Stoneley waves in the monopole acoustic waveform data contain important information about formation anisotropy and permeability. The effects of permeability and the anisotropy can be demonstrated by analyzing the Stoneley wave data. In an isotropic, non-permeable formation, the Stoneley wave behavior can be well predicted from the shear-wave log data. Permeable formation intervals cause Stoneley attenuation and increase the wave's slowness, which provides a permeability indication. Formation anisotropy, conversely, tends to reduce the Stoneley slowness from a value modeled using the shear data. This happens because in a VTI formation the Stoneley wave is controlled by the horizontal shear that is usually faster than the vertical shear measured from dipole logging. These data trends, therefore, provide an effective indication of formation permeability and anisotropy. With advances in theoretical modeling and data analyses, formation anisotropy and permeability information can now be obtained simultaneously from processing the Stoneley wave data with other log data. Incorporating anisotropy in permeability estimation substantially improves the quality of permeability estimates in formations with sand-shale sequences. Field results show that in such formations, the Stoneley wave data yields anisotropy in shales and permeability in sands, confirming the above analysis. Many data sets from various formations have been processed and the results reveal that VTI anisotropy is a general property of sedimentary rocks, especially shales. Many shale formations have a VTI magnitude in the range of 10-40 percent. The shear-wave anisotropy obtained from acoustic logging, after relating it to seismic propagation, can be used to enhance seismic processing and analysis results.
展开▼
