Mechanism of the reversibility of asphaltene precipitation in crude oil

Mechanism of the reversibility of asphaltene precipitation in crude oil

Journal of Petroleum Science and Engineering 78 (2011) 316–320 Contents lists available at ScienceDirect Journal of Petroleum Science and Engineerin...

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Journal of Petroleum Science and Engineering 78 (2011) 316–320

Contents lists available at ScienceDirect

Journal of Petroleum Science and Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / p e t r o l

Mechanism of the reversibility of asphaltene precipitation in crude oil Ali Abedini a,⁎, Siavash Ashoori b, Farshid Torabi a, Yaser Saki b, Navid Dinarvand b a b

Faculty of Engineering and Applied Science, University of Regina, Regina, SK, Canada S4S 0A2 Department of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iran

a r t i c l e

i n f o

Article history: Received 16 March 2011 Accepted 26 July 2011 Available online 6 August 2011 Keywords: asphaltene precipitation dissolution reversibility

a b s t r a c t Asphaltene precipitation and deposition occur in petroleum reservoirs as a change in pressure, temperature and liquid phase composition and reduce the oil recovery considerably. In addition to these, asphaltene precipitates may deposit in the pore spaces of reservoir rock and form plugging, which is referred to as a type of formation damage, i.e. permeability reduction. In all cases above, it is of great importance to know under which conditions the asphaltenes precipitate and to what extent precipitated asphaltenes can be re-dissolved. In other words, to what extent the process of asphaltene precipitation is reversible with respect to change in thermodynamic conditions. In present work, a series of experiments was designed and carried out to quantitatively distinguish the reversibility of asphaltene precipitation upon the change in pressure, temperature and liquid composition. Experiments were conducted in non-porous media. Generally it was observed that the asphaltene precipitation is a partial reversible process for oil under study upon temperature change with hysteresis. However, the precipitation of asphaltene as a function of mixture composition and pressure is nearly reversible with a little hysteresis. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Asphaltenes are high-molecular weight solids which are soluble in aromatic solvents such as benzene and toluene and insoluble in paraffinic solvents (Ashoori et al., 2010; Speight et al., 1985). The subject of asphaltene precipitation and deposition has been studied for more than a half century, but there are still some controversies and disagreements between researchers and investigators. An important disagreement is in the nature of asphaltene in crude oil. There are two different models to describe the nature of asphaltene in the solution. The first approach is the solubility model which considers the asphaltene to be dissolved in a true liquid state (Kawanaka et al., 1991; Burke et al., 1990; Hirschberg et al., 1988; Leontaritis and Mansoori, 1987; Pfeiffer and Saal, 1940). In the second approach, the colloidal model, asphaltenes are considered to be solid particles which are suspended colloidally in the crude oil and are stabilized by large resin molecules (Hirshberg et al., 1984; Mansoori, 1997; Pfeiffer and Saal, 1940). According to the solubility model, asphaltene precipitation is a thermodynamically reversible process, while in the colloidal model, precipitation of asphaltene is considered to be irreversible. The validity of each of these models depends on whether the precipitation process is reversible or not. There is a disagreement in the literature about reversibility of asphaltene precipitation.

⁎ Corresponding author at: University of Regina, 3737 Wascana Parkway, Regina, SK, Canada S4S 0A2. Tel.: + 1 306 337 2263. E-mail address: [email protected] (A. Abedini). 0920-4105/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.petrol.2011.07.010

