Workover fluid suitable for fractured formation as well as preparation method and application of workover fluid

Abstract

The invention belongs to the technical field of petrochemical engineering, and particularly relates to workover fluid suitable for fractured formations and a preparation method and application thereof.The workover fluid is prepared from, by weight, 75-90 parts of water, 0.1-2 parts of polymer, 1.0-5 parts of temporary plugging agent, 0.5-3 parts of surfactant, 5.0-15 parts of density regulator, 0.5-3 parts of filtrate reducer, 0.5-2 parts of oil layer protective agent and 0.2-1 part of corrosion inhibitor. The workover fluid disclosed by the invention is low in preparation cost, relatively high in temperature resistance and salt tolerance, and relatively good in flowback blockage removal capability and oil layer protection effect.

Description

Technical Field

[0001] The invention belongs to the technical field of petrochemical industry, and in particular relates to a well repairing fluid suitable for fractured formations and a preparation method and application thereof. Background Art

[0002] After the development of oil and gas wells to the middle and late stages, the oil and gas layers of most oil and gas wells will experience energy loss. The main reason is that the number of leaking wells and leaking layers increases accordingly, and the pressure coefficient drops rapidly. Well repair operations are used to eliminate the formation blockage that occurs in oil and gas wells and restore the oil and gas wells to normal operating conditions. This is a conventional technical means to remove damage to oil and gas wells. Among them, the working fluid used in well repair operations is the workover fluid. The performance of the workover fluid not only affects the operating efficiency, but also has a direct impact on the degree of damage to the reservoir.

[0003] Although the workover fluid currently used in workover operations has solved some of the leakage problems, reservoir damage problems, and environmental protection problems, it still has the disadvantages of poor stability, weak pressure bearing capacity of the leakage layer, poor flowback capacity, and poor reservoir protection effect. It also has the problems of high preparation cost, complex on-site construction process, and poor environmental protection. Therefore, it is necessary to make improvements. Summary of the invention

[0004] In order to overcome the deficiencies of the above-mentioned prior art, the first object of the present invention is to provide a workover fluid suitable for fractured formations, which has low preparation cost, strong temperature and salt resistance, good backflow and plugging removal ability and oil layer protection effect; the second object is to provide a preparation method of the workover fluid suitable for fractured formations; and the third object is to provide the application of the workover fluid suitable for fractured formations.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows: The present invention firstly proposes a workover fluid suitable for fractured formations, which comprises the following components by weight: Water 75-90, Polymer 0.1-2, Temporary plugging agent 1.0-5, Surfactant 0.5-3, Density regulator 5.0-15, Fluid loss reducer 0.5-3, Oil layer protective agent 0.5-2, Corrosion inhibitor 0.2-1.

[0006] Furthermore, the polymer is one of polyacrylamide, acrylamide copolymer, hydroxyethyl cellulose and xanthan gum.

[0007] Preferably, the molecular weight of the polyacrylamide is 6 to 8 million; the molecular weight of the acrylamide copolymer is 7.5 to 8.5 million.

[0008] Furthermore, the temporary plugging agent is at least one of nano- or micro-sized calcium carbonate and nano- or micro-sized barium carbonate.

[0009] Preferably, the temporary plugging agent is a mixture of nano-sized calcium carbonate and micro-sized calcium carbonate, wherein the nano-sized calcium carbonate accounts for 20-30% and the micro-sized calcium carbonate accounts for 70-80%.

[0010] Furthermore, the temporary plugging agent is a mixture of nano-grade barium carbonate and micro-grade barium carbonate, wherein the nano-grade barium carbonate accounts for 20-30% and the micro-grade barium carbonate accounts for 70-80%.

[0011] Furthermore, the surfactant is one of sodium dodecyl polyoxyethylene ether sulfate, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfonate.

[0012] The fluid loss reducer is one of potassium polyacrylate and hydrolyzed polyacrylonitrile sodium salt; The corrosion inhibitor is one of dodecyl dimethyl benzyl ammonium chloride and imidazoline corrosion inhibitor.

