About This Item
- Full text of this item is not available.
- Abstract PDFAbstract PDF(no subscription required)
Share This Item
The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Seismic Reservoir Characterization of a Gas Shale
Utilizing Azimuthal
Data
Processing, Pre-Stack Seismic
Inversion and Ant Tracking
![Previous Hit](/data/images/arrow_left.gif)
![Next Hit](/data/images/arrow_right.gif)
1Schlumberger Data
& Consulting Services, Houston
2WesternGeco, Denver
3Schlumberger Data
& Consulting Services, Denver
Prospective hydrocarbon-bearing zones in gas shales are characterized by primary gas storage entrapped in the sediment matrix with some additional gas in the open fractures. This gas is economically recovered by horizontal drilling and fracturing. Mineralization of faults is a reservoir risk. Faults also represent significant completion risk, as fracture completion jobs often are “captured” by nearby faults.
A wide azimuth 3D survey was acquired of
the studied shale to highlight areas exhibiting
seismic velocity
anisotropy for detection of
open fractures. Simultaneous prestack inversion
of the seismic
data
to Poisson’s ratio
targets matrix-stored gas. Seismic processing
for the detection of horizontal anisotropy targets the gas stored in
fractures. Reservoir risk related to fault mineralization is addressed
through a detailed imaging of the fault planes
and the detection of anomalously low
anisotropy along and adjacent to faults.
Prospective zones for gas production are
identified by areas of anomalously low
Poisson’s ratio away from faults, with (in
this particular shale) high velocity
anisotropy. This study documents the
results of an integrated workflow of
data
processing, pre-stack seismic inversion and
Ant Tracking to successfully characterize
faults and fractures and to identify sweet
spots in the gas shale.
During data
processing, azimuthal anisotropy
analysis
was conducted
to determine the dominant direction of Vfast and Vslow.
In general, there is good agreement between the azimuthal
seismic data
processing
velocity
analysis
and Ant Tracking results.
Simultaneous amplitude versus offset
(AVO) inversion was done on prestack
data
to invert for acoustic impedance (AI) and
Poisson’s ratio. Areas of low Poisson’s ratio
away from faults are thought to be promising
hydrocarbon leads or prospects.
Delineation of Matrix Storage of Gas
Simultaneous prestack inversion of the
seismic data
for Poisson’s ratio proved to be
effective in delineating areas of low
Poisson’s ratio that are thought to indicate
the primary, more siliceous, relatively more
porous, gas-charged sweet spots.
Because the studied gas shale is thought to produce primarily from the sediment matrix
Measuring
Fractures with Seismic
End_Page 35---------------
rather than open fractures, the inversion was
run on a full-azimuth basis. In a shale where
open fractures are more important, we would
recommend running the inversion twice,
once on the fast-direction data
(as determined
by the azimuthal anisotropic
data
processing) and separately on the slow-direction
data
. The fastdirection would give a good
measure of thematrix-only effects of lithology,
porosity, and charge. The slow direction
would provide, by comparison, a measure of
the effects of gas-charged open fractures.
Delineation of Gas-Charged Open Fractures
Azimuthal velocity
analysis
for anisotropy
was used to delineate areas of open fracturing
and stored stress. Comparisons of well productivity with rock
physics seismic attributes have revealed that wells drilled into
areas of high anisotropy in this particular shale have anomalously
long-lived production, presumably from more effective
fracture completions.
Detailed Imaging of Faulting
Ant Tracking reduces the risk of drilling near faults (a reservoir
risk due to expected mineralization) by providing a high resolution
image of fractures and faults beyond what can be interpreted
from conventional seismic data
. Faults were expected to be
mineralized, with some mineralization extending out into the
surrounding shales.
End_Page 37---------------
Integration with the anisotropic analysis
confirmed
this for most faults. Some faults,
however, lacked the diagnostic low anisotropy
expected with mineralization.
Conclusions
Integration of anisotropic data
processing
with pre-stack seismic inversion and Ant
Tracking provides a superior tool to explore
for gas in gas shale. The integration of
Poisson's ratio, fractogram, azimuthal
anisotropic
analysis
, and Ant Tracking from
surface seismic
data
provides actionable information
for leasing and well placement,
including the delineation of areas with
enhanced porosity and charge, areas with
open fractures, and areas with faulting, revealing
likely sweet spots as well as areas to be avoided in drilling.
Integration-The
Bigger Picture
End_of_Record - Last_Page 39---------------