Effects of Four Important Factors on the Measured Minimum Miscibility


Apr 8, 2013 - University of Regina, Regina, Saskatchewan S4S 0A2, Canada. ABSTRACT: The vanishing interfacial tension (VIT) technique is applied to ...
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Effects of Four Important Factors on the Measured Minimum Miscibility Pressure and First-Contact Miscibility Pressure Yongan Gu,* Pengcheng Hou, and Weiguo Luo Petroleum Technology Research Centre (PTRC), Petroleum Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan S4S 0A2, Canada ABSTRACT: The vanishing interfacial tension (VIT) technique is applied to determine the minimum miscibility pressures (MMPs) and first-contact miscibility pressures (Pmax) of five light crude oil−CO2 systems from the measured equilibrium interfacial tensions (IFTs) at different equilibrium pressures. The equilibrium IFTs are measured under various experimental conditions by applying the axisymmetric drop shape analysis technique for the pendant drop case. It is found that in each IFT test, the measured equilibrium IFT is reduced almost linearly with the equilibrium pressure in two pressure ranges. The MMP of each light crude oil−CO2 system is thus determined from the measured equilibrium IFTs in range I by applying the VIT technique. The first-contact miscibility pressure (Pmax) of each light crude oil−CO2 system is extrapolated from the measured equilibrium IFTs in range II. Moreover, the test temperature, crude oil composition (dead/live oil), gas composition (pure/impure CO2), and initial gas−oil ratio (GOR) are studied to examine their specific effects. The experimental data show that the measured MMPs and Pmax values of five light crude oil−CO2 systems increase linearly with the temperature. The presence of CH4 in the CO2 phase results in substantially higher MMP and Pmax. An increased initial GOR leads to marginally higher MMP and Pmax.



INTRODUCTION For a long time gas injection has been used to displace the residual oil, reduce the oil viscosity, induce the oil-swelling effect, and maintain the reservoir pressure1,2 in an effective enhanced oil recovery (EOR) method. During the gas injection process, the interfacial mass transfer occurs across the interface between the residual oil phase and the injected gas phase until they reach an equilibrium state. As a result, the physicochemical properties of the displacing gas and displaced oil become similar, leading to an efficient displacement process.3 The concept of miscibility is introduced and used for a gas injection process when the interfacial tension between the two phases of interest (i.e., the oil and gas phases) approaches zero and there is not an interface formed between these two phases. The minimum miscibility pressure (MMP) is defined as the lowest operating pressure at which the injected gas and the residual oil-in-place (ROIP) become miscible after a dynamic multicontact process at the reservoir temperature.4 At a pressure higher than the MMP, miscibility can be achieved through a vaporizing process, a condensing process, or a combination of the two.5,6 To ensure a miscible gas flooding process with a high oil recovery factor, accurate determination of the MMP is important in the design and implementation of a gas injection EOR project in a field application. Among many gas injection projects, the carbon dioxide (CO2) flooding process is often applied under the miscible or near-miscible flooding conditions to enhance the oil recovery. In comparison with the other solvents used for the gas flooding © 2013 American Chemical Society

processes, CO2 in its supercritical state can enter the oil-bearing zones that are not swept by the previously injected water and thus displace the trapped oil. Besides, a large fraction of the injected CO2 can be stored underground, which is beneficial to the environment.7 During and after the CO2 injection process, the injected CO2 and the residual crude oil interact with each other. At a reservoir pressure close to or higher than the MMP, the injected CO2 is dissolved into the crude oil to dilute it and reduce its viscosity. Meanwhile, the light-hydrocarbons (HCs) in the crude oil can be extracted by CO2. After the first contact of CO2 and crude oil, the light-HCs enriched CO2 advances to contact the fresh crude oil due to its high mobility and becomes even more enriched. After adequate and multiple contacts, ultimately, the light-HCs enriched CO2 becomes miscible with the fresh crude oil at the displacement front. If the injection pressure is sufficiently high, in principle, the injected CO2 can become miscible with the residual oil at the first contact and this pressure is referred to as the first-contact miscibility pressure.8 Several experimental methods for determining the MMP have been developed and described in the literature. The slimtube test method is considered to be a standard method for measuring the MMP in the petroleum industry.9,10 The major component of the experimental apparatus for the slim-tube test Received: February 1, 2013 Accepted: March 28, 2013 Published: April 8, 2013 1361

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in Table 1. The mass fraction of asphaltene content of the cleaned light crude oil was measured to be wasp = 0.0026

is a long (2 to 40 m) coiled slim tube packed with proper packing materials (e.g., the Ottawa sands or glass beads). After the coiled slim tube is saturated with the crude oil to be tested, CO2 is injected through it to displace the crude oil at different injection pressures and at the actual reservoir temperature. At the end, the measured oil recovery factor is plotted versus the injection pressure, and then the MMP is determined to be the break-over pressure, above which the oil recovery factor almost reaches its maximum value.11 This method simulates the multiphase fluid flow through the porous media under the actual reservoir conditions. However, it is extremely timeconsuming and expensive as well. Alternatively, a series of highpressure CO2 coreflood tests can be used to measure the MMP in a similar manner.12 The third experimental method for determining the MMP uses the rising-bubble apparatus (RBA), which is commonly used as a fast and cost-effective alternative to the slim-tube method.13 In this method, a gas (such as CO2) bubble is injected into a thin transparent column of the crude oil at a different pressure each time. The MMP is inferred from the pressure dependence of the rising-bubble behavior (e.g., size, shape, or color of the gas bubble) inside the oil column. In general, the MMP is determined to be the pressure at or above which the rising-gas bubble ultimately disappears in its rising process. The RBA method is fast and requires a small amount of the crude oil, in comparison with the slim-tube or coreflood test. However, the RBA method simulates the vaporizing process alone in the miscibility development process and neglects the condensing process. This leads to an overestimated MMP of some crude oil−CO2 systems, in which the condensing process also contributes to the miscibility development. More recently, an experimental method called the vanishing interfacial tension (VIT) technique has been used to determine the MMP.14−16 In experiment, the equilibrium interfacial tensions (IFTs) between an oil phase and a gas phase can be accurately measured at different equilibrium pressures and at the actual reservoir temperature by applying the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case. Thus the MMP is determined by applying the VIT technique, which is based on the concept that the equilibrium interfacial tension between the two phases approaches zero when they become miscible. In the literature, it is also proven that the zero interfacial tension is a necessary condition for the miscibility development.17,18 Each MMP determination by using the VIT technique can be accomplished in (4 to 6) h, whereas the slim-tube test usually takes (4 to 6) weeks.9 In this paper, the VIT technique is applied to determine the MMPs and the first-contact miscibility pressures (Pmax) of five dead/live crude oil−pure/impure CO2 systems at different experimental conditions. The following four important factors are experimentally studied to evaluate and compare their detailed effects on the measured MMP and Pmax: the test temperature, oil and gas compositions, and initial gas−oil ratio (GOR) in volume.