[0013] The density regulator is one of sodium chloride, calcium chloride, sodium bromide, calcium bromide, potassium formate, sodium formate and cesium formate.

[0014] The oil layer protective agent is an oil-soluble petroleum resin protective agent.

[0015] Furthermore, the oil-soluble petroleum resin protective agent is a type of temporary plugging oil and gas layer protective agent.

[0016] The present invention further provides a method for preparing a workover fluid suitable for fractured formations, comprising the following steps: S1. Add 75-90 parts by weight of water to a container with a stirrer, add 0.1-2 parts by weight of a polymer while stirring, and stir until it is completely dissolved to form a polymer solution; S2. Slowly add 1-5 parts by weight of the temporary plugging agent to the polymer solution obtained in S1, and continue stirring until the temporary plugging agent is evenly dispersed in the polymer solution; then slowly add 0.5-3 parts by weight of the surfactant, and continue stirring until the mixture is uniform; S3. 5-15 parts by weight of a density modifier is added to the container in 3 batches and stirred to achieve the desired workover fluid tightness; S4 continue to add 0.5-2 parts by weight of the oil layer protective agent, stir evenly, so that it can fully play the role of protecting the oil layer; and then add 0.5-3 parts by weight of the fluid loss agent, continue to stir evenly; S5. Finally, add 0.2-1 parts by weight of corrosion inhibitor and stir evenly to obtain a workover fluid suitable for fractured formations.

[0017] In step S1, the polymer should be added dropwise within 10 minutes; In S2, the dropping speed of the temporary plugging agent is 30-40 drops / min, and the dropping speed of the surfactant is 30-40 drops / min.

[0018] The present invention also proposes an application of a workover fluid suitable for fractured formations, including the above-mentioned workover fluid suitable for fractured formations or the workover fluid suitable for fractured formations prepared by the above-mentioned preparation method. The environmentally friendly workover fluid is suitable for use in workover operations in fractured old well formations. By adopting the above technical solution, the present invention has the following beneficial effects: 1. Environmental protection: The well repair fluid suitable for fractured formations prepared by the present invention is a water-based well repair fluid with water as the main component. Compared with oil-based well repair fluid, it has less pollution to the environment.

[0019] 2. Low cost: The cost of materials such as water, polymers, and density regulators is relatively low, which can reduce the cost of well repair operations.

[0020] 3. Stable performance: The present invention prepares a well repairing fluid with stable performance by reasonably selecting and adjusting the proportion of each component to meet different formation conditions and operation requirements.

[0021] 4. Protect the reservoir: Ingredients such as filtration reducers and oil and gas layer protectors can reduce the damage of workover fluid to the reservoir and improve oil and gas recovery.

[0022] The above description is only an overview of the technical solution of the present invention. In order to more clearly understand the technical means of the present invention and implement it according to the contents of the specification, the preferred embodiments of the present invention are described in detail as follows. DETAILED DESCRIPTION

[0023] The content of the present invention can be further understood by combining the following detailed description of the preferred implementation method of the present invention and the embodiments included. Unless otherwise stated, all technical and scientific terms used in this article have the same meaning as those of ordinary skill in the art to which the present invention belongs. If the definition of a specific term disclosed in the prior art is inconsistent with any definition provided in the present invention, the definition of the term provided in the present invention shall prevail.

[0024] In a typical embodiment of the present application, a workover fluid suitable for fractured formations is provided, which comprises the following components in parts by weight: Water 75-90, Polymer 0.1-2, Temporary plugging agent 1.0-5, Surfactant 0.5-3, Density regulator 5.0-15, Fluid loss reducer 0.5-3, Oil layer protective agent 0.5-2, Corrosion inhibitor 0.2-1.

[0025] In some preferred embodiments of the present invention, the polymer is one of polyacrylamide, acrylamide copolymer, hydroxyethyl cellulose and xanthan gum. The molecular weight of the polyacrylamide is preferably 6 to 8 million; the molecular weight of the acrylamide copolymer is preferably 7.5 to 8.5 million.

