# P/Z plot

## Brief

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:

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

Or:

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

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

## Discussion

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

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

## Workflow

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:

## Nomenclature

$GIIP$ = 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