Remaining gas in place in coalbed methane reservoirs

Input(s)

A: Drainage Area (acres)

Ø: Porosity (fraction)

$\mathrm{h}$: Thickness (ft)

$\mathrm{B}_{\mathrm{w}}$: Water Formation Volume Factor (bbl/STB)

$\mathrm{W}_{\mathrm{p}}$: Cumulative Water Produced (bbl)

$\mathrm{S}_{\mathrm{wi}}$: Water Saturation (fraction)

$\mathrm{P}_{\mathrm{i}}$: Initial Pressure $(\mathrm{psi})$

P: Current Reservoir Pressure (psi)

$\mathrm{c}_{\mathrm{f}}$: Formation Compressibility $(1 / \mathrm{psi})$

$\mathrm{c}_{\mathrm{w}}$: Water Compressibility (1/psi)

$\mathrm{E}_{\mathrm{g}}$: Gas Expansion Factor $(\mathrm{SCF} / \mathrm{bbl})$

Output(s)

$\mathrm{G}_{\mathrm{R}}$: Remaining Gas at Reservoir Pressure (SCF)

Formula(s)

$$ \mathrm{G}_{\mathrm{R}}=7758 * \mathrm{~A} * \mathrm{~h} * \emptyset * \mathrm{E}_{\mathrm{g}} *\left(\frac{\left(\frac{\mathrm{B}_{\mathrm{w}} * \mathrm{~W}_{\mathrm{p}}}{7758 * \mathrm{~A} * \mathrm{~h} * \emptyset}\right)+\left(1-\mathrm{S}_{\mathrm{wi}}\right)-\left(\mathrm{P}_{\mathrm{i}}-\mathrm{P}\right) *\left(\mathrm{c}_{\mathrm{f}}+\mathrm{c}_{\mathrm{w}} * \mathrm{~S}_{\mathrm{wi}}\right)}{1-\left(\mathrm{P}_{\mathrm{i}}-\mathrm{P}\right) * \mathrm{c}_{\mathrm{f}}}\right) $$

Reference(s)

Ahmed, T., McKinney, P.D. 2005. Advanced Reservoir Engineering, Gulf Publishing of Elsevier, Chapter: 3, Page: 227.