Optimum Formulation of Surfactant/Water/Oil Systems for Minimum Interfacial Tension or Phase Behavior

1979 ◽  
Vol 19 (02) ◽  
pp. 107-115 ◽  
Author(s):  
J.L. Salager ◽  
J.C. Morgan ◽  
R.S. Schechter ◽  
W.H. Wade ◽  
E. Vasquez
Keyword(s):  
Molecular Weight ◽  
Phase Behavior ◽  
Oil Recovery ◽  
Middle Phase ◽  
Interfacial Tensions ◽  
Three Phase ◽  
Optimum Formulation ◽  
The Subject ◽  
Tension State ◽  
Low Tension

Abstract A screening test used to help select surfactant systems potentially effective for oil recovery is to identify those formulations that yield middle-phase microemulsions when mixed with sufficient quantities of oil and brine. A correlation is presented to link these variables regarding their presented to link these variables regarding their contributions to middle-phase formation: structure of the sulfonated surfactant, alkane carbon number (ACN), and alcohol type and concentration. WOR and temperature effects are introduced as correction terms added to the empirical correlation.Sets of variables that give middle-phase microemulsions are shown as identical to those defining the low tension state without observable middle phases. This generally occurs for low surfactant phases. This generally occurs for low surfactant concentrations. Introduction Healy and Reed and Healy et al. have shown that the phase behavior of surfactant/brine/oil systems is a key factor in interpreting the performance of oil recovery by microemulsion performance of oil recovery by microemulsion processes. By systematically varying salinity, processes. By systematically varying salinity, they found low interfacial tensions and high solubilization of both oil and water in the microemulsion phase to occur in or near the salinity ranges giving phase to occur in or near the salinity ranges giving three phases. Since both low interfacial tensions and a high degree of solubilization are considered desirable for oil recovery, the conditions for three-phase formation assume added importance. Similar conclusions have been reported in other recent papers.Several investigators have considered the effect of different variables on the range of salinities for which three phases form. This optimum salinity (a more precise definition is given in a subsequent section) has been found to decrease with increasing surfactant molecular weight, and to increase with increasing chain length of the alcohol cosurfactant. Studies on the effect of alcohols by Jones and Dreher and Salter provided results similar to those reported by Hsieh and Shah.The interfacial tension at surfactant concentrations low enough so that a discernible third phase does not form has been the subject of considerable phase does not form has been the subject of considerable investigation regarding surfactant molecular weight and structure, oil ACN, salinity and surfactant concentration, and alcohol addition. A recent paper was a first attempt to tie together the low paper was a first attempt to tie together the low tension state observed at low surfactant concentrations and the three-phase region observed at higher surfactant concentrations. All indications point to an inextricable intertwining of phase point to an inextricable intertwining of phase behavior, surfactant partitioning, solubilization, and low tensions. This paper corroborates the equivalence of three-phase behavior and minimum tension as criteria for optimum formulation and presents a correlation that quantifies the trends presents a correlation that quantifies the trends observed previously. EXPERIMENTAL Aqueous phases containing surfactant, electrolyte (NaCl), and alcohol were contacted with an oil phase by shaking and allowed to stand until phase phase by shaking and allowed to stand until phase volumes became time independent for 2 days. All concentrations are expressed in grams of chemical per cubic centimeter of aqueous phase (g/cm3) per cubic centimeter of aqueous phase (g/cm3) before contacting with the hydrocarbon phase. Unless otherwise noted, the oil phase represents 20% of the initial total volume. All measurements, unless otherwise noted, were conducted at room temperature (25 plus or minus 1 degrees C). SPEJ p. 107

Interfacial Tension and Phase Behavior of Surfactant Systems

1978 ◽  
Vol 18 (04) ◽  
pp. 242-252 ◽  
Author(s):  
W.H. Wade ◽  
James C. Morgan ◽  
R.S. Schechter ◽  
J.K. Jacobson ◽  
J.L. Salager
Keyword(s):  
Molecular Weight ◽  
Interfacial Tension ◽  
Crude Oil ◽  
Phase Behavior ◽  
Oil Recovery ◽  
Surfactant Concentration ◽  
Low Molecular Weight ◽  
Middle Phase ◽  
Surfactant Systems ◽  
Low Tension

