A method for polymer flooding assisted by supercritical carbon dioxide

Abstract

The present application relates to a method for supercritical carbon dioxide-assisted polymer flooding. The method for supercritical carbon dioxide-assisted polymer flooding comprises the following steps: lowering a soluble bridge plug into a string, such that the soluble bridge plug is located between a high polymer-absorbing layer in the lower part of an oilfield and a low polymer-absorbing layer in the upper part of the oilfield; injecting an acidizing plugging remover to clean the low polymer-absorbing layer; and sequentially injecting a solubilization synergist, carbon dioxide, and a surfactant. The present application can greatly improve the vertical utilization degree of a reservoir and the oil displacement efficiency, and further improve the reservoir recovery factor.

Description

Technical Field

[0001] This application relates to the field of oilfield applications, and particularly to a supercritical carbon dioxide pressure flooding process for assisting polymer flooding to improve oil recovery. Background Art

[0002] Polymer flooding is an enhanced oil recovery technique that injects polymers into the formation for oil displacement. Macroscopically, it mainly increases the viscosity of the displacement fluid and reduces the mobility ratio between the displacement fluid and the displaced fluid, thereby expanding the swept volume; microscopically, due to its inherent viscoelasticity, the polymer generates a stretching effect on the oil film or oil droplets during the flow process, increasing the carrying capacity and improving the microscopic oil washing efficiency. In some oil regions, such as the third production area of Shuguang, the lower oil layer is better developed than the upper oil layer. After long-term polymer injection, uneven polymer absorption occurs. The polymer absorption profile shows that the polymer absorption ratio in the lower layer reaches more than 70%, and other layers cannot be effectively utilized. The common measure in the oilfield is to inject in layers, such as eccentric separate injection or concentric separate injection, restrict the injection of the lower high polymer absorption layer, and strengthen the injection of the upper low polymer absorption layer. However, due to the small flow channel holes in the separate injection string, it causes certain shear to the polymer solution, affecting the viscosity of the polymer solution injected downhole; at the same time, the injection pressure in the upper low polymer absorption layer rises, and even injection of polymer becomes difficult. The main reason is the difference in the development of oil layers. The average oil layer thickness of the upper layer is 18m, and the permeability is 588mD; the average oil layer thickness of the lower layer is 10m, and the permeability is 1100mD. In contrast, it leads to uneven utilization of the upper and lower oil layer groups.

[0003] Conventional chemical profile control measures are difficult to solve the problems of large interlayer differences and uneven oil layer utilization in these oil regions. First, the chemical profile control intensity of adding a certain solid particle profile control agent is too large. Although good profile control effects are seen in the initial stage, it causes a sharp rise in the polymer injection pressure, making it difficult to inject polymer at the rated injection pressure. And once blockage occurs, there is no effective means of unblocking; second, simply using a liquid profile control agent, the chemical profile control intensity is too low, and the profile control effective period is very short. After injecting polymer for a period of time, the original polymer absorption profile is restored, and even a polymer absorption profile with a greater utilization difference appears. Summary of the Invention

[0004] The embodiments of this application provide a method for supercritical carbon dioxide-assisted polymer flooding to solve the technical problem of uneven utilization in oil regions where the lower oil layer is better developed than the upper oil layer.

[0005] The embodiments of this application provide a method for supercritical carbon dioxide-assisted polymer flooding, and the method for supercritical carbon dioxide-assisted polymer flooding includes the following steps:

[0006] Lower a soluble bridge plug into the string so that the soluble bridge plug is located between the high polymer absorption layer in the lower part of the oilfield and the low polymer absorption layer in the upper part of the oilfield;

[0007] Inject an acidic plugging removal agent to clean the low-absorbing poly layer;

[0008] Sequentially inject a solubilizing and synergistic agent, carbon dioxide, and a surfactant.

[0009] In some embodiments of the present application, when lowering a soluble bridge plug into the string, at least two soluble bridge plugs connected to each other up and down are lowered.

