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.