A nano plugging drilling fluid system, a preparation method and application thereof

Abstract

The application discloses a kind of nano plugging drilling fluid systems and preparation method and application, belong to petroleum drilling fluid technical field.The raw material of the nano plugging drilling fluid system of the present application includes specific proportion bentonite, organic salt weighting agent, filtrate reducer, cation modified asphalt, coating inhibitor, film-forming solid wall agent, solid lubricant, plugging agent and barite, etc., after uniform mixing of each component, not only play original function, components can also synergistic effect, the prepared nano plugging drilling fluid system has strong compression effect on the adsorption diffusion double electric layer of clay particles, in addition to good inhibitory effect on clay dispersion, it also has outstanding plugging performance, lubricating property and good stability, strong anti-pollution capacity and other effects.The nano plugging drilling fluid system of the present application is suitable for drilling fluid, used for deep coalbed methane horizontal well drilling and plugging operation.

Description

Technical Field

[0001] This invention belongs to the field of petroleum drilling fluid technology, specifically a nano-sealing drilling fluid system, its preparation method, and its application. Background Technology

[0002] Currently, as exploration and development of deep coalbed methane horizontal wells under complex coal seam geological conditions deepens, the risk of coal seam collapse and stuck drill pipe increases. This is because the junctions of gray mudstone, gray fine sandstone, and black coal seams in deep coalbed methane horizontal wells are fragmented, and directional operations take longer, making large-scale mudstone collapses or leaks more likely. Therefore, a robust drilling fluid system for preventing collapse and stuck drilling is crucial. Furthermore, frequent drilling between the coal seam roof and floor leads to significant friction in the wellbore trajectory, placing higher demands on the lubrication performance of the drilling fluid system. In addition, commonly used drilling fluid systems still have many shortcomings.

[0003] Conventional water-based drilling fluid systems suffer from several drawbacks. Firstly, their simple composition, particularly high water content, makes them less stable under varying environmental temperatures, hindering their ability to stabilize the wellbore. Secondly, their susceptibility to contamination is limited; components may react with minerals in the wellbore (such as clay and gypsum), potentially causing wellbore delamination or instability, leading to unclean wells and ultimately contaminating oil and gas. In addition to conventional water-based systems, existing solids-free drilling fluid systems suffer from high costs and insufficient resistance to temperature and calcium buildup. For example, Chinese invention patent application CN104327810A discloses a solids-free, low-friction drilling fluid system. Its active ingredients include a thickener (carboxymethyl starch and polyanionic cellulose) and a shearing agent (xanthan gum). While this system exhibits low friction and good lubrication, its insufficient temperature resistance is a significant drawback. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention, based on in-depth experimental research and on-site construction verification, and leveraging the synergistic effect of specific proportions of raw material components, provides a nano-sealing drilling fluid system. This system enhances stability, anti-pollution properties, lubrication, and sealing and anti-collapse performance under complex environmental conditions such as temperature variations, thereby achieving the goals of safe construction and reservoir protection while meeting the drilling requirements of deep coalbed methane horizontal wells.

[0005] Another objective of this invention is to provide a method for preparing and applying the above-mentioned nano-sealing drilling fluid system, so as to achieve industrial production and better practical application results.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] This invention provides a nano-sealing drilling fluid system, the raw materials for which its effective components are prepared, by weight percentage: bentonite 2%–3.5%, potassium chloride 6%–8%, organic salt weighting agent 10%–25%, filtration loss reducer 1%–3%, cationic modified bitumen 1%–2%, coating inhibitor 0.2%–0.4%, film-forming wall-stabilizing agent 1%–3%, solid lubricant 2%–4%, ultrafine calcium carbonate 2%–3%, sealing agent 1.5%–2.5%, barite 20%–40%, and the balance being water;

[0008] The plugging agent is composed of micron-sized plugging agent and nano-sized plugging agent in a weight ratio of 1:0.8 to 1.5.

