Navigation:  Prospect/Field evaluation > Entering probability distributions > Input Variables > Gas Expansion Factor

See also 'Entering Probability Distributions'.

 

The gas expansion factor (1/Bg) accounts for the volume expansion of the gas it moves from reservoir conditions to surface conditions, caused by the reduction in pressure - though partially counterbalanced by the decrease in temperature:

 

Exp = 1/Bg = {volume occupied at surface conditions} / {volume occupied at reservoir conditions}

 

Standard correlations are available to estimate 1/Bg. These correlations may or may not be accurate and it is always best to use real data if you have it. This is especially the case in high pressure / high temperature environments.

 

Surface or standard conditions are set by the local regulatory agency, or maybe the gas buyer: REP assumes that surface conditions are 60 degF and 14.7 psia, which is commonly the case, or at any rate near enough to be accurate in a REP context. But if in doubt, check with your reservoir engineer - nothing so pleases these pedants as to put a non-engineer right on a point of definition, and is always wise to nip this sort of thing in the bud. You can change the surface conditions in the correlations section.