Reduction in Gas Amount compared to GOR Value for Oil 2

[zamakaze]

I am simulating a well with GOR at standard condition as 2000 standard cubic feet / barrel (SCF per barrel)

The well stream go through separation in high , medium and low pressure separator follow by a tower for stabilizing

the pressures are 500 psig, 300 psig, 90 psig and 55 psig

i have 2000 barrels per day flowrate for the well fluid. however when i simulate total gas i get is 2.76 mmscfd

i do not get 2000 scf / barrel x 2000 barrel per day = 4000000 SCF or 4 MMSCF/d

what could be reaason for this reduction in gas amount

 

[LittleInch]

If your final pressure is still 55 psig, or 4 bar, in your tower then some of the gas / light ends are still dissolved in the oil or haven't vapourised due to the high pressure. E.g. the butane fraction might still be a liquid instead of a gas.

You will only get your 4 MMscfd if you allow the liquid pressure to get very close to atmospheric pressure ( 15 psia or say 1 psig)

GOR is worked out when they take a sample of pressurised Crude from downhole or a well stream and then allow it to flash off at atmospheric pressure.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MortenA]

Very often (most cases) the GOR predicted by the reservoir guys does not match the therdynamic flash prediction by HYSYS (or other process simulation tools).

This is where you become an engineer and not a "typist". Have a talk with the subsurface guys and have them explain how they arrive at their numbers.

Often they do a crude process simulation simulating a 2 or 3 stage pressure let down and summarize the flasd from these stages. Even then it wont match yours. But redo this in HYSYS: What i used to do would then be to "recycle" the combined offgas stream and solve this until the GOR in HYSYS matches the GOR predicted by the subsurface guys (remember this may change over field lifetime but your composition from samples wont - this alone should give you a clue as to something cannat "be right". Remember to document this because you will use a lot of time and resource explaining this and you can be unsure of what you did!

Best regards, Morten

 

[zamakaze]

Hi LittleInch,

the stabilized oil from tower is taken to atmospheric tank (0 psig, 15 psia). However, since vapore pressure is 12.5 psia, no flashing is observed on the tank itself.

is it just because i am going thru stages and bringing liquid and gas to equilibrium at different pressure and temperatures that the 4 mmscfd is not seen. if i flash my well stream in one step to std condition 15 psia, 60 F , i do observe 4 mmscfd.

the operating temp for the real world plant is 90F

 

[LittleInch]

The four steps may well be causing the simulation to not be the same.

It's not very clear if you're talking about an analysis such as Hysys or real life?

Can you expand a bit on what you're doing and what your issue is?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zamakaze]

hi LittleInch,

attached is what i am trying to simulate in hysys

i was trying to see if there is a reason as to why in simulation flashed gas is 2.76 mmscfd. The GOR for reservoir is 2000 scf/barrel under standard conditions (60F, 0 psig)

When I take the oil thru various separation stages in hsys, gas seen is 2.76 mmscfd. Oil flow is 2000 barrel per day.

The well stream go through separation in high , medium and low pressure separator follow by a tower for stabilizing - pressures are 500 psig, 300 psig, 90 psig and 55 psig

 

[LittleInch]

How many barrels of liquid are you getting out the tower?

The G O R regress to stock tank oil, not the reservoir fluid

What pressure is your reservoir?

Is this a black oil model or a proper composition?

I don't know how looks at this really.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zamakaze]

Out of the tower is about 1680 barrel

pressure of reservoir is 2000 psig

pressure downstream of choke is about 1000 psig

GOR is measured by flashing bottomhole sample to standard conditions (60 F 0 psig)

it is a proper composition

 

[LittleInch]

Well on that basis the gas flow should be 1680 x 2000 so 3.36 mmscfd. GOR is a crude measure based on stock tank crude.

So still a gap but not as much.

The issue is you're measuring volume but you need to measure mass to see where its going and look to track molecules/ individual components.

At a steady state conservation of mass is a law.

Follow the mass.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.