[0026] As a further improvement to the present invention, the temporary plugging agent is at least one of nanometer or micrometer calcium carbonate and nanometer or micrometer barium carbonate.

[0027] In some preferred embodiments of the present invention, the temporary plugging agent is a mixture of nano-sized calcium carbonate and micro-sized calcium carbonate, wherein the nano-sized calcium carbonate accounts for 20-30% and the micro-sized calcium carbonate accounts for 70-80%.

[0028] In other preferred embodiments of the present invention, the temporary plugging agent is a mixture of nano-sized barium carbonate and micro-sized barium carbonate, wherein the nano-sized barium carbonate accounts for 20-30% and the micro-sized barium carbonate accounts for 70-80%.

[0029] In some preferred embodiments of the present invention, the surfactant is sodium dodecyl polyoxyethylene ether sulfate. The fluid loss reducer is one of potassium polyacrylate and hydrolyzed polyacrylonitrile sodium salt. The corrosion inhibitor is one of dodecyl dimethyl benzyl ammonium chloride and imidazoline corrosion inhibitor. The density regulator is one of sodium chloride, calcium chloride, sodium bromide, calcium bromide, potassium formate, sodium formate and cesium formate.

[0030] In some preferred embodiments, the oil layer protective agent is an oil-soluble petroleum resin protective agent.

[0031] In a further preferred embodiment, the oil-soluble petroleum resin protective agent is a type of temporary plugging oil and gas layer protective agent.

[0032] In another typical embodiment of the present application, a method for preparing a workover fluid suitable for a fractured formation is provided, comprising the following steps: S1. Add 75-90 parts by weight of water into a container with a stirrer, and slowly add 0.1-2 parts by weight of a polymer while stirring. It is worth mentioning that the polymer is preferably added dropwise within 10 minutes and stirred until it is completely dissolved to form a polymer solution; S2. 1-5 parts by weight of the temporary plugging agent is added dropwise to the polymer solution obtained in S1 at a rate of 30-40 drops / min, and stirring is continued until the temporary plugging agent is uniformly dispersed in the polymer solution; then 0.5-3 parts by weight of the surfactant is slowly added at a rate of 30-40 drops / min, and stirring is continued until the mixture is uniform; S3. 5-15 parts by weight of a density modifier is added to the container in 3 batches and stirred to achieve the desired workover fluid tightness; S4 continue to add 0.5-2 parts by weight of the oil layer protective agent, stir evenly, so that it can fully play the role of protecting the oil layer; and then add 0.5-3 parts by weight of the fluid loss agent, continue to stir evenly; S5. Finally, add 0.2-1 parts by weight of corrosion inhibitor and stir evenly to obtain a workover fluid suitable for fractured formations.

[0033] In another typical embodiment of the present application, an application of a workover fluid suitable for fractured formations is proposed. The environmentally friendly drilling fluid is suitable for use in workover operations in fractured old well formations. The present invention uses water as the basic solvent to provide a medium for dispersion and dissolution of other components. The polymer has the functions of thickening, improving rheology and improving suspension ability. It can increase the viscosity of the liquid, make various components evenly distributed, and control the flow characteristics of the liquid in specific applications. Temporary plugging agents are used to temporarily block pores and cracks in the formation to prevent excessive loss of the working fluid and damage to the formation. After the operation is completed, the temporary plugging agent can be easily unblocked to restore the permeability of the formation. Surfactants can reduce the tension of the liquid on the rock surface, promote the rapid discharge of the working fluid from the formation, and improve production efficiency. It can also prevent the water phase in the formation from blocking the pores and keep the passage of oil and gas flow unobstructed. Density regulator (water-soluble salt): By adjusting the type and content of water-soluble salt, the density of the formula can be accurately controlled to adapt to different formation pressures and operation requirements. Fluid loss reducer: It can effectively reduce the filtration loss of the working fluid when passing through the formation, reduce the penetration of the liquid into the formation, and maintain the performance and effective ingredients of the working fluid. Corrosion inhibitor: used to prevent the chemical ingredients in the formula from corroding metal equipment and pipelines, extending the service life of equipment and reducing maintenance costs.

