Preparation method and application of a kind of ocean drilling fluid inner rigid outer soft core-shell structure silicon dioxide blocking agent

Abstract

This invention relates to a method for preparing and applying a core-shell structured silica plugging agent with a rigid inner structure and a flexible outer structure for marine drilling fluids, belonging to the field of deepwater oil and gas drilling. This invention uses N-vinyl-2-pyrrolidone, dimethyl diallyl ammonium chloride, acrylamide, and 2-acrylamide-2-methylpropanesulfonic acid polymerized and grafted onto the surface of nano-silica to form a core-shell structured silica plugging agent with a rigid inner structure and a flexible outer structure. By utilizing the rigid plugging of nanomaterials and the flexible plugging of polymer deformation, this method effectively seals the micro- and nano-pores of shale and mudstone, thereby maintaining wellbore stability. Simultaneously, it effectively inhibits the secondary formation of hydrates, improving the safety and efficiency of deepwater drilling. Furthermore, the plugging agent of this invention reduces the need for large amounts of thermodynamic inhibitors, saving costs and possessing environmentally friendly properties.

Description

Technical Field

[0001] This invention relates to a method for preparing and applying a core-shell structured silica plugging agent with an inner rigid and outer flexible structure for use in marine drilling fluids, belonging to the field of deepwater oil and gas drilling. Background Technology

[0002] In recent years, with the exploration and development of onshore oil resources reaching a relatively high level, new reserves and production are insufficient to meet the ever-increasing energy demand, making the exploration and development of offshore oil and gas resources a focus of attention. Deep-sea areas, in particular, have become a crucial area for global oil exploration. Statistics show that nearly half of the world's major oil and gas discoveries in recent years have come from deep-sea regions, demonstrating the importance and potential of deep-sea exploration. The Gulf of Mexico, Brazilian waters, West African waters, and the South China Sea are currently the most promising deep-water oil and gas areas. Among them, the South China Sea accounts for one-third of the country's total oil and gas resources, with approximately 70% located in deep water. These data indicate that the exploration and development of deep-sea oil and gas resources is not only a focus of the oil industry but also a cutting-edge field of scientific research. Deep-water drilling environments are more severe than onshore and shallow-sea drilling, and the technical challenges are more complex. These challenges include wellbore instability, wellbore blockage caused by secondary formation of natural gas hydrates, wellbore collapse, drilling fluid thickening under low-temperature conditions, and difficulties in wellbore cleaning. Deepwater formations are typically undercompacted, poorly cemented, and contain abundant shale and micropores, making drilling more complex and challenging. Insufficient drilling fluid inhibition, plugging, or wall-building properties can lead to formation instability and frequent lost circulation. Therefore, effective strategies are needed to plug the micro- and nanopores in deep cement and shale formations, and appropriate particle size plugging materials must be selected to ensure wellbore stability.

[0003] Currently, the application of nanotechnology in drilling has driven the rapid development of the oil exploration industry. Nanomaterials can seal microfractures and micropores in shale and mudstone, thereby reducing permeability and pressure transmission, enhancing formation bearing capacity, and improving wellbore stability. The application of nanoparticles in drilling fluids can not only reduce formation damage but also enhance rheological properties and maintain wellbore stability. However, the silanol groups on the surface of nanoparticles are prone to polymerization, making them difficult to disperse in drilling fluids and thus reducing overall performance. To address this issue, chemical modification can significantly improve the dispersibility of nanoparticles in drilling fluids.

[0004] During deepwater drilling, seabed temperatures are often extremely low, sometimes even below 0°C. Simultaneously, the hydrostatic pressure in deepwater environments is extremely high, reaching over 30 MPa. This low-temperature, high-pressure environment easily leads to the secondary formation of natural gas hydrates in the drilling fluid. These hydrates can clog pipelines, blowout preventers, and other critical equipment, severely impacting drilling operations. Hydrate blockage not only interrupts drilling operations but also increases the risk of equipment failure and may even trigger safety accidents, posing a significant threat to the entire drilling process. Therefore, the secondary formation of natural gas hydrates under low-temperature, high-pressure environments in deepwater drilling is a pressing problem. To address these issues, a plugging agent suitable for low-temperature, high-pressure environments is needed, capable of sealing micro- and nano-pores and inhibiting hydrate formation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a method for preparing and applying a core-shell structured silica plugging agent with an inner rigid and outer flexible structure for marine drilling fluids.

