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Phases of Oil Reservoir Hydrocarbon Production

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Primary phase production

Oil reservoir hydrocarbon production is the most frequently recognized to occur in various production phases, these phases including primary, secondary, and tertiary hydrocarbon production.

The primary production is the first production of the reservoir wherein the sources of natural energy sources of the reservoir are used to transport hydrocarbons towards and out of producing wells and these are known as drive mechanisms. The basis drive mechanisms acting in oil reservoirs during the primary production phase are rock and fluid expansion, gas cap drive, solution gas drive, gravity drainage, water drive as well as the combination or mixed drive.

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In general, the drive mechanisms are unidentified during the early stage of reservoir production and will be determined using production data such as volumetric oil and gas production analyses, time and reservoir pressure

Secondary Phase of Production

A substance (mostly water or gas) in the secondary production phase is injected into the reservoir in order to enhance and improve the oil recovery if drives of the natural gas are dropped to a point at which they are no longer effective as a stress causing movement of hydrocarbons to the producing wells. In the case of water injection, water is injected into the aquifer in order to maintain reservoir pressure or into the oil zone which means water flooding in order to displace oil toward production wells. The processes of water flooding are often efficient specifically in light to moderate oil reservoirs and able to produce considerable volumes of oil, even in some cases greater than that which was produced during the primary phase of production. The two essential factors that affecting the sweep efficiency of water flooding processes are oil-water relative permeability and reservoir rock wettability.

Generally, around 50–70% of oil reserve remains in the reservoir in most reservoirs after the water flooding process since it was bypassed by the water that disables to mix with the oil. Also, in the second phase production, gas might also be injected into the reservoir in addition to water flooding. In such scenarios, gas is injected into reservoirs that commonly have large gas caps in order to maintain reservoir pressure. In secondary phases of production, fluid and rock properties of the reservoir are almost unchanged and there is no interaction between the displacing and displaced fluids in the reservoir or phase behavior reaction.

Tertiary Phase of Production

The tertiary phase of production in known as Enhanced Oil Recovery (EOR), during the tertiary phase of production, the oil recovered by both primary and secondary phases of production and this recoverable oil could be between 30–50% of overall reserve and that is depending on the fluid and reservoir properties, and large volumes of reservoir oil remain untouched in the pore spaces of reservoir rock. Enhanced Oil Recovery (EOR) is mainly from the gases injection and/or liquid chemicals, in addition to the use of thermal energy.

The injected fluid interacted with the reservoir rock and oil system which create favorable conditions for oil recovery. For instance, as a result in lower interfacial tension, oil swelling, hydrocarbon extraction, oil viscosity reduction, wettability modification. Generally, Enhanced Oil Recovery (EOR) processes can be classified into four categories of chemical, thermal, miscible gases.

Chemical processes in EOR are characterized by addition of chemicals to water in order to improve the mobility. Thermal processes provide a driving force and add heat energy to the reservoir to reduce the viscosity of heavy oil and vaporize the lighter oil, leading to the improvement of their mobility. Also, the thermal methods include hot water injection, steam injection, cyclic steam injection, etc.

In miscible gas injections which are CO2, N2, and hydrocarbon gases are injected into the reservoir at the miscible condition and it is known as miscible gas injection if the gas is injected into the reservoir at pressures greater than the minimum miscibility pressure between the oil and injected gas; otherwise, the process is immiscible gas injection.

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