Oil recovery by any natural drive mechanism.
This is the result of both rock and liquid within the reservoir expanding due to the declining reservoir pressure. Such a reservoir is characterized by a constant gas-oil ratio. This is the least efficient driving mechanism.
In this reservoir the principal source of energy is a result of gas liberation from the crude oil, and the subsequent expansion of the solution gas as the pressure is lowered.
Reservoirs with a gas cap and little or no water drive. They are characterized by a slow decline in reservoir pressure. The energy comes from two sources – expansion of the gas cap and solution gas.
This is the result of water moving into the pore paces originally occupied by oil, replacing and then displacing the oil.
Occurs as a result of differences in densities of the reservoir fluids.
One where both water and gas are present to displace the oil toward the well. The two driving forces are depletion with weaker water drive and depletion with gas cap and weak water drive.