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

The overall goal of this experiment is to study the effects of carbon dioxide dissolution on the rheology of crude oils. When CO2’s injected into an oil reservoir, it dissolves into the crude oil. This can strongly affect the oil rheology, a crucial parameter in predicting flow in the reservoir.

The main advantage of this technique is the thorough measurement of the rheology of a gas-liquid mixture at restful conditions. Answering the fundamental question:whether the mixture is Newtonian or non-Newtonian. To begin preparing the double-gap geometry pressure cell system, disconnect and open the high-pressure mixer.

Load a stir bar in 200 milliliters of low-viscosity oil into the mixer. Close the vessel with screws, and reconnect the mixer. Remove the pressure head and measuring cylinder from the pressure cell.

Ensure that the valves to the gas tanks are closed. Close the oil and gas valves connected to the mixer, the gas valve to the pressure cell, and the gas relief valve. Open the relief valve on the recycle path of the circulation loop.

Then, open the mixer to nitrogen gas. Once the gas reaches the mixer, close the nitrogen valve and cylinder. Open the oil valve downstream of the mixer and monitor the double-gap geometry.

Once the inner part of the double-gap geometry is just barely immersed in oil, open the gas relief valve to depressurize the system. Start the gear pump and adjust the rotation speed, so that the oil flow rate into the pressure cell is less than or equal to the gravity-driven flow rate out of the pressure cell. When the inlet flow rate is acceptable and oil is dripping from the relief valve outlet, stop the gear pump.

Close the relief valve, then open the valve on the recycle path of the circulation loop. Start the gear pump and allow the oil to circulate through the system. Mount the measuring cylinder in the pressure head on the pressure cell.

Mount the rotation cup on the spindle and set it to the measuring position. Then, turn off the gear pump and close the oil valves. Connect the vacuum pump to the gas relief valve and evacuate the system for 15 minutes.

Then, close the valve and turn off the vacuum pump. Open the carbon dioxide gas flow, the valve on the recycle path of the circulation, and the gas valve to the pressure cell. Once the system is pressurized, close the carbon dioxide valve and cylinder to prevent backflow.

Set the mixer, pressure cell, and system heater to the desired temperatures. Set the syringe pump to the desired pressure. Once the temperature and pressure have stabilized, open the oil ball valves.

Start the stir motor and gear pump. Every six hours, record the volume in the syringe pump. Turn off the stir motor and gear pump, and allow the oil to settle for five minutes.

Then, measure the viscosity at a constant shear rate of 10 inverse seconds. If the volume of viscosity differ from the previous measurement by more than 4%restart the stir motor and gear pump and continue mixing for another six hours. Once the change in volume and viscosity over a six-hour period is 4%or less, close the oil ball valves.

Pre-shear the equilibrated oil mixture at a shear rate of ten inverse seconds for 30 seconds. Allow the mixture to rest for one minute and then measure the viscosity at shear rates, stepping from 250 inverse seconds to 10 inverse seconds. Close the gas valves to the mixer and the pressure cell.

Allow carbon dioxide to flow into the gas system. Then, close the carbon dioxide valve and cylinder and introduce the carbon dioxide into the mixer and pressure cell. Continue adding carbon dioxide in this way to achieve the next pressure step.

Set the new pressure set point on the syringe pump and wait for the pressure to stabilize. Re-equilibrate the oil mixture before performing the next measurement. To begin preparing the coaxial cylinder geometry pressure cell system, load the high-pressure mixer with 200 milliliters of viscous oil.

Next, tighten the pressure head of the coaxial cylinder geometry pressure cell to close the cell. Mount the rotation cup on the spindle and set it to the measuring position. Ensure that the valves to the gas cylinders are closed.

Close the oil valves, the gas valve to the mixer, and the gas relief valve. Open the relief valve on the recycle path of the circulation. Prime the mixer with the loaded viscous oil using compressed nitrogen gas and then close the nitrogen gas valve and cylinder.

Open the oil valve downstream of the mixer and monitor the relief outlet. When oil begins dripping from the outlet tube, open the gas relief valve to depressurize the system. Close the relief valve and open the valve on the recycle path of the circulation.

Start the gear pump and allow the oil to circulate for one to five hours, depending on the viscosity. Then, turn off the gear pump, close the oil valves, and evacuate the system for 15 minutes via the gas relief valve. Open the carbon dioxide flow and the valve on the recycle path of the circulation.

Once the system is filled with carbon dioxide, close off the gas flow. Set the system temperature and pressure and wait for them to stabilize. Open the oil valve downstream of the mixer and start the stir motor and gear pump.

Take volume and viscosity measurements every six hours until the viscous oil and carbon dioxide gas are in equilibrium. Then, close the oil ball valve. Pre-shear the oil mixture at 10 inverse seconds for 30 seconds and then rest the mixture for one minute.

Measure the viscosity at shear rates, stepping from 500 inverse seconds to 10 inverse seconds. Close the valve between the mixer and the gas system. Introduce carbon dioxide into the gas lines and then transfer the carbon dioxide into the mixer.

Increase the pressure set point and re-equilibrate the system as previously described. Zuata crude oil, saturated with carbon dioxide at 50 degrees Celsius, was subjected to shear strain at shear rates ranging from 500 inverse seconds to 10 inverse seconds at various carbon dioxide pressures. The viscosity decreased with increasing pressure up to 100 bar.

At which point the viscosity slowly increased with increasing pressure. A preparation of Zuata crude oil, diluted with toluene, was evaluated under similar conditions, at shear rates ranging from 250 inverse seconds to 10 inverse seconds. The viscosity decreased with increasing carbon dioxide pressure up to 70 bar.

At which point the viscosity increased with increasing pressure. The crude oil displayed Newtonian behavior at pressures exceeding 40 bar, indicating that the dissolved carbon dioxide disrupted an associative network of macromolecules in the oil. The diluted crude oil displayed shear thinning between 30 and 60 bar of carbon dioxide, suggesting that the dissolved carbon dioxide facilitated the formation of an associative network within a narrow range of pressures.

With this system, we no longer assume that the mixture behaves as a Newtonian fluid, but consider shear rate as another variable. Using this system, we have measured the viscosity of the crude oil in equilibrium with CO2 under restful conditions. The result shows that the CO2 addition has a significant effect on the rheology of crude oil.

The rheological properties of a liquid containing dissolved gas are vital for applications from enhanced oral recovery to polymer processing. And the measurement system reported here can be used to characterize such fluids, even if they’re non-Newtonian. Precautions, such as wearing PVC, should always be taken while performing the procedure.

Before the procedure, it is recommended to perform air check for rheometer to ensure that rheometer is functioning well.