[0010] In some embodiments of the present application, the mass ratio of the injection amount of carbon dioxide to the injection amount of the synergistic and solubilizing agent is 1:0.3 - 0.7.

[0011] In some embodiments of the present application, by mass percentage, the surfactant is 3 - 5% of the sum of the injection amounts of carbon dioxide and the synergistic and solubilizing agent.

[0012] In some embodiments of the present application, the acidic plugging removal agent includes at least one of hydrochloric acid, hydrofluoric acid, mud acid, fluoboric acid, formic acid, formic acid ammonium salt, acetic acid, acetic acid ammonium salt, citric acid, citric acid ammonium salt, carbonic acid, and phosphoric acid.

[0013] In some embodiments of the present application, the synergistic and solubilizing agent includes:

[0014] Alkane;

[0015] Naphthene;

[0016] Benzene and its derivatives;

[0017] Carbonic enol ester;

[0018] An alcohol ether formed by condensing two glycol molecules with the removal of one water molecule.

[0019] In some embodiments of the present application, the alkane includes at least one of pentane, hexane, heptane, octane, nonane, and decane; and / or,

[0020] The naphthene includes at least one of cyclopropane, cyclobutane, cyclopentane, and cyclohexane; and / or,

[0021] The benzene and its derivatives include at least one of benzene, toluene, o-xylene, m-xylene, p-xylene, and ethylbenzene; and / or,

[0022] The carbonic enol ester includes at least one of ethylene carbonate, propylene carbonate, and butylene carbonate; and / or,

[0023] The alcohol ether includes at least one of ethylene glycol-butyl ether, diethylene glycol-butyl ether, butoxy triethylene glycol, and modified ethylene glycol ether.

[0024] In some embodiments of the present application, the surfactant is at least one of fatty alcohol polyoxyethylene ether, nonylphenol polyoxyethylene ether, glycerol monostearate, and trifluoroepoxyhexane amide sulfobetaine.

[0025] In some embodiments of the present application, the carbon dioxide is in a liquid state.

[0026] In some embodiments of the present application, the pumping pressure of the carbon dioxide is greater than the bottom layer fracture pressure.

[0027] The above technical solutions provided by the embodiments of the present application have the following advantages compared with the prior art:

[0028] The method for supercritical carbon dioxide-assisted polymer flooding provided by the embodiments of the present application uses carbon dioxide to assist polymer displacement. After the carbon dioxide enters the oil layer, it is easy to reach the supercritical state. Supercritical carbon dioxide has a density close to that of a liquid, a viscosity close to that of a gas, and a low surface tension, so it is easy to penetrate into the pores in the oil layer. Therefore, compared with eccentric separate injection and concentric separate injection measures, it does not affect the injection viscosity of subsequent polymer flooding; compared with chemical profile control measures, it will neither cause too high an injection pressure and inability to unblock due to excessive strength, nor cause a short effective period and failure after a period of time due to too low strength; compared with the technology of improving oil recovery by simple polymer flooding, it can greatly improve the vertical utilization degree and oil displacement efficiency of the reservoir, and further improve the oil recovery of the reservoir. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings herein are incorporated into the specification and form a part of the specification, showing embodiments consistent with the present application, and are used together with the specification to explain the principles of the present application.

[0030] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, for those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0031] Figure 1 It is a schematic flow chart of a method for supercritical carbon dioxide-assisted polymer flooding provided by an embodiment of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0032] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of this application. Obviously, the described embodiments are part of the embodiments of this application, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in this application without creative efforts belong to the scope protected by this application.

[0033] Unless otherwise specifically stated, the terms used in this article should be understood to have the meanings as commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used in this article have the same meanings as those generally understood by those skilled in the art to which this application belongs. In case of any contradiction, this specification shall prevail.

[0034] Unless otherwise specifically stated, various raw materials, reagents, instruments, and equipment used in this application can all be obtained through market purchases or can be prepared by existing methods.