[0006] This invention employs N-vinyl-2-pyrrolidone, dimethyl diallyl ammonium chloride, acrylamide, and 2-acrylamide-2-methylpropanesulfonic acid polymerized and grafted onto the surface of nano-silica to form a core-shell structure silica plugging agent with a rigid inner core and a flexible outer core. By combining the rigid plugging effect of nanomaterials with the flexible plugging effect of polymer deformation, this method effectively seals the micro- and nano-pores of shale, maintaining wellbore stability and effectively inhibiting secondary hydrate formation, thus improving the safety and efficiency of deepwater drilling. Furthermore, the plugging agent of this invention reduces the need for large amounts of thermodynamic inhibitors, saving costs and possessing environmentally friendly properties.

[0007] The technical solution of the present invention is as follows:

[0008] A method for preparing a core-shell structured silica plugging agent with an inner rigid and outer flexible structure for marine drilling fluids includes the following steps:

[0009] Aqueous solutions of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, dimethyldiallylammonium chloride, N-vinyl-2-pyrrolidone, and KH570 modified nano-silica were mixed evenly. The pH was adjusted, and an initiator aqueous solution was added dropwise under a protective atmosphere and stirring. The mixture was stirred and reacted under a protective atmosphere, and then washed, dried, and pulverized to obtain a core-shell structured silica plugging agent for marine drilling fluids.

[0010] According to a preferred embodiment of the present invention, the mass concentration of the acrylamide aqueous solution is 7-13 wt%.

[0011] According to a preferred embodiment of the present invention, the mass concentration of the aqueous solution of 2-acrylamido-2-methylpropanesulfonic acid is 19~21wt%.

[0012] According to a preferred embodiment of the present invention, the mass concentration of the aqueous solution of dimethyl diallyl ammonium chloride is 60 wt%.

[0013] According to a preferred embodiment of the present invention, the mass concentration of the KH570 modified nano silica aqueous solution is 4~8wt%.

[0014] According to a preferred embodiment of the present invention, the particle size of KH570 modified nano-silica is 10-20 nm.

[0015] According to a preferred embodiment of the present invention, the molar ratio of N-vinyl-2-pyrrolidone, dimethyl diallyl ammonium chloride, 2-acrylamido-2-methylpropanesulfonic acid and acrylamide is (3~1):(2~1):(4~2):(3~6).

[0016] According to a preferred embodiment of the present invention, the mass ratio of N-vinyl-2-pyrrolidone to KH570 modified nano-silica is (1-5):(2-6).

[0017] According to a preferred embodiment of the present invention, the mixing is carried out under stirring conditions, with a stirring rate of 200-400 rpm, a stirring time of 15-30 minutes, and a mixing temperature of 20-30°C.

[0018] According to the present invention, the protective atmosphere is preferably nitrogen or argon.

[0019] According to a preferred embodiment of the present invention, the pH is adjusted to 7.

[0020] According to a preferred embodiment of the present invention, the mass concentration of the initiator aqueous solution is 2%-15%; the initiator is a redox compound initiator of ammonium persulfate and sodium bisulfite, wherein the mass ratio of ammonium persulfate to sodium bisulfite is 1:1.

[0021] According to a preferred embodiment of the present invention, the mass of the initiator is 0.1%-1% of the total mass of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, dimethyldiallylammonium chloride, N-vinyl-2-pyrrolidone and KH570 modified nano silica.

[0022] More preferably, the mass of the initiator is 0.1%-0.5% of the total mass of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, dimethyldiallylammonium chloride, N-vinyl-2-pyrrolidone and KH570 modified nano silica.

[0023] According to a preferred embodiment of the present invention, the initiator aqueous solution is added at a rate of one drop every 2-4 seconds.

[0024] According to a preferred embodiment of the present invention, after the initiator is added dropwise, the stirring reaction temperature is 55~85℃, the stirring speed is 100-400rpm, and the stirring reaction time is 6h.

[0025] According to a preferred embodiment of the present invention, the washing is performed using a mixture of anhydrous ethanol and acetone; the drying temperature is 55-85°C.

