Well cementation filling method, well cementation backfill and preparation method

Abstract

The invention relates to the technical field of petroleum drilling backfill, and particularly discloses a well cementation backfill which comprises an inner core and a water-soluble shell covering the inner core, the inner core comprises quartz sand particles of a cement ash mixture, the mass ratio of the cement ash mixture to the quartz sand particles is 10: 1-20: 1, the water-soluble speed of the water-soluble shell under the normal temperature condition is 4-6 h, and the water-soluble shell is a water-soluble shell. The water-soluble shell is a polyvinyl alcohol fiber coating film or a modified polyvinyl alcohol fiber film. The backfill can be used for filling a well section for well cementation, and is simple to operate, convenient to prepare, good in sealing effect and high in reliability.

Description

Technical Field

[0001] This invention relates to the field of oil well backfilling technology, specifically a method for filling wellbore cementing, cementing backfill material, and preparation method. Background Technology

[0002] Drilling sites frequently encounter cementing and backfilling situations, including pilot hole backfilling, complex downhole accident backfilling, and geological abandonment backfilling. The traditional method involves sending cement slurry trucks and pump trucks to the drilling site and then injecting it into the well via drill pipe. The most significant drawback of this method is its susceptibility to delays caused by special vehicles, road conditions, and weather; waiting times for construction are long and costly. Another method primarily uses external packers and expandable plugs, which are expensive and result in limited effective sealing and poor reliability. Therefore, it is essential to research a cementing and backfilling device and method that can be readily available on-site, used immediately during construction, utilizes readily available and low-cost materials, and is convenient and reliable.

[0003] Furthermore, in some wells, due to geological or engineering reasons, the cement return height did not meet design requirements, resulting in substandard cementing quality and affecting the wellbore's service life. Conventional cementing methods, which involve squeezing cement into the wellbore, lack circulation channels, preventing the cement slurry from reaching the lower part, leading to a situation where only a small section of the annulus has a cement ring, while the lower part is empty. There is an urgent need for a cementing method that can fill the entire annulus to meet actual field requirements. Summary of the Invention

[0004] The purpose of this invention is to provide a cementing backfill material that can be used to fill well sections for cementing, and is simple to operate, easy to produce, has good sealing effect, and high reliability.

[0005] The objective of this invention can be achieved through the following technical solutions:

[0006] A cementing backfill material includes a core and a water-soluble shell covering the core. The core comprises quartz sand particles of a cement ash mixture, wherein the mass ratio of the cement ash mixture to the quartz sand particles is 10:1 to 20:1. The water-soluble shell has a water dissolution rate of 4-6 hours at room temperature. The water-soluble shell is a polyvinyl alcohol fiber coated film or a modified polyvinyl alcohol fiber film.

[0007] In a further embodiment, the thickness of the water-soluble shell is greater than or equal to 0.5 mm.

[0008] In a further embodiment, the diameter of the quartz sand particles is 0.5mm-3mm.

[0009] In a further embodiment, the cement ash mixture comprises 100 parts by weight of cement dry ash powder, 1-3 parts by weight of water loss reducing agent powder, 1-4 parts by weight of setting accelerator powder, 0.5-2.5 parts by weight of dispersant powder, and 1-4 parts by weight of expansion agent powder.

[0010] In a further embodiment, the water loss reducing agent is an AMPS-type water loss reducing agent.

[0011] In a further embodiment, the coagulant is an inorganic salt coagulant.

[0012] In a further embodiment, the dispersant is a sulfonate dispersant.

[0013] In a further embodiment, the expanding agent is a calcium magnesium aluminum oxide expanding agent.

[0014] In a further embodiment, the strength grade of the cement dry ash powder is G or above.

[0015] In a further embodiment, the kernel is spherical or ellipsoidal.

[0016] Another aspect of the present invention is to provide a method for preparing cement backfill material, comprising:

[0017] Prepare the above-mentioned core and water-soluble shell;

[0018] The water-soluble shell is used to cover the core.

[0019] In a further embodiment, the method for preparing the kernel includes:

[0020] A mixture of quartz sand particles and cement ash is prepared, and the quartz sand particles are coated with the cement ash mixture and compacted to form a solid core.

