Abstract: In recent years multilateral well drilling technology has become one of the mainstream technologies of the field of
oil and gas exploration and development. But the technology also faces some new challenges，and borehole stability nearby the
junctions is one of the serious problems. This paper establishes a three-dimensional model using finite element. Calculation and
analysis of in-situ stress re-distribution caused by sidetrack drilling is made with consideration of the impact of initial pressure
and steady-state Darcy flow. The stress concentration and distribution of displacement nearby the junction after sidetrack drilling
are obtained through the finite element analysis of fluid-solid coupling calculation. According to Moore–Coulomb criterion，the
stability of the connection section of wall projections is presented with graphics with the help of finite element post-processing
program. The impact of the geometric parameters on borehole stability，such as borehole diameter，dip and azimuth is also
discussed. The results show that：the most stable junction，independently of the depth of its placement and borehole diameter，
is with the lateral wellbore axis oriented parallel to the maximum principal in-situ stress.

Key words: multilateral wells, fluid-solid coupling, stability of borehole, in-situ stress