Primary recovery:

Primary oil recovery phase describes the production of hydrocarbons under the natural driving mechanism present in the reservoir. The sources of natural reservoir energy are fluid and rock expansion, solution gas drive, gravity drainage, and the influx of water from aquifers.

natural force present to move the oil through the reservoir rock to the wellbore is the pressure differential between the higher pressure in the rock formation and the lower pressure in the producing wellbore.

Water Influx: There may exist an aquifer near reservoir. Water compressed in the aquifer expands once the reservoir pressure decreases due to production. This water causes a natural water drive. Recovery : 50-60%

Solution Gas Drive: As the reservoir pressure decreases with production, gas dissolved in oil evolves and displaces oil. Recovery: 10-15%. Gas is more mobile than oil.

Under saturated Reservoir: Reservoir energy to displace oil is due to fluid & rock compressibility. When the bubble point pressure is reached, solution gas drive begins. Water injection can be applied to maintain the pressure above the bubble point.

Gravity Drainage: In thick & well connected in vertical direction or inclined reservoirs, gas moves upward to replace the space left by oil. It is the rather slow process because gas is more mobile than oil and the mobility of oil controls the process.

Gas Cap Drive: Gas compressed in gas cap (if exists) expands as reservoir pressure declines. Pressure can be kept by gas injection. If water is injected, oil displaced by water may reduce the gas entrance from the gas cap.

Secondary recovery techniques:

- water flooding

- immiscible gas injection (displace oil, and drive it to a production well)

Secondary recovery methods are used when there is insufficient underground pressure to move the remaining oil. The most common technique, water flooding, utilizes injector wells to introduce large volumes of water under pressure into the hydrocarbon bearing zone. As the water flows through the formation towards the producing well bore, it sweeps some of the oil it encounters along with it. Upon reaching the surface, the oil is separated out from water for sale. While somewhat more expensive than primary production, water flooding can recover an additional 10 to 30 percent of OOIP.

When water flooding for secondary recovery reaches a point when production is no longer cost–effective, a decision must be made whether to transition the field to a tertiary recovery phase.

In addition to maintaining reservoir pressure, this type of recovery seeks to alter the properties of the oil in ways that facilitate additional production.

Immiscible displacement occurs when the reservoir has lower pressure.  In this process process in which some of the injected CO2 still dissolves in the oil, while the rest is used as an artificial air cap, pushing oil simultaneously downwards and towards the rim of the reservoir where the producing wells are located. For immiscible floods, significantly more CO2 may be left in the reservoir.

Tertiary, or EOR, techniques:

- chemical flooding (polymer, surfactant or alkaline)

- miscible flooding (CO2 , natural gas or N2 )

- thermal recovery (add heat to the reservoir to reduce oil viscosity)

Since the production costs escalate exponentially for each additional percentage of recovery, the oil and gas field must contain a significant remaining quantity of barrels of oil equivalent (boe) to justify the corresponding capital expenditure.

Oil is not found in underground lakes, but in open spaces between grains of rock (pores), oil is held in these spaces much as water is held in a sponge,