Abstract The conditions necessary for optimum low tension and phase behavior at high surfactant concentrations are compared with those required at low surfactant concentrations, where solubilization effects are not usually visible. Major differences in tension behavior between the high and low concentration systems may be observed when the surfactant used contains a broad spectrum of molecular species, or if a higher molecular weight alcohol is present, but not otherwise in the systems studied. We compared the effects of a number of aliphatic alcohols on tension with phase behavior. An explanation of these results, and also of other observed parameter dependences, is proposed in terms of changes in surfactant chemical potential. Surfactant partitioning data is presented that supports this concept. Introduction Taber and Melrose and Brandner established that tertiary oil recovery by an immiscible flooding process should be possible at low capillary process should be possible at low capillary numbers. In practice, the required capillary number, which is a measure of the ratio of viscous to capillary forces governing displacement of trapped oil, may be achieved by lowering the oil/water interfacial tension to about 10(-3) dyne/cm, or less. Subsequent research has identified a number of surfactants that give tensions of this order with crude oils and hydrocarbon equivalents. Interfacial tension studies tended to fall into two groups. Work at low surfactant concentrations, typically 0.7 to 2 g/L, has established that a crude oil may be assigned an equivalent alkane carbon number. Using pure alkanes instead of crude oil has helped the study of system parameters affecting low tension behavior. Important parameters examined include surfactant molecular structure, and electrolyte concentration, surfactant concentration, surfactant molecular weight, and temperature. At higher surfactant concentrations, interfacial tension has been linked to the phase behavior of equilibrated systems. When an aqueous phase containing surfactant (typically 30 g/L), electrolyte, and low molecular weight alcohol is equilibrated with a hydrocarbon, the surfactant may partition largely into the oil phase, into the aqueous phase, or it may be included in a third (middle) phase containing both water and hydrocarbon. Low interfacial tensions occur when the solubilization of the surfactant-free phase (or phases) into the surfactant-containing phase is maximized. Maximum solubilization and minimum tensions have been shown to be associated with the formation of a middle phase. Both the high and low surfactant concentration studies have practical importance because even though a chemical flood starts at high concentration, degradation of the injected surfactant slug will move the system toward lower concentrations. This study investigates the relationship between tension minima found with low concentration systems, and low tensions found with equivalent systems at higher surfactants concentrations, particularly those in which third-phase formation occurs. Many of the systems studied here contain a low molecular weight alcohol, as do most surfactant systems described in the literature or proposed for actual oil recovery. Alcohol originally was added to surfactant systems to help surfactant solubility, but can affect tensions obtained with alkanes, and with refined oil. Few systematic studies of the influence of alcohol on tension behavior exist. Puerto and Gale noted that increasing the alcohol Puerto and Gale noted that increasing the alcohol molecular weight decreases the optimum salinity for maximum solubilization and lowest tensions. The same conclusions were reached by Hsieh and Shah, who also noted that branched alcohols had higher optimum salinities than straight-chain alcohols of the same molecular weight. Jones and Dreher reported equivalent solubilization results with various straight- and branched-chain alcohols. In this study, we fix the salinity of each system and instead vary the molecule; weight of the hydrocarbon phase. SPEJ P. 242

Mixing Rules for Optimum Phase-Behavior Formulations of Surfactant/Oil/Water Systems

1979 ◽  
Vol 19 (05) ◽  
pp. 271-278 ◽  
Author(s):  
J.L. Salager ◽  
M. Bourrel ◽  
R.S. Schechter ◽  
W.H. Wade
Keyword(s):  
Phase Behavior ◽  
Oil Recovery ◽  
Practical Interest ◽  
Phase Region ◽  
Salinity Range ◽  
Mixing Rules ◽  
Recovery Efficiency ◽  
Middle Phase ◽  
Rich Phase ◽  
Three Phase

