Dynamic variation effects of coal permeability during the coalbed methane development process in the Qinshui Basin, China

The commercial exploitation of coalbed methane (CBM) has been achieved in the Qinshui Basin, China. However, the productivity of wells varies greatly because of their different geological settings and production rules, and the permeability damage to coal reservoirs caused during the development process is one of the most important factors affecting productivity. In this study, the patterns of permeability variation in coal seam No. 3 and their influence on CBM recovery were analyzed, and the degree of permeability damage caused by different factors was calculated. The results show that the burial depth, the bottom hole pressure and the dewatering rate affected the effective stress imposed on the coal reservoir, which further induced changes in permeability and productivity. The elastic modulus of the coal body reflects its ability to withstand compression: the greater the elastic modulus is, the less the permeability decreases. When the pressure drops from 7.05 MPa to 0.20 MPa, the relatively low elastic modulus of coal seam No. 3 can cause a 10% decrease in permeability. Coals at different burial depths have different formation pressures and elastic moduli, causing different levels of permeability damage during the development process. Calculations indicate that the absolute permeability will decrease by 5.5% and 14.3% at depths of 500 m and 1000 m, respectively. If the bottom hole pressure drops too fast or the dewatering rate is too high, there will be a strong stress response, resulting in decreasing permeability and productivity. This study found that, in high-productivity wells, a larger pressure drop (approximately 0.022 MPa/d) and stroke (3.0/min) are favorable in the early drainage period, but a smaller pressure drop (0.002 MPa/d) and stroke (0.4/min) should be applied when the gas peak appears.

► An evaluation model of changes in coal permeability and productivity was built. ► Many factors determine whether a CBM well will achieve high and stable production. ► Permeability damage with different degrees existed during CBM production. ► A reasonable production rule was proposed according to the production features.