[0035] In existing oil regions where the lower oil layer is better developed than the upper oil layer, there is a technical problem of uneven utilization of the upper and lower oil layer groups.

[0036] The technical solutions provided by the embodiments of this application to solve the above technical problems are generally as follows:

[0037] The embodiments of this application provide a method for supercritical carbon dioxide-assisted polymer flooding, and the method for supercritical carbon dioxide-assisted polymer flooding includes the following steps:

[0038] Lower a soluble bridge plug into the pipe string so that the soluble bridge plug is located between the high polymer-absorbing layer in the lower part of the oilfield and the low polymer-absorbing layer in the upper part of the oilfield;

[0039] Inject an acid plugging removal agent to clean the low polymer-absorbing layer;

[0040] Inject a solubilization and synergistic agent, carbon dioxide, and a surfactant in sequence.

[0041] Those skilled in the art can understand that the purpose of lowering the soluble bridge plug is to enable the acid plugging removal agent to specifically clean the low polymer-absorbing layer. The soluble bridge plug is composed of degradable metal and rubber materials. Generally, the machining material composition of the soluble bridge plug is designed according to the formation temperature, salinity, casing size, etc. of the specific well to be implemented, and it is necessary to achieve the complete degradation of the soluble bridge plug to facilitate subsequent carbon dioxide-assisted displacement. The soluble bridge plug generally has the following characteristics: the pressure bearing is not less than 70 MPa, and the temperature resistance is not less than 120 °C; the slips can be dissolved; the rubber cylinder can be completely dissolved; the size of the bridge plug can be processed according to the inner diameter of the casing.

[0042] Those skilled in the art can understand that the injection volume of the acid plugging removal agent is calculated in a conventional manner in this field. As an example, it can be calculated in the following way: calculated according to the cylinder model, the plugging removal radius is calculated as 2 - 3 m, Q = πR2hΦ, where Q is the injection volume with the unit of m3; π is the constant of pi, taking 3.14, R is the plugging removal radius with the unit of m; h is the oil layer thickness with the unit of m, and Φ is the oil layer porosity with the unit of %.

[0043] Those skilled in the art can understand that the purpose of injecting the solubilization and efficiency enhancement agent is as follows: when the carbon dioxide injected into the reservoir contacts the crude oil and dissolves into the remaining oil in the supercritical state, it will preferentially extract the light components, and the remaining heavy components are difficult to produce as they stay in the formation pores. Therefore, the purpose of injecting the solubilization and efficiency enhancement agent is to increase the solubility of carbon dioxide in the crude oil, enable a relatively large amount of carbon dioxide to dissolve in the crude oil, reduce the viscosity of the crude oil, and facilitate the production of the remaining oil.

[0044] Those skilled in the art can understand that the purpose of injecting the surfactant is to reduce the interfacial tension between the crude oil and the formation rock, improve the flow characteristics of the formation crude oil, and enhance the seepage and flow capacity of the crude oil.

[0045] For different physical properties of crude oil, supporting viscosity reducers, pour point depressants, etc. are used to comprehensively improve the characteristics of the crude oil and enhance the seepage and flow capacity of the formation crude oil.

[0046] Those skilled in the art can understand that the injection volume of carbon dioxide is calculated in a conventional manner in this field. As an example, it can be calculated in the following way: the displacement radius is calculated as 6 - 10 m, Q = πR2hΦ, where Q is the injection volume with the unit of m3; π is the constant of pi, taking 3.14, R is the displacement radius with the unit of m; h is the oil layer thickness with the unit of m, and Φ is the oil layer porosity with the unit of %.

