Abstract
Kinetic study of the desulfonation of Dowfax surfactants, disulfonated alkyl diphenyloxide, is carried out in the temperature range of from 177 to 272C, and a pH range of 2.0 to 7.0. The rate data support that the desulfonation is a step-wise reaction catalyzed by hydrogen ion. The desulfonation rate is first-order with respect to both surfactant and hydrogen ion concentrations. As a result, the pH of the reaction system has a very significant effect on the stability of the surfactants and may outweigh the effect of temperature. The half-life of disulfonated alkyl diphenyloxide at 235C in a 5.5-pH buffer solution is 1,035 days, and at 272C in a 7.0-pH buffer solution is 295 days. The results indicate that these surfactants can be used in most oil reservoir steamflood applications, especially when the rock formation may buffer the pH to about 7.0.
Introduction
These surfactants are a mixture of mono- and dialkylated, essentially disulfonated diphenyloxide. The materials are sold commercially either in acid form (pH less than 1.0) or mostly in neutralized form (pH >7) as surfactants for various applications. These surfactants commonly are used in harsh environments such as high temperature, high electrolyte concentration, and wide pH range. The stability of these surfactants has not been studied extensively. Handy et al. evaluated several surfactants for oil-reservoir application. Dowfax 2AO (the acid form) was one of the surfactants they studied. Their data, for 177C and a pH ranging from 3.3 to 3.9, indicate that the desulfonation follows first-order kinetic with a half-life of 5.63 days. Since the pH in most carbonate reservoirs will be buffered to approximately 7 pH, Handy et al. also studied the effect of pH using a petroleum sulfonate, Petronate TRS-10-80. Buffering the material to a pH of 7.04 only marginally improves the half-life from 7 to 10 days. Consequently, they concluded that none of the surfactants tested would perform adequately in the steamflood oil-reservoir applications. perform adequately in the steamflood oil-reservoir applications. It is generally believed that the desulfonation of aromatic sulfonates proceeds by an initial rapid protonation of the aromatic ring followed by a proceeds by an initial rapid protonation of the aromatic ring followed by a rate-limiting desulfonation step to release a mole of sulfuric acid. The reaction depends significantly on the hydrogen ion concentration. Since the desulfonation releases sulfuric acid and decreases the pH of the system, an autocatalytic phenomenon can be expected. The kinetics of the desulfonation of disulfonated alkyl diphenyloxide and the effect of temperature and a wide range of pH on the desulfonation rate are discussed in this paper.
Experimental
The analyses of the two forms of disulfonated alkyl diphenyloxide, the acid form and the sodium salt form, are shown in Table 1. Both of these forms contain C 12 branched hydrophobes. The desulfonation experiments were carried out in tantalum vessels approximately 9 cm3 in volume. The vessels were purged first with nitrogen for 5 minutes. The surfactants were diluted to the appropriate concentration and were adjusted to the desired pH.
SPEJ
p. 993
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