Study on stress sensitivity of coal reservoir during drainage of coal-bed methane well in Southern Qinshui Basin

Coal samples from Southern Qinshui Basin were sampled,and experimental study on stress sensitivity under different experimental condition was performed,in order to analyze stress sensitivity and affecting factors on permeability of coal reservoir at different stage of coal-bed methane drainage. The results show that permeability of coal samples decline 50%- 70% when the effective stress increase from 0 to 10 MPa,and the loss of coal reservoir permeability is about 10% when the effective stress increase from 10 MPa to 20 MPa. The stress sensitivity of coal samples are relatively stronger when the effective stress is below 2. 5 MPa. The damage coefficient of permeability increase quickly and the loss of permeability is about 20%- 30% of the initial permeability from coal samples when the effective stress increase to 3. 5 MPa. The stress sensitivity is strongest when the effective stress increase from 2. 5 MPa to 9 MPa. The damage coefficients of permeability decrease quickly and the initial permeability from coal samples lose 60% when the effective stress increases from 3. 5 MPa to 9 MPa. The damage coefficient of permeability decrease slowly and the lossof permeability is about 10% of the initial permeability from coal samples when the effective stress beyond 9 MPa. The damage rate of permeability from coal samples is between 30% and 65%,and the critical stress is 7 MPa to 11 MPa.The permeability of coal samples increase with increase of pore pressure,when the effective stress is relatively lower and constant. The permeability of coal samples decrease with the decline of effective stress and increase of pore pressure when the confining pressure is constant,which is related to the variation of permeability caused by the variation of effective stress and pore pressure on coal reservoir.