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Coal bed


R eflected signals


B orehole signals


> Sonic-im aging data-acq uisition geom etry . Designed to detect lay er


b oundaries and other inhom ogeneities roughly parallel to the b orehole, the sonic-im aging techniq ue records re ected signals ( red ray s) from interfaces tens of feet aw ay . Borehole signals ( b lack ray s) m ust b e  ltered out.


Consequently, this technique has significant potential application in horizontal wells. To create an image, the tool records waveforms of relatively long duration from the monopole transmitters. Receivers must be distributed around the tool to allow the azimuths of the reflections to be distinguished. Complex data processing similar to that designed for surface seismic surveys is applied in a multistep process. First, a compressional- velocity model of the region in the vicinity of the borehole is created using the P head waves. Then, to extract reflected energy, the traditional sonic arrivals, including P and S head waves and Stoneley waves, must be filtered from the waveforms for each shot. The filtered traces are input to depth migration, a process that positions reflections in their correct spatial location using the velocity model.


The sonic-imaging technique, sometimes called the borehole acoustic reflection survey, provides a high-resolution directional image of reflectors up to tens of feet from the borehole (left).2 3


Alteration in near-wellbore properties can cause velocities to increase or decrease relative to the unaltered, or far-field, formation. Usually, drilling-induced damage reduces formation stiffness, causing velocities to decrease near the borehole. However, when drilling fluid replaces gas


as the pore-filling fluid, the resulting


formation is stiffer, so compressional velocity increases near the borehole.


Radial alteration of rocks and fluids affects compressional and shear velocities differently. Alteration that reduces stiffness of the rock fabric, such as drilling-induced cracking or weakening, causes both P and S velocities to decrease. However, a change in pore fluid has little effect on S velocity, while P velocity may change dramatically. For example, when drilling fluid replaces gas, P-wave velocity increases, but S-wave velocity is relatively unaffected. Complete characterization of radial inhomo- geneity requires analysis of radial variation of compressional and shear slownesses. A radial compressional-slowness profile is generated by collecting P-wave data for multiple depths of investigation, from near the wellbore to the unaltered, far-field formation. This requires recordings from a wide range of transmitter-


receiver spacings. Ray-tracing techniques invert the refracted compressional arrivals to yield compressional slowness versus distance from the borehole.2 1


The difference between near-


wellbore compressional slowness and far-field compressional slowness can be plotted along with depth of radial alteration (previous page). In this example, radial variations of shear slownesses are also plotted. Radial variations in shear slowness are


quantified through inversions of the broadband dispersions of flexural and Stoneley modes.2 2


At


high frequencies, these dispersive modes investigate the near-wellbore region, and at low frequencies, they probe the unaltered formation far from the borehole. Dispersion data from a wide range of


frequencies help produce the


most reliable radial profiles of variations in shear slowness.


Some of the most challenging inhomo- geneities to characterize are those that do not intersect the borehole. These may be vertical fractures or faults near a vertical borehole or sedimentary interfaces near a horizontal well. Detecting such inhomogeneities


requires a


method that looks deep into the formation and that is able to detect abrupt changes in formation properties.


The migration process formally converts a set of amplitude and traveltime measurements into a spatial image of the formation. This can be viewed as a triangulation process in which the distance and the dip of a reflector relative to the borehole are determined by the signals recorded at receivers at different TR spacing. The receivers at different azimuths around the borehole measure different distances to a reflector depending on the azimuth and the dip of the reflector relative to the borehole. The sonic-imaging technique was developed in the 1980s, but results have improved with advances methods.2 4


in sonic tools and processing The technique has been used to image


steeply dipping beds from near-vertical boreholes and sedimentary boundaries from horizontal wells. For examples of sonic imaging and other applications of sonic measurements see “ Sonic Investigations In and Around the Borehole,” page 14.


– LS


Spring 2006


43


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