SEISMIC VELOCITIES IN CARBONATE ROCKS

Seismic velocities were measured in the laboratory vs pressure, temperature, and saturation on over 80 cores from various carbonate reservoirs in Alberta, using an ultrasonic pulse transmission method. Porosity, permeability, electrical properties, and capillary pressure were also measured in order to characterize the pore systems. All the measured data and other information are compiled into a database. The results show that both permeabilities and seismic velocities are related to the porosities of the cores, although the data are scattered. Liquid saturation (e.g. oil and water) increases the compressional velocity but has only nominal effect on the shear velocity. The magnitude of the increase in compressional velocities with liquid saturation is dependent upon the pore geometry of the rocks, reservoir pressure, as well as the distribution and properties of the saturating fluids. The velocity data are also fit to the time-average equation and the Gassmann equation. The results are usually unsatisfactory, which means that the time-average equation used in sonic log interpretation is inadequate. For log interpretation, it might be desirable to establish a porosity-velocity relationship specific to carbonate reservoirs, instead of using the time-average equation. Furthermore, the Gassmann equation does not adequately explain the laboratory results. In most of the cores measured, the compressional velocities increase by more than 5% with oil saturation, with a subsequent decrease by over 5% when flooded with a hydrocarbon solvent. This suggests that seismic monitoring of injected gases and hydrocarbon solvent in carbonate reservoirs should be possible.