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wave slowness exceeds 800 µ s/ft in some sections.9 Nine-component vertical seismic profiles and crossed-dipole sonic logs have detected changing anisotropy magnitude and direction with depth and around the field.1 0


Knowledge of acoustic


velocities and of anisotropy can be important for designing fracture stimulations and enhanced oil- recovery operations. Measurements with the Sonic Scanner tool provide new insight into the acoustic behavior of these complex rocks. Waveforms were recorded in an interval from 972 to 1,65 0 ft [ 296 to 5 03 m] in a well near the crest of the Cymric structure. In the diatomite zone, down to 1,5 00 ft [ 45 7 m] , shear slowness estimated from flexural-mode dispersion processing is at least as great as that found in earlier logging programs, averaging 700 µ s/ft and approaching 900 µ s/ft in some intervals (previous page). Below that, shear slowness decreases to about 400 µ s/ft in the cristobalite zone. Much of the logged interval


1 3 1 2


1 1 1 0


9 8 7 6 5 4 3 2 1 0 2 ,5 0 0 5 ,0 0 0 Time, µs exhibits


azimuthal anisotropy, as indicated by the large separation between minimum and maximum offline energy, and also between the fast and slow shear-wave slownesses. Anisotropy magnitude ranges from 4 to 8% , consistent with results of previous studies.1 1


Slowness anisotropy is


calculated by dividing the difference between fast and slow shear slownesses by their average. The azimuth of the fast shear direction is between N35 W and N15 W, in general agreement with previous studies.1 2 Along with the typical fast and slow shear- slowness curves and slowness-time-coherence (STC) projections seen in many sonic-log plots, displays of Sonic Scanner data include new quality-control


0 0 0 2 ,0 0 0 4 ,0 0 0 F requency, H z 6 ,0 0 0 8 ,0 0 0 tracks showing slowness-


frequency analysis (SFA). To create SFA plots, a dispersion curve is generated at each depth using the recorded dipole flexural waveforms (above right).1 3


The dispersion curve is projected onto the slowness axis, and this projection is 9 . Hatchell PJ , De GS, Winterstein DF and DeMartini DC:


“ Q uantitative Com parison Betw een a Dipole Log and VSP in Anisotropic Rock s from Cy m ric Oil Field, California, ” Ex panded Ab stracts, 65 th SEG Annual International Meeting, Houston ( Octob er 8– 13 , 19 9 5 ) : 13 – 16.


10. De GS, Winterstein DF, J ohnson SJ , Higgs WG and


X iao H: “ Predicting Natural or Induced Fracture Azim uths from Shear-Wave Anisotropy , ” paper SPE 5 09 9 3 -PA, SPE Reservoir Evaluation & Engineering1, no. 4 ( August 19 9 8) : 3 11– 3 18.


11. De et al, reference 10. 12. Hatchell et al, reference 9 . 13 . Plona T, K ane M, Alford J , Endo T, Walsh J and Murray D:


“ Slow ness-Freq uency Proj ection Logs: A New Q C Method for Accurate Sonic Slow ness Evaluation, ” Transactions of the SPWLA 4 6th Annual Logging Sy m posium , New Orleans, J une 26– 29 , 2005 , paper T.


> Construction of a slow ness-freq uency -analy sis ( SFA) log for controlling the q uality of shear-slow ness estim ation from  ex ural w aves. Dipole  ex ural w aveform s at each depth ( topleft) are analy zed for their slow ness at vary ing freq uencies. Resulting data are plotted on a slow ness-freq uency plot


( b ottom left) , w ith circle size indicating am ount of energy . Energies are color-coded and proj ected onto the slow ness ax is. The color strip is plotted at the appropriate depth to create a log ( right) . The slow ness estim ate from dispersive STC processing is plotted as a b lack curve. If this m atches the zero-freq uency lim it of the SFA proj ection, the slow ness estim ate is good.


4 0 0 3 5 0 3 0 0 2 5 0 2 0 0 1 5 0 1 0 0 5 0


3 0 0 2 5 0 2 0 0 1 5 0 1 0 0 5 0 1 5 ,1 0 0 1 5 ,0 5 0 7 ,5 0 0 1 0 ,0 0 0 1 5 ,0 0 0


SF A E nergy W a v e f orm s f rom 1 5 , 0 6 1 f t 1 0 0


Shear Slowness µs/ ft


4 0 0


plotted in a log versus depth presentation, similar to the way an STC projection is constructed. The estimated slowness log from dispersive STC processing is overlaid on the SFA projection, and if the estimated slowness matches the low-frequency limit of the SFA projection, the quality of the slowness estimate is high. In azimuthally anisotropic formations, SFA projections may be plotted for both the fast and slow shear directions.


In this extremely slow formation, the monopole source does not excite a compressional head wave, but rather a strong leaky-P mode. Compressional slowness must therefore be estimated


from slowness dispersive STC processing,


analogous to the technique for determining shear


from flexural modes. Compressional slowness is estimated at 192 µ s/ft


Spring 2006


21


Slowness, µs/ ft W aveform number


Amplitude, dB


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