[0047] The critical temperature of carbon dioxide is 31.1 °C, and the corresponding critical pressure is 7.38 MPa. From the phase diagram of carbon dioxide, when the temperature of carbon dioxide exceeds the critical temperature and at the same time the pressure also exceeds the critical pressure, then carbon dioxide exists in the supercritical state; after carbon dioxide is injected into the oil layer, its pressure and temperature can easily make carbon dioxide reach the supercritical state. Supercritical CO2 has a density close to that of a liquid. At the same time, its viscosity is close to that of a gas, and its diffusion coefficient is larger than that of a liquid, having good mass transfer performance. In addition, the surface tension of supercritical CO2 is zero. Therefore, they can enter any space larger than the supercritical CO2 molecule, so it is conducive to assisted displacement.

[0048] In the present application, carbon dioxide is easily able to reach a supercritical state after entering the oil layer by assisting polymer displacement with carbon dioxide. Supercritical carbon dioxide has a density close to that of a liquid, a viscosity close to that of a gas, and a low surface tension, so that it can easily penetrate into the pores in the oil layer. Therefore, compared with eccentric injection and concentric injection measures, it does not affect the injection viscosity of subsequent polymer flooding; compared with chemical profile adjustment measures, it will not cause excessive injection pressure due to excessive intensity, making it impossible to unblock, nor will it cause a short effective period due to too low intensity, making it ineffective after a period of time; compared with the simple polymer flooding technology for improving oil recovery, it can greatly improve the vertical utilization degree and oil displacement efficiency of the oil reservoir, and further improve the oil reservoir recovery rate.

[0049] In some embodiments of the present application, the step of inserting a soluble bridge plug into the tubing string comprises inserting at least two soluble bridge plugs connected to each other up and down.

[0050] Those skilled in the art will appreciate that sometimes soluble bridge plugs may be directly dissolved by the acidic plugging remover. Therefore, multiple soluble bridge plugs may be inserted according to actual working conditions so that when the upper bridge plug is dissolved, the lower bridge plug can still block the acidic plugging remover to ensure its temporary plugging effect.

[0051] In some embodiments of the present application, the mass ratio of the injected amount of the carbon dioxide to the injected amount of the synergistic solubilizing agent is 1:0.3-0.7.

[0052] Those skilled in the art can understand that the beneficial effect of the mass ratio of the injection amount of carbon dioxide to the injection amount of the synergistic solubilizing agent being 1:0.3-0.7 is to obtain the best ratio of increased production and technical investment, that is, at this injection ratio, the best economic benefit can be obtained.

[0053] In some embodiments of the present application, the surfactant is 3-5% of the sum of the injection amounts of the carbon dioxide and the synergistic solubilizing agent, in terms of mass percentage.

[0054] Those skilled in the art will appreciate that the beneficial effect of the surfactant being 3-5% of the sum of the injection amounts of carbon dioxide and the synergistic solubilizing agent is that at this injection percentage, the surfactant can be well dispersed in the formation crude oil and fully exert its effect of reducing the surface tension between the crude oil and the formation rock, thereby ensuring that the surfactant is not wasted and fully exerting its effect.

[0055] In some embodiments of the present application, the acidic plugging remover includes at least one of hydrochloric acid, hydrofluoric acid, earth acid, fluoroboric acid, formic acid, ammonium formate, acetic acid, ammonium acetate, citric acid, ammonium citrate, carbonic acid, and phosphoric acid.

[0056] Those skilled in the art can understand that the above-mentioned acidic plugging remover is a conventional acid in the art, and those skilled in the art can adjust it according to the on-site conditions.

[0057] In some embodiments of the present application, the synergistic solubilizer includes:

[0058] Paraffin;

[0059] Naphthene;

[0060] Benzene and its derivatives;

[0061] Alkenyl carbonate;

[0062] An alcohol ether formed by condensing two diol molecules with the elimination of one water molecule.

