Effect of pulse gas pressure fatigue on mechanical properties and permeability of raw coal

HOU Peng; GAO Feng; GAO Yanan; YANG Yugui; CHENG Hongmei

Abstract：Using the TAWD-2000 rock mechanics testing system and the self-assembled gas pressurization system,the pulse gas pressure fatigue experiment on the raw coal sample was performed.The effect of the pulse time on the deformation and permeability of the raw coal sample was analyzed,and the mechanical properties between the tested coal sample and the initial coal sample under uniaxial compression were compared.The research results show that the strain of the raw coal sample displays cyclical variation and the volume strain shows a swellshrinkage process in a pulse gas pressure cycling.With the increase of the pulse time,the residual strain gradually accumulates,and the strain grows in the direction of the swell.In the initial stage of pulse fatigue,the strain magnitude changes greatly,while when the pulse fatigue time exceeds the critical range,the strain of the raw coal sample is prone to the basic balance.There is also similar relationship between the permeability of the raw coal sample and pulse time.In the early stage of pulse fatigue,the permeability increases rapidly with the pulse time,and after more than the threshold of the pulse fatigue time,the effect of pulse fatigue on the permeability becomes small.The threshold of the pulse fatigue time is between 50 and 100.Compared with the mechanical properties of the initial coal sample under uniaxial compression,the compressive strength and elastic modulus of the tested raw coal sample significantly reduces and their values fall 84％ and 92％,respectively.While the peak strain increases by 117％.The failure mode of the tested coal sample shows high volume fracture.The test indicates a large number of micro cracks are produced in the matrix of the coal sample and the permeability of the coal sample is improved substantially by the pulse gas pressure fatigue,which is conducive to flow and extraction of gas.