What is the reason for using pseudo-pressure for gas reservoirs?

[pet_spec]

Hello,

Please help me to understand the concept of pseudo-pressure. So can you please answer my following questions. That will really put my progress forward.

1) What the pseudo-pressure gives us in gas reservoir performance analysis?

2) It has been said that to linearize diffusivity equation the real pressure should be replaced with pseudo-pressure . However, for pseudo-pressure we don't know the real actual pressure in reservoir. So how can we get it?

Thanks

[gunner]

hi friend,
firstly you have to understand that gas behavior deviates from oil behavior.
Gas is highly compressible so there is 2 problems
1.) Viscosity is a function of pressure
2.) Density is function of pressure

the concept of pseudo pressure is not new, in other disciplines of engineering this problem has been solved by substituting a function in to the original differential equation.

In the case of oil reservoir we dont have such problems , we have the diffusivity equation and the solution is well document viz. Ei solution.
The Ei solution directly gives pressure.

Now by using pseudo pressure function in case of gas , the diffusivity equation is in the same form , only difference is that we have 'Si' in stead of pressure.

We can use the solution, get 'Si' and reverse calculate P.
Regarding obtaining the 'Si' you must know the PVT properties and generate a table for a range of values of Si vs P.

P is the average reservoir pressure or initial reservoir pressure.

if still did not understand the concept i suggest you read the document " Gas well testing manual by ERCB".
I is available on scribd.com

Good luck

[pet_spec]

Hi,

Thanks for your answer. Sorry, I couldn't find the book you mentioned there

I have PVT properties of gas. How can I obtain the pseudopressure? Can you please write short step-by-step method.