Impact of Fe3O4 nanoparticles on asphaltene precipitation during CO2 injection

Impact of Fe3O4 nanoparticles on asphaltene precipitation during CO2 injection

Journal of Natural Gas Science and Engineering 22 (2015) 227e234 Contents lists available at ScienceDirect Journal of Natural Gas Science and Engine...

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Journal of Natural Gas Science and Engineering 22 (2015) 227e234

Contents lists available at ScienceDirect

Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse

Impact of Fe3O4 nanoparticles on asphaltene precipitation during CO2 injection Y. Kazemzadeh a, M.R. Malayeri a, M. Riazi b, *, R. Parsaei a a b

Department of Petroleum Engineering, School of Chemical, Petroleum and Gas Eng, Shiraz University, Shiraz, Iran Enhanced Oil Recovery (EOR) Research Centre, School of Chemical, Petroleum and Gas Eng, Shiraz University, Shiraz, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 October 2014 Received in revised form 21 November 2014 Accepted 24 November 2014 Available online

Precipitation of asphaltene may occur during injecting CO2 into oil reservoirs for enhanced oil recovery (EOR). A potential means of combating such occurrence is to simultaneously inject metal oxide nanoparticles along with the CO2 stream. The present experimental study investigates the impact of Fe3O4 nanoparticles on the precipitation of asphaltene using interfacial tension (IFT) and Bond number measurements. The variation of the latter versus pressure was found to better characterize the onset and intensity of asphaltene precipitation than that of IFT. This is because both changes in IFT and shape of the pendant drop due to gravity force are more rigorously captured by the Bond number. Two different types of asphaltenes were present in the synthesized solution of n-heptane and toluene. Various mass fractions of Fe3O4 nanoparticles were also used at different temperatures of 50 and 70  C. The experimental results show that the higher the mass fraction of Fe3O4 nanoparticles is, the lower would be the intensity of the asphaltene precipitation for the attempted mass fractions. The characteristics of asphaltene in terms of structure were also found to have profound impact on the performance of nanoparticles. © 2014 Elsevier B.V. All rights reserved.

Keywords: Asphaltene precipitation Interfacial tension (IFT) Enhanced oil recovery (EOR) CO2 injection Nanoparticle

1. Introduction Inexpensive and redundant green-house CO2 gas can be captured then injected into oil reservoirs to enhance oil recovery (Sarma, 2003; Kokal et al., 1992). Heavy oil reservoirs though may contain a large amount of asphaltene, which would precipitate in porous medium, wellbore and wellhead facilities (Escrochi et al., 2013; Leonaritis and Mansoori, 1987). Moreover, due to nonuniformity of oil reservoirs in terms of API and density, the percentage and type of asphaltene in the oil may vary from one location to another resulting in different affinity of asphaltene to precipitate. An appropriate operation of oil extraction along with the right choice of mitigation technique would minimize the occurrence of asphaltene precipitation (Ashoori, 2005; Hammami et al., 1999; Syunyaev et al., 2009). A variety of mitigation techniques have been developed to address the problem of asphaltene depending on the location and severity of the deposition (Karan et al., 2003). These include, but not limited to, solvents and chemical inhibitors and pyrogenics as well as ultrasound methods. Among them, using solvents is

* Corresponding author. E-mail address: [email protected] (M. Riazi). http://dx.doi.org/10.1016/j.jngse.2014.11.033 1875-5100/© 2014 Elsevier B.V. All rights reserved.

widespread since asphaltene is soluble in aromatics such as toluene and xylene (Marczewski and Szymula, 2002; Clarke and Pruden, 1997). This is often carried out by combining various solvents, i.e. mixture of chemical solvents like diesel and xylene (Malayeri and Matourian, 2012). Not surprisingly, the utilization of such techniques cannot be generalized for every well and reservoir. High price tag, detrimental impacts on the surrounding environment and poor efficiency are perhaps among other reasons that restrict their widespread utilization (Marczewski and Szymula, 2002; Clarke and Pruden, 1997; Malayeri and Matourian, 2012). The recent technological advancement in nanotechnology provided opportunities to tackle some of these disadvantages through the injection of nanoparticles of various types (Haroun et al., 2012; Kong and Ohadi, 2010; Matteo et al., 2012). This, nevertheless, requires thorough and careful investigation of their impact on asphaltene precipitation (Haindade et al., 2012; Ogolo et al., 2012). Mechanisms through which nanoparticles improve operational performance include: rock wettability alteration, reduced IFT, oil viscosity reduction, reducing the mobility ratio and permeability changes (Ogolo et al., 2012). For EOR purposes, the nanoparticles are mainly metal or metal oxide particles (Nassar et al., 2011a; Alboudwarej et al., 2005). Such nanoparticles are characterized with i) large surface to volume ratio, ii) high degree of suspension,

