STUDY ON SEEPAGE CHARACTERISTICS OF FRACTAL-LIKE TREE NETWORK STRUCTURE IN WATER-INJECTED COAL FRACTURE UNDER STRESS

The study of water seepage characteristics in coal fracture networks is the key for accurately guiding the development of coal seam water injection technology. Based on the fractal geometry theory, we represented the shape of a fracture network in a water-injected coal body through a tree branch network; furthermore, we established a seepage model for the fracture network, determined the relationship between permeability, overburden pressure, and water pressure according to the relationship between fractal dimension and effective stress, and verified the applicability of the proposed model through a self-developed experimental system. This model was developed based on the "penny-shaped" single-fracture seepage model and did not contain any empirical constants. The results showed that the influence weight of the fracture diameter ratio (beta) on permeability was the highest: when beta < 0.35, the permeability was almost 0, but when beta > 0.35, the permeability increased rapidly. Notably, the local and overall permeabilities of the fracture network decreased with the increase of the radial distance; when the radial distance was constant, however, the permeability of the fracture network decreased with the increase of the bifurcation series (m) and of the fracture radius. Lastly, the ratio of gap depth to length increased with the increase of the ratio of fracture depth to length (lambda). Overall, our research results provide a theoretical basis for defining the seepage law of high-pressure water in coal bodies, as well as the relationship between permeability and several environmental parameters.