65f

Defining SLE and VLE Conditions ofHydrocarbon Fluids Containing Wax and Asphaltenes

Using Acoustic Resonance Technology

A. Sivaraman, D.lmer, F.B. Thomas,D.B. Bennion, A.K.M. Jamaluddin.

Hycal Energy Research Laboratories Ltd.Prepared for Presentation at the 1998 Spring National Meeting

New Orleans, LA, March 8 - 12Thennodynamic and Transport Properties 2. Asphaltenes, Waxes, Gas Hydrates

Hycal Energy Research Laboratories Ltd.Copyright ~

UNPUBLISHED

February 23, 1998

AIChE shall not be responsible for statements or opinions contained in papers or printed in its

publications.

Defining SLE and VLE Conditions of Hydrocarbon FluidsContaining Wax and Asphaltenes Using Acoustic Resonance Technology

A. Sivaraman, D.lmer, F.B. Thomas, D.B. Bennion, A.K.M. JarnaluddinHycal Energy Research Laboratories Ltd.

ABSTRACT

Acoustic resonance technology (ART) is a newly developed method for the highly accurate

non-optical detennination of phase transitions in high temperature and pressure reservoir fluids. The

acoustic resonance experiments were conducted under a depressurization mode to detect the onset

of asphaltene precipitation at a fixed experimental temperature. During the depressurization runs,

acoustic features corresponding to the onset of asphaltene precipitation or solid-liquid equilibrium

(SLE) conditions were detected as the system pressures were decreased from reservoir conditions.

Subsequently, during the same run, the acoustic features were also detected that signified the vapor-

liquid (VLE) equilibrium conditions. In addition, isobaric cooling experiments were also conducted

to identify the onset of wax precipitation. The isobaric cooling experiments detected the features

corresponding to wax precipitation (SLE) conditions.

Results defining the SLE and VLE conditions of reservoir crudes containing asphaltenes and

wax are extremely promising. Comparison of the VLE data obtained using ART with that obtained

using an optical method showed excellent agreement. In addition, comparison of SLE data obtained

using ART showed excellent agreement with that obtained using a light-scattering technique where

the light scattering technique is applicable.

Keywords: Defining, Solids-Liquid Equilibrium, Vapour Liquid Equilibrium, Acoustic ResonanceTechnology, Asphaltenes, Waxes.

INTRODUCflON

Reservoir fluids containing wax and asphaltenes are common in the petroleum industry. The

presence of these materials can cause significant operational problems during the life of the

producing wells. The identification of wax and asphaltene precipitation conditions is the first step

towards applying any remedial options. Asphaltenes are dark brown to black solid compounds with

no definite melting point. They decompose while heating and leave a carbonaceous residue. They

are non-crystalline substances or mixtures of relatively high molecular weight fractions (Briant et

a1 (1983» of bitumen with characteristics of strong aromatic polar substances (Change et a1 (1993)

and Stausz et a1 (1991». Asphaltenes are defined as the n-heptane insoluble fraction of crude oil

(Speight et al (1981, 1984». They are classified by the particular solvent used to precipitate them

(Speight et at (1985». They are generally soluble in benzene and insoluble in low molecular weight

n-alkanes (Speight (1979». Asphaltene precipitation can be determined experimentally.

Waxes are defined as normal paraffins as well as other molecules containing long chain alkyl

groups usually ranging from CI8 to C60° Solid waxes crystallize or precipitate when cooled below

their cloud point temperature. The wax precipitation process is thermodynamically reversible (Reid

et at (1977». The viscosity of crude oil is increased by the presence of the wax crystals (Ronningsen

et a1 (1991)). If the temperature is reduced sufficiently, the crude can become highly viscous and

approach its pour point temperature.

Solid (waxes and asphaltenes) precipitation from reservoir fluids causes severe operational

problems in the subsurface, surface equipment, wellhead equipment, separators and tanks (Shelton

et al (1977), Leontaritis et al (1988)). Clean-up costs can be very high in offshore oil production

(Tuttle (1983».