Table 1. Physical and Chemical Properties of the Pembina Cardium Light Crude Oil (9-11-48-9W5), Where ρoil and μoil Are the Oil Density and Viscosity; (SG)oil, (MW)oil, and wasp Are the Oil Specific Gravity, Molecular Weight, and Asphaltene Content in Mass Fraction, Respectively oilfield crude oil ρoil @ 300.15 K/(g·cm−3) μoil @ 300.15 K/(mPa·s) (SG)oil/(°API) (MW)oil/(g·mol−1) wasp

Pembina Cardium light 0.835 ± 0.001 5.53 ± 0.01 37.96 ± 0.01 212.1 ± 0.1 0.0026 ± 0.0001 (normal-pentane insoluble)

(normal-pentane insoluble) by using the standard ASTM D2007-03 method and filter papers (Whatman No. 5, England) with a pore size of 2.5 μm. The compositional analysis result of the light crude oil was obtained by using the standard ASTM D86 and is given in Table 2. Carbon dioxide was purchased Table 2. Compositional Analysis Result of the Pembina Cardium Light Crude Oil (9-11-48-9W5) with the Analysis Uncertainty of ±0.0001 in Mole Fraction



Cn

mole fraction

Cn

mole fraction

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C15 C16 C17 C18 C19 C20 C21 C22 C23 C24 C25 C26

0.0000 0.0000 0.0020 0.0117 0.0367 0.0501 0.1067 0.0720 0.0761 0.0695 0.0575 0.0501 0.0459 0.0397 0.0374 0.0298 0.0308 0.0309 0.0201 0.0207 0.0196 0.0114 0.0155 0.0128 0.0127 0.0114

C27 C28 C29 C30 C31 C32 C33 C34 C35 C36 C37 C38 C39 C40 C41 C42 C43 C44 C45 C46 C47 C48 C49 C50+ total

0.0107 0.0094 0.0089 0.0064 0.0068 0.0059 0.0046 0.0038 0.0054 0.0047 0.0030 0.0027 0.0037 0.0028 0.0027 0.0022 0.0022 0.0020 0.0020 0.0015 0.0014 0.0013 0.0013 0.0335 1.0000

from Praxair (Canada) and normal-pentane was purchased from VWR International (Canada). Four hydrocarbon solvents, methane, ethane, propane, and normal-butane, were all purchased from Praxair (Canada). The purities of these six solvents used in this study are listed in Table 3. Reconstitution of Live Oil Sample. The live oil was reconstituted by saturating the dead oil with the produced hydrocarbon (HC) gas. The actual produced gas composition is listed in Table 4, which was provided by PennWest Exploration.

EXPERIMENTAL SECTION Materials. In this study, the original light crude oil was collected from the Pembina Cardium oilfield in Alberta, Canada. The obtained original crude oil was cleaned by using a centrifuge (Allegra X-30 Series, Beckman Coulter) in order to remove any sands and brine. Some major physical and chemical properties of the Pembina Cardium Crude oil sample are listed 1362

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Table 3. Purities of Six Solvents Used in This Study solvent

purity (mole fraction)

CO2 normal-C5H12 CH4 C2H6 C3H8 normal-C4H10

0.99998 ± 0.00001 0.9976 ± 0.0001 0.9997 ± 0.0001 0.99 ± 0.01 0.995 ± 0.001 0.995 ± 0.001

Table 4. Compositional Analysis Results of the Actual, Nominal, and Synthetic Produced Gas Samples with the Analysis Uncertainty of ±0.0001 in Mole Fraction component

actual (mole fraction)

nominal (mole fraction)

synthetic (mole fraction)

C1 C2 C3 iso-C4 normal-C4 iso-C5 normal-C5 C6 C7+ N2 CO2

0.6650 0.1141 0.1139 0.0183 0.0390 0.0068 0.0086 0.0055 0.0054 0.0148 0.0086

0.6650 0.1141 0.1139

0.7020 0.1002 0.1026

0.1070

0.0906

Figure 1. Measured equilibrium pressure of the Pembina Cardium live crude oil−produced gas mixture versus its volume data at T = 294.15 K.

