Temperature and Composition Effect on CO2 Miscibility by InterfacialTension MeasurementAli Zolghadr, Mehdi Escrochi, and Shahab Ayatollahi*,§
Enhanced Oil Recovery (EOR) Research Centre, School of Chemical and Petroleum Engineering, Shiraz University, P.O. Box7134851154, Shiraz, Iran
*S Supporting Information
ABSTRACT: Crude oil reservoirs have different temper-atures, compositions, and pressures, therefore oil recoveryperformance by CO2 injection varies from one case to another.Furthermore, it is predicted that lower interfacial tensionbetween injected CO2 and reservoir fluid results in more oilrecovery. In this study, we investigate the effect of temperatureon the equilibrium interfacial tension between CO2 and threedifferent oil fluids at different pressures. Also minimummiscible pressure (MMP) is measured by the vanishinginterfacial tension (VIT) technique to determine the temper-ature effect on the CO2 miscible gas injection. The results ondifferent pure and mixtures of hydrocarbon fluids show that for pressures up to 5.2 MPa, the higher the temperature was, thelower was the interfacial tension (IFT) measured. However, for the cases with pressure higher than 5.2 MPa, as the temperaturewas increased, the IFT increased too. In addition the VIT technique is used to measure the MMP of CO2 and pure paraffin; theheavier paraffin was, the higher was the MMP noticed. Also, we have learned that paraffin groups have an important effect onmulticomponent interfacial tension behavior.
■ INTRODUCTION
Enhanced oil recovery (EOR) processes have becomesignificantly important to the petroleum industry. Carbondioxide injection is one of the most effective proposed methodsto improve oil recovery efficiency.1−3 The CO2 EOR methodhas been proposed for both enhanced oil recovery and CO2sequestration to reduce greenhouse gas emissions.4 The CO2
EOR process includes two procedures: miscible and immiscibledisplacements. The possibility to apply each process dependson reservoir conditions. The miscibility is theoretically referredto the cases when there is no interface between the two phasesinvolved (i.e., zero equilibrium interfacial tension (IFT)), or tocases where the two phases can be mixed with each other at anyratio.5 It is stated that miscible CO2 injection improves the oilrecovery efficiency by the reduction of oil viscosity and densityand also improves volumetric sweep efficiency because of theunique properties of CO2 at high pressure and temperature.6−8
The minimum miscibility pressure (MMP) is the lowestpossible pressure at which the injected gas can achievemiscibility with reservoir oil at reservoir temperature.9 Carbondioxide injection can be either miscible or immiscibledepending on the MMP.10 If reservoir pressure is notmaintained above the minimum miscibility pressure (MMP),the gas flood becomes an immiscible injection.11
Accurate determination of the MMP is crucial to design aneffective and economical miscible injection process. Bothanalytical and experimental approaches have been suggestedfor the prediction and measurement of the MMP.12,13 There
are different experimental methods to determine fluid−fluidmiscibility under reservoir conditions such as the slim tubedisplacement and the rising bubble apparatus. Recently, a quickreview of various MMP prediction methods has beenprovided.14,15 The vanishing interfacial tension (VIT) techni-que has been recently developed and utilized to determine themiscibility conditions of various crude oil with different gasesincluding CO2.
16,17 The VIT technique is based on themeasurement of equilibrium IFT between crude oil and CO2
while the pressure increases. It is shown that the equilibriuminterfacial tension in isotherms is reduced as the pressure isincreased and it can be correlated by a linear equation.18 Anestimation of MMP can be obtained through extrapolating theequilibrium IFT by increasing pressure, which reduces theequilibrium IFT to zero.17 The temperature and composition ofoil reservoirs are different which has an immediate effect on themiscibility and immiscibility condition of CO2 injection. On thebasis of the results presented in the literature,19,20 the reductionof interfacial tension between CO2 and reservoir fluid results inhigher oil recovery. This mechanism known as IFT reductionfor more oil recovery during any enhanced oil recovery (EOR)process is well documented.21 To the best of our knowledgethere is no study on the effects of temperature and compositionon injected CO2 by using the VIT technique. Therefore, in this
Received: December 5, 2012Accepted: March 14, 2013Published: March 25, 2013
study we investigate the effect of temperature on (CO2 +hydrocarbon) interfacial tension at different pressures. Besides,the effect of paraffin content in the oil mixture on the CO2equilibrium interfacial tension and their MMP have beenstudied.In this work, equilibrium IFT for (heptane + CO2),
(hexadecane + CO2), and hydrocarbon mixture (diesel fuel)with CO2 are measured at equilibrium conditions by thependant drop method. Wide ranges of pressure (0.34 to 17.24)MPa and temperature (313.15 to 393.15) K were used to assesstheir effects on the miscibility of CO2. Furthermore, themeasured pure densities of liquid and CO2 were compared withequilibrium densities to explain the different behaviors ofequilibrium IFT, which is affected by the phase densities atequilibrium conditions.
