100 Well seismic surveying and acoustic logging The response of the Stoneley wave is strongly related to the state of continuity of the well wall. Its transmission is guided by the water or mud interface contained in the borehole and its wall. It is therefore particularly affected by the continuity solution of the borehole wall, while its exploitation and processing highlight its fracturing and degree of opening. The attenuation of Stoneley waves (decrease of amplitude and frequency) is used to characterize the fissured medium. In addition, wave conversion phenomena are observed at the boundaries of the fractured zones. These phenomena are very pronounced on the Stoneley waves, especially in the presence of open fractures. 3.7 Conclusion Compared to other logging methods, acoustic logging has an equivalent vertical resolution, but a superior ratio of lateral depth of investigation over vertical resolution. Acoustic logging has a lateral investigation in the order of centimeters for interface modes, decimeters to meters for retracted modes, and around ten meters for reflected modes. Acoustic logging is mainly used to: • measure the formation velocities (compression and shear) and calculate elasticity moduli (dynamic measurement 2-40 kHz); • establish a very high resolution time-depth relationship, by integrating the slowness curve (inverse velocity); • make synthetic seismograms to tie surface seismic reflection (see Chapter 4); • measure the attenuation and anisotropy (dipole mode) of a formation; • identify lithology in combination with other logs; • study fracturing and detect heterogeneities; • evaluate casing cementation (see Chapters 1 and 4); • assess porosity and estimate permeability; • measure dips; • provide a detailed micro-seismic survey (reflected waves) in the vicinity of the well; these operations are important for highly deviated or horizontal wells. The following elements must also be considered: • The acoustic measurement must be made in a well filled with water (mud). It is better to work in open hole (or even a PVC-cased hole). It is desirable to make a continuous measurement of the borehole diameters (logging: calipers) to detect the caved zones. The tool must be centered using centralizers during acoustic measurements. • Logging speed should be slow (4 to 6 m/min) to respect sampling conditions in terms of distance and to avoid scraping noises created by the centralizers. In general, a frequency filter (low cut: 1 kHz) is used at acquisition to filter these noises.
RkJQdWJsaXNoZXIy NjA3NzQ=