Analysis on impact factors of borehole wall stability of coalbed methane well

In view of the low strength of coal seam, cleats development, in the process of drilling borehole wall instability phenomenon seriously,based on the filed sampled, statistical analysis and scanned electron microscopy, the macro and micro structure and mechanical strength characteristics and deformation failure characteristics of coal rock were studied. The general discrete element numerical simulation software was used to simulate the influence actors of wellbore stability in the process of CBM drilling based on the characteristics of coal cleats. The results showed:the cleats in coal rock were developed, the face cleats and the butt cleats were approximately vertical and mostly distributed in mesh and dendrite. The mechanical parameters of coal rock were discrete,The stress and strain curves of coal rock were similar under uniaxial and triaxial conditions, underwent several stages of compaction, elasticity, yielding and brittle fracture. Without considered the seepage action, the maximum shear stress and shear displacement of the borehole wall were consistent with the variation of the bottom hole pressure, and all decreased first and then increased.The bigger the well diameter, the greater the shear displacement at the well, the easier to lose stability.When the well diameter was constant, the greater the density of cleats, the more likely the coal bed blocks were to shear slip and the instability of the borehole. The results could provide certain reference for the safe and efficient coal seam gas drilling.