Maintaining a stable borehole is one of the major problems encountered in the oil and gas industry since wellbore instability will result in additional high drilling costs. 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, mud infiltration into the fractures can lead to a reduction in the fracture friction angle. As a result, the rock masses are more prone to wellbore instability along the fractures. In this paper, coupled numerical analyses are presented to investigate the influence of fractured rock mass, pore pressure and, 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 numerical predictions are compared with continuum solutions to evaluate the (in)adequacy of using continuum rock model for fractured rock masses.
Maintaining a stable borehole is one of the major problems encountered in the oil and gas industry since wellbore instability will result in additional high drilling costs. 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 (Last et al. 1995, Okland and Cook 1998). Such instability occurs when the effective stresses acting on the discontinuities reach their critical values which will result in the slippage and rotation of the blocks. In addition, mud infiltrated into the fractures may interact with reactive formations and/or lubricate the fractures which can lead to a reduction in the fracture friction angle. As a result, the rock masses are more prone to wellbore instability along the fractures.
Few records of numerical analyses to investigate wellbore behaviour in fractured rock masses can be found in the literature, especially with friction angle reduction of the fractures when they are infiltrated with mud. Zhang et al. (1999) conducted a relatively comprehensive study of wellbore behaviour in fractured rock masses. However, no mud infiltration mechanism, which is believed to be critical in fractured rock masses, is considered in their study. In this paper, coupled numerical analyses are presented to investigate the influence of fractured rock mass, pore pressure and, 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 accounthe friction angle reduction of the fractures that occurs when infiltrated with mud. The friction angle of the fractures locating within the mudinfiltrated region is reduced from its initial to residual value. The numerical predictions are compared with continuum solutions to evaluate the (in)adequacy of using continuum rock model for fractured rock masses.
