Wellbore / Shale Creep

Creeping shales often carry in situ stresses that are on or close to their shear capacity. Drilling causes stress change resulting in stress states outside the steady-state shear capacity which leads to creep.  However, sufficiently high mud-weights mitigate the levels of creep and stable openhole wellbores can be maintained. 

Wellbore pressure reduction, e.g. mud-weight sagging, accelerates the creep and can lead to annulus closure onto casing.  The resultant bond can provide a good annular seal.  The time-scale for closure can vary between days and centuries, depending on both the loading conditions (wellbore pressure) and creep rate of the shale.