Static and Dynamic Adsorption of Anionic and Nonionic Surfactants

Abstract

Abstract The adsorption of commercial polyoxyethylene nonyl phenols and alkyl benzene sulfonates was studied by measuring the surfactant breakthrough from Berea cores. A rate model that reduces to a Langmuir-type isotherm at equilibrium represented these dynamic results and predicted successfully the equilibrium isotherms determined by static experiments.The ratios of both adsorption and desorption were determined and were observed to increase with the number of ethylene oxide groups. Adsorption of the nonionic surfactant appeared to be by hydrogen bonding and the amount adsorbed per unit of area was the same on a number of metal oxide substrates.Negligible adsorption was observed for sulfonates with an alkyl chain length of 9 or less. Introduction Surfactant adsorption is one of the important features governing the economic viability of chemical flooding processes. However, the adsorption on mineral oxide surfaces is only one of several possible mechanisms leading to surfactant losses.Other mechanisms include precipitation of surfactant in the presence of divalent ions, diffusion of surfactant into dead-end pores, and surfactant partitioning into the oil phase. It is necessary partitioning into the oil phase. It is necessary to minimize the losses by all mechanisms. The work reported here addresses the problem of surfactant adsorption; other mechanisms are not considered.There are a number of approaches that have the potential for minimizing adsorption. The most potential for minimizing adsorption. The most desirable surfactant is one that does not adsorb at all; however, such surfactants may not be effective oil-recovery agents. Sacrificial agents that adsorb in place of the surfactant can be used in a preflush or as a competitive additive to the surfactant slug, but effective agents have not yet been identified.Two aspects of the adsorption process are of interest the rate and the amount adsorbed. Both are examined here. The measurements include the dynamic adsorption of both anionic and nonionic surfactants in Berea cores that are initially filled with brine. The breakthrough curves are represented successfully using a model that accounts for the surface coverage. The rate expression reduces to a Langmuir-type isotherm. The shape of this curve has been verified by conducting static experiments.The study included both nonionic and anionic surfactants. These were not pure surfactants but, in general, they are well characterized. The anionic surfactants were studied because their behavior should-be representative of more complex mixtures such as the petroleum sulfonates that have been regarded as prime candidates for oil-recovery agents. These sulfonates are sensitive to divalent ions and many chemical slugs include quantities of nonionic surfactants to alleviate this difficulty to some extent. Therefore, this study included a systematic study of a particular class of nonionic surfactants. This study is the first to report rates of adsorption and desorption. From this information, the nature of the adsorption can be better understood. THEORY Michaels and Morelos have established that the adsorption of polyanions on kaolin occurs by hydrogen bonding. The specific sites at which this adsorption takes place were not defined. For the adsorption of surfactants, this mechanism can be represented as follows: ....................... (1) SPEJ p. 337