American Oil and Gas Reporter - November 2018 - 62

SpecialReport: Oil and Gas Computing
make it ideal for 4-D time-lapse reservoir
monitoring to detect subtle changes and
measure the effectiveness of enhanced
oil recovery methods. Detailed 4-D analysis also can identify bypassed reservoir
compartments that are not being depleted
effectively.
Node technology has been extended
to semi-permanent reservoir monitoring.
This type of node has a five-year deployment life, 500 days of data recording capacity and an optical data harvesting system, allowing for multiple time-lapsed
monitoring surveys before node recovery.
Monitor surveys are single-vessel operations, and seafloor node layouts can be
reconfigured or expanded as field development plans are revised.
The development of nodal technology
and the creation of sophisticated imaging
algorithms go hand in hand. These algorithms require the data that only nodes
can deliver. The full-azimuth, long-offset
data are a perfect complement for new
imaging algorithms, such as full-waveform
inversion (FWI), which requires ultralong
offset and low-frequency data to produce
greater detail and insights.
Nodes are highly scalable. That scalability enables the acquisition of fit-forpurpose surveys, and ultimately, large-

scale surveys with long offsets and full
azimuths, which enhance operator decision
making. Fit-for-purpose scalable seismic
surveys are becoming increasingly common. The sources for these ultralong
offset surveys are 20-30 kilometers away
from the nodes and very densely sampled
in azimuth.
The latest trend is acquiring a "velocity
survey" to create highly accurate velocity
models to improve existing older seismic
data that do not have the benefit of longer
offsets. New imaging technologies that
require ultralong offsets, full azimuths
and low frequencies also are creating demand for unique acquisition geometries
that only can be accomplished using
nodes placed on the ocean bottom.
Design Flexibility
The design flexibility inherent in
nodal acquisition enables large-scale,
deepwater ocean-bottom surveys with
multiple imaging objectives. Since the
nodes have no fixed interval constraint,
hybrid receiver geometries are possible
that satisfy the competing objectives of
dense subsurface sampling, long offsets
and large area coverage. Recent studies
have shown that typical node spacing
of 400 meters can be increased to as

New data recording and processing capabilities make it economically feasible to scale
up nodal acquisition systems for deepwater exploration surveys, signiﬁcantly improving
the quality of the seismic data available to guide drilling and development decisions.
The full-azimuth, long-offset data and high survey repeatability make nodes ideal for fullwaveform inversion and other advanced imaging algorithms, as well as time-lapse analysis to detect subtle reservoir changes over time.

62 THE AMERICAN OIL & GAS REPORTER

great as 800 meters outboard of the primary imaging area without compromising the final image.
This is important considering that
the seismic objectives in deepwater
field development often involve 3-D
imaging beneath complex overburden,
particularly below and around salt,
where illuminating a formation of interest can be highly sensitive to the azimuth and offset of the source point
relative to the node location. To optimize
imaging beneath complex overburden,
ocean-bottom node surveys are designed
to illuminate the target area from all
azimuths, and to do so continuously in
offset out to a maximum distance suitable for the survey objectives.
As the field is produced, the seismic
response of the reservoir changes over
time. This enables operators to determine
if parts of their reservoirs are not being
depleted. By comparing year-one with
year-two survey data, for example, inferences can be made about the effectiveness of well placement and how
well the reservoir is being managed.
Armed with this knowledge, operators
can avoid leaving behind undrained
reservoir compartments as well as potentially reducing the number of wells
drilled without negatively impacting the
bottom line.
Nodes are self-contained and require
no cabling, minimizing the subsea footprint
and ensuring high reliability and minimal
gaps in coverage, even in areas with extensive infrastructure or high levels of
surface activity. Moreover, because acquiring seismic data with ocean-bottom
nodes is proven safe, an overwhelming
number of oil and gas companies rely on
the technology to maintain health, safety
and environmental standards, accelerate
production, reduce costs, and ultimately,
increase reserves recovery.
Full-waveform inversion, 4-D analysis,
and data analytics are the most significant
trends impacting the life of a field. The
adoption of ocean-bottom node acquisition
has enabled the increasing use of these
technologies. Velocity surveys, for instance, make FWI more accurate by
adding the long-wavelength components
missing from current models.
Analytics turn base data into tools
that can predict rock properties and other
critical parameters. More importantly,
these trends, taken together, can lead to
fewer wells being drilled without lowering
production output. The combination of

American Oil and Gas Reporter - November 2018

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