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iii) enormous absorption capacity and being catalytically highly active (Nassar et al., 2011b; Jain et al., 2008; Hamedi Shokrlu and Babadagli, 2013). The particles can make asphaltene suspended in the oil and prevent them from being precipitated and can also remove asphaltene precipitation from the surface using its thermal catalytic role (Greff and Babadagli, 2011; Nassar et al., 2011c). Thus the functionality of nanoparticles can be divided into two simultaneous actions of i) their high absorptivity resulting from their ultra-small size that can quickly absorb the suspended asphaltene particles. This improves oil mobility and prevents asphaltene aggregation and coagulation. Parameters influencing on asphaltene adsorption include contact time, asphaltene initial saturation, size, water content, temperature, and other existing molecules (Abu Tarbush, January 2014; Nassar et al., 2011d, 2011e, 2011f; Nassar, 2010); and ii) their improved thermal properties which facilitate the mitigation of asphaltene precipitation (Nassar et al., 2011b; Jain et al., 2008; Hamedi Shokrlu and Babadagli, 2013; Greff and Babadagli, 2011; Nassar et al., 2011c; Abu Tarbush, January 2014). This process contains in-situ heavy oil upgrading by the removal of asphaltenes through catalytic oxidation. Disappearance of IFT method, as an innovative tool, can potentially be used to detect the onset and intensity of asphaltene precipitation during gas injection. The method is rapid and qualitative and it is also very useful for computing optimal injection of gases for EOR purposes (Rao and Lee, 2002, 2003). Moreover, the method is quite accurate, and a small amount of sample would sufficient for the experiment. In this study, the asphaltene precipitation was analyzed by the IFT of oil and CO2 gas and from the changes in the IFT slope in terms of pressure of starting point and intensity (Nobakht et al., 2008; Wang et al., 2010; Zolghadr, 2011; Zolghadr et al., 2013). As soon as precipitation starts, IFT suddenly changes the course in terms of its slope with pressure. In the low pressure region, the IFT decreases as temperature increases (Escrochi et al., 2013). At a certain pressure though, this trend reverses and IFT starts to increase at higher temperatures. The increased solubility of CO2 in the oil solution with increased temperature would also lead to more IFT reduction at lower pressures. Furthermore, the solubility of CO2 in the crude oil decreases at elevated temperatures in particular when the pressure increases. Paraffin contents play also a key role in controlling the IFT behavior. Moreover, better efficiency of CO2 injection can be achieved in the reservoirs at high pressures and low temperatures, while reservoirs at lower pressures are more suitable if the temperature is higher (Escrochi et al., 2013). Wang et al. (Wang et al., 2010) studied the behavior of three Canadian crude oil samples (two light oils and one intermediate oil) in the presence of CO2 using the method of disappearance of IFT. They found that the equilibrium IFT usually decreases linearly through three distinct pressure regions. In the first region, as pressure increases the IFT decreases due to improved solubility of CO2 in oil. In the second region, the IFT increases suddenly and then decreases rapidly before the trend turns into a linear pattern. This behavior is because of asphaltene precipitation and the rapid separation of light components. They concluded that the measured IFT in this range would be between a relatively heavier oil and CO2. In the third region, in which the light components of oil have already been extracted, the IFT measurements would be between the heaviest crude oil components and CO2 (Rao and Lee, 2003). In addition to IFT measurements, the Bond number (Bo) can be used to produce similar analyses but more rigorously compared to IFT. The theory upon which the pendant drop shape analysis is

based on the relation between the pressure gradient across the gaseliquid interface which can be given by the Laplace's equation of capillarity (Fig. 1):

  1 1 2g þ zðDrÞg ¼ g þ R1 R2 b

(1)

where g is the interfacial tension, R1 and R2 are the principal radii of curvature, b is the curve radius at the apex, g is the gravitational acceleration, and Dr is density difference between the two fluids. Equation (1) may be rearranged in dimensionless form of:

