Reservoir Fluid Flow and Natural Drive Mechanisms
Gas Cap Drive and the Material Balance Equation
Material Balance Applications
The material balance equation for a gas cap drive reservoir is obtained by assuming that natural water influx is zero (We=0), and that the effect of rock and water compressibilities in oil zone as well as in gas cap compared to the gas compressibility is negligible. With these assumptions, the MBE becomes:
![N_{p}\, \left [ B_{t}+\left ( R_{p}-R_{si} \right )\, B_{g} \right ]=N\, \left [ \left ( B_{t} - B_{ti} \right ) + \dfrac{mB_{ti}}{B_{gi}}\left ( B_{g}-B_{gi} \right ) \right ]](/wp-content/ql-cache/quicklatex.com-91a2bdd7a0a95f93df25df3ee097b6e8_l3.png "Rendered by QuickLaTeX.com")
…………(45)
This equation is rather cumbersome. A better understanding of the mechanism may be gained by writing the equation in the form suggested by Havlena and Odeh (1963), which is:
…………(46)
where
F= cumulative production in reservoir volumes.
N= initial oil in place in surface volumes.
Eo=Bt−Bti
Eg=Bg−Bgi
Equation 46 is used with production data to determine N and the effective size of the gas cap, m. The way to use the equation is to plot F versus 
for an assumed value of m (Figure 1).
Equation 46 is used with production data to determine N and the effective size of the gas cap, m. The way to use the equation is to plot F versus for an assumed value of m (Figure 1).
If the selected value of m is too small, the plot will curve upward, and if it is too large it will curve downward. A correct value of m will give a straight line that passes through the origin. The importance of the origin as a required point cannot be overemphasized. It is the only known fixed point that guides the plot. The slope of the straight line is N, the initial oil in place.
