Optics: measuring and testing – Oil testing
Reexamination Certificate
2002-02-07
2004-02-10
Smith, Zandra V. (Department: 2877)
Optics: measuring and testing
Oil testing
C250S301000
Reexamination Certificate
active
06690453
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and to a device allowing fast prediction of the flocculation threshold of asphaltenes from the measurement of their optical refraction index.
The method finds applications notably in the production or the transport of petroleum products for predicting the flocculation of asphaltenes and for fighting it by means of various processes.
During extraction or transport, crudes are subjected to temperature, pressure, shear variations, and to operations modifying the composition thereof. These modifications can lead to the flocculation of heavy constituents of crude oils such as asphaltenes. As this precipitate does not redissolve spontaneously, it can eventually, as it accumulates, clog lines, part of the porous matrix of the formation or a catalyst grain. The deposits resulting from the precipitation of asphaltenes are the main causes of servicing operations during processing of the petroleum chain It is therefore necessary to be able to predict in time the flocculation of asphaltenes in order to prevent or to fight it.
BACKGROUND OF THE INVENTION
Measurement of the flocculation threshold consists in adding progressively to an asphaltene solution a flocculant that can be, according to standards, pentane, heptane, etc. The following methods can be distinguished:
a) a test referred to as spot test: the flocculated asphaltenes do not diffuse as quickly as the surrounding liquid when the mixture is deposited on a filter paper. A uniform spot corresponds to an absence of flocculated asphaltenes, whereas a black area in the middle of the spot corresponds to a flocculation;
b) an optical method by light diffusion in the near infrared allowing continuous and in-situ monitoring of the appearance of the threshold, as described for example in patent FR-2,647,903 filed by the applicant. This threshold is defined as the minimum amount of flocculant to be added to the solution for the formation of the first asphaltene aggregates to be observed.
Other techniques have been developed since then: light diffusion (UV and visible), particles size, gravimetric analysis, optical fluorescence, viscosity, electrical conductivity, thermal conductivity analysis. They are all based on the same principle: the addition of flocculant leads to two effects: dilution of the sample and increase in the size of the aggregates. Below this threshold, the dilution effect prevails. Crossing the threshold leads to a great increase in the size of the aggregates and this effect prevails over dilution.
A possible method of determining the flocculation threshold consists in preparing at the time
0
different solutions of asphaltenes in toluene to which various heptane concentrations are added for example. Immediately afterwards (at t
0
+1 mn for example), its refraction index is measured by means of a refractometer of a well-known type. The Abbe refractometer is for example used, whose principle essentially consists in measuring the critical angle of total refraction of a light beam falling on a diopter consisting of a sapphire prism, of high index (1.7), and of the sample to be measured. The diopter thus obtained is <<lit>> by a light source having an angular distribution that allows to reach all of the incidences corresponding to the limits of the measurement range accessible by the device. Then, after a predetermined time interval (t
0
+5 mn for example, according to the experimental protocol selected), the various solutions are examined under the microscope to see if the asphaltenes flocculate (see FIG.
1
). It is well-known that the polarizability of a mixture of constituents, under certain hypotheses, is the sum of the contributions of each one of the fractions that constitute this mixture. If the mixture volumes are ideal (without excess volume), if there is no chemical reaction when the constituents are mixed and if the disturbances of the individual oscillators, linked with the presence of neighbours of different chemical nature are not too great, the number of atoms of each constituent per total volume unit being denoted by N
j
, we can write the Clausius-Mosotti
3
n
2
-
1
n
2
+
2
=
∑
j
N
j
α
j
(
1
)
equation:
where &agr;
j
is the polarizability of constituent j.
We therefore draw (
FIG. 2
) the variation curve of function
(
n
s
=
n
2
-
1
n
2
+
2
)
,
of refraction index n as a function, on the abscissa, of the volume fraction of the titrant of the mixture (1−&PHgr;) (&PHgr; heptane volume fraction of the various solutions) and the first point for which a flocculation onset is observed is located on the curve obtained. The value of the refraction index at this point represents the refraction index n
SF
at the flocculation threshold. The lower volume fraction &PHgr;, the more the asphaltene will tend to flocculate. The asphaltene flocculation risk therefore varies in inverse proportion to &PHgr;.
In order to be accurate, this known method requires comparative examination of a great number of solutions with different heptane titers, and consequently a longer experimentation time.
On the other hand, Wang J. et al describe, in: <<Improved Modeling of the Onset of Asphaltene Flocculation >>, 2
nd
International Conference on Petroleum & Gas Phase Behavior and Fouling, Copenhagen, August 2000, a modelling method wherein asphaltene solutions are considered as a binary mixture of a solvent (maltenes) and of a solute (asphaltenes), and a Flory-Huggins type model, well-known to specialists, is used to predict their flocculation. The free enthalpy of mixing &Dgr;G
m
is given by the relation as follows:
&Dgr;
G
m
=&Dgr;H
m
−T&Dgr;S
m
(2)
The entropy term is the Flory-Huggins terms involving the molar fractions (x) and the volume fractions (&phgr;):
&Dgr;
S
m
=−R
(
x
1
ln&phgr;
1
+x
2
ln&phgr;
2
) (3)
On the other hand, the enthalpy term involves the solubility parameters of the solvent and of the solute through interaction parameter &khgr;:
&Dgr;
H
m
=RTx
1
&phgr;
2
&khgr; (4)
χ
=
v
1
R
T
(
δ
2
-
δ
1
)
2
(
5
)
In these expressions, subscripts 1 and 2 correspond to the solvant and to the solute respectively. Interaction parameter &khgr; involves Hildebrand's solubility parameters &dgr;. Now, these parameters are defined from the cohesive-energy densities. &dgr;
1
and &dgr;
2
are therefore measurements of the cohesive energies of the solvent and of the asphaltene aggregates respectively. The expression of the free enthalpy of mixing allows to determine the chemical potentials of the two species (solvent and solute). The flocculation of asphaltenes is then dealt with conventionally like a phase separation.
When modelling an addition of flocculant nC7 for example, the parameters relative to the solvent, (&dgr;
1
, v
1
) and the molar fractions of solvent and of solute, are modified. In doing so, the free enthalpy curve is modified (see FIG. 4). For low proportions of flocculant (&PHgr;
p
), the free enthalpy is concave at any point and no phase separation is favourable from the viewpoint of energy. On the other hand, with high proportions of flocculant, the free enthalpy has a convex domain and a phase separation occurs. A tangent to the curve can in fact be drawn, which gives two mixtures at phase equilibrium. The model thus allows, knowing the quality of the solvent, to define whether the system is stable or if there is a phase separation.
In this model, the flocculation threshold is defined as the case in which the free enthalpy curve goes from a concave to a convex situation. In other words, we change from a single-phase stable system to a two-phase system. This leads to the appearance of a zero-tangent flex point on the free enthalpy curve. Mathematically, by using the chemical potentials of the two species, we can write:
ln
(
1
-
φ
2
)
+
(
1
-
1
m
)
φ
2
+
χ
(
φ
2
)
2
=
ln
(
φ
2
)
+
(
1
-
m
)
(
1
-
φ
2
)
+
mχ
&af
Antonelli Terry Stout & Kraus LLP
Geisel Kara
Institut Francais du Pe'trole
Smith Zandra V.
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