P/Z plot

Brief

The P/Z plot is a plot of P/z versus Reservoir cumulative gas production, G_p.

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

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

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