Experimental study of asphaltene precipitation prediction during gas injection to oil reservoirs by interfacial tension measurement

Experimental study of asphaltene precipitation prediction during gas injection to oil reservoirs by interfacial tension measurement

Accepted Manuscript Title: Experimental Study of Asphaltene Precipitation Prediction during Gas Injection to Oil Reservoirs by Interfacial Tension Mea...

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Accepted Manuscript Title: Experimental Study of Asphaltene Precipitation Prediction during Gas Injection to Oil Reservoirs by Interfacial Tension Measurement Author: Yousef Kazemzadeh Rafat Parsaei Masoud Riazi PII: DOI: Reference:

S0927-7757(14)00833-4 http://dx.doi.org/doi:10.1016/j.colsurfa.2014.10.053 COLSUA 19507

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

27-6-2014 28-10-2014 29-10-2014

Please cite this article as: Y. Kazemzadeh, R. Parsaei, M. Riazi, Experimental Study of Asphaltene Precipitation Prediction during Gas Injection to Oil Reservoirs by Interfacial Tension Measurement, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014), http://dx.doi.org/10.1016/j.colsurfa.2014.10.053 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Experimental Study of Asphaltene Precipitation Prediction during Gas Injection to Oil Reservoirs by Interfacial Tension Measurement Yousef Kazemzadeh1, Rafat Parsaei1, Masoud Riazi2 1

Department of Petroleum Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz,

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Iran 2

Enhanced Oil Recovery (EOR) Research Centre, School of Chemical and Petroleum Engineering, Shiraz Telephone Number: +987116133714

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Email: [email protected]

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University, Shiraz, Iran

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Abstract

The worldwide increase in energy demand dictates use of enhanced oil recovery (EOR) methods

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to recover more oil from depleted reservoirs. Displacement of oil by gas injection process is one of these methods. Carbon dioxide (CO2) and Methane (CH4) are gases that are mostly used to inject into oil reservoirs. These gases under different reservoir conditions fulfill either miscible

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or immiscible displacement conditions. Asphaltene precipitation, which could take place during

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gas injection, would increase the minimum miscible pressure (MMP) of an oil-gas system. Hence, this could affect the economical aspect of the injection process.

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In this paper, prediction of asphaltene precipitation is studied by measuring the interfacial tension (IFT) between CO2 or CH4 (as the displacing gas) and various oil types with different asphaltene content. Also, the mechanism of CH4 solubility in oil containing asphaltene is analyzed. A proper tool, which is Bond number data versus pressure curve, is introduced to investigate asphaltene precipitation process in presence of different gases. When plotting Bond number against pressure for the CO2-oil system, three distinct intervals could be recognized. In the first interval, the oil-swelling occurs at a low pressure, in the second interval, because asphaltene accumulation happens at the gas-liquid interface, Bond number increases with a gentle slope as pressure increases; and in the third interval, more asphaltene accumulation happens when the surface coverage of the particles surpassed a threshold value (e.g., + 60 % surface coverage) and the rate of change in Bond number is much slower compared to the ones in the other two intervals. However, in the case of CH4-oil system, the Bond number increases linearly with pressure, and no significant slope change is observed. 1   

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Key words: Interfacial tension (IFT), Asphaltene precipitation, Carbon dioxide (CO2), Methane

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(CH4), Bond number, Minimum miscible pressure (MMP), Vanishing interfacial tension (VIT).

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1. Introduction

As production from oil reservoirs is limited [1-2], looking for the methods that can increase the oil production or help to economically displace and produce a part of the remaining oil is crucial

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[3]. Even after primary and secondary recovery periods, a significant volume of oil, which is called residual oil, remains unproduced in reservoir. One of the basic reasons of crude oil

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trapping in oil reservoir is due to interaction of the reservoir rock and fluid through capillary

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force, which ceases oil flow in porous media of the reservoir rock [4]. If the pressure of the injecting fluid increases, the IFT between injected fluid and trapped oil approaches to zero and

