An investigation was performed to determine and compare pore pressure, stresses, and critical mud weights using poroelastic analyses with and without including the undrained loading effect. The undrained loading effect, which enables a fully-coupled poroelasticity analysis for wellbore stability, can be crucial to the stabilization of near-wellbore formations within the critical time even though it will diminish at long time. The critical time, or the duration, of the undrained loading effect is found to be inversely proportional to formation permeability and will vary considerably for formations with different permeabilities. It is also demonstrated in this paper that it is the dimensionless time, not the dimensional time, that controls the duration of the undrained loading effect.

Chemical effects, caused by the imbalance between the water activity in the drilling mud and the shale water activity, are also found to affect shale stability by changing pore pressure and shale strength with exposure time. Including all effects (hydraulic diffusion, undrained loading, and chemical effects), one finds that the required mud weight can increase with time, decrease with time, or first decrease and then increase. Furthermore, the change in required mud weight with time is different at different radial positions. The chemical effect does not alter the undrained loading effect, but it may counterbalance the common destabilization of the undrained loading mode.

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