Pfeiffer and Saal (1940) believed that asphaltene precipitation is not reversible mainly due to experimental observation of the colloidal behavior of asphaltene suspensions. Hirshberg et al. (1984) observed reversibility of asphaltene precipitation with pressure at 94 °C. They assumed that asphaltene precipitation is reversible but is likely very slow. Andersen and Stenby (1996) studied the effect of temperature on asphaltene precipitation/dissolution. They used a mixed solvent (toluene and n-heptane) and performed solvent reversibility runs at 24, 50, and 80 °C. Although the reversibility of precipitation with temperature was not explicitly investigated, the results demonstrated that asphaltenes partially re-dissolve with an increase in temperature. Fotland (1996) and Wang et al. (1999) have speculated that asphaltene precipitation is less likely to be reversible for crude oils subjected to conditions well beyond those of the precipitation onset. Rassamdana et al. (1996) performed experiments at room temperature to study the reversibility of asphaltene precipitation with respect to composition. They concluded that the asphaltene precipitation process is partially reversible. Pacheco-Sanchez and Mansoori (1997) suggested that asphaltene flocculation is irreversible. Hammami (1999) used SDS (solids detection system) for their works and concluded that asphaltene precipitation is generally reversible but is time dependent. Peramanu et al. (2001) reported differences in the reversibility of solvent and temperature-induced aggregation. Their investigation confirmed that solvent treatments can be an effective method for re-dissolving asphaltenes as long as there is sufficient turbulence to break-up the asphaltene particles; another results suggested that temperature treatments may not be the best method for re-dissolving asphaltenes. Ashoori et al. (2005) performed a series

A. Abedini et al. / Journal of Petroleum Science and Engineering 78 (2011) 316–320

1.2

Precipitated asphaltene (Wt%)

Table 1 Thermodynamic condition of reservoir and the fluid properties. Initial static pressure (psia) Reservoir temperature (°C) Bubble point pressure (Pb) (psia) Stock tank density (g/cc) n-Heptane asphaltene content (wt %)

317

9200 143 4810 0.867 1.18

of experiment to investigate the reversibility of asphaltene precipitation. According to their report, the precipitation of asphaltene was found to be completely reversible for the oil under study with respect to change in temperature. In addition to this, hysteresis was not observed in any experiment. The aim of this study is to investigate the reversibility of asphaltene precipitation in crude oil with respect to changes in different thermodynamic factors in non-porous media. The behavior of precipitation and re-dissolution of asphaltene particles in crude oil as a function of liquid composition was investigated. The reversibility of asphaltene precipitation due to temperature change was also examined. Furthermore the PVT cell was used to determine the precipitation and re-dissolution of asphaltene with respect to change in pressure at reservoir condition (high pressure and high temperature).

T=60 C

0.6 0.4 0.2

0

(4)

(5)

(7)

2.1. Effect of temperature on asphaltene precipitation

(8)

Aromatics (wt %)

Resins (wt %)

Asphaltenes (wt %)

75.80

19.73

3.29

1.18

6

9

12

15

(9)

(10) (11)

and step 2 is the exact weight of added crude oil which is denoted by w1. A mixture with specific dilution ratio was prepared by adding adequate volume of solvent to the test tube. Then the test tube was closed by its cap. The tube and its content were shaken for approximately 20 minutes. The tube was then placed in a centrifuge to rotate for about 20 minutes at 10,000 rpm. After remaining about 20 minutes in the centrifuge, the asphaltene particles aggregated and precipitated at the bottom of the test tube. After removing the test tube from the centrifuge, its cap was opened and the solution content, which had a dark color, was discarded. 10 cc of n-heptane was added to the test tube. The test tube with its contents was shaken for approximately 20 minutes in order to separate the adhered solid content from the bottom of test tube and allowed to contact with the newly added nheptane. The test tube was placed in the centrifuge again, and remained there for another 20 minutes under a rotational velocity of 10,000 rpm. Steps 6, 7 and 8 were repeated until the solution becomes colorless. At this time, the colorless solution was removed and the test tube with the solid content was transferred into the oven and remains there for 12 hours at temperature of 100 °C.

Precipitated asphaltene (Wt%)

1.4 Rv=1

1.2

Rv=2 Rv=3

1

Rv=5 Rv=7

0.8

Rv=10

0.6

Rv=12 Rv=14

0.4 0.2 0 20

Saturates (wt %)

3

Fig. 1. Effect of temperature on the amount of asphaltene precipitation for n-heptane.

The oil sample under study was obtained from a well from one of the Iranian southwest oil field reservoirs. The thermodynamic condition of reservoir and the fluid properties are shown in Table 1. The SARA analysis of crude oil is presented in Table 2.

Table 2 SARA analysis of sample crude oil.

