Brines chemical equilibrium

Chemical equilibrium model Most reactive transport formulations use the mass action law to solve the chemical equilibrium equations. In this formulation an alternative (though thermodynamically equivalent) approach is used, based on the minimization of Gibbs Free Energy. This approach has a wider application range extending to highly non-ideal brine systems. 

The concept of local phase equilibrium within a reservoir under chemical flood with fractional flow of each phase governed by its fractional mobility and saturation, is well supported by evidence from laboratory chemical floods. Some of that evidence is presented in this paper and in Reference (1). r the extent that this concept represents actuality, we could predict the course of a chemical flood from starting compositions of the rock (for ion exchange and mineral dissolution characteristics), oil, formation brine, chemical slug and drive—if we had phase volume and mobility data on all phases which would form during the flood. 

Teeple, J.E. (1929). The development of Searles Lake brines with equilibrium data, American Chemical Society Monograph Series, Chemical Catalog Company Press, ISBN 0-07-571934-6, New York. 

To model the brine s chemistry, we need to estimate its oxidation state. We could use the ratio of sulfate to sulfide species to fix ao2 > but chemical analysis has not detected reduced sulfur in the brine, which is dominated by sulfate species. A less direct approach is to assume equilibrium with a mineral containing reduced iron or sulfur, or with a pair of minerals that form a redox couple. Equilibrium with hematite and magnetite, for example,... 

The difference between the extended Debye-Hiickel equation and the Pitzer equations has to do with how much of the nonideahty of electrostatic interactions is incorporated into mass action expressions and how much into the activity coefficient expression. It is important to remember that the expression for activity coefficients is inexorably bound up with equilibrium constants and they must be consistent with each other in a chemical model. Ion-parr interactions can be quantified in two ways, explicitly through stability constants (lA method) or implicitly through empirical fits with activity coefficient parameters (Pitzer method). Both approaches can be successful with enough effort to achieve consistency. At the present, the Pitzer method works much better for brines, and the lA method works better for... 

In the two-phase region, the type II(+) system has an oil-rich micellar phase in equilibrium with an excess brine phase. Surfactant is found almost exclusively in the oil-rich phase, and the concentration of surfactant in that phase can greatly exceed the concentration of surfactant in the injected chemical slug. In the type II(+) environment, the micellar phase remains miscible with the oil but is immiscible with the brine. Oil continues to be recovered by a misciblelike process. The opposite occurs if the phase environment is type II(-). The brine-rich micellar phase is immiscible with the oil phase, and oil recovery is by low IFT immiscible displacement. 

The physicochemical aspects of micro- and macroemulsions have been discussed in relation to enhanced oil recovery processes. The interfacial parameters (e.g. interfacial tension, interfacial viscosity, interfacial charge, contact angle, etc.) responsible for enhanced oil recovery by chemical flooding are described. In oil/brine/surfactant/alcohol systems, a middle phase microemulsion in equilibrium with excess oil and brine forms in a narrow salinity range. The salinity at which equal volumes of brine and oil are solubilized in the middel phase microemulsion is termed as the optimal salinity. The optimal salinity of the system can be shifted to a desired value hy varying the concentration and structure of alcohol. 

In this paper we utilize "Phase Volume Diagrams" in discussing the results of floods with continuous chemical floods. Such diagrams depict equilibrium volumes, compositions and properties for varying combinations of chemical slug, brine and oil. Phase Volume Diagrams are used by others too (7,8). 

Figure 3 presents the Phase Volume Diagram for the oil/chemical slug/formation brine combination used in the flow experiments discussed above. In the upper part of the diagram we plot phase volume fractions observed at equilibrium as a function of the amount of chemical slug replaced by formation brine in each sample tube. The points plotted at X = 0,5,10,15,20,30,40,50 and 60 show phase volume fractions observed for the oil/chemical slug/formation brine set of overall compositions 30/70/0, 30/65/5, 30/60/10, etc. 

The diagram shows that this chemical slug gives three phases at equilibrium in all of the sample tubes except the last one (60). The volume fraction of the surfactant-rich, middle (microemulsion) phase decreases as chemical slug is replaced by formation brine. 

Consistent with the phase equilibrium concept discussed in this section, there was a considerable difference in the time at which surfactant concentration peaked in the effluent liquids from those two chemical floods. Surfactant lag, brought about by the high salinity formation brine in the one flood, caused surfactant concentration in the effluent liquids to peak about 15 percent pore volume later in the flood with 100 percent SDSW formation brine than in the flood with 20 percent SDSW formation brine. 

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