Abstract Many formulations used in surfactant flooding involve blends of surfactants designed to glue the best oil-recovery efficiency. Because oil-recovery efficiency usually is presumed to relate closely to surfactant/brine/oil phase behavior, it is of interest to understand the effect of mixing surfactants or of mixing oils on this phase behavior.We show that a correlation defining optimal behavior as a function of salinity, alcohol type and concentration, temperature, WOR (water/oil ratio), and oil type can be extended to mixtures of sulfonated surfactants and to those of sulfonates with sulfates and of sulfonates with alkanoates, provided the proper mixing rules are observed. provided the proper mixing rules are observed. The mixing rules apply to some mixtures of anionic and nonionic surfactants, but not to all. These mixtures exhibit some properties that may be of practical interest, such as increased salinity and practical interest, such as increased salinity and temperature tolerance. Introduction Recent studies have shown that formulation of the surfactant/brine/oil system is a key factor governing the performance of microemulsions designed to recover residual oil. These studies demonstrate that all optimal formulations exhibit characteristic properties that are remarkably similar. In general, properties that are remarkably similar. In general, the optimal microemulsion can solubilize large quantities of oil and connate water; in the presence of excess quantities of oil and water, a third surfactant-rich middle phase is formed. The interfacial tensions (IFT's) between the excess phases and the surfactant-rich phase are both low - about 10 dyne/cm (10 mN/m). Given an oil/brine system from a particular reservoir, one can achieve this formulation by varying the surfactant or the cosurfactant. Different oils, brines, or temperatures require formulations correspondingly altered to maintain optimal conditions. Previous studies have shown that the three-phase region exists over a range of values when one parameter, such as cosurfactant concentration, parameter, such as cosurfactant concentration, salinity, temperature, etc., is varied systematically (often called a scan). Thus, some ambiguity may exist with regard to the selection of those parameters representing the optimal formulation. Clearly, the optimum is that which recovers the most oil. However, tests are laborious, difficult to reproduce precisely, and sensitive to other factors, such as precisely, and sensitive to other factors, such as mobility, surfactant retention, wettability, etc. Therefore, it is desirable to impose an alternative definition that can be used for screening, though the final design still is dictated by core floods.Healy and Reeds have shown that the optimal formulation for oil recovery closely corresponds to that for which the IFT's between the excess oil and water phases and the surfactant-rich phase are equal. An almost equivalent criterion also was shown to be that point in the three-phase region for which the volume of oil solubilized into the middle phase equals the volume of brine. Furthermore, Salager et al. have used still another criterion that seems to be essentially equivalent to those used by Healy and Reed - an optimal salinity is defined as the midpoint of the salinity range for which the system exhibits three phases.These criteria are useful because they permit the screening of microemulsion systems using simple laboratory tests. SPEJ P. 271

Investigation of Three-Phase Regions Formed by Petroleum Sulfonate Systems

1981 ◽  
Vol 21 (05) ◽  
pp. 581-592 ◽  
Author(s):  
Creed E. Blevins ◽  
G. Paul Willhite ◽  
Michael J. Michnick
Keyword(s):  
Interfacial Tension ◽  
Phase Behavior ◽  
Oil Recovery ◽  
Phase Region ◽  
Phase Boundaries ◽  
Middle Phase ◽  
Optimal Salinity ◽  
Petroleum Sulfonate ◽  
Recovery Processes ◽  
Three Phase

