Vertical Equilibrium

[Bilal Amjad]

Need to know about VERTICAL EQUILIBRIUM, What? Why? Effect?

[Abdelkader]

Bilal,
I guess your are speaking about the equilibrium that happens between fluids in place (in the reservoir) under a static condition.

The effect is called also segregation and is mainly due to the gravitational forces (and the difference between the density of the fluids obviously), as you know in the reservoir we deal with 3 type of forces: gravitation, viscosity and capillarity, each one overcoming the others depending on the status of the reservoir.

In vertical equilibrium it is the gravity that overcomes the other forces and the fluids are segregated vertically depending on their densities.

I hope I've understood well your question and addressed your concern; sorry if not!

Regards,
Abdelkader

[Bilal Amjad]

thanks.
why it is important in Simulation?

[Abdelkader]

In simulation, equilibrium is useful for initialization purposes, that way the computer knows where the contacts are and how to deal with the transition than!

Regards,
Abdelkader

[halim78]

thanks.
why it is important in Simulation?

In addition to what Abdelkader said.
Model initialization is based on VE, initialization means defining the initial conditions of the simulation (pressures and saturations should be defined for each grid block at the start of simulation).
How Eclipse do this?
Answer: Using equilibration procedure and the details are as follows:
1- You have to define an equilibration region using EQUIL keyword, which contains basically pressure @datum, fluid contacts and RSVD.
2- You have to define fluid densities @ sc using DENSITY keyword
3- Enter the capillary pressure table Pc vs saturation and the PVT table.
Initialization in Eclipse:
It is performed in to stages:
1-Pressure calculation:
*First it calculates reservoir densities of oil, water and gas using (surface densities, Rs and Bo)
*Convert these densities to gradient and calculates phase pressures (Po,Pw and Pg) for each grid block.
2-Saturation calculation:
*Calculates Pc for each grid block.
*Look up in Pc table and assign the corresponding saturation.
*If calculated Pc >Pc max of the table then the saturation is the critical one.

Hope this helps

[Bilal Amjad]

it is really very helpful.

[bcoats]

Vertical Equilibrium is a method originally developed 40 years ago by Keith Coats to more accurately compute initial pressure, saturations, and composition (fluids in place) by capillary/gravity equlibrium for gridblocks with a large vertical block dimension (block thickness) that are initially in the gas/oil or water/oil or gas/water transition zones. It is easily confused with capillary/gravity equilibrium. In Sensor's current implementation, the rigorous solution is approximated by pseudo capillary pressure curves generated from the input curves (if any) and fluid properties. The VE correction becomes negligible as the vertical block thickness becomes small. Most simulators have the option and it should be described in the documention.

Regards,
Brian Coats
Coats Engineering