There are broad prospects of CO₂ flooding for enhancing oil recovery and greenhouse gas storage. In this paper, we reviewed the development history and brief situation of CO₂ flooding at home and abroad, analyzed the development status of CO₂ immiscible flooding and CO₂ huff and puff, and summarized the phase state of CO₂ flooding, oil displacement mechanism evaluation and optimization design technology of CO₂ flooding reservoir engineering. The design focused on improving oil displacement efficiency and sweep efficiency, controlling viscosity index and gas breakthrough, achieving miscible or near-miscible flooding, and optimizing well pattern and injection parameters in combination with reservoir characteristics. It was pointed out that CO₂ near-miscible flooding and increasing sweep volume were the development trends of CO₂ flooding. WAG, foam flooding, fracture sealing and local gravity flooding were important means to prevent gas channeling. On the basis of summarizing the current CO₂ flooding technology and field experience at home and abroad, the top-level design of combination of CO₂ flooding and CO₂ geological storage should be done according to different types of reservoir characteristics.

A CO₂ flooding pilot test was carried out on the low permeability fractured reservoirs in Huang-3 block. The research results indicated that there were obvious oil-increasing effects in the overall area, but the gas channeling phenomenon was serious in some wells, resulting in a poor effect of CO₂ flooding. In order to improve the CO₂ flooding efficiency of low permeability fractured reservoirs, the dominant flow channels were identified by tracer monitoring combined with production dynamic analysis and numerical simulation of production history fitting. On this basis, a numerical simulation model of the well group considering the dominant channels was obtained. The effects of the dominant channels on the CO₂ displacement were studied by the numerical simulation technology of the component reservoirs, and four measures to improve the oil displacement efficiency were simulated. The results showed that the dominant channels reduced the CO₂ flooding oil recovery by 7 %. The best effect was obtained by blocking the dominant channels with high-intensity plugging measures, and the RF had increased by 1.8 %. The second best effect was when the dominant channels wells were shut down in the early stage of the gas channeling. Then followed when the water injection maintained the formation pressure. Shut in of the well was relatively poor when the GOR of production well reached 2 000 m ³/m ³, and the RF only increased by 0.11 %. The research results have guiding significance for the design of gas injection drive scheme for low permeability fractured reservoirs.

The produced GOR is a key index for the development and design of gas flooding reservoirs. On the basis of confirming that there are five sources of gas production in the whole life cycle of gas flooding, the prediction formula of produced GOR in low permeability reservoirs matching with three development stages is proposed. From gas injection to pre-gas production, the produced GOR is mainly controlled by the initial dissolved GOR of the reservoir fluid, and method of determining the equivalent GOR of water-soluble gas is given. The produced GOR in the stage from pre-gas production to gas channeling is affected by the gas dissolved capacity of ″oil bank″. The method of describing the physical property of gas flooding″oil bank″ in low permeability reservoirs can be used for reference to determine dissolved GOR of gas flooding″oil bank″. The change of produced GOR after gas channeling is determined by the GOR formed by free gas, and its prediction based on the understanding that the pore space released by the produced liquids would be filled with the injected gas and water. It is calculated by oil and gas fractional flow equation, Corey's model and Stone's equation combined with concepts of gas flooding increasing multiple in low permeability reservoir, and slug size ratio of water-gas during water alternating gas injection. The reservoir engineering method of produced GOR prediction taking the form of ″three-stages″ has been verified by the field test of CO₂ miscible flooding. It can be used in gas injection development planning design, optimization of oil production process or gas injection storage potential evaluation.

CO₂ huff and puff in horizontal wells, especially in small fault block reservoirs with imperfect well pattern and thin inter bed oil reservoirs, is an effective technical supplement for CO₂ flooding. For high water-cut horizontal wells, as there are three limitations of the ″elliptical cylinder model method″—a fast calculation method for the design of CO₂ injection volume in horizontal wells, a new CO₂ injection volume calculation formula based on the improved cylinder model is proposed. Compared with the optimized results of numerical simulation method, the calculated error rate of the new formula is less than 10 % so as to meet the needs of industrial design. Besides, compared with the old model, the calculation accuracy is enhanced considerably. The new formula is used to design the CO₂ huff and puff scheme of remaining oil development of well S49P1, a typical high water-cut horizontal well in J oilfield. And a good incremental oil effect and economic benefit was achieved, which verifies the adaptability and reliability of the improved model. The establishment of the new model can provide a more accurate engineering calculation tool of CO₂ injection volume for CO₂ huff and puff in other horizontal wells with high water-cut.