Preparation of Two Impure CO2 Samples. In this study, two different impure CO2 samples were prepared by mixing pure CO2 with pure CH4 at a predetermined approximate composition of 0.8 CO2 + 0.2 CH4 or 0.9 CO2 + 0.1 CH4 in mole fraction. For example, the first impure CO2 sample with the composition of 0.8 CO2 + 0.2 CH4 was prepared as follows. First, two cylinders (500-10-P-316-2, DBR, Canada) with the volume of 1000 cm3 each were cleaned and vacuumed at the room temperature T = 294.15 K. Pure CO2 was injected into one of the two cylinders by using a displacement pump until its pressure reached P1 = 3.06 MPa. Second, pure CH4 was injected into another cylinder by using a displacement pump until its pressure reached P2 = 0.67 MPa. These two respective pressures were calculated by using the P−R EOS so as to prepare the first impure CO2 sample with the targeted composition of 0.8 CO2 + 0.2 CH4. Third, pure CH4 in the second cylinder was injected into the first CO2 cylinder, which was then further pressurized to P3 = 10.00 MPa to ensure that such a prepared CO2−CH4 mixture was in a homogeneous phase. Thus the first impure CO2 sample with the targeted composition of 0.8 CO2 + 0.2 CH4 was obtained. The second impure CO2 sample with the targeted composition of 0.9 CO2 + 0.1 CH4 was prepared in a similar way. After each impure CO2 sample was prepared, the CO2−CH4 gas mixture was sampled and stored in a miniature sample cylinder (SS-4CS, Swagelok, Canada). The gas sample was then analyzed by applying GC. The actual compositions of these two impure CO2 samples are found to be 0.7487 CO2 + 0.2513 CH4 in mole fraction for the first impure CO2 sample and 0.8406 CO2 + 0.1594 CH4 in mole fraction for the second impure CO2 sample, respectively. IFT Measurement. Figure 2 shows a schematic diagram of the experiment setup used for measuring the dynamic/ equilibrium IFT between the dead/live crude oil and pure/ impure CO2 by applying the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case.20 The major component of this experimental setup was a see-through windowed high-pressure IFT cell (IFT-10, Temco) with a net volume of Vcell = 49.5 cm3. A stainless steel syringe needle was installed at the top of the IFT cell and used to form a pendant oil drop. The light crude oil and CO2 were stored in two transfer cylinders (500-10-P-316-2, DBR), heated to a

0.0046

The actual produced gas composition was simplified as the nominal composition in Table 4 by lumping the HCs whose carbon numbers are equal to or larger than 4 (i.e., C4+) and the non-HCs together as normal-C4. The man-made or synthetic produced HC gas was prepared in laboratory to reconstitute the live oil and its composition is given in Table 4. With the synthetic produced HC gas sample, the live oil was reconstituted by using the actual gas−oil ratio (GOR) of 15:1 in volume under the standard conditions. The detailed experimental procedure for reconstituting the live oil sample is described below. First, a high-pressure cylinder (500-10-P316-2, DBR) with the volume of 1000 cm3 was cleaned and kept inside an air bath at T = 326.15 K. A total of 500 cm3 dead oil sample was injected into the cylinder by using a highpressure displacement pump. Second, the synthetic produced gas was injected into another cleaned and vacuumed cylinder with the volume of 500 cm3 until the gas pressure inside the cylinder reached P1 = 1.50 MPa. This pressure was calculated by applying the Peng−Robinson equation of state (P−R EOS)19 so as to reach the actual GOR of 15:1 in volume for the live oil. Third, the produced HC gas was compressed and injected into the oil cylinder to saturate the dead oil. Finally, the high-pressure cylinder containing the dead oil−synthetic produced gas mixture was kept rotating continuously for at least 7 d to fully mix the dead oil with the synthetic produced gas. The saturation pressure of such reconstituted live oil was determined to be Psat = 2.03 MPa at 294.15 K from two linear regression lines of the measured equilibrium pressure of the live oil−produced HC gas mixture versus its volume data in two different regions, as shown in Figure 1. After the live crude oil was reconstituted, the live crude oil−produced gas mixture inside the high-pressure cylinder was further compressed until its pressure reached P2 = 3.50 MPa, which was far above the measured saturation pressure in order to eliminate the HC gas cap. The live oil sample cylinder was positioned vertically for the live oil−CO2 IFT measurement. 1363

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Figure 2. Schematic diagram of the experimental setup used for measuring the dynamic/equilibrium interfacial tension (IFT) between a pendant dead/live light crude oil drop and pure/impure CO2 by applying the axisymmetric drop shape analysis (ADSA) technique for the pendant drop case.

sufficiently long period (e.g., 1 h) and reach their equilibrium state, which was indicated by the constant equilibrium pressure at the prespecified constant test temperature. It is worthwhile to note that in this study, the ratio of the initial gas phase volume (Vgas) to the initial oil phase volume (Voil) inside the high-pressure IFT cell was defined as the initial gas−oil ratio (GOR) in volume, i.e., Vgas:Voil, prior to each IFT test. In the case for the highest initial GOR of 4000:1, for example, no oil was preinjected before the IFT test. Thus the entire highpressure IFT cell was occupied by pure/impure CO2 in this special case, i.e., Vgas = Vcell = 49.5 cm3. If the average pendant oil drop size was assumed to be Voil = 12.375 × 10−3 cm3, the initial GOR was approximated to be Vgas:Voil = 4000:1 in volume. In the case for the lowest initial GOR of 3:1, for instance, the light crude oil with the volume of 0.25Vcell (i.e., Voil = 0.25Vcell) was injected into the high-pressure IFT cell. The remaining IFT cell space (i.e., Vgas = 0.75Vcell) was occupied by pure/impure CO2 in this case so that the initial GOR was equal to Vgas:Voil = 3:1 in volume. After the injected crude oil and CO2 phases inside the IFT cell reached an equilibrium state, a pendant oil drop was generated at the tip of the syringe needle and surrounded by pure/impure CO2. The dynamic IFT measurement was commenced. Once a well-shaped pendant oil drop was formed, the sequential digital images of the dynamic pendant oil drop at different times were acquired and stored automatically in the personal computer. The time interval for the sequential digital image acquisition was set to be smaller at the beginning and larger when the pendant oil drop and surrounding CO2 were close to an equilibrium state. Finally, the ADSA program for the pendant drop case was executed to analyze the digital image of the dynamic pendant drop. This program requires the local gravitational acceleration and the density difference between two immiscible phases as the input data. The output data include the dynamic IFT, the profile, volume, and surface area of the dynamic pendant liquid drop at any time.