2

There is a strong need for a good model to predict precipitation although some theoretical

models based on the principles of colloidal suspension (Espinant et a1 (1993), Novosad et a1 (1990),

Leontaritis et al (1987)), polymer solution theory (Hirschberg et al (1984), Mansoori et al (1985),

Leontaritis (1988)), and a rigorous approach based on a wax deposition model that allows a fully

compositional representation (Thomas et al (1992), Won (1989)) are found in the literature. Full

reversibility in asphaltene precipitation is always questionable. Only a limited amount of

experimental data is available for asphaltene precipitation. Fluorescence spectrometry (Wehry et al

(1966)), conductive measurements (Nighswander (1993)), fibre optics (portland (1993)) and light-

scattering techniques (Fuhr et al (1991), J amaluddin (1996)) are being used to detect solids dropout

onsets experimentally. Most of these techniques are dependent on the oil characteristics. The most

important criterion is the color of the oil. For dark-colored oil, the popular light-scattering technique

becomes inadequate. In this case, near-infrared (NIR) light spectrum is necessary to identify any

changes in the response due to a phase change phenomenon.

An emerging technology, Acoustic Resonance Technology (ARl), has been tested at Hycal

Energy Research Laboratories Ltd. to identify the onset conditions of solids precipitation. ART is

based on the measurement of the response of a fluid or fluids, contained in a cylindrical cavity, to

acoustic variable stimulation. The most powerful use of ART is to study the state and time evolution

of the resonance response of fluids under variable and well-controlled conditions of pressure, volume

and temperature (Colgate et al (1992, 1992, 1993». In these cases, one can obtain information

related to fluid phase behaviour or phase transitions and transport properties. ART offers a sensitive

and objective method for probing and measuring bulk fluid properties and processes. The system is

applicable for any type and color of oil. The basic principle behind the technique is transmitting a

3

sound wave through the fluid and changing pressure, temperature or composition conditions to

initiate asphaltene and/or wax precipitation conditions.

The acoustic resonance experiments were conducted under a depressurization mode to detect

During thethe onset of asphaltene precipitation at a fixed experimental temperature.

depressurization runs, a sharp change in acoustic features, corresponding to the onset of asphaltene

precipitation or solid-liquid-equilibrium (SLE) conditions, was detected as the system pressures were

decreased from the reservoir conditions. Subsequently, during the same run, a significant change

in acoustic features was also detected which signified vapor-liquid (VLE) equilibrium conditions.

In addition, isobaric cooling experiments were also conducted to identify the onset of wax

precipitation. The isobaric cooling experiments detected the features corresponding to wax

precipitation (SLE) conditions. Results of the depressurization and isobaric cooling experimental

runs are presented in this paper.

EXPERIMENTAL EQUIPMENT AND PROCEDURE

The heart of the ART experimental setup is a cylindrical resonator of 0.25 inches in diameter

made of Hastelloy to resist any corrosion. Acoustic stimulation is applied by a piezo-electric

element with the transmitter forming one end of the cylindrical cavity. This element vibrates in

response to an applied voltage. Another similar element, the receiver, forms the other end of the

resonator cavity and generates a voltage principally in response to the stimulated fluid oscillations.

At certain applied stimulation frequencies, standing waves (resonances) will be set up. The pattern

of these standing waves depends on the geometry of the cavity and the nature and state of the fluids.

4

The cylindrical cavity is oriented vertically so that resonance patterns will depend on the geometry

of any multiple phases present as well as the nature of their interfaces.

The volume of the cylindrical cavity resonator used in this acoustic resonance assembly is

precision variable using a piston that allows for density and pressure sweeps of the contents. The

resonator assembly is housed in a well-insulated circulating air bath, thermally controllable in both

The maintenance and sweep control of pressure, volume andisothennal and sweep modes.

temperature is supplied by precise and very stable elements (high precision strain gauge transducers

for pressure, a Linear Velocity Displacement Transducer (L VDT) for volume and a calibrated

platinum resistance themlometer for temperature) read by precision Keithley digital multimeters each

interfaced to a control computer. Also interfaced to the control computer are the pressure, volume

and temperature control elements (a Stepper motor operating the piston for pressure and volume

control, and a liquid nitrogen servo valve and heater element for temperature control). These, in

combination with the respective sensors and multimeters, provide feedback loops and are used in

the control of the instrument. The control program uses a Proportional Integral Differential type

algorithm. The custom software allows the operator to see immediately the results of any tuning

change for each control variable in real time through a graphical interface. One can control the setup

and operation of pressure, volume and temperature sweeps very precisely through the control

computer. Sweeps of anyone or a combination of two control variables can be performed over nearly