[0034] The composition, preparation process and application effect of the well repairing fluid prepared are exemplarily described below in combination with some preferred embodiments.

[0035] It should be noted that the implementation conditions used in the examples can be further adjusted according to the specific experimental environment, and the implementation conditions not specified are usually the conditions in conventional experiments. The various chemical reagents and chemicals mentioned in the following examples are commercially available unless otherwise specified. For example: the oil layer protectant can be selected from the oil-soluble petroleum resin CBJ produced by Hubei Changda Technology Development Co., Ltd.; the imidazoline corrosion inhibitor can be selected from the imidazoline corrosion inhibitor model HCD-104 produced by Hubei Changda Technology Development Co., Ltd.; the filtration loss reducer can be selected from the hydrolyzed polyacrylonitrile sodium salt (NaPAN) produced by Shandong Juxin New Materials Co., Ltd., etc. Example 1 This embodiment relates to a method for preparing a workover fluid suitable for fractured formations, comprising: First, select the components according to the following weight parts: 85 parts of water, 0.1 parts of polyacrylamide, 1 part of temporary plugging agent, 0.5 parts of surfactant sodium dodecyl polyoxyethylene ether sulfate, 9 parts of density regulator sodium chloride, 2 parts of filtrate reducer potassium polyacrylate, 1.5 parts of oil layer protective agent oil-soluble petroleum resin, and 0.9 parts of imidazoline corrosion inhibitor.

[0036] The temporary plugging agent is a mixture of nano-scale calcium carbonate and micro-scale calcium carbonate, with nano-scale calcium carbonate accounting for 20~30% and micro-scale calcium carbonate accounting for 70~80%.

[0037] According to the above ratio, water is injected into a container with a stirrer. While stirring, polyacrylamide or acrylamide copolymer is slowly added to avoid agglomeration of polyacrylamide or acrylamide copolymer, and stirred until it is completely dissolved to form a polymer solution. The temporary plugging agent is slowly added to the polymer solution and continued to be stirred to ensure that the temporary plugging agent is evenly dispersed in the polymer solution; then sodium dodecyl polyoxyethylene ether sulfate is slowly added and continued to be stirred to fully mix the sodium dodecyl polyoxyethylene ether sulfate with the polymer solution to ensure that it plays its role of drainage and waterproof lock in the operation. Sodium chloride is added to the container in 3 batches and stirred evenly to achieve the required workover fluid density. After the addition of sodium chloride is completed, oil-soluble petroleum resin is added and stirred evenly to fully play its role in protecting the oil layer; then potassium polyacrylate is added and stirred evenly to reduce the filtration loss of the workover fluid during use; finally, imidazoline corrosion inhibitor is added and stirred evenly to ensure that the imidazoline corrosion inhibitor is evenly distributed in the workover fluid to prevent corrosion, and a workover fluid A suitable for fractured formations is obtained.

[0038] Example 2 This embodiment relates to a method for preparing a workover fluid suitable for fractured formations, comprising: First, select the components according to the following weight parts: 90 parts of water, 0.5 parts of hydroxyethyl cellulose, 1 part of temporary plugging agent, 1.5 parts of surfactant sodium dodecyl polyoxyethylene ether sulfate, 5 parts of density regulator calcium chloride, 0.5 parts of filtrate reducer potassium polyacrylate, 1 part of oil-soluble petroleum resin, and 0.5 parts of imidazoline corrosion inhibitor.

[0039] The temporary plugging agent is a mixture of nano-grade barium carbonate and micron-grade barium carbonate, with nano-grade barium carbonate accounting for 20~30% and micron-grade barium carbonate accounting for 70~80%.