prespecified constant test temperature, and injected into the IFT cell to reach a prespecified initial GOR in volume at a targeted equilibrium/test pressure. The detailed IFT experimental procedure will be described below. After the injected light crude oil and CO2 reached an equilibrium state, the light crude oil was introduced from its transfer cylinder to the syringe needle to form an extremely small pendant oil drop for the IFT measurement. A light source and a glass diffuser (240− 341, Dyna−Lume) were used to provide uniform and sufficient illumination for the pendant oil drop. A monochrome microscope camera (MZ6 Leica) was used to observe and capture the sequential digital images of the dynamic pendant oil drop inside the IFT cell at different times. The high-pressure IFT cell was positioned horizontally between the light source and the microscope camera. The entire ADSA system and the IFT cell were placed on a vibration-free table (RS4000, Newport). The digital image of the dynamic pendant oil drop surrounded by CO2 at any time was acquired in a tagged image file format (TIFF) by using a digital frame grabber (Ultra II, Coreco Imaging) and automatically stored in a DELL desktop computer. Prior to each IFT measurement, the high-pressure cell was first cleaned with kerosene, then flushed with nitrogen, and finally vacuumed. It was preheated to an elevated test temperature by using a heating belt with an autotransformer (3PN1010B, STACO Energy Products Co.) and a thermometer (type K, DDT2, Supco.). After the temperature inside the high-pressure cell reached the prespecified constant test temperature, CO2 was injected slowly into the IFT cell from its bottom port by using a manual displacement pump (PMP500-1-10-HB-316-MO-CO, DBR) to reach a high pressure. Then a certain amount of the light crude oil was injected into the IFT cell through the syringe needle at its top port to reach a targeted initial GOR in volume by using a programmable syringe pump (100DX, ISCO Inc.). An extremely low oil injection rate of 6.0 cm3·h−1 was used so that the oil and gas phases inside the IFT cell could interact with each other over a 1364

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Before the pendant oil drop and the surrounding CO2 reached an equilibrium state, the measured dynamic IFT changed with time due to the dissolution of CO2 into the pendant oil drop and the light-HCs extraction by CO2. After approximately 0.5 h, the small pendant oil drop and surrounding CO2 phase reached an equilibrium state, at which the measured dynamic IFT was unchanged and an equilibrium IFT was achieved. In this study, the dynamic IFT measurement was repeated for at least three different pendant oil drops. Only the average value of the equilibrium IFTs of three repeated IFT measurements at the same test conditions was noted and is presented in this paper. The measurement error for such an average equilibrium IFT (γeq) was found to be ±0.05 mJ·m−2 at a prespecified equilibrium pressure (Peq) and a prespecified constant temperature (T), whose measurement errors were equal to ±0.01 MPa and ±0.1 K. After each equilibrium IFT at a lower prespecified equilibrium pressure was measured, more pure/impure CO2 was injected to further pressurize the high-pressure IFT cell. The equilibrium IFT measurement was repeated at a higher prespecified equilibrium pressure by following the same IFT experimental procedure, while the test temperature was kept to be constant in each IFT test. The IFT test was continued in a progressively increasing equilibrium pressure manner until either the measured equilibrium IFT (γeq) reached (1.00 to 2.00) mJ·m−2 or the tested equilibrium pressure (Peq) was as high as 26.00 MPa. For a new dynamic/equilibrium IFT test at a different constant temperature or initial GOR, the above-described experimental steps were repeated and the progressively increasing equilibrium pressure manner was followed from the beginning. In this study, a total of 15 IFT tests for five dead/live crude oil−pure/impure CO2 systems were conducted under different test conditions (T and GOR), which are summarized in Table 5. More specifically, in each IFT test, the equilibrium pressure was tested and increased progressively in a large range of Peq = (0.10 to 26.00) MPa. The constant test temperature was chosen to be T = 294.15, 313.15, and 326.15 K and the initial GOR (Vgas:Voil) was set to be 4000:1, 200:1, 10:1, and 3:1 in volume, respectively. Table 5 also lists the measured minimum miscibility pressures (MMPs) and first-contact miscibility pressures (Pmax) for these 15 IFT tests. These experimental results and the detailed effects of four important factors on the measured MMP and Pmax will be presented and discussed in the subsequent section.

Table 5. Summary of 15 IFT Tests for Five Dead/Live Light Crude Oil−Pure/Impure CO2 Systems at Different Test Temperatures and Initial Gas−Oil Ratios (GORs) in Volumea test no.

T/K

oil

1 2 3 4

294.15 313.15 326.15 294.15

dead dead dead dead

5

326.15

dead

6

326.15

dead

7 8 9 10

294.15 313.15 326.15 294.15

live live live live

11

313.15

live

12

326.15

live

13 14 15

326.15 326.15 326.15

dead dead dead

gas/mole fraction

GOR (Vgas:Voil)

MMP/ MPa

Pmax/MPa

CO2 (0.99998) CO2 (0.99998) CO2 (0.99998) 0.7487 CO2 + 0.2513 CH4 0.8406 CO2 + 0.1594 CH4 0.7487 CO2 + 0.2513 CH4 CO2 (0.99998) CO2 (0.99998) CO2 (0.99998) 0.7487 CO2 + 0.2513 CH4 0.7487 CO2 + 0.2513 CH4 0.7487 CO2 + 0.2513 CH4 CO2 (0.99998) CO2 (0.99998) CO2 (0.99998)

4000:1 4000:1 4000:1 4000:1

6.9 9.1 10.6 14.3

10.7 16.5 23.2 20.8

4000:1

16.3

26.7

4000:1

21.4

29.8

4000:1 4000:1 4000:1 4000:1

7.0 9.3 12.5 15.3

9.9 15.1 23.6 22.3

4000:1

17.6

25.6

4000:1

21.7

30.4

200:1 10:1 3:1

10.4 10.3 9.7

23.5 22.6 22.7

Notes: The uncertainty of T = ± 0.1 K; the uncertainty of impure CO2 composition = ± 0.0001 in mole fraction; the uncertainty of MMP = ± 0.1 MPa; and the uncertainty of Pmax = ± 0.1 MPa.

