Core damage: Can we calibrate pressure response with lab data?

Abstract

Time-lapse (4D) seismic quantitative interpretation is based mainly on measurements of how saturation and pressure changes affect seismic velocities. Effect of saturation can be modeled using Gassmann equations. Pressure effect is usually obtained by laboratory measurements, which can be affected by core damage. In order to assess the adequacy of the core measurements to the properties of the intact reservoir rocks, it is necessary to compare them to in situ measurements. We present a study to assess the adequacy of ultrasonic measurements on core samples by comparing measured ultrasonic velocities at reservoir pressures with sonic log data from two wells located in an oil field in Campos Basin, offshore Brazil. The analysis is performed for these densely cored wells: more than 50 samples were extracted from a turbidite reservoir. We use Gassmann fluid substitution to obtain low-frequency saturated velocities from dry core measurements (thus mitigating the dispersion effects) taken at reservoir pressure. Comparisons of these computed velocities with sonic logs measurements show very good agreement. This confirms that for those particular regions the effect of core damage on ultrasonic measurements is below the measurement error. Consequently stress sensitivity of elastic properties as obtained from ultrasonic measurements is adequate for quantitative interpretation of time-lapse seismic data.