Theoretical analysis and application about permeability-increasing radius of drilling for simultaneous exploitation of coal and gas

Abstract

Under the influence of mining induced loading and unloading stress in simultaneous exploitation of coal and gas, a large number of cracks are generated in the surrounding rock plastic zone of gas drilling, which increases the permeability of gas and forms the permeability-increasing circle whose radius directly affects the effect of gas extraction. Based on the theory of the “butterfly plastic zone”, the model of the borehole plastic zone and the gas permeability-increasing circle was established, and the analytic formula of permeability-increasing circle radius was deduced. The further research on the factors affecting the permeability-increasing radiusreveals that the permeability-increasing radius is linearly positive to the radius of borehole, similarly exponentially positive to the minimum confining pressure and confining pressure ratio, and exponentially negative to the cohesion and the internal friction angle. The minimum confining pressure and confining pressure ratio are the key factors, and the deep mining and high confining pressure ratio are the sufficient and necessary conditions for the formation of effective permeability-increasing circle. This theory provides ascientific basis for the drilling spacing design, location selection, direction determination and the coordination of time and space between mining field and borehole layout.