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Abstract

In this paper experimental results are presented In an investigation of surfactant-oil-brine phase behavior and physical properties. The question of whether single-component surfactants are preferable to diverse mixtures is addressed both from the standpoint of oil and brine solubilization, as well as surfactant component fractionation. This investigation includes a detailed study of the phase behavior of a monoisomerically pure sulfonate, a pure hydrocarbon, water, and sodium chloride. Conclusions are reached regarding both the complexity of the global phase behavior of this system and the number of components necessary to describe this phase behavior.

The phase behavior for mixtures of monoisomerically pure surfactants is then compared to the single-component pure surfactants is then compared to the single-component results. The degree of sensitivity to the composition of these mixtures is shown to be very high. An hypothesis is presented, and supported by experimental data, in which presented, and supported by experimental data, in which the mechanism that controls surfactant component fractionation is proposed. This mechanism relates the importance of structure formation to surfactant fractionation. Results are presented for five component systems (oil, brine, and two surfactants) that illustrate the shape of the three-phase region in phase-space, and the movement and alteration of this region with changes in salinity.

Introduction

Recently a discussion took place concerning the most important unanswered questions regarding the Implementation of chemical EOR processes. Much debate centered around selection of optimized surfactant systems—specifically, the interrelationship of molecular weight distribution to surfactant effectiveness. Certain factions believe that a single species, if one could be identified and isolated, would be optimal for a given application. Other factions argue that a broader molecular weight distribution gives the surfactant system tolerance to changes in process variables (e.g., salinity, hardness, temperature, etc.). During the discussion of "tolerance", it became apparent that what some observers were calling tolerance, the presence of a three-phase region over a wider range of a single scanned variable, is not necessarily an indication of improved properties over a wider range. In fact, a less efficient surfactant could be mistakenly Interpreted as more tolerant.

In field flooding operations the properties actually desired are ultralow interfacial tensions over as wide a range of conditions as possible, low surfactant loss, coupled with the properties which cause a surfactant (mixture) to operate as if it were a single species—i.e., not chromatographically separate either by Interphase transport or adsorption driven mechanisms. All surfactants which would be feasible for commercial application would contain a mixture of different species. These mixtures span the gamut between broad equivalent weight distribution petroleum sulfonates and narrower molecular weight petroleum sulfonates and narrower molecular weight distribution surfactants, often called synthetics. Petroleum sulfonates often contain a mixture of Petroleum sulfonates often contain a mixture of nono- and polysulfonated species. By performing a series of extraction steps in which the excess aqueous phase was replaced with fresh brine, it was shown that the mono- and polysulfonate could be separated by Interphase transport (phase behavior) driven mechanisms. Most commercial nonionic, ethoxylated surfactants contain a Poisson, or nearly Poisson, distribution of ethylene oxide (EO) numbers. Koukounis et al. performed experiments similar to those described performed experiments similar to those described above, and showed that interphase transport mechanisms also tended to Separate nonionic surfactants. For nonionics, species with a low number of EO units tend to fractionate preferentially into the excess oil phase, changing the EO distribution in the phase, changing the EO distribution in the micro-emulsion phase. Such measurements have been performed on a variety of nonionics, and the results performed on a variety of nonionics, and the results have teen shown to be general. The various surfactant species present in any mixture also tend to adsorb onto the rock surface in different amounts. Thus, as any mixture of surfactant species passes through a porous medium, these species can travel at different velocities for at least two reasons. If the efficacy of a surfactant flooding process is related to the ratio of the surfactant species present, then the process efficiency can be altered by present, then the process efficiency can be altered by such chromatographic separation.

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