This study was undertaken in order to correlate the filtration behavior of water based muds with the structural properties of the cake. The structure of the cake is analysed by cryo scanning electron microscopy that enables the visualisation of a section of the frozen cake.

Static and dynamic filtration experiments were performed both through rock slices and paper filters. On rock slices it is possible to visualize the structure of the internal cakes that invade the pores of the rock and in particular to show a selective filtration of the polymer when using a formulation containing bentonite and a fluid loss reducer.

When compared to static filtration, dynamic filtration of a clay suspension gives higher filtrate volumes but leads to a cake texture characterized by a more regular network and smaller pore size. The augmentation of the filtrate volumes with shear rates is the result of both a decrease of the cake thickness and a diminution of connections between the clay sheets induced by shear rates. When polymer is added the structure of the pore walls seems less affected by shear rates, probably because of the high degree of dispersion already reached in the suspension by addition of the polymer and the ability of polymer to establish connections between particles.

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