Method for recovering carbon dioxide in oil field gas through membrane separation

Abstract

The invention relates to the field of petroleum refining, and discloses a method for recovering carbon dioxide in oil field driving gas through membrane separation, which comprises the following steps: (1) carrying out shallow cold oil absorption on the oil field driving gas to obtain an oil component and a first mixed gas; (2) performing membrane separation on the first mixed gas to obtain permeability side gas and retentate side gas; (3) optionally, performing gas-liquid separation on the retentate side gas, refluxing the obtained liquid phase part to the shallow cold oil absorption step in the step (1), and using the gas phase part as fuel gas; according to the method, the device construction and operation cost is reduced, and the defects of a membrane separation technology are overcome.

Description

Technical Field

[0001] The present invention relates to the field of oil refining, and particularly to a method for membrane separation and recovery of carbon dioxide in carbon dioxide flooding oilfield gas, which combines shallow cooling oil absorption and membrane separation technologies to recover carbon dioxide in the semi-biogas of carbon dioxide flooding oilfield. Background Art

[0002] In recent years, the global warming and natural disasters caused by the large emissions of greenhouse gas carbon dioxide have attracted wide attention from various countries. The carbon dioxide flooding technology can not only improve the crude oil recovery rate, but also achieve the sequestration of carbon dioxide, which is a good measure for carbon emission control. By recycling the high-concentration carbon dioxide in carbon dioxide flooding oilfield gas, the cost of carbon dioxide flooding can be reduced, and secondary pollution can be avoided. Therefore, the research on the separation and purification technology of carbon dioxide in carbon dioxide flooding oilfield gas is of great significance for resource utilization and environmental protection.

[0003] At present, the technologies for recycling carbon dioxide in carbon dioxide flooding oilfield gas mainly include absorption method, cryogenic distillation method, membrane separation method, etc. Since the carbon dioxide concentration in oilfield gas is high and often contains sulfides, it causes great damage to the absorbent, resulting in high recovery cost, low recovery rate and high equipment requirements. Therefore, the absorption method is generally not applicable. The cryogenic distillation method is a physical process of separating carbon dioxide by low-temperature condensation using the boiling point differences of each component in the raw gas. Generally, after multiple compressions and coolings, phase changes are caused, and carbon dioxide is separated by distillation. It is the main carbon dioxide recycling technology at present. However, the cryogenic distillation method requires a large number of compressors and cold boxes, and there are still problems of high device and energy consumption costs. The membrane separation method is a process of selectively separating carbon dioxide and other gases under the action of pressure according to the different permeabilities of the membrane material to various gas components. Membrane separation has the advantages of simple operation, long service life and low energy consumption, but has the disadvantages of low separation efficiency and poor corrosion resistance.

[0004] Therefore, as a separation method with extremely high potential, it is urgent to develop carbon dioxide separation membrane materials with high separation performance and corrosion resistance, which has important strategic significance for the recycling of carbon dioxide in carbon dioxide flooding oilfield gas. Summary of the Invention

[0005] The purpose of the present invention is to overcome the problems of low separation efficiency and poor corrosion resistance in the membrane separation and recovery of carbon dioxide in carbon dioxide flooding oilfield gas existing in the prior art, and provide a method for membrane separation and recovery of carbon dioxide in carbon dioxide flooding oilfield gas.

[0006] To achieve the above purpose, the present invention provides a method for membrane separation and recovery of carbon dioxide in carbon dioxide flooding oilfield gas, wherein the method includes:

[0007] (1) Subject the drive oilfield gas to shallow cold oil absorption to obtain oil fraction and a first mixed gas;

[0008] (2) Subject the first mixed gas to membrane separation to obtain permeate side gas and retentate side gas;

[0009] (3) Optionally, subject the retentate side gas to gas-liquid separation, return the obtained liquid phase portion to the shallow cold oil absorption step in step (1), and use the gas phase portion as fuel gas.