[0021] In a further embodiment, the method for preparing quartz sand particles includes:

[0022] Quartz sand particles with a diameter of 0.5 mm to 3 mm were obtained by screening.

[0023] In a further embodiment, the method for preparing the cement ash mixture includes:

[0024] Add 1%-3% of water loss reducing agent powder, 1%-4% of setting accelerator powder, 0.5%-2.5% of dispersant powder, and 1%-4% of expansion agent powder to the cement dry ash powder in a mass ratio of 1%-3% to 4% of the cement dry ash powder, and stir to mix evenly.

[0025] In a further embodiment, the method for preparing the water-soluble shell includes:

[0026] Preparation of polyvinyl alcohol fibers:

[0027] Vinyl acetate is prepared by the ethylene process. Vinyl acetate is polymerized in methanol solution to produce polyvinyl acetate. It is then alcoholyzed under the action of alkali to form polyvinyl alcohol. After washing with water, it is purified by dehydration and then dissolved in water to form a polyvinyl alcohol solution.

[0028] Prepare a 40% polyvinyl alcohol solution, add 3% boric acid (by mass) to the polyvinyl alcohol solution to form a cross-linked structure with the polyvinyl alcohol;

[0029] Polyvinyl alcohol fibers are produced by boric acid gel spinning and high-ratio stretching of a solution having the cross-linked structure.

[0030] In a further embodiment, the method for preparing the water-soluble shell also includes:

[0031] The polyvinyl alcohol fiber was modified by adding 3% ethylene ester monomers and 1% acrylamide by mass ratio to the polyvinyl alcohol fiber.

[0032] In a further embodiment, the method of covering the core with a water-soluble shell includes:

[0033] The core is placed inside molten polyvinyl alcohol fiber or modified polyvinyl alcohol fiber, and a coating film is formed to encapsulate the core through solidification.

[0034] Another aspect of the present invention is to provide a method for cementing a wellbore, the method comprising:

[0035] The aforementioned cementing backfill material is transported to the well section that needs to be backfilled. After being dissolved by the water-soluble shell, the core solidifies upon contact with water, and the entire material is solidified in the well section that needs to be backfilled to form a plug cementing section.

[0036] In a further embodiment, the method for transporting the material to the location of the well section requiring backfilling includes:

[0037] The dissolution rate of the water-soluble crust is obtained, and the thickness of the water-soluble crust is obtained based on the dissolution rate, the temperature and depth of the backfill section; the backfill material is delivered to the section to be backfilled by a pumping drill bit.

[0038] In a further embodiment, the method for transporting the material to the location of the well section requiring backfilling also includes:

[0039] By inserting a high-pressure hose into the annulus;

[0040] The backfill material is delivered to the annulus to be backfilled using a high-pressure pump and a high-pressure hose.

[0041] The beneficial effects of this invention are:

[0042] This invention achieves solidification by dissolving the water-soluble shell upon reaching the sealing location, allowing the encapsulated core to solidify upon contact with water. The backfill material has a simple structure, is easy to manufacture, and can be used for cementing filling well sections.

[0043] The polyvinyl alcohol fiber membrane of this invention serves as a water-soluble shell, which can easily combine with the core to form a sphere. Furthermore, it exhibits different water-dissolving rates at different temperatures and thicknesses, enabling the formation of backfill materials with varying dissolution times. This makes it suitable for well sections with different well depths and sealing time requirements.

[0044] The cement-ash mixture of this invention can seal quickly, has a low expansion rate, is easy to bond with well casing, has strong adhesion and high compressive strength, and provides good sealing effect when used for well casing filling. Attached Figure Description

[0045] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0046] Figure 1 This is a schematic diagram of the drill string being lowered into the straight section in an embodiment of the present invention;

[0047] Figure 2 This is a schematic diagram of replacing drilling fluid with clean water in the straight section in an embodiment of the present invention;

[0048] Figure 3 This is a schematic diagram of the backfill material being lowered into the backfill position in the straight section in an embodiment of the present invention;

[0049] Figure 4 This is a schematic diagram of the backfill material forming a plug section in the straight section in an embodiment of the present invention;

[0050] Figure 5 This is a schematic diagram of the drill string being lowered into the horizontal section in an embodiment of the present invention;

