Determination of the Active Soap Number of Crude Oil and Soap

Sep 29, 2016 - The optimal salinity of the alkali/surfactant/crude oil system in an alkali/surfactant/polymer (ASP) flooding process was found previou...
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Determination of the Active Soap Number of Crude Oil and Soap Partitioning Behavior Lei Ding,*,†,‡ Guicai Zhang,†,§ Jacob Behling,‡ Jose Luis Lopez-Salinas,‡ Jijiang Ge,§ Maura C. Puerto,‡ George J. Hirasaki,‡ and Clarence A. Miller‡ †

State Key Laboratory of Heavy Oil Processing, College of Chemical Engineering and §College of Petroleum Engineering, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China ‡ Department of Chemical and Biomolecular Engineering, Rice University, Houston, Texas 77005, United States S Supporting Information *

ABSTRACT: The optimal salinity of the alkali/surfactant/crude oil system in an alkali/surfactant/polymer (ASP) flooding process was found previously to be a function of the soap/surfactant ratio. Therefore, the soap number is of great importance in formulation design and simulation of ASP flooding processes for enhanced oil recovery. However, there is as yet no established way to quantitatively determine the amount of soap in crude oil relevant to an ASP process. Soaps are the salts of fatty acids, a definition generalized here to include the salts of naphthenic acids. In this paper, we present a method to determine the amount of “active soap”, which consists only of soap that partitions into the aqueous phase at low ionic strength and transfers into the oleic phase at high ionic strength. Two fast and accurate methods, aqueous-phase potentiometric titration and two-phase colorimetric titration, were used to determine the water-soluble active soap number (WSASN), a measure of the active soap. Both methods were proven to be sufficiently precise by titrating a model oil containing known concentrations of oleic acid, both with and without isopropyl alcohol (IPA) present. The total soap number (TSN) with IPA present and water-soluble soap number (WSSN) and WSASN of a crude oil without IPA were measured in Na2CO3 and NaOH solutions. The partition of soap between oil and brine phases was also investigated. It was found that the partition coefficient of water-soluble active soap (WSAS) is near unity at optimal salinity as determined by IFT measurements, a result that supports the use of WSASN to represent the amount of active soap. Moreover, it was found that the logarithm of optimal salinity versus soap fraction for a soap/ surfactant mixture followed the previously proposed mole fraction mixing rule more closely when WSASN was used than if total acid number (TAN) or TSN were used as in previous studies. It was also found that the values of WSSN and WSASN measured at room temperature were different from those measured at high temperature and that the soap generated by NaOH was more hydrophobic than that generated by Na2CO3. Results of this work are helpful for formulation design and simulation of ASP flooding processes.

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surfactant ratio,10−12 which, in turn. depended upon the soap number, water/oil ratio (WOR), and surfactant concentration. Liu et al.10,13 developed a one-dimensional, two-phase, and multi-component finite difference simulator for ASP flooding to evaluate the characteristics of the ASP process considering the synergistic effect of a synthetic surfactant and in situ generated soap. It was also found13 that the existence of the soap/ surfactant gradient in a properly designed ASP process ensured the process passing through optimal conditions, where an active region of minimum IFT would be reached and a low residual oil saturation would be attained. Soap generation was also added in the ASP module in UTCHEM14 and MoReS,15 which are chemical flooding compositional simulators, with particular attention to phase behavior and the effect of soap on optimal salinity and oil/water solubilization ratios.14−17 Therefore, the soap number is of great importance in formulation design and simulation of the ASP flooding process for EOR. However, there is not yet any established procedure to quantitatively determine the soap number of crude oil. Usually,

INTRODUCTION An important mechanism of alkali-related enhanced oil recovery (EOR) processes, e.g., alkaline flooding, alkali/ surfactant/polymer (ASP) flooding, etc., is the generation of soap by in situ saponification of acid components that are naturally present in crude oil.1−6 The reduction in the interfacial tension (IFT) is reported to be a function of the type and concentration of alkali, ionic strength, pH, temperature, acid number, and oil property.7 For a simulation study, typically, the mixture of active acid species in crude oil is adequately assumed to be a single component and an accurate determination of the amount of soap, i.e., soap number, is necessary. DeZabala et al.8 proposed a chemical model for continuous, linear alkaline flooding simulation for acidic crude oil applying fractional flow theory, where the effects of the acid number, mobility ratio, and injected pH on secondary and tertiary alkaline flooding were investigated. Rudin and Wasan9 developed an interfacial activity model to predict the equilibrium interfacial tension of the alkali/acidic crude oil system, accounting for a mixed interfacial layer and mixed micelles formed by ionized and un-ionized acids. It was found that the optimal salinity of alkali/surfactant/ crude oil system in the ASP process is a function of the soap/ © 2016 American Chemical Society

Received: July 3, 2016 Revised: September 12, 2016 Published: September 29, 2016 10106

DOI: 10.1021/acs.energyfuels.6b01603 Energy Fuels 2016, 30, 10106−10116

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However, HPLC is much more expensive and time-consuming and also requires standard samples for the calibration curves, which means that the active material percentage should be accurately known. The objectives of this paper are to determine the active soap number and study how soap is partitioned between the oleic and aqueous phases as a function of salinity and WORs. Two fast and accurate methods, potentiometric and colorimetric titration, to determine the amount of water-soluble active soap (WSAS) are proposed, and soap partition between oil/brine phases was also investigated. Moreover, the correlation of the mixing rule with soap fraction of the alkali/surfactant/crude oil system was performed. Besides, the effects of the temperature, equilibration time, and type of alkali on soap number measurement were also studied.

