Hydrolytic Stability of Alkylethoxy Sulfates

Abstract

Summary. Sulfate surfactants are being proposed by industry for use in moderate-temperature surfactant floods. Sulfates are also being considered by some groups for use in CO2 Mobility control and surfactant/alkaline floods. Data from this study show that the maximum applicable temperatures for sulfates under these pH conditions are much lower than for neutral pH conditions. Rate equations are derived from Arrhenius data on the basis of observed decomposition mechanisms. The equations are used to calculate surfactant lifetimes and maximum temperature limits at various reservoir conditions. The maximum temperatures and lifetimes are functions of pH and brine composition. Introduction Hydrolysis of sulfate surfactants may occur within the range of reservoir conditions at rates that can interfere with the performance of these surfactants. There are several studies that have given partial quantitation to the hydrolysis rates of sulfates in aqueous partial quantitation to the hydrolysis rates of sulfates in aqueous solu-tions. This study was initiated to develop a model that could predict the stability of ethoxy sulfates (EOS's) under many EOR process predict the stability of ethoxy sulfates (EOS's) under many EOR process conditions. Such a model would help decisions to be made, case by case, concerning the use of sulfate surfactants as lower-cost alterntives to the more expensive EOS'S. His work identified three principal independent variables that affect sulfate surfactant stability under anaerobic conditions: tem-perature, pH, and brine composition. The pertinent degradation mechanisms observed in this study include pertinent degradation mechanisms observed in this study include H + -catalyzed hydrolysis, water solvolysis, nucleophilic displacement by Cl-, nucleophilic displacement by sulfide, inhibition of H - catalysis by salt cations, and Ca ++-catalyzed OH - displacement. In addition to the variables mentioned previously, surfactant structure influences hydrolysis rates. The alkyl sulfates require different kinetics model from the alkylethoxy sulfates. The reason for the difference is that the sulfate reactivity is affected by ether oxygens adjacent to the sulfate group. Because of these differences, the present study was expanded to include a kinetics model for alkyl present study was expanded to include a kinetics model for alkyl sulfate surfactants. All the above mechanisms were modeled for EOS'S, but only the first three mechanisms were modeled for dodecyl sulfate. Oxygen reacts extremely fast with surfactants at reservoir temperatures. Data are given here to show that typical levels of oxygen in micellar flooding fluids and reservoir fluids will not cause a significant amount of surfactant decomposition. Therefore, on the premise that the surfactants would be used in nominally oxygen-free environments, the large oxygen rate constant was excluded from the models. The effect of reservoir rock on surfactant stability was excluded so that the model would be more generally applicable. The presence of rock will probably not cause a decrease in the hydrolysis presence of rock will probably not cause a decrease in the hydrolysis rates for the mechanisms observed here any unforeseen reaction mechanism is expected to add to the observed rates. Therefore, the model predictions are considered to represent the maximum intrisic stability. Although the experimental methods used here have general applicability to thermal stability studies of nonsurfactant chemicalsin this study particular attention was given to the phase behavior of the surfactants. Because all the kinetics data were obtained for lower-phase water-external microemulsions, the models are not applicable to all phase states. Theory Kurz published rate constants for water solvolysis, H - catalysis, and OH - displacement reactions of alkyl sulfates. Garnett et al. published Arrhenius data for the H -catalyzed hydrolysis of alkyl sulfate and alkylethoxy sulfate surfactants. The reaction mech-anisms described in both papers were established by Batts, who used isotopically labeled reactants. The reactions cited above fit the first two mechanisms shown in Fig. (1) nucleophilic substitution by water or OH - ion 1 and (2) hydrogen-ion catalysis. The result of either mechanism is the loss of the sulfate group to form 1 mol of alcohol plus 1 mol of acid. The water-solvolysis-rate equation is ..........................................(1) where k, is a pseudo-first-order rate constant and C, is surfactant concentration. The concentration of water is implicitly contained within the constant k,. The rate of nucleophilic substitution by aqueous hydroxyl ion was shown by Kurz to be too slow to observe for micellar sulfates. The hydroxylion reaction rate was shown by Calhoun and Burwell to obey a second-order rate equation, ..........................................(2) for those sulfates that do not micellize. The rate of H + catalyzed hydrolysis is first order in surfactant concentration and first order in hydrogen-ion concentration for dilute solutions. The rate equation is ..........................................(3) at moderate to low H + and surfactant concentrations. When Mechanisms 1 and 2 are present, the observed rate is the sum of rates of both mechanisms: ..........................................(4) where the observed rate is pseudo-first-order in surfactant concentration at constant hydrogen-ion concentration. If the H + concentration is held constant, Eq. 4 may be written as ..........................................(5) where kOB is the constant for the observed rate. The rate constants from Eq. 4 are derived from observed rate constants, kOB, measured at various values of pH. The hydrolysis and nucleophilic substitution reactions of the sulfates involve reactants of various charge types. The water solvolysis reaction is of the anionic-neutral-charge type. Hydrogen-ion catalysis is a combination of an anionic/cationic equilibrium and a neutral/neutral-type step involving polarized sulfate acids. SPERE p. 778