P/Z plot
Brief
The P/Z plot is a plot of P/z versus cumulative gas production, Gp.
The interpretation technique is fitting the data points with the straight line to estimate GIIP.
The P/Z plot is based on the Gas Material Balance equation.
Math & Physics
Applying Real Gas EOS at reservoir conditions:
- (1)
Applying Real Gas EOS at standard conditions:
- (2)
Dividing eq. 2 by eq. 1 and rearranging:
- (3)
Applying eq. 3 for initial conditions and for any point in time:
Applying eq. 3 for any point in time:
Therefore at any time:
- , or
Thus a plot of P/z vs cumulative produced gas is a straight line intersecting X axis at GIIP.
Discussion
Griffith correlation adds a hook to the originally straight Hagedorn and Brown VLP curve.
Nomenclature
- = gas initially in place, ft3
- = reservoir pressure (changing), psia
- = initial reservoir pressure (constant), psia
- = pressure at standard conditions, psia
- = initial reservoir pressure (constant), °R
- = reservoir pressure (constant), °R
- = temperature at standard conditions (constant), °R
- = reservoir volume (constant), ft3
- = volume of gas in reservoir converted to standard conditions (changing), ft3
- = gas compressibility factor (changing), dimensionless
References
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