[0063] Those skilled in the art can understand that the functions of the respective components of the above-mentioned synergistic solubilizer are:

[0064] Paraffin: Increases the solubility of carbon dioxide in crude oil. At the same time, using the principle of similar compatibility, it dissolves with the paraffin in crude oil, playing a role in diluting and reducing the viscosity of crude oil;

[0065] Naphthene: Increases the solubility of carbon dioxide in crude oil. At the same time, using the principle of similar compatibility, it dissolves with the naphthene in crude oil, playing a role in diluting and reducing the viscosity of crude oil;

[0066] Benzene and its derivatives: Increases the solubility of carbon dioxide in crude oil. At the same time, using the principle of similar compatibility, it dissolves with the benzene and its derivatives in crude oil, playing a role in diluting and reducing the viscosity of crude oil;

[0067] Alkenyl carbonate: Increases the solubility of carbon dioxide in crude oil. At the same time, using the principle of similar compatibility, it dissolves with the ester compounds in crude oil, playing a role in diluting and reducing the viscosity of crude oil;

[0068] Alcohol ether: Increases the solubility of carbon dioxide in crude oil. At the same time, as an organic solvent, it can greatly increase the solubility of heavy components.

[0069] In some embodiments of the present application, the paraffin includes at least one of pentane, hexane, heptane, octane, nonane, and decane; and / or,

[0070] the naphthene includes at least one of cyclopropane, cyclobutane, cyclopentane, and cyclohexane; and / or,

[0071] the benzene and its derivatives include at least one of benzene, toluene, o-xylene, m-xylene, p-xylene, and ethylbenzene; and / or,

[0072] the alkenyl carbonate includes at least one of ethylene carbonate, propylene carbonate, and butylene carbonate; and / or,

[0073] The alcohol ethers include at least one of ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, butoxytriglycol, and modified ethylene glycol ether.

[0074] Those skilled in the art can understand that the above-listed raw materials are cheap and easily available, and the number of carbon atoms is relatively appropriate, which can achieve good effects when used as synergistic solubilizers.

[0075] In some embodiments of the present application, the surfactant is at least one of fatty alcohol polyoxyethylene ether, nonylphenol polyoxyethylene ether, glycerol monostearate, and tris(epoxyhexafluoropropane) amide sulfobetaine.

[0076] Those skilled in the art can understand that the beneficial effect of selecting the above components as the surfactant is to reduce the viscosity of crude oil internally and reduce the interfacial tension externally, making the experimental crude oil easy to strip and flow.

[0077] In some embodiments of the present application, the carbon dioxide is in a liquid state.

[0078] Those skilled in the art can understand that liquid carbon dioxide is cheap and easily available, and is easy to pump in.

[0079] In some embodiments of the present application, the pumping pressure of the carbon dioxide is greater than the formation fracture pressure.

[0080] Those skilled in the art can understand that the beneficial effect of the pumping pressure of carbon dioxide being greater than the formation fracture pressure is to fracture the formation, form microfractures in the formation, and improve the permeability of the upper low-absorbing polymer layer well.

[0081] The following further elaborates the present application in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present application and not to limit the scope of the present application. The experimental methods without specific conditions noted in the following embodiments are usually determined according to national standards. If there is no corresponding national standard, they are carried out according to general international standards, conventional conditions, or the conditions recommended by the manufacturer.

[0082] Example

[0083] For polymer injection well 3-06-0005 in a certain oilfield, the effective thickness is 28.5 m, there are 10 layers, the permeability ratio is 2335. Eleven months after polymer flooding conversion, the polymer injection pressure is 12.5 MPa. Affected by the difference in the development of the oil-bearing layer, only 3 layers (10#, 18#, 19#) absorb polymer, and the layer segment utilization degree is 23.2%. Most of the oil layers cannot be effectively utilized. To improve the oil layer utilization degree and further improve the polymer flooding recovery rate, supercritical carbon dioxide-assisted polymer flooding is implemented on injection well 3-06-0005, which specifically includes the following steps:

[0084] Step 1: Shield the lower high-absorbing polymer layer by lowering soluble bridge plugs. Lower two soluble bridge plugs connected up and down, and fabricate soluble bridge plugs that will not dissolve within 72 hours according to the basic parameters of Well 3-06-0005, and make them completely dissolve after 72 hours through soluble mechanisms such as changing the working temperature or increasing the chloride ion concentration, etc.