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capillary pressure reduces to a minimum value [5-6]. Under this condition, the injecting gas will be able to mobilize a massive amount of trapped oil [2, 7]. Miscibility of two fluids is a condition at which two fluids with any desired ratio can be mixed in a way that no separation can be detected. This effect depends on pressure, temperature and the fluids compositions [2]. The mechanisms of oil recovery under miscible condition are reduction of both capillary pressure and reservoir fluid viscosity. From practical point of view, the Minimum Miscible Pressure (MMP) is the optimum injection pressure at which oil recovery is high with the lowest possible cost. If the injection pressure is low, displacement process will be under immiscible conditions with low efficiency. If the injection pressure is high, although the displacement process will be miscible with high efficiency on oil displacement, the extra pressure will significantly increase the cost of displacement process [8]. Therefore, it is crucial to find out the minimum pressure under which miscibility can occur. Laboratory techniques to determine miscible conditions are divided into two categories. The first 2   

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category is measuring the minimum miscible pressure (MMP) using displacing techniques such as Spiral Slim Tube, Rising Bubble, and Pressure, Volume and Temperature (PVT) [1]. In the second category, miscibility is predicted by IFT measurements. This method, which is called IFT disappearance method, was first proposed by Rao in 1997 [9-11]. This method includes measuring IFT between the injecting gas and the reservoir oil at reservoir temperature and

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different pressures [1-4]. In contrast to the other methods, the IFT disappearance method is capable to measure quantitatively the minimum miscible pressure (MMP) of an oil sample even

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when it contains high amount of asphaltene. Therefore, it can help to understand the conditions under which asphaltene would precipitate during a gas injection process [9-11].

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Nobakht et al. showed that equilibrium CO2 gas-oil IFT often reduces linearly with pressure to the pressure from which the IFT-pressure trend changed, this pressure is known as threshold

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pressure. They also observed that if the equilibrium pressure is higher than the threshold pressure, light oil components quickly get out from the oil droplet and turn into the gas phase. This physical phenomenon is known as the extraction of very light components [12-13].

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Sain and Rao measured the IFT between two samples of recombined live oil (provided from stock tank oil) and CO2 with 99% purity at reservoir temperature of 289

F and different

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pressures (above bubble point pressure of 2593 psia). They used the vanishing interfacial tension experimental method (i.e. the IFT disappearance method) and equation of state for defining the

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minimum miscible pressure [14].

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In spite of the advantage of miscible displacement process in reducing capillary force and mobilizing trapped oil, injecting gas into the oil reservoir at high pressures suffers from a big drawback, which is asphaltene precipitation. Carbon to hydrogen ratio of asphaltene molecules is high and the paraffinic solvents with low molecular weight promote their precipitation. Asphaltene content of crude oil could precipitate in reservoir and wellbore that would prevent oil flowing [15-17]. The other important factors that affect asphaltene precipitation during gas injection include crude oil composition, gas composition, reservoir pressure and temperature conditions [17-21].

There are different methods to determine asphaltene precipitation conditions. These methods include: Gravimetric, Light Transmission, Light Scattering, Refractive Index, Straight Observation, Heat Transfer Measurement, Electric Convection Measurement, Viscosity Measurement, IFT Measurement and Dynamic Method [21]. In the IFT measurement method, the starting point of asphaltene precipitation is estimated by 3   

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monitoring the oil and gas IFT data versus pressure at a given temperature. The IFT values decrease linearly as pressure increases, however, the slope would suddenly change when the asphaltene begins to precipitate. The accuracy of this method in determining the pressure and temperature of precipitation is high and depends on the precision of the device, which is used for IFT measurement. The disadvantage of this method is that its performance at high pressure and

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temperature conditions is time consuming and difficult to handle. Also, this method is only used for heavy and intermediate oils [20].

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Wang et al. studied the behavior of three Canadian crude oil samples (two light oil and one intermediate oil) in presence of CO2 using the method of disappearance of IFT. They found that

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the equilibrium IFT usually decreases linearly through three distinct pressure intervals. Whenever p is greater than Pas (pressure at which the third slope starts) asphaltene precipitation

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starts. Due to dissolution of CO2 in oil and start of asphaltene precipitation, oil becomes significantly light weighted. It should be mentioned that the oil swelling happens at low pressure conditions; however, the initial robust light-component extraction becomes dominant at high

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pressure conditions. This process confirms multiple contact miscible conditions to reach to minimum miscible pressure. They also interpreted the three pressure intervals as follows. In the

