The Alba Field Ocean Bottom Cable Seismic Survey: Impact on Development

Abstract

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.

4D study of fluid effects on seismic data in the Gullfaks Field, North Sea

“…On a converted stack (PS) seismic dataset (P‐waves down to the reservoir interface being converted into S‐waves at the reservoir interface and propagating back to the receiver as S‐wave) the response from such a fluid contact should be weaker than the corresponding response on the conventional PP stack. A nice example of this effect can be observed on seismic data from the Alba field (MacLeod et al . 1999).…”

Preview Number 108

“…The impact of the 4-C OBC survey on the development of Alba has been positive (MacLeod, et al, 1999b). To date, a number of successful wells have been drilled based primarily on the interpretation of the new converted-wave data.…”

Imaging through gas clouds: A case history from the Gulf of Mexico

“…Introduction Multicomponent 3D surveys have become one of the leading areas where state of the art technology is being applied to areas where the exploitation potential has not been fulfilled due to difficulties in obtaining an interpretable seismic image (MacLeod et al 1999, Rognoe et al 1999. In areas where conventional datasets suffer from poor data quality due to the presence of gas and/or fluids in the pore spaces, it has been shown that ocean-bottom acquired multicomponent surveys have improved images (Thomsen, et al 1997, Granli et al 1999.…”