a

sensitive to the test temperature at GOR = 4000:1, irrespective of the crude oil−CO2 system tested. It is also found from Figures 3(a to d) that in general, the measured equilibrium IFT reduces more slowly with the equilibrium pressure at a higher test temperature, which results in a higher MMP or Pmax. This is because CO2 solubility in the crude oil is lower at a higher test temperature if the equilibrium pressure is kept the same. Moreover, a lower CO2 solubility leads to a larger density difference between the crude oil phase with less dissolution of CO2 and the CO2 phase. Hence, at a higher test temperature, a higher operating pressure (MMP or Pmax) is required for the crude oil−CO2 system to develop the multi-contact or firstcontact miscibility. With the above-described experimental results, the temperature effects on the MMP and Pmax determined by applying the VIT technique are further plotted in Figure 4. On the basis of the measured data points (symbols) in this figure, the MMP (MPa) and Pmax (MPa) for the four dead/live oil−pure/impure CO2 (0.7487 CO2 + 0.2513 CH4) systems at GOR = 4000:1 are correlated to the test temperature T (K) by applying the linear regression:



RESULTS AND DISCUSSION Temperature Effects. As shown in Figure 3a, the measured MMP increases from 6.9, 9.1, to 10.6 MPa when the test temperature increases from 294.15, 313.15, to 326.15 K for the Pembina Cardium dead oil−pure CO2 system. Figure 3b depicts that the measured MMP for the dead oil−impure CO2 (0.7487 CO2 + 0.2513 CH4) system increases from 14.3 to 21.4 MPa when the test temperature increases from 294.15 to 326.15 K. A large increase in the MMP for the dead oil−impure CO2 system is attributed to the presence of CH4 in the impure CO2 phase at the initial GOR of 4000:1. Figures 3c and 3d show that the measured MMP for the live oil−pure/impure CO2 system is always higher than that for the dead oil−pure/ impure CO2 system. It is noted from Figures 3(a to d) that the measured first-contact miscibility pressures (Pmax) increase with the test temperature much faster than the measured MMPs for the four dead/live oil−pure/impure CO 2 systems. In conclusion, both the measured MMP and Pmax are rather

MMP/MPa = 0.116(T /K) − 27.1 (dead oil−pure CO2 system)

(1a)

MMP/MPa = 0.222(T /K) − 51.0 (dead oil−impure CO2 system)

(1b)

MMP/MPa = 0.168(T /K) − 42.7 (live oil−pure CO2 system) 1365

(1c)

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Figure 3. Measured equilibrium IFTs at different equilibrium pressures and GOR = 4000:1 for (a) the Pembina Cardium dead oil−pure CO2 system: ▼, test no. 1 (T = 294.15 K, MMP = 6.9 MPa, and Pmax = 10.7 MPa); ○, test no. 2 (T = 313.15 K, MMP = 9.1 MPa, and Pmax = 16.5 MPa); ●, test no. 3 (T = 326.15 K, MMP = 10.6 MPa, and Pmax = 23.2 MPa). (b) The Pembina Cardium dead oil−impure CO2 (0.7487 CO2 + 0.2513 CH4) system: ●, test no. 4 (T = 294.15 K, MMP = 14.3 MPa, and Pmax = 20.8 MPa); ○, test no. 6 (T = 326.15 K, MMP = 21.4 MPa, and Pmax = 29.8 MPa). (c) The Pembina Cardium live oil−pure CO2 system: ▼, test no. 7 (T = 294.15 K, MMP = 7.0 MPa, and Pmax = 9.9 MPa); ○, test no. 8 (T = 313.15 K, MMP = 9.3 MPa, and Pmax = 15.1 MPa); ●, test no. 9 (T = 326.15 K, MMP = 12.5 MPa, and Pmax = 23.6 MPa). (d) The Pembina Cardium live oil−impure CO2 (0.7487 CO2 + 0.2513 CH4) system: ▼, test no. 10 (T = 294.15 K, MMP = 15.3 MPa, and Pmax = 22.3 MPa); ○, test no. 11 (T = 313.15 K, MMP = 17.6 MPa, and Pmax = 25.6 MPa); ●, test no. 12 (T = 326.15 K, MMP = 21.7 MPa, and Pmax = 30.4 MPa).

It is found that in general, the measured MMP and Pmax increase almost linearly with the test temperature in the range of T = (294.15, 313.15, to 326.15) K. In the literature, an almost linear correlation between the MMP and temperature was reported up to 423.15 K.21 More specifically, in this study, the MMP increases linearly with an increasing temperature at the rates of (0.116, 0.222, 0.168, and 0.194) MPa/K and Pmax increases linearly with an increasing temperature at the rates of (0.384, 0.281. 0.417, and 0.247) MPa/K for the abovementioned four dead/live oil−pure/impure CO2 (0.7487 CO2 + 0.2513 CH4) systems, respectively. Hence, the reservoir temperature has strong effects on the multicontact minimum miscibility pressure (MMP) and first-contact miscibility pressure (Pmax) of a given crude oil−CO2 system. Precisely speaking, the MMP for the dead/live oil−impure CO2 system is more sensitive to the test temperature than that for the dead/ live oil−pure CO2 system. As expected, Pmax increases with temperature more quickly than the MMP. Moreover, the addition of 0.2513 CH4 (mole fraction) into pure CO2

MMP/MPa = 0.194(T /K) − 42.2 (live oil−impure CO2 system)

(1d)

and Pmax /MPa = 0.384(T /K) − 102.8 (dead oil−pure CO2 system)

(2a)

Pmax /MPa = 0.281(T /K) − 61.9 (dead oil−impure CO2 system)

(2b)

Pmax /MPa = 0.417(T /K) − 113.5 (live oil−pure CO2 system)

(2c)