The temperature andany desired paths within the physical operating limits of the instrument.

pressure limits of the current acoustic resonance assembly are -40°C to 150°C and atmospheric to

10,000 psia, respectively.

s

A second computer controls the acoustic excitation of the resonator and acquires acoustic

response data. An interfaced function generator supplies the signal necessary to excite the

transmitter. The acoustic response is processed through a low noise pre-amplifier and then through

a fast high precision analog to digital converter (ADC). Acoustic data, at a sampling rate of 100

kHz, acquired by ADC is synchronized by a trigger signal generated by the function generator. This

second computer displays the acoustic spectrum (frequency domain) through a graphic interface and

The acquisition computer is interfaced to the control computer in a networkalso stores the data.

configuration. Pressure, volume and temperature data, gathered during acoustic data acquisition, are

also displayed and stored. During sweeps, control of all system functions, including those of the

acquisition computer, is directed by the control computer. The raw time domain data collected are

processed to obtain the frequency domain data using a custom software. The fingerprint of the

spectrum is the representation of various excited modes in the fluid contained in the cylindrical

resonator. Tracking any of the modes with temperature, pressure or volume shows the physical and

chemical changes as well as phase transition happening in the system during the process. During

the onset of phase transition, the acoustic response shows a drastic change.

The resonance frequency in a reservoir fluid confined to a cylinder of length "1" and radius

"a" is given by:

"'21(Xn.

f=~2

-1 a

6

where c is the sonic speed in the fluid, Dz is an integer corresponding to different modes or roots of

the Bessel function (Dz = 1 first radial mode; Dz = 2; second radial mode), and "mn is a tabulated

= 0 for radial mode).eigen value «<mn

The above equation shows that the acoustic response is proportional to the sonic speed.

Hence, the onset of phase transition can be observed as a sharp change in the response behaviour

from the acoustic response track of any of the modes with temperature, pressure or volume. The

schematic illustration of the AR system is shown in Figure 1

- - " --~ ~

1

: 1 ~.O~ h1 1~~&D8 U

IP-.~II

-.:L2

~~

~

~

-_0..------...~J

to J - --

FIgI.- 1: A - ~ of DIe .. - ~ ~ ~b' 8y8I8nIn a typical acoustic resonance experiment, the system is first heated up to the experimental

temperature. Subsequently, the system is charged with the reservoir fluid at a pre set pressure and

at the experimental temperature. The system is then depressurized at an initial rate of 40 psi/minute.

The rate of depressurization decreases with time and reaches about 5 psi/minute towards the end of

The acoustic data with volume, temperature, and pressure are collected, andthe experiment.

subsequently, analyzed to identify the acoustic responses related to various phase change

phenomena.

7

Procedure to Detect the Phase Change Phenomena

j'iE<

I_I

-\.,

1~L~1-~. '.. ~. )~ ~

Figure 2: A Typical Transformed Frequency Domain Spectrum

The following is a step-by-step procedure to identify the specific signature related to a phase

change phenomenon from the acoustic resonance responses. A typical amplitudinal response curve

as a function of frequency is presented in Figure 2.

8

The raw time domain amplitudinal responses collected during the AR runs are processed toobtain the amplitude versus frequency plots using the Fast Fourier Transformation (FF1)technique.

.

By plotting the changes in amplitude as a function of specific frequencies, corresponding tothe modes of the Bessel functions, the points of phase transition can be inferred.

Subsequently, the specific changes in frequencies are plotted as a function of independentvariables (i.e., pressure, temperature, or volume). The specific change in the frequencycorresponding to a phase change phenomenon is clearly identified from the plots offrequency versus pressure, for example.

.

Sample Characterization

The composition of the reservoir crude containing wax is presented in Table I.

