A study on wellbore stability in fractured rock masses with impact of mud infiltration

Maintaining a stable borehole is one of the major tasks encountered in the oil and gas industry due to the fact that wellbore instability-related problems will result in additional high drilling costs and have a severe impact on drilling schedule. Wellbore stability analysis has therefore been included at the well planning stage of many operating companies. However, majority of such wellbore stability analyses are based on continuum mechanics. Factors contributing to wellbore instability, such as trajectory of the wellbore, orientation and magnitude of the in situ stress field, rock properties, in situ and induced pore pressures and mud pressure, are normally considered in the analyses. In addition to failure in intact rock, wellbore instability can also be initiated along natural discontinuities, such as bedding planes and fractures, in rock masses. Furthermore, the rock masses will become more prone to wellbore instability along fractures penetrated by mud filtrate due to reduction in the fracture friction angle and loosening of blocks. In this paper, coupled numerical analyses are presented to investigate the influence of fractures in rock masses, in particular mud infiltration into the fractures, on wellbore stability under both isotropic and anisotropic stress states. Two regular fracture geometries are considered in the analyses. A simplified approach is developed to take into account the friction angle reduction of the fractures that occurs when infiltrated with mud. The results show that the presence of natural fractures and friction angle reduction of fractures due to mud infiltration significantly affect wellbore stability during drilling.