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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: The Winds of Change: Anisotropic Rocks—
Their Preferred Direction of Fluid Flow and Their
Associated
Seismic
Signatures
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By
Consultant
Although 20 years ago it was politically incorrect to admit
that horizontal permeability anisotropy
resulting from
aligned connected porosity was linked with
seismic
anisotropy
(azimuthal
anisotropy
), the winds have changed.
Our industry now has a respectable
worldwide effort in research, acquisition,
processing, interpretation and
modeling that pursues precisely that
linkage. The current thought process is
that unequal horizontal stresses and/or
vertical aligned fractures can provide the
aligned, connected porosity that may
result in horizontal permeability
anisotropy
. The presence of vertical
aligned fractures and/or unequal horizontal
stresses typically causes azimuthal
anisotropy
.
The earliest efforts pursued the azimuthal variation of PP and SS
traveltimes and amplitudes, because these pure-mode seismic
waves measurements are the “easiest” measurements our industry
can process and interpret, and we believe we understand traveltimes
and amplitudes. Thus our documentation of the
relationship of azimuthal PP and split shear-wave measurements
was founded.
As time went on, the PS modes (P-S1 and P-S2) or the split
C-wave (converted wave—P down and S up) were used to
document the shear-wave anisotropy
arising from unequal
horizontal stress and/or vertical aligned fractures.
Now, however, our industry is grappling with what researchers
point out as the “biggest” anomaly that links horizontal permeability
anisotropy
to
seismic
anisotropy
—azimuthal variation in
attenuation. However, attenuation has usually received cursory
dismissal.We don’t like “dim zones” being “pay” because (1) they
are “too hard” to map, (2) there are too
many other reasons for dim zones rather
than azimuthal attenuation and (3) attenuation
is too hard to quantify and
attribute to any one cause per se. In the
past, we have often used trace equalization,
AGC, spectral whitening and other
very powerful processing techniques
to remove dim zones. Processors worth
their salt made those pesky dim zones
look nice and bright and sharp!
In the past, attenuation has been a classic
problem and not a “solution” to anything. Now, however, we can
glide forward on the next wave of multi-component,
multi-mode, multi-azimuth 3D and 4D seismic
powered by the
winds of change.
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