Table I: Compositional Analvsis of Wax v Crude (Samnle 1)

HydrocarbonComponents

CompositionMole Fraction

HydrocarbonComponents

CompositionMole Fraction

c, 0.004 Cl7 0.034

i-C4 0.003 CIa 0.027

n-C4 0.010 Ct, 0.025

i-Cs 0.012 ~ 0.025

n-Cs 0.013 C:u 0.022

Cyclo Cs 0.002 ~ 0.021

c, 0.040 C2i 0.020

Mycyclo Cs 0.001 ~ 0.019

Benzene 0.014 ~ 0.017

Cyclo C, 0.018 c. 0.016

Co, 0.082 Cn 0.014

Mycyclo C6 0.005 Ca 0.013

Toluene 0.002 c. 0.012

c. 0.079 ~ 0.009

EB/MP Xylene 0.017 ~I 0.008

O-Xylene 0.005 ~2 0.006

9

HydrocarbonComponents

CompositionMole Fraction

HydrocarbonComponents

CompositionMole Fraction

0.069 Cu 0.005~TMB 0.011 ~ 0.005

0.053 c,s 0.003C1O

c.. 0.048 ~ 0.003

0.003CIa 0.047 ~Ct) 0.040 c. 0.002

0.042 c. 0.002C'4

0.037 C40 0.001CIS

C1. 0.031

The solids content of all three reservoir fluid samples are presented in Table ll.

Table II: Solids Content of Reservoir Fluid SamDles

Sample Asphaltene Content(% by Weight)

Wax Content

(% by Weight)

1 23.1

2 4.6 1.1

0.73

As seen in the table, the total wax content of the waxy fluid Sample 1 is 23.1 % (by weight).

Two different fluid samples containing asphaltenes were used in these experiments. As seen in the

table, Sample 2 contains both asphaltene and wax. The asphaltene content of Sample 2 is 1.1 % (by

weight) and the wax content is 4.6% (by weight). The asphaltene content of Sample 3 is 0.7% (by

weight).

10

DESCRIPTION OF THE EXPERIMENTS

Identification of Wax Solidification

An isobaric cooling experiment was conducted to detennine the onset of wax precipitation

using Sample 1. In this nIn, the ART data were processed to detect the onset of the first wax crystal

(cloud point), and subsequently detect the temperature condition at which agglomeration of the

individual wax crystals took place (pour point).

Identification of the Onset of Asphaltene Precipitation

In the first set of experiments, depressurization runs were conducted using live oil Sample

2 to verify if the changes in temperature and pressure could cause asphaltene precipitation during the

production phase of the well. In this case, the sample was charged into the cylindrical acoustic

resonator' at a pressure higher than the reservoir pressure and at a preset temperature. Subsequently,

the pressure was reduced from the experimental pressure to a pressure below the expected bubble

point pressure. Acoustic signature corresponding to the appearance of the asphaltene particles and

the bubble point pressure were detected. The AR depressurization runs were repeated at various

temperatures.

In the second sets of experiments, two solvents (CO2 and NGL) have been tested to evaluate

the enhanced oil recovery potential. These EOR solvents have the potential to precipitate asphaltene

in contact with oil. Therefore, the ART experiments were conducted to define the SLE and SLVE

conditions. During these testing, NGL (composition given in Table III) was mixed with Sample 3

at various proportions at a pressure much higher than the expected asphaltene precipitation pressure.

1

Table m: NGL Comoosition

Composition(Mole Fraction)

Components

~ 0.0161

c, 0.4395

i-C4 0.1518

n-C. 0.3623

i-C, 0.0223

n-C, 0.0078

c+6 0.0002

Subsequently, the depressurization nms were conducted at the reservoir temperature of 2 5°C

to detect the onset pressure of asphaltene precipitation. Similarly, Sample 3 was mixed with various

proportions of CO2, again at a pressure much higher than the expected pressure of the onset of

asphaltene precipitation. Depressurization nms were conducted at the reservoir temperature of25°C

In the second sets of experiments, the mixing of solvents with oil was conducted in a cell and

transferred to another cell through a filter to see any asphaltenes have precipitated out during the

mixing process. No asphaltenes were seen to appear in the filter during the sample transfer

RESULTS AND DISCUSSIONS

Cloud and Pour Points Measurements

In these experiments, the cell was first cleaned with solvents and dried with argon and

evacuated. The system was then maintained at a temperature of 65 °C, the live waxy crude Sample

, maintained at a temperature of 65 °C, was transferred to the evacuated cell through a heated line

in liquid state. Then the system valve was closed. Once the system was stabilized, the resonator was

12

cooled down to 30°C at a mte of 0.1 °C/min. Time domain data was collected at each set point along

with pressure, temperature and volume data. Once the run was over, the data was processed to

obtain the frequency spectrum.

