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The Leading Edge; January 2001; v. 20; no. 1; p. 18-26; DOI: 10.1190/1.1438870
© 2001 Society of Exploration Geophysicists
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Cracking the fractures—seismic anisotropy in an offshore reservoir

Robert L. Smith and John P. McGarrity

BP, Aberdeen, U.K.

Corresponding authors: mcgarrj2@bp.com and smithrl@bp.com

The first 20% of the full text of this article appears below.

The Clair Field lies west of the Shetland Islands on the U.K. continental shelf in 140 m of water (Figure 1).


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  		Figure 1. Map of Clair Field offshore United Kingdom.

 
The reservoir comprises a thick (about 600 m) succession of fluvial, aeolian, lacustrine, and marginal marine Devonian-Carboniferous clastics. Studies on orientated core and field analog data indicate that Clair contains a complex variety of fracture types, orientations, and scales (Figure 2).


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  		Figure 2. Fracture types identified from core analysis. Wells intersecting open fractures are the most productive.

 
Of particular importance are the larger, through-going, open conductive fractures. Wells intersecting these fractures are the most productive in the field. Well 206/8-10Z attained maximum flow rates in excess of 10 000 bd, and a nearby pilot well showed pressure depletion over a large vertical section of the reservoir. Conversely, wells in different areas of the field produced little or no flow.

It is against this background that studies were initiated to see whether surface seismic attributes could provide a tool for optimum placement of development wells. The attribute of particular interest was moveout velocity variation as a function of azimuth. In principle, seismic waves travel fastest in directions parallel to open fractures and slowest in the orthogonal direction. This azimuthal anisotropy phenomenon, caused by variations in the shear modulus of the fractured rocks, has encouraged studies throughout the world to investigate variations in shear-wave seismic. However, recent research has shown that P-waves traveling obliquely to these fractures can be influenced strongly by the shear properties of the rock and measurable effects can be observed away from the fractures. This means that conventional P-wave seismic acquired by towed arrays in marine environments may help characterize offshore fractured reservoirs such as Clair, and that expensive multicomponent shear-wave acquisition may not be needed.


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