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tracing to calculate signal arrival times at all the sensors, and updates an initially homogeneous formation model to create a final model that satisfies the observed data. To visualize the resulting compressional-slowness radial profile, the differential percentage between observed slowness and far-field slowness is color-coded and plotted against radial distance from the borehole wall (right).


9


Data from this Chevron well showed that the sandstones of interest exhibited radial variations in compressional slowness approaching 15 %


at


the borehole wall and extending up 1 ft [ 30 cm] into the formation. However, quantifying the radial P-wave slowness variation alone does not identify its cause. Compressional-slowness variations can be caused by fluid changes, such as invasion of drilling fluid, or by radial changes in stress or formation strength. Additional information from the shear-slowness radial profile could help distinguish between these. Shear-slowness radial profiles are constructed in a multistep procedure.1 6


Semblance processing


of flexural waveforms at low frequencies provides an initial estimate of formation elastic param- eters. These parameters are used to model a homogeneous isotropic formation. Differences between measured and modeled slownesses at a large selection of frequencies form the input to an inversion procedure that yields the actual flexural-slowness radial profile. The results are plotted with colors that represent the amount of difference between observed slowness and the slowness of the unaltered, far-field formation. In the South Timbalier case, the shear- slowness radial profile shows a large difference in near-wellbore slowness compared with far- field slowness. Flexural-wave dispersion curves


14 . Bennett L, Le Calvez J , Sarver DR, Tanner K , Birk WS, Waters G, Drew J , Prim iero P, Eisner L, J ones R, Leslie D, William s MJ , Govenlock J , K lem RC and Tezuk a K :


“ The Source for Hy draulic Fracture Characterization, ” Oil eld Review 17 , no. 4 ( Winter 2005 / 2006) : 4 2– 5 7 .


15 . Z eroug S, Valero H-P and Bose S: “ Monopole Radial Pro ling of Com pressional Slow ness, ” prepared for presentation at the 7 6th SEG Annual International Meeting, New Orleans, Octob er 1– 3 , 2006.


Hornb y BE: “ Tom ographic Reconstruction of Near- Borehole Slow ness Using Refracted Sonic Arrivals, ” Geophy sics5 8, no. 12 ( Decem b er 19 9 3 ) : 17 26– 17 3 8.


16. Sinha BK : “ Near-Wellb ore Characterization Using Radial Pro les of Shear Slow ness, ” Ex panded Ab stracts, 7 4 th SEG Annual International Meeting, Denver ( Octob er 10– 13 , 2004 ) : 3 26– 3 3 1.


> Com pressional and shear radial pro ling in a Chevron Gulf of Mex ico w ell. P-w ave data from all three transm itters and 13 receivers are input to tom ographic reconstruction b ased on tracing ray s through a m odeled form ation w ith properties that vary gradually aw ay from the b orehole. The percentage difference b etw een ob served com pressional slow ness and slow ness of the unaltered, far- eld form ation is plotted on color and distance scales to indicate the ex tent of difference aw ay from the b orehole ( Track 6) . In these sandstones, com pressional slow ness near the b orehole varies b y up to 15 % from far- eld slow ness, and the variation ex tends to 1 ft from the b orehole w all. Shear-


w ave radial pro les appear in Track s 3 and 5 for the fast and slow shear differences from far- eld slow ness, respectively . Large differences, attrib uted to plastic y ielding in the near-w ellb ore region, are show n in red, and ex tend out to ab out 10 in. from the b orehole w all. These differences occur only in the sandstone intervals, identi ab le from the gam m a ray log in Track 4 .


X ,4 9 0


Compressional 1 8 0


H ole Diameter in.


1 9 1 .7


Density g/ cm3


2 .7 3 0 0 3 0 0


F ast Shear Slow Shear


µs/ ft µs/ ft


µs/ ft 8 0 1 0 0 0 %


Distance from B orehole Center


1 0 0 2 ft 0 1 0


F ast Shear Differential


2 5 0 %


G amma R ay


gAP 1 1 0I 0


Distance from B orehole Center


ft 2 0


Slow Shear Differential


2 5 0 %


Distance from B orehole Center


ft 2


Compressional Differential


2 5


X ,4 8 0


X ,5 0 0


X ,5 1 0


X ,5 2 0


X ,5 3 0


X ,5 4 0


X ,5 5 0


Spring 2006


23


M easured depth, ft


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