 waveforms for improved slowness and anisotropy estimation compared to previous technologies. This is achieved through improved borehole mode extraction and/or rejection and enhanced wavenumber resolution at all frequencies.

Because of the high-quality data from the new tool, the differentiation between isotropic and anisotropic zones is extremely clear, as determined by the minimum and maximum crossline energy. The anisotropic regions are intervals of 5? 80 m thick and the amount of anisotropy ranges from 1 to 8%. The quantification of anisotropy of less than 5% is made possible by the improved transmitters and the additional receivers (both axially and azimuthally. The fast shear azimuth at low anisotropy is very stable?which is made quite obvious in these data because the tool is continuously rotating in this vertical well.

Slowness frequency dispersion analysis is used to identify the mechanism of the anisotropy. In one section, differential horizontal stresses form the mechanism of azimuthal anisotropy as determined by crossing dipole dispersion curves. This stress-induced anisotropy characterization is confirmed by image log analysis in which the observed tensile wall fractures are in the fast shear direction, which is indicative of the maximum stress direction. In another section, the anisotropy is caused by the layered nature of the formation, as determined by parallel dipole dispersion curves. The image logs also highlight the layered nature of the formations.

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