Azimuthal variation in AVO response for fractured gas sands
- Schlumberger Cambridge Research (United Kingdom)
In the presence of aligned vertical fractures, the reflection amplitude at finite offset varies with azimuth. The effect of natural fractures on the azimuthal AVO response from a gas-sandstone reservoir encased within shale is investigated. A simple expression for the difference in P-wave reflection coefficient from the top of the reservoir parallel and perpendicular to the strike of the fractures is obtained in terms of the normal and tangential compliances, Z{sub N} and Z{sub T}, of the fractures. For a given value of Z{sub T}, the azimuthal variation in reflection coefficient at moderate offsets is found to increase with decreasing Z{sub N}/Z{sub T}. For gas-filled open fractures Z{sub N}/Z{sub T} {approx} 1, but a lower ratio of Z{sub N}/Z{sub T} may result from the presence of cement or clay within the fractures, or from the presence of a fluid with a non-zero bulk modulus. For Z{sub N}/Z{sub T} = 1 and moderate offsets, the variation with offset of the reflection coefficient from the top of the fractured unit is dominated by the contrast in Poisson`s ratio between the gas sand and the overlying shale, the effect of fractures only becoming noticeable as the critical angle for the unfractured sandstone is approached. However, for reflections from the base of the fractured unit, the variation in reflection amplitude with azimuth is much greater at conventional seismic offsets than for the reflection from the top. Azimuthal variations in the strength of the reflection from the top of the reservoir depend only on the variation in reflection coefficient, whereas the raypath is also a function of azimuth for reflections from the base of the fractured unit, leading to stronger, more visible, variations of AVO with azimuth. It follows that an azimuthal variation in AVO due to fractures in the overburden may be misinterpreted as due to the presence of aligned fractures in the reservoir.
- OSTI ID:
- 438885
- Journal Information:
- Geophysical Prospecting, Vol. 45, Issue 1; Other Information: PBD: Jan 1997
- Country of Publication:
- United States
- Language:
- English
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