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Measurement of the Rheology of Crude Oil in Equilibrium with CO₂ at Reservoir Conditions

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JoVE Journal Ambiente

Measurement of the Rheology of Crude Oil in Equilibrium with CO₂ at Reservoir Conditions

Note: Since the experiment operates at high temperature and pressure, safety is paramount. The system is protected against over-pressure by the software limit on the syringe pump controller and bursting discs at the mixer and between the gear pump and the rheometer (see Figure 1 and Figure 2). Furthermore, before each experiment, it is recommended to perform a regular leak check. It is also recommended to perform the friction check of the pressure cell geometry to make sure the rheometer is functioning well⁹^,¹⁰.

1. Preparing the Crude Oil Sample

NOTE: Use the Zuata crude oil sample as received. The following table shows the basic physical properties of the Zuata crude oil.

Characteristics	Value
API Gravity	9.28
Barrel Factor (bbl/t)	6.27
Total Sulphur (% wt)	3.35
Reid Vapor Pressure (kPa)	1
Pour Point (°C)	24
Existing H₂S Content (ppm)	–
Potential H₂S Content (ppm)	115
Potential HCl Content (ppm)	–
Calc. Gross Cal. Value (kJ/kg)	41,855

Table 1: The physical properties of the Zuata crude oil.

Add 128.57 g of toluene to 300 g of Zuata crude oil to prepare the dilution with 70 wt% Zuata crude oil and 30 wt% toluene. Rock the mixture at room temperature for 3 h.

2. Loading the Crude Oil Sample into the Mixer

Disconnect the mixer from the system, and open it up. Place a stirrer at the bottom of the mixer. Load 200 mL of crude oil sample into the mixer. After tightening all the screws, connect the mixer back to the system.

3. Priming the Entire System with the Crude Oil Sample

Prime the system with the coaxial cylinder geometry pressure cell.
NOTE: Please refer Figure 1 to locate the valve.
1. Close the rheometer pressure cell by tightening the pressure head⁹. Mount the rotation cup on the rheometer spindle. Adjust it to the measuring position⁹.
2. Close valves A, D, E, F, G, and H. Open valve C.
3. Open the nitrogen cylinder. Introduce the compressed gas into the mixer by opening valves H and E. When the gas reaches the mixer, close valve H and the gas cylinder.
4. Open valve A. The compressed gas will push the crude oil sample into the circulation loop through the suction tube. When the crude oil sample is dripping down from valve C in Figure 1, the entire system is primed by the crude oil sample.
5. Open valve F to release the remaining gas. Close valve C and open valve D. Turn on the gear pump to circulate the fluid for a while. Depending on the viscosity of the crude oil sample, this could take 1 to 5 h.
  NOTE: The pressure of the compressed nitrogen introduced into the mixer depends on the viscosity of the crude oil sample. If the viscosity of the crude oil sample is beyond 5 Pa∙s, the pressure of compressed gas may be larger than 15 bar.
Prime the system with the double gap geometry pressure cell.
NOTE: Please refer Figure 2 to locate the valve.
1. Remove the pressure head and the measuring cylinder of the pressure cell.
2. Close valves A, D, E, F, G, H and I. Open valve C.
3. Open the nitrogen cylinder. Introduce the compressed gas into the mixer by opening valves H and E. When the gas reaches the mixer, close valve H and the gas cylinder.
4. Open valve A. The compressed gas will push the crude oil sample into the circulation loop through the suction tube. When the crude oil sample just immerse the inner part of the double gap geometry, open valve F to release the pressure in the mixer.
5. Turn on the gear pump. Carefully adjust the gear pump rotation speed. Make sure that the inlet flow rate to the pressure cell, which is determined by the gear pump, is less than or equal to the outlet flow rate from the pressure cell, which is determined by gravity. When a reasonable rotation speed of the gear pump is found and the crude oil sample is dripping down from valve C, the entire system is primed by the oil. Then turn off the gear pump.
6. Mount the measuring cylinder and pressure head on the pressure cell¹⁰. Close valve C and open valve D. Turn on the gear pump to circulate the fluid.
  NOTE: If the crude oil sample has a viscosity similar to water, the compressed gas with pressure of 3 to 4 bar is enough.

4. Evacuating the Remaining Volume in the System

Close valves A and D in Figure 1 or Figure 2. Connect the vacuum pump to valve F. Turn on the vacuum pump for 15 min.
Close valve F and then turn off the vacuum pump.