the total acid number (TAN) determined by non-aqueous titration is used to estimate the soap amount, but TAN determined by non-aqueous titration does not distinguish between acids that can generate soap and compounds that can consume alkali without producing soap. Thus, a method to quantitatively measure the soap number needs to be developed. Because soaps, i.e., sodium naphthenates, are anionic surfactants extremely sensitive to any type of salinity or hardness, they can be extracted into the aqueous phase at low electrolyte concentrations and be measurable by hyamine or TEGO titration for anionic surfactants. Highly water-soluble isopropyl alcohol (IPA) can also be added to the aqueous phase for making the system more hydrophilic, and thus, more soaps are expected to be extracted into the aqueous phase and titrated by hyamine. On the basis of this, Liu and others13 proposed a method to determine the soap number by aqueous-phase potentiometric titration. As expected, it was found that the soap number was less than and indeed about one-half of the TAN value with certain exceptions, which is interpreted that, besides components that can consume alkali without producing soap, the TAN also includes soaps that are too hydrophobic to be extracted into the aqueous phase and/or too hydrophilic to be detected by hyamine titration. The partition coefficient of surfactant or soap should be equivalent to or around unity at optimal salinity.18 However, from preliminary experimental results, the soap number as defined by Liu et al.13 was almost equal at high and low salinities, indicating that no partition of soap into the oleic phase occurred when salinity was increased. Therefore, the soap number determined by this method may be the total amount of soap [total soap number (TSN)] rather than the amount of soap that is active, i.e., can partition between oleic and aqueous phases at reservoir conditions. The linear mole fraction mixing rule, first proposed by Salager et al.,19,20 has been used to estimate the optimal salinity of alkali/surfactant systems.13 It was found that a better agreement between the mole fraction mixing rule and experimental data can be obtained when TSN rather than TAN was used; however, the logarithm of optimal salinity was still not a linear function of the soap fraction, even if the TSN was used. Whether there are more precise methods to calculate the active soap number and, subsequently, a better correlation between the mixing rule and experimental data needs to be studied. As mentioned earlier, soap can be regarded as an anionic surfactant. Usually, there are three types of quantitative analysis techniques used for surfactant concentration: potentiometric titration, colorimetric titration, and high-performance liquid chromatography (HPLC) method. The potentiometric titration13 method with an ion-selective surfactant electrode is a widely used method to analyze both anionic and cationic surfactants, which is fast and convenient because of the automatic titrator. However, the surfactant electrode may be insensitive to some kinds of surfactants and conditions, such as some ionic surfactants with short carbon chain length and high salinity. Colorimetric methods21−23 are suitable for most surfactants with a proper indicator, requiring only a modest assembly of the apparatus. Colorimetric titration, such as Epton’s,27 can also be used for ionic surfactant titration at high salinity with a proper indicator because soaps must be titrated at high pH. However, it is difficult to judge the end point and a hazard to health as a result of using organic solvents, i.e., chloroform. HPLC can give the evaporative light scattering detection (ELSD) signal response for most surfactants.

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EXPERIMENTAL SECTION

Materials, Equipment, and Procedures. Many alkalis have been tested for EOR processes, such as ammonium hydroxide, sodium hydroxide, sodium orthophosphate, sodium carbonate, sodium metaborate, sodium silicate, and organic alkali.24−26 As a weak alkali, sodium carbonate has been considered of greater importance than others, because it is inexpensive and can also reduce the extent of mineral dissolution. Then, sodium carbonate was chosen and studied extensively; the soap generated by sodium carbonate and sodium hydroxide was also compared. In all test cases, alkali is adequate to neutralize all acid components in crude oil and the excess alkali in brine is considered to increase its ionic strength. In this paper, sodium carbonate and sodium hydroxide solutions are the brines tested and the optimal salinity refers to the sodium carbonate (ACS reagent grade, Sigma-Aldrich) or sodium hydroxide (ACS reagent grade, Sigma-Aldrich) concentrations where minimum IFT was obtained. The accuracy of colorimetric and potentiometric titration for soap was initially determined by titrating normal octane (Sigma-Aldrich) with a known concentration of oleic acid (Fisher Scientific), where the influence of IPA (HPLC grade, Sigma-Aldrich) was also studied. Three aliquots of different sample weights were titrated for each sample. Then, the volume of titrant consumed at the end point versus the sample weight can be plotted. The slope can be obtained by linear regression, and the oleic acid concentration is calculated by eq 1

