Primary production of hydrocarbons from petroleum reservoirs are due to three forces . These are typically referred to as drive mechanisms. They are depletion drive, segregation drive, and water drive.
Depletion drive (DDI) is the volumetric expansion of the oil. This drive harnesses the energy of the oil that has been compressed due to the high initial reservoir pressure.
Segregation (gas cap) drive (SDI) is the volumetric expansion of the gas. Like depletion drive, it harnessed the energy of the compressed gas. As gas is much more compressible the oil, this drive often has a greater impact. Segregation drive includes both to the expansion of the original gas in place and the evolution of gas from the oil as the pressure declines.
Water drive (WDI) is the bulk inflow of water from outside the boundaries of the reservoir, typically from an adjacent aquifer.
Not all reservoirs experience all three types of drive. Water drive applies only to reservoirs with an attached aquifer of sufficient magnitude. Depletion drive does not apply to single-phase gas reservoirs. Segregation drive applies to all reservoir types.
The relative magnitude of each drive on the overall production can be measured using the material balance equation rearranged in the form of a drive index as shown below. It can be summarized as DDI + SDI + WDI = 1. Using this method, the contribution of each drive to a field can be quantified and used for selecting appropriate strategies to increase production.