[0026] This invention uses N-vinyl-2-pyrrolidone, dimethyl diallyl ammonium chloride, acrylamide, and 2-acrylamide-2-methylpropanesulfonic acid polymerized and grafted onto the surface of nano-silica to form a core-shell structure silica blocker with a rigid inner core and a flexible outer core. The microstructure is a core-shell structure with modified nano-silica (SiO2) as the core and a polymer shell covering the outside.

[0027] A silica plugging agent with an inner rigid and outer flexible core-shell structure for marine drilling fluids is prepared by the above method.

[0028] The aforementioned application of the core-shell structured silica plugging agent with an inner rigid and outer flexible core in marine drilling fluids is used as a micro-nano plugging agent in deep water drilling to seal micro-nano-scale pores in the formation and simultaneously inhibit the secondary formation of natural gas hydrates in the wellbore.

[0029] The technical features and beneficial effects of this invention are as follows:

[0030] 1. This invention utilizes N-vinyl-2-pyrrolidone, dimethyl diallyl ammonium chloride, acrylamide, and 2-acrylamide-2-methylpropanesulfonic acid polymerized and grafted onto the surface of nano-silica to form a core-shell structure silica plugging agent with a rigid inner structure and a flexible outer structure. By combining the rigid plugging effect of nanomaterials with the flexible plugging effect of polymer deformation, this method effectively seals the micro- and nano-pores of shale, thereby maintaining wellbore stability and effectively inhibiting secondary hydrate formation, thus improving the safety and efficiency of deepwater drilling. Furthermore, the plugging agent of this invention reduces the need for large amounts of thermodynamic inhibitors, saving costs and possessing environmentally friendly properties. The preparation method is simple, the raw materials are inexpensive and readily available, and it is highly efficient and environmentally friendly. It also exhibits good compatibility with drilling fluids.

[0031] 2. Acrylamide molecular chains can flexibly fill and adapt to different shapes and sizes of microcracks or pores, providing a flexible and enhanced sealing effect. The amide groups adsorb at different locations in the clay, creating numerous adsorption sites that prevent the intrusion of the aqueous phase on the clay surface and are less prone to desorption, thus inhibiting clay hydration for a longer period. The amide groups of acrylamide can form a strong adsorption layer on the surface of shale and natural gas hydrate crystals, inhibiting the expansion and dispersion of clay minerals and the decomposition of hydrates. 2-Acrylamide-2-methylpropanesulfonic acid (AMPS) can enhance the polymer's temperature and salt resistance. Simultaneously, AMPS contains sulfonic acid groups and hydroxyl groups, increasing the polymer's thermal stability, and stronger CS and CN bonds also improve temperature resistance. The cations of dimethyl diallyl ammonium chloride (DMDAAC) adsorb onto the clay surface through electrostatic interactions; some enter the interlayer, replacing interlayer cations and thus expelling interlayer water molecules. The positive charge provided by DMDAAC can form a strong charge adsorption with the negatively charged clay minerals in shale, enhancing the polymer's... The invention enhances the plugging and inhibitory effects of the compound. N-vinyl-2-pyrrolidone is the polymer monomer of the kinetic hydrate inhibitor PVP. The introduction of N-vinyl-2-pyrrolidone serves two purposes: firstly, it inhibits the secondary formation of hydrates; secondly, it improves the flexibility of the polymer shell coating on the silica. The specific ratio of acrylamide aqueous solution, 2-acrylamido-2-methylpropanesulfonic acid aqueous solution, dimethyldiallylammonium chloride aqueous solution, N-vinyl-2-pyrrolidone, and KH570 modified nano-silica in this invention results in a plugging agent with both a complete shell and improved flexibility of the polymer shell, forming a core-shell structure silica plugging agent with a rigid inner core and a flexible outer core, exhibiting stronger micron-level plugging capability.

[0032] 3. Experiments have shown that when the sealing agent of this invention is added at a mass fraction of 1% to a 4% freshwater-based slurry, the API filtration loss of the slurry decreases from 22 mL to 10 mL. After aging at 180℃ under high temperature and pressure, the filtration loss decreases from 90 mL to 58 mL. The apparent viscosity increases from 10 mPa·s to 32.5 mPa·s, the plastic viscosity increases from 6 mPa·s to 19 mPa·s, and the dynamic shear force increases from 4 Pa ​​to 13.5 Pa. This indicates that the invention has good filtration loss reduction properties at both room temperature and high temperature and pressure, and can also play a role in increasing viscosity and improving shear force, optimizing the rheological properties of the slurry. This also indirectly shows that it has excellent sealing properties. The sealing experiments of the 160-180 mesh medium-pressure sand bed and the microporous filter membrane experiments of this invention both show that the invention has good micro-nano pore sealing ability. The hydrate secondary formation inhibition performance evaluation experiment shows that the invention also has a certain hydrate formation inhibition performance, reducing the need for a large amount of thermodynamic inhibitors. Attached Figure Description

