Experimental study on characterization hydraulic fracturing coal fracture network and evolution of fracture forming performance

The accurate characterization of coal fracture network can effectively evaluate the fracturing effect of the deep coal seam after hydraulic fracturing. In order to quantitatively evaluate the complexity of coal seam after hydraulic fracturing, the hydraulic fracturing test of coal was carried out by using the self-made true triaxial test system, combined with CT scan, the pore network and fracture network with topological structure are reconstructed, and the complexity of the fracture network is quantitatively characterized by fractal theory and topology. The effect of the intermediate principal stress on the complexity of the fracture networks under the condition of true triaxial stress is explored. The results showed that the change rate of the two-dimensional fractal dimension (K) are 1.03% ~ 7.10%, and the change rate of three-dimensional fractal dimension are 3.50% ~ 9.18%, the two-dimensional and three-dimensional fractal dimensions of coal samples after hydraulic fracturing increase significantly. The method based on fractal theory and topology can effectively characterize the fracture structure and fracture forming ability of hydraulic fracturing. The two-dimensional topological parameter of coal samples after fracturing are1.18 ~ 1.52, which is positively correlated with the change rate of two-dimensional fractal dimension．Increasing rate of change rate of two-dimensional fractal dimension decreases with the increase of topological parameters. The reconstructed internal structure and fracture distribution showed that the three-dimensional fractal dimension after fracturing is more advantaged than the two-dimensional fractal dimension. The three-dimensional topological parameters of coal samples after fracturing are 1.82-1.93, increased with the increase of the change rate of three-dimensional fractal dimension. The intermediate principal stress has a positive effect on the seam forming ability of coal seam. The fractal dimension and topological parameters increase with the increase of intermediate principal stress before and after hydraulic fracturing. In other words, Increased with the intermediate principal stress, the resulting fracture network is more complex, the connectivity is better, and the fracture forming ability of hydraulic fracturing is stronger.