T=50 C

Rv

(6)

(1) An empty test tube was weighed accurately with an electronic laboratory balance. (2) Specific volume of crude oil was injected into the test tube with a glass syringe. (3) The weight of the test tube plus the added crude oil was recorded. The difference between recorded weights from step 1

T=40 C

0.8

0

2. Experiment

The amounts of asphaltene precipitation have been measured at temperatures of 25 °C, 40 °C, 50 °C and 60 °C. At each temperature the n-heptane was used as a precipitant. Fig. 1 illustrates the effect of temperature on the amount of asphaltene precipitation. As it is shown, the amount of asphaltene precipitation decreases as temperature increases. An increase in temperature results in decrease of asphaltene precipitation. This may be determined by the strength of the associating forces responsible for the aggregation of asphaltene molecules. The aggregating or adsorption bonds will at a given temperature start to rupture and the solubility will increase on further increase of the temperature. Fig. 2 depicts the amount of asphaltene precipitation as a change in temperature for constant dilution ratios (Rv). As this figure shows, the amount of asphaltene precipitation decreases by increasing temperature at constant dilution ratio. To determine the amount of asphaltene precipitation in a mixture, the gravimetric method was used. The procedure for this method is outlined as follows:

T=25 C

1

30

40

50

60

70

T ºC Fig. 2. Effect of temperature on the amount of asphaltene precipitation at constant dilution ratio.

318

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SAMPLE POINT PRESSURE GAUGE PVT CELL PUMP OIL CELL SYSTEM CONTROLLER Fig. 3. Set up used to determine reversibility of asphaltene precipitation as a change in pressure.

(12) The dried test tube with its solid content was taken out of the oven, allowed to cool down and then weighed. (13) The difference between recorded weights from step 1 and step 12 is the weight of the precipitated asphaltene content of the mixture which is denoted by w2. (14) Knowing the weight of the added crude oil (w1) and precipitated asphaltene (w2), the weight percent of the precipitated asphaltene is determined as follows:

wt% =

w2 × 100 w1

2.2. Investigation of reversibility of asphaltene precipitation Three sets of tests (T-1, T-2 and T-3) were designed and carried out to determine the reversibility of asphaltene precipitation in nonporous medium. In the first set, the reversibility of asphaltene precipitation was investigated with respect to change in mixture composition. The second set includes the investigation of the reversibility as a change of temperature variation and the last one was concerned with the effect of pressure on the reversibility of asphaltene precipitation. In test T-1 several mixtures of oil and n-heptane were prepared with different dilution volume ratios. After 24 hours, the amount of precipitated asphaltenes along with the density of each mixture was measured (precipitation process). Then a mixture was prepared with Rv = 15 and kept in a dark place for 24 hours. The container cap was removed to allow n-heptane to vaporize. The amount of precipitated

asphaltene and density was then measured at particular times (redissolution process). In test T-2, a mixture of n-heptane and oil with dilution ratio of 2 was prepared. Twenty eight samples were taken from this mixture and put in a temperature-controlled oven. First the temperature was set at 25 °C. After 24 hours, 4 samples were collected and the amount of precipitated asphaltene of each sample was measured. Then an average value was calculated. The temperature was raised to 40 °C while the other 24 samples remained in the oven for another 24 hours. Again 4 samples were collected and the amount of asphaltene precipitated was measured. This procedure was also repeated for 50 °C and 60 °C. Experimental results showed that the amount of precipitated asphaltene decreases by increasing the temperature. To examine the effect of temperature variation in reverse direction, the system temperature of 60 °C was lowered to 50 °C and the mixture was allowed to stay at this temperature for 24 hours and thereafter sample collections and precipitated asphaltene measurements were done. This procedure was also repeated for temperature of 40 °C and 25 °C. Recent measurements showed that the amount of precipitated asphaltene increases as temperature decreases. The whole procedure was also repeated for mixtures with dilution volume ratios of 5, 7 and 10. Test T-3 was designed to investigate the reversibility of asphaltene precipitation due to pressure changes at reservoir condition. To start this test, 200 cc of mono-phase oil sample was transferred into PVT cell isobarically. Then the PVT cell was set in 10,000 psia and 143 °C. It remained one day to reach the maximum stability; it should be notified that the mixer of PVT cell was applied to enhance the stabilization

0.4

Precipitated Asphaltene (Wt%)

Precipitated asphaltene (Wt%)

1.2 Precipitation Redissolution

1 0.8 0.6 0.4 0.2 0 0.6

0.65

0.7

0.75

0.8

0.85

0.9

Density (g/cc) Fig. 4. Comparison of the amounts of asphaltene precipitation/re-dissolution versus mixture density.