1 sin∅ z ¼b þ2 þ R1=b x=b b

(2)

where R2 was replaced by its equivalent x/sin ∅ and the dimensionless quantity b is referred to as Bond number that is defined as the ratio of gravity force to capillary force and is given by the following equation:

Bond number ¼

b2 gDr g

(3)

By fitting equation (2) to the experimentally obtained profile of a pendant drop, the value of b is calculated from which the magnitude of IFT can then be estimated. A high value of b 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 (Rotenberg et al., 1983; 700 Operating Manual, VINCI Technologies, France). A large Bond number means that the shape of the pendant drop is more affected by the gravity force thus the shape of oil droplets is elongated toward the vertical direction. Contrariwise, if the relative strength of capillary force is higher than the gravity force, which is the case for small Bond numbers, then the shape of oil droplets becomes more spherical. Accordingly, the present study uses the Bond number measurements in addition to IFT to discern the onset and intensity of asphaltene precipitation of two synthesized oils at different temperatures.

Fig. 1. A pendant liquid drop surrounded by gas (Adamson and Gast, 1997).

Y. Kazemzadeh et al. / Journal of Natural Gas Science and Engineering 22 (2015) 227e234

2. Experimental setup and procedure The synthesized oil that has been attempted in this investigation contained toluene (75% by volume), n-heptane (25% by volume) and asphaltene extracted from two crude oil samples of B and N. Table 1 provides the composition of each crude oil from which asphaltene was extracted. The oil reservoirs of B and N from which asphaltene was extracted contain 5% mass fraction of asphaltene. Accordingly in the present study the same mass fraction of asphaltene was added to the synthetic solution. CO2 gas was also used with a purity of 99.99%. To extract asphaltene from crude oils, different standard procedures are available. In this study though, ASTM (D2007-80) standard was used that has already been proved to produce consistent results (Malayeri and Matourian, 2012). Once this was done then a magnet stirrer was used to dissolve asphaltene in the synthesized solution. At first, toluene (75% by volume) and nheptane (25% by volume) were mixed then the extracted asphaltene as 5% mass fraction of the total solution was added. The mixture was then stirred for 6 h to dissolve asphaltene in the solution. To make the oil solutions of having evenly suspended nanoparticles, the solution was centrifuged for 30 min at 5000 rpm. Thereafter both solutions of i) without any nanoparticle, and ii) with a certain percentage of nanoparticle were shaken for 24 h at 200 rpm. Both solutions were then sonicated for 2 h to allow even and complete distribution of nanoparticles in the solution. The nanoparticles were Fe3O4, which reported to have excellent absorptivity (Nassar et al., 2011c) with a particle size ranging from 20 to 30 nm and a purity of 99%. A new method for the analysis of the IFT of crude oil and CO2 using the VIT technique showed that the time needed for attaining equilibrium between the small liquid droplets and the surrounding gas is about 100 s, which can further be reduced by temperature. A mass transfer model can also be presented to analyze the dynamic behavior of the droplets during VIT experiments which showed that the concentration of the interface quickly tended toward equilibrium. To measure the IFT at different pressures and temperatures, the subsequent procedure was followed. Prior to each test, the apparatus was cleaned using solvents such as isopropyl alcohol, and/or toluene to dissolve any traces of the remaining oil there. Then, it was dried by purging nitrogen, and quickly evacuated, to ensure that all the remaining contaminants were evaporated. The view cell was filled with CO at the desired pressure from the bulk tank, and then the temperature was adjusted to the required value. A droplet of oil was injected from the droplet tank into the cell containing CO2 after the temperature stabilized. For each test, four different droplets were created and each was monitored for 1 h. The software captured the shape of the droplet every minute, and the IFT was then calculated by the drop shape analysis software. The IFT device as shown in Fig. 2 includes a high-pressure visible chamber, which is made of Hastalloy. The system also possessed a means for injecting the attempted oil. There are also a high pressure pump and a pressure gauge to measure the pressure in the system. The whole measuring part was also located in a duly controllable thermal chamber. A high-pressure glass made of emerald was in

