Greenhouse gas CO2 makes the global warming and constitutes a serious threat survival to mankind as well as the development of social and economic. The most effective way to dispose the CO2 is applied in the oil and gas fields, which can not only seal the CO2, but also improve the oil and gas recovery. The integration of CO2 capture-storage-application technology will be a new trend for the development of the future. A lot of field tests have been carried out on a large scale CO2 flooding in foreign countries and good results have been obtained. It is proved that CO2 flooding is one of the most promising methods to improve oil recovery in the three oil recovery.
2. Injection CO2 oil displacement mechanism
2.1 Reduced the oil-water interfacial tension
The residual oil saturation decreases with the decrease of oil water interfacial tension. CO2 is easily dissolved in crude oil, and its solubility in oil is 3~9 times larger than that in water. In the oil displacement process, a large number of CO2 mixed with light hydrocarbon, which can greatly reduce the oil-water interfacial tension, reduce the residual oil saturation, so as to improve the oil recovery.
2.2 Reduced the viscosity of crude oil
CO2 has a good mutual solubility with crude oil, it can significantly reduce the viscosity of crude oil, as well as reduced to the original viscosity of 1/10. The higher the initial viscosity of crude oil, the lower the viscosity difference is. After the viscosity is decreased, the flow capacity of crude oil is increased as well as the output of crude oil .
2.3 Expansion effect
The results of laboratory experiments in Bradford oilfield and Manorville oilfield showed that the volume expansion of crude oil can be made by injecting CO2 into the crude oil under certain pressure conditions. The volume of crude oil is increased after the injection of CO2, which not only increases the internal energy of the crude oil, but also reduces the capillary resistance and the flow resistance in the crude oil flow process, thus enhancing the flow capacity of the crude oil.
2.4 Dissolved gas driving
Due to the dissolution of CO2 injection in crude oil, there formed the CO2 dissolved gas, with the increasing of temperature in the underground, the part of free CO2 gasification, to store the energy in the form of pressure energy. When the well production begins and the reservoir pressure decreases, a large amount of CO2 from crude oil free, expansion and emerge, thus the oil flooding into the wellbore to dissolved gas flooding effect. In addition, some CO2 drive for crude oil, occupy a certain pore space to become the bound gas, which can also make crude oil production increased, and because of the gas with a higher migration rate, it also can be blocked by the oil layer.
2.5 Improve the mobility ratio
Due to inject a large amount of CO2 dissolved in the crude oil and water, carbon formation water acidification, oil mobility is increased, and the water intensity decreased, as a result of oil and water mobility tends to be close to, so that the water oil displacement capability, also further expand the spread of water flooding the area, greatly improve the sweep efficiency.
2.6 Acidification and plugging removal
CO2 dissolved in water, slightly acidic, and react with the formation, dissolved a part of the magazine by the acid solution, and improved the oil layer permeability. Under a certain pressure, a part of free gas on the formation of the blockage has a strong erosion effect, which can effectively clear the two pollution caused by the formation of blockage, reply to a single well productivity.
2.7 Extraction and vaporization of light hydrocarbons in crude oils
CO2 huff and puff soaking period, CO2 can group light extraction and vaporization of crude oil, formation of CO2 rich gas phase, thereby reducing usu.in two lines into the interfacial tension between crude oil and gas, reducing the crude oil flow resistance, so that the oil is easy to flow, improve the sweep efficiency.
2.8 Mixed phase effect
Miscible effect is that the two kinds of fluid can be dissolved and there is no interface, eliminated the interfacial tension. CO2 is not only able to extract light hydrocarbon from crude oil, but also can form CO2 and hydrocarbon mixture. Oil belt movement is the most effective oil displacement process, it can make the recovery rate of more than 90%.
3 Problems for CO2 flooding in China
3.1 High mixed phase pressure
The minimum miscible pressure of CO2 and crude oil not only depends on the purity of CO2 and the temperature of the reservoir, but also depends on the composition of the crude oil. The higher the content of heavy components in the crude oil, the higher the pressure of the minimum miscible pressure is. The prominent characteristics of reservoirs in China Central Plains oil is "three high" that the high viscosity, wax and high colloid content, high solidification point, which determines the most of the reservoirs in the crude oil and CO2 minimum miscible pressure is too high.
3.2 Corrosion and scaling
Due to lower pressure and temperature, CO2 injection will lead to scaling (mainly carbonate scale); in addition, CO2 and water reaction to the formation of carbonic acid on the pipeline, equipment, wellbore has a greater corrosive, corrosion products are injected into the formation of fluid into the reservoir porosity.
3.3 Gas source
Using CO2 injection to improve oil recovery, we must have sufficient gas source. There are two kinds of gas source, one is the natural CO2 gas source, the other is the industrial waste gas. It is urgent for our country to use CO2 oil recovery to find large gas source. In addition, with the development of CO2 emission reduction research in the world, more and more industrial waste gas will be used to improve the oil recovery.
3.4 Serious channeling
In the process of CO2 oil production, the channeling of CO2 in oil reservoir will seriously affect the efficiency, which leads to the main mechanism of CO2 channeling: one is the viscous finger, two is the heterogeneity of the reservoir and the flow channel. Most of the reservoirs in our country are continental deposits, and the heterogeneity is serious. In addition, many reservoirs have more natural fractures, which are connected with natural fractures.
3.5 Solid phase deposition
CO2 on the bottom of the light hydrocarbon has a strong drawer effect, after repeated CO2 extraction, reduced the bottom oil on the paraffin wax dissolution ability and stability, resulting in paraffin precipitation. In addition, CO2 multiple extraction, so that the formation of low carbon number of paraffin wax component gradually reduced, resulting in a substantial increase in the temperature of the underlying oil wax deposition, even in the formation of paraffin deposits, causing damage to the reservoir.
4. Conclusion and outlook
A. CO2 injection enhanced oil recovery mechanism includes: reduce interfacial tension between oil and water, reduce crude oil viscosity, expansion, dissolved gas drive effect. Improve the mobility ratio, acidizing, light hydrocarbon extraction and vaporization of crude oil, mixed phase effect.
B. CO2 application of enhanced oil recovery by miscible flooding and non miscible flooding in 2 ways, the injection method of water & air injection including air injection and gravity stability.
C. CO2 miscible flooding can be implemented on a small scale, it has a broad application prospects; CO2 non miscible flooding applied for the entire field, compared with miscible flooding, it can bury a large number of CO2.
D. Enhanced oil recovery in our country is confronted with high pressure of mixed phase, corrosion and scaling, gas source and CO2. channeling serious, solid phase deposition and other issues.
E. With the growing concern about environmental protection, the capture - storage - application Integration to enhance the recovery technology will be a new direction of development in the future.
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