Enhanced oil recovery technologies

Enhanced oil recovery (EOR), also called tertiary recovery, is the extraction of crude oil from an oil field that cannot be extracted otherwise. Although the primary and secondary recovery techniques rely on the pressure differential between the surface and the underground well, enhanced oil recovery functions by altering the chemical composition of the oil itself in order to make it easier to extract. EOR can extract 30% to 60% or more of a reservoir's oil, compared to 20% to 40% using primary and secondary recovery. Carbon dioxide and water are injected along with one of three EOR techniques: thermal injection, gas injection, and chemical injection.

There are three primary techniques of EOR: gas injection, thermal injection, and 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.

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. Nanofluid is the basic fluid that disperses nanoparticles into traditional heat transfer media such as water, alcohol and oil. Nanoparticles are small and light enough to remain suspended. As a new oil displacement agent, nanofluids have been reported in recent years to enhance oil recovery in low permeability reservoirs.

However, nanoparticles are greatly affected by salinity, so it is necessary to modify their surface to improve their stability in solution. Modified nanoparticles have nano size effect and excellent surface and interface properties, and have higher application value in the field of EOR.

Nowadays, the world's demand for oil is increasing year by year, which makes the exploitation of oil become a major concern of society. The reserves of low permeability reservoirs are huge, so the EOR technologies in low permeability reservoirs have a broad application and development prospect.

EOR technologies in low permeability reservoirs are a very difficult and complex large-scale system engineering, which needs the integration of multiple disciplines. In the past few decades, EOR technologies for low permeability reservoirs has been extensively and deeply explored. However, various technologies still need to be further improved and optimized. On the basis of deepening the understanding of oil displacement mechanism, innovating and developing EOR theory, we need to develop cost-effective and environmentally friendly green oil displacement agents. Different EOR technologies can be comprehensively utilized and synergistic effects among different technologies can be exerted to form a series of low-cost, efficient and environmentally friendly EOR technologies suitable for different reservoirs of low permeability reservoirs, so as to promote the development of the world petroleum industry and disciplines.