Chemistry: analytical and immunological testing – Hydrocarbon – Aromatic
Reexamination Certificate
2000-09-22
2003-11-04
Soderquist, Arlen (Department: 1743)
Chemistry: analytical and immunological testing
Hydrocarbon
Aromatic
C436S060000, C436S139000, C436S141000, C436S142000, C436S181000, C436S183000
Reexamination Certificate
active
06642056
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for deter-mining the stability of asphaltenes in live oil.
2. Description of the Background
Asphaltenes, substances defined on the basis of their insolubility in paraffinic solvents (for example, n-pentane or n-heptane) are present in almost all crude oils. Asphaltenes have a high aromaticity and molecular weight; they also have a greater content of heteroatoms with respect to the corresponding soluble fraction (malthenes).
The structure and chemical behavior of asphaltenes vary a lot in relation to their origin and preparation process; if particular conditions are reached in the field, they can precipitate, blocking the pores and thus causing serious production problems. They can also precipitate along the production string, increasing the pressure drop and thus reducing the well production.
The problem of well-drilling operators is to know the conditions, particularly temperature and pressure, under which asphaltenes are stable in live oil. The term “live oil” refers to crude oil under temperature, pressure, and optionally, dissolved gas conditions, which are encountered in the reservoir, in which the crude oil is present.
In fact, during the extraction of crude oil, it may happen, owing to variations in pressure and temperature, that the asphaltenes present in the oil separate, thus either partially or even totally blocking the crude-oil extraction pipes.
As a result, the well productivity begins to decrease, with evident economic damage.
SUMMARY OF THE INVENTION
The present invention relates to a process for determining the stability of asphaltenes in oil fields.
In this process, use is made of the following equations:
(
v
s
/RT
) (&dgr;
a
−&dgr;
s
)
2
=&khgr;
(equation 1);
v
s
=v
o
x
o
+v
gs
x
gs
+v
p
x
p
(equation 2);
&dgr;
s
=&dgr;
o
&phgr;
o
+&dgr;
gs
&phgr;
gs
+&dgr;
p
&phgr;
p
(equation 3);
&dgr;
a
(
T
)=&dgr;
a
(
T
o
) exp[−9.1·10
−4
(
T−T
o
)] (equation 4).
In the above equations
&dgr;
s
is the solubility parameter of the “solvent mixture”,
i.e. of everything but asphaltenes,
&dgr;
a
is the solubility parameter of the asphaltenes,
&dgr;
o
is the solubility parameter of the oil (&dgr;
sto
for stock
tank oil and &dgr;
lo
for live oil),
&dgr;
p
is the solubility parameter of the paraffin used in the titration,
&dgr;
gs
is the solubility parameter of the good solvent optionally added to the oil,
&khgr; is the asphaltene-solvent mixture interaction parameter,
v
s
is the molar volume of the “solvent mixture”,
v
o
is the molar volume of the oil (v
sto
for stock tank oil, v
lo
for live oil),
v
p
is the molar volume of the paraffin used in the titration,
v
gs
is the molar volume of the good solvent optionally added to the oil,
R is the universal gas constant,
T is the temperature in Kelvin degrees,
x
o
is the molar fraction of the stock oil under threshold conditions,
x
gs
is the molar fraction of the good solvent under threshold conditions,
x
p
is the molar fraction of the paraffin under threshold conditions,
&phgr;
o
is the volumetric fraction of the oil under threshold conditions,
&phgr;
p
is the volumetric fraction of the paraffin under threshold conditions,
&phgr;
gs
is the volumetric fraction of the good solvent under threshold conditions,
T
o
is the reference temperature, coinciding, from a practical point of view, with the measurement temperature.
The precipitation threshold is defined as the point in which, during the titration, the separation of the asphaltenes begins. The term “titration” means addition to the oil, to which a good solvent has optionally been added, of an asphaltene precipitant n-paraffin (for example n-heptane), until a concentration is reached which induces the precipitation of the asphaltenes.