The huff and puff operation by CO₂ extraction can overcome the production and development problems of tight reservoirs which are difficult to form effective displacement. Crude oil fractions were divided by PVTi module, and the pseudo-component properties of formation crude oil could be obtained by combining with the expansion experimental data of CO₂ injection. Relevant factors affecting the extraction of tight oil by CO₂ were studied by CFD simulation. The results showed that supercritical CO₂ mainly extracted the light components of crude oil in porous media, especially those below C₁₅₊. The higher injection pressure was, and the more CO₂ was injected, the stronger the solubility of crude oil would be, and it could enhance the extraction of heavy components. Meanwhile, the increase of CO₂ injection could also promote the retention of CO₂ in porous media, which was conducive to CO₂ storage. With the increase of porosity, the extraction effect of CO₂ increased first and then decreased, and was still 44 % under the condition of lower porosity. The extraction of tight oil by CO₂ has a certain application prospect.

The minimum miscibility pressure（MMP） of CO₂ flooding is affected by crude oil components and reservoir temperature. And its influence rule has great significance to screen and evaluate gas injection reservoirs. The tie-line analysis is a method to calculate MMP analytical approach which is faster, more accurate, and easier to operate. In this paper, a calculation process of simulating CO₂ flooding by tie-line analysis was proposed, and the influence law of components（C₄, C₁₀, C₁₄, C₂₀） and temperature on MMP was studied. The results showed that the simulation results of the tie-line analysis were similar to those of the slim tube numerical simulation. The MMP increased with the increase of temperature, and the sensitivity of MMP for each component was more obvious in higher temperature. The MMP decreased with the increase of C₄, C₁₀ and C₁₄ components, but increased with the increase of C₂₀ component. The study proposed a new method with simple process and accurate calculation for calculating MMP and evaluating influencing factors.

In order to study the effect of CO₂ flooding on the physical properties of crude oil, understand the mechanism of CO₂ flooding, and provide theoretical basis for the analysis of field test results of CO₂ flooding, the field test of CO₂ flooding in Huang 3 block of Jiyuan oilfield in Changqing was carried out by column chromatography and gas chromatography-mass spectrometry. The physical properties of crude oil produced before and after CO₂ flooding in Huang 3 block of Jiyuan oilfield, Changqing, were studied in detail by means of column chromatography, gas chromatography and mass spectrometry method and other methods. The changes of crude oil group content, different carbon chain alkanes content, solidification point and viscosity-temperature curve were mainly investigated. The experimental results show that after CO₂ injection, the alkane fraction of crude oil first decreases and then increases, while the non-hydrocarbon and asphaltene fraction first increases and then decreases; the light alkane（C₈—C₁₆） in the alkane fraction of crude oil first decreases and then increases, the heavy alkane（C₁₇—C₄₀） content first increases and then decreases; the pour point of crude oil first increases and then decreases, and the viscosity of crude oil at low temperature（35 ℃） increased slightly, but there was no obvious change at high temperature. The above results accord with the mechanism of CO₂ miscible flooding. The change of crude oil physical properties is the result of the interaction of CO₂ extraction mechanism. Through the above research, it provides data support for understanding the mechanism of CO₂ flooding, and provides a certain basis for further formulating plans to improve the efficiency of CO₂ flooding.

To solve the problems of difficult miscibility and low sweep coefficient in CO₂ flooding of ultra-low permeability beach-bar sand reservoir, theories and methods such as geology, seepage mechanics and reservoir engineering and by the combined methods of physical modeling and mathematical simulation are used to formulate the evaluation standard of CO₂ flooding, clarify the EOR mechanism of CO₂ flooding, form the optimum design technique adapted for the well pattern of CO₂ flooding, and optimize the policy limitation of CO₂ flooding in the demonstration zone Gao 89-1 block. It is indicated that the technology has been applied effectively. Up to now, over 2.8×10 ⁵ t of CO₂ have successfully been injected into the reservoir, and the cumulative oil increment is 6.5×10 ⁴ t, and 2.6×10 ⁵ t of CO₂ have been stored in CO₂ flooding and storage demonstration zone.

