Dynamic evolution characteristics of fractures in gas-bearing coal under the influence of gas pressure using industrial CT scanning technology

In order to explore the dynamic evolution characteristics and laws of gas-bearing coal fractures under the influence of gas pressure, the in-situ industrial CT scanning test of gas bearing coal under triaxial compression was carried out by using the industrial CT scanning system of loaded coal and rock.The original CT scanning images and stress-strain curves of gas-bearing coal at different deformation stages were obtained, the digital model of gas-bearing coal was obtained using 3D reconstruction technology, and the fracture structure of the digital model was accurately extracted by using an image analysis and processing software.The spatial morphology of internal fractures of gas-bearing coal was visualized successfully, and the internal fracture structure of gas bearing coal was analyzed qualitatively and quantitatively.The results show that when the confining pressure is constant, the variation trend of the full stress-strain curve of gas bearing coal under different gas pressures is roughly the same.They all go through five deformation stages, namely initial compaction stage, linear elastic deformation stage, plastic yield stage, peak failure stage and residual deformation stage, and the elastic modulus and residual strength decrease linearly with the increase of gas pressure.In the whole stress-strain process, under the action of gas pressure, both the fractal dimension and the fracture pixel ratio of two-dimensional fracture of gas-bearing coal similarly first decrease or are constant, then increase rapidly and then slowly.The change trend of fracture rate, fracture density and three-dimensional fractal dimension of three-dimensional fracture of gas-bearing coal is basically same, showing a three-stage evolution law of slow decline, slow rise and rapid increase.After failure, the pixel ratio and two-dimensional fractal dimension of gas-bearing coal increase linearly with the increase of gas pressure, as well as the fracture density, fracture rate and three-dimensional fractal dimension.The existence of gas promotes the fracture expansion and coalescence, and the number and complexity of two-dimensional and three-dimensional fractures increase with the increase of gas pressure.The non-mechanical action of adsorbed gas and gas wedge expansion of free gas lead to the overall degradation of mechanical properties of gas-bearing coal.