Prediction of asphaltene deposition region in pipeline based on equation of state

Chen Junwen, Jing Jiaqiang School of Petroleum Engineering Southwest Petroleum University

Chengdu, China cjwman@126.com, jjq@swpu.edu.cn

Wang Yi Sinopec gas company

China Petroleum & Chemical Corporation Beijing, China

wangyi0501020132@yahoo.com.cn

Du Huimin, Liu Lisheng South-West Oil-Gas Field Company

China National Petroleum Corporation Chengdu, China

du_huimin@petrochina.com.cn, llsheng@petrochina.com.cn

Abstract—Asphaltene is apt to deposit from crude oil under high pressure condition, and the sediment will reduce the pipeline flow area and the performance of equipments, even block the submarine pipeline in extreme cases. The pressure is relatively high in deepwater submarine oil pipeline, so it is entirely possible to meet the condition for asphaltene deposition and pose a significant operational security challenge on deepwater submarine pipeline. In this paper, the asphaltene deposition prediction model is built based on crude oil thermomechanics and solubility theory to predict the pressure and temperature condition for asphaltene deposition in Middle East Crude Oil and draw its asphaltene precipitation envelop. Meanwhile, the location of asphaltene deposition in an assumed deepwater submarine pipeline located from depth of 3000m to the sea level is predicted combining with the thermo-hydraulic models. The study result shows that the asphaltene deposition condition of Middle East Crude Oil and its deposition location in deepwater submarine pipeline could be predicted well by the prediction model proposed in this study. This study provides a fast and effective method for predicting asphaltene deposition in deepwater submarine oil pipeline.

Keywords- asphaltene deposition; equation of state; prediction model; deposition region; submarine pipe system

I. INTRODUCTION Asphaltene is a major component of the solid deposition

in crude oil, which is defined as a material soluable in benzene, but insoluable in low molecular weight n-alkanes, such as n-pentane and n-heptane [1]. Asphaltene might deposit from crude oil when the temperature, pressure and composition of oil changed to a certain critical value or range. Many cases both at home and abroad showed that asphaltene depositon is very harmful for crude oil production [2-9], mainly manifested as reducing the flow cross-sectional area, lowering the efficiency of production equipments, shortening pipeline maintenance cycle, even ceasing production, which will seriously threaten the safety of oil production and bring great economic losses.

For studying asphaltene deposition in a deep-going way, many scholars had proceeded from asphaltene deposition mechanism and deposition law, then gradually probed the asphaltene deposition prediction methods. Nowadays, the main mechanism for asphaltene deposition could be divided into two broad categories: the phase equilibrium theory basing on EOS and the thermodynamic colloidal theory [10, 11].

For wellbore and deepwater pipeline, asphaltene deposition condition could be exhibited with asphaltene deposition envelop (ADE) [13], which would reflect the influence of temperature and pressure on asphaltene deposition clearly. As shown in Fig.1, asphaltene would deposit when the pressure and temperature of oil system were in a certain range (the region enclosed by upper and lower asphaltene envelop).

Though the correlative researches on asphaltene deposition have yielded some rich harvests, an efficient asphaltene deposition prediction system has not been formed. Moreover, the study on asphaltene deposition in pipeline, such as deposition location, deposition degree and deposition style, are still at the exploration and development stage. Hence, during this paper, an asphaltene deposition prediction model is built basing on phase equilibrium theory, and then the asphaltene deposition location of Middle East Oil in an assumed submarine pipeline will be simulated combing temperature drop with pressure drop model of pipeline.

Figure 1. Asphaltene deposition envelop Temperature

Pres

sure

Upper envelop

Lower envelop

Bubble point line

2011 International Conference on Computational and Information Sciences

978-0-7695-4501-1/11 $26.00 © 2011 IEEE

DOI 10.1109/ICCIS.2011.198

586

II. PREDICTION OF ASPHALTENE DEPOSITION CONDITION Equilibrium phase model will be selected as a major

method for asphaltene deposition in deepwater submarine pipeline, because the asphaltene deposition is caused by change of temperature and pressure primarily and the asphaltene deposition is reversible under this condition.

