| The fact that many of the giant hydrocarbon reservoirs, such as the Ghawar field in Saudi Arabia and the Grosmont formation in Alberta, are formed from carbonates make these rocks and the corresponding reservoirs important research topics. Compressional and shear wave velocities (at 1 MHz) and the quasi-static strains of thirty seven carbonate rock samples were measured as functions of saturating fluid and confining pressure. Furthermore, P- and S-wave velocities of the saturated samples were measured at constant differential pressure of 15 MPa. The quasi-static strains of the samples under jacketed and unjacketed conditions were also simultaneously acquired. The lithology, mineralogy, porosity and pore type and size distribution of each sample were obtained using a combination of thinsection and scanning electron microscopy, helium porosimetry and mercury intrusion porosimetry. Due to the lack of closing microcracks and compliant pores in low porosity samples, the travel times show slight changes with the confining pressure. Whereas the high porosity samples show remarkable reduction of travel time with the increase of confining pressure in both P- and S-wave. The samples show high sensitivity to the applied differential pressure specially the high porosity samples. We found that the sample physically deformed at pressure above 25 MPa. An evidence of inelastic deformation were observed in few samples even at 25 MPa differential pressure. The samples show no changes in travel time with increasing confining pressure under constant differential pressure, and this behavior is taken to be representative of full saturation of the sample and hence used as a measure of quality control. The comparisons of Biot, Gassmann, squirt-Biot and squirt-Gassmann model predictions with the measured water saturated velocities show that the squirt mechanism is not active on all the studied samples. Biot mechanism is likely to be the principle dispersion mechanism in these samples. For S-wave velocities, Gassmann's model consistently over-predict the saturated at low pressure and closely fit the measured velocities at high pressure, whereas, Biot model over-predicts the saturated velocities in most of the studied samples.The strains measured from the vertical and horizontal strain gages are differing by around 27%. The strains over the horizontal axis are higher than the vertical axis suggesting that the majority of the compliant pores and crack-like pores are oriented almost in direction parallel to the length of the sample. The static bulk modulus is always lower than dynamic one for all measured samples. There is no correlation between porosity and static-dynamic ratio. The measured grain bulk modulus obtained from the unjacketed test is reasonably close to the bulk modulus of the constituent mineral phases. |