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the front of the chamber. The high pressure chamber consisted of a capillary tube located at the top of it through which the oil droplets were injected into the gas. The system had a high resolution camera, which was located on the moving surface. The camera lens was set such that a clear picture of the needle (capillary tube) could be observed. A light source was placed on the other side of the chamber, which allowed light and clear pictures to be taken. The camera was connected to a computer monitor that calculated the IFT of oil and gas equipped with droplet shape analysis software. The cleanliness of different parts of device is important for accurate measurement of IFT. Accordingly, at first, different parts, which can possibly be contaminated by fluids, have to be cleaned by toluene, acetone and finally demineralized water. To start a test, the oil phase was placed into the droplet fluid storage tank using a piston and likewise the CO2 gas into the bulk fluid storage tank. The temperature of visualization chamber, fluids cells and droplets was adjusted to set values of 50  C and 70  C in compliance with conditions in the reservoirs. By using a plunger, the oil was then injected into the chamber of droplet fluid injection. To inject gas into the visualization chamber, the high pressure pump was used which allowed the gas to be injected into chamber at a desired pressure. A capillary tube was mounted at the top of the high pressure cell from which the oil droplet was injected to the bulk gas. Once these were done then the system was given enough time to reach steady state conditions. Afterward, the IFT of two fluids were reported as a function of time. IFT measurements continued before equilibrium was reached. Interfacial tension decreases due to mass transfer between oil droplet and CO2 gas with time before reaching a constant value. Equilibrium IFT occurs when IFT does not change with time. This can nevertheless be expedited by improving mass transfer when injecting a few oil droplets into the bottom of cell. The average IFT in the last 100 s was calculated and reported as IFT at any given pressure and temperature when the system was confirmed to be in equilibrium state (see Fig. 3). The device reported also the values of Bond number at any specific temperature and pressure. The average Bond number in the last 100 s, was used for the latter analysis (Nobakht et al., 2008). The IFT and Bond number measurements were repeated four times and only their average value is reported here. The error in measuring IFT at each pressure was less than 0.1 mN/m. The error analysis also indicated that the maximum standard deviation of gaseoil interfacial measurement in all experiments was ±0.1 mN/m. The liquid density was also measured at different pressures and temperatures using a hydrometer (Anton Parr device) with high accuracy of 0.00001 g/ml. The density of CO2 gas at a given pressure and temperature was taken from standards of NIST (700 Operating Manual, VINCI Technologies, France).

3. Results and discussion 3.1. Analysis of asphaltene precipitation using IFT and Bond number measurements Disappearance of IFT was used for measuring the minimum miscible pressure. It served also as an indicator to detect the onset and intensity of asphaltene precipitation. To do so, IFT was

Table 1 The composition of the oils from which asphaltene was extracted. Component

CO2

C1

C2

C3

iC4

nC4

iC5

nC5

C6

C7

C8

C9

C10

C11

Cþ 12

Type B-oil

0

0.00

0.16

0.33

0.17

0.41

0.30

0.33

4.83

7.40

7.90

7.22

6.76

4.13

60.06

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

230

Y. Kazemzadeh et al. / Journal of Natural Gas Science and Engineering 22 (2015) 227e234

Fig. 2. Sketch of the experimental setup (Rotenberg et al., 1983).

IFT (mN/m)

16.0 15.0

Equilibrium IFT= 13.53 (mN/m)

14.0 13.0

12

1.7

11

1.6

10

1.5

9 8

1.4

7

1.3

6

1.2

5

1.1

4

1

3 2

12.0

Bond number (-)

17.0

However, the rate of IFT reduction in the second region is less compared to that of the lower pressure region. Furthermore, in the last region, the dissolved CO2 causes initially IFT to decrease. This though coincides with the accumulation of asphaltene at the interface of the gaseoil that causes IFT to slightly increase. An appreciable increase of the IFT slope in the last region would occur when the surface coverage of the asphaltene particles reaches a threshold value of 60% or so. However, the effect of CO2 solubility on IFT is more profound than the impact of surface coverage. Accordingly at high pressures, the rate of IFT reduction is much smaller to that in the first region (Tambe and Sharma, 1994). The variation of Bond number versus pressure provides the same results but more rigorously as it will be shown later on. 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 tries to elongate the oil droplet while capillarity endeavors to keep the droplet spherical. Consequently, the Bond number is a more expressive parameter than IFT. Fig. 4 shows the variation of both IFT and the Bond number as a function of pressure for a solution with 5% mass fraction of Btype asphaltene without adding Fe3O4 nanoparticles. As it can be seen in Fig. 4, at low pressures ranging from 2.5 to 5.5 MPa, IFT decreases monotonously with a constant slope with