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first interval, as pressure increases the IFT decreases due to improving CO2 solubility in oil. In the second interval, the IFT increases suddenly and then decreases rapidly and the trend turns

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into a linear pattern. This behavior is because of asphaltene precipitation and the rapid separation

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of light components. They concluded that the measured IFT in this range would be between a relatively heavier oil and CO2. In the third interval, in which the light components of oil have already been extracted, the IFT measurements would be between the heaviest crude oil components and CO2. They reached the minimum miscible pressure (MMP) by extrapolating the first slope and showed that the first contact minimum miscible pressure (FCM) can be obtained by extrapolating the third part slope [22-23]. In this study a method, which is Bond number curve versus pressure, is presented to investigate asphaltene precipitation process. In this technique, the beginning of asphaltene precipitation is evaluated by monitoring the oil-gas Bond number data versus pressure. 2. Bond Number Theory Figure 1. The theory upon which the pendant drop shape analysis is based on is as follows. The relation between the pressure gradient across the gas-liquid interface is given by Laplase’s equation of 4   

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is the interfacial tension,

and

are the principal radii of curvature, b is the curve

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Where

(1)

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Figure 1. A pendant liquid drop surrounded by gas.

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capillarity [24]:

radius at the apex, g is the gravitational acceleration, and

is density difference between the

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two fluids. Equation (1) may be rearranged into the form of dimensionless terms:

and the dimensionless quantity

is referred to as

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Where R2 was replaced by its equivalent

(2)

Bond number that is defined as the ratio of gravity force to capillary force and is given by the

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following equation:

(3)

By fitting equation (2) to the experimentally obtained profile of a pendant drop, the value of calculated, from which the magnitude of IFT is then estimated. A high value of

is

means that the

shape of the pendant drop is more affected by the gravity force and it is elongated toward the vertical direction. On the other hand, if the relative strength of capillary force is higher than the gravity force, which is the case for low values of Bond number, the shape of the drop becomes more spherical.

3. Experimental To study asphaltene precipitation mechanism two types of crude oil: type B-OIL and type N-OIL are used. Crude oils with high asphaltene content from oil reservoirs located in the southwest of 5   

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Iran are used in this study. The properties of the oils are shown in table 1 the composition of the oils is also shown in table 2 CO2 and CH4 gases (with a purity of more than 99.99%) were prepared from a local company. Table1. General specifications of the crude oils used TEST METHOD

Type N-OIL

Type B-OIL

°API

ASTM D-40452

21.49

24.46

Asphaltene wt%

IP-143

11

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SPECIFICATION

10

C1

C2

C3

iC4

nC4

iC5 nC5

C6

C7

C8

C9

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Component CO2

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Table2. Composition of the crude oils used (mole %)

C12+

C10

C11

4.13 60.06

0.00

0.00

0.16 0.33 0.17 0.41 0.30 0.33 4.83 7.40 7.90

7.22

6.76

Type N-Oil

0.01

0.38

0.51 1.28 0.44 1.84 1.44 2.33 7.43 8.73 6.74

8.69

7.73 5.04 47.41

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Type B-Oil

A schematic of the IFT measurement apparatus is shown in Figure 2 The apparatus allows

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determination of interfacial tension between gas-liquid and liquid-liquid systems at reservoir conditions. The device includes a viewing chamber inside which the two phases are brought in

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contact as the drop phase and the bulk phase. The shape of the interface could be monitored

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through two high pressure windows embedded at either side of the cell. A light source, which is mounted in front of the viewing glasses, provides the required light for the camera to monitor the

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drop shape. The drop shape is then analyzed by software to give the gas-oil IFT and the pertinent Bond number. Two supply tanks for fluids are connected to the viewing cell. To create a pendant drop, a capillary injector is plugged at the top of the cell [25-26].

