Optimum alkane carbon number

Based on this concept, a crude oil/surfactant/brine system should have phase behavior (e.g., optimum salinity, IFT minima) similar to that of the pure alkane/ surfactant/brine system whose ACN is the same as the crude EACN. However, the concept of EACN is not practically applicable for several reasons. First, all the hydrocarbon compositions of a crade oil are not readily identified. Thus, the EACN of a crude oil cannot be calculated directly using Eq. 7.79. Second, measurement of the EACN of a crude oil requires a series of surfactant solutions to be tested to obtain individual minimum ITT. Then these surfactant solutions are tested against increasing alkane carbon numbers to find minimum IFTs. The ACN at which a surfactant solution also gives the lowest IFT for the crude oil is the EACN of the oil. Finding it is not an easy task. Third, several parameters affect the value of the EACN. Variations in EACN with alcohol cosolvent type, total WOR of the sample, and crude oil composition have been observed (Tham and Lorenz, 1981). In practice, we always select surfactants by scan tests using the actual crude oil for a specific application. 

Figure 3.2 Plots of the salinity (5) versus the alkane carbon number (ACN). (a) Optimum formulation lines as the locus of the minimum interfacial tension, i.e. of the three-phase region centre, (b) Optimum formulation line as the locus in bidimensional S-ACN map for the same water-oil-alcohol systems containing different surfactants at constant temperature. CnOXS stands for alkylorthoxylene sulphonates, ABS for alkyl benzene sulphonate, PS for petroleum sulphonate (the number after PS indicates the average molecular weight).

Crude oils were found to behave as an equivalent alkane as far as the attainment of optimum formulation was concerned. The equivalent alkane carbon number or EACN was then introduced to characterise pure hydrocarbons or mixtures [26]. The EACN of an oil phase is defined as the ACN of the alkane that results in the satisfaction of the correlation in the same conditions of surfactant, salinity, alcohol and temperature. EACN has been experimentally determined for n-alkanes mixtures, resulting in a linear mixing rule on a molar fraction basis, namely... 

Figure 14 Relation between aqueous phase salinity (as a natural logarithm) and oil phase Alkane Carbon Number so that optimum formulation is attained.

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