Coal seams are embedded into other sedimentary structures, such as sands, clays and shales. In coal seam gas reservoir engineering, structures such as layering, fractures, cleat orientations, porosity and fluids within the cleats have an important effect on production. The interpretation of 3D seismic surveys in terms of these fractures characteristics is of great interest.

Weak anisotropy is commonly described using Thomsen-style parameters. From seismic survey data, values for Thomsen-style parameters are determined, usually through non-hyperbolic normal moveout (NMO) and azimuth gathers. The interpretation of the parameters in terms of fracture characteristics is based on restrictive assumptions; sparse, non-interacting, disk-shaped fractures are commonly assumed in cases where fracture geometry is not required. This paper investigates the effect of 3D fracture geometry in coal seams on the effective 3D anisotropic elasticity parameters using a numerical approach. We show some first results to verify existing, commonly used elastic models for fractured media, derived under restrictive assumption and then test their validity for more realistic scenarios; fracture distributions and densities, and material-filled cleats. Our study will help to provide a more reliable interpretation of Thomsen-style parameters obtained from seismic survey data.

The impact of cleats, and other small-scale structures, on the elastic properties of coal is investigated using the discrete element method (DEM). The method is capable of simulating complex fracture distributions, surface roughness and orientations on different spatial scales. Firstly, DEM is calibrated by performing uni-axial compression testing and shearing testing on a virtual coal sample, to determine values for macroscopic model parameters for intact coal. Then, cleats with different fracture densities and occupying materials are added. From these tests, we are able to validate commonly-used elasticity models for fractured media (e.g., Linear-slip models and Hudson's model), which is of first order in fracture density.

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