A Parametric Study of Compressional and Shear Wave Velocities in Ekofisk Reservoir Chalk

Abstract

Abstract A parametric study of compressional and shear wave velocity was conducted on Ekofisk reservoir chalk to gain more insight in the acoustic response of the chalk to changes in temperature, stress state and fluid saturations. The information from the study is used to interpret seismic signature changes associated with a Repeated Seismic Monitoring Program. Three factors have been identified to have a possible impact on acoustic velocities as a result of a waterflooding process. These three factors are 1) changes in effective stress state, 2) changes in water saturation, and 3) changes in temperature. It would appear that these 3 effects act together to increase the compressional wave velocity. Laboratory measurements show that the largest effect on velocity is a reduction in porosity, which results from an increase in stress and/or an increase in water saturation. Changes in temperature and fluid saturation are secondary effects which also produce increases in acoustic velocities. Together, these two parameters can account for about 5% increase in compressional wave velocity. However, compaction alone accounts for approximately 3-4% increase in compressional wave velocity. Experimental data indicate that the low frequency Biot-Gassmann relation predicts compressional and shear wave velocities to within ±2% of the measured values when appropriate rock moduli are used. This relation has therefore been considered to be a suitable approach for modeling fluid substitution in Ekofisk chalk.