Study on hydraulic fracture wall stability of CBM well

In order to demonstrate the coal pulverization mechanism caused by the shear instability of coal cracks wall in coal seams during gas drainage,elasticity mechanics coupled with percolation theory were used to construct the total principal stress in the formation surrounding the hydraulic fracture during discharging process. Based on the D-P shear failure criterion,a method for calculating the bottom hole flowing pressure safety window is proposed,which can effectively avoid the instability of the coal-rock fracture wall.The results show that lowering of bottom hole flowing pressure can increase the net stress around fracture,which can lead to higher induced stress perturbation around fracture wall.In addition,due to poroelastic effect,the lower the bottom hole flowing pressure is,and induced stress is also greater.During the drainage process,the bottom hole flowing pressure is ensured within the safety window,which can avoid the formation of pulverized coal due to shear failure of the fracture wall.