# Presentation of the simulation

The geometry

As three-phase flow simulations are complex calculations for Jadim and Fluent, a simple geometry is used. The geometry represents one cave with oil wedged on the top and has same dimensions as the geometry used for experiments carried out in 2013. The boundary conditions are:

- An inlet velocity which represents  the gas injection, situated on the bottom-right of the domain

- A pressure outlet which represents a connection to another cave, situated on the middle left side of the domain

- Walls otherwize which represent the rocks

The simulation starts with an initiale quantity of oil wedged which represents 37% of the domain.

A solid obstacle is defined in the middle of the rectangular geometry in order to avoid a direct oil flow into the outlet and to force gas flow to go up and therefore replace the oil.

Incompressibility

In order to know if the flow is compressible or incompressible, it is important to look at the value of the Mach number. This dimentionaless number is defined by:

$Ma= \frac {U }{ a}$   where a is the sound velocity (a=340 m/s) and  U is the velocity of the flow.

A flow can only be considered as incompressible if its Mach number is smaller than 1.

As velocities inlet used for the simulation do not exceed 1 m/s, velocities of the three-phase flow will be small enough to consider that a Mach number of the flow smaller than 1.

In the simulations, the flow will be considered as incompressible.

Physical Properties

CFD simulations are based on Navier and Stoles equations. Therefore, it is essential to know the physical properties of the differente phases as they have a direct impact on the flow behavior.

Investigate real properties of phases is difficult. Indeed, due to the large dimension of the Tarim basin, oil properties can change from one well to another. Moreover, fluids and gas properties change in function of the temperature, which is itself variable, especially is function of the depth. Regarding gas viscosity, and as gas injection has not been tested yet in the reservoir, it is not known yet if the gas properties will be modified or not before being used.

After  discussion with our supervising team, and on the advices of Mr Montaron, choices have been made to use:

- The same properties of oil and water used for the experiments carried out in summer 2013

- Gas properties of an unmodified nitrogen gas

- Determinate surface tension value between oil/air/water phases instead of oil/nitrogen water phases.

All the physical properties used for the simulations are recap in the table below: Gravity is constant and equals to -9.81 m/s².

Turbulence

Multiphase flow simulation is a complex topic, still in development and which need long calculation times, lots of memory and high-performance material. Therefore, for this BEI, we choose small velocities in order to have a laminar flown which would not require to add turbulence models to the simulation.

Indeed, it is difficult to calculate the Reynolds Number of the flow because the velocity inside the domain is variable. In order to limit the turbulence phanomenon in the domain, the Reynolds Number of the gas is calculated, which must be small enough not to have a turbulent jet at the inlet.

$$Re_{gas}= \frac{U_{inlet}\times D\times \rho} { \mu }$$

with $\rho_{gas}=1.251 kg/m³$, $D=0.025m$ and ${ \mu_{gas}=12.10^{-6} kg/ms }$

A flow can be considered turbulent for a Re>2000. For each simulation, the Renolds Number of the gas is calculated in order to be sure that velocities used are not too high.

Moreover, for each simulation, it is important to verify that velocities in the domain are not to high.