Experimental Study On The Effect Of Liquid Nitrogen On Mechanical Properties Of Coal

In recent years,with the rapid development of China's economy and the increasing demand for oil and gas,conventional oil and gas exploitation has been unable to meet the growing needs of the society.Coalbed methane in coal seams is one of the main alternative energy sources.The permeability of coal reservoir in China is low,and the extraction rate of conventional pumping method is also low.In recent years,the technical method of fracturing and increasing permeability by injecting liquid nitrogen into low permeability coal seam has been put forward,and good results have been obtained.However,it is not clear how the mechanical properties of coal change before and after the action of liquid nitrogen,which affects the corresponding safety assessment of coal mining.Based on the National Natural Science Foundation of China(Nos.51574139),this paper studies the changes of mechanical properties of coal before and after the action of liquid nitrogen.In this paper,liquid nitrogen freezing and freeze-thaw cycle tests were carried out on the coal of Nalinhe Mine in Ordos,and uniaxial compression tests and fatigue mechanics tests were carried out on the coal samples before and after freezing,respectively.The effects of freeze-thaw cycle times,freezing temperature and water saturation of coal samples on the mechanical properties of coal before and after the action of liquid nitrogen are studied.The conclusions are as follows:(1)The wave velocity of coal samples can be reduced by the action of liquid nitrogen,and macroscopic cracks can be formed on the surface of coal samples after freeze-thaw cycle of liquid nitrogen.(2)The mechanical properties of coal samples deteriorate after freeze-thaw cycle,changing from brittleness to plasticity,which indicates that the action of liquid nitrogen helps to reduce the risk of coal seam impact and rockburst.(3)The cyclic load test results of water-saturated coal samples in liquid nitrogen freeze-thaw cycle show that the total axial strain and fatigue life of coal samples decrease exponentially with the increase of the number of freeze-thaw cycles,but tend to be stable after 5 freeze-thaw cycles.(4)The cyclic load axial deformation test results of frozen coal samples are consistent with the limit deformation law of rock fatigue failure,that is,the coal fatigue failure is controlled by the static whole process curve.The total amount of axial strain during cyclic load fatigue failure can be predicted by using the control strain obtained from the static load test.