[0009] Bentonite is a basic material for preparing water-based drilling fluids, and its main mineral component is montmorillonite.

[0010] Barite is a non-metallic mineral product with barium sulfate (BaSO4) as its main component. When used as a weighting material, other weighting materials such as calcium salts, which are well known to those skilled in the art, can also be used. The dosage of barite and cationic modified bitumen can be adjusted by those skilled in the art as needed, depending on the well depth, formation, and lithology encountered during drilling.

[0011] As a limitation of the present invention, the bentonite is one or more of sodium-based bentonite or calcium-based bentonite.

[0012] As a further limitation of the present invention, the filtration loss reducing agent is a carboxyhydroxyalkane copolymer.

[0013] Preferably, the carboxyhydroxyalkene copolymer is carboxyhydroxyalkene copolymer Redu1, which is a small-molecule polymer formed by graft polymerization of high-temperature resistant vinyl monomers and raw materials such as cellulose.

[0014] As a further limitation of the present invention, the coating inhibitor is a high molecular weight coating inhibitor with a molecular weight ≥ 5,000,000.

[0015] As a further limitation of the present invention, the solid lubricant is a spherical solid lubricant obtained by crosslinking styrene and divinylbenzene.

[0016] As a further limitation of the present invention, the nano-blocking agent is a deformable polymer nano-polymer blocking agent.

[0017] As a further limitation of the present invention, the organic salt weighting agent is at least one of organic salt weighting agent Weigh2 and organic salt weighting agent Weigh3; the organic salt weighting agent is a complex of low carbon alkali metal organic acid salt, organic acid ammonium, and organic acid quaternary ammonium, which has high solubility in water and its saturated solution can reach a high density; the organic salt weighting agent contains reducing groups, which can remove dissolved oxygen and other oxidizing substances in drilling fluid, protect drilling tools from corrosion, and protect other easily oxidized treatment agents.

[0018] The coating inhibitor is the high molecular weight coating inhibitor IND-30;

[0019] The solid lubricant is solid lubricant HZN-102;

[0020] The cationic modified asphalt is cationic modified asphalt YRL-1.

[0021] Among them, the positively charged asphalt particles in cationic modified asphalt are easily adsorbed onto negatively charged solid particles, participate in the formation of mud cake, and improve the quality of mud cake. The positively charged particles can enter the micro-cracks in the well wall, generate adhesion and mutual aggregation, thereby playing the roles of sealing, bridging, preventing swelling, preventing collapse, reducing water loss and protecting oil and gas reservoirs. It has a strong ability to seal and prevent collapse and stabilize the well wall.

[0022] As a further limitation of the present invention, an acid-base adjuster is added to the system to bring the pH value to 8-10. The acid-base adjuster includes sodium hydroxide and acidic treatment agents such as tannin and lignite.

[0023] The present invention also provides a method for preparing the above-mentioned nano-sealing drilling fluid system, wherein the raw materials are weighed according to the weight percentage, added in order of increasing molecular weight, and stirred and mixed to obtain the nano-sealing drilling fluid system.

[0024] The present invention also provides an application of the above-mentioned nano-plugging drilling fluid system, wherein the nano-plugging drilling fluid system is used as a drilling fluid for drilling and plugging operations in deep coalbed methane horizontal wells.

[0025] By adopting the above-described technical solution, the beneficial effects achieved by this invention compared to the prior art are as follows:

[0026] (1) The nano-sealing drilling fluid system provided by the present invention has a simple formula, stable performance, and is easy to maintain. Moreover, it has stronger inhibition and can effectively inhibit the dispersion of purplish-red mudstone, avoiding large sections of mudstone collapse and leakage. It has high anti-collapse performance, keeps the wellbore clean in the horizontal section of the coal seam, and ensures smooth tripping and casing running. It has wide compatibility and can be combined with a variety of unconventional treatment agents without affecting the basic performance. It is suitable for deep coalbed methane horizontal wells.

