Alkaline flooding wettability reversal

Alkaline flooding is based on the reaction that occurs between the alkaline water and the organic acids, naturally occurring in some crudes, to produce in-situ surfactants or emulsifying soaps at the oil/water interface. Recent literature (i-J.) summarizes several proposed mechanisms by which alkaline water-flooding will enhance oil recovery. These mechanisms include emulsification and entrapment, emulsification and entrainment, and wettability reversal (oil-wet to water-wet or water-wet to oil-wet). Depending on the initial reservoir and experimental conditions with respect to oil, rock and injection water properties, one or more of these proposed mechanisms may be controlling. 

In the alkaline flood process, the surfactant is generated by the in situ chemical reaction between the alkali of the aqueous phase and the organic acids of the oil phase The surface-active reaction products can adsorb onto the rock surface to alter the wettability of the reservoir rock and/or can adsorb onto the oil-water interface to lower the interfacial tension. At these lowered tensions (1-10 dyne/cm), surface or shear-driven forces promote the formation of stable oil-in-water emulsions or unstable water-in-oil emulsions the nature of the emulsion phase depends on the pH, temperature, and electrolyte type and concentration. These different paths of the surface-active reaction products have created different recovery mechanisms of alkaline flooding. The four alkaline recovery mechanisms which have been cited in the recent literature are (i) Emulsification and Entrainment, (ii) Emulsification and Entrapment, (iii) Wettability Reversal from Oil-to Water-Wet, and (iv) Wettability Reversal from Water- to Oil-Wet. These four mechanisms are similar in that alkaline flooding enhances the recovery of acidic oil by two-stage processes. 

The incremental production of acidic oil by moderate pH (buffered) /high salinity alkaline systems occurred by what is believed to be a complete wettability reversal mechanism. Complete wettability reversal is suggested by the magnitude of the measured contact angle, 170° but not by the wettability index of the porous media. The results of the secondary and tertiary buffered floods appear to confirm the experimental results of Cooke et al. (4) the work of these researchers is discussed in a later section. It will be shown that this mechanism is not a direct extension of the emulsification and partial wettability reversal mechanism. 

In chemical flooding processes for enhanced oil recovery, alkaline chemicals can be useful for hardness ion suppression or removal, reaction with acidic crude oils to generate surface-active species, reduction in surfactant adsorption on reservoir rock surfaces, changes in interfacial phase properties, mobility control and increased sweep efficiency, oil wettability reversal and increased emulsification. 

Alkaline Floods. Alkaline floods, typically using sodium hydroxide, generate surface active products by an in-situ chemical reaction between the injected alkali and the organic acids of the crude. Four possible mechanisms [95] are responsible for the recovery of oil by alkaline floods (1) emulsification and entrainment, (2) emulsification and entrapment, (3) wettability reversal from oil-wet to water-wet, and (4) wettability reversal from water-wet to oil-wet. One example in the literature of wettability alteration by alkali [96] was reported for an offshore field in the Gulf of Mexico fhat had a low recovery factor from primary production. The wettability of this reservoir was found, using the... 

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