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. 2020 Oct 21;5(43):27944–27952. doi: 10.1021/acsomega.0c03326

Effect of CO2/N2 Mixture Composition on Interfacial Tension of Crude Oil

Rahul Gajbhiye 1,*
PMCID: PMC7643147  PMID: 33163777

Abstract

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CO2-enhanced oil recovery (EOR) has demonstrated significant success over the last decades; it is one of the fastest-growing EOR techniques in the USA accounting for nearly 6% of oil production. A large quantity of CO2 gas is required for the EOR process and sometimes other gases such as hydrocarbons, air, flue gases, CO2, N2, and mixtures of two or more gases are used for injection. It is also realized that the injection of CO2 and N2 combines advantage in reducing CO2 concentrations in the atmosphere and improving the oil recovery by sequestering it underground. However, there are a number of variables involved in the successful design of the CO2-EOR process. The objective of this study is to investigate the effect of CO2/N2 mixture composition on interfacial tension (IFT) of crude oil. Experiments were performed to measure the IFT of the CO2/N2 mixtures and crude oil for different compositions of gas by varying the system pressure at a fixed temperature. The effect of CO2/N2 mixture composition and pressure on the IFT of crude oil is evaluated. The experimental results show that an increase in the mole fraction of CO2 in the gas mixture results in a decrease in IFT between CO2–oil, irrespective of the system pressure. However, because of an increase in the mole fraction of N2 in the gas mixture, an increase in IFT was observed and this change is opposite to the effect of the CO2 mole fraction. Also, the change in IFT is consistent with the pressure, which means that the IFT decreases with an increase in the pressure at a given temperature. The effect of the CO2 mole fraction is more profound compared to the N2 fraction and with the pressure at which experiments were conducted in this study. The finding of this study helps in designing the CO2-EOR process in which achieving miscibility conditions is vital for taking advantage of the CO2 injection. Also, the presence of N2 and its influence on the IFT that must be considered in the CO2-EOR were addressed in this study.

Introduction

As the major global energy provider, the oil and gas industry has faced many challenges to keep up with the increasing global demand. Because most major reservoirs have already been discovered and chances of finding new ones are very low, one of the options left to increase production is to increase recovery of the already producing reservoirs by implementing enhanced oil recovery (EOR) techniques. The implementation of EOR techniques is often necessary to maintain operations in reservoirs realizing depletion and declining production rates. In general, field operators consider two major factors during the design of enhanced recovery operations: technique practicability and economic viability. EOR techniques can be classified into three major categories: thermal, chemical, and gas injection (immiscible or miscible).1

In gas injection techniques, various gases are injected into the reservoir such as hydrocarbons, air, flue gases, CO2, N2, and sometimes mixtures of two or more of the aforementioned gases. However, CO2-EOR gained popularity because of several advantages associated with it such as CO2 can be miscible with oil resulting in a reduction in oil viscosity, causing oil swelling, and lowering interfacial tension (IFT) under specific conditions of pressure, temperature, and oil composition. It can invade the zones that were not invaded by water before resulting in reducing and releasing trapped oil.2 Moreover, CO2-EOR can be used to sequestrate large quantities of CO2 in the reservoirs.3

CO2-EOR is accounting for nearly 6% of U.S. onshore oil production, around 350,000 barrels per day.4 By 2010, the number of CO2-EOR projects around the world had reached 127, from which 112 projects are in the United States. This technique uses CO2, both naturally occurring as well as a byproduct of industrial processes, to increase the production of oil from the existing oil fields. Although CO2-EOR is already an important component of today’s oil production, it has great potential to expand production. An analysis commissioned by the U.S. Department of Energy (DOE) projects potential oil resources recoverable with CO2-EOR of up to 137 billion barrels, with 67 billion barrels economically recoverable at $85 a barrel. This represents more than three times the current U.S. proven reserves.5 The Permian Basin is one of the most active CO2-EOR regions in the world. The field in the region has produced over 30 billion barrels of oil, out of which 1.3 billion have been produced with carbon dioxide.6 The current EOR production estimate on average 350,000 barrels of oil per and estimates current world CO2-EOR production about 450,000 barrels of oil per day.7