[0027] (2) The nano-sealing drilling fluid system provided by this invention is based on the synergistic effect of specific proportions of raw material components. Among them, the organic salt weighting agent, when used together with the carboxyhydroxyalkane copolymer, stabilizes the well wall and significantly improves the anti-pollution performance. At the specific concentration of this invention, the electrolyte concentration of the organic salt weighting agent is higher than the salinity of water in the drill cuttings and well wall, causing water in the drill cuttings and well wall to flow into the drilling fluid, while water is difficult to penetrate into the drill cuttings and well wall. This effectively inhibits the hydration and dispersion of drill cuttings and cracks in the well wall. The electrolyte also compresses the clay double layer, making it difficult for drill cuttings to disperse into a stable colloid, maintaining low viscosity and good rheological properties, and reducing the content of submicron particles. This not only helps to improve the drilling rate, but the carboxyhydroxyalkane copolymer combined with the organic salt weighting agent also has good colloid protection ability. It can combine with clay or a small amount of dispersed drill cuttings to form a thin and dense mud cake, which greatly reduces filtration loss, stabilizes the well wall, and reduces oil and gas pollution caused by drill cutting dispersion.

[0028] The polymer coating inhibitor IND-30 inhibits drill cuttings dispersion and expansion, stabilizes the wellbore, and inhibits the hydration expansion and dispersion of mudstone cement in the reservoir. Together with the film-forming wall-stabilizing agent, it forms a dense film structure, improving the performance of inhibiting drill cuttings dispersion. At the same time, it reduces the entry of water molecules into the formation and increases the mutual cementation between minerals, playing a pressure-bearing and wall-protecting role, making clay difficult to disperse even after long-term soaking.

[0029] A composite plugging agent composed of two plugging agents with different particle sizes, micron-sized and nano-sized, is used to improve the pressure-bearing wall protection effect and prevent collapse. Specifically, the polymer in the nano-plugging agent can rapidly adapt to the layered structure and deform upon encountering formation fractures, exhibiting a large size matching window. The polymer chains of the nano-plugging agent partially extend to form a membrane structure, while the micron-sized particles in the micron-sized plugging agent embed into the membrane pores, making the membrane structure denser and preventing free water from passing through, thereby reducing pore pressure transmission and lowering the permeability of the mud cake. Furthermore, the nano-plugging agent has strong adsorption properties, a high specific surface area, and carries a large amount of charge, effectively inhibiting the decomposition of clay particles and preventing the migration of fine particles, further improving the pressure-bearing wall protection effect.

[0030] (3) The nano-plugging drilling fluid system provided by the present invention does not degrade at high temperature. The carboxyhydroxyalkane copolymer and nano-plugging agent also introduce high temperature resistant groups to improve the temperature resistance of the system and can be used normally at 150℃.

[0031] (4) The preparation method of the nano-plugging drilling fluid system provided by the present invention is simple, highly operable, suitable for industrial production, and has good inhibition performance, anti-pollution performance, plugging performance and lubrication performance, and has achieved good application results.

[0032] In summary, this invention is applicable as a drilling fluid for drilling and plugging operations in deep coalbed methane horizontal wells. Detailed Implementation

[0033] The preferred embodiments of the present invention will be described below. It should be understood that the preferred embodiments described herein are for illustrative purposes only and are not intended to limit the scope of the invention.

[0034] Example 1

[0035] This embodiment describes a nano-sealing drilling fluid system and its preparation method. The raw materials for its effective components, by weight percentage, are: 3% sodium bentonite, 6% potassium chloride, 10% organic salt weighting agent Weigh2, 3% carboxyhydroxyalkane copolymer Redu1 (filtration loss reducer), 1% cationic modified asphalt YRL-1, 0.3% polymer coating inhibitor IND-30, 2% film-forming wall-stabilizing agent LK-4, 2% solid lubricant HZN-102, and 2% ultrafine calcium carbonate, 2.5% sealing agent, 25% barite, and water to bring the total weight of the system to 100%.