[0040] According to the above ratio, water is injected into a container with a stirrer. While stirring, hydroxyethyl cellulose is slowly added to avoid caking of hydroxyethyl cellulose, and stirred until it is completely dissolved to form a polymer solution. The temporary plugging agent is slowly added to the polymer solution and continued to be stirred to ensure that the temporary plugging agent is evenly dispersed in the polymer solution; then sodium dodecyl polyoxyethylene ether sulfate is slowly added and continued to be stirred to fully mix the sodium dodecyl polyoxyethylene ether sulfate with the polymer solution to ensure that it plays its role of drainage and waterproof lock in the operation. Calcium chloride is added to the container in 3 batches and stirred evenly to achieve the required workover fluid density. After the addition of sodium chloride is completed, oil-soluble petroleum resin is added and stirred evenly to fully play its role in protecting the oil layer; then potassium polyacrylate is added and stirred evenly to reduce the filtration loss of the workover fluid during use; finally, imidazoline corrosion inhibitor is added and stirred evenly to ensure that the imidazoline corrosion inhibitor is evenly distributed in the workover fluid to prevent corrosion. Workover fluid B suitable for fractured formations is obtained.

[0041] Example 3 This embodiment relates to a method for preparing a workover fluid suitable for fractured formations, comprising: First, select the components according to the following weight parts: 80 parts of water, 1.5 parts of hydroxyethyl cellulose, 5 parts of temporary plugging agent, 3 parts of surfactant sodium dodecyl polyoxyethylene ether sulfate, 8 parts of density regulator calcium bromide, 1.5 parts of fluid loss reducer hydrolyzed polyacrylonitrile sodium salt, 0.5 parts of oil-soluble petroleum resin, and 0.5 parts of dodecyl dimethyl benzyl ammonium chloride.

[0042] The temporary plugging agent is a mixture of nano-scale calcium carbonate and micron-scale calcium carbonate, with nano-scale calcium carbonate accounting for 20~30% and micron-scale calcium carbonate accounting for 70~80%.

[0043] According to the above ratio, water is injected into a container with a stirrer. While stirring, hydroxyethyl cellulose is slowly added to avoid caking of hydroxyethyl cellulose, and stirred until it is completely dissolved to form a polymer solution. The temporary plugging agent is slowly added to the polymer solution and continued to be stirred to ensure that the temporary plugging agent is evenly dispersed in the polymer solution; then sodium dodecyl polyoxyethylene ether sulfate is slowly added and continued to be stirred to fully mix the sodium dodecyl polyoxyethylene ether sulfate with the polymer solution to ensure that it plays its role of drainage and waterproof lock in the operation. Calcium chloride is added to the container in 3 batches and stirred evenly to achieve the required well repair fluid density. After the addition of calcium bromide is completed, oil-soluble petroleum resin is added and stirred evenly to fully play its role in protecting the oil layer; then hydrolyzed polyacrylonitrile sodium potassium is added and stirred evenly to reduce the filtration loss of the well repair fluid during use; finally, dodecyl dimethyl benzyl ammonium chloride is added and stirred evenly to ensure that dodecyl dimethyl benzyl ammonium chloride is evenly distributed in the well repair fluid to prevent corrosion. Well repair fluid C suitable for fractured formations is obtained.

[0044] Example 4 This embodiment relates to a method for preparing a workover fluid suitable for fractured formations, comprising: First, select the components according to the following weight parts: 75 parts of water, 1.0 part of xanthan gum, 5 parts of temporary plugging agent, 3 parts of surfactant sodium dodecyl polyoxyethylene ether sulfate, 10 parts of density regulator potassium formate, 3 parts of fluid loss reducer hydrolyzed polyacrylonitrile sodium salt, 2 parts of oil layer protective agent (oil-soluble petroleum resin), 1.0 part of dodecyl dimethyl benzyl ammonium chloride.

[0045] The temporary plugging agent is a mixture of nano-scale calcium carbonate and micron-scale calcium carbonate, with nano-scale calcium carbonate accounting for 20~30% and micron-scale calcium carbonate accounting for 70~80%.

