**Abstract**

The phase behavior of microemulsions formed in a surfactant-brine-oil system for a chemical Enhanced Oil Recovery (EOR) application is complex and depends on a range of parameters. Phase behavior indicates a surfactant solubilization. Phase behavior tests are simple but time-consuming especially when it involves a wide range of surfactant choices at various concentrations. An efficient and insightful microemulsion formulation via computational simulation can complement phase behavior laboratory test. Computational simulation can predict various surfactant properties, including microemulsion phase behavior. Microemulsion phase behavior can be predicted predominantly using Quantitative Structure-Property Relationship (QSPR) model. QSPR models are empirical and limited to simple pure oil system. Its application domain is limited due to the model cannot be extrapolated beyond reference condition. Meanwhile, there are theoretical models based on physical chemistry of microemulsion that can predict microemulsion phase behavior. These models use microemulsion surface tension and torque concepts as well as with solution of bending rigidity of microemulsion interface with relation to surface solubilization and interface energy.

**Keywords:** surfactant, microemulsion, phase behavior, solubilization, chemical enhanced oil recovery, computational chemistry

### **1. Introduction**

With growing global energy demand and depleting reserves, enhanced oil recovery (EOR) has become more important. Among various EOR processes, chemical EOR has been labeled an expensive process, with overwhelming parameters needed to describe a chemical EOR process and not practical to measure every one of them [1]. The chemical formulations for chemical EOR process consist of single or a combination of alkaline, surfactant and polymer. The traditional chemical EOR processes are polymer flooding, surfactant and alkaline flooding. Over the years, different modes of chemical flood injections were devised. There are the binary mix of alkali–surfactant (AS), surfactant-polymer (SP), alkaline-polymer (AP), and alkaline-surfactant-polymer (ASP) slug [2].

The research and development effort to design a robust chemical EOR formulation tailored to a specific field is challenging and laborious. To design a successful surfactant related chemical EOR formulation, Pope [3] highlighted 8 surfactant

selection criteria. The surfactant must produce high solubilization ratio [i.e., low interfacial tension (IFT)] at optimum condition, commercialize at low cost, be feasible to be tailored to specific crude oil, temperature and salinity, comprise of highly branched hydrophobe with low adsorption onto reservoir rock, be insensitive to surfactant concentration above critical micelle concentration (CMC) and have minimal inclination to form liquid crystals, gels, macroemulsions and show rapid coalescence to microemulsion.

Surfactant solubilization can be determined via phase behavior laboratory test. The phase behavior of microemulsions formed in a surfactant-brine-oil system is complex. Phase behavior for a particular microemulsion system has been known to be measured experimentally [4]. The study of surfactant phase behavior consists of the determination of the number, composition, and structure of phases formed by surfactant systems at a given set of conditions (pressure, temperature, and system composition), in observance of Gibbs phase rule [5].

The relationship between solubilization and IFT is described commonly by Healy & Reed [6] correlations and the Chun-Huh [7] equation. Nevertheless, the latter equation is more commonly used. The term "solubilization" was introduced by McBain [8] to describe the increased solubility of a compound associated with the formation of micelles or inverted micelles. The mechanism to enhance solubility varies depending on the surfactant structure, the solvent type and the nature of the solubilized compound. The oil and water solubilization parameters, *SPo* and *SPw* respectively, are expressed in *SPo* = *Vo*/*Vs* and *SPw* = *Vw*/*Vs*, where *Vo*, *Vw*, and *Vs* are the volumes of oil, water and surfactant contained within the micellar phase [9]. The solubilization ratio of water and oil phase is measured as either the volume of solubilized water (Vw) or oil (*Vo*) over volume of surfactant (*Vs*) in the microemulsion phase. The solubilization ratio of oil (*Vo*/*Vs*) increases with the increase in salinity, while the solubilization ratio of water (*Vw*/*Vs*) decreases with the increase in salinity [10]. The region in or near where the solubilization of oil and water intersects versus salinity is the optimal salinity.

#### **2. Microemulsion and Winsor phase behavior**

Microemulsions are dispersions of oil and water stabilized by surfactant molecules [11–13]. They can take on many structures such as water droplets in oil, oil droplets in water, sponge like, bicontinuous structures, and lamellar phase [14]. Unlike emulsions, they are thermodynamically stable. This because of the oil-water IFT is low enough (below 10˗<sup>2</sup> nM/M) to compensate the dispersion entropy. The interfacial energy is balanced by the dispersion entropy when the dispersion sizes are small enough, which is below 100 Å [15]. Emulsion has much larger dispersion sizes at approximately 1 μm. The different in size between microemulsion and emulsion explains their difference in properties and appearance; however, their fundamental difference is thermodynamic stability [9].