[0036] The plugging agent consists of a micron-sized plugging agent and a deformable polymer nanoparticle plugging agent in a weight ratio of 1:1.

[0037] Among them, the molecular weight of the polymer-coated inhibitor IND-30 is ≥5,000,000;

[0038] HZN-102 is a spherical solid lubricant obtained by crosslinking styrene and divinylbenzene.

[0039] The above raw materials are added in order of increasing molecular weight, stirred and mixed, and sodium hydroxide and tannin are added until the pH value is 9 to obtain a nano-sealing drilling fluid system, which is referred to as Product 1.

[0040] The specific testing methods and results for the inhibition, plugging, and antifouling performance of Product 1 are as follows:

[0041] (a) Inhibition detection:

[0042] The inhibition performance of this product was verified by comparing the thickness of the adsorption-diffusion double layer of water and clay particles. Better inhibition performance is more beneficial to wellbore stability. The test results are shown in Table 1.

[0043] Table 1 Thickness of the adsorption-diffusion double layer of montmorillonite clay particles

[0044] sample water Product 1 Double layer thickness (nm) 98 1.253

[0045] As shown in Table 1, compared with the group containing water, the product of the present invention has a stronger compression effect, which significantly reduces the thickness of the double electric layer, accelerates sedimentation, and thus inhibits clay dispersion. It has good inhibition performance and is beneficial to well wall stability.

[0046] (II) Sealing performance testing:

[0047] Product 1 was subjected to a permeability plugging test under high temperature and high pressure conditions of 130℃ and 3.5MPa using a 400mD sand disc (average pore size 10μm). The filtrate volume was measured at intervals, and the test was continued for 30 minutes. The results are shown in Table 2.

[0048] Table 2 Filtrate volume at different times

[0049] Time T / min 1 5 7.5 15 25 30 Filtrate volume V / ml 0.2 0.6 1.0 1.6 2.0 2.1

[0050] As shown in Table 2, after a permeability plugging test, the filtrate volume of the product of this invention was only 2.1 ml after 30 minutes, which is significantly lower than the design requirement of 4.2 ml. This indicates that the composite plugging agent composed of two plugging agents with different particle sizes and the polymer coating inhibitor IND-30 work synergistically to effectively plug formation pores and form a dense mud cake. The nano-plugging drilling fluid system of this invention has a good plugging and anti-collapse effect.

[0051] (III) Anti-pollution performance testing:

[0052] Group setup: ① Product 1; ② Group with 0.3% calcium chloride added to Product 1 to simulate a high-salt environment; ③ Group with 10% 100-mesh core powder added to Product 1 to simulate a clastic environment, for a total of three groups.

[0053] Test methods: The following three indicators related to anti-fouling performance were tested under the following conditions: room temperature and 150℃ hot rolling for 16 hours: ρ - drilling fluid density; FV - Marvimeter funnel viscosity; FL - medium pressure water loss; GEI - initial and final shear; PV - plastic viscosity; YP - dynamic shear force; HTHP - high temperature and high pressure water loss (this indicator was only tested under high temperature hot rolling conditions).

[0054] The test results are shown in Table 3.

[0055] Table 3. Anti-pollution performance test conditions and results

[0056]

[0057] As shown in Table 3, the product of the present invention exhibits relatively small changes in various indicators under the conditions of 0.3% calcium chloride and 10% 100-mesh core powder, demonstrating good anti-pollution performance. At the same time, the indicators are relatively stable under the condition of 150℃, demonstrating good high-temperature resistance.

[0058] Comparative Example 1

[0059] This comparative example is a conventional water-based polymer drilling fluid system, referred to as Comparative Example 1. The raw materials used to make its effective components are the same, except that the weight of the cationic modified asphalt YRL-1 is 3 times that of Example 1.

