Stability analysis of fractured coal reservoirs under different stress mechanisms in coalbed methane well drainage

The dynamic adjustment of in-situ stress in the process of drainage and production can easily lead to the fracture, instability and damage of the reservoir, which affects the production efficiency of coalbed methane wells. In order to study the stability of fractured coal reservoirs in the process of coalbed methane well drainage and production under different stress mechanisms, based on the dynamic variation model of in-situ stress,the stability discrimination model of fractured coal reservoirs under different stress mechanisms was deduced. The variation law of fracture stability of coal reservoirs during drainage and production was proposed, and the stability evaluation index (Fcr) of fractured coal reservoirs was proposed. Based on actual data,a dynamic evaluation of the stability of fault-bearing coal reservoirs in the Zhengzhuang block in the southern Qinshui Basin was carried out.It indicates that: ① the stability of fractured coal reservoir was determined by fault friction coefficient. When the ratio of shear stress to normal stress on the fracture surface of the coal reservoir exceeds the friction coefficient of the fracture surface during the drainage and production process, the coal reservoir will undergo relative frictional sliding along the fracture surface. ② In the stage of drainage and pressure reduction and initial gas production, the stability of fractured coal reservoir is enhanced under different stress mechanisms. During the stage of stable gas production and attenuation, CBM is continuously desorbed, resulting in matrix shrinkage effect. The stability of fault-bearing coal reservoirs is constantly weakened under the stress mechanism of normal faults and strike-slip faults, and continuously enhanced under the stress mechanism of reverse faults. The change rate of coal reservoir stability increases with the enhancing of matrix shrinkage effects.③ It is assumed that coal reservoirs of Zhengzhuang block is a horizontally distributed isotropic pore-elastic medium, and there are optimal angle fracture surfaces near different well positions. In the stage of drainage and depressurization and initial gas production, the Fcr value in the normal fault and strike-slip fault stress mechanism area of the fault-bearing coal reservoir decreases and increases respectively, the stability decreases and increases respectively, the change range is small, and the main body is still located in the moderate stability area; in the stable gas production and decay stages, the Fcr value decreases rapidly, the stability decreases significantly, and some areas fall to the area of poor stability, or even destabilize.