Rock Physics Experimental And Modeling Studies On Sandstones' Acoustical Dispersion And Attenuation At Seismic Frequencies

Seismic rock physics builds a quantitative relationship between seismic attributes and reservoir/fluid properties through experiments and theory(models),and plays an important role in reservoir prediction,fluid identification and quantitative seismic interpretation.Seismic rock physics expeiments mainly conduct at ultrasonic frequencies,which is much higher than seismic exploration.The lack of experimental data at seismic band makes it difficult to study the characteristics of modulus dispersion and attenuation in this frequency band.On the other hand,the establishment and verification of rock physcics models are usuaslly based on ultrasonic data.The lack of experimental data at seismic band makes it also difficult to verify or constrain the lower frequency band of the models,and limit the development of new models.To this end,this paper carried out rock physics experimental and modeling studies on sandstones' modulus(velocity)dispersion and attenuation at seismic frequencies.The studies directly investigated the effects of fluid,saturation degree,viscosity,pressure and temperature on the modulus(velocity)dispersion and attenuation of reservoir sandstones at seismic frequencies and relevant mechanism,and developed a more reasonable model.The studies help to build the relationship between elastic parameters and reservoir properties,understand the propagation of seismic wave in complex medium and guide oil/gas exploration and development.Firstly,calibration experiments(aluminum and lucite)validated that the low-frequency seismic rock physics system can accurately and effectively measure samples with different dispersion/attenuation properties,so that low-frequency experiments for reservoir sandstone under different conditions can be effectively conducted.Secondly,experiments on a sandstone with relatively high porosity/permeability at frequencies 1-100 Hz were carried to investigate oil saturation and oil-water mixture effects on modulus(velocity)dispersion and attenuation.As saturated with the low viscous oil(2#oil)at 0-100% saturation degrees or fully saturated with the high viscous oil(68#oil),the measured moduli(Young's,bulk and shear moduli)of the sandstone presented weak dispersion at frequencies 1-100 Hz,and extensional attenuation were close to 0.The bulk modulus measured for this sandstone saturated with oil shows good agreement with that predicted by Gassmann's fluid-substitution theory,implying that the WIFF at different scales plays an unimportant role in controlling the bulk modulus of the oil-saturated sandstone.On the other hand,with oil saturation and viscosity increasing,the shear modulus always keeps increasing,deviating from the Gassmann's prediction,which can be attributed to the viscous coupling mechanism.As the distilled water was gradually injected into the fully high viscous oil-saturated sandstone,obvious moduli dispersion and extensional attenuation were manifested.The variations of the modulus dispersion and attenuation agree well with the prediction by patchy saturation theory,suggesting that the WIFF theory at mesoscopic scale is the primary cause of modulus dispersion and attenuation for this sandstone saturated with oil-water mixture at seismic frequencies.Lastly,to investigate whether the fluid flow at meoscopic and microscopic scales co-exist and interact with each other,low frequency experiments on fluid-saturated Fontainebleau sandstone samples(Fon1,Fon2 and Fon3)over the 1-2000 Hz band at different saturation degree,confining pressure and temperature.Two attenuation peaks can be observed for the sample Fon1 partially saturated with oil A at two different saturation degrees and for the sample Fon3 partially saturated with oil B at 41.3 ?,while a single attenuation peak was observed for the sample Fon2 when partially saturated with glycerin or fully saturated with water and for the sample Fon3 partially saturated with oil B at 21.3 ?.The single attenuation peak is mainly caused by squirt flow.On the other hand,with increasing confining pressure,the first peak(at lower frequency)of the dual peaks moves to higher frequencies while the second peak(at higher frequency)shifts to lower frequencies,suggesting that the wave-induced fluid flow mechanism at different scales co-work.A dual-scale fluid flow model was developed to account for the interaction of wave-induced fluid flow at microscopic and mesoscopic scales.The model provided a reasonable interpretation on the measurement results for the sample Fon1 partially saturated with oil A.This suggests that the wave-induced fluid flow mechanism,related to pore microstructure and fluid distribution,interplays with each other and jointly control the attenuation of partially oil-saturated sandstones at broad frequencies.