| Due to the important role of reflection seismology in oil exploration, in recent years, the study of rock physics focused on seismic properties of the rocks, which are mainly about the relationship between the seismic velocity and attenuation, and the properties and conditions of rocks. Rock physics research provides a connection between the seismic data (seismic parameters) and reservoir properties (reservoir parameters), prompting the formation and development of seismic rock physics. Seismic rock physics research dedicated to clarify the relationship between the properties of reservoir rock and the porous fluid contained in it, and the seismic property parameters, in order to more accurately acquire the rock and fluid properties, and reservoir parameters from seismic data. The study of physical properties of rock can provide physical foundation for seismic exploration for finding oil and oil reservoir seismic monitoring. It can also play a role in insuring the correctness and physical meaning of parameters used in seismic modeling, seismic inversion and seismic interpretation.The study of the seismic wave propagation in porous media has so far been one of the hot spots of the geophysics and oil prospecting. And seismic wave velocity dispersion and attenuation caused by the wave-induced fluid flow is the key problem. This paper carried out a detailed review of the experimental and theoretical model research about the wave-induced flow, to find out the main research directions and the latest developments of the velocity dispersion and attenuation which related to the wave-induced flow. The porous rocks are very complex media. It’s complexity include the heterogeneity in micro-and meso-scale structure of rock, anisotropy, and the presence of the pore fluid, which makes it very difficult to construct a universal porous medium model that based on real physical mechanisms. In the laboratory scenario, seizing the phenomenological phenomena of experiments, we can analyze the rock properties using a phenomenological theory model which based on viscoelastic model. And this is how I intend to start my research on velocity dispersion and attenuation caused by wave-induced fluid flow. Viscoelastic model can simulate the viscoelastic behavior of porous rocks, the model itself is simple and convenient, and can be adjusted for the experimental results, which makes it a very good research tool. Thermal relaxation regularities that consistent with the Arrhenius relationship are observed in the low-frequency experiments. Introducing the thermal relaxation regularities into the standard linear solid model, the thermal relaxation model with effects of both frequency and temperature is obtained. The thermal relaxation model can well simulate the properties of the saturated rocks, such as attenuation and velocity dispersion. In the frequency domain and the temperature domain, the characteristics of wave propagation are analyzed for saturated porous rock, to get saturated rock wave velocity and attenuation versus frequency and temperature variation. The co-existence of Biot dissipation mechanism and thermal relaxation mechanism results in the appearance of two attenuation peaks in the attenuation curve of the saturated rock, i.e., the Biot peak and the thermal relaxation peak. The two attenuation peaks will move toward each other when the frequency or temperature changes. The thermal relaxation peak at low-frequency area(or low-temperature area) will shift to high-frequency area(or high-temperature area) when the temperature (or frequency) increases, which is consistent with the experimental results obtained in low-frequency resonance experiments. The shift of the Biot peak locates at the high-frequency area(or high temperature area) is opposite to the movement of thermal relaxation peak.The influence of parameters on characteristics of the thermal relaxation model is analyzed. The change of rock porosity may remarkably change the Biot attenuation. The reduction of the pore space will result in the reduction of Darcy flow, the Biot attenuation of low porosity saturated rock is much smaller than high porosity saturated rock. The influence of porosity variation on thermal relaxation attenuation is much smaller. The effect of porosity for S-wave is more obvious than for P-wave. Since the attenuation and dispersion of saturated rock are mainly caused by the flow of the fluid, the change of the permeability may bring about the shift of the attenuation peaks. Cole-Cole distribution coefficient βcole-cole is an important parameter for constructing the thermal relaxation model, and its value has a great influence on the thermal relaxation peak value and the peak width. When βcole-cole is small, the attenuation is large and the peak is sharp. When βcole-cole is large, the attenuation is small, and the peak width significantly increases. Thermal relaxation model is compared with the Biot and BISQ model in this dissertation. The thermal relaxation model can produce bigger dispersion and attenuation. The dispersion of thermal relaxation model is about2.5times as that of the BISQ model. The frequency range of dispersion is wider, and there is velocity dispersion almost in full-frequency band, which is consistent with the experimental data. The thermal relaxation model overcome the defects of Biot and BISQ model that the Biot attenuation and dispersion are rather small, and the frequency band of dispersion in BISQ model is narrow. The thermal relaxation model can analyze the velocity and attenuation variation with temperature, which could not be carried out for Biot and BISQ model.In order to verify the applicability of the thermal relaxation model, the experimental study of a series of different fluid-saturated rocks is carried out in a higher frequency range, i.e.,1-1000Hz. The experimental data of velocity and attenuation for these saturated rocks are obtained. Variation of velocity and attenuation with frequency and temperature is similar to the low-frequency resonance experimental results, and is consistent with the thermal relaxation regularities. The thermal relaxation mechanism is extended to higher frequency band, and that verifies the universality of the thermal relaxation regularities. A "local thermal flux" mechanism is proposed to explain the thermal relaxation mechanism. An attenuation peak that is similar to phase transition peak was observed in the high temperature region. We use isomorphism to argue that this may be the phase transition peak. We found attenuation peak that has similar shifts with frequency as the Biot peak in the experimental results. Introducing a mechanism of "local fluid flow", the experimental data are well simulated numerally. The "local fluid flow" mechanism proposed a new attempt for understanding the Biot attenuation.Attenuation in saturated porous rocks is closely related with the micro-and meso-scale structure of the rock samples. Thus we have introduced two rock mesostructure characteristic scales, corresponding to the "local thermal flux" and the "local fluid flow" mechanism, respectively. The characteristic frequencies are showed in general terms. Also ultrasound experiments for saturated porous rock are carried out to obtain the ultrasonic velocity and attenuation data for different fluid-saturated rocks. The experimental results obtained for different rock samples are quite different due to the small sample size. The small size of samples makes the interaction between the wave and the sample very complicated. We made some qualitative explanation for the experimental data.The thermal relaxation model can predict strong attenuation as well as a wide range of velocity dispersion, which can well simulate the viscoelastic behavior of saturated porous rocks. The expansion of the low-frequency experiments has supplemented the relative lack of experimental data in the earthquake frequency band, and is of important significance to the interpretation and inversion of seismic data. These rock physics experiments contain research about the viscoelastic wave and physical properties of saturated rocks. The experimental techniques can be widely used for nondestructive testing techniques, damping and seismic studies. |
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