Pmax /MPa = 0.247(T /K) − 50.8 (live oil−impure CO2 system)

(2d) 1366

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Figure 4. Measured MMPs and Pmax values of the Pembina Cardium dead/live oil−pure/impure CO2 (0.7487 CO2 + 0.2513 CH4) systems at different temperatures [T = (294.15, 313.15, and 326.15) K] and GOR = 4000:1: ●, live oil−impure CO2 system; ○, dead oil−impure CO2 system; ▼, live oil−pure CO2 system; ▽, dead oil−pure CO2 system.

dramatically increases the measured MMP and Pmax. On the other hand, the live oil−pure/impure CO2 system has apparently but marginally higher MMP and Pmax than those for the dead oil−pure/impure CO2 system at GOR = 4000:1. Oil Composition Effects. As shown in Figure 5a, the measured MMP for the live oil−pure CO2 system is moderately higher than that for the dead oil−pure CO2 system because the live oil sample was saturated with the synthetic produced HC gas (0.7020 CH4 + 0.1002 C2H6 + 0.1026 C3H8 + 0.0906 normal-C4H10 + 0.0046 N2). Figure 5b shows that the measured MMP for the live oil−impure CO2 (0.7487 CO2 + 0.2513 CH4) system is only slightly higher than that for the dead oil−impure CO2 system, both of which are far higher than those for the dead/live oil−pure CO2 systems. It is seen from these two figures that Pmax is strongly dependent on the gas phase composition (pure/impure CO2) but insensitive to the crude oil composition (dead/live oil). The oil composition effects on the measured MMP and Pmax are further summarized in Figure 6, and three conclusions can be made accordingly. First, it is found that the oil composition effects on the measured MMP and Pmax are measurable but minimal. It is worthwhile to note that at an extremely high initial GOR of 4000:1, no dead/live oil sample was preinjected into the high-pressure IFT cell and saturated with the existing CO2 phase to reach an equilibrium state before a pendant dead/live oil drop was formed. In this special case, the dead/ live oil composition was solely represented by an extremely small (≈ 0.010 cm3) pendant dead/live oil drop used in the IFT tests. This fact indicates that the ADSA technique for the pendant drop case used in the IFT measurements is accurate and the VIT technique applied in the MMP and Pmax determination is reliable. Second, the oil composition effects on the MMP and Pmax obtained in this study at GOR = 4000:1 are minimized and thus cannot represent those for a CO2-EOR project in an oil reservoir. In the field application case, the residual live oil under the actual reservoir conditions is fully saturated with the solution gas of primarily CH4, which can significantly increase the MMP and Pmax especially when the GOR is rather low at the beginning of CO2 flooding. Finally, Figure 6 also shows that impure CO2 with 0.2513 CH4 (mole fraction) causes the MMP and Pmax to drastically increase. The

Figure 5. Measured equilibrium IFTs at different equilibrium pressures, T = 326.15 K, and GOR = 4000:1 for (a) the Pembina Cardium dead/live oil−pure CO2 system: ●, test no. 3 (dead oil, MMP = 10.6 MPa, and Pmax = 23.2 MPa); ○, test no. 9 (live oil, MMP = 12.5 MPa, and Pmax = 23.6 MPa). (b) The Pembina Cardium dead/ live oil−impure CO2 (0.7487 CO2 + 0.2513 CH4) system: ●, test no. 6 (dead oil, MMP = 21.4 MPa, and Pmax = 29.8 MPa); ○, test no. 12 (live oil, MMP = 21.7 MPa, and Pmax = 30.4 MPa).

Figure 6. Measured MMPs and Pmax values of the Pembina Cardium dead/live oil−pure/impure CO2 (0.7487 CO2 + 0.2513 CH4) systems at T = 326.15 K and GOR = 4000:1: ●, impure CO2; ○, pure CO2.

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with CH4 content. It is worthwhile to note that at such an extremely high GOR, the gas composition effects on the MMP and Pmax are maximized. Figure 7b indicates that the MMP and Pmax for the Pembina Cardium live oil at T = 326.15 K and GOR = 4000:1 increase respectively from 12.5 MPa to 21.7 MPa and from 23.6 MPa to 30.4 MPa when 0.2513 CH4 is added into pure CO2. Obviously, as the lightest HC gas, CH4 can significantly increase the MMP and Pmax. This is because CH4 has a much lower solubility in the crude oil and a much weaker ability to extract light to intermediate HCs from the crude oil than CO2. It is well-known that CO2 is an excellent extracting solvent for reducing the crude oil viscosity and crude oil−solvent IFT under the actual reservoir conditions. Thus CH4 in an impure CO2 stream can severely prevent the multicontact or first-contact miscibility development. With the above-described experimental results, the gas composition effects on the MMP and Pmax are compared in Figure 8. On the basis of the measured data points (symbols) in

specific gas composition effects on the measured MMP and Pmax will be analyzed in the following section. Gas Composition Effects. It is well-known that both the multicontact MMP and the first-contact miscibility (Pmax) strongly depend on the gas composition or the impurities of CO2. In an actual field application of a CO2-EOR project, the injected pure CO2 will be likely coproduced with some solution gas of HCs, the primary component of which is methane.22 In this work, the effects of CH4 in an impure CO2 stream on the MMP and Pmax are purposely studied by adding approximately 0.16 and 0.25 CH4 in mole fraction into pure CO2 to prepare two different impure CO2 samples, respectively. Figure 7a shows that the MMP for the Pembina Cardium dead oil at T = 326.15 K and GOR = 4000:1 increases quickly from (10.6, 16.3, to 21.4) MPa if CH4 content increases from zero (i.e., pure CO2), 0.1594, to 0.2513. Meanwhile, Pmax moderately increases

Figure 8. Measured MMPs and Pmax values of the Pembina Cardium dead/live oil−pure/impure CO2 systems with different CH4 contents (yCH4 = 0, 0.1594, and 0.2513), T = 326.15 K, and GOR = 4000:1: ●, live oil; ○, dead oil.

