Micellar Aggregation Behavior of Alkylaryl Sulfonate Surfactants for Enhanced Oil Recovery

Alkylaryl sulfonate is a typical family of surfactants used for chemically enhanced oil recovery (EOR). While it has been widely used in surfactant–polymer flooding at Karamay Oilfield (40 °C, salinity 14,000 mg/L), its aggregation behavior in aqueous solutions and the contribution of aggregation to EOR have not been investigated so far. In this study, raw naphthenic arylsulfonate (NAS) and its purified derivatives, alkylaryl monosulfonate (AMS) and alkylaryl disulfonate (ADS), were examined under simulated temperature and salinity environment of Karamay reservoirs for their micellar aggregation behavior through measuring surface tension, micellar size, and micellar aggregation number. It was found that all three alkylaryl sulfonate surfactants could significantly lower the surface tension of their aqueous solutions. Also, it has been noted that an elevation both in temperature and salinity reduced the surface tension and critical micellar concentration. The results promote understanding of the performance of NAS and screening surfactants in EOR.


Infrared spectra
Infrared spectra of raw NAS, AMS, and ADS samples were recorded with Abb Inc-MB 3000 FTIR spectrophotometer. Figures S1S3 show that the peaks at 3441-3445 cm -1 are from the stretching vibrations of water molecules. The peaks around 2925 cm -1 are the stretching vibration adsorption of C-H bonds. The peaks at 1583-1630 and 1442-1458 cm -1 display the stretching vibration of the skeleton of the benzene ring. The peaks at 1182-1186, 1050-1058, and 632-644 cm -1 were the standard peaks for sulfonate groups. The infrared analysis indicates that the sodium petroleum sulfonate is the main component of the surfactant.

1 H NMR spectra
The 1 H NMR spectra were obtained on a Bruker Avance-II 400 MHz NMR spectrometer. The 1 H NMR spectra of raw NAS, AMS, and ADS samples are shown in Figures S4-S6.
The 1 H NMR spectrum of AMS is shown in Figure S4, where the peaks at 0.87, 1.26, 2.38, 3.62, 6.74, and 7.58 ppm are the chemical shifts of the sodium petroleum sulfonate. The peaks at 3.57, 1.50, 1.31, and 7.58 ppm are the chemical shifts of n-butyl alcohol.
The 1 H NMR spectrum of ADS is demonstrated in Figure S5, where 0.80, 1.20, 2.50, 3.60, and 7.00 ppm are the chemical shifts of sodium petroleum sulfonate.
The 1 H NMR spectrum of raw NAS is displayed in Figure S6, where the peaks at 0.87, 1.26, 2.38, 3.62, 6.74, and 7.58 ppm are the chemical shifts of sodium petroleum sulfonate.

GPC Results
Molecular weight data are essential for studying the chemical composition of surfactant for enhanced oil recovery (EOR). Given that the studied surfactants are three types of different complicated mixtures, the molecular weights of the contained compounds are not the same, and there is a wide range in molecular weights. Hence, only the average molecular weight could be used to represent the surfactant. GPC measurements (Shodex 3000 GPC system consisting of an HPLC pump) were performed to measure the molecular weight of samples. According to GPC, the molecular weight of sodium petroleum sulfonate in AMS and ADS was mainly distributed at approximately 400 D (Figures S7-S8).

Thermogravimetric Analysis (TGA)
According on the analysis above, the three surfactant samples all contain sodium petroleum sulfonate. The TGA on the samples was carried out to determine the thermostability of sodium petroleum sulfonate. The measurement was performed by TGA 2850 thermogravimetric analyzer (TA instruments) under N2 in the temperature range 0800 C with an increase rate of 5 C/min. The TGA results of the three types of dried surfactant samples are shown in Figures S9-S11.
For AMS, the dialysis temperatures of n-butyl alcohol and sodium petroleum sulfonate tested by TGA are 138 and 456 C, respectively.
For ADS, the dialysis temperatures of sodium petroleum sulfonate and sodium carbonate tested by TGA are 444 and 717 C.
For raw NAS, the dialysis temperature of sodium petroleum sulfonate tested by TGA is 447 C.

Mass spectrometry (MS)
MS was performed on the three samples to analyze whether the samples contain substances with a low molecular weight. MS analyses were performed on an HP 5890 Series II gas chromatograph (Hewlett-Packard, Les Ulis, France).