A. hydrocarbon-plus fractions extension Crude oil characterization is mainly referred to measure

the mole fraction of C1-C6 and C7+. However, asphaltene is usually termed to be C31+ [14], whose content is difficult to measure. Hence, it is necessary to extend C7+ and determine the contents of each single carbon component and the C31+. Afterwards, deposition condition of crude oil can be predicted using EOS and phase equilibrium model.

The research results abroad [15] shown that the molar distribution of single carbon can be calculated by Pederson model for light oil or condensate oil, and by Ahmed model for middle or heavy oil.

Ahmed, Cady, and Story [16] devised a simplified method for splitting the C7+ fraction into pseudo components (Eq.1).

���

�

���

�

−−

=++

++++

nn

nnnn MM

MMzz

)1(

)1( (1)

Where Mn+ can be determined by empirical equations.

B. pseudo-composition splitting and lumping Whitson [17] proposed a regrouping scheme whereby the

compositional distribution of the C7+ fraction was reduced to only a few multiple carbon number (MCN) groups (Eq.2).

( )[ ]nNN g −+= log3.31tln (2)

Where, N——number of carbon atoms of the last component in the

hydrocarbon system, n——number of carbon atoms of the first component in the

plus fraction. The molecular weights of each different MCN group are

calculated from Eq.3: gNI

C

NCI M

MMM ��

���

= +

7

7

(3)

C. Determination of pseudo-composition characterization parameter Lee et al. [18], in their proposed regrouping model,

employing Kay’s mixing rules (Eq.4) as the characterizing approach for determining the properties of the lumped fractions.

�∈

∗ = L

Lii

ii

z

zz

(4)

Where, Zi——characterization parameter of i component, Zi

*——characterization parameter of i pseudo-component. From this equation, critical pressure, critical temperature

and critical eccentric factor of each pseudo-component can be calculated with the empirical critical parameter of each

single component, which is summarized completely by Pedersen [19].

D. Fugacity calculation of asphaltene(C31+) in liquid phase Using PR equation of state to calculate the compression

factor Z in liquid phase, and its cubic equation can be written as Eq.5:

3 2 2 2 3(1 ) ( 2 3 ) ( ) 0Z B Z A B B Z AB B B− − + − − − − − = (5) With

2 2/A aP R T= (6) /B bP RT= (7)

Where, P——system pressure, T ——system temperature, R ——gas constant, Z——system compressibility factor.

The fugacity of i component in liquid phase can be calculated as follows:

2 2.414ln ( 1) ln( ) ln0.4142 2

i iji i i

i

x af b bA Z BZ Z Bx P b a b Z BB

+ = − − − − −� �� � � �� � −� � �

� (8)

Where, fi——fugacity of i component.

E. Fugacity calculation of asphaltene(C31+) in solid phase Fugacity of asphaltene (C31+) in solid phase can be

determined as follows: ( )

��

���

� −=

∗∗

RTppV

ff SSS exp

(9)

Where, fs solid fugacity at P and T, fs

* solid fugacity at P* and T, Vs solid molar volume, P system pressure, P* reference pressure, T system temperature.

In this equation, the fugacity of asphaltene in solid phase can be calculated under different pressure and temperature. In addition, fs

* and Vs can also be determined by EOS.

F. Determination of asphaltene deposition onset According to the fugacity of asphaltene in liquid/solid

phase, the asphaltene deposition onset/condition (pressure and temperature) can be determined as follows:

If lnfC31+,L lnfC31+,S, then asphaltene deposits, If lnfC31+,L lnfC31+,S, then asphaltene remains stable, If lnfC31+,L=lnfC31+,S, then the temperature and pressure of

system refers to asphaltene deposition onset.

G. Prediction of asphaltene deposition lower envelop When system pressure is lower than bubble point

pressure, light components will evaporate from liquid phase and run into vapor phase. Hence, the composition of liquid phase should be recalculated with equilibrium ratio K. Once the composition of liquid phase is determined, the

587

asphaltene deposition lower envelop could be drawn by comparing the C31+ fugacity in liquid phase with that in solid phase. It might also be noted that the judgment standard of asphaltene deposition under bubble point pressure is the opposite of that beyond bubble point pressure.

III. PREDICTION OF PIPE FLOW CHARACTERISTIC For applying prediction method in real production

system, it is necessary to build a simple pressure and temperature drop model of wellbore and pipeline to determine the asphaltene deposition region in the production system..

