Wellbore stability is still a primary concern in the oil/gas well drilling industry in spite of several decades of research. Wellbore instability may incur large financial losses, especially for offshore wells or ultra-deep onshore wells. Naturally fractured shale formation has been recognized as one of the major sources of wellbore instability issues. Thus, a better understanding of the mechanisms of wellbore instability in naturally fractured shale formations is highly desirable. In this paper, we present a three-dimensional hydro-mechanical finite element model for analyzing the evolution of damage zone around the wellbore and evaluating the stability of wells drilled in naturally fractured shale formations with a single system of parallel fractures. A jointed material plasticity model is adopted to account for the strength anisotropy due to the existence of fractures. Permeability of the formation is also treated as anisotropic to honor the fact that drilling fluid infiltration along the fracture direction is easier than that along the direction normal to the fracture plane. Furthermore, an inertia-based stabilizing method that we have proposed previously is employed to overcome the numerical convergence difficulty. The finite element model has been applied to analyze the stability of an actual well drilled in Bohai oilfield of China where wellbore instability issues have been frequently encountered within the naturally fractured shale intervals. The impacts of drilling fluid density and drilling fluid infiltration on the shape and size of the damage zone around the wellbore are demonstrated, and some implications for maintaining wellbore stability in these naturally fractured shale formations have been gained.

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