Alkaline interfacial tension reduction

Figure 5. Comparison of alkaline interfacial tension reduction vs, pH of Illinois crude, 0.25% Petrostep 450, 1.5% N l. (Reproduced, with permission, from Ref. 17. Copyright 1980, Society of Petroleum Engineers of AIME.)...

Mechanistic interpretations The results of the dynamic and equilibrium displacement experiments are used to evaluate and further define mechanisms by which alkaline floods increase the displacement and recovery of acidic oil in secondary mode and the tertiary mode floods. The data sets used in the mechanistic interpretations of alkaline floods are (a) overall and incremental recovery efficiencies from dynamic and equilibrium displacement experiments, (b) production and effluent concentration profiles from dynamic displacement experiments, (c) capillary pressure as a function of saturation curves and conditions of wettability from equilibrium displacement experiments, (d) interfacial tension reduction and contact angle alteration after contact of aqueous alkali with acidic oil and, (e) emulsion type, stability, size and mode of formation. These data sets are used to interpret the results of the partially scaled dynamic experiments in terms of two-stage phase alteration mechanisms of emulsification followed by entrapment, entrainment, degrees and states of wettability alteration or coalescence. 

For Distillate cut 3, which accounts for 30% of the total crude, the narrow "gate nature (15) has been broadened considerably in alkalinity range. It is possible that under certain conditions, the very low interfacial tension levels obtainted from the caustic solution-crude oil interaction enabled the oil to be entrained in a continuous flowing alkaline-water phase, resulting in a substantial reduction in residual oil saturation (16,17). 

When an alkaline solution is mixed with certain erode oils, surfactant molecules are formed. When the formation of surfactant molecules occurs in situ, the interfacial tension between the brine and oil phases could be reduced. The reduction of interfacial tension causes the microscopic displacement efficiency to increase, which thereby increases oil recovery. 

The reactions between the alkaline solution and reservoir oil generate the complex oil recovery mechanisms of AF postulated to date (1) in-situ surfactant generation by neutralization (saponification), (2) reduction of the interfacial tension (IFT) at the oil-water interface, (3) temporary wettability alteration, (4) emulsification with entrainment, (5) emulsification with entrapment, (6) emulsification with coalescence, (7) oil phase swelling, and (8) breaking out the rigid films. Thus, the consumption of alkalinity through the reactions of alkaline solution with reservoir waters and rock constituents has been accepted as the most important disadvantage of the AF process. ... 

The decreases in the water relative permeabilities of the high pH/high salt alkaline floods are directly contrasted with the increases in the relative permeabilities to water at the end of the moderate pH/high salt flood (compare the end point relative permeabilities column in Table 2). The increased permeability to water is believed to be caused by the formation of rigid interfacial films (which increases the resistance to flow in oil filled pores) and by the oil-wet conditions (under which water flows in the less restrictive flow paths). Such a reduction in permeability, which has been used to indicate the existence of a low tension mechanism (18), is not a valid low tension index since the interfacial tension minimum is only 3.5 dynes/cm and the capillary number is 1 x 10" for the buffered alkali/salt-oleic acid system. 

Ultra-low tension In the alkaline flooding of acidic acids, some reduction in interfacial tension (from 30 to approximately 10 1 dynes/cm) is necessary for the emulsification and subsequent mobilization of waterflooded residual oil by the previously discussed phase alteration mechanisms. The residual oil may also be mobilized and produced by a low-tension displacement process which is similar to surfactant flooding if the interfacial tension can be further reduced to ultra-low values (10 to 10 dynes/cm). 

The recovery of acidic oils via an Emulsification and Coalescence mechanism is very similar to the recovery of nonacidic oils by petroleum sulfonate solutions (6). Although high coalescence rates are prerequisites to the success of both systems, the recovery efficiencies of alkaline-acidic systems appear to be inversely related to the rate of coalescence. It is speculated that the relaxation time for coalescence must be greater than the flow time required for the contacting of adjacent ganglia of residual oil which have been mobilized by emulsification and wettability alteration processes. Yet coalescence was a necessary condition for enhanced recoveries since systems (such as hydroxides with univalent cations) in which the emulsion phase flocculated into the original oil volume did not improve the recovery efficiency. Some reduction in interfacial tension (10 " -100 dynes/cm) is required for the emulsification step although ultralow tension (10 -10 dynes/cm) was not necessary to the alkaline recovery processes. This observation concurs with an identical result of Schechter and coworkers (6). The two steps to this recovery mechanism. Emulsification and Coa,lescence, appear to be necessary and sufficient conditions for the enhanced production and recovery of nonacidic (6) and acidic oils. 

The data from these tests show that sodium orthosilicate is more effective than sodium hydroxide in recovering residual oil under the conditions studied, both for continuous flooding and when 0.5 PV of alkali was injected. The mechanisms through which sodium orthosilicate produced better recovery than sodium hydroxide in this system have not been completely elucidated. Reduction in interfacial tension is similar for both chemicals, so other factors must play a more important role. Somasundaran (26) has shown that sodium silicates are more effective than other alkaline chemicals in reducing surfactant adsorption on rock surfaces. Wasan (27,28) has indicated that there are differences in coalescence behavior and emulsion stability which favor sodium orthosilicate over sodium hydroxide. Further work is being done in this area in an attempt to define the limits of physically measurable parameters which can be used for screening potential alkaline flooding candidates. 

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