Emulsions in EOR processes and refining

The enhanced oil recovery using solutions of surfactants or their mixtures has attained relatively little application. This is, first of all, due to the fact that surfactants adsorb from the solution on porous media of the reservoir, the specific surface of which may range from 150 to 3000 cmVcm, therefore, the use of emulsions, microemulsions and the so-called micellar-polymer flooding turned out to be more effective. In all of these processes, the flow 

The emulsion behaviour in porous media is discussed in [235]. O/w emulsions with volume fractions of up to 50% show Newtonian behaviour, whereas those with more than 50% are non-Newtonian liquids, the apparent viscosity of which depends on the shear rate. The viscosities of such emulsions are more than 20 times that of water and sometimes can be even comparable with that of oil. When the emulsion is moving, a temporary permeability reduction of the reservoir may occur due to the capture of small droplets by the surface of the porous medium. In this case, stable o/w emulsions may flow not as a continuous liquid, i.e. the emulsion flow largely depends on the nature of the porous medium. Therefore, it is necessary to know about the structure and physicochemical characteristics of the oil reservoir (porous medium) porosity, the mean pore diameter, the mean pore size and pore size distribution, chemical composition of the minerals ( acidic , basic , neutral ), the nature of the pore surface, first of all wettability, for a successful application of the emulsion flooding method. 

Several micellar-polymer flooding models as applied to the EOR are discussed in [237]. It is noted that the co-solvent ordinarily used in this process considerably influences not only the microemulsion stabilisation, but also the removal of impurities in the pores of the medium. The idea of using an alkali in micellar-polymer flooding is discussed in [238] in detail. The alkali effect on the main oil components was studied aromatic hydrocarbons, saturated and unsaturated compounds, light and heavy resin compounds and asphaltenes. It is demonstrated that at pH 12 surfactants formed from resins allow to achieve an interfacial tension value close to zero. For asphaltenes, such results are achieved at pH 14. In the system alkali solution (concentration between 1300 to 9000 ppm)/crude oil at 1 1 volume ratio a zone of spontaneous emulsification appears, which is only possible at ultra-low interfacial tensions. 

Optimum parameters for a successful accomplishment of the EOR process are defined in [239]. First, they can be realized in reservoirs consisting of sandstones, sands, carbonates or mixtures of these materials with a layer thickness of under 25 m and permeability of over 20 mD, containing crude oil with a viscosity of 60 mPa s. In this case, a formulation containing, along with alkali, 20 - 30 g/1 salts, ethoxysulphates and alkyl sulfonates, used for injection at a temperature below 80 °C. The authors are optimistic about the wide commercial use of EOR, since oil will remain the main energy source up to the second half of the 21 century. Surfactants can also be used to recover oil containing gas for the purpose of gas hydrate formation control [240]. 

One of the most important steps of oil refining is demulsification of oil emulsions both of o/w and w/o types. O/w emulsions may contain up to 40% water, and their breaking is, as a rule, accomplished under field conditions at special stations where the field oil arrives. W/o emulsions of 4 - 5% water content are subjected to demulsification directly at refineries. General approaches to the problem and theories of the process are given in [37, 38]. 

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