The advantages associated with the CO2-EOR have driven the research in this field and it is still been considered the important subject of recent studies. The oil recovery by the injection of CO2 is mainly due to the interaction between the CO2 and reservoir fluid. As oil and water are the dominant fluids present in the reservoir, many studies were conducted to understand the CO2–oil or CO2–water or oil–water–CO2 interaction.8−15 The CO2-injection scheme is dictated by the condition of the miscibility of CO2 with the reservoir fluid. Also, some hybrid schemes of injecting carbonated water or water-alternating gas or sometimes chemical or surfactant was employed to enhance the process efficiency. Several experimental and modeling studies were presented in the literature.15−20

The deployment of more efficient and economical extraction methods and processing facilities of oil and gas requires the accurate knowledge of the IFT of fluid phases in contact.21−24 It is very important while dealing with the multiphase systems such as CO2-EOR, in which resultant fluid properties and a mechanism to extract the oil from the reservoir are governed by the fluid interaction. The IFT knowledge can assist in describing the behavior and characteristics of interfacial phenomena and surface chemistry involved in a variety of chemical and petroleum processes.25−27 IFT existed at the interface between two phases because of the imbalance of the molecular forces at the interface; it is generally expressed as the force at the interface per unit length in units of dyne/cm or mN/m.

In the petroleum industry, many processes require knowledge of IFT such as multiphase flow, separation process, and extraction of crude oil from the reservoir with different methods such as water flooding and gas injection as well as chemical injection. Also, surface processes such as gas–oil-separation plant as well as refining processes in the petrochemical plant are influenced by the IFT. There are several reservoir properties such as wettability, capillary pressure, and relative permeabilities that require determination of the IFT, and these properties are dependent on the accuracy of the IFT. As a result, the IFT can be considered as a key parameter controlling the hydrocarbon distribution in the reservoir pore, as well as the recoverable hydrocarbon from the subsurface.25−28

The IFT plays a vital role, especially in the EOR processes. Among the different EOR techniques, chemical/surfactant injection and gas injection which are aimed to reduce capillary pressure are governed by the resulting IFT because of the introduction of chemicals or gases in the reservoir. The lowering of the IFT is considered as one of the prime factors in the success of EOR.29−35 In the case of the gas injection process such as CO2, the IFT dictates the schemes of the injection as whether the process is miscible or immiscible. Many studies utilized IFT as a parameter to study the interaction of gases such as CO2, CH4, and N2, with water, crude oil, or n-decane.36−44 Also, IFT has been utilized for determining the miscibility or immiscibility condition of the CO2-EOR processes.45−48

There are different ways to measure the IFT that exists between the different phases such as pendant drop, capillary rise, Du Noüy ring, and slim tube. Each of these methods has its limitation and accuracy in the measurement of IFT. The measurement of IFT using the pendant drop method requires having a high-density difference between different phases; however, the measurement is done under high-pressure conditions. One of the limitations of this method is that it is not suitable for the measurement of the very low value of IFT. On the other hand, the capillary rise measurement method can be utilized for the measuring low value of IFT. There are other factors that also influence the selection of IFT measurement methods. These factors are important especially dealing with the petroleum fluid, which is multicomponent in nature. The complex nature of the multicomponent system makes measurements of the IFT difficult; moreover, IFT measurement requires phase densities, which is difficult to obtain in the multicomponent systems. For example, for a system with low solubility (hydrocarbon + water) and (nitrogen + water) mixtures, the density of the saturated phases has been commonly approximated to that of pure substances. For the miscible system such as (carbon dioxide + water) and (carbon dioxide + n-alkanes), it further complicates the system leading to inaccurate estimation of the IFT.36−43

