The main classes of EOR technologies are: • CO2 EOR: CO2 is injected into the subsurface. • Other gas injection EOR: similar to CO2-EOR, but with other gases injected such as natural gas or nitrogen. Gases used include CO2, natural gas or nitrogen. The fluid most commonly used for miscible displacement is carbon dioxide because it reduces the oil
viscosity and is less expensive than
liquefied petroleum gas. The process was first commercially attempted in 1977 in
Scurry County,
Texas. Since then, the process has become extensively used in the
Permian basin region of the US and is now more recently is being pursued in many different states. It is now being more actively pursued in China and throughout the rest of the world. Most CO2 injected in CO2-EOR projects comes from naturally occurring underground CO2 deposits. Some CO2 used in EOR is captured from industrial facilities such as
natural gas processing plants, using
carbon capture technology. In high pressure applications with lighter oils, CO2 is miscible with the oil, with resultant swelling of the oil, and reduction in viscosity, and possibly also with a reduction in the surface tension with the reservoir rock. In the case of low pressure reservoirs or heavy oils, CO2 will form an immiscible fluid, or will only partially mix with the oil. Some oil swelling may occur, and oil viscosity can still be significantly reduced. In these applications, between one-half and two-thirds of the injected CO2 returns with the produced oil and is usually re-injected into the reservoir to minimize operating costs. The remainder is trapped in the oil reservoir by various means. Carbon dioxide as a solvent has the benefit of being more economical than other similarly miscible fluids such as
propane and
butane.
Water-alternating-gas (WAG) Water-alternating-gas (WAG) injection is another technique employed in EOR. Water is used in addition to carbon dioxide. A saline solution is used here so that carbonate formations in oil wells are not disturbed. Water and carbon dioxide are injected into the
oil well for larger recovery, as they typically have low miscibility with oil. Use of both water and carbon dioxide also lowers the mobility of carbon dioxide, causing the gas to displace more oil. According to a study done by Kovscek, using small slugs of both carbon dioxide and water allows for quick recovery of the oil.
Thermal injection In this approach, various methods are used to heat the crude oil in the formation to reduce its viscosity and/or vaporize part of the oil and thus decrease the mobility ratio. The increased heat reduces the surface tension and increases the permeability of the oil. The heated oil may also vaporize and then condense forming improved oil. Methods include
cyclic steam injection, steam flooding and combustion. These methods improve the sweep efficiency and the displacement efficiency. Steam injection has been used commercially since the 1960s in California fields. In
solar thermal enhanced oil recovery, a solar array is used to produce the steam.
Steam flooding Steam flooding (see sketch) is one means of introducing heat to the reservoir by pumping steam into the well with a pattern similar to that of water injection. Eventually the steam condenses to hot water; in the steam zone the oil evaporates, and in the hot water zone the oil expands. As a result, the oil expands, the viscosity drops, and the permeability increases. To ensure success the process has to be cyclical. This is the principal enhanced oil recovery program in use today.
Fire flooding Fire flooding works best when the oil saturation and porosity are high. Combustion generates the heat within the reservoir itself. Continuous injection of air or other gas mixture with high oxygen content will maintain the flame front. As the fire burns, it moves through the reservoir toward production wells. Heat from the fire reduces oil viscosity and helps vaporize reservoir water to steam. The steam, hot water, combustion gas and a bank of distilled solvent all act to drive oil in front of the fire toward production wells. There are three methods of combustion: Dry forward, reverse and wet combustion. Dry forward uses an igniter to set fire to the oil. As the fire progresses the oil is pushed away from the fire toward the producing well. In reverse the air injection and the ignition occur from opposite directions. In wet combustion water is injected just behind the front and turned into steam by the hot rock. This quenches the fire and spreads the heat more evenly.
Chemical injection The injection of various chemicals, usually as dilute solutions, have been used to aid mobility and the reduction in
surface tension. Injection of
alkaline or
caustic solutions into reservoirs with oil that have
organic acids naturally occurring in the oil will result in the production of
soap that may lower the
interfacial tension enough to increase production. Injection of a dilute solution of a
water-soluble polymer to increase the viscosity of the injected water can increase the amount of oil recovered in some formations. Dilute solutions of
surfactants such as petroleum
sulfonates or
biosurfactants such as
rhamnolipids may be injected to lower the
interfacial tension or
capillary pressure that impedes oil droplets from moving through a reservoir, this is analyzed in terms of the
bond number, relating capillary forces to gravitational ones. Special formulations of oil, water and surfactant,
microemulsions, can be particularly effective in reducing interfacial tension. Application of these methods is usually limited by the cost of the chemicals and their
adsorption and loss onto the rock of the oil containing formation. In all of these methods the chemicals are injected into several wells and the production occurs in other nearby wells.
Polymer flooding Polymer flooding consists in mixing long chain polymer molecules with the injected water in order to increase the water viscosity. This method improves the vertical and areal sweep efficiency as a consequence of improving the water/oil mobility ratio. Surfactants may be used in conjunction with polymers and
hyperbranched polyglycerols; they decrease the interfacial tension between the oil and water. This reduces the residual oil saturation and improves the macroscopic efficiency of the process. Primary surfactants usually have co-surfactants, activity boosters, and co-solvents added to them to improve stability of the formulation. Caustic flooding is the addition of
sodium hydroxide to injection water. This lowers the surface tension, reverses the rock's wettability,
emulsifies and mobilizes the oil, and helps in drawing the oil out of the rock.
Low salinity nanofluids EOR processes can be enhanced with
nanoparticles in three ways: nanocatalysts,
nanofluids, and nanoemulsions. Nanofluids are base fluids that contain nanoparticles in colloidal suspensions. Nanofluids perform many functions in EOR of oil fields, including pore
disjoining pressure, channel plugging, interfacial tension reduction, mobility ratio, wettability alteration, and
asphaltene precipitation prevention. Nanofluids facilitates disjoining pressure to remove sediment entrapped oil via aggregation at the interface. Alternatively, wettability alteration and interfacial surface tension reduction are other alternative mechanism of EOR.
Other EOR methods Microbial injection Microbial injection is part of
microbial enhanced oil recovery and is rarely used because of its higher cost and because the
development is not widely accepted. These
microbes function either by partially digesting long
hydrocarbon molecules, by generating
biosurfactants, or by emitting carbon dioxide (which then functions as described in
Gas injection above). Three approaches have been used to achieve microbial injection. In the first approach, bacterial cultures mixed with a food source (a carbohydrate such as
molasses is commonly used) are injected into the oil field. In the second approach, used since 1985, nutrients are injected into the ground to nurture existing microbial bodies; these nutrients cause the bacteria to increase production of the natural surfactants they normally use to metabolize crude oil underground. After the injected nutrients are consumed, the microbes go into near-shutdown mode, their exteriors become
hydrophilic, and they migrate to the oil-water interface area, where they cause oil droplets to form from the larger oil mass, making the droplets more likely to migrate to the wellhead. This approach has been used in oilfields near the
Four Corners and in the
Beverly Hills Oil Field in
Beverly Hills, California. The third approach is used to address the problem of
paraffin wax components of the crude oil, which tend to precipitate as the crude flows to the surface, since the Earth's surface is considerably cooler than the petroleum deposits (a temperature drop of 9–10–14 °C per thousand feet of depth is usual).
Plasma-pulse In 2013, a technique called
plasma-pulse technology was introduced into the United States from Russia. This technique can result in another 50 percent of improvement in existing well production. ==Economic costs and benefits==