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6 6 0 X ,2 9 0


W ashout B it Size in.


Caliper in.


1 6 1 6


G amma R ay gAP I


1 5 0 8 0


Shear Slowness STC Coherence


0 µs/ ft 5 4 0 0


V ariable Density L og


Arrival Time µs


µs 1 0 ,0 0 0 6 1 0 ,0 0 0 0 X ,2 9 0


W aveform


B it Size in.


STC Coherence 1 6 0


G amma R ay gAP I


1 5 0 8 0


Shear Slowness µs/ ft


5 4 0 0


V ariable Density L og


Arrival Time µs


µs 1 0 ,0 0 0 1 0 ,0 0 0


W aveform


X ,3 0 0


X ,3 0 0


X ,3 1 0


X ,3 1 0


X ,3 2 0


3 0 0 2 5 0


2 0 0 1 5 0 1 0 0 5 0 0 0 0 0 0 2 ,0 0 0 4 ,0 0 0 6 ,0 0 0 F requency, H z 8 ,0 0 0 1 0 ,0 0 0 0 2 ,0 0 0 4 ,0 0 0 6 ,0 0 0 F requency, H z


> Openhole ( left) and cased-hole ( right) results in a Statoil North Sea w ell. The Sonic Scanner tool m easures P-, S-and Stoneley -w ave slow nesses in open hole and b ehind casing, even w here the caliper ( Track 1) indicates a w ashed-out zone ( b etw een X , 29 6 and X , 3 05 m ) in the openhole logs. Flex ural-m ode slow ness display ed in Track 2 of each set is m ore sharply de ned, w ith a narrow er color b and, in the cased-hole ex am ple than in the openhole logs. In the dispersion curves ( b ottom ) , com pressional-w ave slow ness is in dashed green and shear-w ave slow ness is in dashed b lue.


8 ,0 0 0 1 0 ,0 0 0


3 0 0 2 5 0


2 0 0 1 5 0 1 0 0 5 0


X ,3 2 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


Extreme Slowness


Some formations are so slow that not only is the S-wave slowness greater than that of the mud, but the P-wave slowness approaches that of the mud. In these circumstances, the P-wave loses energy to the formation, in what is known as a leaky-P mode, and is dispersive. At the low-


4 . The X - and Y -dipole sources are separated b y 1 ft. While this avoids electrical cross-talk , it also m eans that


w aveform s m ust b e shifted b y 1 ft b efore Alford rotation. This reduces the num b er of collocated w aveform s from 13 to 11.


Alford RM: “ Shear Data in the Presence of Azim uthal Anisotropy : Dilley , Tex as, ” Ex panded Ab stracts, 5 6th SEG Annual International Meeting, Houston ( Novem b er 2– 6, 19 86) : 4 7 6– 4 7 9 .


frequency limit, the leaky-P dispersion curve tends toward the P-wave slowness, and at the high-frequency limit, it reaches the borehole- fluid slowness.7


The Antelope formation in the Cymric oil field


in the San Joaquin Valley, California, is such a case, combining extreme slowness with other


5 . For anisotropy to b e identi ed in this w ay , the anisotropy sy m m etry ax is m ust b e perpendicular to the b orehole ax is. For ex am ple, crossed-dipole logging tools in vertical w ells can detect anisotropy caused b y aligned vertical fractures, and in horizontal w ells can detect anisotropy caused b y horizontal lam inations.


6. Sinha BK and K ostek S: “ Stress-Induced Azim uthal Anisotropy in Borehole Flex ural Waves, ” Geophy sics61, no. 6 ( Novem b er-Decem b er 19 9 6) : 189 9 – 19 07 .


Wink ler K W, Sinha BK and Plona TJ , “ Effects of Borehole Stress Concentrations on Dipole Anisotropy Measurem ents, ” Geophysics63 , no. 1 ( J anuary -Feb ruary 19 9 8) : 11– 17 .


complications that make sonic logging chal- lenging.8 and


cristobalite— forms opalized


The formation lithology is diatomite of


silica.


Permeability is low, averaging 2 mD. From earlier studies, compressional-wave slowness in this formation is known to approach 200 µ s/ft, which is near the slowness of the mud wave, and shear-


7 . Valero H-P, Peng L, Y am am oto M, Plona T, Murray D and


Y am am oto H: “ Processing of Monopole Com pressional in Slow Form ations, ” Ex panded Ab stracts, 7 4 th SEG International Meeting, Denver ( Octob er 10– 15 , 2004 ) : 3 18– 3 21.


8. Walsh J , Tagb or K , Plona T, Y am am oto H and De G:


“ Acoustic Characterization of an Ex trem ely Slow Form ation in California, ” Transactions of the SPWLA 4 6th Annual Logging Sy m posium , New Orleans, J une 26– 29 , 2005 , paper U.


Spring 2006


19


Slowness, µs/ ft


Depth, m


Amplitude, dB


Slowness, µs/ ft


Depth, m


Amplitude, dB


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