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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Interpretation and Modeling of Time-Lapse
Seismic
Data: Lena Field, Gulf of Mexico
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By
1ExxonMobil Upstream Research Co.
2Western Geophysical
Abstract
Two 3D seismic
data sets from the Lena Field, Gulf of Mexico, are analyzed
for time-lapse effects. The
seismic
analysis involves cross equalization and
residual migration of the post-stack
seismic
data, as well as full reprocessing
and
attribute
analyses. The time-lapse differences for the B80 reservoir are
compared with production data, geologic models, flow simulations, and forward
seismic
models. The time-lapse
seismic
difference anomaly is interpreted to be a
region of gas invasion. Areas bypassed by the injected gas are identified from
4D
seismic
data as opportunities for infill drilling. Successful interpretation
of this time-lapse
seismic
data illustrates the importance of integrating the
results of modeling and simulation with
seismic
processing
and interpretation.
Introduction
Seismic
monitoring (time-lapse or 4D
seismic
) has the potential
to significantly increase recovery in existing and new fields. One
important issue is the significance of the
seismic
difference
anomaly relative to nonrepeatable noise. While future field
developments should benefit from
seismic
acquisition designed
for time-lapse monitoring, current
seismic
monitoring opportunities
consist of existing fields for which one or more 3D
seismic
surveys have already been acquired. The reliability of a 4D interpretation
is measured by the repeatability and the reconciliation
of the time-lapse anomaly with geologic and production data.
The objective of this paper is to interpret the
seismic
difference
observed in the Lena B80 reservoir through the use of geologic
modeling, flow simulation, and
seismic
modeling.
B80 Reservoir and Production History
The Lena Field (Mississippi Canyon Block 251) is located south of the modern Mississippi delta in 1,000 feet of water. The field is situated on the western flank of a salt diapir within a fault-bounded intraslope basin. The B80 reservoir is located about 10,500 feet below sea level and is interpreted as a low-stand fan systems tract representing deposition in distributary lobes composed of amalgamated and channelized turbidites. The average total porosity of the B80 sands is 27% and the permeability ranges from 30-200 md. The average reservoir thickness is 100 feet with a net-to-gross of 47%.
Oil production in the B80 reservoir began in 1984. The B80 has been depleted by a combination of bottom water and gas-cap expansion drive, supplemented with up-dip gas injection. Pressure decline below the bubble is believed to have trapped about 5% gas in the entire oil leg. In 1987 gas injection was initiated just below the original gas-oil contact. Gas quickly broke through to producers resulting from gravity. By 1995, most down-structure wells had watered out and many producers had high GOR production.
A preproduction 3D seismic
survey was acquired over the Lena
Field in 1983 and a regional 3D spec survey covering the field
was acquired in 1995, after 11 years of production. The 1983
survey was acquired in an east-west direction and the 1995
survey was shot in a N58°E direction. Initial differences in the
two
seismic
data volumes are substantial and are due primarily
to different acquisition and processing parameters.
A stepwise approach was taken regarding the processing of the two data volumes. Post-stack reprocessing represents an inexpensive, rapid analysis technique, whereas reprocessing both data sets represents a more rigorous, expensive, and time-consuming methodology.
One of the obstacles to full reprocessing is that the navigation data for the 1983 data are unavailable. Navigation information
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was generated based on knowledge of the acquisition parameters,
the final seismic
grid, and observer's notes. The fidelity of
the reprocessed volumes exceeds the original processing for both
the 1983 and 1995 surveys, especially for steeply dipping reflectors
at the salt flank.
For the relatively low-dip 880 reservoir, which is removed from the salt dome flank, the time-lapse difference anomaly is similar for each processing stream.
4D Difference and Interpretation
Differences of the 1995 and 1983 surveys are calculated from
interpolated time-aligned seismic
traces and illustrated in Figure
1. There is a large difference anomaly unambiguously associated
with the 880 reservoir. The anomaly is restricted to the reservoir
(outlined by the polygon). The difference is nearly zero away
from the reservoir, demonstrating that the data are repeatable
and that the
seismic
difference is significant.
Reservoir flow simulation and the 3D geologic model are used to
generate a synthetic seismic
difference volume. Petrophysical
analyses based on sonic and density logs relate the reservoir
properties in the geologic and simulation models to
seismic
properties. A comparison of the synthetic and actual
seismic
differences
is used to facilitate the interpretation of reservoir
changes imaged by 4D
seismic
data.
Geologic Models and Simulation
Geologic models of effective porosity and shale volume are initially
constructed independently for each parasequence using
Sequential Gaussian simulation. Because the reservoir is below
seismically resolvable thickness, collocated cokriging with
Bayesian updating is used to incorporate seismic
amplitude
attribute
information in the geologic model. The
seismic
attribute
is corrected for the effect of reservoir fluids using forward
seismic
modeling. The resulting reservoir flow model has a
good match between the simulated and actual cumulative
production history of the B80 reservoir.
Petrophysics
Petrophysical analysis shows that horn 1983 to 1995 the original water leg sees a very slight increase in impedance because the formation fluid pressure has declined, increasing the effective stress on the reservoir. Where oil has been swept by water, the impedance is almost unchanged because of the compensating effects of trapped gas, water displacing oil, and pressure decline on the rock frame. In the remaining oil leg, the small decrease impedance is again the result of trapped gas competing with the effect of pressure. Impedance in the original gas cap increases as a result of pressure decline. The gas-invaded zone, originally the up-dip portion of the oil leg, has the largest impedance change.
Synthetic 3D seismic
volumes representative of the 1983 and
1995 reservoir conditions are derived from the geologic models,
reservoir flow simulations, and petrophysical analysis. The most
significant change in the
seismic
response between 1983 and
1995 occurs in the gas cap expansion or gas injection zone. The
seismic
difference anomaly in Figure 2 is located in the
area invaded by gas and represents regions of significant gas
saturation changes.
Interpretation
As shown in Figure 3, the anomaly is restricted to the central
portion of the reservoir, suggesting that there may be regions of
bypassed oil or areas not contacted by gas to the north and to
the south. The area to the north may be an area of poor reservoir
quality or an area swept by water as suggested by the flow
simulation. Both conditions will result in little seismic
change.
End_Page 12---------------
Thus, an area of bypassed oil is identified to the south near the A29ST well. The interpretation is consistent with well production data.
Conclusions
Lena represents a significant challenge for the application of
time-lapse seismic
methodology. Even so, the time-lapse
seismic
analysis at Lena represents an important success. Post-stack
processing and full reprocessing of the
seismic
data have shown
that time-lapse differences in the B80 reservoir are distinct and
robust. These differences are interpreted using reservoir simulation
and forward
seismic
modeling to be the result of gas cap
expansion and/or gas injection. By comparing measured
time-lapse
seismic
differences with model predictions, areas
bypassed by the injected gas can be identified. The identification
of potentially bypassed oil may affect future drilling decisions.
Acknowledgment
We thank Exxon USA New Orleans Production Office for its support in this study.
Figure 1. Seismic
difference volume. The average absolute amplitude
map is calculated around the B80 reflection from the difference volume.
The polygon outlines approximately the B80 reservoir.
Figure 2. Cross-sections /ram 1983, 1995, and difference seismic
models.
The greatest change occurs in the gas-invaded zone. The top B80 horizon
time is shown by the line on the difference.
Figure 3. 3D visualization of the B80 seismic
difference. Wells A28, A25,
A17 have watered out, wells A5, A7, A25ST are gas injectors or producers,
wells A28ST, A18, A17ST are oil producers and the well A29ST was lost during a workover in 1994.
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