Stress-strain characteristics of coal mine gas hydrate-coal mixture under confining pressure unloading

Dynamic disasters such as coal and gas outburst have become more frequent and intense due to the increases in mining depth caused by the increases in mining intensity and energy demand.Coal mine gas prevention based on hydrate method is termed as coal mine gas hydrate formation using medium-high pressure injected water and promoters.When most coal mine gas is transformed to hydrate phase, the gas pressure in the coal seam decreases, and the coal strength increases.This can theoretically reduce the risk of coal and gas outburst.The measurement of mechanical properties of coal mine gas hydrate-coal mixture is one of key issues in the application of coal and gas outburst prevention technology based on the hydrate method.In addition, a large-scale coal mining causes a repeated disturbance to the coal and rock mass in the excavation space.Therefore, the mechanical properties of coal mine gas hydrate-coal mixture mainly depend on the loading-unloading condition.In view of the above, the mechanical properties of coal mine gas hydrate-coal mixture under conventional triaxial compression tests and confining pressure unloading tests were investigated to provide theoretical reference for coal mine gas hydrate method application in coal and gas outburst prevention.Firstly, a series of triaxial compression tests was conducted on coal mine gas hydrate-coal mixture samples under different confining pressures(12,16,20 MPa) and hydrate saturations(50%,80%),using the integrated device for in-situ hydrate formation in coal and mechanical properties measurement of coal sample.Then, a series of confining pressure unloading tests was conducted, with the initial axial stress of confining pressure unloading tests determined from conventional triaxial compression tests.The results show that hydrates have greater saturation with higher water content under the same confining pressure.The stress-strain curves of coal mine gas hydrate-coal mixture exhibit quite different profiles under different stress paths.Specifically, the curves show strain hardening behavior while stress platform occurring in a confining pressure unloading condition.Furthermore, the strength of coal mine gas hydrate-coal mixture is lower under confining pressure unloading condition.The stress platform duration approximately increases linearly with the increase of hydrate saturation or confining pressure.Moreover, the lower the confining pressure and the saturation are, the shorter the duration of stress platform and easier failure will be.Finally, a multiple linear regression equation is well fitted for correlating the stress platform duration with the hydrate saturation and confining pressure.Shear failure mainly occurs under unloading condition, accompanied by splitting failure.The severity of coal failure increases with the increase of confining pressure.In conclusion, the strength of coal mine gas hydrate-coal mixture is lower under a confining pressure unloading condition compared to the triaxial compression tests, leading to easy and sudden failure and severe failure of coal samples.The higher the confining pressure and the saturation are, the longer the duration of stress platform and more difficult failure will be, which means the dynamic disasters such as coal and gas outburst are more difficult to occur.