Simulation 2: Real properties of oil, gas and water

A second simulation is carried out with real properties of gas. In order to avoid convergence issues, the timestep is reduced to $10^{-5}$ and a small velocity inlet of 0.1 m/s is used. Because of this small velocities, the computational time of the simulation is really high. Therefore, only 5% of nitrogen is injected in the domain before convergence issues appear in the Jadim simulation.

The table below recaps the main caracteristics of the simulation:

  Physical properties Velocity inlet Time step Scheme Order Time simulated Nitrogen Injected (% of the domain)
Fluent Real properties 0.1 m/s $10^{-5}$ Second Order 2.8 sec 5%
Jadim Real properties 0.1 m/s max. $10^{-5}$ Second Order 2.8 sec 5%

The gas injection is characterized by a Gas Reynolds Number of 26. Moreover, as for the simulation 1, the velocity field in the domain observed on paraview shows that the velocity remains small in the domain in function of time.

Thus, it is relevant to conclude that the flow  can be consider as laminar and that turbulence models are not needed.

The two videos below show the same case simulated with Fluent and Jadim. As explained previoulsy, gas is injected on the lower right side of the domain, and the outlet is situated on the middle left side. The physical time simulated is 2.8 seconds, which correponds to a dimentionless time of 0.4 

                                                                                Simulation carried out with Fluent   

                                                                              Simulation carried out with Jadim

On videos, differencies between local flow features can be observed with the Fluent and Jadim simulations.

In the Jadim simulation, the gas interface is not accurate and it seems that Jadim has difficulties to handle flow features. While bubble of gas should be expected, the gas is diffused and seems to mix with oil and water.

In the Fluent simulation, the interface is more accurate but the gas seems to "desappear", probably because of numerical diffusion.

If  numerical diffusion can explained the diffusion of the gas observed with Jadim, another explanation can be found in the scale of the mesh. Indeed, the size of cells is close to 1 mm. Therefore, if the gas forms bubbles smaller than 1 mm, Jadim is not able to represent the topology changes properly. If Fluent seems to provide a accurate interface, it is important to note that it is a really strong commercial software design to always give a result.

The graph below represents the repartition of  phases in function of time. The comparison between Jadim and Fluent shows that after an injection of 5% of nitrogen, the repartition of gas and water in the domains already starts to be different.  If the simulation has been run longer, the repartirion of phases obtained with Fluent and Jadim would have probably been even more different.

                               

If we plot the outlet oil flow rate in function of time, results obtained with Jadim and Fluent are also different. When only oil goes out of the domain, which corresponds to a maximum oil flow rate, the flow rate ratio of the Fluent simulation as a value of 0.9, which represents an error of 10%.

                                

Due to the small velocity, only 5% of nitrogen is injected in the domain and only 4% of oil is recovered.

To conclude, the results obtained by Fluent and Jadim does not seem as accurate as in the Simulation 1. As the viscosity of the gas is small, bubbles of gas are difficult to represent and the interface between gas and other phases is hard to predict.

However, the computational time of Fluent is smaller than the one with Jadim and Fluent is a strong software which had a good capability to handle topology changes. Therefore, the parametric study will only be simulated with Fluent.