Equivalent Medium Model And Seismic Wavefield Propagation Characteristics In Fractured Reservoirs

Fractured reservoirs play an important role in the oil and gas exploration. The presence of cracks in the reservoirs strongly in?uences seismic wave propagation, giving rise to the changes of seismic reflection. The characteristics can be used to indicate the presence of cracks. Utilizing numerical simulation of seismic wave field, we can master seismic wave propagation laws in the fractured reservoirs, and thus it can provide theoretical basis for identifying cracks by using the seismic response characteristics. The key technology is how to accurately characterize fractured reservoir medium. At present, the general method that we study the fractured medium is fracture equivalent method.According to the presence of matrix porosity in the fractured media, two types of fractured reservoirs are studyied. The first is fractured reservoir with the lower matrix porosity (or only in the case of cracks). From the description of spatial relationships of different crack angles (such as crack dip, strike, distribution, etc.), macroscopic fractured equivalent medium theory can be used to study the relationship between the elastic parameters and fracture parameters (such as crack density, crack width, etc.) and the corresponding fractured equivalent medium model is established. Then place the model in the corresponding positions of actual formation, and get real formation fractured equivalent medium model. Thus, the seismic response of fractured reservoirs in different structures and different rock can be obtained by numerical simulation. The results are shown that the real fracture network can be simulated by the method and the method can characterize the seismic wavefield propagation characteristics in the subsurface fractured media.Another type is the fractured reservoir with a large matrix porosity. The above fractured equivalent medium model become inadequate once the matrix porosity is large enough to play a signi?cant role in determining the effective behavior of the medium. Thus, the effects of pores on seismic wave propagation also must be considered. In the oil seismic exploration band, the paper presents a fractured-porous equivalent medium model to describe the media containing cracks and pores. In the model, the fractures are thought as be the integral part of the solid skeleton and can be described in terms of fracture-induced anisotropy. Then, based on Biot's equations of poroelasticity, considered two sets of porosity and permeability respectively, the fractured porous equivalent medium is established and effective constants and equations for this model are derived. The results show that the fractures and pores strongly influence seismic wave propagation, and cause anisotropy and two-phase characteristics of the medium respectively. In particular, it is observed that the presence of the fractures gives rise to the changes of the phase velocities and attenuation.The above studies show that the equivalent medium model established in the paper can characterize the fractured reservoirs accurately.