■ EXPERIMENTAL SECTIONMaterials. The CO2 used for the experiments was supplied
by Pars Balloon Co, with a mole fraction purity of > 0.999.Heptane (mole fraction purity > 0.990) and hexadecane (molefraction purity > 0.990) were provided from MERCK, anddiesel fuel was produced from the final diesel fuel blend fromShiraz Refinery. Some information of the samples used in thiswork is listed in Table 1, and diesel fuel composition ispresented in Table 2.
Density Measurement. Liquid and gas densities weremeasured using high-pressure-high-temperature vibrating tubedensitometer DMA HPM (Anton-Paar, Austria).
The schematic view of the mentioned apparatus is shown inFigure 1. It is shown that the DMA HPM is connected to the
interface module by a metal-jacketed cable. The interfacemodule that generates and measures the period of oscillation isalso responsible for temperature measurement. To evaluate themeasured raw data (oscillation period, cell temperature) of theDMA HPM (e.g., to calculate the density), these data have tobe transferred to the evaluation unit mPDS 2000 V3. Accordingto the required pressure and temperature, the piston has to beselected. The setup should be accurately designed so as to avoiddead volume, also no gas should be trapped there. Densitymeasurement in this equipment is based on the dependence ofthe period of oscillation of a constant volume unilaterally fixedU-tube on its mass. This mass consists of the U-tube materialand the mass of the fluid filled into the U-tube. This equipmentworks at high pressure and temperature, close to the real oilreservoir condition (temperatures up to 403K, and pressures upto 137.89 MPa). The equipment was initially calibrated by aseries of standard gas and liquid samples in the pressure rangeat each temperature before running the experiments.
Equilibrium Interfacial Tension Measurement. Theequilibrium interfacial tension was measured using IFT 700equipment. Figure 2 show the schematic diagram of theexperimental setup for IFT measurement between the liquidand CO2 samples.A customized design was used for high-pressure view cell of
20 cm3 inner volume to conduct interfacial tension measure-ments at elevated pressures and temperatures. The high-pressure view cell was connected by means of 1/8 in. o.d. high-pressure tubing with manual pressure isotherms correlated togenerators (HP) equipped with a pressure manometer, with 30cm3 capacity. Two containers are used to introduce differentfluids into the cell; each was used for bulk and drop injections(BT and DT). The view cell was a hollow cylindrical vessel withflat ends, which was fitted with a 36 mm diameter O-ring,which holds the sapphire windows and the view cell beside eachother. The view cell is equipped with an entry port at the top,for high-pressure tubing connections. The temperatures of thecell and containers were controlled with an electrical temper-ature sensor (PT100). These sensors are placed in the holesfilled with conductive paste, connected to the wall of the view
Table 1. Specification of Chemical Samples
chemical name sourcemole fraction
purityanalysismethoda
heptane MERCK 0.99 GChexadecane MERCK 0.99 GCcarbon dioxide Pars Balloon Co 0.999 GCaGC = gas chromatography.