oleic acid concentration (%) = slope × C TEGO × 282.46/10 (1) where 282.46 is the molecular weight of oleic acid. The crude oil with 27.5° American Petroleum Institute (API) gravity was from an oilfield in California. Samples with different WORs and sodium carbonate concentrations (aqueous-phase concentration) were prepared in 17 mL glass vials (Thermo Scientific) and shaken on an automatic shaker for 48 h. In this case, the desired Na2CO3 concentrations were prepared using deionized (DI) water and concentrated Na2CO3 solution (4% Na2CO3). It is important to record the mass of aqueous phases precisely for future soap number calculation. Then, they were left for equilibrium at reservoir temperature (54 °C), unless mentioned otherwise. To compare to the results by Liu et al.,13 several sets of phase behavior samples were added with a 1:1 volume ratio (by aqueous phase) of IPA after shaken 48 h and before being put into the water bath at 54 °C. The aqueous phases were sampled using a glass syringe with a long needle at 54 °C and then collected individually in glass vials for potentiometric and colorimetric titration at room temperature. pH of samples was measured before titration to ensure that they were titrated under alkaline conditions. Colorimetric Titration. The method is derived from Epton’s lowpH two-phase titration,27 with certain modifications,22 because soap is only active in alkaline conditions. A solution of cationic surfactant, such as TEGO or Hyamine, can be used as a titrant under high-pH buffer conditions. Chloroform (HPLC grade, Sigma-Aldrich) was used as the immiscible organic phase. The indicator as well as dye used for 10107

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Figure 1. Schematic of two-phase colorimetric soap titration. high-pH two-phase titration is 60 mg/L bromocresol green (BG, Sigma-Aldrich) in DI water (18.2 MΩ). Because soap is only active in alkaline conditions, a high-pH buffer solution constituted with 300 mL of 28.98 g/L (in DI water) sodium diphosphate (Na4P2O7·10H2O, Sigma-Aldrich), 100 mL of 23.91 g/L (in DI water) sodium tripolyphosphate solution (Na5P3O10, Sigma-Aldrich), and 80 mL of n-propanol (Sigma-Aldrich) were prepared.22 It is also found that TEGO can give a more clear inflection point near the titration end point than Hyamine, although the former is much more expensive than the latter. Therefore, the titrant used for colorimetric and potentiometric titration is 2 mM TEGO (TEGO Titrant A 100, Metrohm, catalog number 6.2317.000), whose exact concentration can be calibrated by low-pH Epton’s method27 or potentiometric titration with a standard 10 mM sodium lauryl sulfate solution (Metrohm). Calibration of TEGO by the potentiometric titration method is available in the Supporting Information. The end point for colorimetric titration is when the aqueous phase is becoming completely clear; a blank test is always needed because the colorimetric titration measures both soap (A−) and anionic indicator (BG−) in this technique. A schematic illustration of this process was given in Figure 1. In the initial phase of titration, as exhibited in Figure 1a, BG is in the top aqueous layer, which is colored blue. The added titrant reacts first with the anionic surfactant in the aqueous phase to form insoluble ion pairs, which will transfer to the chloroform phase, as shown in Figure 1b. After all surfactant species are in chloroform, then the titrant will start to react with the dye, so that a blue complex will be formed and solubilized in chloroform, as indicated in Figure 1c. The blank test can be performed, as shown in Figure 1d, and should be subtracted from the end point. More specifically, a 17 mL vial was used to contain the mixture of 4.0 mL of chloroform, 1.0 mL of BG solution, 5 mL of high-pH buffer, and specific amount of sample (msample). The sample size needed to be large enough such that the titrant consumed at the end point is larger than 2 mL to minimize experimental error. The titrant (2 mM TEGO) was added by a buret (Kimax, catalog number 17110F-10), with the smallest scale of 0.02 mL. After each addition of titrant, the sample was rigorously shaken by hand for 1 min and then was centrifuged for 2 min at 3000 rpm in a centrifuge (Eppendorf, 5804) to speed up the emulsion coalescence. Near the end point, small amounts of the aqueous phase were transferred to a micro quartz cuvette (minimum required volume of 50 μL, catalog number 268-809900, Fisher Scientific Co.) to measure light absorbance at the characteristic

wavelength of BG at 620 nm using an ultraviolet−visible (UV−vis) spectrometer (Genesys 10S UV-Vis spectrometer, Thermo Scientific). The aqueous solution in the cuvette was poured back to the vial after this measurement. The colorless end point was reached when the absorbance at 620 nm fell below a threshold value of 0.138. The added volume of titrant was recorded as Vtitrant. A blank test, which accounts for the titrant consumed by BG in the aqueous phase, was conducted to correct Vtitrant. Amounts of 4.0 mL of chloroform, 1.0 mL of BG solution, 5 mL of high-pH buffer, and sodium carbonate solution at a volume of Vsample were added to a 17 mL vial. When IPA is added to the sample to measure TSN, a blank test with IPA is also necessary because IPA is found to consume TEGO at high pH. The same titration process was repeated as mentioned earlier. The titrant consumed in the blank test was recorded as Vblank. Thus, the corrected volume of titrant is described by Vcor.tit. = Vtitrant − Vblank. Three aliquots of sample weights were studied, and a linear curve of Vcor.tit. versus msample can be plotted for each sample. The slope can be obtained by linear regression and is used to calculate the sample concentration based on eq 2, assuming that the soap concentration is the same in the aqueous phase and the stoichiometry is 1:1 during colorimetric titration. The soap number calculated in this way has the same unit as TAN. It should be noted that the intercept should coincide with the blank tests. An example of the calculation is also available in the Supporting Information soap number (mg of KOH/g of oil) = slope × maqueous × C TEGO × 56.1/moil

