Experimental study on drag reduction-permeability increase and enhanced displacement of low-rank coal gas

In order to achieve drag reduction-permeability increase and enhanced displacement of low-order coal gas，based on the mechanism of gas adsorption and displacement under the inherent properties of coal porous media，the liquid-gas coupling medium was firstly obtained by using the coexistence characteristics of water medium, surfactant CTAB and micro and nano bubbles. The optimization performance and quality improvement characteristics of the liquid-gas coupling medium were characterized by the flow rate of the circular pipe and the rotational viscosity. Combined with the instantaneous flow of seepage and gas permeability measurement, the effects of liquid gas coupling medium on reducing drag and increasing permeability and strengthening displacement of low-rank coal gas were quantified. The results show that the liquid phase surface tension can be reduced by 50.41% with 0.05% CTAB and the average liquid phase surface tension can be reduced by 5.50 mN/m with micro-nano bubbles. Based on the surface activity of CTAB and micro and nano bubbles，the annular flow rate of liquid-gas coupled medium increases by 1.44 times and 1.27 times respectively compared with distilled water medium and surfactant containing medium. The rotational viscosity at 100，200，300 and 600 r/min decreased by 60.00%，71.43%，50.00% and 45.45% compared with distilled water medium and 42.86%，60.00%，37.50% and 25.00% compared with surfactant medium respectively. In the case of methane seepage displacement in distilled water medium, water medium containing surfactant and liquid gas coupling medium, the instantaneous flow rate and gas permeability gradually increase with the increase of annular flow and the decrease of rotational viscosity. Based on the good drag reduction and flow performance of the liquid gas coupling medium，when the axial pressure is 3, 5 MPa and confining pressure is 3, 5 MPa，the instantaneous flow rate increases by 1.43 and 1.67 times and the permeability measured by gas increases by 1.44 and 1.68 times respectively compared with the distilled water medium，and the seepage difference between different axial pressure/confining pressure is significantly reduced. The coupling medium of liquid and gas further activates the surface properties, strengthens the drag reduction, permeability increase and displacement of low-grade coal gas under the action of high flow and low viscosity, and has clear engineering guidance for the coal gas increase, desorption and extraction standards, and the gas emission prevention and control of high-yield working face.