American Oil and Gas Reporter - August 2019 - 69
SpecialReport: Seismic & Geophysics
Approach Improves Reservoir Modeling
By Anat Canning
and Yair Gordin
HOUSTON-When an unconventional
reservoir is modeled, many important
physical characteristics can be estimated
using advanced seismic azimuthal analysis,
including fracture density and orientation,
pore pressure, stress perturbation, and
principal stress directions. Two main types
of azimuthal analysis can be performed
using a 3-D dataset or seismic offset gathers: seismic velocity (or travel time) variations with azimuth (VVAZ), and seismic
amplitude variations with azimuth (AVAZ).
These two analysis types then can be
combined with the results of seismic inversion (rock physics) to derive the greatest
benefit from the seismic data.
As an example, Figure 1 shows an
Eagle Ford horizon vector map presenting
the azimuth (vector direction) and intensity
(vector length and color) of VVAZ analysis. However, these types of azimuthal
inversions can suffer from a significant
degree of unreliability. Fortunately, uncertainty analysis can be performed to
eliminate unreliable results from the azimuthal data, and thereby increase its reliability. This process aims to develop a
robust unconventional reservoir model
that can be used to delineate sweet spots
and optimize well location.
Horizontal drilling and multistage hydraulic fracturing technologies have
changed the energy sector dramatically
by enabling hydrocarbon recovery from
extremely low-permeability rocks. Nevertheless, developing unconventional reservoirs is expensive, and therefore requires
optimal designs in order to be economically justified when oil prices are relatively
low. Recent studies show that well production performance can change significantly from one well to another within
the same reservoir, and that economic
justification often relies on the most productive wells. Wells' performance variations may be attributable to various subsurface conditions and changes in the
formation target's local properties, such
as clay content, existing natural fractures,
overpressured zones (high pore pressure),
stress regime, etc.
An obviously efficient way to optimize
well performance is to avoid drilling in
less productive zones. This can be achieved
by integrating different geophysical seismic
imaging techniques, using seismic data
and well logs, to delineate sweet spots
with the best production potential. The
most effective way of using geophysical
data for imaging and characterization of
potential targets is to combine different
and independent geophysical observations,
which then are examined from a geological
point of view.
Integrating petrophysical information
based on laboratory studies with seismic
data analysis may supply additional valuable insights into the investigation process,
such as by providing boundaries for possible values to help calibrate the geophysical models.
The seismic data attributes that can
be used to model an unconventional reservoir may be divided into several general
categories of increasing complexity:
· Post-stack geometrical (structural)
attributes such as dip, dip azimuth, continuity and curvature used to visualize
the geological structures and the main
faults in the reservoir;
· Diffraction imaging;
· Amplitude versus offset (AVO) type
inversion;
· Rock physics seismic inversion attributes created by using well data to
provide the missing low frequencies and
enable representation of the rock layer's
physical properties (instead of seismic
boundary properties); and
· Azimuthal 360-degree seismic
anisotropy analysis of both amplitudes
(AVAZ) and velocities (VVAZ).
Innovative Approach
The latter two categories-inversion
and azimuthal analysis-can be used to
estimate fracture density, fracture orientation, pore pressure, stress perturbations
and stress directions, all of which are
very important physical reservoir characteristics for unconventional exploration
and production. Figure 2 illustrates VVAZ
and AVAZ analysis, showing real data
VVAZ and AVAZ cross-sections (panels
A and C, respectively), a synthetic VVAZ
cross-section (B), and an AVAZ 3-D
gather view (D).
As noted, however, both VVAZ and
AVAZ azimuthal analysis methods can
suffer from substantial unreliability. Conventional AVAZ workflows are highly
sensitive to instability/reliability problems
because of the standard data organization
often used (azimuth sectoring), or because
the "azimuth space" is inadequately sampled. Another problem with standard
practices is that azimuths are estimated
from the surface, while the analysis assumes that azimuths are measured at the
subsurface (reflection point).
To solve these issues, an innovative
approach for migrating multiple azimuth
data has been developed, opening new
horizons for more reliable azimuthal
AVAZ and VVAZ analysis. This approach
ensures that densely sampled, full-azimuth
3-D angle gathers in depth are available
for analysis, providing high-resolution
and azimuth-angle domain information
in true subsurface coordinates. The ac-
FIGURE 1
Eagle Ford Horizon Vector Map Showing Azimuth
And Intensity of VVAZ Analysis
AUGUST 2019 69
American Oil and Gas Reporter - August 2019
Table of Contents for the Digital Edition of American Oil and Gas Reporter - August 2019
Contents
American Oil and Gas Reporter - August 2019 - Intro
American Oil and Gas Reporter - August 2019 - 1
American Oil and Gas Reporter - August 2019 - 2
American Oil and Gas Reporter - August 2019 - Contents
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