(2)

where 56.1 is the molecular weight of KOH. Potentiometric Titration. The automatic titrator 716 DMS Titrino connected to a computer is used as an automatic titrator for titration and data collection. An ion-selective electrode (ISE, pHoenix, catalog number SUR1502) is used for aqueous soap titration, and its electrolyte solution is 4 M KCl (catalog number R0010011). Anionic surfactants (soap) are titrated with cationic surfactants (TEGO Titrant A 100, Metrohm, catalog number 6.2317.000). Two drops of Triton X-100 (Metrohm) were added to the sample solution to keep the electrode “clean”. Weighted aliquots of the aqueous phase from the equilibrated samples at 54 °C were diluted to ∼60 mL with DI water. The stirring speed should be selected to avoid foaming. Near the inflection point, precipitation occurs and the solution becomes turbid. The potential−volume curve obtained is usually S-shaped, and the 10108

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Energy & Fuels inflection point(s) where the local maximum derivative of potential occurs is automatically given by the titrator. Three aliquots of different sample weights were titrated for each sample. When IPA is present to measure the TSN, a blank test was also conducted and the volume at end point was subtracted from titration results because IPA can be titrable at high pH. Then, the volume of titrant consumed at the inflection point versus sample sizes can be plotted. The soap number was calculated by eq 2.

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RESULTS AND DISCUSSION Threshold Absorbance of the Colorless End Point for Colorimetric Titration. BG solution was diluted with different volumes of DI water, and the color and absorbance were determined by visual check and a spectrometer, respectively. The test results of the absorbance spectra of BG in DI water are exhibited in Figure 2, from which we can clearly see that the

Figure 3. Overall accuracy of potentiometric and colorimetric titration (WOR = 5 and 1% Na2CO3).

Potentiometric titration also gives well-defined end points around 100 mV (Figure 4b).

Figure 2. Absorbance spectra of BG.

absorbance is high in the range of 610−640 nm wavelength. Thus, the sensitivity of the measurement and the accuracy for determining the colorless end point can be enhanced if a wavelength in this range is considered as the characteristic value of BG. In this paper, 620 nm was used as the characteristic wavelength. When the dilution factor reached 0.004, the green color was nearly undetectable by visual check and the corresponding absorbance was 0.138. Therefore, it is hypothesized that the sample is colorless when its absorbance at 620 nm is lower than 0.138, which was used as the criterion to determine the end point of titration. Accuracy of Potentiometric and Colorimetric Titration for Oleic Acid. Before measuring the soap number of crude oil, the accuracy of colorimetric and potentiometric titration methods was investigated by titrating model oil samples. Normal octane samples with known oleic acid concentration were prepared. Oleic acid concentrations in octane range from 0.025 to 0.5%, and the corresponding TANs are 0.05−1 mg of KOH/g of oil. The sodium carbonate concentration is fixed at 1% (aqueous-phase concentration), and WOR is 5 in these samples. It can be seen from Figure 3 that both methods are accurate to determine the concentration of oleic acid in the model oil system in the absence of IPA. The potentiometric titration method is still precise to determine that the oleic acid concentration in the presence of IPA after a blank test (IPA with alkali) is performed, as seen from the data of Figure 3. The blank test is needed because IPA can be titrable at high pH. Empirically, 1 g of IPA can consume approximately 0.20 mL of 2 mM TEGO based on potentiometric titration experiments.

Figure 4. Influence of IPA on (a) colorimetric and (b) potentiometric titration of 0.125% oleic acid in octane.

However, colorimetric titration curves exhibit overshoot in the presence of IPA even when a blank test is performed. This is probably because IPA interferes with the partition of ion pairs formed during colorimetric titration. IPA can also adversely affect the titration results even with a blank test, as indicated in Figure 4a. However, when the amount of IPA is less than 1.0 g, the result is relatively precise. Therefore, the sample size should be carefully chosen such that the IPA in the sample is less than 1.0 g for the present case. It should be noted that the amounts of IPA in titrated samples with different sample sizes are different. Soap Number as a Function of WORs and Salinity. Soap is the salt of a fatty acid and is generalized here to include the salts of naphthenic acids. Sodium carbonate in water is the only brine used in the results disclosed in this section. Three definitions of different soap numbers are defined as follows: (1) 10109

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Energy & Fuels Water-soluble soap number (WSSN): The amount of soap partitioned into the aqueous phase in the absence of IPA can be measured by two-phase colorimetric titration at high pH or aqueous-phase potentiometric titration. This amount of soap is defined as the WSSN in this paper. It is a function of the Na2CO3 concentration. (2) TSN: If brine samples are treated by IPA, more soap is expected to be extracted into the aqueous phase. Soap partitioned into this alcoholic aqueous phase can also be titrated by the methods described above. Blank tests are required. The soap extracted by sodium carbonate solution and IPA is defined as the TSN, consisting of the soap making up the WSSN and some of the more hydrophobic soap that cannot be extracted by brine alone but can be extracted by alcoholic brine (IPA + brine). The test results are not sensitive to the volume ratio of brine/IPA, and the ratio is 1 in this case. The criteria for selecting sample sizes for colorimetric titration are the same as in the last section. (3) Water-soluble active soap number (WSASN): At optimal conditions, it is hypothesized that soap has equal attraction to aqueous and oleic phases and the soap partition coefficient between oleic and aqueous phases is unity. From the discussion above, it is clearly seen that not all of the soaps are active enough to partition between phases. Thus, in this paper, we have further restricted the term “active soap” to be only the components that partition into the aqueous phase at low ionic strength and the oleic phase at high ionic strength. The WSASN is subsequently defined as the amount of watersoluble soap that can partition between oil and aqueous phases at low and high salinities, i.e., the difference of the amount of water-soluble soap between that at low and high sodium carbonate concentrations. Figures 6 and 7 show the TSN, WSSN, and WSASN determined by potentiometric and colorimetric titration at 54