Abstract The three-phase region of the Witco TRS 10-80 sulfonate/nonane/isopropanol (IPA)/2.7% brine system was investigated in detail. A method is described to locate phase boundaries on pseudoternary diagrams, which are slices of the tetrahedron used to display phase boundaries of the four-component system.The three-phase region is wedge-like in shape extending from near the hydrocarbon apex to a point near 20% alcohol on the brine/alcohol edge of the tetrahedron. It was found to be triangular in cross section on pseudoternary diagrams of constant brine content, with its base toward the nonane/brine/IPA face. The apex of the three-phase region is a curved line where the M, H + M, and M + W regions meet. On this line, the microemulsion (M*) is saturated with hydrocarbon, brine, and alcohol for a particular sulfonate content. A H + M region exists above the three-phase region, and an M + W region exists below it.Relationships were found between the alcohol concentration of the middle phase and the sulfonate/alcohol and sulfonate/hydrocarbon ratios in the middle phase. These correlations define the curve that represents the locus of saturated microemulsions in the quaternary phase diagram. Alcohol contents of excess oil and brine phases also were correlated with alcohol in the middle phase.Pseudoternary diagrams for sulfonates are presented to provide insight into the evolution of the three-phase region with salinity. Surfactants include Mahogany AA, Phillips 51918, Suntech V, and Stepan Petrostep(TM) 500. Differences between phase diagrams follow trends inferred from comparisons of equivalent weights, mono-/disulfonate content, optimal salinity, and EPACNUS values. Introduction The displacement of oil from a porous rock by microemulsions is a complex process. As the microemulsion flows through the rock, it mixes with and/or solubilizes oil and water. The composition of the microemulsion is altered by adsorption of sulfonate, leading to expulsion of water and/or oil. Multiphase regions are encountered where phases may flow at different velocities depending on the fluid/rock interactions. Knowledge of phase behavior of microemulsion systems is required to understand the displacement mechanisms, to model process performance, and to select suitable compositions for injection.Microemulsions used in oil recovery processes consist of five components: oil, water, salt, surfactant (usually a petroleum sulfonate and a cosurfactant (usually an alcohol). Brine frequently is considered to be a pseudocomponent. When this assumption is valid, a microemulsion may be studied as a four-component system.Windsor developed a qualitative explanation and classification scheme for microemulsion phase behavior. Healy and Reed showed that Windsor's concepts were applicable to microemulsions used in oil recovery processes. Healy et al. introduced the concept of optimal salinity to define a particular characteristic of surfactant system. The optimal salinity for phase behavior was defined as the salinity where the middle phase of a three-phase system has equal solubility of oil and brine. They also found that optimal salinity determined in this manner was close to the salinity where the interfacial tension between the upper and middle phases was equal to the interfacial tension between the middle and lower phases.Salager et al. developed a correlation of optimal salinity data for a particular surfactant. SPEJ P. 581^

Phase Behavior Studies of Two Model Surfactant Systems

1982 ◽  
Vol 22 (01) ◽  
pp. 53-60 ◽  
Author(s):  
William J. Benton ◽  
Natoli John ◽  
Syed Qutubuddin ◽  
Surajit Mukherjee ◽  
Clarence M. Miller
Keyword(s):  
Aqueous Solutions ◽  
Phase Boundary ◽  
Phase Behavior ◽  
Oil Recovery ◽  
Develop Model ◽  
Middle Phase ◽  
Lower Phase ◽  
Interfacial Tensions ◽  
Petroleum Sulfonate ◽  
Surfactant Systems