Figure2. Schematic illustration of the experimental setup [27]. 6   

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A high pressure pump is used to maintain the cell pressure at a desired value. The whole system is covered by a heating jacket to establish a high temperature condition. The temperature of the sight glassed cell and fluid storage tanks were then set to the desired one. Using a piston, the oil phase was transferred into the droplet fluid storage tank (DT) and the gas phase was transferred into the bulk fluid storage tank (BT). A pressure producer is used to inject the gas into the cell

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and to pressurize it at a desired value. Enough time is then given to the whole system to reach equilibrium conditions. After the oil temperature remained constant in the storage tank, it is

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injected from the top of the cell to form a drop. Drop shape analysis software is used to calculate the gas-oil interfacial tension and Bond number based on the shape of the drop. The IFT values

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are recorded automatically versus time until it reaches a relatively constant value. The average of the IFT data in the last 100 seconds is calculated and reported as the IFT at that pressure and

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temperature (thermodynamic equilibrium) conditions. To achieve thermodynamic equilibrium quickly, several oil droplets (10-20 droplets) are placed at the bottom of the cell before injecting gas into the cell. For the precise measurement of gas-oil IFT at a specific temperature and

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pressure, it is necessary that the oil density to be measured under the same conditions. Density of crude oil was determined experimentally by a high pressure oscillating tube densitometer (DMA

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HPM Anton-Paar) [28].

4. Results and Discussion

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The results of density measurement at temperature of 323.15 K are shown in Figure 3 As it can be seen from this figure, oil density varies linearly with pressure at a particular temperature. Therefore, one can use the pertinent correlation to estimate oil density. The density of CO2 and CH4 was obtained from National Institute of Standards and Technology source data [29].

(a)

(b)

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Figure3. Oil density measured at temperature of 323.15 K and different pressures for (a) N-OIL and (b) B-OIL

Similar to the IFT values, Bond number values and droplet volume are recorded versus time and their average values at the last 100 seconds is considered as the Bond number and the droplet

Bond Number =1.15 (-)

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(a)

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volume at that particular pressure and temperature (Figure 4).

(b) Droplet Volume= 6.74 (mm3)

(c) 8   

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Figure 4. IFT (a), Bond Number (b), Droplet volume (c) versus time for B-OIL drop in the presence of CH4 at temperature of 323.15 K and pressure of 4.13 MPa.

After the IFT measurement versus time at a specific pressure was completed, the equilibrium value of IFT was estimated. The experiment was repeated for different pressures and the

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equilibrium IFT values were plotted against pressure for various fluid pairs.

To validate our results, the experimentally measured data of this study was compared with the

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outcomes of the previous studies. Here, IFT between Merck n-Heptane and CO2 gas vs. pressure

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at 333.15 and 323.15 K was measured. The measured values were then plotted against pressure

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to investigate the interfacial behavior of the system at different pressures. IFT values were

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carefully extrapolated linearly to zero, which was previously defined as the VIT-MMP of the crude oil−gas system from which the minimum miscible pressure was estimated and compared with the literature data. The results are compared in table 3 that shows a close match between the

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two sets of data [24, 29]. This table compares two quantities with previous studied data:

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i) The slope of the IFT vs. pressure curve (DR, decline rate).

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ii) Minimum miscible pressure (MMP) that is obtained from IFT vs. pressure curve.

Table3. Comparing the experimental results with literature data Experimental

Ref

Percent

Experimental

Ref

Percent

Result

[25]

difference

Result

[30]

difference

DR, decline rate (109 m)

1.90

2

-5 %

1.63

1.64

-0.6%

MMP (MPa)

8.4

8.5

-1.2 %

8.93

9

-0.77%

As the data of table 3 shows, the estimated differences of two quantities are negligible, confirming the repeatability and accuracy of the experimental results of this study. The estimated IFT data between B-OIL and CH4/CO2 at the temperature of 323.15 K and different pressures is shown in Figure 5. As can be seen, the equilibrium IFT reduces with pressure for both gas types. In Figure 5.a, the trend of IFT versus pressure for both gases was 9   

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correlated similarly, however, in Figure 5.b; the curve for CH4 was correlated using a second order polynomial, while for CO2, the IFT versus pressure data was treated as three linear parts.

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This trend has been pointed out previously by different authors [22].

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(a)

(b) 10   

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Figure 5. IFT curve vs. pressure for B-OIL in the presence of CH4 and CO2 gases at the temperature of 323.15 K (a) both sets of data are fitted to linear function,(b) oil-CH4 gas IFT vs. pressure is fitted to a second order polynomial function.

