Kinetic Study of the Decomposition of Surfactants for EOR

Abstract

Summary The effect of temperature, pH, surfactant purity, and air on the decomposition rate of two classes of surfactants typically used in steam EOR processes has been investigated. With synthetic alkylaryl sulfonates, isolation and characterization of the hydrocarbon reaction products established that the principal reaction is desulfonation of the surfactant. The kinetics of the decomposition of certain alkylaryl sulfonates under carefully controlled pH and temperature conditions are discussed and compared with the behavior observed for alpha- olefin sulfonates (AOS's) under the same conditions. The synthesis and decomposition of key pure isomeric alkylaromatic sulfonates of known structure led to a mechanistic description of the decomposition reaction. An explanation for the differing stability of alkylaromatic sulfonates and AOS's at high temperatures and at differing pH is suggested. Introduction The enhanced recovery of heavy oil with steam has been a commercial reality for many years. The more recent inclusion of surfactants in these processes has attracted considerable interest because indications are that surfactants in the steam may lead to additional recovery of oil. For a surfactant to be used in steam operations, however, it must possess some measure of thermal and chemical stability, the degree of which is determined by the conditions of use. The results of several investigations of the so-called "thermal stability" of various sulfonates have been reported in recent literature, and it appears from the data presented that certain alkylaromatic sulfonates have unusual thermal stability under conditions anticipated in most reservoirs. Our original intention was to study the stability of surfactants in environments similar to those expected in their applications. The alkylaryl sulfonates chosen proved so stable at reasonable reservoir temperatures and acidities, however, that obtaining meaningful data would have required excessively tong reaction times extending over several months per experiment. Consequently, the data reported herein represent results obtained in the laboratory under rather severe conditions of high temperature (the upper region of commercial steam operations) and strongly acidic environments to bring about reasonable reaction rates. Ample evidence exists to indicate that the decomposition reaction of alkylaryl sulfonates is not a purely thermal reaction (i.e., resulting from the homolytic rupture of bonds), but is one of hydrolytic desulfonation. This reaction was studied as early as 1901 by Crafts, 10 who suggested a catalytic influence on the reaction by strong acids in concentrated acidic media. More recent studies on the desulfonation reaction also support an autocatalytic decomposition mechanism for aromatic disulfonates. However, other workers claim that for both mono- and disulfonates the desulfonation reaction is first order with respect to the sulfonate concentration and independent of the acid concentration; different mechanisms were suggested to be operative in concentrated vs. dilute solution. The desulfonation reaction is suggested to proceed according to the following equation : (1) The decomposition of the sulfonate results in acid generation; thus if the hydrolysis reaction is actually acid-catalyzed, one might expect to encounter autocatalytic kinetics. In the literature, there seems to be general agreement that the disappearance of sulfonate follows first- or pseudo-first-order kinetics; the influence of the hydrogen ion is less clear. One objective of this paper is to clarify the role of the hydrogen ion in this reaction. Experimental The sulfonate decomposition studies were carried out in a 300-mL stainless steel Parr reactor equipped with a sampling tube, pressure relief valve, pressure gauge, and thermowell. The reactor was pressure relief valve, pressure gauge, and thermowell. The reactor was heated with an external proportionating heater. Samples were withdrawn from the reactor and passed through a coiled 3.2-mm [1/8-in.] stainless steel tube immersed in an ice bath, thereby providing only condensed samples. This system was purged between samples. The reactor was charged with 200 to 220 mL of 1-wt% surfactant solution, and the entire system swept with nitrogen unless otherwise noted. In the case of alkylaryl sulfonates, 3- to 4-mL aliquots were removed; in the AOS studies, 7- to 9-mL samples were withdrawn. The sulfonate solutions were prepared in deionized water, with buffers and salt added as required. Samples were analyzed for surfactant by high-performance liquid chromatography (HPLC) and by two-phase titration (AOS's). The analysis of very low concentrations of alkylaryl sulfonates was accomplished rapidly and reproducibly on either a Hewlett-Packard 1082B or Varian 5000 instrument using a UV detector. All samples for analysis were sonicated to ensure uniformity and carefully filtered before analysis. Each sample was analyzed in triplicate, and the average peak area used to calculate the concentration. Standard response curves were prepared for each aromatic sulfonate analyzed, and in all cases the prepared for each aromatic sulfonate analyzed, and in all cases the concentration vs. peak area response was linear in the concentration region of interest. Samples with sulfonate concentrations at the hundredths percent level can be readily and reliably analyzed by this method. For the experiments carried out in buffered solutions, potassium dihydrogen phosphate/sodium hydroxide was used to keep the pH at 7, sodium borate buffers for pH 8 to 9, excess sodium hydroxide for pH 11, and acetic acid/sodium acetate for pH in the 3 to 5 region. The pH remained unchanged even after several hundred hours at temperatures as high as 300C [572F], indicating that the buffers are stable at the temperatures used. All pH measurements were taken at room temperature with a Pope Model 1500 meter. The pH at reaction temperature is not known; however, for the phosphate buffer a temperature coefficient of +0.0003/C [+0.00054/F] has been reported" covering ranges from 25 to 95C [77 to 203F]. If this trend is obtained at higher temperatures, it would result in less than 0.1 pH unit change at 300C [572F]. Similarly, a solution of HCl of pH 1.1 at 25C [77F] is reported to have a pH of 1.2 at 275C [527F]. Other studies also report very small pH deviations with temperature, albeit the range of temperatures reported was not as large as in our experiments. We believe, however, that for a given buffer system, the deviation at reaction conditions from the room-temperature measured value will be uniform and probably small, thereby making the observed reaction rates valid on a relative basis. SPERE P. 613