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Time-B ased Anisotropy


-1 0 deg 9 0 Sonde Deviation


O ffline E nergy


0


M aximum 0 E nergy


0 0 1 0 0


M inimum E nergy


5 1 0 0 0


Depth, m


A 1 ,6 0 0 C


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


0 1 ,6 5 0 B


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


0


H ole Azimuth deg 3 6 0


0 % Anisotropy F lag, %


Total Azimuth deg 3 6 0


0 2 4 6


H ole Diameter in.


G amma R ay gAP I 1 5 0 2 0 -9 0


F ast Shear Azimuth Azimuth U ncertainty


deg 9 0 3 5 0 3 5 0 µ s/ ft 1 6


F ast Shear ∆T Slow Shear ∆T


µ s/ ft 1 ,2 0 0 1 ,2 0 0 0 2 0 % 0 ∆T-B ased Anisotropy 2 0


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


D e p th = 1 , 5 9 3 . 0 4 m


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


2 ,0 0 0 4 ,0 0 0 F requency, H z D e p th = 1 , 6 5 8 . 8 7 m


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


2 ,0 0 0 4 ,0 0 0 F requency, H z D e p th = 1 , 6 6 5 . 2 7 m


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


2 ,0 0 0 4 ,0 0 0 F requency, H z


> A crossed-dipole log ( left) from the Pem ex Cuitlahuac-83 2 w ell, show ing zones w ith isotropy and w ith differing am ounts of anisotropy . Z one A, an isotropic zone, has low of ine energy ( depth track ) and eq ual fast and slow shear-w ave slow nesses ( Track 3 ) . Anisotropic Z ones B and C have nonzero of ine energies and different fast and slow shear-w ave slow nesses. Anisotropy m agnitude, either slow ness-b ased or tim e-b ased ( edges of Track 3 ) , is ab out 8% in Z one B and ab out 2% in Z one C. The azim uth of the fast shear w ave ( Track 2) rem ains constant through the anisotropic intervals, even though the tool is rotating ( Track 1) , giving con dence in the anisotropy values. Dispersion curves from the three intervals ( right) show characteristic relationships. In Z one A ( top) , as in other isotropic form ations, the dispersion curves for  ex ural w aves recorded in the tw o dipole directions ( red and b lue circles) overlie each other. At the b ottom of Z one B ( b ottom ) , the dispersion curves cross each other. The  ex ural w ave that is fast near the b orehole, at low freq uencies


( red dots) , b ecom es the slow er w ave w ith distance from the b orehole ( b lue dots) . This indicates that stress-induced anisotropy is the dom inant m echanism of anisotropy in this section. Shallow er in Z one B ( m iddle) , the dispersion curves look as though they could cross, b ut the high-freq uency com ponents of the fast shear w ave are lost. At this depth, open, induced fractures w ere visib le in OBMI Oil-Base MicroIm ager logs. ( Modi ed from Wielem ak er et al, reference 25 . )


6 ,0 0 0 8 ,0 0 0 6 ,0 0 0 8 ,0 0 0 6 ,0 0 0 8 ,0 0 0


imbalance.2 4


Until now, wireline sonic tools have been able to quantify the magnitude and orientation of elastic anisotropy only if


the


difference in velocities was at least 5 % . The high quality of data provided by the Sonic Scanner tool allows reliable measurement of anisotropy as small as 1% , and also helps interpreters determine the cause of the anisotropy.


Pemex Exploració n y Producció n wanted to


evaluate the amount and direction of anisotropy in tight gas-producing sandstone formations in the Burgos basin of northern Mexico. These formations have low permeabilities and must be stimulated to produce gas in commercial quantities. Optimal development depends on correctly orienting hydraulic


fractures in the local stress field so that each vertical well drains


its designated volume. Knowledge of elastic anisotropy orientation and magnitude would help in the design and application of oriented- perforating techniques prior to fracture treatments and would also improve the success of infill-drilling campaigns.2 5


28


Oilfield Review


Slowness, µ s/ ft


Slowness, µ s/ ft


Slowness, µ s/ ft


Amplitude, dB


Amplitude, dB


Amplitude, dB


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