William J. Benton, Carnegie-Mellon U. John Natoli, Carnegie-Mellon U. Qutubuddin, Syed SPE, Carnegie-Mellon U. Mukherjee, Surajit, Carnegie-Mellon U. Miller, Clarence M., SPE, Carnegie-Mellon U. Fort Jr., Tomlinson, Carnegie-Mellon U. Abstract Phase behavior studies were carried out for two systems containing pure surfactants but exhibiting behavior similar to that of commercial petroleum sulfonates. One system contained the isomerically pure surfactant sodium-8-phenyl-n-hexadecyl-n-sulfonate (Texas 1). The other contained sodium dodecyl sulfate (SDS). Additional components used in both systems were various pure short-chain alcohols, NaCl brine and n-decane. Aqueous solutions containing surfactant, cosurfactant, and NaCl were studied over a wide range of compositions with polarizing and modulation contrast microscopy, as well as the polarized light screening technique. Viscosity measurements were conducted on selected scans of the Texas 1 system. Maxima and minima of the scans were correlated with textural changes observed with microscopy. The aqueous solutions were contacted with equal volumes of n-decane, and phase behavior and interfacial tensions were determined. The middle microemulsion phase was found to be oil continuous close to the upper phase boundary and water continuous close to the lower phase boundary. Both the Texas 1 and SDS systems showed similar behavior in that the middle microemulsion phase was observed over the entire range of surfactant concentrations studied. Introduction Surfactant systems usually consisting of petroleum sulfonate, an alcohol, salt, and water have been used for enhanced oil recovery. Various parameters important to oil recovery by surfactant flooding, such as interfacial tension and viscosity, are related strongly to the phase behavior of the microemulsion systems. The relationship of ultralow interfacial tensions to phase separation has been treated in our laboratory. The recovery of petroleum from laboratory cores and field tests appears to be related directly to phase behavior. It is important to understand phase behavior to identify the mechanisms involved and improve the efficiency of the oil-recovery process. The physicochemical aspects of the phase behavior of microemulsion systems containing commercial petroleum sulfonates as surfactants have been well documented by Healy and Reed and others. However, the systems studied were not pure, and the commercial surfactants sometimes contained as much as 40% inactive ingredients. There is a need to develop model microemulsion systems using pure components. Such systems would provide an experimental platform for verifying or interpreting the implications of any model for the phase behavior of multicomponent microemulsion systems and also allow the behavior of commercial systems to be predicted and understood. The objective of our work has been to fulfill these needs. Microemulsions have been classified as lower phase (l), upper phase (u), or middle phase (m) in equilibrium with excess oil, excess brine, or both excess oil and brine, respectively. Transitions among these phases have been studied as functions of salinity, alcohol concentration, temperature, etc. The middle-phase microemulsion is particularly significant because microemulsion/excess brine and microemulsion/excess oil tensions can be ultra low simultaneously. The concept of an optimal parameter as proposed originally by Reed and Healy when equal amounts of oil and brine are solubilized in the middle phase has been followed in this paper. We have shown earlier that the structure of petroleum sulfonate solutions exhibits a general pattern of variation with salinity. SPEJ P. 53^

Estimation of Optimal Salinity and Solubilization Parameters for Alkylorthoxylene Sulfonate Mixtures

1977 ◽  
Vol 17 (03) ◽  
pp. 193-200 ◽  
Author(s):  
M.C. Puerto ◽  
W.W. Gale
Keyword(s):  
Molecular Weight ◽  
Enhanced Oil Recovery ◽  
Mole Fraction ◽  
Oil Recovery ◽  
Carbon Number ◽  
Constant Weight ◽  
Optimal Salinity ◽  
Interfacial Tensions ◽  
Molecular Weight Distributions ◽  
Weight Distributions

