P/Z plot

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The P/Z plot is a plot of P/z versus Reservoir 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.

P/Z plot at ep.pengtools.com

Math & Physics

Applying Real Gas EOS at reservoir conditions:

 PV_r=z\frac{m}{M} RT_r (1)

Applying Real Gas EOS at standard conditions:

 P_{SC}V_g=1\frac{m}{M} RT_{SC} (2)

Dividing eq. 2 by eq. 1 and rearranging:

 V_g=\frac{P}{z} \frac{V_rT_{SC}}{P_{SC}T_{r}} (3)

Applying eq. 3 for initial conditions and for any point in time:

 GIIP=\frac{P_i}{z_i} \frac{V_rT_{SC}}{P_{SC}T_{r}}

Applying eq. 3 for any point in time:

 GIIP-G_p=\frac{P}{z} \frac{V_rT_{SC}}{P_{SC}T_{r}}

Therefore at any time:

 \frac{G_p}{GIIP}=1-\frac{P}{z} \frac{z_i}{P_i}


 \frac{P}{z}=\frac{P_i}{z_i} \left (1- \frac{G_p}{GIIP}\right )

Thus a plot of P/z vs cumulative produced gas is a straight line intersecting X axis at GIIP.


P/Z plot is a part of the Reservoir Management workflow in the E&P Portal used to estimate Reservoirs GIIP and recovery.

Example 1. Multiple Reservoirs on the same P/Z plot in the E&P Portal

Poverz multiple reservoirs.png

Gas Flowing Material Balance is the more advanced tool to determine the Reservoirs GIIP and recovery as well as Well's EUR and JD.


  1. Upload the data required
  2. Go to the Reservoir Management -> Performance Plots
  3. Select the Reservoirs you want to see and the Data range and click "Search"
  4. Scroll down the Performance Plots to see the P/Z plot

Data Required

In case you need to calculate the Monthly Measures from the Daily Measures:

See also

Gas Flowing Material Balance
Gas Material Balance


 GIIP = gas initially in place, scf
 G_p = cumulative gas produced, scf
 P = reservoir pressure (changing), psia
 P_{i} = initial reservoir pressure (constant), psia
 P_{SC} = pressure at standard conditions, psia
 T_i = initial reservoir temperature (constant), °R
 T_r = reservoir temperature (constant), °R
 T_{SC} = temperature at standard conditions (constant), °R
 V_g = volume of gas in reservoir converted to standard conditions (changing), scf
 V_r = reservoir volume (constant), ft3
 z = gas compressibility factor (changing), dimensionless
 z_i = initial gas compressibility factor (constant), dimensionless