The Forward Modelling On The Wave Propagation Characteristics In Single Fractures

Fractured reservoir is the focus of oil and gas exploration and development.As a simplified theoretical model of actual fracture,the studying on single fracture is of great guiding significance to the understanding of the propagation law and response characteristics of seismic wave in single fracture,and the improvement of the existing fracture medium theoretical model,as well as the identification and prediction of fluid filled fractures.Aiming at the influence of fracture filling and scale on the propagation characteristics of single fracture wave,two kinds of single fracture models with different fillings and different scales are designed and made in this paper,and the shear wave source is used.Seismic forward modeling techniques(numerical simulation and physical simulation)are used to study the propagation characteristics of waves in a single crack at different angles of polarization and incidence.Ultrasonic wave measurements under different polarization angles of shear waves show that the waveforms of the crack free model do not change with the polarization angle,and the main wave distortion of the fracture model with fluid filling is obvious when the polarization angle is 40°.When the polarization angle is 50°,the main wave is distorted obviously in the fracture model with solid filling.The coda of all the models does not change with the change of polarization angle,and the coda shape of the fracture model with different fillings is different,which indicates that the coda is only related to the filling of the single fracture model.For the filling model,the polarization angle increases gradually,but the single fracture model with different filling has two peak frequencies,and the low frequency main frequency of different fracture model is close,and the larger P-wave velocity of the filler,the higher the main frequency.Ultrasonic wave was tested at different incident angles of shear wave and time-frequency analysis was carried out.In addition to the crack free model,the other models had a narrow band low-frequency coda with longer duration after strong first arrival shear wave.The larger the incident angle of the shear wave is,the stronger the coda energy is.When the incident angle is greater than 45°,the amplitude of the seismic coda is larger than the amplitude of the shear wave.In order to better receive the signal from a single crack,numerical simulation usually places the receiving line inside the crack,but the receiving transducer used in the physical experiment can only be set outside the model.Because of the difference of observation methods,the numerical simulation and physical simulation have not been able to correspond well with each other in the research of single crack wave.