[0046] According to the above ratio, water is injected into a container with a stirrer. While stirring, xanthan gum is slowly added to avoid agglomeration of xanthan gum, and stirred until it is completely dissolved to form a polymer solution. The temporary plugging agent is slowly added to the polymer solution and continued to be stirred to ensure that the temporary plugging agent is evenly dispersed in the polymer solution; then sodium dodecyl polyoxyethylene ether sulfate is slowly added and continued to be stirred to fully mix the sodium dodecyl polyoxyethylene ether sulfate with the polymer solution to ensure that it plays its role of drainage and waterproof lock in the operation. Potassium formate is added to the container in 3 batches and stirred evenly to achieve the required well repair fluid density. After potassium formate is added, oil-soluble petroleum resin is added and stirred evenly to fully play its role in protecting the oil layer; then hydrolyzed polyacrylonitrile sodium potassium is added and stirred evenly to reduce the filtration loss of the well repair fluid during use; finally, dodecyl dimethyl benzyl ammonium chloride or imidazoline corrosion inhibitor is added and stirred evenly to ensure that dodecyl dimethyl benzyl ammonium chloride or imidazoline corrosion inhibitor is evenly distributed in the well repair fluid to prevent corrosion. The well repair fluid D suitable for fractured formations is obtained.

[0047] Well repair fluid performance: (1) Rheological properties test The rheological properties of the workover fluid A prepared in Example 1 before and after high-temperature aging were tested using a ZNN-D6 six-speed rotation viscometer, including the rheological properties before aging and after aging at 160°C for different aging times. The results are shown in Table 1.

[0048] Table 1 Rheological properties of workover fluid system

[0049] The results in Table 1 show that after aging at high temperature for 3 days, the 3 The density of the workover fluid system changes little with the test temperature, and the rheological properties of workover fluid systems with different densities do not change significantly. In other words, the green, environmentally friendly and low-damage workover fluid system has excellent heat resistance and stability, and is suitable for deep reservoirs, high temperature and high pressure formations; at the same time, the workover fluid system has good surface fluidity and low temperature rheology, and is suitable for operations in cold and harsh climate areas.

[0050] (2) Surface tension / interfacial tension test Take 1000 ml of the green, environmentally friendly and low-damage workover fluid system prepared in Examples 1-3, respectively, with densities of 1.20, 1.4 and 1.6 g / cm 3 , standby; debug the fully automatic surface / interface tension meter; test the gas-liquid surface tension and oil-liquid interfacial tension of the green, environmentally friendly and low-damage workover fluid system.

[0051] Table 2 Well repair fluid system and kerosene / interfacial tension

[0052] According to the data in the above table, it can be seen that the gas-liquid surface tension and oil-liquid interfacial tension of the new workover fluid system are relatively low, which greatly reduces the capillary resistance, thereby facilitating the return of the workover fluid and restoring the production capacity of oil and gas wells.

[0053] (3) Oil and gas layer protection effect test The HTHP dynamic damage evaluation device was used to evaluate the change of the permeability of the core in the field by the new workover fluid system, so as to evaluate the protective effect of the workover fluid on the reservoir. The experimental results are shown in Table 3. It can be seen from the table that the permeability recovery value of the core by the new workover fluid system is ≥90%, and the reservoir protection effect is excellent.

[0054] Table 3 Effect of workover fluid on reservoir protection

[0055] It should be noted that the above preparation method is only an example. After the preparation of the above embodiments is completed, the various performance indicators of the well repair fluid need to be tested, including density, viscosity, filtration loss, corrosiveness, biological toxicity, reservoir protection performance, etc.; at the same time, the predetermined performance standards are compared to check whether the well repair fluid meets the requirements. If not, the reasons need to be analyzed and corresponding adjustments need to be made. If the test results show that the performance of the well repair fluid does not meet the standards, for example, the density is too high or too low, the amount of density regulator can be appropriately increased or decreased, and the mixture can be re-stirred and tested until it meets the standards.

[0056] The above description is only a preferred embodiment of the present invention, which is merely illustrative of the present invention and does not limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made by ordinary technicians in this field to the technical solution of the present invention should fall within the scope of protection determined by the claims of the present invention.

