1. Chapter 3 Material Balance Applied to Oil Reservoirs
§ 3.1 Introduction
-The Schilthuis material balance equation
- Basic tools of reservoir engineering
=> Interpreting and predicting reservoir performance.
-Material balance
1. zero dimension – this chapter
2. multi-dimension (multi-phase) – reservoir simulation

2. § 3.2 General form of the material balance equation for a hydrocarbon reservoir
Underground withdrawal (RB)
= Expansion of oil and original dissolved gas (RB)………(A)
+ Expansion of gascap gas (RB) ……………… ………(B)
+ Reduction in HCPV due to connate water expansion and decrease in
the pore volume (RB)……………………… …….……(C)

4. Expansion of oil & originally dissolved gas

5. Expansion of the gascap gas
Expansion of the gascap gas =gascap gas (at p) –gascap (at pi)

6. Change in the HCPV due to the connate water expansion & pore volume reduction

7. Underground withdrawal

8. The general expression for the material balance as

9. where

10. No initial gascap, negligible water influx
With water influx eq(3.12) becomes
Eq.(3.12) having a combination drive-all possible sources of energy.

11. § 3.4 Reservoir Drive Mechanisms
- Solution gas drive
- Gascap drive
Natural water
drive
- Compaction drive
In terms of
reducing the M.B to a compact form to
quantify reservoir performance
determining the main producing
characteristics,
for example, GOR; water cut
determining the pressure decline in the
reservoir
- estimating the primary recovery factor

12. § 3.5 Solution gas drive
(a) above the B.P. pressure (b) below the B.P. pressure

13. Above the B.P. pressure - no initial gascap, m=0 - no water flux, We=0 ; no water production, Wp=0 - Rs=Rsi=Rp
from eq.(3.7)

14. Exercise3.1 Solution gas drive, undersaturated oil reservoir Determine R.F.
FromTable2.4(p.65)
Eq(3.18)

15. Table 2.4 Field PVT P(psia) Bo (Rb/STB) Rs(SCF/STB) Bg( Rb/SCF)
4000 (pi)
3300 (pb)

16. Bo as Function of Pressure

17. Rs as Function of Pressure

18. Bg and E as Function of Pressure

19. Producing Gas-oil Ratio (R) as Function of Pressure

21. Below B.P. pressure (Saturation oil) P<Pb =>gas liberated from saturated oil

22. Exercise3.2 Solution gas drive; below bubble point pressure Reservoir-described in exercise Pabandon = 900psia (1) R.F = f(Rp)? Conclusion? (2) Sg(free gas) = F(Pabandon)? Solution: (1) From eq(3.7)

23. Eq(3.7) becomes
Conclusion:

25. (2) the overall gas balance
liberated
gas in the reservoir
total
amount
of gas
gas
produced
at surface
gas still
dissolved
in the oil
= − −