dx.doi.org/10.1021/je301283e | J. Chem. Eng. Data 2013, 58, 1168−11751169
cell and the tank. A covering jacket was used to prevent loss ofheat. The overall temperature control had an accuracy of 0.1 K.The pressure was monitored by means of a pressure transducer(Keller, model PA-33X, Winterthur, Switzerland) with anoperating range up to 100 MPa (with a coverage factor k = 2).The transducer had full scale up to 70 MPa with relativeuncertainty of 0.1 %. A CCD color camera (1.4 Mpixel, macrozoom lens, and panel light) was used for monitoring the drops.The camera is supported by micrometer motion tables(rotation and translation). The equilibrium interfacial tensionwas determined from digitized images of the drops by analysissystem software which allows the fast calculation of equilibriumIFT.The o.d. of the capillary tube injector used for creating the
pendant drops was measured to be 1.5 mm with an uncertaintyof ± 0.01 mm. After each image the diameter of the capillarytube injector is selected to calibrate the software’s output.Eventually the IFT is calculated by the software based on thisscaling correction.
■ EXPERIMENTAL PROCEDURETo calculate IFT, oil and gas densities are needed to bemeasured accurately at the desired temperatures and pressures.For density measurement, DMA HPM (Anton-Paar, Austria)equipment was utilized. The cell is connected to a high-temperature circulating bath (Peter Huber Kal̈te maschinenbauGmbH, Germany), which is capable of maintaining the cell atthe desired temperature. The U-tube cell is connected to ahigh-pressure cylindrical vessel and a pump, which introducesthe fluids into the cell at the desired pressure.First, the vibrating U-tube is completely cleaned using
toluene and acetone then dried using nitrogen stream. Thecalibration of the densitometer was carried out with deionizedwater, dodecane, and nitrogen. The temperature ranges of thecalibration were (313.15 to 353.15) K for water and (313.15 to393.15) K for dodecane and nitrogen was used also. Thedensities of pure fluids, heptane, hexadecane, and diesel fuelwere then measured at various pressures (0.34 to 11.38) MPaand at five different temperatures (313.15, 333.15, 353.15,373.15, and 393.15) K. The experimental procedure wasfollowed by equilibrium IFT measurements. Since the cleaningstep is very important to measure equilibrium IFT(s); theequipment (capillary injector, line connections, bulk container,
drop and view chamber) were carefully cleaned with tolueneand acetone followed by deionized water, finally dried bypurging high grade nitrogen. The interfacial tension betweenwater and CO2 was measured by this instrument and comparedwith the data reported in the literature to verify the accuracy ofthe tests.22 This verification is reported in the SupportingInformation. According to this comparison, the accuracy of theapparatus is more than 98 %. Then, pure CO2 was injected intothe bulk tank using a syringe pump at the required pressure andthen liquid entered into the drop tank through the cylindricalvessel. The heater was set to reach the required temperature(covering jacket were installed). When the equilibriumcondition was reached (at selected temperatures andpressures), pure CO2 was slowly injected into the viewchamber. Then the view chamber was allowed to reachequilibrium condition. At this stage, the diameter of thecapillary injector, as scaling reference to calculate the IFT, wasintroduced into the software. The capillary injector is stabilizedby an antivibration device in order to be vertical to increase theaccuracy of IFT measurement. Using a pressure generatorcharge, an accurate amount of the fluid is injected from thedrop container slowly into the top of the view chamber.Once a well-shaped pendant oil drop was formed, the
sequential digital images of the dynamic pendant oil drop wereacquired and automatically stored. Simultaneously, the imageprocessing software calculates equilibrium IFT with the usersupported phase densities. The equilibrium IFT wasdetermined at the condition where the IFT did not change,which took about 45 min. To be more confident the timeperiod was extended more than 1 h. Measurements wererepeated for at least three different pendant oil drops to ensuresatisfactory repeatability at every pressure and temperature. Theuncertainty of the interfacial tension associated with theuncertainties in pressure and temperature is given by23
γ γ γ= ∂∂
+ ∂∂
⎜ ⎟⎛⎝⎜
⎞⎠⎟
⎛⎝
⎞⎠u
pu p
Tu T( ) ( ) ( )c
22
22
2
(1)
where uc is the combined standard uncertainty, u(p) = 5·10−4pis the uncertainty in pressure, and u(T) = 0.1 K is theuncertainty in temperature. In the case of the hydrocarbon−CO2 system the measurement uncertainty reached to 0.3mN·m−1. Note that the uncertainties of pressure andtemperature have been presented in the reported Tables (seethe Figures S5 to S7 of the Supporting Information). It isworthwhile to point out that measured equilibrium IFTs for the(liquid + CO2) systems at different equilibrium pressures andtemperatures were used for MMP determination as it ispresented in this work.