For a given set of conditions (temperature, composition), microemulsion displays well defined structures. The physical properties of microemulsion often undergo an abrupt change over a narrow concentration range [16]. It is generally accepted that this rapid change in the property over the range of concentration is due to the formation of surfactant aggregates or micelle in solution [9]. The nucleation of micelles is spontaneous, forming a structure that can vary between spherical and cylindrical depending on the surfactant molecular structure, solution composition and temperature. **Figure 1** depicted the intermicellar equilibrium and the associated phase changes. Micellar structure, S1 is formed when the hydrophilic group of a surfactant are in contact with water while the hydrophobic group are

*Experimental and Computational Modeling of Microemulsion Phase Behavior DOI: http://dx.doi.org/10.5772/intechopen.101482*

**Figure 1.** *Intermicellar equilibrium and associated phase changes [9].*

gathered within the interiors of micelles to create small regions from which water is essentially excluded. When aggregates of surfactant form in apolar solvent, it is called inverted micelles, S2. Inverted micelles promote the solubility of water in apolar solvents. Micellar aggregates have a finite mean lifetime, where their structures are mobile, the interfaces are flexible with a rapid exchange of molecules between the neighboring region and the aggregate [9]. Therefore, both S1 and S2 do form isotropic solutions with a bicontinuous and fluctuating structure in a given optimal condition. The microstructure of the sequence of states progressing from S1 to S2 is difficult to study and hence, these systems were large ignored until recent years, when it has been recognized that the isotopic solutions between S1 and S2 are agents which can significantly enhance oil recovery [9].

In less frequent cases, the microemulsion structure is made of elongated cylinders, eventually interconnected, or of distorted lamellar (sponge like), as depicted as M1 and M2 in **Figure 1**. These structures are encountered when the spontaneous curvature of the surfactant layer is small and approaches zero. On the contrary, sponge like structure can be significantly swollen by both oil and water. The G phase in **Figure 1** is liquid crystal with lamellar structure [15].

A microemulsion can exist in three types of systems. Winsor's introduced three types of simple phase diagram that are characterized by the nature of the polyphasic zone at low and moderate concentration. They are Winsor I (WI), Winsor II (WII) and Winsor III (WIII) [17]. Winsor I and II are also commonly known as II˗ and II+. Below a certain salinity, *Cseu*, the system is WI. Above a certain salinity, *Csel*, the system is WII. If the salinity is between *Cseu* and *Csel*, the system is WIII. In a WIII system, the IFT is lower than WI and WII [4]. WI and WII diagrams shows a characteristic of 2-phase behavior with water or oil microemulsion in equilibrium with the excess phase. WIII diagram shows a bicontinuous microemulsion in equilibrium with both the excess water and oil phases. WIII is considered to have the best probability of recovering additional oil. WII is considered to have the secondbest chance to recover additional oil because it shows interaction between the aqueous phase and crude oil. Even though WI demonstrates interaction between the crude oil and the aqueous phase, it is considered to have poorer oil recovery potential than WII.

Winsor's phase behavior studies in the early 1950s introduced the R ratio of interactions of the adsorbed surfactant at the interface with the neighboring oil and water molecules as a criterion to take into account along with the effects of all formulation variables found in a ternary surfactant-oil-water (SOW) system, i.e., the surfactant head and tail characteristics, the nature of the oil, the aqueous phase salinity, as well as temperature and pressure [18].

In micellar solutions, 3 distinct regions can be identified: an aqueous region, W, an oil or organic region, *O*, and a surfactant region, *C*. The variation of the dispersing tendencies at the *O* and *W* faces of the *C* region is expressed qualitatively by Winsor [17] as:

$$R = \frac{(A\_{CO} - A\_{OO})}{(A\_{CW} - A\_{WW})} \tag{1}$$

where *ACO* and *ACW* are the interaction of surfactant molecules per unit area at the interface with oil and water respectively, *AOO* is the interaction between two oil molecules, and *AWW* is the interaction between two water molecules. Winsor described that an optimal microemulsion (Winsor III) is formed when the microstructure surface is flat, i.e., *R* = 1. When *R* < 1, there is a tendency to form oil-inwater emulsion (Winsor II or II+), whereas when *R* > 1, the tendency is to form water-in-oil emulsion (Winsor I or II˗). Salager [18] presented a diagram (**Figure 2**) to link the Winsor R ratio with the observe phase type. *R* < 1, *R* = 1 and *R* > 1 correspond to WI (II˗), WIII, and WII (II+) diagrams, respectively. This shows that

#### **Figure 2.**

*Ternary phase diagram, test tube phase behavior and R ratio variations along a 1-dimensional formulation scan [18].*

#### *Experimental and Computational Modeling of Microemulsion Phase Behavior DOI: http://dx.doi.org/10.5772/intechopen.101482*

any formulation change that alters one of the interactions indicated in the ratio can increase or decrease *R*. When the formulation variation is properly selected to change *R* from *R* < 1 to *R* > 1 or vice versa, it changes the phase behavior from WI to WII or vice versa, with an intermediate WIII three-phase behavior at *R* = 1 [18].
