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New Developments in Acoustic Logging
Compressional velocities under difficult borehole conditions, shear velocities in unconsolidated formations and estimates of in situ formation permeability can now be obtained using two new logging tools. By combining improvements in transducer technology and electronic equipment design a long spaced acoustic logging tool (LSAL) has been developed to produce superior quality digital waveform data. These data are analyzed to obtain accurate compressional velocities free of skip cycles in large diameter boreholes and through zones of damaged shale or gas. Permeabilities from core analyses have been found to correlate with tube wave attenuation and velocity providing for the first time a direct estimate of in situ permeability even in the water portion of a reservoir.
When formation shear velocities are greater than the borehole fluid compressional velocity, the digitally recorded LSAL data are analyzed to extract shear velocities from the refracted shear wave. In unconsolidated formations where the shear velocity is less than the borehole fluid velocity there is no refracted shear wave. For this reason and because sometimes the refracted shear wave is not readily recognizable on LSAL data, a direct shear wave acoustic logging tool (SWAL) was developed. Data have been obtained in lithologies ranging from well lithified sediments to unconsolidated sandstones. In hard rocks, agreement of SWAL data to refracted shear wave data from the LSAL is excellent. In formations with little or no lithification excellent shear wave data have also been obtained. Such data are useful for strength of cementation for proper design of sand control measures.
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