Precipitation Redissolution

0.35 0.3 0.25 0.2 0.15 0.1 20

30

40

50

60

70

T (ºC) Fig. 5. Effect of temperature on reversibility of asphaltene precipitation for a mixture of n-heptane and oil with Rv = 2.

A. Abedini et al. / Journal of Petroleum Science and Engineering 78 (2011) 316–320

1.1 Precipitation

Precipitation

Precipitated Asphaltene (Wt%)

Precipitated Asphaltene (Wt%)

0.85

319

Redissolution

0.8 0.75 0.7 0.65 0.6 0.55 20

30

40

50

60

Redissolution

1.05 1 0.95 0.9 0.85 0.8 20

70

30

40

T (ºC)

50

60

70

T (ºC)

Fig. 6. Effect of temperature on reversibility of asphaltene precipitation for a mixture of n-heptane and oil with Rv = 5.

Fig. 8. Effect of temperature on reversibility of asphaltene precipitation for a mixture of n-heptane and oil with Rv = 10.

process. After stabilization of the system, the pressure was lowered at constant increments of 1000 psia. At each pressure step, the system was mixed for 6 hours and then remained at its condition for 24 hours. Then, around 15 cc of sample expelled from PVT cell under constant pressure to flash to atmospheric condition. Finally the flashed sample was analyzed to determine its asphaltene content. To investigate the reversibility of asphaltene precipitation as a function of pressure, the pressure of PVT cell was increased from bubble point pressure to initial pressure of 10,000 psia with pressure increment of 1000 psia. A schematic of the used set up is depicted in Fig. 3.

the process has some hysteresis with respect to the direction of temperature variation. This hysteresis decreases by increasing the dilution ratio of the mixture. Fig. 9 depicts the precipitation and re-dissolution of asphaltene in crude oil at reservoir condition. As this figure reveals, the precipitation of asphaltene as a change in pressure is nearly a complete reversible process with some hysteresis. The result of this test (T-3) suggests that pressurizing of under-saturated oil reservoirs may be an appropriate method to solve the problems caused by asphaltene precipitation.

3. Results and discussion

4. Conclusion

The amounts of asphaltene precipitated measured in test T-1 are represented in Fig. 4. The squares show the amount of precipitated asphaltene in the first step of the test (i.e., precipitation), while the triangular points show the measured values during re-dissolution process. There is a good agreement between the amounts of asphaltene precipitated measured in two precipitation and re-dissolution processes, which reveals that the asphaltene precipitation is reversible upon the addition and removal of the precipitant. Results of test T-2 are presented in Figs. 5 to 8. As these figures show, reversibility of asphaltene precipitation/deposition is a partial process with respect to temperature variation. It is concluded that

The reversibility of asphaltene precipitation was investigated with respect to change in some thermodynamic factors which include liquid composition, temperature and pressure in non-porous medium. Results of this study showed that the precipitation of asphaltene decreases by increasing the temperature. Furthermore it is observed that asphaltene precipitation is a partial process with respect to change in temperature, while it is nearly reversible as a change in composition and pressure with a little hysteresis. The result of this study shows that the thermodynamic solubility approach can be a satisfying model for prediction and simulation of asphaltene precipitation and deposition in crude oil.

1.2

1.2

Precipitated Asphaltene (Wt%)

Precipitated Asphaltene (Wt%) from Flashed Oil

Precipitation

1

Redissolution

0.95 0.9 0.85 0.8 0.75 0.7 20

40

50

60

70

T (ºC) Fig. 7. Effect of temperature on reversibility of asphaltene precipitation for a mixture of n-heptane and oil with Rv = 7.

1

0.8

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0 30

Redissolution

1

0

2000

4000

6000

8000

10000

Precipitated Asphaltene (Wt%) in PVT Cell

Precipitation

1.05

0 12000

Pressure (psia) Fig. 9. Reversibility of asphaltene precipitation due to pressure change at reservoir condition.

320

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