Enhanced Oil Recovery Methods

Thermal EOR

Injection of steam has historically been the most widely applied EOR method. Heat from steam or hot water dramatically reduces the viscosity of viscous oils, making it flow more readily. There are many variations for this process including Cyclic Steam Injection ("huff 'n puff", where steam is first injected, followed by oil production from the same well), Continuous Steam Injection (where steam injected into wells drives oil to separate production wells), hot water injection, and Steam Assisted Gravity Drainage (SAGD) using horizontal wells, among others. Another set of thermal methods, in situ combustion or "fire flooding", involves injection of air or oxygen. In this process, oxidation of some of the oil in place does the following:

• Produces heat that reduces viscosity for the remaining oil
• Cracks some high-molecular weight hydrocarbons into smaller molecules
• Vaporizes some of the lighter hydrocarbons to help miscible displace oil
• Creates steam that may steam-distill trapped oil

Miscible EOR

Commonly applied, this method usually employs supercritical CO2 to displace oil from a depleted oil reservoir with suitable characteristics (typically containing "light" oils). Through changes in pressure and temperature, carbon dioxide can form a gas, liquid, solid, or supercritical fluid. When at or above the critical point of pressure and temperature, supercritical CO2 can maintain the properties of a gas while having the density of a liquid. Injected miscible CO2 will mix thoroughly with the oil within the reservoir such that the interfacial tension between these two substances effectively disappears. CO2 can also improve oil recovery by dissolving in, swelling, and reducing the viscosity of oil.

In deep, high-pressure reservoirs, compressed nitrogen has been used instead of CO2.Hydrocarbon gases have also been used for miscible oil displacement in some large reservoirs.

CO2, nitrogen, hydrocarbon gases, and flue gases have also been injected to immiscibly displace oil. At one extreme of conditions, these displacements may simply amount to "pressure maintenance" in the reservoir (a secondary recovery process). Depending on oil character, gas composition and pressure, and temperature, the displacements could have a range of efficiencies up to and approaching a miscible displacement. CO2 has also been injected in a "huff 'n puff" or cyclic injection mode, like cyclic steam injection.

Chemical EOR

Three chemical flooding processes include polymer flooding, surfactant-polymer flooding, and alkaline-surfactant-polymer (ASP) flooding. In the polymer flooding method, water-soluble polymers increase the viscosity of the injected water, leading to a more efficient displacement of moderately viscous oils. Addition of surfactant to the polymer formulation may, under very specific circumstances, reduce oil-water interfacial tension to almost zero-displacing trapped residual oil. Although no large-scale surfactant-polymer floods have been implemented, the process has considerable potential to recover oil.

A variation of this process involves addition of alkaline to the surfactant-polymer formulation. For some oils, alkaline may convert some acids within the oil to surfactants that aid oil recovery. The alkaline may also play a beneficial role in reducing surfactant retention in the rock. For all chemical flooding processes, inclusion of a viscosifier (usually a water-soluble polymer) is required to provide an efficient sweep of the expensive chemicals through the reservoir. Gels are also often used to strategically plug fractures (or other extremely permeable channels) before injecting the relatively expensive chemical solutions, miscible gases, or steam.

Other EOR Processes

Over the years, a number of other innovative EOR processes have been conceived, including injection of carbonated water, microorganisms, foams, alkaline (without surfactant), and other formulations. These methods have shown varying degrees of promise, but require additional development before such applications will become common.