5. Introducing CO₂ into the Mixer

With coaxial cylinder geometry pressure cell
1. Open valve G and the CO₂ cylinder in Figure 1. Open valve D in Figure 1.
2. After the CO₂ fills up the remaining space in the system, close valve G and the CO₂ cylinder to prevent CO₂ from back flowing to the cylinder.
With double gap geometry pressure cell
1. Open valve G and the CO₂ cylinder in Figure 2. Open valve D and I in Figure 2.
2. After the CO₂ fills up the remaining space in the system, close valve G and the CO₂ cylinder to prevent CO₂ from back flowing to the cylinder.

6. Setting the Temperature and Pressure

Input the desired temperature value to the mixer and rheometer. Input the desired temperature value to the heating system of the pipeline network. Input the desired pressure value to the syringe pump.
Wait for the temperature and pressure to stabilize.

7. Turning on the Stirrer and Gear Pump

Open the valves in the downstream and upstream of the gear pump.

8. Monitoring the Volume in the Mixer and the Mixture Viscosity

Record the volume reading in syringe pump for every 6 h.
After every 6 h, turn off the stirrer and gear pump. Measure the viscosity of the mixture through the rheometer. The viscosity measurement starts with a 5 min settling time, and then measure viscosity at constant shear rate of 10 s^-1.
When the volume and viscosity values show considerable differences (> 4%) between two consequent measurements, turn on the gear pump and stirrer again to continue the mixing. When both the volume and viscosity measurements show no change in the values (≤ 4%), the equilibrium between the CO₂ and crude oil sample is confirmed.
Turn off the gear pump and stirrer for the rheology measurement.
NOTE: The mixing period could last for 1 to 2 days, depending on the viscosity of the crude oil sample.

9. Performing the Rheology Measurement

With coaxial cylinder geometry pressure cell⁹
1. Close valves A and D in Figure 1 for the rheology measurement. Pre-shear the mixture at shear rate of 10 s^-1 for 0.5 min. Rest the mixture for 1 min.
2. Measure the mixture viscosity at shear rate from 500 s^-1 to 10 s^-1. At each shear rate, the shear rate adjusting time is 0.2 min. The measurement duration at each shear rate step is logarithmically increased from 0.5 min to 1 min, excluding the shear rate adjusting time.
With double gap geometry pressure cell¹⁰
1. Close valves A and D in Figure 2 for the rheology measurement. Pre-shear the mixture at shear rate of 10 s^-1 for 0.5 min. Rest the mixture for 1 min.
2. Measure the mixture viscosity at shear rate from 250 s^-1 to 10 s^-1. At each shear rate, the shear rate adjusting time is 0.2 min. The measurement duration at each shear rate step is logarithmically increased from 0.5 min to 1 min, excluding the shear rate adjusting time.

10. Increasing the Pressure to the Next Desired Value

With the coaxial cylinder geometry pressure cell
1. Close valve E in Figure 1.
2. Introduce more CO₂ into the syringe pump by opening valve G and the CO₂ cylinder. Close valve G and the CO₂ cylinder. Open valve E to add more CO₂ to the mixer.
3. If the pressure is less than the desired value, repeat to introduce more CO₂.
4. Input the new pressure set point into the syringe pump. Wait for the pressure to stabilize.
With the double gap geometry pressure cell
1. Close valves E and I in Figure 2.
2. Introduce more CO₂ into the syringe pump by opening valve G and the CO₂ cylinder. Close valve G and the CO₂ cylinder. Open valves E and I to add more CO₂ to the mixer.
3. If the pressure is less than the desired value, repeat step to introduce more CO₂.
4. Input the new pressure set point into the syringe pump. Wait for the pressure to stabilize.
  NOTE: Repeat steps 7 to 10 for the rheology measurement at higher pressures.

Measurement of the Rheology of Crude Oil in Equilibrium with CO₂ at Reservoir Conditions

Learning Objectives

The rheology measurement of the Zuata crude oil and its CO₂ saturated mixture, at 50 °C using the coaxial cylinder geometry pressure cell, is shown by Figure 5 and Figure 6. Figure 5 shows the measurement from ambient to 100 bar, while Figure 6 shows the measurement from 120 bar to 220 bar. Furthermore, Figure 7 illustrates the relative viscosity, which is the ratio of the viscosity at a given shear rate to the viscosity at the lowest shear rate. The dashed lines in Figure 7 are the maximum measurement error caused by the friction of the bearings of the geometry.