Euilibrium IFT (mN/m)

measured while increasing pressure. The change of slope in IFT curve in terms of pressure provides valuable information. For instance, asphaltene precipitation causes IFT to increase compared to circumstances where no precipitation is anticipated. The IFT measurement of crude oils and CO2 may provide three pressure gradients with different slopes. In the first region (at low pressures), any reduction of IFT implies the separation of light components from the attempted oil solution (vaporizing) and solubility of CO2 in oil droplet (condensing). The change of slope in the third region (at high pressures) is due to the precipitation of asphaltene in form of large molecules in the interface between the two fluids (oil and CO2 gas). The pressure gradient in the third region is caused by the disturbance of intermolecular forces balance followed by a sudden increase in IFT. In the second region (at intermediate pressures), which is usually observed in crude oil systems with a wide variety in oil compositions, the IFT slope is less than the first region and greater than the one in the third region. In this region, large molecules of asphaltene begin to aggregate at the interface of two fluids. In this study though, for the synthesized oil solution (toluene and n-heptane with a certain percentage of asphaltene that has a narrower range of oil composition compared to that in crude oil systems) with CO2, there were only two regions (i.e. the first and the last regions without a noticeable transition region) with different IFT slops. As pressure increases, more CO2 dissolves in the oil phase which in turn causes the oil phase to swell and IFT of gaseoil to decrease.

3

4

5 6 Pressure (MPa)

7

8

9

11.0 0

200

400

600

Time (s) Fig. 3. Typical variation of the IFT versus time.

800

1000

IFT

Bond number

Fig. 4. Variation of IFT and Bond number versus pressure for the oil solution containing 5% of B asphaltene without nanoparticles at 50  C.

Y. Kazemzadeh et al. / Journal of Natural Gas Science and Engineering 22 (2015) 227e234

3.2. Impact of asphaltene type on the performance of Fe3O4 nanoparticles The propensity of asphaltene to precipitate is presented in Fig. 6a as a function of only IFT for the solution containing B-type asphaltene with/without nanoparticles. Evidently, as a result of asphaltene precipitation, the slope of IFT decreases at high pressures. The addition of 0.5% of Fe3O4 nanoparticles to the oil solution caused asphaltene particles to be absorbed by the surface of nanoparticles. The higher that pressure is the lesser chance would be for the asphaltene particles to get aggregated and then precipitated. Thus, addition of nanoparticles increased the slope but not as much as the one in the first region. This means that nanoparticles reduced the asphaltene precipitation but did not completely eliminate it. Fig. 6b demonstrates the variation of Bo versus pressure for the same solution shown in Fig. 6a. It is apparent that the addition of Fe3O4 nanoparticles increases the slope at higher pressures compared to that without nanoparticles. Thus, the asphaltene precipitation is being reduced with the addition of nanoparticles so that the slope increases in the second region. Fig. 7a demonstrates how IFT changes versus pressure for the solution containing 5 wt% of N-type asphaltene with/without Fe3O4 nanoparticles. As Fig. 7a evidently shows, addition of nanoparticles to the solution of the N reservoir asphaltene increases slightly the second region slope but with no considerable effect on the IFT in the first region. A slight increase in the slope in the second region shows the asphaltene precipitation.

The variation of Bo versus pressure for the same solutions is shown in Fig. 7b. The values of Bond number are approximately equal at low pressures. In the second region i.e. high pressures, nonetheless, the absorption of asphaltene particles by the nanoparticles retards the aggregation of its large molecules. The presence of nanoparticles in the oil solution prevents asphaltene particles from accumulation at the gaseoil interface causing asphaltene to remain in the oil bulk. Meanwhile the slope of IFT decreases if asphaltene particles accumulate at the interface when pressure increases. Since the nanoparticles reduce the particle movement toward the interface by absorbing asphaltene then the IFT further reduces when pressure increases compared to the solution with no nanoparticles. This implies that Fe3O4 nanoparticles will not be able to completely eliminate the asphaltene precipitation. Adsorption of these types of asphaltene on the surface of nanoparticles is enhanced by low H/C ratio and high nitrogen content. B-type asphaltene has more nitrogen and less H/C ratio than N-type asphaltene. Thus the adsorption capacity of Btype asphaltene was greater than the N-type asphaltene. Consequently, in the presence of nanoparticles, asphaltene precipitation in oil containing B-type asphaltene was lower to that of the N-type asphaltene (Nassar et al., 2012). As stated before, the change and rate of change of the IFT and Bo slopes indicate the onset and intensity of asphaltene precipitation, respectively. Table 2 shows the impact of nanoparticles in the synthesized solutions of containing two types of asphaltene on the change of IFT slope in the second region (high pressure) and consequently the intensity of asphaltene precipitation.

