Modeling seismic wave propagation in a coal-bearing porous medium by a staggered-grid finite difference method

A staggered-grid finite difference method is used to model seismic wave records in a coal bearing, porous medium. The variables analyzed include the order of the difference calculations, the use of a perfect match layer to provide absorbing boundary conditions, the source location, the stability conditions, and dispersion in the medium. The results show that the location of the first derivative of the dynamic variable with respect to space is coincident with the location of the first derivative of the kinematic varable with respect to time. Outgoing waves are effectively absorbed and reflection at the boundary is very weak when more than 20 perfect match layer cells are used. Blot theory considers the liquid phase to be homogeneous so the ratio of liquid to solid exposure of the seismic source depends upon the medium porosity. Numerical dispersion and generation of false frequencies is reduced by increasing the accuracy of the difference calculations and by reducing the grid size and time step. Temporal second order accuracy, a tenth order spatial accuracy, and a wavelength over more than ten grid points gave acceptable numerical results. Larger grid step sizes in the lateral direction and smaller grid sizes in the vertical direction allow control of dispersion when the medium is a low speed body. This provides a useful way to simulate seismic waves in a porous coal bearing medium.

Tectonically deformed coal types and pore structures in Puhe and Shanchahe coal mines in western Guizhou

To evaluate the effect of tectonic deformation on coal reservoir properties, we provide an analysis of the types of tectonically deformed coal, macroand microscopic deformation and discuss pore structural characteristics and connectivity based on samples from the Puhe and Shanchahe coal mines. Our research shows that the tectonically deformed coal mostly includes cataclastic structural coal, mortar structural coal and schistose structural coal of a brittle deformation series. The major pore structures of different types of tectonically deformed coal are transitional pores and micropores. The pore volumes of macropores and visible fracture pores produced by structural deformations vary over a large range and increase with the intensity of tectonic deformation. Mesopores as connecting passages develop well in schistose structural coal. According to the shapes of intrusive mercury curves, tectonically deformed coal can be divided into parallel, open and occluded types. The parallel type has poor connectivity and is relatively closed; the open type reflects uniformly developed open pores with good connectivity while the occluded type is good for coalbed methane enrichment, but has poor connectivity between pores.