■ RESULTS AND DISCUSSIONDensity Measurement. In the gas injection process the
mass transfer occurs between the injected gas and the reservoirfluids. This mass transfer continues until it reaches theequilibrium condition. At equilibrium state the density ofCO2 and reservoir oil change from their initial states and effectson interfacial tension between them. To measure preciseequilibrium IFT one needs to determine accurate equilibriumdensity. In this study, the equilibrium density is compared withthe pure density of the fluids to find their effects on themeasured IFT. As it is shown in Figure 3a, the CO2 density atequilibrium condition with heptane has a small deviationcompared to the density of pure fluids, however for (CO2 +
Figure 2. Schematic of the experimental apparatus used to measureinterfacial tension: 1, view cell; 2, pressure generator; 3, pressuremanometer; 4, bulk tank; 5, drop tank.
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hexadecane) case (Figure 3b) the deviation is small up to353.15 K. The results indicate that at (373 and 393) K the linesof equilibrium densities do not overlap with pure densitiesunder the same conditions. Although they do not overlap, thesmall change (0.03 g·cm3) is because of the hexadecane’smolecular weight, as well as the fact that hexadecane evaporatesmore in CO2 at higher temperature to reach to the equilibriumstate.Density measurements of CO2, heptane, hexadecane, and
diesel fuel have been performed at different temperatures(313.15, 333.15, 353.15, 373.15, and 393.15) K and pressures(0.34 to 17.24) MPa. The obtained densities were comparedwith the data in the literature.24,25 (see Figures S2 and S3 of theSupporting Information). The measured densities of CO2increase linearly with pressure up to 4.14 MPa at alltemperatures. In this pressure range, pure and equilibriumdensities of fluids were found to be very close to each other asshown in Figure 3a. Supercritical conditions for CO2 wereexperienced with a sharp increase in the density at pressuresaround (7.07 to 11.38) MPa, until it reached high densityvalues of a semiliquid phase. This jump was sharper at lowtemperatures and has been smoother at higher temperatures,where the CO2 density at 393.15 K experienced almost linearchanges with pressure as shown in Figure 3.Accurate IFT prediction depends on the density measure-
ment which is calculated by the SRK equation of state for eachphase at the test conditions. As the densities change withpressure and temperature when the two phases are in contact,equilibrium densities are supposed to be used for the
calculation of very precise IFT; however it has smallcontribution in the IFT value and can be ignored.26 Theresults presented in Figures 3 and 4 show the equilibrium
densities for the materials used in this study at pure andequilibrium conditions. In Figure 4 the results show that as thepressure increases the pure fluid density increases linearly andequilibrium density decreases after 5.17 MPa; also the densityat the equilibrium state is less than pure density. It was alsoshown that as the temperature increases, the reduction inequilibrium densities becomes smoother. The solubility of CO2in the liquid phase is the main reason for equilibrium densityreduction. Since the CO2 solubility decreases as the temper-ature increases, the density changes become less significant. As
Figure 3. Comparison of CO2 pure densities with equilibriumdensities (ρ) at different pressures (p): (a) (CO2 + heptane); (b)(CO2 + hexadecane). ⧫, pure density at 313.15 K; Δ, pure density at333.15 K; ●, pure density at 353.15 K; ○, pure density at 373.15 K;▲, pure density at 393.15 K; −·−·−·, equilibrium density at 313.15 K;..., equilibrium density at 333.15 K; − − −, equilibrium density at353.15 K; , equilibrium density at 373.15 K; − − − (bold),equilibrium density at 393.15 K.
Figure 4. Comparison of heptane, hexadecane, and diesel fuel puredensities with equilibrium densities (ρ) at different pressures (p): (a)(Heptane + CO2); (b) (hexadecane + CO2); (c) (diesel fuel + CO2).⧫, pure density at 313.15 K; Δ, pure density at 333.15 K; ●, puredensity at 353.15 K; ○, pure density at 373.15 K; ▲, pure density at393.15 K; −·−·−·, equilibrium density at 313.15 K; ..., equilibriumdensity at 333.15 K; − − −, equilibrium density at 353.15 K; ,equilibrium density at 373.15 K; − − − (bold), equilibrium density at393.15 K.