As it can be seen from Figure 5, CO2 has a stronger effect on the IFT compared to CH4. The strong affinity of carbon dioxide to the interface could be associated to its accumulation at the CH4-oil IFT data versus pressure

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interfacial region that consequently decreases IFT [31].

follows a second order polynomial function, which is different from that of CO2–oil system. This

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difference is likely because of accumulation and precipitation of asphaltene for the case of CO2oil, which results in significant gas-oil IFT reduction. This phenomenon changes minimum

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miscible pressure (MMP). However, for CH4, asphaltene does not precipitate. But it, likely, gradually accumulates at the interface of two fluids. Consequently, IFT does not approach zero

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and two fluids remain immiscible. As it is shown in Figure 5 (b), the second order function fitted to the IFT data versus pressure does not have a root. Therefore, the two fluids are immiscible in each other. Under the situation that the fitted function of IFT data for CH4 gas and high

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asphaltene content oil is considered similar to CO2, three distinct linear intervals are recognized. These intervals correspond to light compositions separation and asphaltene accumulation

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conditions. As pressure increases, more CO2 dissolves in the oil phase and hence the oil phase

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swells and gas-oil IFT decreases. However, the slope of IFT reduction at second intervals range is smaller compared to that of the low pressure interval. At second intervals, although dissolving

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CO2 causes the reduction of IFT value, accumulation of asphaltene at the interface of the oil phase and CO2 occurs, which results in increasing the IFT. Increasing trend of the IFT in the third interval is critically amplified when the surface coverage of the particles surpassed a threshold value (e.g., + 60 % surface coverage) [32-33]. However, the effect of CO2 solubility on IFT is more intense than surface coverage. Therefore, at high pressure range the slope of IFT reduction is much smaller than that of the first interval. However, for CH4 gas, from low pressure to high pressure, the two abovementioned mechanisms exist, but it would not reach the threshold point (e.g., + 60 % surface coverage), hence the extreme change in the IFT versus pressure curve is not observed. As change in intermolecular forces and the surface area vs volume ratio of the drop would affect the surface coverage of asphaltene’s at the liquid/gas interface. The variation and rate of change of the IFT data could specify asphaltene accumulation. Figure 6 shows the impact of asphaltene

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in a synthesized solution (75 volume percent toluene and 25 volume percent n-heptane) on

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change of IFT slope at different pressure regions and asphaltene content.

Figure 6. The equilibrium IFT versus pressure for CO2 and oil solutions with different asphaltene

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content at 323.15 K [34].

As it is shown in Figure 6, IFT reduction at low pressure interval could be mainly due to higher

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mass transfer between the gas and oil phases as the system is further pressurized. As pressure

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increases, more CO2 dissolves in the oil phase and consequently the oil phase swells and gas-oil IFT decreases. However, the slope of IFT reduction at high pressure range is gentler compared to

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that of the low pressure interval. At high pressure range, although CO2 dissolution reduces the IFT value, accumulation of asphaltene at the interface of the oil phase and CO2 occurs, which results on increasing the IFT. The trend of the IFT data is critically amplified when the surface coverage of the particles surpassed a threshold value (e.g., + 60 % surface coverage) [32-33]. However, the results shows that the effect of CO2 solubility on IFT is more pronounced than surface coverage. Therefore, at high pressure conditions the slope of IFT reduction is much smaller than that at lower pressure. The variation of Bond number versus pressure provides the same results, however, more rigorously. The Bond number is the ratio of gravitational to capillary forces that makes it dimensionless, while IFT directly contains only the gravity force in its measurements. The gravity tends to elongate the oil droplet while capillarity tends to keep the droplet at its minimum specific area (i.e. in spherical shape). Consequently, the Bond number is a more expressive parameter than IFT. Figure 7 shows the variation of both IFT and the Bond number as a function of pressure for a solution with 5% mass fraction of Asphaltene [34]. 12   

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Figure 7. Variation of IFT and Bond number versus pressure for the oil solution containing 5% of B

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asphaltene without nanoparticles at 50°C.

