The image above represents pressure and initial compressibility factor ratio (P/Z Ratio).

To compute for the pressure and initial compressibility factor ratio, four essential parameters are needed and these parameters are **initial pressure (P**_{i}), Initial Compressibility Factor (Z_{i}), Cumulative Gas Production (G_{P}) and initial gas in place (G).

The formula for calculating the P/Z Ratio:

^{P}/_{Z} = ^{Pi} / _{Zi} (1 – ^{Gp} / _{G})

Where;

^{P}/_{Z} = P/Z Ratio

P_{i} = Initial Pressure

Z_{i} = Initial Compressibility Factor

G_{P} = Cumulative Gas Production

G = Initial Gas in Place

Let’s solve an example;

Given that the initial pressure is 20, the initial compressibility factor is 28, the cumulative gas production is 30 and the initial gas in place is 34.

Find the pressure and initial compressibility factor ratio (P/Z Ratio)?

This implies that;

P_{i} = Initial Pressure = 20

Z_{i} = Initial Compressibility Factor = 28

G_{P} = Cumulative Gas Production = 30

G = Initial Gas in Place = 34

P/Z = ^{Pi} / _{Z}_{i} (1 – ^{Gp} / _{G})

P/Z = ^{20} / _{28 (1 – 30 / }_{34})

P/Z = ^{20} / _{28 (1 – 0.88)}

P/Z = ^{20} / _{28 (0.117)}

P/Z = 0.714(0.117)

P/Z = 0.084

Therefore, the **P/Z Ratio** is **0.084.**

**Calculating for Initial Pressure when P/Z Ratio, Initial Compressibility Factor, Cumulative Gas Pressure and Initial Gas In Place is Given.**

P_{i} = ^{P/Z x Zi} / _{(1 – Gp / }_{G})

Where;

P_{i} = Initial Pressure

^{P}/_{Z} = P/Z Ratio

Z_{i} = Initial Compressibility Factor

G_{P} = Cumulative Gas Production

G = Initial Gas in Place

Let’s solve an example;

Given that the P/Z Ratio is 50, the initial compressibility factor is 18, the cumulative gas production is 25 and the initial gas in place is 32.

Find the initial pressure ?

This implies that;

^{P}/_{Z} = P/Z Ratio = 50

Z_{i} = Initial Compressibility Factor = 18

G_{P} = Cumulative Gas Production = 25

G = Initial Gas in Place = 32

P_{i} = ^{P/Z x Zi} / _{(1 – Gp / }_{G})

P_{i} = ^{50 x 18} / _{(1 – 25 / }_{32})

P_{i} = ^{900} / _{(1 – 0.78125)}

P_{i} = ^{900} / _{(0.21875)}

P_{i} = 4114.28

Therefore, the **Initial Pressure** is **4114.28**.

**Calculating for Initial Compressibility Factor when P/Z Ratio, Initial Pressure, Cumulative Gas Pressure and Initial Gas In Place is Given.**

Z_{i} = ^{Pi (1 – }^{Gp} / _{G}) / _{P/Z}

Where;

Z_{i} = Initial Compressibility Factor

P_{i} = Initial Pressure

^{P}/_{Z} = P/Z Ratio

G_{P} = Cumulative Gas Production

G = Initial Gas in Place

Let’s solve an example;

Given that the P/Z Ratio is 40, the initial pressure is 18, the cumulative gas production is 15 and the initial gas in place is 22.

Find the initial compressibility factor ?

This implies that;

P_{i} = Initial Pressure = 18

^{P}/_{Z} = P/Z Ratio = 40

G_{P} = Cumulative Gas Production = 15

G = Initial Gas in Place = 22

Z_{i} = ^{Pi (1 – }^{Gp} / _{G}) / _{P/Z}

Z_{i} = ^{18 (1 – }^{15} / _{22}) / _{40}

Z_{i} = ^{18 (1 – 0.681)} / _{40}

Z_{i} = ^{18 (0.319)} / _{40}

Z_{i} = ^{5.742} / _{40}

Z_{i} = 0.1435

Therefore, the **initial compressibility factor **is ** 0.1435.**

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