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it is shown in Figure 4a we found out that the liquidequilibrium density has the maximum value at 5.17 MPa formost temperatures and after this point the liquid equilibriumdensity starts to decline. At this pressure the solubility of CO2 isminimum because the interfacial forces between liquid and CO2is maximum. At this point the solubility of CO2 is at the lowestlevel where the highest liquid equilibrium density was obtained.It seems that 5.17 MPa is not a suitable pressure for gasinjection into the reservoirs containing hydrocarbon paraffin,described in the following section, because the highestinterfacial forces between fluid and CO2 were noticed at thispressure. The interfacial tensions between hydrocarbon liquidsand CO2 are measured in next part to clarify this finding.Equilibrium IFT and MMP Measurements. The
interfacial tensions between CO2 and light pure hydrocarbon(heptane + CO2), heavy pure hydrocarbon (hexadecane +CO2) and multicomponent mixture (diesel fuel + CO2), weremeasured at five different temperatures (313.15, 333.15, 353.15,373.15, and 393.15) K and pressure ranges from 0.34 MPa upto the miscible condition. The equilibrium IFTs were measuredafter sufficient equilibrium time, which took about 45 min foreach test. Figure 5 verifies the results of the equilibriuminterfacial tension of (heptane + CO2) at 353.15 K comparedwith the result of Nasr-El-Din et al.,25 which shows very goodoverlap.
Figures 6 to 10 show the measured equilibrium IFTs for(heptane + CO2), (hexadecane + CO2), and (diesel fuel +CO2) systems at different equilibrium pressure ranges of (0.34
to 7.58) MPa, (0.34 to 11.38) MPa, and (0.34 to 15.51) MPa,respectively.Results presented in Figure 6 show that the measured
equilibrium IFT is reduced linearly as the pressure increases for(heptane + CO2) system. All equilibrium IFT isotherms for thissystem meet exactly at 5.52 MPa except for 313.15 K. Thecombined results at different temperature and pressureconditions for (heptane + CO2) system are shown in Figure7. Equilibrium IFT was decreased as the temperature wasincreased at the pressure ranges from (0.34 to 5.17) MPa.Interestingly, for the pressure at 5.17 MPa the equilibrium IFTsare found to be very close at different temperatures. This is an
Figure 5. Verification of (heptane + CO2) interfacial tension (γ)verses pressures (p) at 353 K which compared with previous literature:○, this work; ⧫, Nasr-El-Din et al.25
Figure 6. Measured equilibrium interfacial tensions (γ) of (heptane +CO2) system versus different pressures (p) at different temperatures:⧫, 313.15 K; Δ, 333.15 K; ▲, 353.15 K; ○, 373.15 K; ●, 393.15 K.
Figure 7. Measured equilibrium interfacial tensions (γ) of (heptane +CO2) system versus temperatures at various pressures (p). ⧫, 0.34MPa; ●, 2.76 MPa; ▲, 4.14 MPa; Δ, 4.48 MPa; ⧫, 5.17 MPa; ○, 6.2MPa; ◊, 6.9 MPa.
Figure 8.Measured equilibrium interfacial tensions (γ) of (hexadecane+ CO2) system versus different pressures (p) at different temperatures:⧫, 313.15 K; Δ, 333.15 K; ▲, 353.15 K; ○, 373.15 K; ●, 393.15 K.
Figure 9.Measured equilibrium interfacial tensions (γ) of (hexadecane+ CO2) system versus temperatures at various pressures (p): ⧫, 0.34MPa; ●, 2.07 MPa; ○, 5.17 MPa; ◊, 5.52 MPa; ▲, 8.62 MPa; ■,11.38 MPa.