The CO₂ miscible flooding project in the Taizhou formation of Caoshe oilfield is one of the most successful CO₂ EOR projects in China. The feasibility study on the secondary miscible flooding was carried out to settle the issues of complex remaining oil distribution and severe gas channeling in the later stage after first miscible flooding. The characteristic of microscopic fractal dimension in the CO₂ miscible flooding process of real core sample was observed through metalloscope. The remaining oil types attaching to the pore surface of rock took on the forms of thin film and isolated beads. The displacement efficiency of the secondary CO₂ miscible flooding was 93.4 %, which was 9.73 % higher than that of the first CO₂ miscible flooding. The average residual oil saturation was 30.8 % in sealed coring test, indicating great potential of III formation. A feasible development plan for secondary miscible-phases flooding was proposed to develop the stratified reservoirs at the bottom position. The CO₂ injection on the higher position was adopt to suppress gas gravity overlap, and the water alternating gas was applied to slow down the gas channeling. Furthermore, the injection profile was adjusted during the whole process of gas injection. The incremental recovery ratio was estimated to be 8 % ~ 10 %.

At present, the water cut of fault block 14-1 reservoir in Banqiao Oilfield is 98.1 % and the recovery degree is 10.93 %. Conventional water control and oil stabilization technology can not solve the problems of high water cut, low productivity and poor economic benefit. Compared with conventional water control and oil enrichment technology, CO₂ huff and puff cone has the advantages of simple technology, short construction period and high success rate. The mechanism of precipitation and oil increase by CO₂ huff and puff is studied by numerical simulation technology. On this basis, the technological parameters of CO₂ huff and puff are optimized, and the optimal technological scheme of CO₂ huff and puff for horizontal wells in reservoirs is determined. On the basis of scheme optimization, CO₂ huff and puff scheme was implemented on site. The results show that CO₂ huff and puff achieves water control and oil increase in horizontal wells at extra high water cut period by extracting the intermediate components of crude oil through gas drive pressure cone and reducing the viscosity mechanism of crude oil. By comparing the cumulative oil increase and oil exchange rate of single well under different process parameters, the optimal injection rate of CO₂ is 600 t, injection rate is 4.0 t/h, soaking time is 17 d, fluid production is 16 m ³/d, and the number of huff and puff cycles is two. Field pilot test of well 45H shows that the fluid production of well 45H is 12 m ³/d, the water cut is reduced from 98.1 % to 62.5 %, and the oil production is increased from 1.6 m ³/d to 4.73 m ³/d. CO₂ huff and puff can control bottom water coning, reduce water cut and restore productivity effectively. The results show that horizontal well CO₂ huff and puff has good adaptability and application prospect for similar reservoirs in Banqiao Oilfield.

Low permeability reservoirs are widely distributed in Subei（northern Jiangsu province） Basin. There are some problems such as insufficient natural energy and fragmented structures, which make it difficult to form an effective injection-production well pattern. Therefore, the effect of water injection is poor. The CO₂ huff and puff technology can effectively solve these problems with less investment, simple process and rapid results. Choosing the proper wells to execute is the key to the success of this technology. Based on the comparative analysis of CO₂ huff and puff in low permeability reservoirs in Subei Basin in recent years, combined with indoor physical simulation experiments and numerical simulation results, it is considered that closed reservoirs with medium-deep burial depth and fracture development are more suitable for CO₂ huff and puff. Fractures have a greater impact on huff and puff effect. There is an appropriate interval for formation pressure, not the bigger the better, whether miscibility can be achieved or not. It has no decisive effect on the throughput effect. Oil wells with high initial production and water cut of 30 %~70 % in lower structure are more conducive to huff and puff, and the huff and puff effect of horizontal wells is better than that of vertical wells.