[0060] The inhibition and lubrication properties of Product 1 and Comparative Example 1 were tested, as follows:

[0061] (1) Inhibition detection:

[0062] The inhibitory performance of the product was verified by conducting standard linear expansion experiments, high-temperature rolling recovery experiments, and water activity tests on Product 1 and Comparative Example 1. The test results are shown in Table 4.

[0063] Table 4 Results of Anti-inhibition Test

[0064]

[0065]

[0066] As shown in Table 4, compared with Comparative Example 1, Product 1 of the present invention has a higher rolling recovery rate and a lower water activity, indicating that it has better inhibition performance and is more conducive to wellbore stability. This shows that the cationic modified asphalt YRL-1 can achieve better inhibition by using the ratio of the present invention with other components.

[0067] (2) Lubricity test:

[0068] The lubrication performance of each group was verified by conducting torque simulation tests on three groups of products: water, Product 1, and Comparative Example 1. The torque of each group was measured three times, and the average torque was calculated. The percentage reduction in torque compared to water was calculated based on the average value. The test results are shown in Table 5.

[0069] Table 5 Lubricity test results

[0070]

[0071] As shown in Table 5, compared with the lubrication performance of Comparative Example 1, the torque reduction percentage of Product 1 of the present invention is increased by 7.52 percentage points, indicating that the nano-sealing drilling fluid system of the present invention has good lubrication performance and can reduce the occurrence of stuck drill bit problems.

[0072] Example 2

[0073] This embodiment is basically the same as Example 1, except that the proportion of raw materials used to make its effective components is different. Specifically, by weight percentage, it is: 2% sodium bentonite, 7.5% potassium chloride, 12% organic salt weighting agent Weigh2, 1% carboxyhydroxyalkane copolymer Redu1 (filtration loss reducer), 1% cationic modified asphalt YRL-1, 0.3% polymer coating inhibitor IND-30, 1% film-forming wall-fixing agent LK-4, 2% solid lubricant HZN-102, and 2% ultrafine calcium carbonate, 2.5% sealing agent, 40% barite, and water to make up to 100% of the total weight of the system.

[0074] The plugging agent consists of a micron-sized plugging agent and a deformable polymer nano-polymer plugging agent in a weight ratio of 1:1.2. The above raw materials are stirred and mixed thoroughly, and sodium hydroxide and tannin are added until the pH value reaches 9, thus obtaining a nano-plugging drilling fluid system.

[0075] Example 3

[0076] This embodiment is basically the same as Example 1, except that the proportion of raw materials used to make its effective components is different. Specifically, by weight percentage, it consists of: 3.5% of an equal mixture of sodium-based bentonite and calcium-based bentonite, 6.5% of potassium chloride, 15% of organic salt weighting agent Weigh2, 1.5% of carboxyhydroxyalkane copolymer Redu1 (filtration loss reducer), 1.5% of cationic modified asphalt YRL-1, 0.35% of polymer coating inhibitor IND-30, 3% of film-forming wall-fixing agent LK-4, 3.5% of solid lubricant HZN-102, and 2.8% ultrafine calcium carbonate, 1.5% sealing agent, 20% barite, and water to bring the total weight of the system to 100%.

[0077] The plugging agent consists of a micron-sized plugging agent and a deformable polymer nano-polymer plugging agent in a 1:1 weight ratio. The above raw materials are stirred and mixed thoroughly, and sodium hydroxide and tannin are added until the pH value reaches 8, thus obtaining a nano-plugging drilling fluid system.

[0078] Example 4

[0079] This embodiment is basically the same as Example 1, except that the proportion of raw materials used to make its effective components is different. Specifically, by weight percentage, it is: 2.8% calcium-based bentonite, 8% potassium chloride, 20% organic salt weighting agent Weigh2, 2% carboxyhydroxyalkane copolymer Redu1 (filtration loss reducer), 2% cationic modified asphalt YRL-1, 0.4% polymer coating inhibitor IND-30, 1.5% film-forming wall-fixing agent LK-4, 3% solid lubricant HZN-102, and 2.5% ultrafine calcium carbonate, 2% sealing agent, 35% barite, and water to make up to 100% of the total weight of the system.