The term “good solvent” refers to any solvent capable of solvating the asphaltenes, for example aromatic solvents such as toluene; the various titrations are effected by initially adding different quantities of a specific good solvent to the oil.
The term “stock tank oil” means oil under surface conditions (pressure of about 1 atmosphere, temperature of about 25° C., absence of dissolved gases).
The present invention relates to a process for determining the stability of asphaltenes in oil fields, characterized in that it comprises the following steps:
a) titrations are carried out of stock tank oil, diluted with good solvents, with C
5
-C
20
aliphatic hydrocarbons, preferably C
5
-C
10
paraffins, even more preferably with n-heptane, thus determining the precipitation threshold of the asphaltenes, said threshold being determined at a temperature ranging from 20° C. to the field temperature, preferably the field temperature; subsequently by means of equations (1), with &khgr;=0,5, (2) and (3) (referring stock tank oil), &dgr;
sto
(T) and &dgr;
a
(T), are obtained;
b) from the physico-chemical analyses of the stock tank oil and &dgr;
sto
(T) determined in step (a), the boiling point of the residue (T
bp
of the residue) is obtained, by means of an equation of state, preferably the RKS equation (Redlich, Kwong, Soave);
c) from the T
bp
of the residue determined in (b) and from physico-chemical analyses of the live oil, the experimental data relating to the phase behavior of the live oil are interpolated, in order to improve the representation of the live oil by means of the above equation of state;
d) using the equation of state with the parameters determined in step (c), the v
lo
and &dgr;
lo
values of the live oil are determined under different T and P conditions of interest;
e) the &khgr; parameter is evaluated for every condition using:
(i) &dgr;
a
referring to the temperature T, this value being equal to that determined in step (a) if the measurement has been effected at the temperature T or, if the measurement in (a) has been effected at a different temperature, calculated from that obtained in (a) by means of equation (4);
(ii) v
lo
and &dgr;
lo
obtained in (d), these parameters being used in equation (1);
f) the stability of the asphaltenes being correlated to the &khgr; parameter, the asphaltenes are stable when &khgr;<0.5, and unstable when &khgr;≧0.5.
The precipitation of the asphaltenes can be interpreted as a precipitation of a sterically stabilized colloid; the precipitation start condition (theta point) is expressed by equation (5):
&khgr;=&khgr;&thgr; (equation 5);
wherein &khgr;=the asphaltene-solvent mixture interaction parameter; &khgr;&thgr;=0.5.
On applying the Bragg-Williams expression to the above equation, expression (6) is reached:
(
v
s
/RT
) (&dgr;
a
−&dgr;
s
)
2
=0.5 (equation 6).
Equation (6) does not have any dependence on the concentration of the starting solution, but makes the precipitation threshold of the asphaltenes depend only on the interaction parameter—i.e. on the difference of the solubility parameters (&dgr;
a
−&dgr;
s
) and on the molar volume of the solvent mixture v
s
.
This independence of the threshold from the concentration is experimentally confirmed by all determinations described in literature (see for example Hotier G., Robin M. (1983), Revue de l'IFP, 3-8 (1):101); a strong indication of the model capacity is therefore to reproduce the physics of the phenomenon.
Equation (6) was applied to the process of the present invention.
The first step (a) of the present invention consists in a series of titrations of a stock tank oil diluted with good solvents, preferably aromatic, for example toluene, with various C
5-
C
20
hydrocarbons, preferably C
5-
C
10
paraffins, even more preferably n-heptane. The titrations are carried out at a temperature ranging from 20° C. to the field temperature, preferably the field temperature. A typical example of this titration is indicated in
FIG. 1
, which shows the absorbance trend in a titration with paraffin of an initially stable solution. E
Bersano Delfina
Correra Sebastiano
Donaggio Francesca
Enitecnologie S.p.A.
Soderquist Arlen
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