[0033] Figure 1This is a SEM image of the micro-nano plugging agent prepared in Example 1 after plugging the microporous filter membrane. Detailed Implementation

[0034] The present invention will be further described below with reference to specific embodiments, but is not limited thereto.

[0035] Furthermore, unless otherwise specified, the experimental methods described in the following embodiments are all conventional methods; and unless otherwise specified, the reagents, materials and equipment are all commercially available.

[0036] Example 1

[0037] A method for preparing a core-shell structured silica plugging agent with an inner rigid and outer flexible structure for marine drilling fluids, the plugging agent being suitable for deepwater drilling fluids, the steps of which are as follows:

[0038] (1) Mix 3g of KH570 modified nano silica with 50g of pure water at 25℃ and stir for 20 minutes at a stirring rate of 200r / min to obtain KH570 modified nano silica aqueous solution.

[0039] (2) Mix 6.3g (0.089mol) acrylamide with 60g pure water at 25℃ and stir for 10 minutes at a stirring rate of 200r / min to obtain an acrylamide aqueous solution;

[0040] (3) Add 11.02 g (0.053 mol) of 2-acrylamido-2-methylpropanesulfonic acid to the acrylamide aqueous solution obtained in step (2), and stir for 10 minutes at a stirring rate of 200 r / min and a temperature of 25 °C to obtain a mixed aqueous solution of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid.

[0041] (4) Add 9.56 g (0.035 mol) of dimethyl diallyl ammonium chloride aqueous solution (60 wt%), 3.94 g (0.035 mol) of N-vinyl-2-pyrrolidone and the KH570 modified nano silica aqueous solution obtained in step (1) to the acrylamide and 2-acrylamido-2-methylpropanesulfonic acid mixed aqueous solution obtained in step (3) to obtain a mixed solution;

[0042] (5) The pH of the mixed solution was adjusted to 7 using 20% ​​NaOH solution. The solution was placed in a constant temperature water bath and nitrogen gas was continuously introduced for 30 minutes at a temperature of 65℃ and a rotation speed of 300 rpm to purge the air from the flask while stirring.

[0043] (6) Weigh 0.045g ammonium persulfate and 0.045g sodium bisulfite, dissolve them in 10mL of pure water to obtain 10mL of redox complex initiator aqueous solution of ammonium persulfate and sodium bisulfite; under stirring conditions and nitrogen protection, add the initiator aqueous solution dropwise to the system in step (5) at a rate of 2 drops per second; after the dropwise addition is completed, continue to react for 6 hours under stirring conditions of 300rpm, temperature of 65℃ and nitrogen; after the reaction is completed, wash with a mixture of anhydrous ethanol and acetone, dry in a vacuum drying oven at 70℃ for 24h, and pulverize to obtain a core-shell structured silica sealing agent with an inner rigid and outer flexible structure.

[0044] Example 2

[0045] A method for preparing a core-shell structured silica plugging agent with an inner rigid and outer flexible structure for marine drilling fluids, the plugging agent being suitable for deepwater drilling fluids, the steps of which are as follows:

[0046] (1) Mix 3g of KH570 modified nano silica with 50g of pure water at 25℃ and stir for 20 minutes at a stirring rate of 200r / min to obtain KH570 modified nano silica aqueous solution.

[0047] (2) Mix 6.39g (0.090mol) acrylamide with 60g pure water at 25℃ and stir for 10 minutes at a stirring rate of 200r / min to obtain an acrylamide aqueous solution.

[0048] (3) Add 12.43 g (0.060 mol) of 2-acrylamido-2-methylpropanesulfonic acid to the acrylamide aqueous solution obtained in step (2), and stir for 10 minutes at a stirring rate of 200 r / min and a temperature of 25°C to obtain a mixed aqueous solution of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid.