[0085] The basic parameters of Well 3-06-0005 are shown in Table 1:

[0086] Well depth Temperature Casing outer diameter Casing wall thickness Casing steel grade Downhole salinity Construction time Injection pressure 1370-1450m 45℃ 177.8 mm 8.05 mm N80 1000 - 2000 mg / l 48 h - 72 h 25 - 35 MPa

[0087] Table 1

[0088] Step 2: Inject an acid plugging removal agent to carry out pressure reduction and cleaning treatment on the upper low-absorbing polymer layer. By mass percentage, the specific composition of the acid plugging removal agent is 10% hydrochloric acid, 5% hydrofluoric acid, 1% acetic acid, 3% fluoboric acid, 0.5% corrosion inhibitor (HAI-90 type corrosion inhibitor provided by Nanjing Huazhou Materials Co., Ltd.), and the balance is water. The injection volume is the treatment volume of the upper low-absorbing polymer layer. Calculated according to the cylinder model, the plugging removal radius is calculated as 3m, Q = πR 2 hΦ = 3.14 * 5 2 * 24.4 * 0.25 = 172m 3 , and the pumping pressure is 18MPa.

[0089] Step 3: Inject a solubilizing and synergistic agent. By mass percentage, the specific composition of the solubilizing and synergistic agent is 10% pentane, 8% hexane, 15% cyclopropane, 15% cyclopentane, 2% ethylene glycol monobutyl ether, 10% ethylene carbonate, and the balance is modified ethylene glycol ether. The injection volume of the solubilizing and synergistic agent is 400m 2 .

[0090] Step 4: Inject liquid carbon dioxide. The injection volume of carbon dioxide is calculated according to the cylinder model, and the displacement radius is calculated as 8m, Q = πR 2 h = 3.14 * 8 2 * 24.4 * 0.25 = 1225m 3 , and the construction displacement is 2 - 4m 3 / min, and the pumping pressure is 32MPa.

[0091] Step 5: Inject a surfactant, and the injection volume of the surfactant is 80m 2 . By mass percentage, the specific composition of the surfactant is 15% fatty alcohol polyoxyethylene ether, 15% tris(epoxyhexafluoropropane) amide-based sulfobetaine, and the balance is water.

[0092] After performing the above operations, resume polymer injection. After the injection pressure stabilizes, record the polymer injection pressure of the upper layer as 10.8 MPa. Since the injection pressure of the upper layer is lower than that of the lower layer, start the dissolving mechanism of the soluble bridge plug to resume the general polymer flooding process.

[0093] After performing the first to fifth steps, three more polymer absorption layers are added vertically in this well, the reservoir utilization degree increases from 23.2% to 61.1%, an increase of 37.9%. And two more effective directions are added horizontally. The stage production lasts for 153 days, and the well group increases oil production by 1255 tons. It is predicted that on the basis of the polymer flooding increasing the recovery rate by 10.3%, the recovery rate will be further increased by 5 - 8%.

[0094] The various embodiments of the present application may exist in the form of a range; it should be understood that the description in the form of a range is only for convenience and brevity, and should not be construed as a rigid limitation on the scope of the present application; therefore, it should be considered that the description of the range has specifically disclosed all possible sub - ranges and single values within that range. For example, the description of the range from 1 to 6 should be considered to have specifically disclosed sub - ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., and single numbers within the range, such as 1, 2, 3, 4, 5, and 6, regardless of the range. Additionally, whenever a numerical range is indicated herein, it means including any cited number (fraction or integer) within the indicated range.

