This page contains a Flash digital edition of a book.
same effect at a much smaller scale leads to

“ ground roll” noise in surface seismic surveys. In 1924, Stoneley looked at waves propa- gating at the interface between two solids and found a similar type of surface wave.5


R eceiver array

The waves traveling at the fluid- borehole interface are nonetheless known as Stoneley waves. In other areas of geophysics, such as marine seismic surveys, waves traveling at a fluid-solid interface are called Scholte or Scholte-Stoneley waves.7

particular case corresponding to a fluid-filled borehole, that is, the interface between a solid and a liquid, was described not by Stoneley, but by Scholte.6

A Stoneley wave appears in nearly every monopole sonic log. Its speed is slower than the shear- and mud-wave speeds, and it is slightly dispersive, so different frequencies propagate at different speeds.


The decay of Stoneley-wave amplitude with distance from the interface is also frequency- dependent; at high frequencies, the amplitude decays rapidly with distance from the borehole wall. However, at low frequencies— or at wave-

Z one of alteration

U naltered formation

> Ray tracing using Snell’ s law to m odel ray paths. Here, ray s are traced through a form ation that has radially vary ing velocity in a zone of alteration. Velocity is low er near the b orehole and grow s larger w ith distance, a situation that arises w hen drilling induces near-w ellb ore dam age. Ray s traveling to the receivers nearest the transm itter travel only through the altered zone ( dark

b row n) , and ray s traveling to distant receivers sense the velocity of the unaltered form ation ( light b row n) .

spacing and near-wellbore altered-zone thick- ness and velocity contrast (above). In addition, ray tracing is used in inversion techniques such as tomographic reconstruction, which solves for slowness models given arrival-time information. After the P and S head waves, the next waves to arrive at the receivers from a monopole source are the direct and reflected mud waves. These are followed by trapped modes and interface waves that owe their existence to the cylindrical nature of the borehole. Trapped modes arise from multiple internal reflections inside the borehole. Wavefronts of particular wavelengths bouncing between the walls of the borehole interfere with each other constructively and produce a series of resonances, or normal modes. Trapped modes are not always seen on logs and may be affected by borehole condition. In slow formations, trapped modes lose part of their energy to the formation in the form of waves that

radiate into the formation. These are called leaky modes, and propagate at speeds between P and S velocities. Leaky modes are dispersive, meaning their different frequency components travel at different speeds.

Stoneley Waves

The last arrivals from a monopole source are interface, or surface, waves. Surface waves were first suggested by Lord Rayleigh in 1885 .4


investigated the response at the planar surface of an elastic material in contact with a vacuum and found that a wave propagated along the surface with particle motion that decreased in amplitude with distance from the surface— a property called evanescence. Rayleigh’s findings predicted the existence of waves that propagate along the Earth’s surface and give rise to the devastating shaking caused by earthquakes. The


> The Stoneley w ave, traveling at the interface bet w een the bor ehole and the form ation. The Stoneley w ave is dispersive and its particle

m otion is sy m m etric ab out the b orehole ax is. At low freq uencies, the Stoneley w ave is sensitive to form ation perm eab ility . Waves traveling past perm eab le fractures and form ations lose  uid, and viscous dissipation causes attenuation of

w ave am plitude and an increase in w ave slow ness. At open fractures, Stoneley w aves are b oth re ected and attenuated. Red arrow s in the center of the b orehole sy m b olize Stoneley -w ave am plitude.

P ermeable formation

Attenuated and slowed down

Cond ition R eceiver E f f e c t

Attenuated R eflected

Stoneley wave


Oilfield Review

F racture

Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68