Abstract Economic constraints are such that it is unlikely a pure surfactant will be used for major enhanced oil recovery projects. However, it is possible to manufacture at competitive prices classes of syntheic and natural petroleum sulfonates that have fairly narrow molecular-weight distributions. Under some reservoir conditions, one of these narrow-distribution sulfonates may serve quite well as the basic component of a surfactant flood, however, in many instances a mixture of two or more of these may be required. Since evaluation of a significant subset of "all possible combinations" is a formidable undertaking screening techniques must be established that can reduce the number of laboratory core floods required. It is well known that interfacial tension plays a dominant role in surfactant flooding. It has recently been shown that minimal interfacial tensions occur at optimal salinity, Cphi, where the solubilization parameters VO/Vs and Vw/Vs are equal. Additionally, it has been shown that interracial tensions are inversely proportional to the magnitude of the solubilization parameters. This paper demonstrates that optimal salinity and solubilization parameters for any mixture of orthoxylene sulfonates can be estimated by summation of mole-fraction-weighted component properties. Those properties, which could not be properties. Those properties, which could not be measured directly, were obtained by least-squares regression on mixture data. Moreover, for surfactants of known carbon number distributions, equations that are linear in mole fractions of components and logarithmic in alkyl carbon number were found to be excellent estimators of both Cphi and solubilization parameters evaluated at Cphi. parameters evaluated at Cphi. Optimal salinity and associated solubilization parameters were measured using constant weight parameters were measured using constant weight fractions of alcohol cosolvents and mixtures of seven products with narrow molecular weight distributions. The average alkyl carbon number of these products varied from about 8 to 19. Alkyl chain lengths of individual surfactant chemical species ranged from 6 to 24 carbon atoms. Introduction Optimal salinity and the amounts of oil and water contained in a microemulsion have been shown to play important roles in obtaining low interfacial tensions and high oil recoveries. Since economics of enhanced oil recovery projects demand use of inexpensive surfactants, broad-distribution products likely will be chosen. Knowledge of how to estimate optimal salinity and oil-water contents of microemulsions prepared from such products would reduce time involved in laboratory screening procedures. This paper presents a method for procedures. This paper presents a method for obtaining such estimates that should prove useful for all types of surfactant mixtures that involve homologous series. The basic concept used is that a given property of a mixture of components (Yi) is related to the sum of products of mole fraction of components in the mixture (Xij) and the "mixing value" of the property in question for that component (Y'j). In property in question for that component (Y'j). In other words, (1) This approach is similar, for example, to the pseudocritical method used by Kay to calculate pseudocritical method used by Kay to calculate gas deviation factors at high pressures. The properties of interest in this paper are optimal properties of interest in this paper are optimal salinity and solubilization parameters, Vo/Vs, and Vw/Vs, at optimal salinity. Two separate approaches were developed that depended on the degree of detail of the available surfactant-composition data. In the first approach, only average molecular weights of several surfactant products were assumed known. Optimal salinity and products were assumed known. Optimal salinity and solubilization parameters could be measured for some, but not all, of the products. Regression on mixture data was used to estimate these quantities for the remainder of the products. Those properties, either measured experimentally or estimated from mixture data, are referred to as surfactant product contributions since they can be used as mixing values of the property in question in Eq. 1 or Eq. 2. SPEJ P. 193

Development of the Surfactant-Based Chemical EOR Formula for Low Permeability Reservoirs

2021 ◽  
Author(s):  
Nancy Chun Zhou ◽  
Meng Lu ◽  
Fuchen Liu ◽  
Wenhong Li ◽  
Jianshen Li ◽  
...  
Keyword(s):  
Phase Behavior ◽  
Oil Recovery ◽  
Aqueous Solubility ◽  
Low Permeability ◽  
Chemical Flooding ◽  
Residual Oil ◽  
Key Factors ◽  
Low Salinity ◽  
Low Tension ◽  
Low Permeability Reservoirs

Abstract Based on the results of the foam flooding for our low permeability reservoirs, we have explored the possibility of using low interfacial tension (IFT) surfactants to improve oil recovery. The objective of this work is to develop a robust low-tension surfactant formula through lab experiments to investigate several key factors for surfactant-based chemical flooding. Microemulsion phase behavior and aqueous solubility experiments at reservoir temperature were performed to develop the surfactant formula. After reviewing surfactant processes in literature and evaluating over 200 formulas using commercially available surfactants, we found that we may have long ignored the challenges of achieving aqueous stability and optimal microemulsion phase behavior for surfactant formulations in low salinity environments. A surfactant formula with a low IFT does not always result in a good microemulsion phase behavior. Therefore, a novel synergistic blend with two surfactants in the formulation was developed with a cost-effective nonionic surfactant. The formula exhibits an increased aqueous solubility, a lower optimum salinity, and an ultra-low IFT in the range of 10-4 mN/m. There were challenges of using a spinning drop tensiometer to measure the IFT of the black crude oil and the injection water at reservoir conditions. We managed the process and studied the IFTs of formulas with good Winsor type III phase behavior results. Several microemulsion phase behavior test methods were investigated, and a practical and rapid test method is proposed to be used in the field under operational conditions. Reservoir core flooding experiments including SP (surfactant-polymer) and LTG (low-tension-gas) were conducted to evaluate the oil recovery. SP flooding with a selected polymer for mobility control and a co-solvent recovered 76% of the waterflood residual oil. Furthermore, 98% residual crude oil recovery was achieved by LTG flooding through using an additional foaming agent and nitrogen. These results demonstrate a favorable mobilization and displacement of the residual oil for low permeability reservoirs. In summary, microemulsion phase behavior and aqueous solubility tests were used to develop coreflood formulations for low salinity, low temperature conditions. The formulation achieved significant oil recovery for both SP flooding and LTG flooding. Key factors for the low-tension surfactant-based chemical flooding are good microemulsion phase behavior, a reasonably aqueous stability, and a decent low IFT.

