Effect of pore structure on the dispersion and attenuation of fluid-saturated tight sandstone

Abstract

The pore structure of tight sandstones has significant influence on the elastic properties, and also determines the fluid-flow-related wave dispersion and attenuation mechanisms. However, in those velocity dispersion models, only the compliant pore or crack with fixed aspect ratio and concentration has been taken into account, which is not completely realistic in reservoir rock. A procedure is presented to obtain the pore aspect distribution of compliant pore (crack) from the pressure dependence of velocities. Based on the pore aspect distribution of compliant pore, the existing squirt fluid model of Gurevich is extended to consider complex pore structure of reservoir sandstones, especially when the aspect ratio has a relatively wide distribution. The extended Gurevich's squirt-flow model is consistent with Gassmann's equation at low-frequency limit, and with Mavk-Jizba model at high-frequency. To illustrate the validation of the extended Gurevich's squirt-flow model, we compare predictions of our squirt model with laboratory measurement of two water-saturated tight sandstones at ultrasonic frequency. The new model on the basis of the pore structure is more accurate in predicting the pressure dependence of compression and shear velocities for these two samples. Considering aspect ratio distribution of compliant pore in the tight sandstone samples, the new model for predicting dispersion curve shows a relatively wide squirt-flow relaxation frequency range, which even covers the typical seismic frequency and sonic logging frequency. This observation suggests that squirt flow be still important even in the seismic frequency band for tight sandstone with special pore structure.