On Interfacial Tensions Measured With Alkyl Aryl Sulfonate Surfactants

1979 ◽  
Vol 19 (02) ◽  
pp. 71-82 ◽  
Author(s):  
J.E. Puig ◽  
E.I. Franses ◽  
H.T. Davis ◽  
W.G. Miller ◽  
L.E. Scriven
Keyword(s):  
Phase Behavior ◽  
Drop Size ◽  
Physicochemical Study ◽  
Alkyl Aryl Sulfonate ◽  
Alkyl Aryl ◽  
Interfacial Tensions ◽  
Petroleum Sulfonate ◽  
Mixing Effects ◽  
Ultralow Tension ◽  
Low Tension

Abstract Tensions of aqueous solutions and dispersions of sodium p (1-heptylnonyl/benzene sulfonate against decane are not ultralow in the absence of salt, but can fall below 0.01 dyne/cm (0.01 mN/m) with 0.3 wt% (3 g/kg) NaCl, depending on order of mixing, preparation age, and decane drop size. A dispersed preparation age, and decane drop size. A dispersed liquid crystalline phase revealed by spectroturbidimetry and microscopy appears responsible for ultralow tensions. Results with petroleum sulfonate Witco TRS 10-80 support this conclusion. Introduction The importance of ultralow [less than 0.01 dyne/ cm (0.01 mN/m)] interfacial tensions between oil and water as the means of achieving high capillary-number displacement in one mechanism of enhancing oil recovery is well recognized. Low tension is related closely to the phase behavior of suitable surfactant with oil, water, and often an alcohol, as well as other additives. Information in the technical and patent literature concentrates on commercial sulfonate surfactant systems. Few studies of the relationship between tension and phase behavior with pure surfactants have appeared. Since the pioneering work of Hartley on molecular structure dependence of interfacial tension, the influences of WOR, salinity, temperature, surfactant molecular-weight distribution, and other factors have been investigated. However, the origins of ultralow interfacial tensions are not yet known.This paper deals with the low-tension regime associated with low surfactant concentrations, i.e., no more than 1 wt% (10 g/kg). In contrast to the second regime, in which microemulsion plays the central role, this regime does not require the presence of a suitable alcohol or other cosurfactant. presence of a suitable alcohol or other cosurfactant. In this regime it is not clear whether ultralow tensions are equilibrium or nonequilibrium properties. Even with compositions capable of producing ultralow tensions, the measured tensions depend on the way the system is prepared, and time effects are seen. Nevertheless, reproducible measurements can be made by observing a fixed experimental protocol. Tensions measured this way with a given surfactant correlate strongly with semiempirical assignment of carbon numbers to a wide range of hydrocarbons. The correlation extends across related sulfonate surfactants and is a potentially useful means of characterizing low-tension formulations. Understanding this correlation may shed light on the mechanism of ultralow tension in the low-surfactant concentration regime and thus may aid process studies. Understanding the factors responsible for nonequilibrium effects is crucial to interpret laboratory and field data and to design surfactant flooding processes rationally. Commercial surfactants are complex mixtures and are characterized inadequately for basic physicochemical study of order-of-mixing effects physicochemical study of order-of-mixing effects and time-varying interfacial tensions. Experience suggested that a representative pure surfactant would display much of the behavior of commercial mixtures, thereby exposing the practically important aspects to systematic scientific study. The U. of Texas group synthesized an agreed-on alkyl aryl sulfonate that is the subject of extensive studies in this laboratory and elsewhere. Results on its phase behavior with brine and with decane are phase behavior with brine and with decane are reported elsewhere. Parallel studies are under way with a petroleum sulfonate surfactant to establish the extent to which a pure surfactant can mimic the behavior of a commercial mixture. Companion investigations are aimed at explaining the unknown microstructures of ultralow tension interfaces. SPEJ p. 71

Microemulsion Phase Behavior Observations, Thermodynamic Essentials, Mathematical Simulation