a 12 Equilibrium IFT (mN/m)

increased pressure and so does the Bond number increases with a constant slope. Once asphaltene starts to precipitate then the IFT decreases as pressure increases but with a lower slope and likewise for the Bond number but with opposite direction. It should also be pointed out that for both curves of IFT and the Bond number, the pressure, at which the precipitation occurs, is the same. The arrow in this figure indicates the pressure where the slope changes the course (in this figure 6.2 MPa) and implies the onset of asphaltene precipitation. Fig. 5 presents similar results of IFT and the Bond number but when 0.5 wt% of Fe3O4 nanoparticles were added to the solution of B-type asphaltene. The slope changed again for the two curves (IFT and Bo) at the same pressure. The reduction of the slope for both curves is due to the addition of Fe3O4 nanoparticles. Moreover, the change in the IFT slope occurs at a higher pressure of 6.55 MPa (indicated by arrow) compared to the test with no nanoparticles (see Fig. 4).

231

11 10 9 8 7 6 5 4 3 2

3

4

5 6 Pressure (MPa) Without nanoparticles With nanoparticles

3

4

7

8

1.7

1.6

11

1.6

1.5

10

1.5

9 8

1.4

7

1.3

6

1.2

5

1.1

4

1

3 2

3

4

5 6 Pressure (MPa) IFT

7

8

9

Bond number

Fig. 5. IFT and Bond number versus pressure with a 5% of B asphaltene and 0.5% of nanoparticles at 50  C.

Bond number(-)

12

Bond number (-)

Equilibrium IFT (mN/m)

b 1.7

1.4

1.3 1.2 1.1 1

2

5

6

7

8

Pressure (MPa) Without nanopartciles

With nanoparticles

Fig. 6. a. IFT versus pressure of two solutions of 1) without nanoparticles and 2) 0.5% nanoparticles with 5 wt% B asphaltene at 50  C. b. Bond number versus pressure of two solutions 1) without nanoparticles and 2) 0.5% nanoparticles with 5 wt% B asphaltene at 50  C.

232

Table 2 Comparison of the IFT slope of the 1st to the 2nd region for B and N asphaltene types in the absence and presence of Fe3O4 nanoparticles at 50  C.

12 11

10 9 7 6

Slope of the 1st region

Slope of the 2nd region

Ratio of the IFT slope of 2nd to the 1st region

B-Oil (5 wt%)

0 0.5

2.05 2.03

0.45 0.83

21.95% 40.89%

N-Oil (5 wt%)

0 0.5

2.12 2.18

0.43 0.61

20.28% 27.98%

5 4 2

3

4

5 Pressure (MPa)

Without nanopartices

6

7

8

With nanoparticles

1.7

1.6 Bond number (-)

Nanoparticle wt%

8

3

b

Type of asphaltene

1.5 1.4

1.3 1.2 1.1

1 2

3

4

5 Pressure (MPa)

Without nanoparticles

6

7

8

With nanoparticles

Fig. 7. a. IFT versus pressure of two solutions of 1) without nanoparticles and 2) with 0.5% nanoparticles for an oil solution of 5 wt% N asphaltene at 50  C. b. Bond number versus pressure of two solutions of 1) without nanoparticles and 2) with 0.5% nanoparticles for an oil solution of 5 wt% N asphaltene at 50  C.

According to Table 2 in the solution containing B-type asphaltene, the slope of the second region is only 21.95% of the one in the first region. This changes though to 40.89% when nanoparticles were added to the solution. Furthermore, in the first region, the slope is approximately equal for both solutions of with/without nanoparticles. The addition of nanoparticles increases the slope of the second region by two-fold compared to that of without nanoparticles case. In the solution containing N-type asphaltene with the addition of nanoparticles the second region slope is about 1.5 times more to that without nanoparticles. Therefore, the impact of nanoparticles in retarding the precipitation of asphaltene is more pronounced in the solution containing B-type asphaltene compared to that of N-type. Table 3 presents similar results but for the Bond number. Likewise to the IFT analysis, the addition of nanoparticles to a solution containing B-type asphaltene causes the slope ratio of the second to the first regions to change from 24.7% to 82.44%. For the N-type asphaltene though it changes only from 25.52% to 46.25%. The order of magnitude of changes in the Bo slope is also far greater to that of IFT.

