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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.
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Reducing reservoir model uncertainty.
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Reducing dry holes and targeting optimal completions.
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As a result of these benefits, many oil companies are aggressively pursuing the application of time-lapse seismic data.
uGeneral StatementuFigure captionsuPhysical basisuSeismic repeatability
u4-D
acquisition,
|
The Physical BasisSeismic velocity and density changes in a producing reservoir depend on rock type, fluid properties, and the depletion mechanism. Time-lapse seismic responses may be caused by:
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 data. Seismic Repeatability
The
difference between two seismic surveys is not only sensitive to changes
in reservoir rock properties but also to differences in acquisition and
4-D Seismic
Acquisition,
|