1981 ◽  
Vol 21 (06) ◽  
pp. 747-762 ◽  
Author(s):  
Karl E. Bennett ◽  
Craig H.K. Phelps ◽  
H. Ted Davis ◽  
L.E. Scriven
Keyword(s):  
Experimental Data ◽  
Phase Behavior ◽  
Experimental Studies ◽  
Mathematical Framework ◽  
Alkyl Aryl Sulfonate ◽  
Phase Volume ◽  
Displacement Efficiency ◽  
Alkyl Aryl ◽  
Theoretical Understanding ◽  
Tie Lines

Abstract The phase behavior of microemulsions of brine, hydrocarbon, alcohol, and a pure alkyl aryl sulfonate-sodium 4-(1-heptylnonyl) benzenesulfonate (SHBS or Texas 1) was investigated as a function of the concentration of salt (NaCl, MgCl2, or CaCl2), the hydrocarbon (n-alkanes, octane to hexadecane), the alcohol (butyl and amyl isomers), the concentration of surfactant, and temperature. The phase behavior mimics that of similar systems with the commercial surfactant Witco TRS 10–80. The phase volumes follow published trends, though with exceptions.A mathematical framework is presented for modeling phase behavior in a manner consistent with the thermodynamically required critical tie lines and plait point progressions from the critical endpoints. Hand's scheme for modeling binodals and Pope and Nelson's approach to modeling the evolution of the surfactant-rich third phase are extended to satisfy these requirements.An examination of model-generated progressions of ternary phase diagrams enhances understanding of the experimental data and reveals correlations of relative phase volumes (volume uptakes) with location of the mixing point (overall composition) relative to the height of the three-phase region and the locations of the critical tie lines (critical endpoints and conjugate phases). The correlations account, on thermodynamic grounds, for cases in which the surfactant is present in more than one phase or the phase volumes change discontinuously, both cases being observed in the experimental study. Introduction The phase behavior of a surfactant-based micellar formulation is one of the major factors governing the displacement efficiency of any chemical flooding process employing that formulation. Knowledge of phase behavior is, thus, important for the interpretation of laboratory core floods, the design of flooding processes, and the evaluation of field tests. Phase behavior is connected intimately with other determinants of the flooding process, such as interfacial tension and viscosity. Since the number of equilibrium phases and their volumes and appearances are easier to measure and observe than phase compositions, viscosities, and interfacial tensions, there is great interest in understanding the phase-volume/phase-property relationships. Commercial surfactants, such as Witco TRS 10-80, are sulfonates of crude or partially refined oil. While they seem to be the most economically practicable surfactants for micellar flooding, their behavior, particularly with crude oils and reservoir brines, can be difficult to interpret, the phases varying with time and from batch to batch. Phase behavior studies with a small number of components, in conjunction with a theoretical understanding of phase behavior progressions, can aid in understanding more complex behavior. In particular, one can begin to appreciate which seemingly abnormal experimental observations (e.g., surfactant present in more than one phase or a discontinuity in phase volume trends) are merely features of certain regions of any phase diagram and which are peculiar to the specific crude oil or commercial surfactant used in the study.We report here experimental studies of the phase behavior of microemulsions of a pure sulfonate surfactant (Texas 1), a single normal alkane hydrocarbon, a simple brine, and a small amount of a suitable alcohol as cosurfactant or cosolvent. The controlled variables are hydrocarbon chain length, alcohol, salinity, salt type (NaCl, MgCl2, or CaCl2), surfactant purity, surfactant concentration, and temperature. Many of these experimental data were presented earlier. SPEJ P. 747^

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

Multiphase Microemulsion Systems

1976 ◽  
Vol 16 (03) ◽  
pp. 147-160 ◽  
Author(s):  
R.N. Healy ◽  
R.L. Reed ◽  
D.G. Stenmark
Keyword(s):  
Interfacial Tension ◽  
Physicochemical Properties ◽  
Phase Behavior ◽  
Oil Recovery ◽  
Displacement Efficiency ◽  
Oil Composition ◽  
Alkyl Aryl ◽  
Optimal Salinity ◽  
Interfacial Tensions ◽  
Multiphase Behavior