Evaluation of the Interfacial Tension Between a Microemulsion and the Excess Dispersed Phase

1981 ◽  
Vol 21 (05) ◽  
pp. 593-602 ◽  
Author(s):  
E. Ruckenstein
Keyword(s):  
Interfacial Tension ◽  
Oil Recovery ◽  
Phase Inversion ◽  
The Other ◽  
Dispersed Phase ◽  
Middle Phase ◽  
Inversion Point ◽  
Interfacial Tensions ◽  
Dispersed Medium ◽  
Oil Water

Abstract From a consideration of the thermodynamic stability of microemulsions, one can establish a relation between the interfacial tension y at the surface of the globules and the derivative, with respect to their radius re, of the entropy of dispersion of the globules in the continuous medium. Expressions for the entropy of dispersion are used to show that gamma is approximately proportional to kT/r2e, where k is Boltzmann's constant and T is the absolute temperature. Since the environment of the interface between the microemulsion and the excess dispersed medium is expected to be similar to that at the surface of the globules, these expressions are used to evaluate the interfacial tension between microemulsion and excess dispersed medium. Values between 10 and 10 dyne/cm that decrease with increasing radii are obtained, in agreement with the range found experimentally by various authors. The origin of the very small interfacial tensions rests ultimately in the adsorption of surfactant and cosurfactant on the interface between phases. The effect on the interfacial tension of fluctuations from one type of microemulsion to the other, which may occur near the phase inversion point, is discussed. Introduction The system composed of oil, water, surfactant, cosurfactant, and salt exhibits interesting phase equilibria. For sufficiently large concentrations of surfactant, a single phase can be formed either as a microemulsion or as a liquid crystal. In contrast, at moderate surfactant concentrations, two or three phases can coexist. For moderate amounts of salt (NaCl), an oil phase is in equilibrium with a water-continuous microemulsion, whereas for high salinity, an oil-continuous microemulsion coexists with a water phase. At intermediate salinity, a middle phase (probably a microemulsion) composed of oil, water, surfactants, and salt forms between excess water and oil phases. Extremely low interfacial tensions are found between the different phases, with the lowest occurring in the three-phase region. These systems have attracted attention because of their possible application to tertiary oil recovery. It has been shown that the displacement of oil is most effective at very low interfacial tensions.Microemulsions have been investigated with various experimental techniques, such as low-angle X-ray diffraction, light scattering, ultracentrifugation, electron microscopy, and viscosity measurements. These have shown that the dispersed phase consists of spherical droplets almost uniform in size. While it is reasonable to assume that the microemulsions coexisting with excess oil or water contain spherical globules of the dispersed medium, the structure of the middle-phase microemulsion is more complex. Experimental evidence obtained by means of ultracentrifugation indicates, however, that at the lower end of salinity the middle phase contains globules of oil in water, while at the higher end the middle phase is oil continuous. A phase inversion must occur, at an intermediate salinity, from a water-continuous to an oil-continuous microemulsion. The free energies of the two kinds of microemulsions are equal at the inversion point. Since their free energy of formation from the individual components is very small, small fluctuations, either of thermal origin or due to external perturbations, may produce changes from one type to the other in the vicinity of the inversion point. As a consequence, near this point, it is possible that the middle phase is composed of a constantly changing mosaic of regions of both kinds of microemulsions. SPEJ P. 593^

scholarly journals Ionic liquids for low-tension oil recovery processes: Phase behavior tests

2017 ◽  
Vol 504 ◽  
pp. 404-416 ◽  
Author(s):  
Iria Rodriguez-Escontrela ◽  
Maura C. Puerto ◽  
Clarence A. Miller ◽  
Ana Soto
Keyword(s):  
Ionic Liquids ◽  
Phase Behavior ◽  
Oil Recovery ◽  
Recovery Processes ◽  
Low Tension
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