The complexities associated with the measurement of IFT due to CO2 interaction with oil or water and associated chemicals were addressed by measuring the dynamic IFT. Also, the effect of temperature and pressure was investigated on the dynamic IFT between the CO2, crude oil, and water with different ionic compositions as well as in the presence of chemicals. However, there is no clear relationship between the CO2 mole fraction and oil/brine IFT, as this physical property depends on complex intermolecular forces including electrostatic, acid–base, and other interactions.49−54

The IFT investigations are not limited to laboratory measurement studies. There are studies on predictive and theoretical approaches for IFT that are presented in the literature. Among the different approaches, one of the simple and commonly used approaches is presented by Parachor.55−57 However, it is not suitable for many systems such as IFT calculation of the CH4 + H2O system. Another theoretical approach is the density gradient theory (DGT), and linear gradient theory (LGT) is available for predicting the IFT values of the system. Moreover, studies pertaining to the assessment of the theoretical models with the laboratory measurement were carried out by the investigators.58−68 However, the DGT and LGT have not been widely tested for the IFT between gas mixtures because of the unavailability of the data in the literature. A more rigorous approach through integrated experimental measurements is required for validation of the theoretical and predictive models.

There are studies on understanding the phase behavior and to predict the saturated densities of CO2, N2, and CH4 with n-decane presented in the literature, which can be correlated with the IFT as the density of the phases in contact is essential for the estimation of IFT. Also, the effect of the pressure and temperature on the densities of saturated phases was investigated to have accurate models for IFT.60,69−78 The experimental investigation of the IFT in these systems was investigated only for the pure system.79 Moreover, some studies were carried out in relation to the gas solubility in the oil, as well as IFT of the gas mixture with water.80 Despite all studies conducted to better understand IFT, only a few were concerned with the effect of gas composition on the IFT between gas and oil. A study performed on North Sea oils was concerned with nitrogen/oil IFT and conclusions were made that the developed method was not sufficient for accurate predictions because of large deviations from experimental values.80,81 It arises a need for further investigation to be concerned with the effect of gas composition on oil surface tension. Therefore, this study aims to investigate the effect of gas composition on the IFT of crude oil by subjecting crude oil samples to gas mixtures of different CO2 and N2 ratios in a range of pressures. The IFT properties of the gas mixture with oil were not investigated in the literature, especially, when the mixture of gases contains the components with wide variation in the properties such as solubility as well as phase behavior. This study attempts to fulfill this gap by evaluating the effect of gas composition on the IFT of crude oil in a wide range of pressure conditions through experimental measurement of the IFT of crude oil with the gas mixture.

Methodology

The effect of CO2/N2 mixture composition on IFT of crude oil was evaluated by performing a series of experiments by measuring gas–oil IFT with the pendant drop technique. The IFT measurement was performed for different compositions of the gas mixture under a specific condition of pressure and temperature. Attention was given to the composition of injection gas to determine its effect on IFT during the interaction between injected gas mixture composition and crude oil. The following sections explain the details of the experimental setup, materials, and procedure used in the study.

Experimental Setup

The experimental apparatus used in the study is manufactured by Vinci Technologies IFT Meter (IFT 700). The apparatus is designed to withstand high pressure (10,000 psi) and a high temperature of 356 °F (180 °C). It can measure the IFT in the range of 0.1–72 mN/m by drop shape analysis. To facilitate the variation in the composition of the gas mixture, additional gas accumulator, syringe pump, and pressure gauges were installed to modify the system as shown in Figure 1A,B, schematic of the experimental setup. The modified system enables to measure the IFT of crude oil with variation in gas mixture composition over a range of pressure and temperature conditions. The composition variation of the gas mixture is achieved by injecting the particular gas in the cell under a controlled pressure condition.

Figure 1.

Figure 1

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(A) Experimental setup used in the IFT measurement of crude oil and gas mixture. (B) Schematic of the setup used in the IFT measurement of crude oil and gas mixture.

Materials

The materials used in the study are

  • 1.

    CO2 gas (99.99% purity), supplied from Saudi Industrial Gas Company

  • 2.

