Due to the low permeability of shales, excess pore pressures will be induced in the material in response to the volume change of the rock matrix during drilling. These excess pore pressures will reduce the effective confining pressure applied on the material which will lead to a less stable wellbore condition. However, the extent to which the induced pore pressure influences wellbore stability varies dramatically with the in-situ stress regime, wellbore trajectory, and the poroelastic and strength anisotropies of the shale.
This paper examines wellbore stability under different drainage conditions taking into account variations in the in-situ stress environment, wellbore trajectory and shale properties. Design charts derived from the analyses are then presented and their application discussed. Finally, the effects of induced pore pressure on wellbore stability are demonstrated through numerical modelling of borehole deformation of a thick-walled hollow cylinder shale sample under different drainage conditions. Comparison of numerical results with laboratory measurements shows good agreement.
The investigation provides further insight and understanding of the effects of induced pore pressure on stability of wellbores drilled in shales. The design charts enable assessment of the effects of in-situ stress regime, wellbore trajectory and poroelastic properties of the formation on wellbore stability to be readily made. In addition, recommendations for appropriate drainage conditions for use in wellbore stability analyses enable more accurate prediction of the safe mud weight window.