Figure 6. Potentiometric titration at 54 °C: TSN, WSSN, and WSASN (purple arrow) as a function of the sodium carbonate concentration.

Figure 7. Colorimetric titration at 54 °C: TSN, WSSN, and WSASN (purple arrow) as a function of the sodium carbonate concentration.

difference between TAN and TSN may be phenols and very large-molecular-weight acid components with a relatively small dissociation constant and other acid components that can consume alkali without producing soap. The WSSN values measured at 0.2 and 0.5% Na2CO3 are almost constant at a plateau value, which indicates the maximum value of WSSN of this crude oil. Moreover, the overall trend for the WSSN is that it decreases with an increasing Na2CO3 concentration for all WORs studied, which indicates that, with an increase of salinity, soap gradually partitioned into the oil phase. The gradual partition of soap into the oil phase with increasing salinity is consistent with the lighter color in the aqueous phase with increasing salinity, and a lighter color, in turn, corresponds to a lower WSSN value. One picture of phase behavior samples for soap number measurement with 0.5−1.4% Na2CO3 at WOR = 3 is exhibited in Figure 5. This picture was taken after these samples had been equilibrated for 21 days at 54 °C. It should be noted that the WSSN defined by colorimetric titration is higher than that determined by potentiometric titration. This is probably because the soap or surfactant carbon chain length can affect the potentiometric and colorimetric titration results and the solubility and partition coefficient of the surfactant cation−anion salt change with its molecular weight. It is found that the potentiometric titration method is only suitable for the surfactant/soap with a carbon chain length larger than 8 when TEGO is used as a titrant. The amount of hydrophilic soap with carbon chain length less than 8 may be not titrable by potentiometric titration but can still be titrated by colorimetric titration. This is also probably why the WSSN

Figure 5. Alkali/crude oil samples in 17 mL vials for soap number measurement after equilibrated for 21 days at 54 °C, at WOR = 3.

°C, respectively. It may be seen from Figures 6 and 7 that the TSN (generated by Na2CO3 and then extracted by IPA and brine) measured by both potentiometric and colorimetric titration is around 0.6 mg of KOH/g of oil, being much similar, although they are determined by different methods. The TSN is almost independent of WOR and salinity within the salinity range studied. Moreover, the TSN is consistently higher than the WSSN. This is because IPA can make the system more hydrophilic; thus, more soap is extracted into the aqueous phase. It should be noted that the TAN is 0.84 mg of KOH/g of oil, which is determined by the spiking method.28 Therefore, most of the acid components contained in this crude oil sample can be surface-active, but crude oils may differ significantly. The 10110

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Energy & Fuels determined by potentiometric titration is almost zero, while the WSSN determined by colorimetric titration is not zero at the Na2CO3 concentration higher than 2%. It can also be seen from Figure 7 that, at a high sodium carbonate concentration (larger than 2% Na2CO3), there is still a certain amount of soap being partitioned in brine, and it did not decrease much with a further increase of the sodium carbonate concentration (to 4% Na2CO3). This is probably the amount of soap with a small molecular weight or short carbon chain length. However, this amount of soap may be too hydrophilic to contribute to lowering of IFT and, thus, is considered to be not active enough at reservoir conditions. The WSASN obtained from colorimetric titration can be subsequently calculated. It can be seen from Figure 6 (potentiometric titration) and Figure 8

Figure 9. Potentiometric titration at 54 °C: WSAS soap partition coefficient as a function of the sodium carbonate concentration.

Figure 8. Colorimetric titration at 54 °C: WSASN as a function of the sodium carbonate concentration. Figure 10. Colorimetric titration at 54 °C: WSAS soap partition coefficient as a function of the sodium carbonate concentration.

(colorimetric titration) that the WSASNs determined by potentiometric and colorimetric titration are 0.36 and 0.34 mg of KOH/g of oil, respectively, being much similar between two methods. During potentiometric titration, more than one inflection point may be given by the titrator while measuring the soap number of crude oil and the end point should be carefully selected. In the case of this crude oil sample, the inflection point is around 270 mV. However, it still needs to be investigated if 270 mV is applicable for soap components in other crude oil samples. Soap Partitioning Coefficient. Previous results reveal that TAN may not be accurate to estimate the amount of surfaceactive soap, because certain acid components can consume alkali without producing soap and some soaps generated are either too hydrophilic or too lipophilic to partition from one phase to the other with a changing electrolyte concentration. The partition coefficient of surfactant or soap should be equivalent to or around unity at optimal salinity.18 However, TSN was almost equal at high and low salinities,13 as also found here, which indicated that no partition of soap into the oleic phase occurred when salinity was increased. Therefore, the soap number determined by TSN may be the total amount of soap rather than the amount of soap that is active, i.e., can partition between oleic and aqueous phases, at reservoir conditions. On the basis of WSASN, which can partition between oil and brine, the soap partitioning coefficient can be calculated, as indicated in Figure 9 for potentiometric titration and Figure 10 for colorimetric titration, at WORs equal to 3, 5, and 10. In this section, sodium carbonate solution is the only brine used and the optimal salinity refers to the sodium carbonate concentration, where minimum IFT was obtained. It can be seen that