The normalized acoustic response obtained from a particular frequency track of the FFT data

as a function of temperature is presented in Figure 3

u

lIt..u

F\c1Ire 3: Aceastic Detenaiaatioe of aolld aod Pour Poi.to of Sample 1

As seen in the figure, the acoustic response shows some variations as the temperature is

reduced. At about 40°C temperature, the acoustic response starts to increase and this point is

characterized as the cloud point temperature. This increase in the acoustic response is due to the

increase in the density of the fluid undergoing a cooling cycle. Further reduction in temperature

provided another sharp change in the acoustic response, and this corresponds to the pour point of the

system. The cloud point was determined to be 41.4OC. An optical laser method was used to detect

the cloud point of this oil sample and this method detected the cloud point at a temperature of

13

42.5°C. Therefore, the results are comparable. The pour point of the system was determined to be

20°C.

Onset of Asphaltene Precipitation Pressure

Depressurization Experiments Using Live Reservoir Fluid

Three acoustic resonance depressurization experiments were conducted at three specified

In these runs, the experimentaltemperatures of 49, 57, and 66°C using live oil Sample 2.

temperature was held constant and the system pressure was decreased from 55 MPa to 10 MPa. The

55 MPa pressure is higher than the reservoir pressure of 43 MPa. The acoustic response is basically

the sonic frequency in Hertz and the normalized values of the acoustic responses are presented in the

scale of 0 to I in the example figure. The nominal values of these acoustic responses varied between

0 a 50 kHz.

A typical normalized acoustic response curve is presented as a function of pressure in

Figure 4. The onset of solids precipitation (SLE) and bubblepoint pressures (VLE) are identified by

their respective sharp and drastic changes in acoustic response and these specific signatures are

labelled in Figure 4.

Figure 4: A.oati. Deter8li.alioa or 0- or Asphaltetle Precipitation..d Bubblepoint In Uve 011 @ 66OC

14

As seen in Figure 4, the acoustic response starts to slightly increase with a decrease in

pressure from an experimental pressure of 55 MPa. The AR response attains a sharp increase at

around a pressure of 42 MPa. This sharp increase in acoustic response is characterized as the onset

of solids precipitation. Further reduction in pressure results in decreasing the acoustic response until

a low AR response is attained. This low AR response is characterized as the bubble point pressure

of the reservoir fluid (37 MPa). The increases in the AR response after the appearance of the solids

are believed to be due to the higher sonic speed in solids than that in the liquid phase.

The onset of asphaltene precipitation (solid-liquid equilibrium, SLE) and the bubble point

pressures (vapor-liquid equilibrium, VLE) at various temperatures are summarized in Table IV and

graphically presented in Figure 5.

The bubble point pressures measured using a visual high pressure cell are also presented inFigure 5.

15

-M,.-M~

45 50 55 eo 85 70T-- ('C)

r...re 5: At ~.rizatioa Sa Z at Varleu T_peo-atar..

As seen in this figure and, as expected, the bubble point pressure increases slightly with an

The bubble point pressure measured by two methods compare well.increase in temperature.

As seen in Figure 5, the onset pressure of solids precipitation decreases with an increase in

This indicates that, at a higher temperature, the solids precipitation will occur at atemperature.

lower pressure, and hence, increase the range of operating pressure conditions (from sand face

pressure to the solids onset pressure).

Depressurization Experiments Using a Mixture of Live Reservoir Oil and Solvents

In these experimental runs, the live crude oil, Sample 3, was pre-mixed with various

proportions of solvents at a pressure of 40 MPa and a reservoir temperature of25°C. Subsequently,

the AR depressurization runs were conducted to identify the SLE and VLE conditions of these live

oil and solvent mixtures. Two different solvents (NGL and COJ were tested in these experiments.

The objective of the study was to evaluate whether the addition of solvent to the live oil would

precipitate any asphaltenes from the live oil solvent mixture. This study was conducted prior to

16

injecting solvents in the reservoir to evaluate the potential for enhanced oil recovery using two types

of solvents injection.

A. SLE and VLE Usina NGL Solvent

The composition ofNOL is given in Table ill. As seen in the table, NOL primarily consists

of propane and nomlal butane and these two components are very effective in altering the

thennodynamic equilibrium of asphaltenes, and hence, precipitate asphaltenes.The acoustic

depressurization results are presented in Table V and graphically presented in Figure 6.

