Reservoir gas cap shrinkage is the reduction in the volume of a gas cap within an oil or gas reservoir over time, typically as a result of production activities or natural processes. In a reservoir, a gas cap is a layer of free gas that sits above an oil layer due to the difference in density. This gas cap can play a crucial role in maintaining reservoir pressure, which helps drive oil toward production wells.
As oil is extracted from the reservoir, the pressure in the system often decreases. This drop in pressure can cause the gas dissolved in the oil (solution gas) to come out of solution and migrate upward, joining the gas cap. However, if the gas cap itself is produced (intentionally or unintentionally) or if the reservoir pressure declines significantly, the gas cap can "shrink." This shrinkage occurs because the gas either dissolves back into the oil under changing pressure conditions, is extracted along with the oil, or disperses due to reservoir dynamics.
It's important in reservoir management because the gas cap often provides energy to push oil toward the wells (a process called gas cap drive). If it shrinks too much or too quickly, the reservoir's ability to produce oil efficiently may decline, requiring techniques like gas injection to restore pressure. Engineers study this behavior to optimize recovery and predict long-term production trends.
Gas Cap Shrinkage Formula
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| \( G_s \;=\; ( G_{pc} \cdot B_g) - (m \cdot N \cdot B_{oi} ) \cdot \left( \left( \dfrac{ B_g }{ B_{gi} } \right) - 1 \right) \) | ||
| Symbol | English | Metric |
| \( G_s \) = Gas Cap Shrinkage | \(bbl \;/\;ft^3\) | - |
| \( G_{pc} \) = Cumulative Gas Production from Gas Cap | \(SCF\) | - |
| \( B_g \) = Gas Formation Volume Factor | \(bbl \;/\;SCF\) | - |
| \( m \) = 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) | \(bbl \;/\;SCF\) | - |
