Depositional and remobilized sandstone units are identified in core from the Eocene sand-rich deep-water Nauchlan Member and termed stratified and unstratified facies, respectively. The unstratified facies association records an increased intensity of sand remobilization, and inferred fluidization, upward. Unstratified facies have lower average porosity and permeability than stratified facies. Bulk density and acoustic velocity are higher in unstratified facies than in stratified facies. The general geometric relations of the reservoir can be inferred from a correct identification of the facies. Correlation of borehole data with (3D PS) seismic data enables the seismic to be used as a lithology indicator. A modified interpretation of sandbody geometry is made that incorporates sand injection features and provides a more accurate reservoir model.
In early 1998, Chevron acquired one of the world's first full-field four-component ocean bottom cable (3D/4C OBC) surveys at the Alba field in the central UK North Sea. The primary objective of the survey was to use converted shear waves to provide a better image of the sandstone reservoir and shales within the reservoir. Pre-survey technical studies based on a dipole sonic log and 2D OBC seismic lines gave us confidence that converted waves (PS) could provide a better image of the reservoir relative to conventional P-wave seismic data. The secondary objective of the new survey was to map water movement in the reservoir after four years of production and water injection by comparing the new P-wave OBC data with the original 1989 streamer data. A strong original oil-water contact reflector seen throughout much of the field and pre-survey technical studies suggested that production-related saturation changes would be observable on the new P-wave OBC seismic data. The new data shows that both objectives were achieved with dramatic results - the converted-wave images provide the clearest image of the Alba reservoir sands ever seen and production effects are obvious on the new P-wave OBC data near several producing and injector wells. In addition, the converted-wave data has offered new insights into the shape of the reservoir. The previous lens-shaped interpretation based on P-wave data has been replaced with a far more complex shape that is at least in part related to post-depositional structural alteration. This new interpretation is also supported by improved reservoir images seen on far-offset P-wave sections. To date, two successful wells have been drilled based primarily on the converted-wave seismic data. Both wells prove the existence of ‘wings’ – structurally high sand at the margins of the channel that may represent re-mobilized and injected reservoir sand. Neither of these wings had been previously identified on the conventional P-wave streamer seismic data.
No abstract
The 1998 Alba 3D Ocean Bottom Cable (OBC) survey was designed to accomplish multiple objectives. The primary goal was to image low impedance reservoir sands with converted wave (PS) reflections; one important secondary goal was to image fluid movement by comparing the OBC data with a 1989 streamer survey. Modelling shows that a strong original oil–water contact reflector should be visible throughout much of the field and that water saturation changes should be observable by analysing the time-lapse differences between the 1989 streamer data and 1998 OBC survey. Differences between the 1989 and 1998 seismic field data confirm that fluid changes are clearly visible near several producing and injector wells. However, extracting additional quantitative saturation information from the seismic data has proven difficult, possibly because of: (a) complex interaction between the fluids, sands and shales within the Alba reservoir; (b) moderate to poor repeatability of the seismic response to reservoir fluids. The focus of this paper is the acquisition and analysis of Alba time-lapse data. We show that production- and injection-related effects are predicted by modelling and observed in the data and then we make an attempt to relate these effects quantitatively to oil production and water injection. Despite the challenges in using the Alba time-lapse data quantitatively, the data have been successfully used qualitatively for well planning risk assessment and for guiding reservoir simulation efforts. Lessons from this work will be used in any future time-lapse surveys at Alba.
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