As brine composition profoundly influences reservoir wettability and hence microscopic sweep, careful design of injection brine is part of a strategy to improve on oil production in existing and future water flooding projects, in both sandstone and carbonate reservoirs and in combination with follow-up EOR projects.

The following results were found: (1) Formation water with higher salinity level correlates to a higher content of multivalent cations. This causes the (sandstone) reservoir wettability to be more oilwet; (2) The field-observed temporary reduction in watercut during breakthrough of so-called "Designer Waterflood" water in a Middle Eastern sandstone reservoir with highly saline formation water was interpreted to be caused by an oil bank ahead of the slug of injected water; (3) The oil bank results from improved sweep by wettability modification to more waterwet state. The interpretation was confirmed by laboratory experiments; (4) Experiments in limestone core plugs demonstrate similar wettability modification, if the sulphate ion content in the invading brine is far in excess of the calcium ion content.

Based on these results the following conclusions were drawn: (1) Designer Waterflooding may increase the Ultimate Recovery of oil by at least a few percent; (2) There is scope for further improvement in oil production by flood front stabilization by adding low concentration polymer to the optimised slug; (3) If future EOR projects are planned, a Designer Waterflooding pre-flush is recommended to obtain more favourable oil desaturation profiles and savings on polymer costs; (4) In case of seawater injection into reservoirs with formation water of low salinity level, removal of multivalent cations from the seawater should be considered to avoid the potential risk that the reservoir becomes more oilwet, which will result in reduced sweep.

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