(Extract) Currently, the world has formed four tertiary oil recovery technology series, namely chemical flooding, gas flooding, heat flooding and microbial flooding. The rich resources of heavy oil determines that steam flooding is still the main method of the future tertiary oil recovery. The use of polymer is sharply declining, but has some increase in recent years with the rise in oil prices. Because of its environmental protection characteristics and cost advantages, flooding of injection of carbon dioxide has huge space for development. Each of oil recovery technology has its limitations, so combining the advantages of different technologies, to develop highly effective technology is a new direction for enhancing oil recovery.

With the deepening of exploration and development, the difficulty of crude oil extraction is gradually increasing, therefore enhancing oil recovery is a concern not only for the oil industry, but also for the whole industrial community. Tertiary oil recovery technology is a high-tech developed in China over the past decade, and its application has taken important role in enhancing oil recovery and stabilizing the production of old oilfield.

1. Concept

Tertiary oil recovery refers that the physical-chemical methods, after primary production (depending on the original reservoir energy) and secondary oil recovery (injecting water to replenish their energy), are used to change the nature and phase of the fluid, to change the interface interaction between gas-liquid, liquid-liquid, liquid-solid, so to expand the geographic scope of water injection to improve the flood efficiency, thereby once again to significantly enhance oil recovery.

2. Mechanism

For dilute oil reservoirs, the tertiary oil recovery generally refers to as chemical flooding, including polymer flooding, surfactant flooding, alkaline flooding and the compound flood of them in any portions. Polymer flooding is a method that by adding water-soluble polymer to increase the viscosity of the injected water, thus to improve mobility ratio, and to increase the swept volume injected to expand the ultimate oil recovery. Surfactant flooding is to reduce the oil-water interfacial tension to a ultra-low level (less than 10 ~ 2 mNrm), so to drive the flow of residual oil. For the reservors containing organic acid, alkaline aqueous solution at a concentration of from 0.05% to 4% of NaOH, Na2C03 or Na4Si04 can be injected to react with these organic acids in the oil to generate a surface-active agent, and this is known as alkaline flooding. Pure alkaline water flooding is to extract the residue oil by mechanisms of wettability flip, emulsifying trapping, emulsion entrainment, spontaneous emulsification and coalescence and dissolution of hard films generated by reducing interfacial tension between oil and water. Compound flooding is to add polymers to the aqueous alkaline or surfactant, to improve the viscosity of the aqueous solution, to improve the unfavorable mobility ratio, to increase the contact of the solution with the crude oil, so to improve sweep efficiency and improved driving oil efficiency. Various methods applicable standards in Table 1.

3. Development of Tertiary Oil Recovery

Our country highly stresses the development of Tertiary Oil Recovery technology. Using Tertiary Oil Recovery technology to enhance oil recovery is for the strategic needs of reducing the speed of the decline of most oilfields, maintaining stable production of crude oil. Our country not only pays attention to indoor study, also arranged a number of field trials, making part of our technologies reach the world advanced level.

Currently, the world has formed four tertiary oil recovery technology series, namely chemical flooding, gas flooding, heat flooding and microbial flooding. Chemical flooding includes polymer flooding, surfactant flooding, alkaline flooding and compound binary, ternary flooding, foam flooding, etc. Gas flooding includes carbon dioxide miscible/non-miscible flooding, nitrogen flooding, hydrocarbons and flue gas flooding, etc;  heat flooding includes steam stimulation, hot water flooding, steam flooding and combustion oil reservor; microbial flooding includes microorganisms or microbial enhanced oil adjustment. The four technologies have formed some industrial applications, and some are ongoing pilot field test, as well as in the theoretical study.

3.1 Chemical Flooding

During the 20 years after the United States reached a peak by application of chemical flooding in the 1980s, chemical flooding was used less and less in the United States, but in China it has been successfully applied. China's chemical flooding technology is representing the world's advanced level, and the polymer flooding technology formed industrial application in 1996. During "the fifteenth five” period, Daqing Oilfield formed the binary flooding technology of alkali + polymer + surfactant with alkyl benzene sulfonate as the main agent. Shengli Oilfield formed an alkali-free binary flooding technology of polymer + surfactant, now is carrying out laboratory study and field trials of compound floodings of alkali + polymer + surfactant + natural gas foam.

Chemical flooding currently has three different research directions. First, to improve the mobility ratio of the oil-water, in addition to reduce the viscosity of crude oil, the appropriate approach is to increase the viscosity of oil flooding agent, reducing its mobility ratio, and by the application of this principle, methods as polymer solution, foam fluid flooding etc. Secondly, to improve the washing ability of oil flooding agent and the adverse wettability of rocks, active water flooding method was developed. Third is the method between the first two, known as alkaline water flooding, which is to generate active water by using alkali water and crude oil components, to improve wettability or emulsify crude oil

3.2   Heat Flooding

From 1980s to 1990s, the development of thermal drive technology in China underwent the three phases of steam stimulation tests, the promotion of steam stimulation and pilot test of steam flooding, and steam stimulation and steam flooding industrial applications. Steam stimulation and steam flooding have become the main method for heavy oil, and the country's major heavy oil production mainly come from Liaohe, Xinjiang, victory, Henan, the four oil fields.

In recent years, there is a new thermal recovery method known as geothermal oil exploitation. It is the use of rich geothermal resources, to bring the heat of deep high temperature fluids (oil, gas, water and mixtures) into the shallow reservoirs of, so to reduce oil viscosity and improve oil flow capacity. This method is developed when the efforts of energy conservation were increasing, the conflicts between oil resources supply and demand became outstanding, and the international oil prices continued to rise.

3.3 Gas flood

In other countries, carbon dioxide injection technology is mainly used for late high water cut reservoir, reservoir heterogeneity, and heavy oil reservoirs not suitable for thermal recovery. the main constraints of promotion carbon dioxide flooding are the limited natural resource of carbon dioxide, carbon dioxide transport and oil wells and equipment corrosion, safety and environmental issues etc. To solve the above problems, the injection of carbon dioxide for enhanced oil recovery technology, which is injecting reaction solution into the formation to fully react at reservoir conditions and to release carbon dioxide gas, and carbon dioxide dissolved in the crude oil to reduce crude oil viscosity, so as to enhance oil recovery.

3.4 Microbial flooding

After years of development, microbial removing wax and reducing the viscosity of heavy oil, microbial selective plugging formation, microbial stimulation, microbial strengthen flooding have become mature technologies. Microbial flooding has become a the fourth largest methods of EOR after traditional thermal recovery, chemical flooding, gas flooding. The development of microbial flooding technology has three main directions, first one is microbial efficiency of water flooding, the second is activation of reservoir microbial flooding, the third is microbial profile flooding.

4. Conclusion

It is predicted that in the coming several decades, the oil production of the tertiary oil recovery will have larger and larger portion of the total production. Rich heavy oil resources determines the thermal recovery mainly of steam flooding is still the main method of the future. The use of polymer is sharply declining, but has some increase in recent years with the rise in oil prices. Because of its environmental protection characteristics and cost advantages, flooding of injection of carbon dioxide has huge space for development. Each of oil recovery technology has its limitations, so combining the advantages of different technologies, to develop highly effective technology is a new direction for enhancing oil recovery.

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