Sulfonate Retention by Kaolinite at High pH - Effect of Inorganic Anions

Abstract

Summary At high pH and with surfactant concentrations well above the critical micelle concentration (CMC), the pH-dependent part of adsorption is small. Retention depends especially on the sodium-ion activity and on the type and valence of inorganic anions. As the anionic charge increases, adsorption at a given ionic strength decreases. This behavior correlates with the effect of anions on the sulfonate activity in solution. Introduction Retention of a petroleum sulfonate by sodium kaolinite is investigated at high pH (up to 13) and with surfactant concentrations in alcohol-containing brines well above the solubility, giving rise to suspensions of liquid crystals. Usual alkaline agents, sodium hydroxide, carbonates, silicates, and phosphates are compared in various salt environments. Sulfonate adsorption is determined in batch experiments. Adsorption isotherms show no plateau value after the CMC but a more or less pronounced maximum with a shape that varies with the ionic environment. The pH-dependent part of adsorption, assumed to occur on the charged crystal edge of kaolinite, is small with respect to total retention. Thus, at these surfactant concentrations, increases in pH have practically no effect. On the other hand, adsorption depends both on the sodium-ion activity and on the types and valences of inorganic anions present in the aqueous fluid. As. the anionic charge increases, adsorption at a given ionic strength decreases considerably. It is smaller than that expected with a decrease in sodium-ion concentration alone. The results are correlated with the effect of anions on the sulfonate activity in solution. One major point is that sodium carbonate reduces adsorption more than sodium hydroxide does, although the pH is lower by more than one unit, which helps reduce the dissolution of kaolinite and therefore the consumption of the alkaline agent. Background In EOR by surfactant flooding, one major preoccupation is to maximize the chemicals injected because the profitability of the process depends largely on additive loss. Hence, the degradation of the micellar solution must be minimized, especially by reducing the adsorption of surfactant on the rock surface as much as possible. One way to do this (because anionic surfactants are mostly used) is to increase the pH of the medium with alkaline agents. This technique has been recommended for a long time.1,2 An increase in pH reduces the number of positive sites on mineral surfaces (silica, clays) and then decreases the electrostatic attraction of anionic surfactants, particularly in the acidic pH range and around neutrality. Adsorption of surfactants under alkaline conditions has been studied mainly with such relatively water-soluble surfactants as sodium dodecylbenzene-sulfonate3 at concentrations near the CMC. Furthermore, the pH of investigation used to determine adsorption isotherms was generally 11 or less. This study aims at examining the adsorption of a petroleum sulfonate that is not very brine-soluble by a kaolinite at low to medium salinity and with surfactant liquid-crystal suspensions in alcohol-containing brines. Specific effects of standard alkaline agents, such as sodium hydroxide, carbonates, silicates, and phosphates, are compared in various salt environments and in a pH range from neutrality to around 13. Experimental The results reported here focus on adsorption of the petroleum sulfonate TRS 10-80TM on a homoionic sodium kaolinite at 30°C in the absence of multivalent cations. Kaolinite. The kaolinite used came from Guizengeard (Charentes, France) and contains 86% pure kaolinite [Al2O3, (SiO2)2, (H2O)2]. Excess aluminum and exchangeable multivalent cations, as well as most organic materials, were eliminated with the following procedure. The kaolinite was washed with a concentrated (50 g/L) NaCl brine, a sodium hydroxide solution (pH˜13.5), a 10-g/L NaCl brine up to pH 12, and finally with a 5-g/L NaCl/0.1-N HCl brine up to pH 7. After the resulting suspension was centrifuged, the clay was dried at 45°C and stored. The Brunauer/Emmett/Teller specific surface area was found to be 23 m2/g, with the crystal edge area representing about 22 % of the total surface area. 4 Chemicals. Witco Chemical Co. provided the petroleum sulfonate TRS 10-80. Its average molecular weight is 405. The chemical composition is approximately 80 wt% sulfonate (active material), 11 wt% unsulfonated oil, 8 wt% water, and 1 wt% inorganic salts. The petroleum sulfonate was mainly used as received. In fact, no difference in adsorption behavior was observed between the crude sulfonate and the sulfonate purified by deoiling and desalting. Aqueous solutions of TRS 10-80 are slightly alkaline. The sulfonate solution concentration was determined by turbidimetry at 620 nm with Hyamine 1622 as the reagent or, for