As it can be seen from Figure 7, at low pressure conditions (ranging from 2.5 to 5.5 MPa), IFT

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decreases monotonously with a constant slope with pressure and so does the Bond number increases with a constant slope. Once asphaltene starts to accumulation then the IFT would

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decrease as pressure increases, however, with a lower slope and likewise for the Bond number. It should also be pointed out that for both curves of IFT and the Bond number, the pressure, at

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which the asphaltene accumulation takes place, is the same. The arrow in this figure shows the

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pressure where the slope changes (in this Figure 6.2 MPa) and could be a proper indication of the onset of asphaltene accumulation. It increases as the pressure goes up since the gravity force overcomes the capillary force resulting in a larger slope. In the second interval, because of asphaltene accumulation at the interface of oil-gas, droplet shape changes with pressure. The gravity force and capillary force changes reduce their corresponding impacts and Bond number increases with gentle slope as pressure increases. Once asphaltene precipitation occurs at the interface of two fluids then capillary overcomes the gravity force thus a lower slope of the Bond number is expected at higher pressures. Figure 8 shows that the oil droplet volume in presence of both types of gas reduces linearly with pressure. The equilibrium shape of a pendant drop is affected by the relative amount of gravity force to the capillary force, i.e. Bond number. The gravity forces tend to pull the drop downwards, while the capillary forces have the tendency to keep the spherical shape of a drop. Therefore, we made an attempt to track changes in the Bond number with increasing the pressure. 13   

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(a)

(b)

Figure8. Oil droplet volume curve vs. pressure during IFT measurement between B-OIL and CH4 gas (a),

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and B-OIL and CO2 gas (b) at temperature of 323.15 K

A sample of these results is shown in Figure 9 for a drop of B-OIL in presence of CH4 and CO2.

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Figure 9 shows that for CH4-oil system, the Bond number increases linearly as the cell pressure increases. The data of Figure 9 shows no considerable slope change for CH4. This increase is merely due to the reduction in the IFT values as a result of improving gas solubility in the oil

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phase. Similar to the case of CH4, the Bond number increases by increasing the cell pressure for

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the case of CO2, however, with different trend. For the case of CO2, three distinct intervals can be distinguished. In the first interval, assuming a relatively constant density, the increase in the

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Bond number is due to reduction in the drop size and hence decreasing the gravity force that is more considerable compared to that of CH4. The IFT values would also decrease with pressure. In the other hand, higher CO2 dissolution in oil at higher pressure results in more oil swelling. These two reasons cause the Bond number to change rapidly for the case of CO2 compared to that of CH4.

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Figure9. Bond number vs. pressure for B-OIL drop in the presence of CH4 and CO2 at temperature of 323.15 K

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In the second interval, because of asphaltene accumulation at the two phase interface, the surface forces change and hence the IFT amount changes with a gentle slope; therefore, Bond number

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changes with pressure with a gentler slope than that of the first interval. In the third interval, the IFT values reduce slightly with pressure. Hence, Bond number curve versus pressure in this

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interval is almost horizontal and increases with a near zero slope. This accumulation may occur at oil−water, oil−rock and oil−gas interfaces, that can alter the interfacial properties (i.e.

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interfacial tension and wettability). The interfacial properties of a fluid system having colloidal particles such as asphaltenes, changes as the interfacial concentration of the particles increases. Increasing trend of the IFT is critically amplified when the surface coverage of the particles surpassed a threshold value (e.g., + 60 % surface coverage) [32-33]. It was also specified that the asphaltene forms two dimensional thin islands or nets at the interface at low concentrations (low surface coverage), the structure transfigures to three-dimensional arrangements at higher concentrations. It is concluded that the multiple decline rate Bond number curves occur if the liquid phase contains small colloidal particles or if it was contaminated by any surface active materials during the tests (e.g., it may be due to practically negligible contaminations in the apparatus) [32-33]. Pressurizing the CO2 gas causes the precipitation of asphaltene molecules at the interface, which makes the IFT values to decrease slightly. Accumulation of the asphaltene molecules at the two-phase interface also causes the drop to be prolonged slightly. Because the gravity force decreases linearly by volume reduction, the ratio of these two forces (i.e., Bond 15   

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number) will be relatively constant (3rd slope). For the CH4 gas, because the accumulation of asphaltene happens less than CO2 gas, it do not reach the threshold surface coverage. Hence, there is no considerable change in the slope of the Bond number data. For validating the experimental results, the tests performed on type B oil were repeated for type N-OIL. Figure 10 shows the plot of IFT value against pressure for type N-OIL. Since the

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asphaltene content of both types of oil is almost similar, the type N-OIL exhibited very similar behavior in terms of the plot of CH4-oil and CO2-oil IFT versus pressure, as well as the Bond

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number versus pressure as they are shown in Figure 10 and Figure 11, respectively.

