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GCTime-Lapse
4-D Technology:
Reservoir
Surveillance*
By
David H. Johnston1
Search and Discovery Article #40142 (2005)
Posted February 9, 2005
*Adapted from the
Geophysical Corner column in AAPG Explorer, December, 2004, entitled
"4-D
Gives
Reservoir
Surveillance,”
and prepared by the author. Appreciation is expressed to the author, as well as
to Alistar R. Brown, editor of Geophysical Corner, and Larry Nation, AAPG
Communications Director, for their support of this online version.
1ExxonMobil Exploration Company, Houston, Texas ([email protected])
Reservoir
surveillance during production is a key to meeting goals of reduced
operating costs and maximized recovery. Differences between actual and predicted
performance are typically used to update the
reservoir's
geological model and to
revise the depletion strategy. The changes in
reservoir
fluid saturation,
pressure, and temperature that occur during production also induce changes in
the
reservoir
acoustic properties of rocks that under favorable conditions may
be detected by seismic methods.
The
key to seismic reservoir
surveillance is the concept of differential imaging
using time-lapse, or 4-D measurements. Time-lapse seismic methods are usually
based on differences in seismic images that minimize lithologic variations and
emphasize production effects. The concept is illustrated in
Figure 1, where a base 3-D survey acquired before
production is compared with a monitor 3-D survey acquired at a later time,
dependent on the recovery process to be monitored.
The difference
between the seismic surveys can then be interpreted in terms of the
production-related changes in reservoir
properties. Time-lapse seismic data have
been shown to increase reserves and recovery by:
-
Locating bypassed and undrained reserves.
-
Optimizing infill well locations and flood patterns.
-
Improving
reservoir
characterization -- identifying
reservoir
compartmentalization and permeability pathways.
Four-D also can decrease operating costs by:
-
Reducing initial development well counts.
-
Optimizing phased developments using early
field
-wide surveillance data.
-
Reducing dry holes and targeting optimal completions.
-
As a result of these benefits, many oil companies are aggressively pursuing the application of time-lapse seismic data.
uGeneral StatementuFigure captionsuPhysical basisuSeismic repeatabilityu4-D acquisition, processingu4-D interpretation
uGeneral StatementuFigure captionsuPhysical basisuSeismic repeatabilityu4-D acquisition, processingu4-D interpretation
uGeneral StatementuFigure captionsuPhysical basisuSeismic repeatabilityu4-D acquisition, processingu4-D interpretation
uGeneral StatementuFigure captionsuPhysical basisuSeismic repeatabilityu4-D acquisition, processingu4-D interpretation
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The Physical Basis
Seismic
velocity and density changes in a producing
The
optimal times for repeat seismic surveys depend on detectability and the
Seismic Repeatability
The
difference between two seismic surveys is not only sensitive to changes
in
4-D Seismic Acquisition, Processing
The
objective of 4-D seismic acquisition and processing is to minimize
differences in the seismic data that are unrelated to production -- and
to preserve and resolve those differences in the A number of strategies have been developed to maximize acquisition repeatability for both land and marine data, and permanent monitoring systems -- such as the BP's installation at Valhall -- can result in high repeatability. While there is a large up-front cost associated with fixed receivers, these systems can permit the acquisition of lower-cost monitor surveys with short repeat intervals or "on demand."
Four-D processing is best described as co-processing or parallel processing of base and monitor surveys. This implies:
A key to successful time-lapse processing is continual comparison of the base and monitor surveys to ensure that repeatability is not being compromised. Often, "fast track" data (e.g., decimated, post-stack migrated and/or using parameters based on earlier processing) are used to evaluate the processing flow and refine interpretation concepts. Also, the objective to maximize repeatability may be at the expense of other processing objectives, such as high-resolution imaging. As a result, it is not uncommon that separate flows are used for time-lapse data. 4-D Interpretation
The
interpretation of time-lapse seismic differences in terms of
This approach is used because 4-D seismic interpretations are non-unique.
An example
of 4-D interpretation is from the North Sea Jotun
Figure 3 compares the results of inverting
the seismic difference acquired after three years of production to
obtain impedance change with the oil saturation change predicted by the
Infill or
sidetrack opportunities are found where there is no change in the
seismic data and where
Other
published 4-D case studies show that seismic data can image production
changes in a variety of geological settings and production scenarios,
including water and gas sweep, pressure changes and compaction, and
enhanced recovery. Further, 4-D interpretation is evolving toward a more
quantitative analysis of the data. By incorporating time-lapse shear
wave information, either from AVO analysis, elastic inversion, or PS
data, it is possible to estimate saturation and pressure changes in the
More predictive
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