A thermal-hydraulic-mechanical-damage coupling model of layer coal fracturing by liquid nitrogen
The mechanical properties of coal are heavily influenced by the primary bedding fractures.To investigate the crack expansion and destabilization mechanism of liquid nitrogen(LN2)low-temperature fractured layer coal,a thermal-fluid-solid-damage coupling model was established based on meso-element theory and damage mechanics.The evolution of coal damage,permeability and temperature were analyzed throughout the fracturing process,along with the distribution characteristics of crack propagation in layer coal induced by LN2 fracturing during various stress ratios,as obtained by COMSOL software simulation.The results indicate that the temperature of the coal in contact withLN2 near the drill hole drops sharply to form a small range of ultra-low-temperature zone,which generates thermal stresses exceeding the tensile strength of the coal,and produces damage around the drill hole and destroys the damage area in the initial stage ofLN2 injection(5 s).With the increase ofLN2 injection pressure,multiple main cracks appeared inside the coal.The main cracks were mainly developed along layer direction and generated secondary fissures and a complex damage area was formed around the borehole.The number of coal damage begins to increase,accompanied by a corresponding increase in permeability.With a continuous increase in injection pressure,the coal enters an instability stage.A large number of cracks extensively penetrate the coal specimen,leading to coal damage.During this process,both permeability and fracture pressure gradually reach the peakvalues.The coal damage,fracture pressure,and permeability exhibit a trend of increasing and then decreasing with increasing layer angle,and reach the maximum value when the layer angle is 45°.The coal damage,fracture pressure and permeability reached the maximum when the stress ratio was 0.5,decreased dramatically when the stress ratio increased from 0.5 to 1,and gradually leveled off after the stress ratio exceeded 1.The existence of coal layer has a greater influence on the initiation pressure of LN2 fracturing of coal and the change rule of initiation pressure is similar under the conditions of different layer angles.With the increase of layer angle,the initiation pressure shows a U-shaped change rule,and the initiation pressure shows an increasing trend with the increase of stress ratio.The research results provide a basis for further mastering the LN2 fracturing coal technology and determining the process parameters.
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