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Figure10. IFT curve vs. pressure for N-OIL in the neighbor of CH4 and CO2 gases.

Figure 11. Bond number curve vs. pressure for N-OIL drop in the neighbor of CH4 and CO2 gases. 16   

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As it can be seen from Figure 11, the Bond number for type N-OIL droplet in the presence of CH4 increases linearly with pressure, similar to the case of type B-OIL. When plotting Bond number against pressure for the fluid pair CO2 and type N-OIL, three distinct intervals could be recognized. Similar to the type B-OIL, in the first interval, the oil-swelling occurs at a low pressure. In the second interval, because of asphaltene aggregation at the oil droplet interface,

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Bond number rises slightly with pressure. In the third interval more asphaltene accumulation would take place and the rate of change in Bond number is much lower compared to the ones in

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the other two intervals.

Each IFT and Bond number measurement test was repeated four times and their

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average value was reported here. The error in measuring interfacial tension at each pressure is less than 0.1 mN/m. Also, the results of error analysis indicated that the

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maximum standard deviation of gas-oil interfacial measurement in all the experiments was ±0.2 mN/m.

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6. Conclusions

From the experimental results of this study the following conclusions can be drawn:

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1. In the case of CO2 gas and an oil type with high asphaltene content the plot of Bond Number data versus pressure could be treated as three distinct intervals: The first interval corresponds to the significant increase in Bond number with increasing

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• •

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pressure because of CO2 gas solubility in oil. In the second interval, because of asphaltene accumulation at the interface of oil-gas, droplet shape changes with pressure. The gravity force and capillary force changes reduce their corresponding impacts and Bond number increases with gentle slope as pressure increases. •

Finally in the third interval, the small reduction in gravity force and also, low descending in IFT values causes the Bond number to be relatively constant and the plot of Bond number versus pressure have a near zero slope with compared to the other two intervals.

2. Considering the IFT curve versus pressure for CH4 and different types of oil with high asphaltene content, IFT reduces as a second order polynomial function. This function has no root that means that oil and gas are immiscible. However, the plot of IFT versus pressure for CO2 and the same types of oil could be divided into three distinct linear relations with three different slopes. This means that there is no asphaltene precipitation when injecting CH4 gas to the 17   

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reservoir with high asphaltene content oil although asphaltene aggregation may happen. 3. The Bond number curve versus pressure is a suitable tool for determining asphaltene precipitation condition. Asphaltene precipitation not only changes the amount of IFT between two immiscible phases, but also affects the shape of pendant drop. Both of these effects are captured through Bond number; therefore, variation of Bond number with pressure is an

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appropriate indication of asphaltene precipitation.

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7. Acknowledgments

The authors are grateful to Prof. Sh. Ayatollahi and Dr. M. Escorochi for their valuable advices.

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We would also like to thank Ms. S. Noorbakhsh, Mr. A. Golkari and Mr. E. Mahdavi for their helps over the course of this study. Special thanks to the core analysis laboratory director, Ms.

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Sh. Meratian, for providing a suitable situation and work space.

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8. References

[1] Kazemzadeh, Y., Riazi, M. “Comparison between Minimum Miscibility Pressure (MMP) of Different Gases using Slim Tube and Rising Bubble Methods”, Petroleum Technical Conference and Exhibition,

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[2] Green, D. W.; Willhite, G. P. “Enhanced Oil Recovery”, Henry L. Doherty Memorial Fund of AIME,

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Society of Petroleum Engineers: Richardson, TX, (1998). [3] Stalkup Jr., F.I., “Miscible Displacement”, SPE Monograph, Vol.8, SPE of AIME (1983), New York. [4] Holm, L.W.: “Miscible Displacement,” in H.B. Bradley (Ed.), in Petroleum Engineering Hand Book, Society of Petroleum Engineers, Richardson, TX, (1987) 1-45. [5] Lake, L.W., “Enhanced Oil Recovery”, Prentice-Hall Englewood Cliffs, NJ (1989) 234. [6] Danesh, A. “PVT and Phase Behavior of Petroleum Reservoir Fluids”, 1st ed.; Elsevier Science, B.V: Oxford, U.K., (1998); pp 253−255. [7] Stalkup, Jr., F.I.: “Miscible Displacement”, Henry L. Doherty Series, SPE Monograph, (1983). [8] Drop, Bubbles, Pearls, Waves, “Capillarity and Wetting Phenomena”, Springer, (2004).