[0095] In this application, unless otherwise specified, the orientation terms such as "upper" and "lower" specifically refer to the drawing directions in the attached drawings. Additionally, in the description of the specification of this application, the terms "include", "comprise", etc. mean "include but not limited to". Moreover, the term "include", "comprise" or any other variant thereof is intended to cover non-exclusive inclusion, such that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising..." does not exclude the presence of additional identical elements in the process, method, article or device comprising the said element. In this text, relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. In this text, "and / or" describes the associated relationship of associated objects and indicates that three relationships can exist. For example, A and / or B can mean: A exists alone, A and B exist simultaneously, or B exists alone. For the associated relationship of more than three associated objects described by "and / or", it means that these three associated objects can exist alone for any one of them, or any at least two of them exist simultaneously. For example, for A, and / or B, and / or C, it can mean that any one of A, B, and C exists alone, or any two of them exist simultaneously, or all three of them exist simultaneously. In this text, "at least one" means one or more, and "multiple" means two or more. "At least one kind", "at least one item (piece) below" or similar expressions refer to any combination of these items, including any combination of single item (piece) or plural items (pieces). For example, "at least one item (piece) among a, b, or c", or, "at least one item (piece) among a, b, and c" can both mean: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0096] The above are only specific embodiments of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be obvious to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application will not be limited to these embodiments shown herein, but rather to the broadest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for polymer flooding assisted by supercritical carbon dioxide, characterized in that The method of supercritical carbon dioxide-assisted polymer flooding is applicable to oil reservoirs in oil areas where the lower oil reservoirs are better developed than the upper oil reservoirs and there is uneven utilization, and includes the following steps: Lower a soluble bridge plug into the string so that the soluble bridge plug is located between the high polymer-absorbing layer in the lower part of the oilfield and the low polymer-absorbing layer in the upper part of the oilfield; Inject an acid plugging remover to clean the low polymer-absorbing layer; Sequentially inject a solubilization and synergistic agent, carbon dioxide, and a surfactant; The mass ratio of the injection amount of the carbon dioxide to the injection amount of the solubilization and synergistic agent is 1:0.3 - 0.7; By mass percentage, the surfactant is 3 - 5% of the sum of the injection amounts of the carbon dioxide and the solubilization and synergistic agent; The acid plugging remover includes at least one of hydrochloric acid, hydrofluoric acid, mud acid, fluoboric acid, formic acid, formic acid ammonium salt, acetic acid, acetic acid ammonium salt, citric acid, citric acid ammonium salt, carbonic acid, and phosphoric acid; The solubilization and synergistic agent includes: Alkanes; Cycloalkanes; Benzene and its derivatives; Carbonate enol esters; Alcohol ethers formed by condensing two diol molecules by removing one water molecule; The surfactant is at least one of fatty alcohol polyoxyethylene ether, nonylphenol polyoxyethylene ether, monoglyceryl stearate, and tris(epoxyhexafluoropropane amide) sulfobetaine; The pumping pressure of the carbon dioxide is greater than the formation fracture pressure.

2. The method for supercritical carbon dioxide-assisted polymer flooding according to claim 1, wherein In the step of lowering the soluble bridge plug into the string, at least two soluble bridge plugs connected to each other up and down are lowered.

3. The method for supercritical carbon dioxide-assisted polymer flooding according to claim 1, wherein The alkanes include at least one of pentane, hexane, heptane, octane, nonane, and decane; and / or, The cycloalkanes include at least one of cyclopropane, cyclobutane, cyclopentane, and cyclohexane; and / or, The benzene and its derivatives include at least one of benzene, toluene, o-xylene, m-xylene, p-xylene, and ethylbenzene; and / or, The carbonate enol esters include at least one of ethylene carbonate, propylene carbonate, and butylene carbonate; and / or, The alcohol ethers include at least one of ethylene glycol-butyl ether, diethylene glycol-butyl ether, butoxy triethylene glycol, and modified ethylene glycol ether.

4. The method of supercritical carbon dioxide-assisted polymer flooding according to claim 1, characterized in that, The carbon dioxide is in a liquid state.

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