Reservoir Fluid Properties - Cased-Hole Log Analysis and Reservoir Performance Monitoring - page 22

Fluid Properties

Downhole flow rates can differ quite markedly from surface-recorded flow rates.
For example, water is compressible, thus less water flows at downhole conditions
than at surface. This can be expressed as:

=*
where B w is the water formation volume factor (FVF). By contrast, oil flow rates
downhole are greater than oil flow rates at surface. Although oil too is compressible,
it can also accept greater volumes of dissolved gas at reservoir conditions and there-
fore expands. This can be expressed as:

q q B

wwf wsc w

=*
where B o is the oil formation volume factor. Gas is highly compressible, hence
downhole gas flow rates will be much smaller than those recorded on surface at
standard conditions. Thus we have:

q q B

owf osc o

=*
However, the fact must also be taken into account that there are really three sources
of the gas seen at the surface. Some will have come out of solution in the oil and/or
water and some will be free gas at reservoir conditions (assuming reservoir pressure
is below the bubble-point). Hence,

q q B

gwf gsc g

= +
Figure 3.8 illustrates these concepts. In order to calculate downhole flow rates from
the surface rates a series of charts may be used. Computer software is also widely
available for this task.

gasatsurface freegas solutiongas