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[9] Rao, D.N.: “A New Technique of Vanishing interfacial Tension for Miscibility Determination,” Fluid Phase Equilibria, 139, (1997) 311-324. [10] Rao, D.N. and Lee, J.I.: “Application of the New Vanishing Interfacial Tension Technique to Evaluate Miscibility Conditions for the Terra Nova Offshore Project,” Journal of Petroleum Science and

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Precipitation”, AIChE J., 53 (11), (2007), 2940−2947. [17] Firoozabadi, A. “Thermodynamics of hydrocarbon reservoirs”, McGraw-Hill: New York, (1999). [18] Mohammadi, A. H.; Eslamimanesh, A.; Richon, D. Monodisperse, “Thermodynamic Model based on Chemical Flory−Hu ggins Polymer Solution Theories for Predicting Asphaltene Precipitation”, Industrial and Engineering Chemistry Research, 51 (10), (2012), 4041−4055. [19] Cao, M.; Gu, Y. “Oil Recovery Mechanisms and Asphaltene Precipitation Phenomenon in Immiscible and Miscible CO2 Flooding Processes”, Fuel, 109 (0), (2013), 157−166. [20] Escrochi, M. Mehranbod, N. and Shahab Ayatollahi, “The Gas−Oil Interfacial Behavior during Gas Injection into an Asphaltenic Oil Reservoir”, Journal of Chemical Engineering Data, 58 (9), 2013, 2513– 2526 [21] Malayeri, M. Matourian, R, “Asphaltene in oil industry”, Setayesh, Tehran, Iran, (2012). 19   

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[22] Wang, X.; Zhang, S.; Gu, Y. “Four Important Onset Pressures for Mutual Interactions between Each of Three Crude Oils and CO2”. Journal of Chemical Engineering Data, 55 (10), (2010), 4390–4398. [23] Wang, X.; Gu, Y. “Oil Recovery and Permeability Reduction of a Tight Sandstone Reservoir in Immiscible and Miscible CO2 Flooding Processes”. Industrial and Engineering Chemistry Research, 50,

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[28] Anton Paar Instruction Manual, “DMA HP Density Measuring Cell for High Pressure and High

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[29] National Institute of Standards and Technology, www.nist.gov. it was available on 6/4/2014

[30] Jaeger, Ph.T. Eggers, R. “Interfacial Properties at Elevated Pressures in Reservoir Systems 80−85.

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Containing Compressed or Supercritical Carbon Dioxide”. Journal of Supercritical Fluids, 66, (2011),

[31] O.G. Niño Amézquita, S. Enders, P.T. Jaeger, R. Eggers “Interfacial properties of mixtures containing supercritical gases” Journal of Supercritical Fluids 55.2 (2010): 724-734. [32] Tambe, D. E.; Sharma, M. M. “The Effect of Colloidal Particles on Fluid−Fluid Interfacial Properties and Emulsion Stability”, Advance Colloid Interface Science, 52, 1−63, (1994), 1847−1853.

[33] Cadena-Nava, R. D.; Cosultchi, A.; Ruiz-Garcia, J. “Asphaltene Behavior at Interfaces”, Energy Fuels, 21, (2007), 2129−2137.

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[34] Riazi, M., Kazemzadeh, Y., Parsaei, R., “Experimental Investigation of the Effect of Asphaltene and

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Normal Paraffin on CO2-Oil Interfacial Tension.” Journal of Dispersion Science and Technology, (2014),

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Highlights

Asphaltene precipitation occurs after the surface coverage beat a threshold value



Bond number (Bo) vs pressure is a tool for determining asphaltene precipitation



Three distinct regions could be identified in Bo curve for the CO2-oil system



In the Bo vs pressure for the CH4-oil, no major slope change is observed



There is no asphaltene precipitation when injecting CH4 to the asphaltenic oil

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*Graphical Abstract (for review)

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