Abstract Economical microemulsion flooding inevitably involves microemulsion phases immiscible with oil or water, or both; oil recovery is largely affected by displacement efficiency during the immiscible regime. Therefore, it is essential to understand the role of interfacial tension in relation to multiphase microemulsion behavior. Three basic types of multiphase systems are identified and used to label phase transitions that occur when changes are made in salinity, temperature, oil composition, surfactant structure, cosolvent, and dissolved solids in the aqueous phase. Directional effects of these changes on phase behavior, interfacial tension, and solubilization parameter are tabulated for the alkyl aryl sufonates studied. A relationship between interfacial tension and phase behavior is established. This provides the phase behavior is established. This provides the basis for a convenient method for preliminary screening of surfactants for oil recovery. Interfacial tensions were found to correlate with the solubilization parameter for the various microemulsion phases, a result that can substantially reduce the number of interfacial tensions that must be determined experimentally for a given application. Introduction A previous paper established that microemulsion flooding is a locally miscible process until slug breakdown and is an immiscible, rate-dependent displacement thereafter; furthermore, for an effective flood, most of the oil recovered is acquired during the immiscible regime. An extensive study of single-phase regions defined classes of micellar structures for a particular surfactant; however, it was subsequently shown these did not affect oil recovery, provided viscous, lamellar structures were avoided. Optimal salinity was introduced as defining a ternary diagram having the least extensive multiphase region, a desirable feature in that locally miscible displacement is prolonged. Immiscible displacement after slug breakdown is known to depend on interfacial tension through its inclusion in the capillary number. A brief study showed chat interfacial tension varied widely throughout the multiphase region; accordingly, it is anticipated that oil recovery will depend on details of multiphase behavior in relation to interfacial tension, as well as on injection composition. Consider a flood sufficiently advanced that the microemulsion slug has broken down. A microemulsion phase remains that is immiscible with water or oil, phase remains that is immiscible with water or oil, or both, and displacement has assumed an immiscible character. The problem is twofold: to design a microemulsion slug that effectively displaces oil at the front and that is effectively displaced by water at the back. Both aspects are essential and, therefore, both microemulsion-oil and microemulsion-water interfacial tensions must be very low. The condition where these two tensions are low and equal will be of particular significance. The purpose of this paper is to explore physicochemical properties of multiphase physicochemical properties of multiphase microemulsion systems with a view toward understanding immiscible aspects of microemulsion flooding, and with the expectation of developing systematic screening procedures useful for design of optimal floods. Equilibration is an essential part of this study. Even the simplest of these systems is so complex it may well happen that nonequilibrium effects will never be understood sufficiently to be usefully accommodated in mathematical simulation of microemulsion flooding. In any event, equilibration, although time consuming, leads to a coherent picture of multiphase behavior that can be correlated with flooding results. Multiphase behavior of "simple" ternary systems divides into three basic classes. Dependence of phase behavior on salinity, with respect to these phase behavior on salinity, with respect to these classes, leads to correlations of interfacial tension with the solubilization parameter. These correlations are studied in relation to surfactant structure, temperature, cosolvents, oil composition, and brine composition. Optimal salinity again plays an important role, especially in relation to interfacial tension. SPEJ P. 147

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^

Phase Behavior of a Pure Alkyl Aryl Sulfonate Surfactant

1979 ◽  
pp. 35-74 ◽  
Author(s):  
E. I. FRANSES ◽  
H. T. DAVIS ◽  
W. G. MILLER ◽  
L. E. SCRIVEN
Keyword(s):  
Phase Behavior ◽  
Alkyl Aryl Sulfonate ◽  
Alkyl Aryl

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.