[0010] By the above technical solution, the present invention ingeniously couples shallow cold oil absorption technology, carbon dioxide membrane separation technology and optional gas-liquid separation and recovery technology, and can at least obtain the following beneficial effects:

[0011] (1) Optimize the carbon dioxide recovery process for drive oilfield gas, without the need for cryogenic rectification operation, and significantly reduce the device construction and operation costs;

[0012] (2) Through shallow cold oil absorption operation, separate the acidic gas and high-carbon organic matter in the drive oilfield gas, thereby reducing the damage to the carbon dioxide separation membrane and making up for the deficiencies of membrane separation technology;

[0013] (3) In the preferred embodiment of the present invention, by setting up a gas-liquid separation and recovery process to recover the absorbent, the process cost is significantly reduced. Specific Embodiments

[0014] The endpoints and any values within the ranges disclosed herein are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and individual point values, and between individual point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0015] The following details the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only for the purpose of illustrating and explaining the present invention, and are not used to limit the present invention.

[0016] The present invention provides a method for membrane separation and recovery of carbon dioxide from drive oilfield gas, wherein the method comprises:

[0017] (1) Subject the drive oilfield gas to shallow cold oil absorption to obtain oil fraction and a first mixed gas;

[0018] (2) Subject the first mixed gas to membrane separation to obtain permeate side gas and retentate side gas;

[0019] (3) Optionally, perform gas-liquid separation on the retentate-side gas, and return the obtained liquid phase portion to the shallow cold oil absorption step of step (1), and use the gas phase portion as fuel gas.

[0020] In the present invention, the shallow cold oil absorption in step (1) can be carried out in an absorption tower, and the absorption tower is selected from a packed tower, a plate tower, a spray tower, a bubble column or a liquid column tower, preferably a packed tower; the packing in the packed tower is one of Raschig rings, Pall rings, cascade rings or corrugated packing.

[0021] In the present invention, the pressure of the gas-phase feed tray of the absorption tower is 100 - 1000 kPa; the pressure of the liquid-phase feed tray is 500 - 1000 kPa; the temperature of the absorption tower is -20°C - 10°C.

[0022] In the present invention, the absorbent used in the shallow cold oil absorption in step (1) is selected from C3 - C4 organic substances; preferably a C4 absorbent.

[0023] In the present invention, the oil fraction in step (1) is discharged from the bottom of the absorption tower; the first mixed gas is discharged from the top of the absorption tower.

[0024] In the present invention, the separation membrane used in the membrane separation in step (2) can be selected from at least one of flat membranes, hollow fiber membranes and tubular membranes; preferably, the material of the separation membrane is selected from at least one of polysulfone, polyethersulfone, polyamide, polypropylene, polyethylene, synthetic resin, polyvinylidene fluoride, polytetrafluoroethylene, polyetheretherketone, polybenzimidazole, polydimethylsiloxane, cellulose acetate, polycarbonate, polymethyl methacrylate, silica, zeolite molecular sieve, carbon molecular sieve, metal-organic framework material. In order to further improve the carbon dioxide flux and selectivity of the carbon dioxide separation membrane for carbon dioxide, preferably, the separation membrane is selected from a composite separation membrane including a support layer, a silicone rubber intermediate layer and a polymer layer stacked in sequence or a polyamide-based separation membrane; the crosslinked polymer constituting the polymer layer contains structural unit I from a polyamine compound, structural unit II from a polyacyl chloride compound and structural unit III from a polyphenol compound (see application number: 202311127284.6, which is incorporated herein by reference).

[0025] In the present invention, in the membrane separation in step (2), the carbon dioxide flux of the separation membrane used is 300 - 800 GPU, and the separation coefficient of carbon dioxide / nitrogen is 40 - 80; more preferably 55 - 65.

[0026] In the present invention, there are no special requirements for the conditions of the membrane separation. For example, the pressure on the positive pressure side of the membrane separation in step (2) can be 100 - 2000 kPa; preferably 300 - 1500 kPa; the pressure on the permeate side is lower than the pressure on the retentate side, and the pressure on the retentate side can also be negative.

[0027] In the present invention, the positive pressure is the intake pressure on the upstream side of the separation membrane.

[0028] In the present invention, preferably, the method may further include: before the first mixed gas undergoes membrane separation, it is pressurized by a compressor, and the pressure after pressurization is 100 - 2000 kPa.

[0029] In the present invention, the membrane separation adopts a single - stage or multi - stage membrane separation technology; preferably 2 - 3 stages. When multi - stage membrane separation is carried out, the positive pressure of the separation membrane can be the same or different.