[0051] Figure 6 This is a schematic diagram of water replacing drilling fluid in the horizontal section in an embodiment of the present invention;

[0052] Figure 7 This is a schematic diagram of the backfill material being lowered into the backfill position in the horizontal segment in an embodiment of the present invention;

[0053] Figure 8 This is a schematic diagram of the backfill material forming a plug segment in the transverse section in an embodiment of the present invention;

[0054] Figure 9 This is a schematic diagram of inserting a high-pressure hose into the backfill section inside the annulus of the well casing in an embodiment of the present invention;

[0055] Figure 10 This is a schematic diagram of the backfill material being lowered into the backfill position within the annulus of the well casing in an embodiment of the present invention;

[0056] Figure 11 This is a schematic diagram of the plug segment formed by backfill material in the annulus of the well casing in an embodiment of the present invention;

[0057] In the diagram: 1. Drilling tool; 2. Wellbore; 3. Drilling fluid; 4. Clean water; 5. Backfill material; 6. Plug section; 7. High-pressure hose; 8. Original cementing cement; 9. Annulus. Detailed Implementation

[0058] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0059] A cementing backfill material includes a core and a water-soluble shell, the water-soluble shell covering the core, the thickness of the water-soluble shell being determined according to the dissolution rate, and the dissolution rate being determined according to the cementing backfill depth.

[0060] Its working principle is that the core and water-soluble shell are transported together to the backfill site by pumps and other tools. The water-soluble shell prevents the core from breaking when it comes into contact with water during transportation, which could even contaminate the well. Only when it reaches the designated backfill site does it begin to dissolve at a specific rate, allowing the large number of cores piled together to expand when they come into contact with water, break and bond with each other, and then solidify to form a set seal. At the same time, due to the expansion and bonding effect, it can form a compression and bond with the casing wall, which can withstand high downhole pressure, making the set seal reliable and not easy to be breached.

[0061] Based on the above working principle, some preferred implementation structures or methods are provided. The core comprises a mixture of quartz sand particles and cement ash. The surface of the quartz sand particles is coated with the cement ash mixture to form a compacted solid core, which solidifies and bonds upon contact with water. After being sprayed with water, the quartz sand particles easily bond and compact with the cement ash mixture to form a solid core. After dissolution and solidification, it exhibits strong compressive strength. Furthermore, due to the high density of the quartz sand particles, it easily sinks in well fluid. Through a unique preparation process and optimized material ratio, rapid solidification and high strength of the quartz sand-cement ash mixture core are achieved upon contact with water, significantly improving construction efficiency and product durability. Simultaneously, this technical method is simple to implement, low in cost, and has broad market application prospects.

[0062] The diameter of the quartz sand particles is 0.5mm-3mm. This makes it easier to compact into balls and mix with cement ash to produce a high-strength binder.

[0063] The cement ash mixture includes 100 parts by weight of dry cement ash powder, 1-3 parts by weight of water loss reducing agent powder, 1-4 parts by weight of setting accelerator powder, 0.5-2.5 parts by weight of dispersant powder, and 1-4 parts by weight of expansion agent powder.

[0064] The fluid loss control agent is an AMPS-type fluid loss control agent, which refers to fluid loss control agents with 2-acrylamido-2-methylpropanesulfonic acid (AMPS) as the main component. This type of fluid loss control agent can be used in oilfield cementing and other petroleum engineering applications to improve the performance and stability of cement slurry by reducing its fluid loss. The AMPS-type fluid loss control agent is designed to address the problems of traditional fluid loss control agents, such as high viscosity, easy thickening, and poor salt resistance, to improve its application effect under complex geological conditions. By using different synthesis methods and adding different monomers, such as N,N-dimethylacrylamide (DMAA) and itaconic acid (IA), the temperature resistance and salt resistance of AMPS-type fluid loss control agents can be further improved, making them suitable for a wider range of oilfield cementing needs. The coagulant is an inorganic salt coagulant, such as calcium chloride.

[0065] The dispersant is a sulfonate dispersant such as A-olefin sulfonate; the swelling agent is a calcium magnesium aluminum oxide swelling agent such as magnesium oxide, calcium sulfoaluminate, etc.

