Experimental study on overburden pore permeability of mediumand high rank coal reservoirs in Qinshui Basin

In order to study the variation characteristics of porosity,permeability and stress sensitivity of medium and high rank coal reservoirs in net confining stress,four medium and high rank coal samples were collected from Qinshui Basin in Shanxi Province.Comparative study of the porosity and permeability of coal in net confining stress experiments.This study discussed the difference of stress sensitivity between medium-rank coal and high-rank coal.And it studied the change rule of stress sensitivity with the change of coal rank.The results show that the porosity of coal increases first and then decreases with the increase of Ro,max.There is no obvious relationship between permeability and Ro,max.The porosity and permeability of medium and high coal rank coal in net confining stress decrease exponentially with the increase of effective stress.The average pore compressibility coefficient and stress sensitivity coefficient of high-rank coal are 1.61 times and 1.26 times of medium-rank coal respectively.The damage rate of porosity and permeability of high-rank coal is 1.42 times and 1.1 times of that of medium-rank coal,respectively.The permeability curvature of medium and high coal grade coal decreases with the increase of effective stress.When the effective stress is 12 MPa,the average permeability curvature of high rank coal is 1.33 times of that medium coal rank coal.With the increase of Ro,max,the stress sensitivity of coal tends to increase,that is,the stress sensitivity of high-rank coal is higher than that of medium-rank coal.Therefore,in the process of drainage and mining of different coal-rank coal reservoirs,the production pressure differential adopted for different coal-rank coal reservoirs should be different.Compared with medium-rank coal reservoirs,high-rank coal reservoirs show that as porosity increases with effective stress and permeability damage rate is high,permeability curvature decreases by a large margin.Therefore,in order to obtain a higher CBM output,the production pressure differential used in the drainage and mining process of high-rank coal reservoirs should be lower than that of medium-rank coal reservoirs.