Surfactant formulations for enhanced oil recovery

During the past decade, it has been reported that many surfactant formulations for enhanced oil recovery generally form multiphase microemulsions (18-20). From these studies, it is evident... 

Solubllization. The effectiveness of surfactant formulations for enhanced oil recovery depends on the magnitude of solubilization. 

Phase Behavior. The surfactant formulations for enhanced oil recovery consist of surfactant, alcohol and brine with or without added oil. As the alcohol and surfactant are added to equal volumes of oil and brine, the surfactant partitioning between oil and brine phases depends on the relative solubilities of the surfactant in each phase. If most of the surfactant remains in the brine phase, the system becomes two phases, and the aqueous phase consists of micelles or oil-in-water microemulsions depending upon the amount of oil solubilized. If most of the surfactant remains in the oil phase, a two-phase system is formed with reversed micelles or the water-in-oil microemulsion in equilibrium with an aqueous phase. 

The phase behavior of surfactant formulations for enhanced oil recovery is also affected by the oil solubilization capacity of the mixed micelles of surfactant and alcohol. For low-surfactant systems, the surfactant concentration in oil phase changes considerably near the phase inversion point. The experimental value of partition coefficient is near unity at the phase inversion point (28). The phase inversion also occurs at the partition coefficient near unity in the high-surfactant concentration systems (31). Similar results were also reported by previous investigators (43) for pure alkyl benzene sulfonate systems. 

The effectiveness of surfactant formulations for enhanced oil recovery depends on the magnitude of solubilization. By injecting a chemical slug of complete miscibility with both oil and brine present in the reservoir, 100% recovery of oil should be possible. 

The phase behavior of surfactant formulations for enhanced oil recovery is also affected by the oil solubilization capacity of the mixed micelles of surfactant and alcohol. For low concentration surfactant systems, the surfactant concentration in the oil phase changes considerably near the phase inversion point. 

We have recently reported (6, 7) that those surfactant formulations which yield good oil recovery exhibit both low interfacial tensions and low interfacial viscosities. Our experiments have shown that surfactant formulations which ensure low interfacial viscosity will promote the coalescence of oil droplets and thereby decrease the emulsion stability, thus enhancing the formation of a continuous oil bank. It has been demonstrated that the requirements for emulsion stability are the presence of an interfacial film of high viscosity and a film of considerable thickness. We have observed that the surfactant concentration which minimizes the interfacial tension may not simultaneously minimize the interfacial viscosity. Hence, our results indicate both interfacial tension and interfacial rheology must be considered in selecting surfactant formulations for tertiary oil recovery. 

Optimizing the formulation of micellar surfactant solutions used for enhanced oil recovery consists of obtaining interfacial tensions as low as possible in multiphase systems, which can be achieved by mixing the injected solution with formation fluids. The solubilization of hydrocarbons by the micellar phases of such systems is linked directly to the interfacial efficiency of surfactants. Numerous research projects have shown that the amount of hydrocarbons solubilized by the surfactant is generally as great as the interfacial tension between the micellar phase and the hydrocarbons. The solubilization of crude oils depends strongly on their chemical composition [155]. 

In order to vary interfacial tension over more than four orders of magnitude, several fluid systems were chosen that ranged from high tension surfactant-free formulations to middle phase microemulsions that were at optimal conditions for enhanced oil recovery and had ultralow tensions with the excess brine and oil. Table I lists the specific components used along with their corresponding physical properties. In each case a red water-soluble food coloring dye was added before equilibration to enhance the contrast between phases during microscopy. 

Salinity Tolerance. As the petroleum reservoir salinity can be very high, the surfactant formulations should be designed for high salt tolerance. The widely used petroleum sulfonates for enhanced oil recovery exhibit relatively low salt tolerance in the range 2-2.5% NaCl concentration, and even smaller for the optimal salinity. The presence of divalent cations in the brine decreases the optimal salinity of surfactant formulations (44). 

Microemulsion research has since its inception been stimulated by the great potential for practical applications. In particular, considerable research interest has been invested in the possibility of using microemulsions for enhanced oil recovery (EOR). It was observed that surfactant formulations forming three-phase microemulsion systems, often termed Winsor III systems [29], in the oil well could increase the oil yield considerably. Important contributions to the understanding of the mechanisms involved were made by Shah and Hamlin [30] and the Austin group led by Schechter and Wade (see Bourrel et al. [31]). 

The formulation scan and dual compensating scan techniques were carried out on an experimental basis first with anionic surfactant s) tems such as alkyl aryl sulfonates, which were the primary candidates for enhanced oil recovery because of their low price and availability. 

Polymer-surfactant systems have found wide-spread practical applications, e.g., in paints, in pharmaceutical formulations, and in systems for enhanced oil recovery. Their practical importance and the fundamental intricacies in these systems have triggei extensive studies of the interactions between polymers and surfactants, and several reviews have appeared [1-6]. The sodium dodecylsulfate (SDS)-poly(ethylene oxide) (PEO) system has been particularily well studied, both by classical [7,8] and modem methods [9-12]. 

