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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
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.
Seismic Data
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.
Seismic Models
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.
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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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