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 undersaturaion 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 hydrostaic pressure implies a degree of undersaturation below the goc ( albeit ~ 1-1.5 psi/metre).
Depending on where the oil was sampled, large degrees of undersaturation can be inferred in a reservoir with a gas-cap. This phenomena is related to compositional grading - and the (decreasing) degree of underaturation can easily be up to 5 ( or more) psi per metre below the 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)
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 metre gradient ..... so it isnt all that unusual .....
As you see more reservoirs - the existence of such gradients becomes increasingly common ...... its just a question of degree ( not all reservoirs show large gradients). It makes most difference when the oil column is thick and the resvoir has strong temperature gradients. Similar arguments mena that is it also quite normal for solution gas ratios to decrease with increasing depth ..... again: Sometimes its important to capture this and sometimes it doesnt matter. Thsi is where engineering judgement comes in.
itag
Reply With Quote
Bookmarks