Shear wave amplitude versus offset (AVO)<br>analysis was used in Phase VII Reservoir<br>Characterization Project in Vacuum Field, New<br>Mexico, for porosity and permeability estimation and<br>time-lapse investigation of the Upper San Andres reservoir.<br>DeVault (1998) pointed out that plane-wave<br>reflection coefficients for shear waves can be<br>inverted for density contrast, fracture density<br>contrast and shear velocity contrast.<br>The interpretation of the AVO attributes can<br>help to estimate the porosity and permeability of the<br>reservoir. The result obtained for porosity agrees with<br>the data from well logs, especially in the center of the<br>reservoir area, around the producers and injector<br>wells. Upper San Andres reservoir is a dolomite<br>where most of the permeability is provided by<br>vertical fractures. The fracture density was<br>particularly efficient in portraying the intense<br>fracturing between CVU-97 and CVU-200,<br>confirmed by the oil production increase after CO2 injection.<br>The time-lapse analysis showed variation in the<br>density between the pre and the post CO2 injection in<br>the northern part of the reservoir, with agreement<br>with the production wells in this area.<br>AVO attributes inversion proved to be a reliable<br>method for reservoir attribute determination and<br>time-lapse analysis, and provides an important link<br>between seismic data and rock properties.


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