Study On Seismic Wave Field Forward Modeling And AVO Characters Cracked Monoclinic Media

Recently, people found that oil and gas resources are extremely rich in cracks of underground reserves. Exploration and development of the oil and gas in cracks is the main method of improving oil and gas production. Cracks exist widely in subsurface, which is one of the main causes of subsurface anisotropy and heterogeneity, and also the theoretical starting point of this paper.For underground anisotropic media, for the purpose of understanding the ground rules and characteristics of seismic wave propagation in anisotropic media, from simple to complex, domestic and foreign scientists build a lot of theoretical medium model according to the characteristics of underground media, such as TI, TTI, HTI, orthotropy, monoclinic media, etc, to solve the wave equation for each model and give the stiffness coefficient matrix of the anisotropic medium model and gain the phase velocity and group velocity of various types anisotropic media model. Through forward modeling of these models, research and analysis of seismic wave field, we found that the theoretical model of seismic wave field is a very good description of certain real subsurface seismic response and geological structure. However, the complexity of the subsurface medium is difficult to imagine, the same is the distribution and developed zones of fracture, especially the anisotropy of cracks inducement. The presence of cracks produces a strong scattering effect on seismic wave, and small-scale fracture affects the reservoir heterogeneity and anisotropy strongly, which produce huge disturbance to the seismic waves. So the general method of seismic exploration can hardly identify, which increases the difficulty of exploration. Because of the detachment of oil and gas and the changes of environmental factors, such as pressure, and also, reservoir fractures are damaged and new fractures are created, which are the key factors affecting harvest ratio. To learn fissure oil, gas seismic wave field characteristics and seismic attributes, forward modeling is necessary to do the work, so choosing a theoretical model for forward modeling of medium is critical. By comparison, we found that monoclinic anisotropy fissure medium model is a good one theoretically, and the parameters of this model are relatively complete, including the velocity and density of longitudinal and transverse wave of isotropic medium, distribution density, aspect ratio and the fracture angle of the two fractures, and the velocity and density of longitudinal and transverse wave of fracture filling. We found that it is a better model to simulate oil and gas fracture reservoir.This article first used higher order staggered-grid finite difference method to forward model by the air and water crack filling respectively, which is the complete fracture of the monoclinic anisotropic medium fissure of elastic and viscoelastic anisotropic media. The observation angle is 0°of 2 three-dimensional component forward modeling, and we can get four models snapshots of the wave field. Through comparative analysis with the complete flexibility and viscoelastic monoclinic medium seismic data of the same crack filling cracks, we found that the different crack fillings affect the qP waves and the qS differently, and a viscoelastic factor increases the energy dissipation of seismic waves much more.As the cracks in the local small scale have highly heterogeneous and anisotropic effect, which can hardly be identified by general exploration methods. General deterministic parameters model is difficult to describe the medium with such characteristics. To understand this small-scale anisotropy of the non-homogeneous, the introduction of the theory of random media is a good way. Combining with Thomsen random medium theory, we use a smooth space of zero mean random process to describe the heterogeneity of small-scale fractured media. In the small scale, we use a few statistics to describe the space disturbance of elastic medium parameters for a detailed study on seismic wave field characteristic of non-homogeneous and anisotropic.To further study the non-homogeneous and anisotropic of this fracture of the small scale, we combine the crack monoclinic media with the random theory, and choose the random fractured monoclinic anisotropic medium of Von Karman type as the research object to randomly deal with the three parameters of the model background rate, fracture density and fracture angle, so as to get three random theory models of cracks parameters. Through three-dimensional anisotropic wave equation making two-dimensional three-component anisotropic equation, we use the higher order staggered-grid finite-difference seismic numerical modeling method to do the two-dimensional three-component well seismic modeling for the three random crack parameters model of the standard deviation's changes from 1%-18%. Well seismic is higher resolution, which is the great means to understand the fine structure of underground media, we can study the underground small scale media of non-uniformity and anisotropy. For a more detailed analysis of internal links of these seismic data to find out the fractured nature of monoclinic media information of these data, this paper uses the correlation coefficient in statistics to analyze the series of simulation data between wells in these three parameters. After that the seismic wave Z component is more sensitive than the X component to the subsurface heterogeneity and anisotropy. While seismic waves are in random media propagation, the seismic waves changes degree of the fracture rock physical parameters with the standard deviation will be affected by crack filling. When the cracks are filled with air, the order of the influence degree is the background velocity, crack density and crack angle. When the cracks are filled with water, the order of the influence degree is the background velocity, crack angle and crack density. On study of reservoir fracture, in accordance with different sensitivities of ground anisotropy each component, using multi-dimensional and multi-component seismic exploration technology helps us understand the underground media properties and fractured rock heterogeneity more dentally. Through the analysis of the correlation coefficient curve, we can affectively compare the similarity on the simulation records. For the selected three fractures parameters of this thesis, the curve changes of correlation coefficient are not the same, and X component and Z component of the trend are also different. Heterogeneity of the background medium is still the main factor effecting seismic response and the forefront in fractured oil and gas reservoirs of wave field. Based on the fracture reservoir studies, the heterogeneity of background medium is the prerequisite to ensure the reliability of the results. So whatever we study the fractured oil and gas reservoir seismic wave response or the reservoir fracture heterogeneity in the oil and gas reservoirs production, the models in this paper are available as a useful reference.For the research of fractured reservoir, identification and prediction fractures and development zones is also one of the important purposes. This paper uses reflectance formula of PP and PS waves of azimuthal anisotropy to study, analyze, and compare HTI media and monoclinic anisotropic medium. We found that, with the changes of azimuth angle and incidence angle, monoclinic anisotropy of fractured media model is weaker than a single fracture, and PS wave is more sensitive to the fracture anisotropy. The presence of cracks causes the fundamental difference of the dielectric anisotropy at different azimuth. These studies on the identification and prediction of fracture have important significance.