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indication that a specific type of microstructure forms at thispoint in which the fluid density has the maximum value (seeFigure 4a). Thus, the temperature dependence of equilibriumIFT relies highly on pressure; however, at the 5.17 MPa thechanges are negligible. The trend of equilibrium IFTs isinverted for the pressures above 6.20 MPa as the temperature isincreased. These results indicate that CO2 injection isrecommended for the reservoir cases with pressures higherthan 6.20 MPa and low temperature, whereas reservoirs withpressures less than 6.20 MPa at high temperatures maintainbetter conditions for CO2 injection. On the basis of themeasured data (symbols) in Figure 6, the equilibrium IFT (γ) iscorrelated to the equilibrium pressure for (heptane + CO2) byapplying the linear regression. (see Table S8 of the SupportingInformation).The trend of equilibrium IFTs obtained for (hexadecane +
CO2) system is almost the same as (heptane + CO2). It isshown in Figure 8 that the IFT decreases linearly as thetemperature increases at pressure ranges from (0.34 to 5.17)MPa except for, as we noticed, different behavior at 313.15 K.The equilibrium IFT is increased at the pressure range from(7.24 to 11.38) MPa by increasing temperature. At pressuresfrom (5.17 to 6.2) MPa IFTs are found to be close to eachother except at a temperature of 313.15 K. It is noticed at313.15 K as the pressure increases, lower equilibrium IFT wasmeasured because of the change in equilibrium density, whichwas already described for the (heptane + CO2) case. Besides,higher pressure was needed to obtain (hexadecane + CO2)equilibrium IFT near miscible conditions compared to thepressure needed to obtain (heptane + CO2). As it is shown inFigure 9 the equilibrium IFT for (hexadecane + CO2) is almostthe same at 5.17 and 5.52 MPa in all temperature ranges, whichshows that at these pressures a stable interface is formedbetween CO2 and hexadecane which is not affected bytemperature.On the basis of the measured data (symbols) in Figure 8, the
equilibrium IFT (γ) is correlated to the equilibrium pressure for(hexadecane + CO2) by applying the linear regression (see theTable S9 of the Supporting Information).To check the effects of multicomponent mixture, diesel fuel
was selected as a multicomponent mixture in the next tests.Figure 10 shows the measured IFT at the range of pressuresfrom (0.34 to 15.51) MPa for diesel fuel. It can be seen that themeasured IFTs with respect to pressure create two differentlines as it has already been mentioned by Gu et al.27 They haveshown that the second slope is related to interfacial tension
between crude oil’s heavy components and CO2, and also theyindicate when the second slope is extrapolated to zero, thispoint is close to the so-called first-contact miscibility pressure.The obtained data of the first line are in the range of P = (0.34to 7.24) MPa at 313.15 K, P = (0.34 to 8.62) MPa at 333.15 K,P = (0.34 to 10) MPa at 353.15 K, P = (0.34 to 11.38) MPa at373.15 K, and P = (0.34 to 11.38) MPa at 393.15 K, and thesecond line refers to P = (8.62 to 11.38) MPa at 313.15 K, P =(10 to 12.75) MPa at 333.15K, P = (11.38 to 14.13) MPa at353.15 K, P = (12.75 to 15.51) MPa at 373.15 K, and P =(12.75 to 15.51) MPa at 393.15 K. The equilibrium IFTreduction is attributed to the increased solubility of CO2 in thediesel fuel with pressure. Diesel fuel as a multicomponentmixture contains light and heavy constituents. The first slope isthe indication of lighter components presented at the interfacewith CO2, and the second slope indicates the presence ofheavier components after the light components are extracted byCO2. Comparison of equilibrium IFTs for (heptane + CO2),(hexadecane + CO2), and (diesel fuel + CO2) systems at 393.15K are shown in Figure 11 (for other temperatures see the
Figure S4 of the Supporting Information) The interfacialtensions obtained for (hexadecane + CO2), and (diesel fuel +CO2) systems are close to some extent at each temperature.Accordingly, it can be concluded that the heavier componentsof diesel fuel tend to expose to the interface and this tendencyincreases as the pressure increases. Besides, the slope of IFTlines with respect to pressure for heptane, hexadecane, and thefirst slope of diesel fuel are almost the same at constanttemperature. Thus, it is concluded that the paraffins are mostlypositioned at the (oil + CO2) interface when the oil is amulticomponent mixture.The MMP was determined by applying the VIT technique
with extrapolation of measured IFT to zero.17 The MMPs for(hexadecane + CO2), (heptane + CO2) and (diesel fuel + CO2)are compared in Figure 12 indicating very close values for theMMP of the diesel fuel and hexadecane. It is clearly shown thatthe MMPs increase as the temperature is increased. Also theMMP values are linear with R-square = 0.99 for all oil samples.
