# Gas material balance equation

## Input(s)

$$\frac{\mathrm{P}_{\mathrm{i}}}{\mathrm{z}_{\mathrm{i}}}$$: Ratio of Pressure to Compressibility Factor at Initial Conditions (psi)

$$\mathrm{P}_{\mathrm{SC}}$$: Pressure at Standard Conditions (psi)

$$\mathrm{T}_{\mathrm{SC}}$$: Temperature at Standard Conditions $$\left({ }^{\circ} \mathrm{R}\right)$$

T: Current Temperature $$\left({ }^{\circ} \mathrm{R}\right)$$

V: Original Gas Volume $$\left(\mathrm{ft}^{3}\right)$$

$$\mathrm{G}_{\mathrm{p}}$$: Cumulative Gas Production $$(\mathrm{SCF})$$

## Output(s)

$$\frac{\mathrm{P}}{\mathrm{z}}$$: Ratio of Pressure to Compressibility Factor at Current Conditions for $$\mathrm{P} / \mathrm{z}$$ vs $$\mathrm{G}_{\mathrm{p}}$$ plot (psi)

## Formula(s)

$\frac{\mathrm{P}}{\mathrm{z}}=\left(\frac{\mathrm{P}_{\mathrm{i}}}{\mathrm{z}_{\mathrm{i}}}\right)-\left(\frac{\mathrm{P}_{\mathrm{SC}} * \mathrm{~T}}{\mathrm{~T}_{\mathrm{SC}} * \mathrm{~V}}\right) * \mathrm{G}_{\mathrm{p}}$

## Reference(s)

Ahmed, T., McKinney, P.D.2005. Advanced Reservoir Engineering, Gulf Publishing of Elsevier, Chapter: 3, Page: 203.

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