[0030] In the present invention, in step (3), the gas - liquid separation method is condensation, and the temperature of the condensation is - 20°C to 15°C; preferably - 10°C to 10°C.

[0031] In the present invention, the condensation in step (3) can be carried out using various common condensates in the art. For example, the condensate can be at least one of water, methanol, ethanol, ethylene glycol, and sodium chloride solution.

[0032] In the present invention, the method is applicable to recovering carbon dioxide from carbon - dioxide - containing oil - field driving gas. The composition of the oil - field driving gas is preferably: carbon dioxide 50 - 80 vol%, and the other gas components are not limited. For example, the composition of the oil - field driving gas can be: carbon dioxide 50 - 80 vol%, nitrogen 1 - 5 vol%, methane 10 - 20 vol%, C2 2 - 6 vol%, C3 1 - 3 vol%, C4 2 - 5 vol%, C5 and above 1 - 7 vol%, sulfur dioxide 0.1 - 1 vol%.

[0033] In a preferred embodiment of the present invention, the separation membrane is selected from a composite separation membrane including a support layer, a silicone rubber intermediate layer, and a polymer layer stacked in sequence or a separation membrane made of polyamide material; the separation coefficient of carbon dioxide / nitrogen of the separation membrane is 55 - 65; the positive pressure side pressure of the membrane separation is 300 - 1500 kPa; the membrane separation adopts a 2 - 3 - stage membrane separation technology. Using the above - mentioned preferred embodiment, the purity of CO 2 is above 99%, and the recovery rate of CO 2 is above 75%.

[0034] The present invention will be described in detail below through examples. For those not specifying specific conditions in the following examples and comparative examples, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments without indicating the manufacturer, they are all conventional products that can be obtained through commercial channels.

[0035] In the following preparation examples, examples, and comparative examples,

[0036] The weight average molecular weight of the material of the support layer can be determined by GPC method;

[0037] The porosity of the support layer is measured by scanning electron microscopy, and the porosity is obtained by dividing the pore area within the field of view by the total surface area;

[0038] The test method for the average pore size of the support layer is: measure the diameters of all visible pores in the field of view through a scanning electron microscope and calculate the average value;

[0039] The test method for the viscosity of silicone rubber at 25°C is: using a rotor viscometer to measure;

[0040] The test method of the cross-linking degree of the cross-linked polymer in the separation composite membrane C1 is: calculation by XPS and infrared spectrum;

[0041] The average pore size of the polymer layer is tested according to the standard GB / T42269-2022 "Separation membrane pore size test method gas permeation method";

[0042] IsoparG, purchased from Aladdin Reagents;

[0043] Polysulfone (mass fraction: 20wt%), purchased from Solvay;

[0044] Polydimethylsiloxane, purchased from Adamas-Beta;

[0045] 5,5',6,6'-Tetrahydroxy-3,3,3',3'-tetramethyl spirobiindane, diethylene glycol di(3-aminopropyl) ether, purchased by TCI;

[0046] Triethylamine and trimesoyl chloride, purchased by Anaiji Chemical;

[0047] Sodium hydroxide, purchased from Sinopharm Reagents;

[0048] The gas composition analysis method uses a gas analyzer to test the gas concentration.

[0049] Preparation Example

[0050] (1) Add 200g of polysulfone, 680g of dimethylacetamide, 60g of polyvinyl pyrrolidone, and 60g of polyethylene glycol, heat to 70°C for 24h, prepare support layer A1, and then wash with water for 48h;

[0051] Wherein, the weight average molecular weight of the material of the support layer A1 is 3500Da, the porosity is 59%, and the average pore size is 21nm.

[0052] (2) Using the casting method on the above 400cm 2 ​​​8 mL of a 1 wt% polydimethylsiloxane solution was coated on the support layer A1, the blade thickness was 100 μm, the coating and curing temperature was 30 °C, the coating and curing time was 12 h, and a middle layer was formed by curing to obtain a composite membrane B1;

[0053] Among them, the viscosity of the silicone rubber in the middle layer of the composite membrane B1 at 25 °C was 820 cP.

