Seismic Numerical Simulation And Inversion Of Fractured Coal Reserviors

In the coalfield exploration, the degree of development of fractures usually is a key factor, which affects many problems in coal mines such as the roof and floor stability of the coal seam, prevention and treatment of coal gas and water, and coalbed methane exploration and development as well. And in this paper, seismic wave propagation of fractured coal reservoirs under different conditions and status has been in a detailed study by high-order staggered grid finite-difference which is one of seismic numerical simulation methods.At first, according to elastic wave fundamental equation of two-phase medium and the stress and strain equation given by Biot at 1962, elastic wave equation of fractured coal reservoirs has been deduced. On this basis, high-order finite-difference formulas of first-order stress-velocity and velocity-stress for fractured coal reservoirs are obtained. And an improved absorbing boundary condition for perfectly matched layers is achieved by modifying its damping factor on the basis of the general absorbing boundary condition. Case study states that this method has not only achieved a very good absorbing effect of the boundary, but also impoved its computational efficiency. Besides, the numerical dispersion has been suppressed during the simulation of two-phase medium of coal reservoirs by using the Flux-corected transport (FCT) method. And its advantages and disadvantages also have been analyzed through examples contrast.Secondly, elastic constants of anisotropic and two-phase medium of fractured coal reservoirs are gained with effective media theory and some other experiential formulas, which provide a theoretical foundation for parameters. And thus forward flows of numerical simulation for fractured coal reservoirs are desighed including the calculation of petrophysical parameters and numerical simulation.Then, with high-order staggered grid finite-difference, wave field distribution and propagation characteristics of fractured coal reservoirs and its adjacent aquifers containing different states (natural, gas-saturated, water-saturated) have been made a systematic discussion, and the seismic response characteristics have been qualitatively analyzed with the seismic attributes. It states that elastic constants of fractured coal reservoirs containing different states have some differences. And these differences are not obvious at the seismic records, but can be qualitatively distinguished throughout the seismic attributes techonology. Therefore, forward theoretical foundation has been provided for the inverse calculation of fractured coal reservoirs.Finally, research has been done for the porosity and fractures in the fractured coal reservoirs and Ordovician limestone top by using seismic impedance and azimuth anisotropic inversion with the actual seismic data. Research findings state that porosity of fractured coal reservoirs and Ordovician limestone top can be calculated through the seismic inversion method, and density and orientation of fractures also can be predicted with the azimuth anisotropic method.