    N2 gas (99.99% purity), supplied from Saudi Industrial Gas Company

  • 3.

    Arab light oil (API 40)

Procedure

The following steps were involved in the experimental work.

  • 1.

    The oil density was measured as a function of temperature, and correlations were utilized to determine its value at various pressures.

  • 2.

    The gas mixture densities were determined at each value pressure and temperature using the correlation.

  • 3.

    The equipment was made ready for the experimental runs under the specific condition of pressure, temperature, and gas mixture, each experiment was performed, and IFT data were collected for the analysis.

  • 4.

    Two different sets of experiments were performed in this work. First, the IFT measurements were carried out by fixing the gas mixture composition and the pressure was changed from low to high value until the lowest value of IFT is recorded. The composition of the gas mixture at a specific value of pressure is maintained using the gas compressibility factor.

  • 5.

    In the second set of experiments, the pressure was increased by the injection of N2 gas in a cell which is filled with CO2 at a particular pressure. The resultant gas composition is determined using real gas law for specific pressure at which IFT is measured. A similar procedure was repeated by injecting CO2 gas in a cell filled with N2 at a particular pressure. The resultant gas composition is determined using real gas law, and IFT measurement is performed at a specific value of pressure.

  • 6.

    The IFT measurement was performed using the pendant drop method. Initially, all the traces of the oil and impurities were removed from all the tubing connections and from the visual cell. Then, oil is transferred to the oil accumulator from which oil is injected into the cell for creating the pendant drop in the presence of the gas phase. The gas phases are injected into the visual cell from the accumulator connected to the syringe pump to provide the required pressure and the composition of the gas mixture is controlled by controlling the pressure of the individual gas phase. After starting the software, the gas and oil densities as well as the required temperature is set through the software and the equipment is left to stabilize for the temperature. Then, the oil is carefully displaced from the drop chamber through the connections until it comes out of the needle as a hanging droplet inside the gas-filled chamber. The equipment automatically records the readings. After the reading is stabilized and enough data points are recorded, the pressure can be changed to perform a new experiment.

  • 7.

    The result was analyzed to evaluate the effect of gas composition on gas–oil IFT as a function of pressure at a fixed value of temperature.

Results and Discussion

Density of Oil

Initially, the density of the oil is measured at 77 °F (25 °C) using a densitometer. Considering this measurement as a base value of the oil density, Standing–Katz correlation is used to obtain the value of the oil density for a different set of experimental conditions. Considering that oil density (dead oil) does not vary significantly as a function of pressure, the densities for different values of pressure were determined. Table 1 shows the oil density as a function of temperature and pressure. For oil density determination, the Standing–Katz method was used.

Table 1. Density of Oil as a Function of Pressure and Temperature.

temperature (°F (°C)) pressure (psi) Δρp (lb/ft3) ΔρT (lb/ft3) oil density (lb/ft3) oil density (g/cm3)
77 (25) 14.7 0 0.35 51.10 0.819
  250 0.05 0.35 51.15 0.820
  500 0.1 0.35 51.20 0.821
  600 0.12 0.35 51.22 0.821
  750 0.15 0.35 51.25 0.822
  1000 0.2 0.35 51.30 0.822
  1250 0.25 0.35 51.35 0.823
  1500 0.3 0.35 51.40 0.824
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Standing–Katz correlation for oil density

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Density of the Gas Phase

The density of the gas phase (mixture of CO2/N2) was determined for a particular set of the experimental condition by determining the corresponding values of compressibility using Papay’s correlation and applying the Kays mixing rule to obtain the density of the gas mixture as a function of composition, pressure, and temperature.27 The density of the 80% CO2 and 20% N2 at 25 °C for different values of pressure is shown in Table 2.