Claims

1. A workover fluid suitable for fractured formations, characterized in that: The well repairing fluid comprises the following components by weight: Water 75-90, Polymer 0.1-2, Temporary plugging agent 1.0-5, Surfactant 0.5-3, Density regulator 5.0-15, Fluid loss reducer 0.5-3, Oil layer protective agent 0.5-2, Corrosion inhibitor 0.2-1.

2. The well repairing fluid suitable for fractured formations according to claim 1, characterized in that: The polymer is one of polyacrylamide, acrylamide copolymer, hydroxyethyl cellulose and xanthan gum.

3. The well repairing fluid suitable for fractured formations according to claim 2, characterized in that: The molecular weight of the polyacrylamide is 6 to 8 million; the molecular weight of the acrylamide copolymer is 7.5 to 8.5 million.

4. The well repairing fluid suitable for fractured formations according to claim 1, characterized in that: The temporary plugging agent is at least one of nanometer or micrometer calcium carbonate and nanometer or micrometer barium carbonate.

5. The well repairing fluid suitable for fractured formations according to claim 1, characterized in that: The temporary plugging agent is a mixture of nano-scale calcium carbonate and micro-scale calcium carbonate, wherein the nano-scale calcium carbonate accounts for 20-30% and the micro-scale calcium carbonate accounts for 70-80%.

6. The well repairing fluid suitable for fractured formations according to claim 1, characterized in that: The temporary plugging agent is a mixture of nano-grade barium carbonate and micro-grade barium carbonate, wherein the nano-grade barium carbonate accounts for 20-30% and the micro-grade barium carbonate accounts for 70-80%.

7. The well repairing fluid suitable for fractured formations according to claim 1, characterized in that: The surfactant is one of sodium dodecyl polyoxyethylene ether sulfate, sodium dodecylbenzene sulfonate, and sodium dodecyl sulfonate; The density regulator is one of sodium chloride, calcium chloride, sodium bromide, calcium bromide, potassium formate, sodium formate and cesium formate; The fluid loss reducer is one of potassium polyacrylate and hydrolyzed polyacrylonitrile sodium salt; The corrosion inhibitor is one of dodecyl dimethyl benzyl ammonium chloride and imidazoline corrosion inhibitor; The oil layer protective agent is an oil-soluble petroleum resin protective agent.

8. A method for preparing a workover fluid suitable for fractured formations according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1. Add 75-90 parts by weight of water to a container with a stirrer, add 0.1-2 parts by weight of a polymer while stirring, and stir until it is completely dissolved to form a polymer solution; S2. Add 1-5 parts by weight of the temporary plugging agent to the polymer solution obtained in S1, and continue stirring until the temporary plugging agent is evenly dispersed in the polymer solution; then add 0.5-3 parts by weight of the surfactant, and continue stirring until the mixture is uniform; S3. 5-15 parts by weight of a density modifier was added to the container in 3 batches and stirred to achieve the desired density of the workover fluid; S4 continue to add 0.5-2 parts by weight of the oil layer protective agent, stir evenly, so that it can fully play the role of protecting the oil layer; and then add 0.5-3 parts by weight of the fluid loss agent, continue to stir evenly; S5. Finally, add 0.2-1 parts by weight of corrosion inhibitor and stir evenly to obtain a workover fluid suitable for fractured formations.

9. The method for preparing a workover fluid suitable for fractured formations according to claim 1, characterized in that: In step S1, the polymer should be added dropwise within 10 minutes; In S2, the dropping speed of the temporary plugging agent is 30-40 drops / min, and the dropping speed of the surfactant is 30-40 drops / min.

10. An application of a workover fluid suitable for fractured formations, characterized in that: The invention comprises the workover fluid suitable for fractured formations as described in any one of claims 1 to 7 or the workover fluid suitable for fractured formations prepared by the preparation method as described in any one of claims 8 to 9, and the workover fluid is suitable for use in workover operations in fractured old well formations.

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