Wellbore stability is an important issue for drilling a horizontal well or an extended reach well. In this paper, the distribution of the breakdown pressure and collapse pressure is calculated with varying well inclination and azimuth angles. Considering the formation seepage, the mud density window for wellbore stability is obtained for different cases of in situ stress. The open-hole extension limit of the horizontal section in an extended reach well can be determined using these results. The results show that when σv>σH>σh, the mud density window will become narrower with increasing deviation angle. Meanwhile, the mud density window becomes narrower or disappears as the in situ stress disparity increases. In addition, when the horizontal well section is drilled along the direction of σh, the wellbore stability achieves the safest state. The extension limit length of the open-hole horizontal section in the extended reach well can also reach its maximum value. The open-hole extension limit will continue to increase as the annular wellbore friction is reduced. The research presented in this paper provides guidance for the optimal design of wellbore stability during drilling operations.
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