Borehole stability analysis: A new model considering the effects of anisotropic permeability in bedding formation based on poroelastic theory

The mechanical properties of bedding planes in laminated shale rocks have attracted great attention especially when assessing borehole stability. However, the anisotropic permeability effects, though proved significant in bedding rocks by many experiments, are seldom indicated for the borehole-stability problem in the literature. In this paper, a new borehole-stability model was developed to thoroughly analyze the anisotropic permeability effects: the equivalent permeability coefficient of bedding formation is first defined according to the permeability tensor theory in anisotropic medium; the poroelastic solutions of the stress state around inclined boreholes are then obtained incorporating the anisotropic permeability effects; the failures of both rock matrix and bedding planes are considered to evaluate borehole stability. The results show that the permeabilities around deviated boreholes are quite different in bedding formation, which will lead to the change of stress state, failure area around the borehole and the change of collapse pressure and fracture pressure. The pore pressure around the borehole calculated under anisotropic permeability condition (APC) is usually lager than that calculated under isotropic permeability condition (IPC), while for other effective stresses the opposite holds true. The failure depth around borehole is quite different for APC and IPC under the Mohr-Coulomb and Mogi-Coulomb criteria. In general, the collapse failure of rock matrix can be initiated at some distance away from the borehole while the collapse failure of weak planes is usually initiated at the borehole wall. It is more crucial to consider the anisotropic permeability effects to calculate collapse pressure than fracture pressure when assessing borehole stability. Besides, the time growth may lead to collapse or fracture failures near the borehole. The developed analytical model can provide a theoretical basis for drilling design in laminated shale formation.