3.3. Effect of nanoparticle concentration on the asphaltene precipitation rate

it can be seen in this figure, the IFT slope changes with respect to the quantities of nanoparticles mainly in the second region. At low pressures i.e. the first region, IFT decreases as pressure increases with a steep slope but almost similar for all three attempted solutions. Nonetheless at high pressures, with possibility of getting asphaltene precipitated, the IFT slope was lowered for all three solutions. In the solution with no nanoparticles the slope of second region is the smallest compared to other two because of asphaltene precipitation. Intense precipitation of large asphaltene molecules in this solution at the interface of CO2 gas and oil has led to the disruption of intermolecular forces. However, the presence of nanoparticles in the solution causes asphaltene molecules to be absorbed on the surface of nanoparticles during the preparation of solution. At high pressures, asphaltene precipitation is expected to take place in the synthetic solution. Nonetheless, the nanoparticles diminish their aggregation and precipitation at the interface. As shown in Fig. 8, with the addition of 0.5 wt% of nanoparticles, the IFT change rate increases compared to the solution with no nanoparticles. This indicates that the intensity of asphaltene precipitation at the interface of both fluids has been reduced. If one increases the amount of nanoparticles in the solution then more

Table 3 Comparison of the Bond number slope of the 1st to the 2nd region for B and N asphaltene types in the absence and presence of Fe3O4 nanoparticles at 50  C. Type of asphaltene

Nanoparticle weight percent

Slope of the 1st region

Slope of the 2nd region

Ratio of the Bond number slope of 2nd to the 1st region

Type B-oil (5 wt%)

0 0.5

0.1055 0.1016

0.0261 0.0841

24.74% 82.77%

Type N-oil (5 wt%)

0 0.5

0.1136 0.1254

0.0290 0.0580

25.52% 46.25%

12 Equilibrium IFT (mN/m)

Equilibrium IFT (mN/m)

a

Y. Kazemzadeh et al. / Journal of Natural Gas Science and Engineering 22 (2015) 227e234

11 10 9 8 7 6 5 4

The performance of nanoparticles is influenced by various factors among them their quantity in terms of mass fraction. Fig. 8 shows the variation of IFT versus pressure for different mass fractions of 0 wt%, 0.5 wt% and 1.0 wt% of Fe3O4 nanoparticles in the solution along with the injected CO2 gas. The temperature was set to 70  C according to conditions in the attempted oil reservoirs. As

3

4

Without nanoparticles

5

6 Pressure (MPa)

With 0.5% wt nanoparticles

7

8

9

With 1% wt nanoparticles

Fig. 8. Variation of IFT versus pressure for the N type asphaltene in solutions with different concentration of nanoparticles at 70  C.

Y. Kazemzadeh et al. / Journal of Natural Gas Science and Engineering 22 (2015) 227e234

Table 4 Changes in the IFT slope ratio for the N type asphaltene in solutions with different mass fractions of nanoparticles at 70  C.

1.6 Bond number (-)

asphaltene molecules are expected to be absorbed by the nanoparticles. At high pressures (2nd region) nanoparticles absorb more asphaltene molecules, consequently retard them from moving towards the interface of the two fluids. Table 4 shows the relation of IFT in terms of pressure for each solution and CO2 at two regions. This table also compares IFT slopes at different regions for various solutions. According to Table 4, the synthetic oil solution with no nanoparticles, the IFT in the two regions with two different slopes decreases by increasing the pressure. In the first region, the IFT reduction occurs with a larger slope. In the second region though, with asphaltene precipitation, the IFT drops with a lower slope. Thus the slope in the second region is characterized with a lower slope of 13.42% to that of the first slope. Once 1.0 wt% of nanoparticles was added to the synthetic solution, then the IFT in the first region does not change so much and drops approximately with the same slope compared to the solution with no nanoparticles. But the IFT in the second region of the solution containing 1.0 wt% of nanoparticles drops with a larger slope compared to the solution with no nanoparticles (i.e. 33.06%). If the percentage of slope change in the second region is considered as the intensity of asphaltene precipitation compared to the first region, then by adding the nanoparticles to the oil sample, the intensity of asphaltene precipitation decreases. The impact of nanoparticles of different concentration on wettability based on the pendant oil droplets is investigated by the present authors but yet the final analysis is still pending. As already discussed, the variation of Bond number data versus pressure is another way to diagnose the intensity of asphaltene precipitation. Fig. 9 shows the Bond number in terms of pressure for three solutions and CO2. The Bond number changes versus pressure in two regions with different slopes. In the second region, because of asphaltene accumulation at the interface of oilegas, 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 occurred 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. In the first region (low pressure), the Bond number slope is almost identical for all three solutions. In the second region where asphaltene is prone to precipitation then the nanoparticles play a greater role. With the addition of nanoparticles, the slope in second region increases. The very same reasons that were brought up for the variation of IFT, could also be applied here but more noticeably. Table 5 provides changes in the slopes of Bond number in different regions for various solutions of the N type asphaltene.

