Page 216 - Hydrocarbon

Reservoir Dynamic Behaviour 203

oil (rb)
underground
gas (rb)
withdrawal
water (rb)

free gas free gas
connate water oil Connate Water oil

aquifer aquifer

Initial Conditions After Production
Figure 9.1 Expansion of £uids to replace produced volumes.
Reservoir engineers describe the relationship between the volume of fluids
produced, the compressibility of the fluids and the reservoir pressure using material
balance techniques. This approach treats the reservoir system like a tank, filled with oil,
water, gas and reservoir rock in the appropriate volumes, but without regard to the
distribution of the fluids (i.e. the detailed distribution or movement of fluids inside
the system). Material balance uses the PVT properties of the fluids described in
Section 6.2.6, Chapter 6, and accounts for the variations of fluid properties with
pressure. The technique is firstly useful in predicting how reservoir pressure will
respond to production. Secondly, material balance can be used to reduce uncertainty
in volumetrics by measuring reservoir pressure and cumulative production during
the producing phase of the field life. An example of the simplest material balance
equation for an oil reservoir above the bubble point will be shown in the next
section.

9.2. Reservoir Drive Mechanisms

The previous section showed that the fluids present in the reservoir, their
compressibilities, and the reservoir pressure all determine the amount of energy