Experimental investigation on visualization and quantitative characterization of filling and plugging performances of fractures in coal

Underground coalbed methane extraction is an important way for preventing coal mine gas disasters and reducing greenhouse gas emission, but air leakage around extraction boreholes is a critical factor that limits efficient coalbed methane extraction. Dynamically filling and plugging fractures around extraction boreholes can prevent air leakage. Due to the invisibility and complexity of underground geological formations, observing the process and effectiveness of plugging is extremely challenging. To visualize and quantitatively characterize the filling and plugging performances of fractures in coal. First, experiments are conducted to fill the coal damaged by the uniaxial and the triaxial loading-induced using a novel fracture filling experimental system under the constraint conditions. Then the multidimensional (point, surface, volume) plugging performances of fractures in coal before and after filling is visualized by the method of the X-ray computed tomography imaging. Finally, the quantitative characterization of filling and plugging performances of fractures in coal is conducted by testing changes in weight, fracture volume, and permeability before and after filling process. The experimental results show that after filling, the width and quantity of fractures in the coal significantly decreases, and the damaged coal is repaired. The filling ratio of fractures in coal fractured by uniaxial loading is 72%, and the fracture filling ratio in coal under triaxial loading is 68.1%. Under the influence of seepage and diffusion of plugging materials, the fractures are continuously filled, leading to a continuous decrease in the permeability of the fractured coal. The significant reduction in permeability of the fractured coal body is evident. The filling and plugging performances of fractures in coal is effective. The findings in this paper provide meaningful references and guidance for determining the filling and plugging parameters in underground gas extraction engineering practices.