Mechanisms and control methods of depletion-induced shear failure of coalbed seams

Pressure depletion can induce the shear failure of coalbed seams,consequently resulting in downhole acci- dents such as borehole instability,casing failure and fines production. In order to prevent the shear failure,the stress path and failure behavior of coalbed seams were analyzed based on the uniaxial strain model with No. 3 coal of Shanxi Formation in the Qinshui Basin,and then the influence factors of shear failure were determined. Finally,a method through controlling the bottom hole flow pressure (BHFP) and CO2 -ECBM was proposed to prevent the shear failure. The results show that the effective horizontal stress of coalbed seam linearly increases with the decrease of pore pres- sure at the dewatering phase,whereas it nonlinearly reduces at the desorption stage,indicating that the gas desorption can accelerate the shear failure of coalbed seams;the critical pore pressure when coal failure happens is positively as- sociated with the vertical stress,initial pore pressure,critical desorption pressure,sorption-induced swelling,elastic modulus and Poisson’s ratio,whereas it increases with the decrease of initial horizontal stress and uniaxial compressive strength;CO2 injection before coal failure,as a well operation time of CO2 -ECBM,can not only prevent the shear fail- ure,but also enhance the CBM recovery.