Investigation of core data reliability to support time-lapse interpretation in Campos Basin, Brazil

Abstract

A comparison between laboratory experiments and log measurements was done to investigate the effect of core damage on compressional wave velocities. This was done using 43 rock samples extracted from a 45 meters thick turbidite reservoir. We used Gassmann fluid substitution to obtain low-frequency saturated velocities from dry core measurements (thus mitigating the dispersion effects) taken at reservoir pressure. To mitigate the scale effect (or under-representation of reservoir properties by a number of small cores), we apply a correction for small mismatch between the core and log porosities. Comparison of these computed velocities with the sonic log measurements shows very good agreement. This confirms that for this particular portion of the reservoir the effect of core damage on ultrasonic measurements is below the measurement error. The results suggest that stress sensitivity of elastic properties as obtained from ultrasonic measurements is adequate for quantitative interpretation of time-lapse seismic data.