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8 0 0 7 0 0 6 0 0 5 0 0 4 0 0 0 1 ,0 0 0 2 ,0 0 0 3 ,0 0 0 F requency, H z 8 0 0 7 0 0 6 0 0 5 0 0 4 0 0 1 2 3 4 5 6 7 Alteration radius/ borehole radius


> Com parison of  ex ural-w ave dispersion seen in a South Tim b alier w ell w ith m odeled results ( top) . Ob served  ex ural-w ave slow nesses ( red and


b lue circles) show m uch larger dispersion than the m odel for a hom ogeneous isotropic form ation ( b lue curve) . The large difference at higher freq uencies indicates near-w ellb ore dam age. Stoneley -w ave slow nesses appear as green circles. In the b ottom  gure, the difference b etw een ob served and


m odeled  ex ural slow ness is plotted against distance, in b orehole-radius ratio units. The difference b etw een ob served and m odeled  ex ural slow ness am ounts to 20% out to a distance eq uivalent to ab out tw o b orehole radii.


~ 2 0 % shear alteration 4 ,0 0 0 5 ,0 0 0


yielding of grain contacts. The caliper shows no wellbore enlargement through this zone, so the damaged material has not yet fallen into the borehole, but the increase in shear slowness near the borehole wall indicates that it is near failure. The Sonic Scanner data indicate a wide zone of damage that will require extra care when the time comes to design a well completion. Compressional and shear radial profiles bring new information not previously available from any logging tool. Borehole imaging tools and calipers have been able to deliver images or evidence of drilling-induced borehole irregular- ities such as breakouts and fractures, but are useful only after the borehole shape has changed. The Sonic Scanner tool probes deep into the formation to reveal mechanical damage beyond the borehole wall.


Radial profiling may also help to fine-tune programs for acquisition of fluid samples. In an example from the North Sea, Sonic Scanner compressional radial profiles were computed for two intervals from which samples were subsequently acquired using the MDT Modular Formation Dynamics Tester. Z one A showed little difference between near-wellbore and far-field slowness (next page). Two fluid samples were taken from this interval after pumping times of 75 and 80 minutes and no sampling problems. In


Z one B, the radial profile indicated formation damage out to 12 in. from the borehole wall. During the attempt to obtain a fluid sample, the probe on the sampling tool became plugged, and no sample was obtained.


also indicate a high degree of near-wellbore alteration (above). The analysis is complicated somewhat by the addition of anisotropy; the fast shear and slow shear waves exhibit distinct differences relative to the unaltered, far-field slowness. In the sandstones, both fast and slow shear slownesses are up to 20% greater than the far-field slowness in a zone roughly 10 in. [ 25 cm] from the borehole wall.


17 . Brie A, Endo T, J ohnson DL and Pam puri F: “ Q uantitative Form ation Perm eab ility Evaluation from Stoneley Waves, ” paper SPE 4 9 13 1, presented at the SPE Annual Technical Conference and Ex hib ition, New Orleans, Septem b er 27 – 3 0, 19 9 8.


The radial heterogeneity in shear slowness rules out invasion or other fluid-related causes of near-wellbore alteration, because shear waves are almost insensitive to changes in pore fluid. Fluid-related


Formation damage does not necessarily mean that samples cannot be acquired, but sampling in these zones may have an increased risk of tool plugging or sticking. To minimize these risks, sampling from potentially damaged zones should be delayed and attempted later in the sampling program, so that samples from other intervals can be collected first with less risk.


changes would cause only


compressional-slowness radial variation. The measurable radial variation in shear slowness indicates that the formation has undergone mechanical damage in the form of plastic


18. K im b all CV and Endo T: “ Q uantitative Stoneley Mob ility Inversion, ” Ex panded Ab stracts, 68th SEG Annual International Meeting and Ex hib ition, New Orleans ( Septem b er 13 – 15 , 19 9 8) : 25 2– 25 5 .


Liu H-L and J ohnson DL: “ Effects of an Elastic Mem b rane on Tub e Waves in Perm eab le Form ations, ” J ournal of the Acoustic Society of Am erica101, no. 6 ( J une 19 9 7 ) : 3 3 22– 3 3 29 .


Characterizing Permeable Zones and Fractures Petrophysicists and reservoir engineers have long sought a continuous measurement of permeability to optimize well completions and production scenarios, but continuous permeability is one of the most difficult properties to measure in an oil well. Using empirical relationships calibrated to core measurements, permeability or mobility— the ratio of permeability to viscosity— can be inferred from other measurements such as porosity or nuclear magnetic resonance logs.


24


Oilfield Review


Slowness, µs/ ft


Slowness, µs/ ft


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