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Undersaturated oil reservoir with a gas cap? Possible? 
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Joined: Sat Jan 15, 2011 2:42 pm
Posts: 2
Post Undersaturated oil reservoir with a gas cap? Possible?
I am writing to know your thoughts on the possibility of having an undersaturated oil reservoir with a gas cap. You may wonder but I know an oil reservoir where the reservoir pressure is above its bubble point and it has a huge gas cap.


Sat Jan 15, 2011 2:58 pm

Joined: Thu Sep 16, 2010 8:38 pm
Posts: 14
Post Re: Undersaturated oil reservoir with a gas cap? Possible?
In my opinion they must be either not connected, or your PVT sampling/testing has errors. By definition an undersaturated reservoir is above the bubble point, therefore no free gas (i.e. a gas cap) can exist. In an undersaturated reservoir all gas is in solution.


Sat Jan 15, 2011 4:57 pm

Joined: Sun Aug 15, 2010 4:12 am
Posts: 2
Post Re: Undersaturated oil reservoir with a gas cap? Possible?
I'm not totally on board with you Bill....
Depending on where the oil was sampled, large degrees of under-saturation can be measured in a reservoir with a gas-cap. This phenomena is related to compositional grading - and the (decreasing) degree of under-sturation can easily be up to 5 ( or more) psi per metrer below the gas-oil-contact (GOC). This gradient exceeds the (positive) hydrostatic with the result that locally the reservoir pressure can significantly exceed the local bubblepoint (even with a gascap)

The surprising answer (to many) is that only at the GOC is oil generally saturated - and the gas and oil phases in equilibrium. Below a GOC it is quite normal for a degree of under-saturation to set in. I'm assuming that it is this effect that you question refers to.

The normal 'undergraduate' definition of saturated tends to be an oversimplification in some cases - as even if the bubble point stayed constant - the increasing hydrostatic pressure implies a degree of under-saturation below the GOC (about ~1-1.5 psi/metre).

See SPE 13719 that discusses just such a case from Oman, where some 825psi difference was noted (about 200m below a GOC) this is around 4 deg C per meter gradient. So it is not all that unusual I believe.

As you see more reservoirs, the existence of such gradients becomes increasingly common. It makes most difference when the oil column is thick and the reservoir has strong temperature gradients. Similar arguments mean that is it also quite normal for solution gas ratios to decrease with increasing depth. Again, sometimes it is important to capture this and sometimes it does not matter. The choice to do so differs on a case by case basis.


Sun Jan 16, 2011 10:58 am
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