Table 2. Gas Mixture (80% CO2 and 20% N2) Density as a Function of Pressure at 77 °F (25 °C).

temperature 77 °F (536 °R)
pressure (psi) Ppr Tpr Z gas mixture density (lb/ft3) gas mixture density (g/cm3)
14.7 0.014 1.030 0.995 0.072 0.0012
250 0.244   0.918 1.324 0.0213
500 0.489   0.841 2.890 0.0465
750 0.733   0.769 4.742 0.0763
1000 0.978   0.701 6.932 0.1115
1250 1.222   0.639 9.518 0.1531
1500 1.467   0.580 12.565 0.2021
1750 1.711   0.527 16.144 0.2596
2000 1.955   0.478 20.327 0.3269
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Papay’s correlation for the compressibility factor

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IFT of the Oil and Gas Mixture

To evaluate the effect of gas composition on gas–oil IFT, five experiments for the various gas compositions and at a different value of pressure were carried out. The first experiment was carried out to determine the IFT of the CO2 gas as a function of pressure by keeping the temperature constant at 77 °F (25 °C). In the second experiment, the ratio of CO2/N2 was kept constant at 80:20, and IFT was determined as a function of pressure by keeping the temperature constant at 77 °F (25 °C). Similarly, a third experiment was performed for the ratio of CO2/N2 as 50:50 and IFT was determined as a function of pressure by keeping the temperature constant at 77 °F (25 °C). The fourth and fifth experiments were performed by varying both composition and pressure of the system at a constant temperature of 77 °F (25 °C).

It should be noted that for the IFT measurement, densities of the pure phases were used instead of equilibrium densities of the oil-dominated liquid phase and gas mixture-dominated vapor phase. The phase densities are difficult to obtain for the multicomponent system. The phase densities were not measured, as a high-pressure high-temperature densitometer was not accessible. Also, the density of the oil with dissolved CO2 is assumed to be constant, as variations of the oil density with the CO2 dissolution at these pressures are minimal. It may lead to variation in the measured value of IFT from the actual IFT value. The average IFT values based on the shape of the captured pendant drop at the equilibrium pressure and temperature are reported in this paper. Similar results were also reported for other crude oil/CO2 systems in the literature.79−85

CO2/Oil Equilibrium IFT (Experiment 1)

In this experiment, the IFT was measured for the pure CO2 system by keeping the temperature constant at 77 °F (25 °C) and pressure was increased from the lowest value until it reaches the stage where the pendant drop is unable to form within equilibrium with CO2. This point refers to the maximum pressure at which IFT can be measured with the pendant drop method. The results are shown in Figure 2. As can be seen that for the oil utilized in this study that has IFT 16.58 ± 0.38 measured at the lowest pressure (250 psi), the pendant drop method can be used to measure the value of IFT for the pure CO2 up to a pressure of 750 psi (IFT = 7.6 ± 0.11) for the oil used in this study. The IFT decreases with an increase in pressure of the system, and similar results were presented in the literature in which IFT decreases with an increase in the system pressure.39,45,47,48 For the comparison, the IFT measurement of pure CO2 with crude oil was presented from the study by Yang and Gu9 in which IFT was measured at 80 °F (27 °C). In the study by Yang and Gu,9 the density of the oil was assumed to be constant, and the small variation due to CO2 dissolution in oil was not accounted for the IFT measurement. The density of the CO2 was calculated at a particular temperature and pressure condition of IFT measurement. Similarly, in the present study, the gas densities were calculated based on the densities of the pure component system; however, the density of oil was corrected for the pressure and temperature condition. It can be seen that trends of IFT with pressure are comparable, in which IFT is decreased with an increase in pressure. The IFT of the pure CO2 at higher pressure is not feasible with the pendant drop method, which is based on the drop shape analysis, and required drop shape, which is unable to form at high pressure (850 and 950 psi), as shown in Figure 3. Similar phenomena were observed in several studies on IFT of CO2 with liquid hydrocarbon.13,84−87

Figure 2.

Figure 2

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IFT as a function of pressure for the pure CO2 and crude oil system.

Figure 3.

Figure 3

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Pendant drop shape as a function of pressure for the pure CO2 and crude oil system.

