August 31 to September 3, 1997 Rio de Janeiro, Brazil
Paper # 12
Modeling the Behavior of Asphaltene-Micelle in PetroleumFluids
byJ.H. Pacheco-Samchez and G.A. Mansoori
PROCEEDINGS ofTHE SECOND INTERNATIONAL SYMPOSIUM ON
COLLOID CHEMISTRY IN OIL PRODUCTION
Modeling the behavior of asphaltene-micelle in petroleum fluids
Juan H. Pacheco-Sanchez a and G.Ali Mansoori b
• . Permanent Address: lnstitulo Mexicano de/ Petroleo, Subdireccion de Exploracion y Produccion, Eje CentralLazaro Cardenas No. 152, Col. San Bartolo Atepehuacan, Deleg. Gustavo A. Madero 07730 Mexico, DF.;
Asphaltene particles can assume various fonns when mixed with petroleum fluids depending on the relative sizes and polarities of the particles present in such complex mixtures. Small asphaltene particles can be dissolved in a petroleum fluid. Whereas relatively large asphaltene particles can fonn steric colloids in the presence of excess amounts of resins and paraffin hydrocarbons (see Figure 1). However, asphaltene particles may fonn micelles in the presence of excess amounts of aromatic hydrocarbons (see Figure 2).
Experimental observations and theoretical calculations have indicated that asphaltene monomers in the presence of aromatic (polar) solvents fonn micelles l .2_ Considering that paraffin hydrocarbons are non-polar, asphaltenes in their presence will flocculate forming steric colloids (in case of the presence of resins in the crude oil) and eventually it will deposit out of the solution3
,4
,5
.
An existing micelle fonnation model, originally developed for water/amphiphile systems, is applied to predict the phase behavior of asphaltene micelle fonnation in pure aromatic hydrocarbons at various temperatures and nonnal pressure. Several mechanisms for asphaltene micelle formation are investigated. The calculations are indicative of the existence of an upper and a lower boundary for the micelle-formation region. The results are compared with limited critical micelle concentration data for asphaltenes-in-toluene systems. These experimental data cover the lower boundary of the asphaltene-micelle-formation region. Should be mentioned that there are no experimental data for the upper boundary.
Based on the model for optimum composition of asphaltenes in aromatic (polar) solvents, the behaviour of phase separation for asphaltene-micelles in petroleum fluids is proposed in this report for practical field applications. In order to know the conditions of either deposition or asphaltene micellization, it is necessary to perform phase equilibrium calculation between asphaltene in micelle and asphaltene in molecular state in petroleum fluids. Such equilibrium in this model is corresponding with the following coupled system of equations2 for the separation of the micellar solution into two coexisting phases having different total
concentrations Y .. and z .. of asphaltene
and
=
(1-Z )(Z..-Y..)+ln l-Y: I z
2 y 2
+ 2 J3rC ( aa )2 - (' u )2l+(y-l)Z01 l+(y-l)Y ..
(1)
ISCOP '97
Page 1 of Paper # 12
b. University of Illinois at Chicago, (M/C 063) Chicago, IL 60607. 7052;[email protected]
_I (-!- __ I_) + � - .jY: =./K {z:; .f[:. ./K
where,
I (1-z .. )2
z,.-Y .. +2f3C 2 (I+ (y- l)z •• )
K(T,P) = exp[j3(µ*0mic/n • n�"0mic)l, 0
(1 - Y.,) 2
(2)
(3)
n0 is the minimum number of asphaltenes in the micelle. Assuming sphero-cylindrical form of the asphaltene micelles: µ •0 mici"o is the chemical potential of reference per asphaltene monomer associated to the end region and µ •o mic is the chemical potential of reference per asphaltene monomer associated to the cylindrical region. In general, parameter K is responsible of the size and form of each micelle. Analysing the expression l .2,6 (3), K can assume the following values: K =f: If µ*0mic/no = noµ*Omic then asphaltenes are proportionally distributed in bothcylindrical and end regions of the micelles being formed, i.e., asphaltene micelles form is spherical-like. O<K<l: If µ*omic> µ•0mic/n then asphaltenes are distributed in the cylindrical region of the
0 micelles being fonned, i.e., asphaltene micelles form is disk-like. �: If µ •0 mic/"o > µ •o mic then asphaltenes are distributed in the end region of the micellesin fonnation, i.e. asphaltene micelles form is cylindrical-like. It must be mentioned that K is related to the theory of moments,
M - I K(a-ll/2 X (a+IJ/2a - a. as
Finally, near of the critical point a.= 0,1,2, ...
(4)
where A0 "' 4{ 1 +3/[5(3y-2)Xd - (2/5)ln[4(3y..2)/3] - lnXc} 112.
Equations (I) and (2) must be solved simultaneously for Y ,, and Z85
• To accomplish this, Equation (1) is solved by the Newton-Raphson (NR) method for one of the variables (either Y aa or Z..) assigning an initial value to the other variable. Then Equation (2) is solved by the NR method using the solution to the equation (I) as the initial value. This iterative procedure converges for E = 1Xi-Xi_ 1 I << I.
For the case when K = I, y = 10.4038 (asphaltenes derived from tar of Kotur-Tepinski crude) and j3C = 314.5/(yT): It is important to mention that, in the absense of data for the critical point, the value of j3C=314.5/(yT) was treated as an adjustable parameter. The NR simultaneous iterative solution of Equations (I) and (2) for Y as and z .. , is convergent, when the
Page 2 of Paper # 12
J.H. Pacheco-Samchez and G.A. MansooriModeling the Behavior of Asphaltene-Micelle in Petroleum Fluids Paper # 12, Proceed. 2nd Int’l Symp. on Colloidal Chem. In Oil Prod. (ISCOP’97)
Rio de Janeiro, Brazil, 8/31-9/03, 1997
ISCOP '97
Page 3 of Paper # 12
(5)
(6)
(7)
(8)
Page 4 of Paper # 12
J.H. Pacheco-Samchez and G.A. MansooriModeling the Behavior of Asphaltene-Micelle in Petroleum FluidsPaper # 12, Proceed. 2nd Int’l Symp. on Colloidal Chem. In Oil Prod. (ISCOP’97)
Rio de Janeiro, Brazil, 8/31-9/03, 1997
2 ( ') Paraffins-
111p11an•n•
- - - -
- - - - - - - - - - -
- - - -
n( Paraffins-
flocculaled Rsphaltene
Figure 1
p( ) Aromatics ..
Rsphalten
• Figure 2
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Figure 3
ISCOP '97
Page 5 of Paper # 12
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Figure 4
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Page 6 of Paper # 12
J.H. Pacheco-Samchez and G.A. MansooriModeling the Behavior of Asphaltene-Micelle in Petroleum FluidsPaper # 12, Proceed. 2nd Int’l Symp. on Colloidal Chem. In Oil Prod. (ISCOP’97)
Rio de Janeiro, Brazil, 8/31-9/03, 1997
