Abstract

Quantitative interpretation of time-lapse seismic data requires knowledge of saturation and pressure effects on seismic velocities. Although the former can usually be modeled adequately using the Gassmann equation, the latter is obtained mainly by laboratory measurements, which can be affected by core damage. We investigate the magnitude of this effect on compressional-wave velocities by comparing laboratory experiments and log measurements. We use Gassmann fluid substitution to obtain low-frequency saturated velocities from dry core measurements (thus mitigating the dispersion effects) taken at reservoir pressure. The analysis is performed for an unusual densely cored well from which 43 cores were extracted over a 45-m -thick turbidite reservoir. These computed velocities show very good agreement with the sonic-log measurements. This confirms that, for this particular region, the effect of core damage on ultrasonic measurements is less than the measurement error. Consequently, stress sensitivity of elastic properties as obtained from ultrasonic measurements is adequate for quantitative interpretation of time-lapse seismic data.

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