Failure modes of drilling fluid filter cake

Filter cakes formed on well bore walls during the drilling process need to be ruptured to start oil production. In this study, the failure of external filter cakes from water-based drilling fluids was investigated when the damage incurred by the formation is due almost entirely to brine and polymer invasion during filtration. It is proposed that in such cases the two-phase flow behind the filter cake, in the rock, combined with the cake's mechanical properties determines the mode in which it will fail. The two different failures, ‘pinholing’ of the cake and ‘lift-off’, are both observed in the experiments, with the mode of failure depending on the pressure drop across the cell. At a low pressure drop the cake deforms locally, separating from its substrate. Pinholes subsequently appear above these detached zones. For a large pressure drop the original deformed zones grow in size and ultimately merge. This leads to a large portion of the cake separating from the rock. A simple model of filter cake failure, based on a classical elastic cavity expansion analysis, is proposed. The model predicts pinholing or lift-off behaviour, depending on the cake's stress state and the degree of fingering in the two-phase formation.