The rheology measurement at 50 °C of the diluted Zuata crude oil, using double gap geometry pressure cell, is illustrated by Figure 8 and Figure 9, while Figure 10 shows the relative viscosity for pressure up to 70 bar. Furthermore, Figure 10 shows that the diluted crude oil at ambient pressure behaves as a Newtonian fluid. However, when the CO₂ pressure is from 30 bar to 60 bar, the shear thinning effect is observed. At CO₂ pressure above 60 bar, the shear thinning disappears and the mixture behaves as a Newtonian fluid again.

De Figure 5 and Figure 6 one can see that the CO₂ dissolution significantly decreases the viscosity of the crude oil mixture until 100 bar. When the CO₂ pressure is beyond 100 bar, the oil mixture viscosity increases with increasing CO₂ pressure, but at a much lower rate.

Figure 7 reveals that the Zuata crude oil shows a shear thinning effect without CO₂ addition. When CO₂ is dissolved into the crude oil, the shear thinning effect is weakened, given that the curves at higher CO₂ pressures are flatter. At CO₂ pressures higher than 40 bar, the viscosity change with shear rate is within the measurement error range, thus the mixture can be considered to be Newtonian. CO₂ dissolution weakens and eventually eliminates the shear thinning effect of the Zuata crude oil. This indicates that the CO₂ molecule dissolved into the crude oil can eventually disrupt the associating network generated by the macromolecules in the crude oil, such as asphaltenes.

Regarding the diluted crude oil as shown by Figure 8, the CO₂ addition dramatically reduces the oil mixture viscosity to a minimum at 70 bar. As the CO₂ pressure increases beyond 70 bar (Figure 9), the higher CO₂ pressure causes an increase in the oil viscosity.

According to the study by Seifried et al.¹¹, in both the original and diluted Zuata crude oil, the onset of asphaltene precipitation occurs at CO₂ pressures above 80 bar. However, in our rheology experiments when the pressure is higher than 80 bar, the crude oil/CO₂ mixture behaves as a Newtonian fluid. This implies that asphaltene precipitation does not alter the rheological properties of this mixture.

The rheology results for the diluted crude oil are also interesting: in this case CO₂ dissolution gives rise to the non-Newtonian behavior, which only appears in a certain range of CO₂ pressure. Two speculations are given here for the shear thinning effect induced by CO₂ addition.

The first speculation is that the non-Newtonian behavior is caused by micelles formed by the asphaltene molecules under CO₂ dissolution. The CO₂ dissolved in the crude oil can reduce the critical micelle concentration (CMC) of the system by its action on the structure of asphaltene aggregates, and this can lead to greater interaction between micelles¹². At pressures from 30 to 60 bar, the distance between asphaltene micelles may be within the effective range of the van der Waals attraction force¹³. Thus, an associating network is formed among the micelles and causes the shear thinning effect. However, when the pressure is above 60 bar, CO₂ effect on the solvent or the non-asphaltene molecules is dominating, which leads to increase the CMC. Therefore, the asphaltene micelles are destabilized, and consequentially the associating network disappears.

The second speculation is based on the phase behavior point of view. At CO₂ pressures between 30 and 60 bar, a CO₂ rich liquid phase may have been generated, which makes the mixture form a liquid-liquid-vapor (LLV) system. An emulsion of these two liquids could be formed through the mixing by stirring and circulation due to the similar density of the two liquid phases. As the dispersed phase of the emulsion, the CO₂ rich liquid phase may be stabilized by the asphaltene in the crude oil. This emulsion shows non-Newtonian behavior because the dispersed phase gives rise to an associating network. However, when more CO₂ is dissolved into the oil mixture at pressure above 60 bar, the two liquid phases become miscible again. The result is a liquid-vapor (LV) system comprised of a crude oil rich liquid in equilibrium with a CO₂ rich vapor and the crude oil rich liquid phase behaves as a Newtonian fluid.

Figure 5. Viscosity measurement for the Zuata heavy crude oil with CO₂ at 50 °C and various shear rates. , lower shear rate limit; , ambient; , 20 bar; , 40 bar; , 60 bar; , 80 bar; , 100 bar. Reprinted with permission from Hu et al.¹⁵. Copyright 2016 American Chemical Society. Please click here to view a larger version of this figure.

Figure 6. Viscosity measurement for the Zuata heavy crude oil with CO₂ at 50 °C and various shear rates. , lower shear rate limit; , 120 bar; , 140 bar; , 160 bar; , 180 bar; , 200 bar; , 220 bar. Reprinted with permission from Hu et al.¹⁵. Copyright 2016 American Chemical Society. Please click here to view a larger version of this figure.