233

1.5 1.4 1.3 1.2 1.1 1 3

4

5

6

7

8

9

pressure (MPa) Without nanoparticles

With 0.5% wt nanoparticles

With 1% wt nanoparticles

Fig. 9. Variation of the Bond number versus pressure for the N type asphaltene in different solutions at 70  C.

According to Table 5, in the solution with no nanoparticle, the second slope of the Bond number versus pressure diagram is approximately 27.22% of the first slope and in the solution containing 1 wt% of nanoparticles, the slope of the second region is approximately 55.22% of the slope of the first region. In other words, if the percentage of slope change in the second region is comparable to the intensity of asphaltene precipitation then by adding the nanoparticles to the oil sample, the rate of asphaltene precipitation decreases. 4. Conclusions The following conclusions can be drawn from the experimental results presented in this study:  Two slopes were observed in both measurements of IFT and Bond number of the synthetic solution (toluene and n-heptane) and CO2. Changes in the slope of both parameters occurred at the same pressure. The pressure, at which the slope changes, can be considered as an indication of the onset of asphaltene precipitation. Furthermore, the slopes of both parameters were used in the second region at high pressures to examine the intensity of precipitation. The presence of Fe3O4 nanoparticles caused the slope of the second region to increase implying the reduction in intensity of asphaltene precipitation.  The performance of nanoparticles varied substantially in retarding the asphaltene precipitation by the type of asphaltene in the solution. For the attempted two types of asphaltene extracted from B and N reservoirs, the nanoparticles preferably reduce the intensity of asphaltene precipitation for the B type.

Table 5 Changes in the Bond number slope ratio for the N type asphaltene in solutions with different mass fractions of nanoparticles at 70  C. Nanoparticle Region (wt%)

Equation Bond number (), P (MPa) Ratio of the Bond number slope of 2nd to the 1st region

Nanoparticle (wt %)

Region

Equation IFT (mN/m), P (MPa)

Ratio of the IFT slope of 2nd to the 1st region

0

1st 2nd

IFT ¼ 1.5776 P þ 16.050 IFT ¼ 0.2117 P þ 6.735

13.42%

0

1st Region Bond number ¼ 0.0687 P þ 0.9224 2nd Region Bond number ¼ 0.0187 P þ 1.2878

27.22%

0.5

1st 2nd

IFT ¼ 1.5902 P þ 16.108 IFT ¼ 0.3307 P þ 7.475

20.80%

0.5

1st Region Bond number ¼ 0.0714 P þ 0.9097 2nd Region Bond number ¼ 0.0377 P þ 1.1700

52.80%

1.0

1st 2nd

IFT ¼ 1.6231 P þ 16.312 IFT ¼ 0.5366 P þ 8.885

33.06%

1.0

1st Region Bond number ¼ 0.0746 P þ 0.9064 2nd Region Bond number ¼ 0.0412 P þ 1.1590

55.22%

234

Y. Kazemzadeh et al. / Journal of Natural Gas Science and Engineering 22 (2015) 227e234

 Larger mass fraction of Fe3O4 nanoparticles in the attempted solutions improved their performance in stemming asphaltene precipitation. The experimental results showed that the higher the mass fraction of nanoparticles is, the steeper would be the slopes of both IFT and the Bond number with respect to pressure in the second region, i.e. higher pressures. Acknowledgments The authors would like to thank Ms. S. Noorbakhsh, Mr. A. Golkari and Mr. S. Hasanpour, for their help in performing some of the experiments. Special thanks also go to Ms. Sh. Meratian for setting up the test rig. Nomenclature b Bo g R1, R2

Dr g

curve radius at the apex, m Bond number, e gravitational acceleration, m/s2 principal radii of curvature, m density difference between the two fluids, kg/m3 interfacial tension, mN/m

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