Abstract: The unique mechanical deformation characteristics of the soft shale under a high pressure and in the hydraulic environment are always important issues of researches of the wellbore stability and control engineering.In view of low strength and high plasticity of the soft shale formation, a numerical evaluation model of the wellbore stability is established with consideration of the effects of the hydraulics-deformation coupling, the plastic damage and the permeability variation. The influences of the fluid seepage, the mud pressure and the horizontal stress difference on the wellbore stability are studied. It is shown that for overbalanced drilling, when the mud pressure exceeds ten percent of the formation pore pressure, the hydraulics-deformation coupling effect leads to an expansion of the plastic damage area, by at least sixty-five percent. Moreover, the increase of the horizontal stress difference promotes the plastic damage zone to concentrate in the direction of the minimum horizontal principal stress, and the damage zone becomes deeper.

Key words: soft shale|plastic strain softening|wellbore stability|hydraulics-deformation coupling|mud pres-sure