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W ashout 0 4 4 1 ,4 7 0 1 ,4 8 0 2 2 5 1 ,4 9 0 2 0 0 1 ,5 0 0 1 7 5 1 ,5 1 0 1 5 0 1 ,0 0 0 2 ,0 0 0 3 ,0 0 0 Time, µs 4 ,0 0 0 5 ,0 0 0 1 5 0


1 ,0 0 0 2 ,0 0 0 3 ,0 0 0 4 ,0 0 0 Time, µs


5 ,0 0 0 6 ,0 0 0


> Estim ating com pressional slow ness b y processing leak y -Pdispersion data in the slow Antelope form ation ( left) . Traditional m onopole processing in Track 2 does not give slow ness estim ates as reliab ly as does dispersive STC processing ( Track 3 ) . STC plots ( right) from tw o different depths show the im proved coherence delivered b y dispersive STC processing ( right) com pared w ith traditional STC processing ( left) . Track 4 show s slow ness-freq uency analy sis ( SFA) using leak y -Pdispersion data, such as those show n in the dispersion curves ( b elow ) . ( Modi ed from Walsh et al, reference 8. )


1 7 5 2 0 0


G amma R ay gAP I 1 5 0


B it Size in.


Caliper in.


1 4 1 4


Compressional Slowness M onopole P rocessing


1 4 0 µs/ ft Coherence 2 4 0


Compressional Slowness L eaky-P


1 4 0


P rocessing µs/ ft


Coherence 2 4 0 1 4 0 2 2 5


F requency Analysis


µs/ ft E nergy Slowness-


2 ,0 0 0 µs 4 ,0 0 0 Time


2 4 0


W aveform Amplitude


2 0 0 1 7 5 1 5 0 1 ,0 0 0 2 ,0 0 0 D e p th = 1 , 5 1 0 f t 2 2 5 3 ,0 0 0 Time, µs 4 ,0 0 0 2 0 0 1 7 5 1 5 0


1 ,0 0 0 2 ,0 0 0 3 ,0 0 0 4 ,0 0 0 Time, µs


D e p th = 1 , 5 1 0 f t 5 ,0 0 0 6 ,0 0 0


T ra d itiona l S T C P roc e ssing D e p th = 1 , 4 7 0 f t


2 2 5


D isp e rsiv e S T C P roc e ssing 3 . 5 k H z to 6 . 5 k H z


D e p th = 1 , 4 7 0 f t


2 2 0 2 1 0 2 0 0 1 9 0 1 8 0 1 7 0 1 6 0 1 5 0


0


3 0 0 Depth = 1, 4 7 0 ft 2 5 0 2 0 0


Compressional slowness from monopole leaky P


1 5 0 1 0 0 5 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 2 0 2 1 0 2 0 0 1 9 0 1 8 0 1 7 0 1 6 0 1 5 0


3 0 0 Depth = 1, 5 10 ft 2 5 0


Compressional slowness from monopole leaky P


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


> Dispersion curves for com pressional arrivals in the upper, diatom ite zone ( left) and the low er, cristob alite zone ( right) . Com pressional-w ave slow ness is estim ated b y the slow ness of the leak y -Pm ode at low freq uency . [ Modi ed from Walsh et al, reference 8] .


8 ,0 0 0 1 0 ,0 0 0


understood, and decisions can be made regarding how to take advantage of or mitigate the situation.


In a recent exploration well in the South


Timbalier area of the Gulf of Mexico, Chevron successfully penetrated a target sandstone. In other wells, the same formation had presented completion challenges, so the logging program in this well included measurements to assess its mechanical properties.


in the shallow diatomite section and at 175 µ s/ft in the cristobalite section (top).


Following on the initial success of the Sonic


Scanner tool, Chevron is planning to run the tool in more wells in this field in 2006. Sonic velocities will support microseismic fracture- mapping techniques.1 4


Radial Profiles of Slowness Variation Variations


in formation properties may be


natural or induced by the drilling process, and may be beneficial or detrimental to the E& P activity at hand. By fully characterizing P- and S-wave slownesses in a large volume around the borehole,


the cause of the variation can be


Radial profiles of compressional and shear slownesses can reveal important information about the state of the formation near the borehole. Radial variation in compressional slowness is revealed by examining the difference in P slowness detected by the receiver array from the near and far monopole transmitters. Rays from the near transmitter sample the altered zone near the borehole, while rays from the far transmitter sample the unaltered zone, also called the far field.


A clear picture of radial variation emerges


when the P-wave data from all three transmitters and 13 receivers undergo tomographic recon- struction.1 5


This inversion technique uses ray


22


Oilfield Review


Slowness, µs/ ft


M easured depth, ft


Amplitude, dB Slowness, µs/ ft


Slowness, µs/ ft


Slowness, µs/ ft


Amplitude, dB


Slowness, µs/ ft


Slowness, µs/ ft


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