Empirical and numerical approaches for geomechanical characterization of coal seam reservoirs

The effects of fractures or joints on geomechanical properties of coal when transitioning from small samples to larger field scales have long been recognized. Two geomechanical characterization methodologies for of a coal seam for reservoir engineering are presented which attempt to account for these scale effect. The first is a well established empirical rock mass classification method, the Geological Strength Index (GSI). The second is a newly developed numerical technique, termed the Synthetic Rock Mass (SRM).GSI was developed to empirically account for the influence of rock joint density and joint surface conditions on the behaviour of the rock mass. The established GSI to strength and deformation theories are reviewed and an approach to account for joint stiffness based on the minimum confining stress is presented. The SRM approach to jointed rock mass geomechanical characterization is a numerical modeling technique which combines the Bonded Particle Model for rock and Discrete Fracture Network simulation. The intact or matrix of the SRM is calibrated to laboratory data, the DFN inserted creating a numerical representation of the rock mass which can then be tested at any scale. In each case, the approaches attempt to geomechanically characterize (strength, modulus, etc) the large scale behaviour of the coal seam, where the properties can then be used for continuum type simulation (reservoir simulation, borehole stability, etc).