[0049] (4) Add 9.56g (0.030mol) of dimethyl diallyl ammonium chloride aqueous solution (60wt%), 3.94g (0.030mol) of N-vinyl-2-pyrrolidone and KH570 modified nano silica aqueous solution obtained in step (1) to the mixed aqueous solution of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid obtained in step (3) to obtain a mixed solution.

[0050] (5) The pH of the mixed solution was adjusted to 7 using 20% ​​NaOH solution. The solution was placed in a constant temperature water bath and nitrogen gas was continuously introduced for 30 minutes at a temperature of 65℃ and a rotation speed of 300 rpm to purge the air from the flask while stirring.

[0051] (6) Weigh 0.045g of ammonium persulfate and 0.045g of sodium bisulfite, dissolve them in 10mL of pure water to obtain 10mL of aqueous solution of ammonium persulfate and sodium bisulfite redox initiator. Under stirring and nitrogen protection, add the aqueous solution of initiator dropwise to the system in step (5) at a rate of 2 drops per second. After the addition is complete, continue to react for 6 hours under stirring at 300rpm, temperature of 65℃ and nitrogen. After the reaction is complete, wash with a mixture of anhydrous ethanol and acetone, dry in a vacuum drying oven at 70℃ for 24h, and pulverize to obtain a core-shell structured silica sealing agent with an inner rigid and outer flexible structure.

[0052] Example 3

[0053] A method for preparing a core-shell structured silica plugging agent with an inner rigid and outer flexible structure for marine drilling fluids, the plugging agent being suitable for deepwater drilling fluids, the steps of which are as follows:

[0054] (1) Mix 3g of KH570 modified nano silica with 50g of pure water at 25℃ and stir for 20 minutes at a stirring rate of 200r / min to obtain KH570 modified nano silica aqueous solution.

[0055] (2) Mix 6.3g (0.089mol) acrylamide with 60g pure water at 25℃ and stir for 10 minutes at a stirring rate of 200r / min to obtain an acrylamide aqueous solution.

[0056] (3) Add 11.02 g (0.053 mol) of 2-acrylamido-2-methylpropanesulfonic acid to the acrylamide aqueous solution obtained in step (2), and stir for 10 minutes at a stirring rate of 200 r / min and a temperature of 25°C to obtain a mixed aqueous solution of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid.

[0057] (4) Add 9.56g (0.035mol) of dimethyl diallyl ammonium chloride aqueous solution (60wt%), 3.94g (0.035mol) of N-vinyl-2-pyrrolidone and KH570 modified nano silica aqueous solution obtained in step (1) to the mixed aqueous solution of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid obtained in step (3) to obtain a mixed solution.

[0058] (5) The pH of the mixed solution was adjusted to 7 using 20% ​​NaOH solution. The solution was placed in a constant temperature water bath and nitrogen gas was continuously introduced for 30 minutes at a temperature of 65℃ and a rotation speed of 300 rpm to purge the air from the flask while stirring.

[0059] (6) Weigh 0.015g ammonium persulfate and 0.015g sodium bisulfite, dissolve them in 10mL of pure water to obtain 10mL of redox complex initiator aqueous solution of ammonium persulfate and sodium bisulfite. Under stirring and nitrogen protection, add the initiator aqueous solution dropwise to the system in step (5) at a rate of 2 drops per second; after the addition is complete, continue to react for 6 hours under stirring at 300rpm, temperature at 80℃ and nitrogen conditions; after the reaction is completed, wash with a mixture of anhydrous ethanol and acetone, dry in a vacuum drying oven at 70℃ for 24h, and pulverize to obtain a core-shell structured silica sealing agent with an inner rigid and outer flexible structure.

[0060] Comparative Example 1

[0061] A method for preparing a micro / nano plugging agent is carried out according to the preparation method of Example 1, except that the amount of 2-acrylamido-2-methylpropanesulfonic acid used is 6g (0.029mol), and the other steps and conditions are the same as in Example 1.

[0062] Comparative Example 2

[0063] A method for preparing a micro / nano plugging agent is carried out according to the preparation method of Example 1, except that the amount of N-vinyl-2-pyrrolidone used is 1.12 g (0.010 mol), and the other steps and conditions are the same as in Example 1.

[0064] Comparative Example 3

[0065] A method for preparing a micro / nano plugging agent is carried out according to the preparation method of Example 1, except that the amount of N-vinyl-2-pyrrolidone used is 7.84g, and the other steps and conditions are the same as in Example 1.

