Borehole collapse analysis incorporating time-dependent pore pressure due to mud penetration in shales

This paper presents a model to predict time-dependent instability of arbitrarily oriented wellbores. The underlying factor to make the borehole collapse time-dependent is the increase of pore pressure around the wellbore due to mud penetration over time. The time-dependent pore pressure around the wellbore is estimated by analyzing two-phase fluid flow in porous media. The petrophysical parameters required for two-phase flow analysis, such as capillary pressure and absolute and relative permeabilities are characterized from experimental data. The model for time-dependent borehole collapse analysis is then developed by incorporating the transient pore pressure in the widely used wellbore failure analysis model. Potential wellbore failure modes are mathematically defined and guidance is incorporated to identify the upper and the lower bounds of the safe mud window at a particular time. The developed model is applied to wells in shale under different stress regimes. Results demonstrate that an initial safe mud window becomes narrower as the time progresses. Consequently, a mud weight, which was initially safe, may become unsafe and results in borehole collapse after a certain period of time. Such a failure is further influenced by the orientation of the wellbore and different stress regimes.