Favorable Attributes of Alkaline-Surfactant-Polymer Flooding

SPE Journal ◽  
2008 ◽  
Vol 13 (01) ◽  
pp. 5-16 ◽  
Author(s):  
Shunhua Liu ◽  
Danhua Zhang ◽  
Wei Yan ◽  
Maura Puerto ◽  
George J. Hirasaki ◽  
...  
Keyword(s):  
Crude Oil ◽  
Phase Behavior ◽  
Anionic Surfactants ◽  
Naphthenic Acids ◽  
Mobility Control ◽  
Alkaline Solutions ◽  
Synthetic Surfactant ◽  
Optimal Salinity ◽  
Interfacial Tensions ◽  
Alkaline Surfactant Polymer

Summary A laboratory study of the alkaline-surfactant-polymer (ASP) process was conducted. It was found from phase-behavior studies that for a given synthetic surfactant and crude oil containing naphthenic acids, optimal salinity depends only on the ratio of the moles of soap formed from the acids to the moles of synthetic surfactant present. Adsorption of anionic surfactants on carbonate surfaces is reduced substantially by sodium carbonate, but not by sodium hydroxide. The magnitude of the reduction with sodium carbonate decreases with increasing salinity. Particular attention was given to a surfactant blend of a propoxylated sulfate having a slightly branched C16-17 hydrocarbon chain and an internal olefin sulfonate. In contrast to alkyl/aryl sulfonates previously considered for EOR, alkaline solutions of this blend containing neither alcohol nor oil were single-phase micellar solutions at all salinities up to approximately optimal salinity with representative oils. Phase behavior with a west Texas crude oil at ambient temperature in the absence of alcohol was unusual in that colloidal material, perhaps another microemulsion having a higher soap content, was dispersed in the lower-phase microemulsion. Low interfacial tensions existed with the excess oil phase only when this material was present in sufficient amount in the spinning-drop device. Some birefringence was observed near and above optimal conditions. While this phase behavior is somewhat different from the conventional Winsor phase sequence, overall solubilization of oil and brine for this system was high, leading to low interfacial tensions over a wide salinity range and to excellent oil recovery in both dolomite and silica sandpacks. The sandpack experiments were performed with surfactant concentrations as low as 0.2 wt% and at a salinity well below optimal for the injected surfactant. It was necessary that sufficient polymer be present to provide adequate mobility control, and that salinity be below the value at which phase separation occurred in the polymer/surfactant solution. A 1D simulator was developed to model the process. By calculating transport of soap formed from the crude oil and injected surfactant separately, it showed that injection below optimal salinity was successful because a gradient in local soap-to-surfactant ratio developed during the process. This gradient increases robustness of the process in a manner similar to that of a salinity gradient in a conventional surfactant process. Predictions of the simulator were in excellent agreement with the sandpack results. Background Although both injection of surfactants and injection of alkaline solutions to convert naturally occurring naphthenic acids in crude oils to soaps have long been suggested as methods to increase oil recovery, key concepts such as the need to achieve ultralow interfacial tensions and the means for doing so using microemulsions were not clarified until a period of intensive research between approximately 1960 and 1985 (Reed and Healy 1977; Miller and Qutubuddin 1987; Lake 1989). Most of the work during that period was directed toward developing micellar-polymer processes to recover residual oil from sandstone formations using anionic surfactants. However, Nelson et al. (1984) recognized that in most cases the soaps formed by injecting alkali would not be at the "optimal" conditions needed to achieve low tensions. They proposed that a relatively small amount of a suitable surfactant be injected with the alkali so that the surfactant/soap mixture would be optimal at reservoir conditions. With polymer added for mobility control, the process would be an alkaline-surfactant-polymer (ASP) flood. The use of alkali also reduces adsorption of anionic surfactants on sandstones because the high pH reverses the charge of the positively charged clay sites where adsorption occurs. The initial portion of a Shell field test, which did not use polymer, demonstated that residual oil could be displaced by an alkaline-surfactant process (Falls et al. 1994). Several ASP field projects have been conducted with some success in recent years in the US (Vargo et al. 2000; Wyatt et al. 2002). Pilot ASP tests in China have recovered more than 20% OOIP in some cases, but the process has not yet been applied there on a large scale (Chang et al. 2006).

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