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generates no direct shear wave but does generate leaky-compressional waves. At low frequencies, the monopole source again generates Stoneley waves, but, in addition, there is a strong leaky- compressional wave generated. The X- and Y- dipole transmitters generate flexural waves with the characteristic low-frequency response of a slow formation. The dispersion data include the slightly dispersive Stoneley mode and the leaky- compressional wave, but no shear head wave, as expected in a slow formation. In the absence of a shear head wave, the shear slowness is estimated from the low-frequency limit of the flexural mode. The flexural mode is not as dispersive as in a fast formation, but more dispersive than that expected from a homogeneous, isotropic forma- tion. At low frequency, the two flexural-wave dispersion curves level off at different slow- nesses, indicating azimuthal anisotropy. The flexural waveforms have been mathematically rotated into fast and slow shear-wave directions.4


In formations that have undergone drilling-induced damage and are near failure but are otherwise homogeneous and isotropic, the two dispersion curves are identical but show much greater slowness at high frequencies than the modeled dispersion for a homogeneous isotropic formation. In formations with stress- induced anisotropy, the fast and slow shear-wave dispersion curves cross. This characteristic feature is caused by near-wellbore stress


Analysis of flexural-wave dispersion curves from the Sonic Scanner tool classifies formations according to anisotropy type by comparing observed dispersion curves to those modeled assuming a homogeneous isotropic formation (below). In a homogeneous isotropic formation, shear waves do not split into fast and slow components, so the two observed flexural-wave dispersion curves have identical slowness-versus- frequency signatures, and overlie the modeled curve. In cases of intrinsic anisotropy, such as shales or fractured formations, the fast and slow shear-wave dispersion curves are separate everywhere and tend to the true slowness at zero frequency.5


concentrations.6


These simplified relationships


between dispersion curves are valid when only one physical mechanism controls wave behavior. When multiple mechanisms are involved, such as if both stress-induced and intrinsic anisotropy are present, the curves can be different. In addition to acquiring openhole measure-


ments in isotropic, anisotropic, homogeneous and inhomogeneous formations,


the Sonic Scanner H om og e ne ous I sotrop ic I nh om og e ne ous I sotrop ic


Damaged formation, near failure


F ast shear F requency H om og e ne ous A nisotrop ic


I ntrinsic anisotropy: shales, layering, fractures


F ast shear Slow shear


F requency F requency


> Flex ural-w ave dispersion curves for classify ing form ation anisotropy and inhom ogeneity . In a hom ogeneous isotropic m edium ( top left) , ob served dispersion curves for  ex ural w aves recorded on orthogonal dipole receivers


( red and b lue curves) m atch m odeled  ex ural-w ave dispersion ( b lack circles) . In an inhom ogeneous isotropic form ation ( top right) , b oth ob served dispersion curves show greater slow ness w ith increasing freq uency than the hom ogeneous isotropic m odel. Greater slow ness w ith increasing freq uency indicates that the near-b orehole region has b ecom e slow er, a sign of dam age all around the b orehole. In a hom ogeneous anisotropic m edium


( b ottom left) , such as one w ith intrinsic anisotropy , the fast  ex ural-w ave dispersion curve ( red) m atches the hom ogeneous isotropic m odel ( to a  rst approx im ation) , w hile the slow  ex ural-w ave dispersion curve ( b lue) has the sam e shape b ut is translated to higher slow ness. In an inhom ogeneous anisotropic m edium ( b ottom right) , the tw o ob served  ex ural-w ave dispersion curves cross. This phenom enon is a result of near-w ellb ore stress concentration, and indicates stress-induced anisotropy .


F requency I nh om og e ne ous A nisotrop ic


Stress-induced anisotropy


F ast shear Slow shear


F ast shear


tool provides high-quality results behind casing. The improved tool design records waveforms through casing with high signal-to-noise ratio. Powerful transmitters and large bandwidth allow acquisition of formation slowness data through casing and cement of varying thickness. The ability to measure formation properties through casing allows companies to monitor the mechanical effects of production on the produc- ing formation. Many formations undergo compac- tion, weakening or other changes with time as a result of pressure depletion or water injection. In an example from a Statoil well in the North Sea, Sonic Scanner data were acquired in both 8.5 -in. open hole and behind 8-in. OD casing before any production (next page). The openhole logs in the zone of interest indicate a slower, softer formation between X,296 and X,305 m. The caliper log flags a washout in this interval. When compared with the openhole logs, the cased-hole compressional and shear slownesses are markedly similar, even through the washed- out zone. The dispersion curves in the two cases are also similar.


In the Middle East, the Sonic Scanner tool


has been used multiple times to acquire slowness through 133


⁄ 8 -in. casing in hole sizes larger than


20 inches. In each case, despite poor cement, good shear-wave slowness data were acquired over the entire interval, and adequate compressional slowness was recorded over at least half the interval.


The Sonic Scanner tool not only obtains slowness results behind casing, but can also simultaneously evaluate the quality of the cement bond and the top of cement. Signals recorded by receivers 3 and 5 ft [ 0.9 and 1.5 m] from the two near monopole transmitters are processed to produce a discriminated attenua- tion measurement that is free of tool- normalization fluid effects and pressure and temperature drifts. The results are comparable to those of the CBT Cement Bond Tool, but are also corrected for casing and cement properties. Evaluation of well integrity and formation properties in the same tool run can avoid separate logging runs and reduce rig-time and mobilization costs.


18


Oilfield Review


Slowness


Slowness


Slowness


Slowness


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