Anisotropic Wave Field Simulation And Characteristic Analysis Of Coal Reservoirs

In coalbed methane exploration, the presence of porosity, fractures and beddingin a coal seam will lead to the anisotropy of the coal seam. In this paper, seismic wavefield characteristics and wave propagation in anisotropic media of coal reservoirs hasbeen detailed studied by using the method of higher-order staggered gridfinite-difference seismic numerical simulation and AVO forward modeling.First of all, we take the coal samples underground from Sihe mine of ShanxiProvince, and test the longitudinal wave velocity and shear wave velocity of coalsamples in different directions, then calculate the elastic constants of the coal samplesand the Thomsen anisotropic coefficients, then analyze the relationships betweenwave velocities and different confining pressures of the test data, analyze therelationships between porosity and the Thomsen anisotropic coefficients. Throughstudying the coal samples for microscopic analysis and scanning electron microscopy,analyze qualitatively the formation mechanism of coal seam anisotropy.Secondly, the wave field response of the different media model is analyzed byusing the high-order staggered grid finite-difference numerical simulation. Weresearch the anisotropic characteristics of elastic wave field on different thickness,different confining pressure and fracture fluid of a coal seam by analyzing the shotrecord and snapshot of the wave field.Studies have shown that the coal seams thatcontain cracks, have a significant impact on the seismic records. These effects displaymainly the energy of the reflected wave, the time difference, and the characteristicsdiffer materially between the Vx component and Vz component. However, thisdifference in the thin seam was not apparent records of seismic wave field.Then, AVO analysis of a coal seam based on Zoeppritz equation has a lot ofproblems. AVO analysis for thin seam must use of the formulas of the superimposedmedia. From the applicable formula of layered media established by Berkhovski,derivative a simplified formula for elastic wave propagation in thin coal seams, andthen study the AVO curves characteristic of the different thin seam models by the thinseam formula, analyze AVO curves of the thin seam on the incident sub-wavefrequency, seam thickness and media absorption. The underground media actually isviscoelastic anisotropic media. A thin seam of the viscoelastic layered media formulais derived by the Viscoelastic dynamics Carcione theory, and using the viscoelasticanisotropic thin formula analyze the AVO curves characteristic of the coal seammodel, and analyzes the fracture fluid influences on AVO curves of the anisotropic thin coal seam.Finally, based upon the test data of coal samples and collecting a variety ofgeological data, we establish a coal seam anisotropic layered model. Using themethod of azimuthal AVO forward modeling simulates azimuthal anisotropy in theroof and floor of mine under different confining pressure. The forward modeling ofthe maximum amplitude of azimuthal anisotropy of a coal seam proved that the coalseams are azimuthal anisotropic media, providing a theoretical basis for the P-waveazimuthal anisotropy detection.