In these tests, three experiments were conducted using three solvent concentrations (10, 40

and 800/0 by volume). The onset of solids precipitation pressures from these three nms were obtained

from the respective acoustic response data and summarized in Figure 6. As seen in this figure, the

Thisonset of solid precipitation pressure increases with an increase in solvent concentration.

phenomenon indicates that the more solvent will enhance the onset of asphaltene precipitation and

minimi7.e the operating pressure limit above the SLE curve.

17

As expected, the bubble point pressures decreases with an increase in the solvent

concentration. The bubble point pressures at these three solvent concentrations identified using the

acoustic resonance technology were compared with that measured using conventional PVT analysis.

As seen in Figure 6, the bubble point pressures measured by two methods compare very well.

B. SLE and VLE USin2 CO2 Solvent

Similar to the NGL experiments, live oil Sample 3 was premixed with CO2 at three different

concentrations (10, 40 and 80% by volume). Subsequently, the AR depressurization runs were

The solid onset pressures (SLE) and saturation pressure (VLE) conditions areconducted.

summarized in Table V and graphically presented in Figure 7. As seen in the figure, the solids onset

pressure shows an upward trend with an increase in the CO2 concentration. If the CO2 injection

pressure is above the SLE curve, there will not be any problem with solids precipitation. Once the

injection pressure drops below the SLE curve, the asphaltene particles will appear and possibly cause

plugging problems in the porous medium.

18

The saturation pressures were also identified from the acoustic response and are presented

in Figure 7.

As seen in the figure, none of the saturation pressures identified using the ART system were

The saturation pressure measurements conducted using thehigher than the solids onset pressure.

conventional PVT method indicated two bubblepoint pressures and one dewpoint pressure. The

dewpoint pressure is found to be higher than the onset of solids precipitation pressure. This makes

logical sense because solids cannot remain in solution in the dense phase above the dewpoint. The

solids precipitation will occur in the liquid phase. The measured data point shows that the solids

appears in the liquid phase below the dewpoint. In these nms, saturation pressures identified using

the acoustic technology were also compared with that measured using the conventional PVT method.

As seen in Figure 7, the values compare very well.

The appearance of the solids precipitation pressure below the dew point pressure is logical

because of the fact that solids can only exist in the liquid phase. The crossover point of the SLE and

19

VLE curves is speculated to be the critical point. However, no analysis at hand indicates that the

crossover point could be the critical or near critical point to validate the hypothesis.

DISCUSSION

Because of dark color of the oil, the asphaltenes were not visible during the bubblepoint

experiments using the visual method. However, at the end of the AR experiments, the equipment

was reported to be plugged during the flushing and cleaning of the cell.

At the end of each experimental run (10 MPa), the system was bled off using the bottom

valve at the experimental temperature. During the bleeding process, the pressure did not drop at the

beginning. This is due to the plugging of exit lines with solids. Subsequently, the cell was charged

with toluene from the top valve at a pressure of 10 MPa and allowed to soak for one hour. After the

soak period when the bottom valve was opened, the cell content was bled off with intermittent flow.

The solids were extremely small, and therefore, were not visible under naked eyes. Following this

bleeding step the cell was rinsed with methanol and dried before the next run.

During the production phase of the live reservoir fluid or during processing of the produced

reservoir fluid, it is critical to understand the pressure-temperature conditions at which the solids will

precipitate out. Especially during the production phase of the live reservoir fluid, if the pressure falls

below the SLE condition, the solids will start to precipitate out, and hence, cause operational

problems. In addition if the pressure falls below the SLE condition in the near wellbore region, the

problem would be severe due to the solids plugging in the porous media. Therefore to take proper

preventive measure, it is recommended that the SLE conditions be determined when the operating

company is aware of the fact that the producing fluid contains solids.

20

CONCLUSION

The acoustic resonance technology is a state-of-the-art technique to precisely identify the

pressure and temperature conditions at which the solids precipitation will occur. The solid-liquid

equilibrium (SLE) condition can be defined in a live reservoir fluid using the ART system where

other techniques become inadequate due to dark color of the oil. The vapor-liquid equilibrium

Comparison of results with the(VLE) condition can also be identified using the ART system.

conventional method shows excellent agreement in the VLE data.

21

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22

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