IFT of the CO2/N2 (80:20) Mixture with Crude Oil (Experiment 2)

The second experiment was performed with a CO2-rich gas mixture; it represents the case of some quantity of impurities present in the CO2 gas during the CO2-EOR process. The IFT was measured for the CO2/N2 (80:20) gas mixture equilibrium with crude oil by keeping the temperature constant at 77 °F (25 °C) and the pressure was increased from the lowest value until it reaches the stage where the pendant drop is unable to form in the presence of the CO2/N2 (80:20) gas mixture in equilibrium with crude oil. The results are shown in Figure 4. It can be seen that the IFT of the system is increased with the addition of N2 to the CO2. The change in IFT is not significant and it might be due to the small quantity of N2 present in the system. Also, the pendant drop was able to form at higher pressure (900 psi) compared to the pure CO2 (750 psi), and it indicates that the addition of N2 gas to the CO2 system leads to an increase in the miscibility pressure of the system.

Figure 4.

Figure 4

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IFT as a function of pressure for the CO2/N2 (80:20) mixture with crude oil.

IFT of the CO2/N2 (50:50) Mixture with Crude Oil (Experiment 3)

In this experiment, the percentage of N2 increased to 50%, and it will further affirm the effect of N2 addition on the IFT of the system. It represents the case of gas injection operations that use carbon dioxide (CO2) and nitrogen (N2) mixtures in a cyclic gas injection process. The IFT was measured for the CO2/N2 (50:50) gas mixture equilibrium with crude oil by keeping the temperature constant at 77 °F (25 °C) and pressure was increased from the lowest value until it reaches the stage where the pendant drop is unable to form in the presence of the CO2/N2 (50:50) gas mixture in equilibrium with crude oil. The results are shown in Figure 5. It can be seen that the IFT of the system is increased with the addition of N2 to the CO2. The change in IFT is significant because of the reasonable quantity of N2 present in the system. Also, the pendant drop was able to form at higher pressure (1000 psi) compared to the pure CO2 (750 psi), and it indicates that the increase in the percentage of the N2 gas to the CO2 system increases the miscibility pressure of the system. The IFT comparison for the pure CO2, gas mixture CO2/N2 (80:20), and gas mixture CO2/N2 (50:50) is shown in Figure 6.

Figure 5.

Figure 5

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IFT as a function of pressure for the CO2/N2 (50:50) mixture with crude oil.

Figure 6.

Figure 6

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Effect of gas composition on IFT.

IFT Behavior with an Increase in the N2 Gas Fraction (Experiment 4)

In this experiment, initially, CO2 is charged into the cell at low pressure. Then, the N2 gas is introduced into the cell at a predetermined value of pressure by controlling the valve. The introduction of N2 gas in the visual cell reduces the CO2 gas fraction and subsequently increases the N2 gas fraction in the visual cell. The system is allowed to come in equilibrium for a set of temperature and pressure and then IFT was measured by introducing the oil drop in the visual cell through the needle to form a pendant drop. The procedure is repeated until the pendant drop was not able to form the shape required for IFT measurement. In each step, pressure and N2 gas fraction increase (reduction in CO2 gas fraction) in the system. The results are shown in Figure 7. It can be seen that the IFT of the system is increased with an increase in the N2 gas fraction, which corresponds to a decrease in the CO2 gas fraction in the system. In other words, the increase in the CO2 gas fraction results in a reduction in the IFT with crude oil, which is opposite to the effect of N2 gas. However, for the constant gas composition, the IFT decreases with an increase in pressure of the system.

Figure 7.

Figure 7

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IFT as a function gas composition with crude oil (increase in N2 gas fraction).