[0054] (3) Interface polymerization reaction was carried out at 25 °C: 0.4 g of trimesoyl chloride was added to 99.6 g of IsoparG solution, 35 mL of the prepared oil-phase solution was taken, and the above 400 cm 2 of the composite membrane B1 was infiltrated in 35 mL of the oil-phase solution for 3 min. In the presence of 0.05 g of triethylamine, 0.1 g of 5,5’,6,6’-tetrahydroxy-3,3,3’,3’-tetramethylspirobisindane and 0.1 g of diethylene glycol bis(3-aminopropyl) ether were added to 99.75 g of water, and the pH was adjusted to 12 with sodium hydroxide. The composite membrane B1 infiltrated in the oil phase was infiltrated in 35 mL of the aqueous solution for 5 min. After the interface polymerization reaction was completed, the residual solvent was washed thoroughly with water to form a polymer layer containing a cross-linked polymer (structural unit I was from diethylene glycol bis(3-aminopropyl) ether, structural unit II was from trimesoyl chloride, and structural unit III was from 5,5’,6,6’-tetrahydroxy-3,3,3’,3’-tetramethylspirobisindane) to obtain a separation composite membrane C1; Attenuated total reflection infrared absorption spectroscopy analysis showed that there were linking groups a and b in the cross-linked polymer;

[0055] Among them, the cross-linking degree of the cross-linked polymer in the separation composite membrane C1 was 55.3%;

[0056] Among them, the average pore size of the polymer layer was 0.37 nm.

[0057] Example 1

[0058] The composition of the carbon dioxide flooding oilfield gas in a certain oilfield was roughly as follows: carbon dioxide 65 vol%, nitrogen 1 vol%, methane 19 vol%, C2 4.5 vol%, C3 2.5 vol%, C4 3.9 vol%, C5 and above 4 vol%, sulfur dioxide 0.1 vol%. The carbon dioxide recovery operation was carried out on a certain flooding oilfield gas by using the technology of the present invention.

[0059] The flooding oilfield gas first entered the absorption tower for shallow cold oil absorption operation. The absorption tower used a packed tower, in which the packing used Raschig rings and n-butane was used as the absorbent. The inlet pressure of the absorption tower was 100 kPa, the feed pressure of the absorbent was 500 kPa, and the temperature of the packed tower was 0 °C. Oil was obtained at the bottom outlet of the absorption tower, and a first mixed gas was obtained at the top outlet.

[0060] After the first mixed gas is pressurized to 1000 kPa by a compressor, it enters the carbon dioxide membrane separation step for separation operation. The separation composite membrane C1 prepared in the preparation example is used as the separation membrane, and the separation coefficient of carbon dioxide / nitrogen of the separation composite membrane C1 is 65. Two-stage membrane is used for purification, and the positive pressure is 500 kPa and 500 kPa respectively. The gas on the membrane permeation side is the carbon dioxide product, and the gas on the retentate side is mixed to obtain the second mixed gas. The second mixed gas is condensed, and a water-ethylene glycol mixed solution (60 vol% water and 40 vol% ethylene glycol) is used as the condensing agent, and the condensing temperature is 0 °C. After condensation, it enters the two-phase separator. The liquid phase part is mainly C4 and is refluxed to the inlet of the shallow cold oil absorption unit, and the gas phase part is used as fuel gas.

[0061] The main gas compositions of each step of carbon dioxide recovery are shown in Table 1.

[0062] Table 1 Gas composition table at each stage

[0063]

[0064]

[0065] As can be seen from the above table, through the preferred process technology of the present invention, a carbon dioxide product with a purity of 99.8% is obtained by using the preferred separation membrane, and the carbon dioxide recovery rate reaches 77%.

[0066] Example 2

[0067] Carbon dioxide is recovered according to the method of Example 1, except that: the separation membrane is a hollow fiber membrane made of polyamide material, and the separation coefficient of carbon dioxide / nitrogen is 55.

[0068] The main gas compositions of each step of carbon dioxide recovery are shown in Table 2.

[0069] Table 2 Gas composition table at each stage

[0070]

[0071] As can be seen from the above table, through the process technology of the present invention, a carbon dioxide product with a purity of 99.2% is obtained, and the carbon dioxide recovery rate reaches 75%.

[0072] Example 3

[0073] Carbon dioxide is recovered according to the method of Example 2, except that: in the carbon dioxide membrane separation step, one-stage membrane purification is carried out, and the positive pressure is 1500 kPa.

[0074] The main gas compositions of each step of carbon dioxide recovery are shown in Table 3.