CO₂ injection is a commonly used technique for recovering oil from mature reservoirs as it can not only greatly improve oil recovery, but also achieve the goal of greenhouse gas emission reduction. Most of the reservoirs in China are continental deposits, and the serious heterogeneity leads to serious CO₂ gas channeling, which makes the effect of CO₂ flooding worse. In this paper, the simulation experiment of continuous gas injection, WAG, intermittent gas injection and pulse injection of CO₂ is carried out by using the heterogeneous model in the layer, and the effects of the characteristics of the CO₂ flooding in the heterogeneous oil reservoir and the dynamic environment on the CO₂ gas blow-by are studied. The experimental results show that WAG injection flooding is the best, the effect of intermittent gas injection and continuous gas injection is second, and the pulse gas injection effect is the worst; CO₂ flooding has no gas and low gas oil. In the production stage, extending the production time of the two stages can improve the development effect; by changing the hydrodynamic conditions, the CO₂ gas channeling can be effectively suppressed, and the WAG injection is more capable of suppressing the gas channeling, and the disturbance effect of the pulse gas injection is intensified. The above experimental results show that WAG injection in heterogeneous reservoirs is the optimal injection mode for CO₂ flooding.

In view of the technical difficulties of CO₂ flooding completion string, the research process and development situation of completion string of CO₂ flooding injection-production well were systematically expounded. It was concluded that the sealing performance of the completion string of injection well and anti corrosion and gas proof performance of production well were the key technologies for the normal production of CO₂ flooding injection-production well. The pilot test of CO₂ flooding in domestic oilfield showed that the continuous optimization of packer and the gas tightness detection of tubing contributed to improve sealing of injection wells, and the technologies like CO₂ corrosion inhibitor formulation, injection strengthening, tube controlling by gas lifting, downhole oil and gas separation were helpful to improve the pumping efficiency of the production wells. Meanwhile, in view of the technical problems faced by the injection and production wells on site, the further research direction of production by CO₂ flooding injection and production technology was pointed out.

CO₂ flooding is one of the effective methods for EOR. CO₂ flooding test has been started in Subei（northern Jiangsu province） oilfield since 2005. Y221 packer was used to seal pipe column in the early stage, which basically met the requirements of gas injection. But there still existed problems such as the poor gas sealing performance of gas injection pipe string, the backwashing valve could not be opened, or the casing annulus corrosion inhibitor could not be replaced in circulation, which led to serious corrosion of tubing and casing, and it brought difficulties to the later operation. Since 2014, by improving and optimizing the tubing material, a new CO₂ flooding pipe string （Φ73.02 mm N80-3Cr-BGT1 gas sealing tubing+sliding sleeve switch+Y445J packer+Φ73.02 mm N80 thickening tubing+gas admittance valve of multi-function） has been designed based on the development and application of Y445J packer. And field tests have been conducted for 8 wells, all of which can ensure the good condition of casing pressure being zero for a long time. The experiment shows that the pipe string has the advantages of good gas sealing performance, circulating replacement of oil casing annulus corrosion inhibitor, and preventing back flow. The performance meets the design requirements, and is worth popularizing.

The greenhouse effect caused by excessive CO₂ emissions has led to many adverse effects on human life. As an effective CO₂ disposal technology, CO₂ geological storage technology has aroused more and more attention. The solubility of CO₂ in simulated deep saline solution and its variation with burial depth（800~2 800 m）were studied. The results showed that, when the buried depth was in the range of 800~1 700 m, the solubility of CO₂ decreased with the increase of buried depth. When the buried depth was greater than 1 700 m, the solubility of CO₂ increased with the increase of saline depth. In addition, the change of CO₂ solubility with burial depth could be fitted by the equation. And based on this equation, the storage capacity of CO₂ in a certain area of storage site could be calculated.

Aiming at the feasibility of time-lapse seismic monitoring in the CO₂ flooding-EOR process of tight reservoirs in the Ordos basin, and based on the tight oil-bearing sandstone reservoirs in the Sulige field, the volume modulus and shear modulus of dry rock were calculated by Digby model which considered the pressure changes. Then, Digby model and Gassmann equation were used to predict the primary and shear wave velocity, and to calculate their value varying with the pressure after CO₂ injection. After that, a two-layer medium model was established based on the reservoir logging data of well Huang-234. Finally, the AVO gradient intercept varied with the formation pressure and CO₂ saturation after the CO₂ injection was calculated. It was found that with the increase of injection pressure, the gradient decreased and the intercept increased. As CO₂ saturation increased, the gradient and intercept decreased. The reflection coefficients before and after CO₂ injection were significantly different. As for the tight sand oil reservoir, its AVO features were accorded with the third class of gas-bearing sandstone, which could be effectively detected by the secondary seismic monitoring.