[0080] The plugging agent consists of a micron-sized plugging agent and a deformable polymer nano-polymer plugging agent in a weight ratio of 1:1.5. The above raw materials are stirred and mixed thoroughly, and sodium hydroxide and tannin are added until the pH value reaches 10, thus obtaining a nano-plugging drilling fluid system.

[0081] Example 5

[0082] This embodiment is basically the same as Example 1, except that the proportion of raw materials used to make its effective components is different. Specifically, by weight percentage, it is: 3.2% bentonite, 7% potassium chloride, 25% organic salt weighting agent Weigh2, 3% carboxyhydroxyalkane copolymer Redu1 (filtration loss reducer), 1.8% cationic modified asphalt YRL-1, 0.2% polymer coating inhibitor IND-30, 2% film-forming wall-solidifying agent LK-4, 4% solid lubricant HZN-102, and 3% ultrafine calcium carbonate, 1.5% sealing agent, 25% barite, and water to make up to 100% of the total weight of the system.

[0083] The bentonite is composed of a mixture of sodium-based bentonite and calcium-based bentonite in a weight ratio of 1:0.7; the plugging agent is composed of a micron-sized plugging agent and a deformable polymer nanoparticle plugging agent in a weight ratio of 1:0.8.

[0084] The inhibition, anti-fouling, plugging, and lubrication properties of the nano-plugging drilling fluid systems prepared in Examples 2 to 5 were tested respectively. The results showed that, compared with Comparative Example 1, the method of the present invention is simple to prepare, highly operable, and suitable for industrial production. The drilling fluid system prepared has good inhibition, anti-fouling, plugging, and lubrication properties, and has outstanding anti-collapse and anti-sticking effects.

[0085] Example 6

[0086] This embodiment describes the application of Product 1 obtained in Embodiment 1. Specifically, Product 1 is used as a drilling fluid in the drilling operation of a horizontal well for deep coalbed methane exploration. The target area is prone to mudstone collapse and dispersion of purplish-red mudstone, and the well design places high demands on the drilling operation.

[0087] In practical application, the method effectively suppressed the dispersion of purplish-red mudstone, preventing large-scale mudstone collapse and leakage on-site. The wellbore in the horizontal section of the coal seam remained clean, and the tripping and casing running processes were smooth, successfully setting a new record for the first large-angle, seven-section, deep coalbed methane horizontal well in the region. This demonstrates that the nano-plugging drilling fluid system prepared by the method of this invention exhibits outstanding anti-collapse and anti-sticking properties, including strong inhibition, anti-fouling, and lubrication.

[0088] In other embodiments, those skilled in the art can adjust the proportions of raw materials in a nano-plugging drilling fluid system according to different lithologies, using the following amounts: bentonite 2%–3.5%, potassium chloride 6%–8%, organic salt weighting agent 10%–25%, filtration loss reducer 1%–3%, cationic modified bitumen 1%–2%, coating inhibitor 0.2%–0.4%, film-forming wall-stabilizing agent 1%–3%, solid lubricant 2%–4%, ultrafine calcium carbonate 2%–3%, plugging agent 1.5%–2.5%, barite 20%–40%, and the balance water, all of which yield good operational results.

[0089] Comparative Example 2

[0090] This comparative example is basically the same as Example 1, except that the proportions of the raw materials used to prepare its effective components are different. Specifically, the ratios of the carboxyhydroxyalkane copolymer Redu1 (filtration loss reducer), film-forming wall-stabilizing agent LK-4, solid lubricant HZN-102, and plugging agent, which were used in the ratio of 3:2:4:1.5 in Example 1, are changed to 8:3:6:1.5. The resulting drilling fluid system is designated as Comparative Example 2.

