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When the vertical stress is the maximum stress, hydraulic fractures propagate in the direction of the maximum horizontal stress and they open in the direction of minimum horizontal stress. Shear waves travel fastest when polarized in the direction of maximum horizontal stress (SH) and slowest when polarized in the direction of minimum horizontal stress (Sh). This is because additional stress stiffens the formation, increasing velocity, and reduced stress conversely decreases velocity. Measuring the direction of the fast shear waves yields the preferred direction of fracture propagation.

The directions, or azimuths, of fast and slow shear waves can be seen in a crossed-dipole log. A crossed-dipole log from the Cuitlahuac-832 well shows both isotropic and anisotropic zones (previous page). Z one A, an isotropic zone, is identified by near-zero offline energies and equal fast and slow shear-wave slownesses.2 6 Anisotropic Z ones B and C are identified by nonzero offline energies and diverging fast and slow shear-wave slownesses.

The two anisotropic zones have different amounts of anisotropy. In Z one B, anisotropy magnitude is about 8% . In Z one C, the amount of anisotropy is about 2% . Although 2%

is lower than

has been reliably detected by other tools, interpreters

have confidence in the value

because the waveforms are so clear and because the fast shear azimuth remains constant, between 30° and 40° over the interval, even with the tool continually rotating.

Knowing the magnitude and azimuth of

anisotropy is vital, but this does not identify the cause. The anisotropy may be intrinsic to the rock or may be stress-induced; identifying the cause is important for understanding how stable the drilling process will be and how a borehole will respond to stress. Usually, additional information, such as borehole images or core analysis, is required to pinpoint the cause of anisotropy. Analysis of flexural-wave dispersion curves

provided by the Sonic Scanner tool helps to identify anisotropy mechanisms at three depths in the Cuitlahuac-832 well using only sonic measurements. Dispersion curves at 1,5 93.04 m, within Z one A, overlie each other closely and match the model for a homogeneous isotropic formation. Curves from 1,665 .27 m, one of the most anisotropic intervals near the bottom of

Z one B, show the crossover characteristic of stress-induced anisotropy. Slightly shallower, at 1,65 8.87 m, the fast and slow shear dispersion curves are separated at low frequencies, but the

high-frequency data are missing, so it is impossible to determine the curve trend or the anisotropy type. OBMI Oil-Base MicroImager images at this depth indicate the presence of open, induced fractures, which are the likely cause of the loss of high-frequency data and also strongly suggest stress-induced anisotropy. The 45 ° azimuth of

fractures seen in OBMI

images correlates well with the 40° azimuth of maximum horizontal stress inferred from the fast shear direction.

In the Burgos basin, maximum horizontal stress has traditionally been taken to be parallel to the strike of the nearest faults. The results from Sonic Scanner logging in five wells in this basin indicate that local stress direction can vary significantly— up to 20° from the strike of nearby faults— accentuating the importance of making localized sonic measurements before designing perforation, stimulation and infill- drilling operations.

Imaging Well Beyond the Wellbore The superior quality of waveforms acquired with the Sonic Scanner tool allows for improved imaging away from the borehole. Sonic imaging uses reflected P-waves to detect reflectors that are subparallel or at low angle relative to the borehole.

Norsk Hydro has used the imaging capability of the Sonic Scanner tool in a highly deviated well in the Norwegian Sea (above). Following acquisition of standard sonic waveforms in one TLC Tough Logging Conditions wireline pass, a separate TLC imaging pass recorded waveforms every 0.5 ft [ 15 cm] from the three monopole sources firing sequentially to the 104 receivers

24 . Elastic anisotropy is som etim es called acoustic anisotropy or velocity anisotropy . It can b e ex pressed in term s of a difference of velocities, slow nesses, stresses or elastic param eters.

Arm strong P, Ireson D, Chm ela B, Dodds K , Esm ersoy C, Miller D, Hornb y B, Say ers C, Schoenb erg M, Leaney S and Ly nn H: “ The Prom ise of Elastic Anisotropy , ” Oil eld Review 6, no. 4 ( Octob er 19 9 4 ) : 3 6– 5 6

25 . Arroy o Franco J L, Gonzalez de la Torre H, Mercado Ortiz MA, Weilem ak er E, Plona TJ , Saldungaray P and Mik haltzeva I: “ Using Shear-Wave Anisotropy to Optim ize Reservoir Drainage and Im prove Production in Low -Perm eab ility Form ations in the North of Mex ico, ” paper SPE 9 6808, presented at the SPE Annual Conference and Technical Ex hib ition, Dallas, Octob er 9 – 12, 2005 .

Wielem ak er E, Saldungaray P, Sanguinetti M, Plona T,

Y am am oto H, Arroy o J L and Mercado Ortiz MA: “ Shear- Wave Anisotropy Evaluation in Mex ico’ s Cuitlahuac Field Using a New Modular Sonic Tool, ” Transactions of the SPWLA 4 6th Annual Logging Sy m posium , New Orleans, J une 26– 29 , 2005 , paper V.

26. The difference b etw een slow nesses is called slow ness anisotropy , and the difference b etw een arrival tim es is called tim e-b ased anisotropy .

W ellbore

> Geologic cross section w ith traj ectory of a deviated w ell in w hich Norsk Hy dro acq uired Sonic Scanner im aging data. The high deviation angle req uired TLC Tough Logging Conditions w ireline logging.

Spring 2006


I nterval logged with Sonic Scanner tool

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