Time-lapse (4D) analysis of legacy seismic data presents unique challenges, as neither the acquisition nor processing is designed for seismic monitoring. Two legacy seismic data sets from the Lena Field, Gulf of Mexico, are analysed for time-lapse effects. The analysis involves post-stack processing of the legacy seismic data including cross equalization and residual migration, and the definition of a new suite of 4D seismic attributes. These new attributes are used in both processing and interpretation. The time-lapse differences are interpreted using forward modeling and production data. The 4D difference anomaly is interpreted to be the result of gas cap expansion. The identification of potentially bypassed oil based on this interpretation may affect future drilling decisions. Introduction Seismic monitoring (time-lapse or 4D seismic) has the potential to significantly increase recovery in existing and new fields. However, there are many issues associated with the application of time-lapse seismic data. Two of the most significant are repeatability of the seismic data in the non-reservoir portion of the data volume and the robustness and credibility of the seismic difference anomaly within the reservoir (Ross et al. 1996, 1997). While future field developments should benefit from seismic acquisition designed for time-lapse monitoring, the portfolio of current seismic monitoring opportunities for most companies consists of existing fields for which one or more 3D seismic surveys have already been acquired. These legacy seismic data sets were not acquired for the purposes of seismic monitoring and are often very different in terms of acquisition and processing parameters. In addition, the seismic acquisition is rarely timed to optimally map reservoir changes or impact development decisions. Seismic repeatability is sufficient for time-lapse interpretation if the seismic differences in the region of interest are substantially greater than the differences outside the region of interest. The smaller the change in the seismic response due to production, the greater the repeatability required of the seismic data. Seismic modeling incorporating rock physics and reservoir simulation can help estimate the magnitude of reservoir changes but repeatability and interpretability can only be determined by the analysis of multiple seismic surveys. The main goal of this study is to understand the magnitude ofthe processing effort required to obtain reliable time-lapse differences. The reliability of the seismic difference is measured by repeatability in the seismic volume and the reconciliation of the time-lapse anomaly with geologic and production data. Geologic Setting The Lena Field (Mississippi Canyon Block 281) 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 dome within a fault-bounded intraslope basin. Hydrocarbon production is from six Pliocene-age sands. The B80 reservoir is located about 10,000 feet below SL at about 3 seconds seismic TWT. The interval is interpreted as a low-stand fan systems tract representing deposition in distributary lobes composed of amalgamated and channelized turbidities. The updip limit of the sands lies about 2,000 feet west of the salt flank and the reservoir thickens basin-ward to the west.
IntroduetionThere has been a resurgence of interest lately in using repeated seismie surveys as a reservoir description and production monitoring tooI. Although studies of repeated seismie surveys for mapping steam fronts go back more than 10 years, recent advances have extended time lapse or 4-D seismie to map gas-oil and oil-water contacts with significant economie bene fit (Greaves and Fulp, 1987;Pullin et al., 1987;Dunlop et al., 1991;Johnstad et al., 1995). Recent studies, e.g., Watts, 1995, have made fortunate use of legacy 3-D data not originally acquired for a 4-D study and found that the varying quality of the different vintages of 3-D data was a factor limiting the resolution of their studies. As aresuit, the question arises how best to carry out the data acquisition and processing for a 4-D seismie program so that the data are repeatable to the extent that differences are attributable to movement of the reservoir fluids rather than acquisition and processing.In this paper we present a case study ofthe repeatability ofthe ocean bottom cable (aBC) system based on two colocated 3-D surveys acquired and processed independently.First we compare off-the-shelf 3-D migrated data volumes. Then, beginning with migration, we back up in the processing sequence until we arrive at analogous flows for the two surveys. We found that in this case it was essential to reprocess the data from the prestack stage to account for known differences to achieve the best results. AcquisitionTwo aBC surveys were carried out with a separation of four months over an area that contained no known hydrocarbon production or other subsurface mechanism for changing the seismie response. The survey plan was identical for the two surveys, but for a variety of reasons, such as varying currents and wind, the receiver positions were not identical between the two surveys. Nevertheless, the flexibility of the aBC method allowed changes in receiver positions to be offset by corresponding changes in shot locations yielding fmal fold, offset, and azimuth distributions that were very similar. The same recording instruments were used for the two surveys with one significant difference: while one survey employed a 3 Hz instrument low cut filter, the other used an 8 Hz filter. As we shall see, dealing with such seemingly minor differences in the initial stages of processing is key to insuring repeatability. Off-the-shelf 4-DIn some areas with extensive 3-D coverage, existing 3-D surveys overlap and thus the appealing possibility arises to do 4-D seismie simply by studying the existing different surveys. To test the notion of such raw repeatability, we began our investigation with the existing data which were processed with similar, but independent sequences. Adding to the acquisition differences noted above, veloeities were interpreted independently, reflection staties, though smalI, had been derived from and applied to one data volume but not the other, and, although identical migration algorithms were used, the migration velocity fields and aerial e...
slolage of any part of this papel 101 comm~rclal purpo~e~WIthout the W1ltt~n consent of the ONshore Technology Conlelence is prohibIted. Pennlsslon to reproduce In print is restricted to an absllact of nOI mOle Ihan 300 WOlds; iIlustlalions may not be compiled. The abstlact must conlain conspicuous acknowledgment of whele and by whom Ihe paper was presented. AbstractThe repeatability of seismic data acquisition and processing has become an important question in determining the role of conventional land, marine, and transition zone systems in seismic monitoring of reservoirs. In this paper we examine a case study involving two repeated 3-D seismic surveys acquired using an ocean bottom cable (aBC) system. Although not originally acquired as part of a monitoring project, the survey design and preplanned source and receiver patterns were identical over a significant area. Even though there were slight differences in the source and receiver positions, the final fold, offset and azimuth distributions were very similar. We also found, not surprisingly, that since the two surveys had some differences in acquisition and processing, "off-the-shelf' final migrated data volumes showed significant differences, which could only be slightly reduced by the use of matching filters. However, by careful reprocessing of the prestack data with a sequence designed to maximize repeatability of the signal, the differences were reduced dramatically. The areal distribution of the dual-sensor summation scalars for the base and monitoring surveys were found to be similar. We also found that small phase differences can give large amplitude differences and that it is essential to account for known differences such as instrument responses and noise characteristics at the start of prestack reprocessing. The level of repeatability required for a given monitoring project depends on the specifics of the reservoir and the monitoring goals; however, in this case, repeatability achieved between the two data volumes, after careful prestack reprocessing, indicates that aBC data would be suitable for analysis of reservoir fluid movement.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.