A. Model of oil flow pressure drop The overall pressure drop in vertical pipe contain

friction pressure drop, gravity pressure drop and acceleration pressure drop, while the overall pressure drop in horizontal pipe contain only friction pressure drop and acceleration pressure drop. The pressure drop law of oil flow pipe could be determined with some relevant calculation method. The detail computation flow process of oil flow pressure drop is shown in Fig.2.

B. Model of oil flow temperature drop The heat exchange between crude oil in pipeline and the

external medium will be in stable state, if the crude oil flows steadily in pipeline and the external environment conditions change little. At the same time, the axial temperature difference of crude oil in pipeline is hypothesized to be little, and the axial heat conduction is neglected. The detail computation flow process of oil flow temperature drop is shown in Fig.3.

Hence, with the above-mentioned method, the region of asphaltene deposition from crude oil in pipeline system could be determined, which is helpful for the flow assurance of deepwater submarine pipeline.

IV. EXAMPLE AND ANALYSIS The asphaltene deposition region of Middle East Oil in

an assumed submarine pipeline (Fig.4) is calculated with the asphaltene deposition prediction model and pipe flow model built in this paper. The characteristic parameters of crude oil are listed in Table 1 [20]. The prediction and calculation results are shown in Fig.5.

It can be seen from Fig.5 that the predicted results are very similar to the real situation (black dots), which could prove the accuracy of prediction model. The four asphaltene deposition onsets of Middle East oil are almost coincident with the predicted upper envelop, which is the most important part of asphaltene deposition prediction and control. The pressure-temperature drop curve of virtual wellbore and pipeline run across the asphaltene deposition region, which means that there will be asphaltene deposition existing in the pipe system.

Basing on the result of asphaltene deposition prediction and pipe flow characteristics calculation, some key

information could be achieved and exhibited with accurate values: 1) the asphaltene would begin to deposit about 700 m from the well bottom (in the wellbore); 2) as far from asphaltene deposition onset point, more asphaltene would deposit from oil; 3) asphaltene would be the most instable at bubble point, 12500m from well bottom along the pipe system (in the riser); 4) asphaltene might be stable when crude oil flow past the location which is 13700m far from the wellbore bottom (in the riser).

TABLE I. CHARACTERIZATION OF MIDDLE EAST OIL

Category Middle East Oil Reservoir Temperature ( ) 116 Reservoir Pressure (MPa) 62.95

C1 (mole %) 43.43 C2 (mole %) 11.02 C3 (mole %) 6.55 C4 (mole %) 4.49 C5 (mole %) 3.53 C6 (mole %) 2.70 C7+ (mole %) 26.88

Deposition point 1 99 , 47.26MPa Deposition point 2 104 , 45.42MPa Deposition point 3 110 , 44.26MPa Deposition point 4 116 , 42.92Mpa

V. CONCLUSIONS AND SUGGESTION In this paper, the asphaltene deposition onset could be

determined through fugacity calculation and contrast of asphaltene component in two phases.

The asphaltene deposition region in pipe system could be calculated combing asphaltene deposition envelop with pipe flow characteristic curves.

The predicted asphaltene deposition conditions of Middle East Oil accord with the real situation, which could prove the accuracy of prediction model.

The asphaltene deposition region of Middle East Oil in virtual pipeline system is from 700m to 11700m along with pipeline, with means that there will be serious asphaltene deposition in the virtual pipeline.

ACKNOWLEDGMENT This study was supported by National Natural Science

Foundation of China (No.51074136), by Southwest Petroleum University, State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation. The authors would like to acknowledge gratefully the research team.

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Figure 2. Calculation process chart of oil flow pressure drop

Figure 3. Calculation process chart of oil flow temperature drop

Figure 4. Asphaltene deposition region in assumed pipe system of

Middle East Oil Figure 5. Asphaltene deposition envelop and flow characteristic chart

Temperature /

Pres

sure

Upper Bubble point

Lower Wellbore P-TSubmarine P-T

Riser P-TDeposition

Wellbore: 3000m Submarine pipeline: 8000m Riser: 3000m

8 0 0 0 m

2700

m

1500

m

2300

m

Sea surface

Formation

Bubble Point

Submarine pipeline

Wel

lbor

e

Ris

er

Asphaltene deposition region Assumed pipe system

Upper onset point

Lower onset point

589