Wettability reversal

In the water-wet to oil-wet type of wettability reversal, low residual oil saturation is attained through low IFT and viscous water-in-oil emulsions working together to result in a high capillary number. Obviously, the salinity in alkaline water should be high so that W/0 emulsion can be generated with the help of low IFT caused by soap, and the rock surfaces are made to be oil-wet (Cooke et al 1974). 

The wettability reversal from water-wet to oil-wet needs to be discussed further because it is opposite to our perception that rock surfaces should be made more water-wet for improved oil recovery. To the best of our knowledge, only Cooke et al. provided a detailed discussion of this mechanism. 

Injected alkaline concentration and volume appear to vary depending on the recovery mechanism involved. Concentrations are generally lowest for emulsification mechanisms, from about 0.001 to 0.500 wt.%. Higher concentrations ranging from about 0.5 to 3.0 wt.% or even as high as 15.0 wt.% usually have been required for wettability reversal. Generally, a slug of alkaline solution can... 

The caustic method as a means of improved waterflooding for enhanced oil recovery is a complex process. Johnson has outlined four recovery mechanisms (6). Presumably, besides the ultralow tension mode, there are other requirements to ensure efficient and stable recovery of an oil in a given reservoir. To name a few spontaneous emulsification, entrainment, entrappment, wettability reversal in both directions, etc. In order to maintain a particular set of pro-... 

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. 

By tradition, most of the efforts to change wettability with either strong acids or alkali have been directed toward making substances more water-wet. However, Cooke, Williams and Kolodzie (29) described a field trial wherein a water-wet reservoir was actually changed to a system which was more oil-wet with a small increase in recovery resulting from the injection of an alkaline material. In order to effect this unusual wettability reversal from water-wet to oil-wet, it was necessary to achieve the right conditions of temperature, pH and, especially, salt concentration. 

Perhaps the most successful field trial utilizing inexpensive sodium hydroxide is that reported by Graue and Johnson in 1974 (30). Although it is not considered to be a wettability reversal project, it is included here because of the use of sodium hydroxide. Graue and Johnson (30) reported an increased oil recovery of approximately 400,000 barrels of crude oil in the Whittier Field in California by what they describe as an emulsification and entrapment mechanism. 

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. 

High pH/high salt alkaline (such as 0.25M NaOH/l.OM NaCl) systems appear to improve the rate of recovery of acidic oils by an emulsification and partial wettability reversal mechanism. Partial wettability reversal is suggested by the intermediate magnitude of the externally measured contact angle, 120° and the wettability index of the porous media, -0.023 (Table 4). 

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. 

Civ) COMPLETE WETTABILITY REVERSAL (Water- to Oil-Wet) increases the recovery of acidic crudes in moderate pH flooding in high salinity environments. 

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... 

Winter, A., Stability of Thin Wetting Films and Wettability Reversal in Reservoir Rocks, presented at the Symposium on Fundamentals of Fluid Transport in Porous Media organized by Institut Prancais du Petrole, May 14 18, 1990, Arles, Prance (in preparation). 

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