Microemulsions in enhanced oil recovery

Microemulsions in enhanced oil recovery 10.3.1 Why enhanced oil recovery and not alternative fuels ... 

Mashiko, A. B. E., Microemulsions in enhanced oil recovery Middle-phase microemulsion formation with some typical anionic surfactants, in Industrial Applications of Microemulsions, Solans, C. and Kunieda, H. (Eds), Surfactant Science Series, Vol. 66, Marcel Dekker, New York, 1997, pp. 279-303. 

Emulsifiers are used in many technical applications. Emulsions of the oil-in-water and the water-in-oil type are produced on a large scale in the cosmetic industry. Other fields of employment are polymerization of monomers in emulsions and emulsification of oily and aqueous solutions in lubricants and cutting oils. In enhanced oil recovery dispersing of crude oil to emulsions or even microemulsions is the decisive step. 

Solubilizing activity are also used in enhanced oil recovery. Tar and extremely viscous hydrocarbons are recovered by the injection of an aqueous solution of an anionic orthophosphate ester surfactant into a petroleum formation, retaining the surfactant in the formation for about 24 h, and displacing the solubilized hydrocarbons toward a recovery well. The surfactant forms an oil microemulsion with the hydrocarbons in the formation. An anionic monoorthophosphate ester surfactant which is a free acid of an organic phosphate ester was dissolved in water. The input of surfactant solution was 2-25% of the pore volume of the formation [250]. To produce a concentrate for the manufacture... 

In using microemulsions to enhance oil recovery from petroleum reservoirs (see Section 11.2.2) the concept of optimal salinity has evolved. By optimal salinity is meant the salinity for which O/W interfacial tension is lowest and oil recovery is... 

See Emulsion. In enhanced oil recovery nomenclature, the term macroemulsion is employed sometimes to identify emulsions having droplet sizes greater than some specified value and sometimes simply to distinguish an emulsion from the microemulsion or micellar emulsion types. 

The polymerization of acrylamide (AM) and the copolymerization of acrylamide-sodium acrylate in inverse microemulsions have been studied extensively by Candau [10,11,13-15], Barton [16, 17], and Capek [18-20]. One of the major uses for these inverse microlatexes is in enhanced oil recovery processes [21]. Water-soluble polymers for high molecular weights are also used as flocculants in water treatments, as thickeners in paints, and retention aids in papermaking. 

Crude oil becomes trapped in porous media as a result of capillary forces. The reduction of these forces is required for the recovery of residual oil, and this is the basis of enhanced oil recovery. In practice capillary forces are reduced primarily by lowering interfacial tension between oil and water phases, although increasing the viscosity of the water is also important. Lowering interfacial tension leads to the formation of emulsions and microemulsions, which are of great importance in enhanced oil recovery techniques. 

Although the contacting experiments were performed with surfactant systems typical of those used in enhanced oil recovery, application of the results to detergency processes may be possible. For example, the growth of oil-rich intermediate phases is sometimes a means for removing oily soils from fabrics. Diffusion path theory predicts that oil is consumed fastest in the oil-soluble end of the three-phase regime where an oil-rich intermediate microemulsion phase forms. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

When an aqueous system containing a surfactant, cosurfactant and of intermediate salinities is allowed to equilibrate with crude oil, the mixture sometimes separates into three phases. One of these phases is the aqueous phase which contains very little surfactant. This is called the lower phase. The second phase is called the middle phase. This phase is a microemulsion which contains large amounts of both oil and water and nearly all the surfactant. The third (upper) phase contains the oil. Systems of oil and aqueous phases which show this phase behavior are said to exist in the "beta" region of the phase diagram. The "beta" type systems have been shown to form the least stable emulsions and thereby result in enhanced oil recovery (30). 

Microemulsions were first discovered empirically by Schulman, who found that the addition of a fourth component (often an alcohol) to an emulsion containing oil, water, and a surfactant led to the formation of a clear, apparently homogeneous phase. This additional component is usually called the co-surfactant. Microemulsions have been the subject of intense study in recent years, especially in view of their possible use in enhanced oil recovery. A typical recipe for forming a microemulsion is given in Appendix I. 

Microemulsion research has since its inception been stimulated by the great potential for practical applications. In particular, considerable research interest has been invested in the possibility of using microemulsions for enhanced oil recovery (EOR). It was observed that surfactant formulations forming three-phase microemulsion systems, often termed Winsor III systems [29], in the oil well could increase the oil yield considerably. Important contributions to the understanding of the mechanisms involved were made by Shah and Hamlin [30] and the Austin group led by Schechter and Wade (see Bourrel et al. [31]). 

Further development of recovery methods contemplated physical and chemical procedures, such as application of pressure, water injection, or implementation of techniques that alter system miscibility. All aspects that impact on the flnal recovery yield must be considered, for example capillary forces oil viscosity contact angle between the adsorbed oil and the solid surface permeability, wettability, and porosity of the solid reservoir among others. Hence, it is obvious that huge perturbations are provoked in the oil reservoirs when surfactant-based systems are used in enhanced oil recovery operations. The potential role of microemulsions in such activities is once again highlighted. 

The properties of microemulsions can be exploited with many advantages in enhanced oil recovery apphcations. hi this topic, two basic approaches are discussed ... 

Inverse microemulsion polymerization is used for the production of water-soluble polymers of high molecular weight, with applications in enhanced oil recovery, as flocculants in water treatments, thickeners for coatings, and retention aids in papermaking. 

In addition to the four mechanisms for surfactant action in enhanced oil recovery cited above, work continues on the use of polymeric additives to control the rheological properties of the aqueous flooding solutions. In the use of such processes in conjunction with surfactant additives, it is important to consider potential interactions between the polymer and the surfactant. As more surface-active chemicals are employed in the recovery process, control of the formation and breaking of emulsions and microemulsions produced in the recovery process can become important as a postproduction problem. 

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