Experimental study on porosity and permeability of anthracite coal under different stresses

• Variation laws of porosity and permeability of anthracite under stress were revealed.
• Correlations between porosity, permeability and effective stress were established.
• Relationship between porosity and permeability under effective stress was established.
• Influences of temperature and moisture on coal permeability were analyzed.
• A few indexes were proposed to evaluate effective stress-dependent permeability.

Coal porosity and permeability are key factors influencing coal-bed methane well production. In order to investigate the permeability behavior during anthracite coal seam methane production, the porosity and permeability of anthracite coal sample from No. 3 coal seam in Southern Qinshui Basin of China in net confining stress were measured in laboratory. The correlations between porosity, permeability and effective stress were analyzed. Permeability damage rate, stress sensitivity coefficient and pore compressibility factor were proposed to evaluate the effective stress-dependent sensitivity characteristics of anthracite coal. It turns out that, both porosity and permeability of coal sample decrease exponentially with the increase of effective stress. If the effective stress is less than 5 MPa or 6 MPa, stress sensitivity coefficient of coal reservoir changed greatly, and the stress sensitivity coefficient decreases rapidly with effective stress increased. The permeability damage rate increases rapidly with increasing effective stress, the stress sensitivity of coal reservoir enhanced; while in the effective stress is greater than 5 MPa or 6 MPa, the stress sensitivity coefficient of the coal reservoir decreases as effective stress increases slowly, and there is fluctuation, the stress sensitivity of coal reservoir is reduced; while permeability damage rate with the increase of effective stress increased more slowly. With the increase of moisture content and temperature, the permeability damage rate of coal reservoir and stress sensitive coefficient increase, and the stress sensitivity of coal reservoir enhanced.