Figure 7. The relative viscosity for the Zuata crude oil with CO₂ at 50 °C and various shear rates. – –, measurement fluctuation range; , ambient pressure; , 20 bar; , 40 bar; , 60 bar; , 80 bar; , 100 bar; , 120 bar; , 140 bar; , 160 bar; , 180 bar; , 220 bar. Reprinted with permission from Hu et al.¹⁵. Copyright 2016 American Chemical Society. Please click here to view a larger version of this figure.

Figure 8. Viscosity measurement for the diluted crude oil with CO₂ at 50 °C and various shear rates. , lower shear rate limit; , 1 bar; , 10 bar; , 20 bar; , 30 bar; , 40 bar; , 50 bar; , 60 bar; , 70 bar. Please click here to view a larger version of this figure.

Figure 9. Viscosity measurement for the diluted crude oil with CO₂ at 50 °C and various shear rates. , lower shear rate limit; , 80 bar; , 100 bar; , 120 bar; , 140 bar; , 160 bar; , 180 bar; , 200 bar; , 220 bar. Please click here to view a larger version of this figure.

Figure 10. The relative viscosity for the diluted crude oil with CO₂ at 50 °C and various shear rates. – –, measurement fluctuation range; , 1 bar; , 10 bar; , 20 bar; , 30 bar; , 40 bar; , 50 bar; , 60 bar; , 70 bar. Please click here to view a larger version of this figure.

List of Materials

Heavy Crude Oil	Shell	N/A	Produced from the Zuata oil flied. Used without further treatment
Toluene	Sigma-Aldrich	244511-2L	Anhydrous, 99.8%. Used without further treatment
CO₂	BOC	111304-F	CP Grade. Used without further treatment
Name	Company	Catalog Number	Comments
Syringe Pump	Teledyne ISCO	65D
Mixer	Parr Instruments	4651	Vessel volume 250 ml. Mounted on a series 4923EE bench-top heater
Gear Pump 1	Polymer Systems Inc.	CIP-12/1.5	Used with CC29/Pr pressure cell for high viscosity fluids.
Gear Pump 2	Micropump	GAH X21	Used with DG35.12/Pr pressure cell for low viscosity fluids.
Rheometer	Anton Paar	MCR301
Pressure cell 1	Anton Paar	CC29/Pr	With flow-through configuration. Used for high viscosity fluids. Coaxial cylinder geometry
Pressure cell 2	Anton Paar	DG35.12/Pr	With flow-through configuration. Used for low viscosity fluids. Double gap geometry

Preparação do Laboratório

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO₂) at high temperatures and pressures is described. The system comprises a high-pressure rheometer which is connected to a circulation loop. The rheometer has a rotational flow-through measurement cell with two alternative geometries: coaxial cylinder and double gap. The circulation loop contains a mixer, to bring the crude oil sample into equilibrium with CO₂, and a gear pump that transports the mixture from the mixer to the rheometer and recycles it back to the mixer. The CO₂ and crude oil are brought to equilibrium by stirring and circulation and the rheology of the saturated mixture is measured by the rheometer. The system is used to measure the rheological properties of Zuata crude oil (and its toluene dilution) in equilibrium with CO₂ at elevated pressures up to 220 bar and a temperature of 50 °C. The results show that CO₂ addition changes the oil rheology significantly, initially reducing the viscosity as the CO₂ pressure is increased and then increasing the viscosity above a threshold pressure. The non-Newtonian response of the crude is also seen to change with the addition of CO₂.

Measurement of the Rheology of Crude Oil in Equilibrium with CO₂ at Reservoir Conditions

PREPARAÇÃO DO INSTRUTOR

CONCEITOS

PROTOCOLO DO ALUNO

Measurement of the Rheology of Crude Oil in Equilibrium with CO₂ at Reservoir Conditions

Learning Objectives

List of Materials

Preparação do Laboratório

Procedimento

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Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

PREPARAÇÃO DO INSTRUTOR

CONCEITOS

PROTOCOLO DO ALUNO

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

Learning Objectives

List of Materials

Preparação do Laboratório

Procedimento

Tags

Measurement of the Rheology of Crude Oil in Equilibrium with CO₂ at Reservoir Conditions

Measurement of the Rheology of Crude Oil in Equilibrium with CO₂ at Reservoir Conditions