REARRANGEMENT OF FINE PARTICLES IN POROUS-MEDIA CAUSING REDUCTION OF PERMEABILITY AND FORMATION OF PREFERRED PATHWAYS OF FLOW - EXPERIMENTAL FINDINGS AND A CONCEPTUAL-MODEL

The rearrangement of matrix configuration due to the redistribution of clay particles was studied by introducing different suspensions into porous media. Clay (kaolinite) with low CEC (cation exchange capacity) and small flocs of high CEC clay (smectite) accumulated in regions of slow flow. In some experiments this was followed by rapid accumulation in regions of high velocity, impairing flow to a considerable degree. Clay with high CEC formed more voluminous structures which could bridge over a passage and impair the flow. In either case, whether small or large units of clay are involved, the final flow occurred mainly through open, preferred pathways. Such processes of redistribution and appearance of flow pathways may occur in subsurface porous media aquifers where forced gradients exist (e.g. in aquifers near the perforation of pumping wells and in places where large quantity of water are naturally or artificially recharged and in producing gas and oil fields).