A model study of clay mobilisation and permeability reduction during oil reservoir flooding
Formation damage effects have been reported in a number of oil reservoirs in secondary and enhanced recovery stages of production. The loss in permeability has been widely attributed to the swelling or pore-plugging action of colloidal clay minerals which are present in varying quantities in the pore spaces of reservoir rocks. This research has been aimed at predicting the action of non-swelling clay particles such as those commonly found in North Sea reservoir sandstones. The literature from the areas of formation damage, colloidal suspensions, and deep bed filtration has been examined with particular attention being given to the subject of clay mineralogy. An experimental test rig was designed, comprising a cylindrical packed sand bed used as a model reservoir to study the permeability changes associated with the action of clay fines under various flooding regimes. The pH, salinity and valency of ionic species present in the flooding fluids were found to be the controlling factors for clay mobilisation. Fines dispersal coincided with spontaneous decrease. in permeability when sandpacks treated with monovalent brine were subjected to fresh water flow. Analysis showed that'the fines released from the bed were primarily kaolinite in the size range 1-5 micron. A minimum critical+ salinity of flooding water was shown to exist, above which particle dispersal is prevented. Further, experimental work provided a measure of the ion exchange capacity of the clay fines in the sandpack as well as of pure clay materials. The close-range interaction of kaolinite particles with silica surfaces was measured in a continuous flow glass apparatus which allowed in-situ observation and measurement by optical microscopy. Particle deposition in this system was found to be very sensitive to the method of cleaning of the silica surface. Deposition was also a function of the suspension flowrate. Clay mobilisation behaviour, as observed in the sandpack experiments, was confirmed with suspension pH, salinity and flowrate to a lesser extent, governing the particle release process. The research has identified the conditions under which clay mobilisation is initiated. It is proposed that any model of permeability reduction should be adapted to include the effects of specific ion-exchange processes between clays and flooding liquids.