[0066] Comparative Example 4

[0067] A method for preparing a micro / nano plugging agent is carried out according to the preparation method of Example 1, except that the amount of acrylamide used is 1.2g, and the other steps and conditions are the same as in Example 1.

[0068] Comparative Example 5

[0069] A method for preparing a micro / nano plugging agent is carried out according to the preparation method of Example 1, except that the amount of KH570 modified nano silica is 0.5g, and the other steps and conditions are the same as in Example 1.

[0070] Comparative Example 6

[0071] A method for preparing a micro / nano plugging agent is described in Example 1, except that the initiator mass fraction is 1%, and the other steps and conditions are the same as in Example 1.

[0072] Experimental Example 1

[0073] Referring to the national standard GB / T 29170-2012 "Laboratory Testing of Drilling Fluids for Petroleum and Natural Gas Industry", the rheological parameters of the base slurry of the examples or comparative samples, as well as the filtration loss at room temperature and pressure and the filtration loss under high temperature and pressure after aging, were measured. The experimental results are shown in Table 1.

[0074] Table 1. Rheological filtration test of the base slurry of the examples and comparative samples.

[0075]

[0076] As can be seen from the results in Table 1, the plugging agents prepared using Examples 1-3 of the present invention have little effect on the rheological properties of the base slurry before and after aging at 180℃ for 16 hours, while effectively reducing the high-temperature and high-pressure filtration loss of the base slurry. Before hot rolling, the filtration loss is 10-3 mL, and after aging at 180℃ for 16 hours, the filtration loss is controlled at 40-52 mL, indicating that the plugging agent of the present invention has good temperature resistance. In contrast, the amount of 2-acrylamido-2-methylpropanesulfonic acid used in Comparison 1 is too small, and the monomer ratio is small, resulting in insufficient temperature resistance of the product, so the plugging effect is not ideal. In Comparative Example 2, the amount of N-vinyl-2-pyrrolidone was too small, resulting in an incomplete product shell and poor sealing effect. In Comparative Example 3, the amount of N-vinyl-2-pyrrolidone was too large, resulting in low shell flexibility and poor sealing effect. In Comparative Example 4, the amount of acrylamide was too small, resulting in an incomplete shell and few sealing sites, leading to poor sealing effect. In Comparative Example 5, the amount of KH570 modified nano-silica was too small, resulting in insufficient rigidity of the sealing agent, which was prone to collapse and thus poor sealing effect. In Comparative Example 6, the initiator mass fraction was too high, resulting in an overly vigorous reaction, severe monomer self-polymerization, and low conversion rate, thus the sealing effect was not ideal.

[0077] Experimental Example 2

[0078] Using 160-180 mesh sand to simulate formations with micron-sized pores, the sealing performance of the samples was tested. The results are as follows: Figure 1 As shown in Table 2.

[0079] Table 2. Medium-Pressure Sand Bed Plugging Experiment

[0080]

[0081] As shown in Table 2, the base slurry after hot rolling was completely lost. The penetration depth of the base slurry + 1% in Example 1 in the sand bed was 4 cm, while the penetration depth of the base slurry + 1% in Comparative Example 1 in the sand bed was 10.2 cm. This indicates that Example 1 of the present invention has a stronger micron-level sealing ability compared to Comparative Example 1.

[0082] Experimental Example 3

[0083] Microporous filter membranes were used to simulate the nanoscale pore development environment of deep-water shale mudstone. The filtration loss of samples was measured at room temperature and pressure using microporous filter membranes with different pore sizes. The results are shown in Table 3.

[0084] Table 3 Microporous membrane filtration loss experiment

[0085]

[0086] As shown in Table 3, compared with pure water and Comparative Example 4, Example 1 can more effectively block nanoscale pores of different sizes, thereby significantly reducing the filtration loss of the microporous membrane, indicating that Example 1 of the present invention has better nanoscale blocking ability.

[0087] Test Example 4

[0088] An experiment was conducted to evaluate the inhibitory performance of the sample on secondary hydrate formation. The gas pipeline and methane cylinder were turned on, and the pressure was increased to 20 MPa using a gas pressurization device. A 1 wt% inhibitor aqueous solution was prepared and placed in the reaction vessel. After the reaction vessel was evacuated, methane gas was introduced. Hydrates were formed in an environment of 14 MPa and 0℃. A data acquisition system was used to record data in real time, including liquid temperature, gas temperature, pressure, and stirring torque. When the pressure change inside the vessel was less than or equal to 0.01 MPa after half an hour, the hydrate formation was considered complete. The experimental results are shown in Table 4.