[0091] Comparative Example 3

[0092] This comparative example is basically the same as Example 1, except that the proportions of the raw materials used to prepare its effective components are different. Specifically, the ratio of the carboxyhydroxyalkane copolymer Redu1 (filtration loss reducer), film-forming wall-stabilizing agent LK-4, solid lubricant HZN-102, and plugging agent used in Example 1 (3:2:4:1.5) is changed to 1:4:4:5. The resulting drilling fluid system is designated as Comparative Example 3.

[0093] Comparative Examples 2 and 3 were subjected to inhibition tests, and the results were compared with those of Product 1. The test results are shown in Table 6.

[0094] Table 6 Results of Anti-inhibition Test

[0095] sample Expansion rate / mm Rolling recovery rate / % Water activity test results / aw Product 1 1.01 96.3 0.772 Comparative Example 2 1.12 89.7 0.953 Comparative Example 3 1.14 90.8 0.957

[0096] As shown in Table 6, compared with Comparative Examples 2 and 3, Product 1 of the present invention has a higher rolling recovery rate and a lower water activity, indicating that it has better inhibition performance and is more conducive to wellbore stability. This shows that the specific raw material composition of the present invention is well compatible, especially the synergistic effect of the filtration loss reducer, film-forming wall-stabilizing agent LK-4, solid lubricant HZN-102 and plugging agent. The addition ratio needs to be within a specific range for each component to achieve better inhibition.

[0097] It should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still modify the technical solutions described in the above embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A nano-sealing drilling fluid system, characterized in that: The raw materials used to make its effective components, by weight percentage, are: 2%–3.5% bentonite, 6%–8% potassium chloride, 10%–25% organic salt weighting agent, 1%–3% filtration loss reducer, 1%–2% cationic modified asphalt, 0.2%–0.4% coating inhibitor, 1%–3% film-forming wall-fixing agent, 2%–4% solid lubricant, 2%–3% ultrafine calcium carbonate, 1.5%–2.5% sealing agent, 20%–40% barite, and the balance being water; The plugging agent is composed of micron-sized plugging agent and nano-sized plugging agent in a weight ratio of 1:0.8 to 1.

5.

2. The nano-sealing drilling fluid system according to claim 1, characterized in that: The bentonite is one or more of sodium-based bentonite or calcium-based bentonite.

3. The nano-sealing drilling fluid system according to claim 2, characterized in that: The filtration loss reducer is a carboxyhydroxyalkane copolymer.

4. The nano-sealing drilling fluid system according to claim 3, characterized in that: The coating inhibitor is a high molecular weight coating inhibitor with a molecular weight ≥ 5,000,000.

5. The nano-sealing drilling fluid system according to claim 4, characterized in that: The solid lubricant is a spherical solid lubricant obtained by crosslinking styrene and divinylbenzene.

6. The nano-sealing drilling fluid system according to claim 5, characterized in that: The nano-blocking agent is a deformable polymer nano-polymer blocking agent.

7. The nano-sealing drilling fluid system according to any one of claims 1 to 6, characterized in that: The organic salt weighting agent is at least one of organic salt weighting agent Weigh2 and organic salt weighting agent Weigh3; The coating inhibitor is the high molecular weight coating inhibitor IND-30; The solid lubricant is solid lubricant HZN-102; The cationic modified asphalt is cationic modified asphalt YRL-1.

8. The nano-sealing drilling fluid system according to claim 7, characterized in that: An acid-base adjuster is added to the system to bring the pH value to 8-10.

9. A method for preparing the nano-plugging drilling fluid system according to any one of claims 1 to 8, characterized in that: Weigh the raw materials according to their weight percentage, add them in order of increasing molecular weight, and stir until well mixed to form the nano-sealing drilling fluid system.

10. The application of the nano-plugging drilling fluid system according to any one of claims 1 to 8, characterized in that: The nano-plugging drilling fluid system is used as a drilling fluid for drilling and plugging operations in deep coalbed methane horizontal wells.

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