IFT Behavior with an Increase in the CO2 Gas Fraction (Experiment 5)

In this experiment, initially, N2 is charged into the cell at low pressure. Then, the CO2 gas is introduced into the cell at a predetermined value of pressure by controlling the valve. The introduction of CO2 gas in the visual cell reduces the N2 gas fraction and subsequently increases the CO2 gas fraction in the visual cell. In each step, pressure and CO2 gas fraction increase (reduction in N2 gas fraction) in the system. The results are shown in Figure 8. It can be observed that the IFT of the system decreases with an increase in the CO2 gas fraction, which corresponds to an increase in the N2 gas fraction in the system. In other words, a decrease in the N2 gas fraction results in an increase in the IFT with crude oil, which is opposite to the effect of CO2 gas. Similar to previous experiments, for the constant gas composition, the IFT decreases with an increase in pressure of the system. Also, in this experiment, IFT of the pure N2 gas is measured at 200 psi which is the highest (∼21 mN/m) for all the range of IFT observed in this study. Similarly, the IFT for the CO2/N2 (∼90:10) ratio is ∼3.63 mN/m at 900 psi, lowest in the range of IFT measurement performed in this study.

Figure 8.

Figure 8

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IFT as a function of gas composition with crude oil (increase in CO2 gas fraction).

Conclusions

In this study, rigorous experiments were carried out to evaluate the effect of gas composition on the IFT of crude oil. The constant gas composition (for pure CO2 and CO2/N2 ratio of 80:20 and 50:50) IFT was measured as a function of pressure. Also, the IFT was measured by varying the fraction of individual gas from ∼10% to nearly ∼90% along with ∼99.99% pure form of the gases with maximum pressure (1000 psi) allowed by the system. The following are the conclusion from this study.

  • 1.

    The IFT between the crude oil and gas system is significantly affected by the composition gaseous phase. The IFT appears to be the strong function of the gas composition.

  • 2.

    The change in IFT depends on the type of gas; for CO2, IFT decreases with an increase in the fraction of CO2 in the mixture of CO2/N2. However, for N2, IFT increases with an increase in the fraction of N2, which is opposite to the effect of CO2 gas on IFT of the crude oil and CO2/N2 gas mixture.

  • 3.

    The IFT decreases with an increase in pressure of the system irrespective of the gas composition. The addition of N2 gas may affect the CO2 miscibility pressure indicated by an increase in the IFT value. For the CO2/N2 gas mixture system, an increase in the CO2 gas fraction may help in achieving the miscibility at a lower pressure; however, an increase in the N2 gas fraction can worsen the miscibility of the gas mixture with oil. It should be noted that minimum miscibility pressure was not measured in this study. It can be referred as the zero value of IFT used in VIT.

  • 4.

    The IFT does not show any hysteresis effect, and an increase in the CO2 fraction or N2 fraction gradually gives the same value of the IFT as long as gas composition remains constant at the equilibrium with the crude oil under the specific condition of pressure and temperature.

  • 5.

    The results from this study are very useful in designing the CO2-EOR as the determination of IFT is crucial for capillary pressure estimation and designing the EOR process. Similarly, IFT is a critical parameter to understand the fluid–fluid interaction (solubility) in the cyclic gas injection processes in which a mixture of gases was injected into the reservoir for improving recovery. In addition, knowing the IFT of the system is very important in multiphase flow and surface production facility designing.

Limitations

It is acknowledged that the density used in the measurement of the IFT is based on the pure component densities. The changes in the densities of the gaseous and liquid phases in the equilibrium due to the interaction of the gases especially the solubility of carbon dioxide were not considered because of the complex multicomponent system presented in the study. Also, one of the aspects of applying the results in a real reservoir in which water is present as the third phase was not considered as the equipment can measure the IFT between two phases. Although, the IFT in the presence of third phase (water saturated with CO2) was reported in the literature, where IFT between the oil and CO2-saturated water rather than IFT between the gaseous and oleic phase.

Acknowledgments

The author would like to acknowledge the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum and Minerals (KFUPM) for funding this research work through the research grant (Project # IN171026). The author would also like to acknowledge the Center of Integrated Petroleum Research (CIPR) at the College of Petroleum Engineering and Geosciences for providing access to their laboratories for performing the experiments. Special thanks to Aly Moustafa for initiating the experiment of this work.

The author declares no competing financial interest.

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