Gas-cap-drive reservoirs can be identified by the presence of a gas cap with little or no water drive. Due to the ability of the gas cap to expand, these reservoirs are characterized by a slow decline in the reservoir pressure. The natural energy available to produce the crude oil comes from the following two sources:
The reservoir pressure falls slowly and continuously. Pressure tends to be maintained at a higher level than in a depletion drive reservoir. The degree of pressure maintenance depends upon the volume of gas in the gas cap compared to the oil volume.
Absent or negligible water production.
The gas-oil ratio rises continuously in up-structure wells. As the expanding gas cap reaches the producing intervals of up structure wells, the gas-oil ratio from the affected wells will increase to high values.
Ultimate oil recovery
Oil recovery by gas-cap expansion is actually a frontal drive displacing mechanism that, therefore, yields considerably larger recovery efficiency than that of depletion-drive reservoirs. This larger recovery efficiency is also attributed to the fact that no gas saturation is being formed throughout the reservoir at the same time. The expected oil recovery ranges from 20% to 40%.