Click
to article in PDF format.
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 reservoir model uncertainty.
-
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
|
The Physical Basis
Seismic
Reservoir factors that affect the seismic detectability of production changes can be evaluated in order to determine which geological settings and production processes are most suited for reservoir monitoring. Each field is unique, and modeling of the seismic response to production, based on reservoir flow simulation, is used to evaluate the interpretability of seismic differences and to determine how early in field life a time-lapse survey can be used to monitor reservoir changes.
The
optimal times for repeat seismic surveys depend on detectability and the
field's development and depletion plan. Planning for repeat surveys in
the context of field surveillance will maximize the value of the Seismic RepeatabilityThe difference between two seismic surveys is not only sensitive to changes in reservoir rock properties but also to differences in acquisition and processing. As suggested in Figure 2, the chance of success for a 4-D project depends on both detectability and seismic repeatability. Some of the factors that affect repeatability include:
4-D Seismic Acquisition, Processing
The
objective of 4-D seismic acquisition and processing is to minimize
differences in the seismic
A number
of strategies have been developed to maximize acquisition repeatability
for both land and marine
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"
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 4-D Interpretation
The
interpretation of time-lapse seismic differences in terms of reservoir
changes requires integration of the
This approach is used because 4-D seismic interpretations are non-unique.
An example of 4-D interpretation is from the North Sea Jotun Field, where oil is being depleted through a strong natural water drive. Water sweep in the reservoir results in a 10-12 percent increase in the seismic impedance.
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
reservoir flow simulation. At this location, the simulator suggests that
the reservoir is fully swept -- but the seismic
Infill or
sidetrack opportunities are found where there is no change in the
seismic
Other
published 4-D case studies show that seismic More predictive simulations will result in more efficient reservoir management.
|