[0075] Table 3 Gas composition table at each stage

[0076]

[0077] As can be seen from the above table, through the process technology of the present invention, a carbon dioxide product with a purity of 96.04% is obtained, and the carbon dioxide recovery rate reaches 71%.

[0078] Example 4

[0079] Recover carbon dioxide according to the method of Example 2, the difference is that: in the carbon dioxide membrane separation step, purification is carried out through three-stage membranes, and the positive pressure is 500 kPa, 300 kPa, and 300 kPa respectively. The gas on the membrane permeate side is the carbon dioxide product, and the gas on the retentate side is mixed to obtain the second mixed gas, and the second mixed gas is condensed. Water is used as the condensing agent, and the condensing temperature is 10°C.

[0080] The main gas compositions of each step of carbon dioxide recovery are shown in Table 4.

[0081] Table 4 Gas Composition Table at Each Stage

[0082]

[0083]

[0084] As can be seen from the above table, through the process technology of the present invention, a carbon dioxide product with a purity of 99.75% is obtained, and the carbon dioxide recovery rate reaches 85%.

[0085] Example 5

[0086] Recover carbon dioxide according to the method of Example 2, the difference is that: the inlet pressure of the absorption tower is 500 kPa, and the feed pressure of the absorbent is 800 kPa; in the carbon dioxide membrane separation step, purification is carried out through two-stage membranes, and the positive pressure is 1000 kPa and 500 kPa respectively; water is used as the condensing agent in the condensation step, and the condensing temperature is -10°C.

[0087] The main gas compositions of each step of carbon dioxide recovery are shown in Table 5.

[0088] Table 5 Gas Composition Table at Each Stage

[0089]

[0090] As can be seen from the above table, through the process technology of the present invention, a carbon dioxide product with a purity of 99.04% is obtained, and the carbon dioxide recovery rate reaches 79%.

[0091] Example 6

[0092] Recover carbon dioxide according to the method of Example 2, except that: in the carbon dioxide membrane separation step, purification is carried out through two-stage membranes, and the positive pressures are 200 kPa and 200 kPa respectively; in the condensation step, water is used as the condensing agent, and the condensation temperature is -10°C.

[0093] The main gas compositions of each step of carbon dioxide recovery are shown in Table 6.

[0094] Table 6 Gas Composition Table at Each Stage

[0095]

[0096] As can be seen from the above table, through the process technology of the present invention, a carbon dioxide product with a purity of 90.86% is obtained, and the carbon dioxide recovery rate reaches 68%. It can be seen from the data that the carbon dioxide purity and recovery rate of the products obtained in Examples 1-5 are higher, and the usage amount of the carbon dioxide separation membrane is less. Thus, it can be seen that using the positive pressure of the preferred separation membrane of the present invention for carbon dioxide separation has a better separation effect.

[0097] Example 7

[0098] Recover carbon dioxide according to the method of Example 2, except that: in the carbon dioxide membrane separation step, the separation membrane is a polydimethylsiloxane hollow fiber membrane, and the separation coefficient of carbon dioxide / nitrogen is 31.

[0099] The main gas compositions of each step of carbon dioxide recovery are shown in Table 7.

[0100] Table 7 Gas Composition Table at Each Stage

[0101]

[0102]

[0103] As can be seen from the above table, through the process technology of the present invention, a carbon dioxide product with a purity of 78.37% is obtained, and the carbon dioxide recovery rate reaches 64%. It can be seen from the data that the carbon dioxide purity and recovery rate of the products obtained in Examples 1-6 are higher, and the usage amount of the carbon dioxide separation membrane is less. Thus, it can be seen that using the preferred separation membrane of the present invention for carbon dioxide separation has a better separation effect.

[0104] Example 8

[0105] Recover carbon dioxide according to the method of Example 2, except that: the condensation step is not carried out.

[0106] The main gas compositions of each step of carbon dioxide recovery are shown in Table 8.

[0107] Table 8 Gas Composition Table at Each Stage

[0108]

[0109] As can be seen from the above table, in Example 8, a carbon dioxide product with a purity of 99.2% was obtained, and the carbon dioxide recovery rate reached 75%, which was the same as that in Example 2. However, since the preferred scheme was adopted in Example 2 for the condensation recovery unit, the C4 light cold oil was recovered, and the cost of supplementing the absorbent in the light cold oil absorption tower was reduced by about 50% compared with this example, and the process cost was further reduced accordingly.