In general, the surfactant formulations used for enhanced oil recovery contain a short chain alcohol. The addition of alcohol can influence the viscosity, IFT and birefringent structures of micellar solutions as well as coalescence rate of oil ganglia. The present paper reports the effect of addition of isobutanol to a dilute petroleum sulfonate (< 0.1% cone) solution on IFT, surface shear viscosity, surfactant partitioning, the rate of change of IFT (or flattening time) of oil drops in surfactant solutions and oil displacement efficiency. The two surfactant systems chosen for this study indeed exhibited ultralow IFT under appropriate conditions of salinity, surfactant concentration and oil chain length (11,15,19). 

For enhanced oil recovery (EOR) and environmental remediation, an important property of any surfactant is its critical microemulsion concentration, or CfiC, because this is the minimum surfactant concentration needed to achieve ultralow interfacial tensions (<0.1 mN/m) [37, 38]. Recently, it has been determined that for rhamnolipids with a CMC of 10 mg/1, the C/rC is close to 100 mg/1 [39], which is approximately 10 times lower than the C/rC of anionic surfactants [38]. The study of the C/rC for rhamnolipids and other biosurfactants obtained from waste sources has not been performed yet. The work of Nguyen et al. [39] on the formulation of microemulsions with rhamnolipids also suggests that they are relatively hydrophilic (i.e. tend to form micelles but not reverse micelles), and that it is best to use them in combination with other surfactants. 

Ionic surfactants are sensitive to water hardness whereas polyethoxylated surfactant are not. Hence the mixing of both types often result in formulations that are salt tolerant for applications such as detergency or enhanced oil recovery. 

Surfactants are widely used for a variety of reasons, including surface wetting agents, detergents, emulsifiers, lubricants, gasoline additives, and enhanced oil-recovery agents. The type of surfactants selected for a particular application often depends on the chemical and physical properties required and on economics or other considerations such as environmental concerns. To meet these requirements, a typical surfactant formulation may contain blends of a variety of commercial products, which could include ionic and nonionic ethoxylated surfactants, alkylsulfonates, and alkylaryl-sulfonates, and petroleum sulfonates. 

In this section, several important aspects of microemulsions in relation to enhanced oil recovery will be discussed. It is well recognized that the success of the microemulsion flooding process for improving oil recovery depends on the proper selection of chemicals in formulating the surfactant slug. 

Within the last 30 years, micro emulsions have also become increasingly significant in industry. Besides their application in the enhanced oil recovery (see Section 10.2 in Chapter 10), they are used in cosmetics and pharmaceuticals (see Chapter 8), washing processes (see Section 10.3 in Chapter 10), chemical reactions (nano-particle synthesis (see Chapter 6)), polymerisations (see Chapter 7) and catalytic reactions (see Chapter 5). In practical applications, micro emulsions are usually multicomponent mixtures for which formulation rules had to be found (see Chapter 3). Salt solutions and other polar solvents or monomers can be used as hydrophilic component. The hydrophobic component, usually referred to as oil, may be an alkane, a triglyceride, a supercritical fluid, a monomer or a mixture thereof. Industrially used amphiphiles include soaps as well as medium-chained alcohols and amphiphilic polymers, respectively, which serve as co-surfactant. 

In this chapter, we will focus on the formulation of systems in which the surfactant has equal affinity for both O and W phases. These formulations do form bicontinuous microemulsions of zero mean curvature and have important properties such as minimum interfacial tension and maximum solubilisation. Such condition has been called optimum formulation in the 1970s, because it matches the attainment of an ultralow interfacial tension that guarantees an enhanced oil recovery from petroleum reservoirs, which was the driving force behind the research effort on microemulsions (see Chapter 10, Section 10.3 of this book) [3,4]. High solubilisation performance micro emulsions which are able to cosolubilise approximately equal amount of oil and water with less than 15-20% surfactant are attainable only at an optimum formulation. 

Mixtures of anionic and nonionic surfactants were proposed to provide temperature-insensitive systems [37], a suggestion that has considerable practical interest not only for microemulsion systems but also in emulsion polymerization and enhanced oil recovery It was recently shown that since both the anionic and nonionic surfactants can be selected, this double degree of freedom can be used to attain both temperature insensitivity and mixture composition insensitivity so that the formulation is a particularly robust one as far as the applications are concerned [39]. 

Abstract Amphiphilic block copolymers (BCPs) are used in a steadily growing number of applications and formulations such as cosmetics, detergents, coatings, and enhanced oil recovery. In most of these applications, BCPs are used in complex mixtures with normal surfactants to control the solution properties of the mentioned systems. In addition, these systems are used as templates for nanoparticle and mesoporous silica synthesis. Hence, a deeper understanding of the self-assembly processes and the formed structures is desirable to achieve a better control of the properties of the obtained inorganic materials. This article reviews the recent literature describing physicochemical aspects of the BCP/surfactant mixtures and attempts to identify some general features of the behavior of these systems. 

Scaling theory can be used to design microemulsions for important applications such as enhanced oil recovery. The salinities of brines in oil reservoirs range from potable to saturated at elevated temperature and pressure. When a reservoir of high salinity has been flooded previously with fresh water, the brine salinity can also vary greatly within the reservoir. To find a suitable surfactant requires a laborious search for a formulation with the correct optimal salinity for each reservoir. Thus (see Fig. 16.8), there has been a desire to find a surfactant that would form three phases over the broadest range of salinities and have ultralow interfacial tensions for those phases at the same time. However, there are thermodynamic limits on the extent to which these goals can be simultaneously met. 

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