this figure, the MMP (MPa) and Pmax (MPa) measured at T = 326.15 K and GOR = 4000:1 are linearly correlated to CH4 content in an impure CO2 phase in mole fraction, yCH4, for the dead/live oil: MMP/MPa = 42.2yCH + 10.3 (dead oil)

(3a)

MMP/MPa = 36.6yCH + 12.5 (live oil)

(3b)

4

4

and Pmax /MPa = 25.8yCH + 23.0 (dead oil)

(4a)

Pmax /MPa = 27.1yCH + 23.6 (live oil)

(4b)

4

Figure 7. Measured equilibrium IFTs at different equilibrium pressures, T = 326.15 K, and GOR = 4000:1 for (a) the Pembina Cardium dead oil−pure/impure CO2 system: ▼, test no. 3 (pure CO2, MMP = 10.6 MPa, and Pmax = 23.2 MPa); ○, test no. 5 (0.8406 CO2 + 0.1594 CH4, MMP = 16.3 MPa, and Pmax = 26.7 MPa); ●, test no. 6 (0.7487 CO2 + 0.2513 CH4, MMP = 21.4 MPa, and Pmax = 29.8 MPa). (b) The Pembina Cardium live oil−pure/impure CO2 system: ●, test no. 9 (pure CO2, MMP = 12.5 MPa, and Pmax = 23.6 MPa); ○, test no. 12 (0.7487 CO2 + 0.2513 CH4, MMP = 21.7 MPa, and Pmax = 30.4 MPa).

4

The above four correlations show that both the MMP and Pmax are rather sensitive to the gas composition, that is, CH4 content in CO2. They increase almost linearly with CH4 content up to 0.2513 mol fraction. More specifically, the MMP and Pmax increase linearly with an increasing CH4 content at the rates of 0.422/0.366 MPa per 0.01 CH4 and 0.258/0.271 MPa per 0.01 CH4 for the dead/live oil, respectively. These rates indicate that for a CO2 flooding project, the contents of 1368

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reduction with the equilibrium pressure is mainly caused by CO2 dissolution into the crude oil and the light-HCs extraction from the crude oil to CO2. These two physical processes occur simultaneously and at the end jointly cause the equilibrium IFT reduction as the test pressure increases, especially at a low initial GOR. In this case, both dead oil and CO2 phases are substantially modified so that they are much similar and thus can easily reach their multicontact or even first-contact miscibility. At a high initial GOR of 200:1 or 4000:1, on the other hand, Figure 9b gives slightly higher MMP and Pmax. At a high initial GOR, the light-HCs extraction by CO2 can make the crude oil heavier but pure CO2 phase remains almost unchanged because there was a small amount of dead oil inside the IFT cell, both of which lead to marginally higher MMP and Pmax. The experimental data in Figures 9(a and b) are combined together and plotted in Figure 10 to manifest the initial GOR

the produced HCs (especially CH4) in the produced CO2 should be monitored and controlled during the entire CO2EOR project. In this way, the reinjected CO2 with some produced HCs is miscible or near-miscible with the reservoir oil at any time.23 In case the total HC content (especially yCH4) is higher than a threshold value, some makeup pure CO2 should be added into the produced gas (CO2 + HCs) to increase its CO2 content prior to its reinjection. On the other hand, the detrimental effect of CH4 on the miscibility development can sometimes be offset by the beneficial effects of the intermediate HCs (C3 to C6) if they are present in the produced gas (CO2 + HCs) to some extent. In this case, even though the produced gas (CO2 + HCs) from a CO2 flood may contain a relatively large amount of HCs (including CH4), it can still be miscible or at least near-miscible with the residual crude oil under the actual reservoir conditions.23,24 Initial Gas−Oil Ratio (GOR) Effects. Figure 9a shows lower equilibrium IFTs, relatively lower MMP and Pmax of the dead oil−pure CO2 system measured at GOR of 3:1 or 10:1. For the dead oil−pure CO2 system, its equilibrium IFT

Figure 10. Measured MMPs and Pmax values of the Pembina Cardium dead oil−pure CO2 system at T = 326.15 K and different initial GORs in volume (GOR = 3:1, 10:1, 200:1, and 4000:1).

effects on the MMPs and Pmax values of the dead oil−pure CO2 system at T = 326.15 K and GOR = 3:1, 10:1, 200:1, and 4000:1. It is seen from this figure that the initial GOR used in the IFT tests has little effect on either the MMP or Pmax. However, it is expected that the initial GOR effects on the MMP and Pmax of the live oil−impure CO2 system will be much stronger due to the presence of CH4 in both the live oil and impure CO2. In fact, an initial GOR determines to what extents the oil composition (e.g., dead/live oil) and the gas composition (e.g., CH4 content) will affect the measured MMP and Pmax.



CONCLUSIONS The advanced axisymmetric drop shape analysis (ADSA) technique for the pendant drop case is used to measure the equilibrium interfacial tensions (IFTs) of the light crude oil− CO2 system at different equilibrium pressures. With the measured equilibrium IFT versus pressure data, the vanishing interfacial tension (VIT) technique is applied to determine the minimum miscibility pressures (MMPs) and the first-contact miscibility pressures (Pmax) of five dead/live oil−pure/impure CO2 systems under various experimental conditions. In particular, the test temperature, crude oil composition, and gas composition, and initial gas−oil ratio (GOR) in volume are studied to determine their specific effects on the measured MMP and Pmax. It is found that both the MMP and Pmax

Figure 9. Measured equilibrium IFTs of the Pembina Cardium dead oil−pure CO2 system at different equilibrium pressures, T = 326.15 K, and (a) at GOR = 3:1 or 10:1: ●, test no. 15 (GOR = 3:1, MMP = 9.7 MPa, and Pmax = 22.7 MPa); ○, test no. 14 (GOR = 10:1, MMP = 10.3 MPa, and Pmax = 22.6 MPa). (b) At GOR = 200:1 or 4000:1: ●, test no. 13 (GOR = 200:1, MMP = 10.4 MPa, and Pmax = 23.5 MPa); ○, test no. 3 (GOR = 4000:1, MMP = 10.6 MPa, and Pmax = 23.2 MPa). 1369