the soap partitioning coefficient is near unity at optimal salinity defined by the IFT measurement. Optimal salinity defined by the IFT measurement at 54 °C is 0.9% Na2CO3, where a minimum IFT occurred, as shown in Figure 11. The IFT values

Figure 11. Equilibrium IFT as a function of salinity and WORs from pre-equilibrated samples at 54 °C.

are equilibrium values measured with pre-equilibrated samples. It is believed that the equilibrium IFT values should be used to find the optimal salinity of the alkali/crude oil system, which will be discussed in a future publication. The finding that the WSAS partition coefficient is unity at optimal salinity indicates that WSAS may be used to characterize the amount of soap that is active at reservoir conditions. Theoretically, the partitioning for the surfactant or soap does not depend upon the WOR or soap concentration in the dilute solution limit.29 However, it does depend upon the salinity 10111

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Energy & Fuels (concentration of electrolyte) and the properties of crude oil. The partition coefficient of soap is unity at the optimum conditions. For over-optimum conditions, most of the soap is in the oleic phase, so that the partition coefficient of soap is larger than unity for over-optimum conditions. Contrarily, when phase behavior is under-optimum, the partition coefficient of soap is less than unity. The partition coefficient can be calculated by the following empirical equations:10 above optimal salinity

K part = 102(Sal/Salopt − 1)

(3)

below optimal salinity K part = 10−2(Salopt /Sal − 1)

Figure 13. Equilibrium IFT as a function of salinity and WORs using pre-equilibrated samples at room temperature.

(4)

where Sal is the local salinity and Salopt is the optimum salinity of soap (0.9% Na2CO3), obtained by IFT measurements as discussed before. Kpart is unity at optimal salinity. The calculated partition coefficient curve as a function of salinity is given in Figure 12 based on the equations above, which is in good agreement with the experimental data at 54 °C.

Figure 14. Colorimetric titration at room temperature: TSN, WSSN, and WSASN (purple arrow) as a function of the sodium carbonate concentration.

salinity, which is different at room temperature and reservoir temperature (54 °C). The WSASNs at room temperature and 54 °C are around 0.21 and 0.35, respectively. Therefore, the WSASN at different temperatures needs to be studied specifically at the target temperature. The WSAS partition coefficient by colorimetric titration (Figure 15) is around unity at the optimal salinity at room temperature, which further shows that the WSAS can be a good indicator of the amount of active soap contained in crude oil.

Figure 12. Comparison of experimental results and calculated soap partition coefficient as a function of salinity at 54 °C.

Effect of the Temperature. The potentiometric and colorimetric soap number measurements and soap partition coefficient measurement were also conducted at room temperature. Samples were prepared and equilibrated at room temperature, and two-phase colorimetric titration and aqueousphase potentiometric titration were also performed. The IFT values used here are also equilibrium IFT values measured with pre-equilibrated samples at room temperature, as shown in Figure 13. Optimal salinities defined by IFT measurement at room temperature and 54 °C are 1.4 and 0.9% Na2CO3, respectively. The soap numbers determined by colorimetric titration are exhibited in Figure 14. It is found that the TSN measured at room temperature is almost the same as that at reservoir temperature (54 °C), being nearly constant at 0.6 mg of KOH/g of oil, which indicates that the total amount of soap generated at room temperature is the same as that at 54 °C. However, the WSSN and WSASN at room temperature are found to be different from those at reservoir temperature (54 °C). The WSSN is different because the partition coefficient largely depends upon how a given salinity is related to optimal

Figure 15. Colorimetric titration at room temperature: WSAS soap partition coefficient as a function of the sodium carbonate concentration. 10112

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Energy & Fuels Effect of the Equilibration Time. After these samples equilibrated for 21 days at 54 °C, there was almost no emulsion. However, if emulsion has not been completely coalesced, the soap concentration in the aqueous phase may be different. For the WSSN measurement, the soap is assumed to be partitioned in either the aqueous phase or the oleic phase, although in actuality, emulsion exists in most samples and adsorbs a certain amount of soap. As exhibited in Figure 16, the

emulsion can influence the soap number measurement performed at different equilibration times. After these samples equilibrated for 2 weeks, the soap number measured almost did not change with time. Similar experiments were performed at WOR = 24, and it was found that the influence of time on the soap number measurement was more striking at high WOR, where only a small amount of soap was generated because the volume of oil was small. In this case, a small difference of the soap concentration will cause a large deviation. However, the equilibration time may largely depend upon the oil sample. Presumably, heavy oil with long-chain or large-molecularweight components may take a longer time to equilibrate because the adsorption of these components at the oil/water interface may result in a more rigid emulsion film. Centrifugation is recommended to facilitate the coalescence of emulsion. For TSN measurement, no emulsion existed after the samples were treated by IPA/brine and then equilibrated for 2 days. IPA is known to facilitate emulsion coalescence because it can adsorb at the oil/water interface, making the oil/ water film less rigid. Comparison of the Soap Generated by NaOH and Na2CO3. It is assumed that the mixture of hydrophobic acid species in crude oil can be represented by a single component, denoted as HA with concentration CHAo. When the acidic oil is

Figure 16. Potentiometric titration at 54 °C: effect of the equilibration time on the soap number measurement, at WOR = 3.

