Gas Cap Shrinkage

Reservoir gas cap shrinkage is the reduction in the volume of the gas cap in a reservoir due to various factors such as production activities and changes in reservoir pressure.  The gas cap is a layer of natural gas that sits above the oil zone in a petroleum reservoir.  It serves as a source of drive energy to help push oil towards the production wells.

Shrinkage Can Occur Due To

Pressure Decline  -  As oil and gas are produced, the pressure in the reservoir decreases.  This pressure drop can cause the gas in the gas cap to condense back into the oil phase, reducing the volume of the gas cap.
Gas Production  -  If gas is produced along with oil, the volume of the gas cap will decrease.
Water Encroachment  -  Water can move into the reservoir, replacing the volume formerly occupied by the gas cap and causing it to shrink.

The shrinkage of the gas cap can affect reservoir performance by reducing the gas drive mechanism, potentially leading to lower oil recovery rates and necessitating changes in production strategy to maintain optimal production levels.

 

Gas Cap Shrinkage formula Oilfield units are barrels (bbl)  -  \(1\; bbl = 5.615 \;ft^3\) or \(0.159 \;m^3\). STB (stock tank barrel) is the same volume defined at some surface standard conditions in a stock tank, which are usually 60°F and 14.7 psi.
\( G_s \;=\; G_{pc} \; B_g - m \; N \; B_{oi} \; [\; ( B_g \;/\; B_{gi} ) - 1 \;] \)
Symbol	English	Metric
\( G_s \) = Gas Cap Shrinkage	\(bbl \;/\;ft^3\)	-
\( G_{pc} \) = Cumulative Gas Production from Gas Cap	\(bbl\)	-
\( B_g \) = Gas Formation Volume Factor	\(bbl \;/\;ft^3\)	-
\( m \) =  Gas Cap Ratio	\(dimensionless\)	-
\( N \) =  Oil in Place	\(STB\)	-
\( B_{oi} \) =  Initial Oil Formation Volume Factor  (barrels of oil per barrel of reservoir oil)	\(bbl \;/\; STB\)	-
\( B_{gi} \) =  Initial Gas Formation Volume Factor (cubic feet of gas at standard conditions per cubic foot of gas at reservoir conditions)	\(bbl \;/\;ft^3\)	-