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(8) Holm, L. W. Miscibility and Miscible Displacement. J. Pet. Technol. 1986, 38 (8), 817−818. (9) Elsharkawy, A. M.; Poettmann, F. H.; Christiansen, R. L. Measuring Minimum Miscibility Pressure: Slim-Tube or Rising-Bubble Method? Proceeding of the SPE/DOE Symposium on Enhanced Oil Recovery, Tulsa, OK, 1992; paper SPE 24114. (10) Dong, M.; Huang, S. S.; Dyer, S. B.; Mourits, F. M. A Comparison of CO2 Minimum Miscibility Pressure Determinations for Weyburn Crude Oil. J. Pet. Sci. Eng. 2001, 31 (1), 13−22. (11) Flock, D. L.; Nouar, A. Parameter Analysis on the Determination of the Minimum Miscibility Pressure in Slim-Tube Displacements. J. Can. Pet. Technol. 1983, 23 (9−10), 80−88. (12) Huang, E. T. S. The Effect of Oil Composition and Asphaltene Content on CO2 Displacement. Proceeding of the SPE/DOE Symposium on Enhanced Oil Recovery, Tulsa, OK, 1992; paper SPE 24131. (13) Christiansen, R. L.; Kim, H. Rapid Measurement of Minimum Miscibility Pressure Using the Rising Bubble Apparatus. Proceeding of the SPE Annual Technical Conference and Exhibition, Houston, TX, 1987; paper SPE 13114. (14) Rao, D. N. A New Technique of Vanishing Interfacial Tension for Miscibility Determination. Fluid Phase Equilib. 1997, 139 (2), 311− 324. (15) Rao, D. N.; Lee, J. I. Application of the New Vanishing Interfacial Tension Technique to Evaluate Miscibility Conditions for the Terra Nova Offshore Project. J. Pet. Sci. Eng. 2002, 35 (3), 247− 262. (16) Rao, D. N.; Lee, J. I. Determination of Gas−Oil Miscibility Conditions by Interfacial Tension Measurements. J. Colloid Interface Sci. 2003, 262 (2), 474−482. (17) Holm, L. W. Miscible Displacement. In Petroleum Engineering Hand Book; Bradley, H. B., Ed.; Society of Petroleum Engineers: Richardson, TX, 1987, pp 1−45. (18) Benham, A. L.; Dowden, W. E.; Kunzman, W. J. Miscible Fluid Displacement−Prediction of Miscibility. Pet. Trans. Repr. Ser. 1960, 21 (9), 229−237. (19) Peng, D. Y.; Robinson, D. B. A New Two-Constant Equation of State. Ind. Eng. Chem. Fundam. 1976, 15 (1), 58−64. (20) Cheng, P.; Li, D.; Boruvka, L.; Rotenberg, Y.; Neumann, A. W. Automation of Axisymmetric Drop Shape Analysis for Measurements of Interfacial Tensions and Contact Angles. Colloids Surf. 1990, 43 (2), 151−167. (21) Johnson, J. P.; Pollin, J. S. Measurement and Correlation of CO2 Minimum Miscibility Pressure. Proceeding of the SPE/DOE Symposium on Enhanced Oil Recovery, Tulsa, OK, 1981; paper SPE 9790. (22) Sebastian, H. M.; Wenger, R. S.; Renner, T. A. Correlation of Minimum Miscibility Pressure for Impure CO2 Streams. J. Pet. Technol. 1985, 37 (11), 2076−2082. (23) Pyo, K.; Damian-Diaz, N.; Powell, M.; van Nieuwkerk, J. CO2 Flooding in Joffre Viking Pool. Proceeding of the Canadian International Petroleum Conference, Calgary, Alberta, 2003; Paper No. 2003-109. (24) Johns, R. T.; Kaveh, A. A Practical Method for Minimum Miscibility Pressure Estimation of Contaminated CO2 Mixtures. Soc. Pet. Eng. Res. Eng. 2010, 13 (5), 764−772.

increase almost linearly with an increasing test temperature in the range of T = (294.15, 313.15, and 326.15) K. Thus the reservoir temperature has a strong effect on the crude oil−CO2 miscibility development. Moreover, both the oil composition (from dead to live oil) at GOR = 4000:1 and the initial GOR (from 3:1 to 4000:1) for the dead oil−pure CO2 system have measurable but weak effects on the MMP and Pmax. In the field applications, nevertheless, the residual live oil saturated with the solution gas of CH4 under the actual reservoir conditions can substantially increase the MMP and Pmax, especially at the beginning of CO2 flooding. Also it is expected that the initial GOR will strongly affect the MMP and Pmax of the live oil− impure CO2 system due to the presence of CH4 in both the live oil and impure CO2. Finally, the addition of CH4 into pure CO2 drastically increases the MMP and Pmax. Therefore, CH4 content in the CO2 injection stream has to be closely monitored and well controlled during an oilfield CO2-EOR project. If CH4 content is higher than a threshold value, the makeup pure CO2 should be added into the produced gas (CO2 + HCs) so that the reinjected produced gas is miscible or at least near-miscible with the reservoir oil under the actual reservoir conditions at any production time.



AUTHOR INFORMATION

Corresponding Author

*Tel.: 1 (306) 585-4630. Fax: 1 (306) 585-4855. E-mail: Peter. [email protected] Funding

The authors acknowledge the discovery grant and the collaborative research and development (CRD) grant from the Natural Sciences and Engineering Research Council of Canada and the industrial R&D fund from the PennWest Exploration to Dr. Yongan Gu. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors thank Ms. Meng Cao at the University of Regina for editing some tables and figures. REFERENCES

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