Figure 17. Contour map of the fraction of dissociated acids in crude oil as functions of Kp, Ka, and pH, at WOR = 1. 10113

DOI: 10.1021/acs.energyfuels.6b01603 Energy Fuels 2016, 30, 10106−10116

Article

Energy & Fuels exposed to alkali, a reaction occurs to produce the water-soluble anionic surfactant, A−. It is the species A− that is presumed to be surface-active. Consider crude oil in equilibrium with an aqueous phase consisting of DI water and either NaOH or Na2CO3. Assume that all ions are confined to the aqueous phase but that HA partitions between oil and water with the partition coefficient Kp being very large, as in eqs 5 and 6. HA o ⇔ HA w

(5)

K p = C HA o/C HA w

(6)

The usual expression for acid dissociation holds as the following eqs 7 and 8, with a dissociation constant Ka. HA w ⇔ H+ + A− Ka =

C H+C A− C HA w

Figure 18. Colorimetric titration at 54 °C: comparison of the soap number measured by NaOH and Na2CO3, at WOR = 3 (dashed lines indicate optimal salinities).

(7)

(8)

surfactant electrode has an operating pH range of 2−12; otherwise, the membrane is adversely affected, and the interference of the electrode is severe. Therefore, colorimetric titration is the only suitable method to measure the soap number generated by NaOH. It can also be derived from Figure 18 that the qualitative behavior of NaOH is similar to that of Na2CO3. The optimal salinity of soap generated by NaOH may be around 0.5% NaOH based on the soap partition coefficient calculation. Sodium activity is found to be suitable to indicate the optimal salinity among various sodium salts and its mixtures when sodium ion is the only counterion of the anionic surfactant or soap, as will be discussed in detail in a future publication. The sodium activity can be calculated in PHREEQC software under different conditions, such as temperature and pH. The sodium activities calculated in this way at optimal conditions of soap are 0.096 and 0.115 mol/L for NaOH and Na2CO3, respectively. On the basis of that, the soap generated by NaOH is more lipophilic than that by Na2CO3. This is because the pH is higher with NaOH than that with Na2CO3. Moreover, the soap formed with NaOH contains a larger proportion of more hydrophobic soaps (larger molecular weight and long chain length). Both of these factors are in the direction to explain why less NaOH is needed than Na2CO3 to reach the inversion point from lower phase to upper phase microemulsions. Correlation of Optimal Salinity and Soap Fraction. The mixing rule, as shown in eq 12, was found to work reasonably well to estimate optimal salinity of mixtures of anionic surfactants.19,20 Because soap is also an anionic surfactant, it is reasonable to assume that the blend of soap and synthetic anionic surfactant in an alkali/surfactant/crude oil system might follow the same mixing rule, as shown in eqs 13 and 14.

Combining the equations, we have eq 9

Ka C A− = C HA o K pC H+

(9)

which is equivalent to eq 10 Ka C A− = (C A− + C HA o) (K a + K pC H+)

(10)

HAw is negligible as a result of the large Kp, which means that eq 10 is approximate to the fraction of dissociated acids compared to the original acids in oil for WOR = 1. For generalized WORs, the fraction of dissociated acids compared to the original acids in oil can be calculated based on eq 11. C A− 1 = (C A− + C HA o/WOR) 1 + (K pC H+)/(K a WOR)

(11)

Figure 17 presents the fraction of dissociated acids at different pH, Ka, and Kp for WOR = 1. It is indicated from Figure 17 that the extent of dissociation is largely dependent upon aqueous pH, Ka, and Kp. A typical value of Ka for carboxylic acids is about 10−5, and Kp may be ranging from 104 to 107 for individual acids with different molecular weights and carbon chain lengths.8 However, it should be noted that the oil/water surface pH may be much lower than the bulk aqueous pH. This results from the adsorption of negatively charged carboxylic acid at the oil/water interface, which further induces a more negative ζ potential and, thus, a higher concentration of hydronium ions at the vicinity of the oil/water interface than that in the bulk aqueous phase. Therefore, more hydrophobic soap with a large molecular weight or long carbon chain length may be active at high pH with NaOH, and the emulsion formed may be more stable. As indicated in Figure 18, TSN and WSASN measured by NaOH at 54 °C are 0.67 and 0.42 mg of KOH/g of oil, respectively, being greater than those by Na2CO3. The difference of TSN and WSASN between NaOH and Na2CO3 may be because certain soaps with a larger molecular weight can be active at high pH. For NaOH, all samples were between a pH of 12.1 and 12.6, while for Na2CO3, all samples had a pH between 10.7 and 11.0. Therefore, if Kp and/or Ka are large enough, more soap can be titrated with NaOH than Na2CO3. However, no clear inflection point was found during potentiometric titration of soap generated by NaOH. This is probably because this

log(Opt mix) =

∑ Xi log(Opti)

