Time-dependent wellbore stability has been observed in the field. Yet, a combined effect of pore fluid advection, swelling shale, and thermal diffusion may not be sufficient to explain and simulate this process adaquately. A coupled chemico- geomechanics constitutive model combined with a model with an ion-concentration dependent strength is developed in this paper. Strength reduction due to water hydration seems to dominate the time-dependent process in wellbore collapse. The ultimate borehole instability is proposed to be determined by the damage radius not the onset of wellbore wall mechanics failure, the latter is usually practiced. A field case is analyzed, and from experimental testing and simulations, it is concluded that the time-dependent nature is dominated both by the time- dependent strength reduction and damage zone propagation.

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