Page 224 - Principles of Applied Reservoir Simulation 2E

Part HI: Case Study 209

Taking the product of porosity and bulk volume gives the following estimate
of pore volume:
7 3 6
= * 9.18 x io ft * 16.4 x W RB
V p $V B
The product of oil saturation and pore volume gives an estimate of oil volume
in reservoir barrels. Dividing this volume by an average oil formation volume
factor B 0 for the reservoir gives an estimate of oil volume in stock tank barrels.
The value of oil FVF at an initial average reservoir pressure of 3935 psia is
1.3473 RB/STB. This value is obtained from laboratory data that has been
corrected for use in a reservoir simulator (Chapter 20.5). The resulting oil
volume is
6
11 5 10 RB
SV P 0.7V p x iu 6
V = -2_£ « £ ~ -ii£ *** ~ 8.5 x 10 STB
0
5 B. 1.3473 RB/STB

21.2 Material Balance

Volumetrics provides one measure of the quality of a reservoir model,
but it is based on information that does not change with time. Another estimate
of original oil volume can be obtained from a material balance study if a
reasonable amount of production data is available, such as the historical data
presented in Chapter 20. At this point we have surmised that the reservoir was
initially undersaturated, but it may not have aquifer support.
The presence of a few barrels of water during the latter months of the first
year of production indicates that mobile water is present, but its source is
unknown. The volume of produced water is small enough to be water mobilized
by swelling as reservoir pressure declines, or it could be the first indication of
water production from aquifer influx. Both of these scenarios can be assessed
if we consider the possibilities of depletion with and without aquifer influx.
We begin by deriving the material balance equation for the more general
case: depletion of an undersaturated oil reservoir with water influx. The
derivation is simplified by assuming formation compressibility is negligible and
then setting the decrease in oil volume at reservoir conditions equal to the