Abstract:Air drilling is a kind of under-balanced drilling technology with air,whose rock breaking form is different from conventional mud drilling. Based on rock mechanics and elastoplastic mechanics,a 3D non-linear dynamic model of dynamic rock breaking with full gauge PDC bit is established using finite element method (FEM),in which anisotropy,strata dip and friction factor of drilling bit and rock are considered. The mechanism and process of dynamic rock breaking in air drilling are studied. And the stress and strain of borehole、shaft wall and bottom hole after rock breaking are analyzed as well. It shows that the rate of rock breaking in air drilling is faster in bottom hole where the tensile stress is larger,and the shaft wall is quiet stable under dry air;however,it would collapse in blocks in case of instability. And the change law of hole deviation and azimuth in anisotropic formation in air drilling is obtained by investigating the displacement of drilling bit with time variation. The model provides a new method for the study of breaking mechanism,the change of deviation and stability of shaft wall,and it offers theoretical basis for the development and optimization of bit and drilling tool in air drilling.