The existence of natural fractures in low permeability reservoir has a negative effect on water injection development in oil field. Using area or linear water injection well network, recovery ratio is less than 30%. The recovery ratio of ultra low permeability reservoir is lower, less than 19% [1] . The development of many oilfields abroad and domestic shows that CO₂ flooding can improve more recovery than water flooding [2] . The CO₂ flooding project in the Kelly-Snyder oilfield and the Chao-She oilfield in China is an example. They basically belong to the medium-high permeability reservoir, the CO₂ flooding oil recovery ratio is more than 10%. However, for low permeability reservoir, especially ultra-low permeability reservoir (permeability less than 1 × 10⁻³ um²), there are natural fractures and artificial fractures, reservoir heterogeneity is strong. The reservoir performance in oilfield development is faster than that of water, and the liquid mining index decreases with the increase of water production, and the oil recovery index decreases more quickly [3] . The remaining oil saturation in the plane is banded distribution after water drive, and the low permeability reservoir in the profile is in the unused state. Improving the effect of reservoir development is mainly to improve displacement efficiency and increase sweep volume. CO₂ displacement mechanism shows that CO₂ has a strong permeability, which is easy to enter micro pores, and the effective displacement of crude oil in the formation [4] [5] . The characteristics of interphase mass transfer, oil and gas mixing phase and volume expansion are beneficial to improve oil recovery [6] . The EOR and buried rate of CO₂ flooding after water flooding in low permeability reservoir require both research and mine practice. This paper discusses the effect of CO₂ flooding to improve the recovery ratio of low permeability reservoir and the effect of CO₂ sequestration [7] .

The study area is located in H3 area of Jiyuan oilfield, western Ordos basin, China. The reservoir is a Triassic formation, belonging to an ultra low permeability reservoir [8] . The area is 3.5 km², and has 9 injections for 37 oil wells. The permeability is 0.39 × 10⁻³ μ∙m² .The porosity is 7.1%. The crude oil density of the formation is 0.7 t/m³, the viscosity of crude oil is 0.73 MPa/s, the original gas-oil ratio is 85 m³/t. The formation water is CaCl₂ water type and the content of Ca²⁺ and Mg²⁺ are high, the total mineralization degree is 35.42 g/l. The formation temperature is 85˚C, the original formation pressure is 19.7 MPa, the pressure coefficient is 0.7, which belongs to low pressure reservoir. The initial production capacity of the oil well is 2.6 tons.

Prior to the early adoption of oil production, water injection began to maintain sufficient ground energy. Influenced by natural fractures, the water production of oil well rises rapidly. After the control of water injection, the increase of water volume decreases, but the formation pressure cannot be maintained, the pressure drops to 14.4 MPa, only 66.3% of the original pressure. Oil well productivity is 1.5 tons, water drive recovery rate is only 9.5%.

Low permeability reservoir physical property is poor, CO₂ injection capacity can meet the requirements of complementary formation energy. The reservoir water salinity is high, Including Ca²⁺ and Mg²⁺. The deposition and dissolution of the chemical reaction with carbonate have an effect on the oil displacement effect. The development of reservoir fracture, how to improve the sweep volume of CO₂ flooding in oil reservoir with oil saturation after water drive, need to carry out the research of CO₂ flooding miscible technology [9] [10] . At the same time, the geological safety risk assessment study of CO₂ sequestration and the economic benefit of CO₂ flooding to improve recovery efficiency are also required.

The selection of test area mainly consider the following four factors: reservoir can achieve the conditions of miscible pressure. The reservoir has a certain area, reserves and number of central wells, convenient for comparative analysis. Select the reservoir block close to the gas source to reduce the transportation cost. The reservoir has the sealing property of CO₂.

The H3 area belongs to the structure of inclined to west, and the sand body distribution of reservoir is consistent with the fault. The upper part of the reservoir

is lithology sheltered, and the top is the mudstone with thick thickness and stable distribution. The study of the top surface structure of reservoir (Figure 4) shows that the fault direction is parallel to the extension of the sand body, so the sealing property of the CO₂ storage area is not destroyed. And the thickness of the interlayer is 16.3 m, and the distribution range is wide. Therefore, the CO₂ storage stability of reservoir is better. However, the stability monitoring of CO₂ sequestration still needs to be strengthened.

The CO₂ flooding field test shows that it is easier to inject liquid CO₂ into the ultra-low permeability reservoir than water injection. Whether the dissolution ability after the chemical reaction of carbonate is greater than the deposition, further study is needed.

The test area can reach miscible conditions, and the oil recovery ratio of the oilfield can be increased by more than 10% by CO₂ flooding.

Reservoir is also a good CO₂ storage area. The CO₂ sequestration rate was 73.2%. Support can be provided for CO₂ abatement governance. But it is also necessary to strengthen the monitoring of formation CO₂ stability.

Cheng, Q.G., Li, Z.X., Zhu, G.S. and Zhang, H.T. (2017) Research and Application of CO₂ Flooding Enhanced Oil Recovery in Low Permeability Oilfield. Open Journal of Geology, 7, 1435-1440. https://doi.org/10.4236/ojg.2017.79095