American Oil and Gas Reporter - August 2019 - 72

SpecialReport: Seismic & Geophysics
the successful production of unconventional reservoirs. Therefore, azimuthal
seismic anisotropy investigation may be
beneficial for developing unconventional
reservoirs with multistage hydraulic fracturing. These essential conditions can include the presence of vertically aligned
fractures, pore pressure inside fractures
and pores, fracture density (number of
fractures per unit area of rock), and the
direction of principal in situ stress.
Azimuthal anisotropy investigation
can shed light on these reservoir conditions and attributes to optimize well location and ensure high-performance well
production. For example, the direction
of the principal in situ stress is highly
important because the propagated faults
created by multistage hydraulic fracturing
stimulation always propagate in the direction perpendicular to the least principal stress. Therefore, horizontal drilling
of a production well should be in the
same direction.
A recent study conducted by Shell
Canada in the Duvernay Shale compared
two wells, one drilled in the direction of

the least principal stress, and the other
drilled at an oblique angle (~45 degrees)
to the least principal stress. Although
both wells showed good fracture propagation, the well drilled in an oblique
angle to the least principal stress produced
at considerably lower rates.
The difference between the wells'
performance seemed to be the result of
much better proppant distribution during
hydraulic fracturing when wells are
drilled in the direction of the least principal stress. The proppant is carried by
the injected frac fluid in order to keep
the fracture open when the fluid pore
pressure drops, and allows hydrocarbons
to flow back to the surface. Therefore,
proper proppant distribution is a crucial
step in hydraulic stimulation. (For detailed
results of this study, see SPE 189863,
"Geometry and Failure Mechanisms from
Microseismic in Duvernay Shale to Explain Changes in Well Performance with
Drilling Azimuth," by Ben Stephenson,
et. al., from the Society of Petroleum
Engineers' 2018 Hydraulic Fracturing
Technology Conference & Exhibition).

FIGURE 4
Eagle Ford AVAZ Reliability Analysis using Vertical Cross-Sections
(A)

(B)

(C)

Estimating Reliability
Apart from the need to migrate raw
data properly to produce reliable results
for azimuthal analysis, the azimuth information derived from prestack seismic
data at target layers is very sensitive to
many aspects of the analysis process,
making reliability a serious concern. This
is true for both AVAZ and VVAZ analysis.
Also, unlike velocity analysis or migrations, in which the assessment of the
quality and robustness of the results is
relatively straightforward and can be done
by observing them in the image domain,
analyzing attributes along a horizon-oriented map does not provide clear insight
into the data's reliability. This statement
is generally true for every "horizon-oriented attribute map," but in the case of
azimuthal analysis, the lack of such clear
insight begins with the data itself, even
before it is extracted onto a horizon slice.
This becomes the main obstacle in the
analysis of azimuthal information and is
dangerous because it may lead the interpreter to depend on unreliable data.
The new approach to address the reliability problem includes reliability estimates in the performed VVAZ and AVAZ
analyses. This process is illustrated by
vertical cross-sections (Figure 4) and horizontal maps (Figure 5) in the Eagle Ford
dataset. Reliability is defined as a combination of amplitude, semblance and
anisotropy intensity criteria, and is included
in both the VVAZ and AVAZ algorithms.
Points marked unreliable are assumed to
be isotropic, and for those, an isotropic
analysis replaces the anisotropic one.
These points are marked in the azimuthal
anisotropy attribute volume, and when
extracted onto the final result, the unreliable tags are maintained.
This process is illustrated for both the

FIGURE 5
Eagle Ford AVAZ Reliability Analysis using Maps
(A)

(B)

High

72 THE AMERICAN OIL & GAS REPORTER

(C)

Low

Coming In September
Part III of the Horizontal Well Architecture series examines the results
of a well spacing trial project in the
Bakken/Three Forks that included
comprehensive multidisciplinary data
acquisition and extensive hydraulic
fracture, geomechanical and reservoir
modeling. Reports also spotlight a
case study of improving the performance of Eagle Ford infill wells.

American Oil and Gas Reporter - August 2019

Table of Contents for the Digital Edition of American Oil and Gas Reporter - August 2019