Alkali-Polymer (AP) flooding involves adding an alkaline agent along with polymer to softened injection water. The alkaline agents are usually either caustic soda (NaOH) or soda ash (Na2CO3) at concentrations usually less than 2%. The water must be softened because the alkaline agents would cause any divalent cations, such as calcium and magnesium, to precipitate, and this solid precipitate will plug most formations. The alkali reacts with components of some oils to form "soap" (saponify) that in the right environment will reduce the interfacial tension sufficiently to overcome capillary forces trapping the oil. The AP solution is injected for 20% to 40% pore volume and followed by a similar volume of polymer flush.
AP flooding has been and is currently being used in full field projects with incremental recoveries of 10% OOIP to 20% OOIP. Of all the chemical flooding processes, it probably has the most limited number of potential applications because not all oils have components that saponify. Also, softening water itself can be complex and costly, depending upon the water hardness and salinity. However, AP flooding is more economic than processes using surfactant because the cost of alkali is much lower than surfactant.
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