■ CONCLUSIONSThe effects of temperature on the equilibrium interfacialtension between the CO2 and three different oil fluids atdifferent pressures have been investigated. Additionally, theMMP was measured by the VIT technique to determine the
Figure 10.Measured equilibrium interfacial tensions (γ) of (diesel fuel+ CO2) system versus different pressures (p) at different temperatures:⧫, 313.15 K; Δ, 333.15 K; ▲, 353.15 K; ○, 373.15 K; ●, 393.15 K.
Figure 11. Comparison between equilibrium interfacial tensions (γ)versus pressure (p) for (heptane + CO2), (hexadecane + CO2), and(diesel fuel + CO2) systems at 393.15 K: ●, hexadecane; ▲, dieselfuel; ⧫, heptane.
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temperature effect on MMP when injecting CO2 to thereservoir. It was found that low pressures (up to 5.17 MPa) hadthe best results at higher temperatures for CO2 oil recoverybecause the interfacial tensions for all samples were decreasedas the temperature was increased. At a pressure of 5.17 MPa theinterfacial tension is independent of the temperature, and astable interface is formed; therefore it is concluded that thispressure is not recommended for CO2 injection. When thepressure increased even more, the reservoirs with lowertemperature were better candidates for CO2 injection becausea positive slope for IFTs versus temperature was observed.The experimental results showed that IFTs versus pressure
for (hexadecane + CO2) and (diesel fuel + CO2) systems werealmost the same, but IFT changed in high pressures in whichmost of the paraffin compositions turned miscible in CO2, andif we consider the above-mentioned high pressure a turningpoint, at pressures higher than the above-mentioned pressurethe slope of diesel fuel’s IFT versus pressure has changed. Thechanges of diesel fuel IFT in high pressures showed thepresence of nonparaffin substances in the interface, and thereare two reasons for this. First the findings of this researchshowed that the slopes of the IFT between two different kindsof paraffin and CO2 versus pressure at constant temperaturewere the same, and the slope of these IFTs were different fromthe slope of IFT versus pressure between diesel fuel and CO2 inhigh pressures. Second at these high pressures the two differentkinds of paraffin were miscible in CO2. This conclusion wasbased on the MMP of hexadecane, which was the heaviestcomponent of the different kinds of paraffin.Since the first slope of diesel fuel IFT versus pressure is
necessary for measuring MMP, in this research we tried todetermine what substance has the biggest effect on this slope.We realized that different kinds of paraffin had the highestinfluence, because the slope of IFT versus pressure betweendifferent kinds of paraffin and CO2 was the same as the slope ofIFT versus pressure between CO2 and diesel fuel.In conclusion, the results showed that the trend of IFT
versus pressure between two kinds of paraffin and CO2 werethe same at each constant temperature. Also the sameconditions existed for a multicomponent mixture containingparaffin, which shows the influence of paraffin in the slope ofIFT versus pressure between CO2 and a multicomponentmixture. The reduction of the slopes of IFTs versus pressure forCO2 and other compositions are observed as the temperature isincreased. According to this finding we can conclude that the
MMPs of CO2 with heptane, hexadecane, and diesel fuelincreased as the temperature was increased.
■ ASSOCIATED CONTENT*S Supporting InformationAdditional tables and data as described in the text. This materialis available free of charge via the Internet at http://pubs.acs.org.
Present Address§S.A.: Sharif University of Technology, Tehran, Iran.
FundingFinancial support from Enhanced Oil Recovery (EOR) Centreand The Colleges of Engineering is greatly acknowledged.
NotesThe authors declare no competing financial interest.
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Journal of Chemical & Engineering Data Article
dx.doi.org/10.1021/je301283e | J. Chem. Eng. Data 2013, 58, 1168−11751175