concentrations less than 100 ppm (with the deoiled sulfonate), by ultraviolet (UV) spectrophotometry at 230 nm. All the sulfonate concentrations mentioned here refer to active material. The inorganic chemicals used were pure-grade reagents. Merck supplied the secondary butanol. Deionized water (pH=6.7) was used. Procedure - Isotherm Determination. The sulfonate solubility was determined at an ionic strength, I, of 0.448 M, corresponding to a (10-g/L NaCl)/(10-g/L Na2CO3 brine), arbitrarily chosen as a reference.Equation 1 where Ci and Zi are the molar concentration and the valence of Ion i, respectively. At this salinity level, sulfonate solubility is very low, a few parts per million. If solubility is exceeded, liquid-crystal suspensions form, especially in the presence of alcohol. With 30 g/L of 2-butanol, sulfonate suspensions up to a sulfonate concentration of 20 g/L are quite stable, with no decantation or heterogeneity in the bulk solution after a I-week storage at 30°C. In 21-g/L NaCl brine and 30-g/L 2-butanol, the CMC was found to be around 7 ppm (Fig. 1). This value corresponds with the CMC values that Salager5 found. Adsorption was measured in batch experiments. Mixtures of surfactant solution and kaolinite (solid/liquid ratio=0.1) were agitated for 40 hours and then were centrifuged at 1500 g for 20 minutes. The supernatant liquid was filtered through 0.45-µm Millipore Millex HV™. The amount of sulfonate adsorbed was determined by measuring the difference in the sulfonate concentration in the solution before and after contact with kaolinite. In the presence of NaCl and Na2SO4 or Na2SO4, only (i.e., without any alkaline agents), the pH of the supernatant liquid increases from about 6.7 to 7.3 as a function of sulfonate equilibrium concentration. p. 123–127 Kaolinite. The kaolinite used came from Guizengeard (Charentes, France) and contains 86% pure kaolinite [Al2O3, (SiO2)2, (H2O)2]. Excess aluminum and exchangeable multivalent cations, as well as most organic materials, were eliminated with the following procedure. The kaolinite was washed with a concentrated (50 g/L) NaCl brine, a sodium hydroxide solution (pH˜13.5), a 10-g/L NaCl brine up to pH 12, and finally with a 5-g/L NaCl/0.1-N HCl brine up to pH 7. After the resulting suspension was centrifuged, the clay was dried at 45°C and stored. The Brunauer/Emmett/Teller specific surface area was found to be 23 m2/g, with the crystal edge area representing about 22 % of the total surface area. 4 Chemicals. Witco Chemical Co. provided the petroleum sulfonate TRS 10-80. Its average molecular weight is 405. The chemical composition is approximately 80 wt% sulfonate (active material), 11 wt% unsulfonated oil, 8 wt% water, and 1 wt% inorganic salts. The petroleum sulfonate was mainly used as received. In fact, no difference in adsorption behavior was observed between the crude sulfonate and the sulfonate purified by deoiling and desalting. Aqueous solutions of TRS 10-80 are slightly alkaline. The sulfonate solution concentration was determined by turbidimetry at 620 nm with Hyamine 1622 as the reagent or, for concentrations less than 100 ppm (with the deoiled sulfonate), by ultraviolet (UV) spectrophotometry at 230 nm. All the sulfonate concentrations mentioned here refer to active material. The inorganic chemicals used were pure-grade reagents. Merck supplied the secondary butanol. Deionized water (pH=6.7) was used. Procedure - Isotherm Determination. The sulfonate solubility was determined at an ionic strength, I, of 0.448 M, corresponding to a (10-g/L NaCl)/(10-g/L Na2CO3 brine), arbitrarily chosen as a reference.Equation 1 where Ci and Zi are the molar concentration and the valence of Ion i, respectively. At this salinity level, sulfonate solubility is very low, a few parts per million. If solubility is exceeded, liquid-crystal suspensions form, especially in the presence of alcohol. With 30 g/L of 2-butanol, sulfonate suspensions up to a sulfonate concentration of 20 g/L are quite stable, with no decantation or heterogeneity in the bulk solution after a I-week storage at 30°C. In 21-g/L NaCl brine and 30-g/L 2-butanol, the CMC was found to be around 7 ppm (Fig. 1). This value corresponds with the CMC values that Salager5 found. Adsorption was measured in batch experiments. Mixtures of surfactant solution and kaolinite (solid/liquid ratio=0.1) were agitated for 40 hours and then were centrifuged at 1500 g for 20 minutes. The supernatant liquid was filtered through 0.45-µm Millipore Millex HV™. The amount of sulfonate adsorbed was determined by measuring the difference in the sulfonate concentration in the solution before and after contact with kaolinite. In the presence of NaCl and Na2SO4 or Na2SO4, only (i.e., without any alkaline agents), the pH of the supernatant liquid increases from about 6.7 to 7.3 as a function of sulfonate equilibrium concentration. p. 123–127