Seismic Fracture Detection
Exploiting the Range of Seismic Signatures
Using Rock Physics Principles
www.rocksolidimages.com
Rock Solid Images is a leader in applying rock physics principles to map fractured reservoir physical properties from seismic data.
The sensitivity of seismic wave propagation to cracks and fractures is one of the fundamental observations of Rock Physics. Cracks and
fractures:
•
•
•
•
•
Decrease P- and S-wave velocity,
Increase velocity dispersion and wave attenuation,
Increase pressure-dependence of velocity,
Increase velocity and attenuation anisotropy,
Increased potential for stress-induced anisotropy
Rock Solid Images’ strategy for fracture detection is to exploit these many different seismic signatures of fractures.
Seismic P- and S-waves tend to “see” fractures when:
•
•
their direction of propagation or
their direction of polarization is perpendicular (or nearly perpendicular) to the fracture faces. Hence, anisotropy is often a key
indicator of fractures.
S fast
S slow
P fast
P slow
S slow
Fracture Related Azimuthal Variation of AVO
Shear Wave Splitting
There is no question that P-waves “see” fractures. Vertically
propagating P-waves will generally not be very sensitive to vertical
fractures. However, as offsets increase a natural fracture-induced
velocity anisotropy can exist. Source-receiver azimuths parallel to
the fracture strike will tend not to see the fractures, while sourcereceiver azimuths perpendicular to the fractures will be slowed.
Since fractures affect both P- and S-waves, azimuthal variations in
AVO can be a powerful tool to not only detect fractures, but also
to identify the pore fluid.
Common field targets are sites with approximately vertical,
subparallel fractures. Vertically propagating shear waves that are
polarized parallel to the fracture planes will travel faster than
shear waves polarized perpendicular to the fracture planes. Any
vertically propagating shear wave incident on the medium at
arbitrary polarization will immediately split into the two modes,
called the fast shear wave and the slow shear wave. This is shear
wave splitting or birefringence.
P-to-P reflectivity versus azimuth and angle of incidence
The example below shows shear wave data of different polarizations. The
travel time difference between fast and slow shear arrivals (lower left) can
be an indicator of fracture density (lower right).
0.0
8
xline #205
xline #205
4
F
0
-4
-8
215
Crack Density
Gas saturated cracks
S-wave Travel time Anisotropy (%)
Water saturated cracks
0.0
0
-0.04
-0.08
220
225
CDP
230
235
215
220
225
CDP
230
235
P-wave travel-time picks versus. azimuth (left) and azimuthal variation of far-offset reflectivity (right), indicating fracture-related
anisotropy. Multiple curve fits are from a bootstrap method, used to explore the range of plausible models that are consistent with the
data. (From Teng and Mavko, 1998 )
Fracture Related P-Wave Attenuation Seen as Loss of
A common strategy is to infer the most likely fracture orientation Frequency
Emphasis on the Rock Physics Fundamentals
from the seismic symmetry directions, and the regions of highest
fracture density from the highest anisotropy.
However,
quantifying the underlying fracture density, shape, aperture,
connectivity, and pore fluid in geologically reasonable terms is
difficult.
Chen (1995), using simple spectral analysis of stacked traces,
found a rough correlation of lower average frequency and fracture
occurrence on 2D P-wave lines. Rock physics mechanisms for
attenuation include wave-induced pore fluid movements and
scattering.
As with many geophysical problems, the most reasonable and
realistic fracture interpretation requires a careful rock physics
analysis, coupled with solid geologic input.
1601
Rock Physics Fracture Models
1501
Penny
shaped
crack
models
Discontinuous
slip models
Using proper rock and fluid properties is critical (more so than the
mathematics) to getting realistic results.
Fluid Effects
40Hz
1401
35
30
1301
High Frequencies
(unfractured)
1201
25
1101
We use state-of-the art analyses of fluid effects on seismic
fracture signatures. Adiabatic estimates of gas, oil, and brine
compressibilities and densities are combined with frequencydependent fracture-fluid models for seismic interpretation and
modelling.
0.025
Adiabatic
Isobaric
0.02
brine oil
AVOZ anisotropy increases
with crack density, crackfilling fluid bulk moduli, and
seismic frequency
source station numbers
PCI incorporates the two accepted approaches for quantifying the
elastic signatures of fracture orientation and density, recognizing
that both are only idealized analogs of real reservoir fractures:
Low Frequencies
(fractured)
0.015
30MPa gas
15MPa gas
0.01
1101
1201
1301
1401
1501
1601
receiver station numbers
Rock Solid Images is a seismic reservoir characterization
company specializing in the integration of rock physics
and seismic attributes. The services we provide are the
work of a talented and experienced inter-disciplinary team
who partner closely with our clients. Our final deliverable
is a calibrated reservoir volume(s), used to expedite
prospect evaluation and improve decision-making
processes of our clients.
0.1MPa gas
0.005
brine
oil
0
30, 15, 0.1 MPa gas
-0.005
0
0.02
0.04
0.06
Crack density
0.08
0.1
2600 South Gessner Ste 650
Houston, TX 77042
713-783-5593
www.rocksolidimages.com