Experimental analysis of stress-strain,permeability and acoustic emission of coal reservoir under different confining pressures

The characteristics of stress-strain,permeability and acoustic emission(AE) for coal reservoir are the basis for the study of hydraulic fracturing and the productivity evaluation of coal reservoir. As an effective method to study the instability,failure and its evolution process of coal and rock materials,the acoustic emission technology has been widely used. In this study,the stress-strain,permeability and acoustic emission tests of coal samples under different confining pressures were carried out by using No. 8 coal seam samples from Carboniferous of Taiyuan Formation in Xishan mining area of Qinshui Basin. The influence of confining pressure on the stress-strain,permeability and acoustic emission of coal and its control mechanism were revealed. The results show that the deformation and failure process of coal is divided into three stages:pore compression and elastic deformation stage,plastic deformation stage and failure instability stage. In the stage of pore compression and elastic deformation,the pores and fractures in coal are gradually compacted at the initial stage of loading,the permeability of coal sample decreases,and when it enters the stage of elastic deformation,the permeability of coal sample is low,and the AE activity is not obvious. In the plastic deformation stage,the permeability of coal sample increases with the increase of axial stress and fracture propagation,and the AE activity intensity increases significantly and reaches a peak value. At the stage of failure and instability,the axial stress of coal decreases with the increase of strain,the permeability of coal sample begins to decline,and the AE intensity also gradually decreases. The axial failure load,effective elastic modulus and residual strength of coal sample increase with the increase of confining pressure. The initial permeability,peak permeability and residual permeability of coal sample and the accumulated AE counts decrease with the increase of confining pressure. The stress-strain,permeability and AE characteristics of coal samples under different confining pressures are caused by the failure mechanism of coal under different confining pressures.