[0089] Table 4 Hydrate Formation Inhibition Test

[0090]

[0091] As shown in Table 4, compared with pure water and Comparative Example 3, adding 1% of Example 1 can extend the time for large-scale hydrate formation to 6.95 h and reduce the large-scale formation temperature from 9.56 °C to 3.24 °C. This illustrates the importance of the proportion of kinetic hydrate inhibitory monomers and also shows that Example 1 has good hydrate secondary formation inhibition performance.

Claims

1. A method for preparing a core-shell structured silica plugging agent with an inner rigid and outer flexible structure for marine drilling fluids, characterized in that, The silica plugging agent is used as a micro-nano plugging agent in deep water drilling to seal micro-nano pores in the formation and inhibit the secondary formation of natural gas hydrates in the wellbore. The steps include the following: Aqueous solutions of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, dimethyldiallyl ammonium chloride, N-vinyl-2-pyrrolidone, and KH570 modified nano-silica were mixed evenly. The pH was adjusted, and an initiator aqueous solution was added dropwise under a protective atmosphere and stirring. The mixture was stirred and reacted under a protective atmosphere. After washing, drying, and pulverizing, N-vinyl-2-pyrrolidone, dimethyldiallyl ammonium chloride, acrylamide, and 2-acrylamido-2-methylpropanesulfonic acid were polymerized and grafted onto the surface of nano-silica, forming a core-shell structure silica plugging agent with a rigid inner core and a flexible outer core. The microstructure is a core-shell structure with modified nano-silica SiO2 as the core and a polymer shell covering the outside. This yields a core-shell structure silica plugging agent with a rigid inner core and a flexible outer core for marine drilling fluids. The mass concentration of the acrylamide aqueous solution is 7~13wt%, the mass concentration of the 2-acrylamido-2-methylpropanesulfonic acid aqueous solution is 19~21wt%, the mass concentration of the dimethyl diallyl ammonium chloride aqueous solution is 60wt%, the mass concentration of the KH570 modified nano silica aqueous solution is 4~8wt%, and the particle size of the KH570 modified nano silica is 10-20nm. The molar ratio of N-vinyl-2-pyrrolidone, dimethyl diallyl ammonium chloride, 2-acrylamido-2-methylpropanesulfonic acid, and acrylamide is (3~1):(2~1):(4~2):(3~6); The mass ratio of N-vinyl-2-pyrrolidone to KH570 modified nano-silica is (1-5):(2-6). Mixing is carried out under stirring conditions, with a stirring rate of 200~400 rpm and a stirring time of 15~30 minutes. The mixing temperature is 20~30℃, and the protective atmosphere is nitrogen or argon. The pH is adjusted to 7.

2. The preparation method according to claim 1, characterized in that, The mass concentration of the initiator aqueous solution is 2%-15%; the initiator is a redox compound initiator of ammonium persulfate and sodium bisulfite, wherein the mass ratio of ammonium persulfate to sodium bisulfite is 1:1, and the mass of the initiator is 0.1%-1% of the total mass of acrylamide, 2-acrylamido-2-methylpropanesulfonic acid, dimethyldiallylammonium chloride, N-vinyl-2-pyrrolidone and KH570 modified nano silica.

3. The preparation method according to claim 1, characterized in that, The initiator aqueous solution was added at a rate of one drop every 2-4 seconds. After the initiator was added, the stirring reaction temperature was 55-85℃, the stirring speed was 100-400 rpm, and the stirring reaction time was 6 hours.

4. The preparation method according to claim 1, characterized in that, Washing is performed using a mixture of anhydrous ethanol and acetone; drying temperature is 55-85℃.

5. A silica plugging agent with an inner rigid and outer flexible core-shell structure for marine drilling fluids, prepared by any one of the methods described in claims 1-4.

6. The application of the core-shell structured silica plugging agent with an inner rigid and outer flexible core as described in claim 5 for marine drilling fluids, as a micro-nano plugging agent applied to deep-water drilling, to seal micro-nano pores in the formation and simultaneously inhibit the secondary formation of natural gas hydrates in the wellbore.

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