[0110] Comparative Example 1

[0111] Carbon dioxide was recovered according to the method of Example 2, except that: the oilfield gas displacement did not perform the light cold oil absorption operation. Since there was no need for light cold oil recovery, the condensation recovery unit was not set up either. Instead, through the carbon dioxide membrane separation step, three-stage membrane purification was carried out, and the positive pressure was 1000 kPa, 500 kPa, and 500 kPa respectively.

[0112] The main gas compositions of each step of carbon dioxide recovery are shown in Table 9.

[0113] Table 9 Gas Composition Table at Each Stage

[0114]

[0115] As can be seen from the above table, in Comparative Example 1, a carbon dioxide product with a purity of 75.9% was obtained, and the carbon dioxide recovery rate reached 33%. The carbon dioxide purity and recovery rate of the product obtained in Comparative Example 1 were both very low, and the first-stage membrane was severely damaged, showing a serious swelling phenomenon. This was because the organic matter above C6 was not pretreated in the early stage, resulting in damage to the carbon dioxide separation membrane, thus having a huge impact on the separation effect. This shows that the light cold oil absorption unit plays a very important role in the process technology of the present invention.

[0116] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A method for recovering carbon dioxide from oilfield gas by membrane separation, characterized in that: The method includes: (1) shallowly cooling and absorbing the oil field gas to obtain oil and a first mixed gas; (2) subjecting the first mixed gas to membrane separation to obtain permeate gas and retentate gas; (3) Optionally, the gas on the retentate side is subjected to gas-liquid separation, the obtained liquid phase is refluxed to the shallow cold oil absorption step of step (1), and the gas phase is used as fuel gas.

2. The recycling method according to claim 1, wherein: In step (1), the shallow cold oil absorption is carried out in an absorption tower, and the absorption tower is selected from a packed tower, a plate tower, a spray tower, a bubbling tower or a liquid column tower; preferably a packed tower.

3. The recycling method according to claim 2, wherein: In step (1), the gas phase feed plate pressure of the absorption tower is 100-1000 kPa; the liquid phase feed plate pressure is 500-1000 kPa; and the temperature is -20°C to 10°C.

4. The recovery method according to any one of claims 1 to 3, wherein: In step (1), the absorbent used in the shallow cold oil absorption is selected from C3-C4 organic matter; preferably C4 absorbent.

5. The recycling method according to claim 1, wherein: In step (2), the separation membrane used in the membrane separation is selected from at least one of a flat membrane, a hollow fiber membrane and a tubular membrane; Preferably, the material of the separation membrane is selected from at least one of polysulfone, polyethersulfone, polyamide, polypropylene, polyethylene, synthetic resin, polyvinylidene fluoride, polytetrafluoroethylene, polyetheretherketone, polybenzimidazole, polydimethylsiloxane, cellulose acetate, polycarbonate, polymethyl methacrylate, silica, zeolite molecular sieve, carbon molecular sieve, and metal organic framework materials; more preferably, the separation membrane is selected from a separation composite membrane comprising a support layer, a silicone rubber intermediate layer, and a polymer layer stacked in sequence, or a separation membrane made of polyamide material; the cross-linked polymer constituting the polymer layer comprises structural unit I from a polyamine compound, structural unit II from a polyacyl chloride compound, and structural unit III from a polyphenol compound.

6. The recovery method according to claim 1 or 5, wherein: In step (2), in the membrane separation, the carbon dioxide flux of the separation membrane used is 300-800 GPU, and the separation coefficient of carbon dioxide / nitrogen is 40-80.

7. The recovery method according to any one of claims 1, 5 or 6, wherein: In step (2), the pressure on the positive pressure side of the membrane separation is 100-2000 kPa, preferably 300-1500 kPa.

8. The recycling method according to claim 7, wherein: In step (2), the pressure on the permeate side of the membrane separation is lower than the pressure on the positive pressure side.

9. The recycling method according to claim 1, wherein: In step (3), the gas-liquid separation method is condensation, and the condensation temperature is -20°C to 15°C; preferably -10°C to 10°C.

10. The recycling method according to claim 1, wherein: The carbon dioxide content in the oilfield gas is 50-80 vol%.

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