Fracture pressure in traditional wellbore stability analysis is based on tensile failure. When the drilling fluid density is high, there is not only tensile fracture but also shear fracture may occur on the wellbore wall of horizontal wells. The possible fracture modes of horizontal wells were analyzed. Furthermore, the influence of drilling azimuth, in-situ stress value and formation strength on the fracture modes of horizontal wells was analyzed. The results show that: the possibility of tensile fracture is greater when drilling horizontal wells near the maximum horizontal stress direction, and the possibility of shear fracture grows when the drilling azimuth is close to the minimum horizontal stress; when the value of horizontal stress is low, the wellbore is more likely to occur tensile fracture, but with the horizontal stress increasing, the possibility of shear fracture increases; the possibility of shear fracture increases as the formation strength decreasing.
Wellbore instability while drilling is a common but important problem that has puzzled the petroleum industry for long. The economic losses caused by wellbore instability reaches more than one billion dollar every year (Mohammad 2012). Therefore, the wellbore stability technique is on the top of the research list of the petroleum companies all over the word (Lu& Tang 2000). The aim of wellbore stability research is to determine the range of drilling fluid density that can maintain the wellbore stable and to provide the foundation for the well structure design and selection of the proper drilling fluid density and property (McLean 1990, Tang et al. 2007). During the drilling operation, proper drilling fluid density should prevent downhole accidents, such as overflow, wellbore collapse, shrinkage and lost circulation. The selection of the drilling fluid density should keep to the following rules: the drilling fluid density should be bigger than the lower limit of the safe drilling fluid density window while less than the upper limit of the safe drilling fluid density window. The lower limit is equal to the bigger one of the collapse pressure and the pore pressure, and the upper limit is the formation fracture pressure. Previous wellbore stability research mostly focused on collapsing pressure and revealed wellbore collapsing mechanism from different aspects such as mechanics and chemistry et al (Bradley 1979, Aadnoy et al. 1987, Liu et al. 1998, Deng et al. 2003, Qiu et al. 2007, Zhao & Chen 2011, Yu et al. 2011, Roshan et al. 2012). Research on the fracture pressure was comparably less, though some achievement has presented (Eaton 1969, Huang et al. 1984, Guo et al. 2004,Wang et al. 2005, Lian et al. 2008, Zhang et al. 2008, Deng et al. 2009, Roshan et al. 2012). However, the theoretical foundation was derived from the hydraulic fracture theory (Hubbert & Willis, 1972), and only took the tensile fracture into consideration with overlooking of the shear fracture which may occur when tangential stress is the minimum principal stress.