(12)

i

where Xi is the mole fraction of surfactant i, Optmix is the optimum salinity of the surfactant mixture, and Opti is the optimum salinity of surfactant i log(Opt mix) = Xsoap log(Opt soap) + (1 − Xsoap) log(Opt surfactant) Xsoap = 10114

soap soap + surfactant

(13)

(14) DOI: 10.1021/acs.energyfuels.6b01603 Energy Fuels 2016, 30, 10106−10116

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Energy & Fuels

Figure 19. Optimal salinity curve as a function of the soap fraction.

where Xsoap is the mole fraction of natural soap. It was found previously that better agreement between the mixing rule and experimental data can be obtained when TSN rather than TAN was used.11,13 However, the logarithm of optimal salinity was still not a linear function of the soap fraction, even if the TSN was used. Whether an even better correlation would be obtained if WSAS is used to calculate the soap fraction needs to be studied. Figure 19 is for illustrating how the soap content of the oil could shape the correlation of the optimal salinity versus soap fraction. It is assumed that the natural soap has an optimal salinity of 0.9% Na2CO3 and the surfactant has an optimal salinity of 4.0% Na2CO3.30 This surfactant system corresponding to the NI blend reported in a former article10,13 and will be discussed more extensively in a future publication. In Figure 19, there are plots to compare the dependence of optimal salinity for the Na2CO3/surfactant/crude oil system on Xsoap when the latter is determined by the TAN, TSN and WSASN, respectively. It is assumed that WSASN will result in a linear correlation. In the example, the surfactant concentration is 1% and is assumed to have an average molecular weight of 661. Meanwhile, the mole fraction at soap can be calculated by TAN, TSN, or WSASN. Subsequently, the soap fraction at different WORs can be calculated. The straight line in Figure 19 presents Equation 13 using WSASN as the assumed effective soap content of the oil, while the markers represent calculated optimal salinity of alkali/surfactant/crude oil system when the amount of soap is calculated by 0.84 mg KOH/g (TAN), 0.6 mg KOH/g (TSN), 0.2 mg KOH/g and 0.35 mg KOH/g (WSASN) respectively. At large and small values of this ratio, optimal salinity approaches those of the soap alone (0.9% Na2CO3) and surfactant alone (4% Na2CO3), respectively. As shown in Figure 19, the soap fraction calculated by WSASN, as indicated by black asterisks, is assumed to give exact agreement with the mole fraction mixing rule. An over-estimation of soap number, such as TSN or TAN, results in a convex correlation while an under-estimation of soap number (0.2 mg KOH/g) results in a concave correlation.

For the ASP process, the optimal salinity is a function of the soap/surfactant ratio. However, designing an ASP formulation needs extensive phase behavior tests and IFT measurements. The finding of WSASN as an appropriate estimation of active soap can be applied to the mixing rule, which will save much time in formulation design. Moreover, for chemical flooding simulators, such as UTCHEM, MoReS and CMG, the acid number is often used as an input parameter to estimate the amount of soap. The WSASN will provide a more reasonable approach and will be beneficial for simulation of the ASP process, as will be reported in a future publication.

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CONCLUSION (1) Two methods, aqueous-phase potentiometric titration and two-phase colorimetric titration, were used to quantitatively determine the amount of active soap. The WSASN was found to provide a reasonable estimate of the amount of active soap contained in crude oil. (2) The temperature can influence the optimal salinity of soap and its partition between oleic and aqueous phases. The WSASN is different at different temperatures. (3) The WSAS partitioning coefficient is close to unity at optimal salinity for all WORs at both room temperature and reservoir temperature for the system studied. (4) The WSSN decreases with increasing Na2CO3 or NaOH concentration, which means that soap gradually partitions into the oil phase. The gradual partition of soap into the oil phase with increasing salinity is consistent with the lighter color in the aqueous phase with increasing salinity. (5) The TSN generated by Na2CO3 or NaOH and then extracted by IPA is almost independent of WOR and salinity, which is because IPA can make the system be more hydrophilic, extracting more hydrophobic soap from the oleic phase to the aqueous phase than the alkali alone. (6) More soap can be generated by NaOH than by Na2CO3; the soap generated by NaOH is more hydrophobic than that by Na2CO3. (7) It is assumed that the logarithm of optimal salinity versus soap fraction follows the mole fraction ‘mixing rule’, when WSASN is used to calculate the soap fraction. If that is the case, the logarithm of optimal 10115

DOI: 10.1021/acs.energyfuels.6b01603 Energy Fuels 2016, 30, 10106−10116

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Energy & Fuels

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salinity is not a linear function of soap fraction when either total acid number (TAN) or total soap number (TSN) is used. In the illustrated case, the deviation is significant.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01603. Calibration of TEGO, photographs of samples during colorimetric titration, and examples of soap number calculation by colorimetric and potentiometric titration (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Rice University Consortium for Processes in Porous Media and the Chinese Scholarship Council (201206450036) is greatly acknowledged.

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REFERENCES

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DOI: 10.1021/acs.energyfuels.6b01603 Energy Fuels 2016, 30, 10106−10116