True triaxial rockburst experiments with four different unloading rates were performed on four prism specimens of granite sampled from Beishan, China. The damage evolution in the rockburst test was investigated from two aspects including fracture surface crack and fragment characteristics. The scanning electron microscopy was used to observe the micro crack information on fragment surface. Combing binarization and box counting dimensions, the fractal dimensions of cracks were obtained. Meanwhile,the fragments were collected and a sieving experiment was conducted. We weighed the fragments qualities, counted the amount of fragments and measured the fragments length, width and thickness.Utilizing four methods to calculate damage fractal dimensions of fragments, the trend of fractal value changing with unloading rates can be roughly described. It can be concluded from these experiments that the fractal dimension either for crack or for fragment holds a decreasing trend with the decreasing unloading rate, indicating a reduction of damage level.
The effect of the fracture distribution on CO 2 injection into coal seams was studied with a heterogeneous model having dual porosity to represent both the primary medium (the coal matrix) and the secondary medium (the fractures) under variable stress conditions. A numerical generation method and a digital image processing method were used to model the heterogeneous fracture distribution in the coal. The model solutions demonstrate that: (1) the fractures are the main channel for gas flow and their distribution has an important impact on the gas injection rate; (2) the fractures only affect the injection rate of CO 2 into the coal but not the final storage amount; (3) when gas is injected into coal the fractures will first expand and then close due to the changing effective stresses and the adsorption induced swelling of coal grains. This fully coupled dual-porosity model with a heterogeneous fracture distribution provides a way to predict the CO 2 injection into a coal seam.