[0066] The strength grade of the dry cement ash is G or above.

[0067] By optimizing the component ratio and selecting high-performance additives, the high-performance cement-ash mixture of this technology exhibits significant advantages in terms of strength grade, impermeability, workability, and durability. Traditional cement-ash mixtures typically contain only cement and a small amount of additives, whose functions are relatively limited and cannot meet the complex and ever-changing downhole pressure resistance requirements. Furthermore, the preparation process of traditional mixtures is simple and lacks in-depth optimization of the mixture's properties.

[0068] The water-soluble shell consists of a polyvinyl alcohol (PVA) fiber-coated membrane, with a spherical or ellipsoidal core. This facilitates transport via pumps and pipelines. The PVA fiber-coated membrane is elastic and hydrophilic. The core has an outer diameter of approximately 5 mm, and the outer coating material is PVA fiber with a thickness of 0.5-1 mm. A key characteristic is that different molecular weights of PVA fibers produced using different preparation methods, at different temperatures, and with different core-membrane thicknesses, result in varying water-soluble rates. For on-site application, a water-soluble rate of 4-6 hours is preferred for the coating material.

[0069] The core is wrapped with mixed cement ash. After the coating dissolves in water, the cement ash comes into contact with water to form cement slurry. After setting, it forms cement stone, which serves to seal the well.

[0070] The core contains quartz sand particles, preferably with a diameter of about 1 mm. Its functions are: first, to increase the density of the cement ball, making it easier for it to sink to the bottom of the well; second, to improve the strength and stability of the cement stone; and third, to reduce the shrinkage and expansion rate of the cement slurry during solidification.

[0071] A method for preparing cement backfill material, comprising:

[0072] Prepare the aforementioned core and water-soluble shell, and then coat the core with the water-soluble shell.

[0073] Methods for preparing kernels include:

[0074] A mixture of quartz sand particles and cement ash is prepared. The quartz sand particles are coated with the cement ash mixture and compacted to form a solid core. Specifically, uniformly sized quartz sand particles are sprayed with water mist to moisten their surface, then poured into the cement ash mixture. The quartz sand particles are then coated with the cement mixture and compacted to form cement ash balls with a desired diameter, typically 4 mm.

[0075] Methods for preparing quartz sand particles include:

[0076] Quartz sand particles with a diameter of 0.5mm-3mm were obtained by screening. The specific screening method is as follows: first, use a 20-mesh sieve to screen the quartz sand, and then use a 16-mesh sieve to remove the quartz sand. Collect the sieved quartz sand to obtain quartz sand particles with a diameter of about 0.5mm-3mm that are uniform in size.

[0077] Methods for preparing cement ash mixtures include:

[0078] Add 1%-3% of water loss reducing agent powder, 1%-4% of setting accelerator powder, 0.5%-2.5% of dispersant powder, and 1%-4% of expansion agent powder to the cement dry ash powder in a mass ratio of 1%-3% to 4% of the cement dry ash powder, and stir to mix evenly.

[0079] Methods for preparing water-soluble shells include:

[0080] Polyvinyl alcohol (PVA) fiber-coated films were prepared. Because PVA fibers are resistant to the alkali properties of cement and have good adhesion and affinity to cement, resulting in high bonding strength, they can be used as reinforcing materials for cement products, improving the strength of cement paste. This material exhibits low fiber strength loss under prolonged sunlight exposure, is not easily deteriorated, and is non-toxic and harmless to humans and the environment.

[0081] Methods for preparing polyvinyl alcohol fibers include:

[0082] Vinyl acetate is prepared by the ethylene method. Vinyl acetate is polymerized in methanol solution to produce polyvinyl acetate. It is then alcoholyzed in the presence of an alkali such as NaOH to form polyvinyl alcohol. Impurities and low molecular weight components are removed by washing in water at 40°C. The product is then purified by dehydration and dissolved in water to form a polyvinyl alcohol solution.

[0083] Prepare a 40% polyvinyl alcohol solution, add 3% boric acid (by mass) to the polyvinyl alcohol solution to form a cross-linked structure with the polyvinyl alcohol, thereby enhancing the strength of the polyvinyl alcohol fiber.

[0084] Heating a solution with a cross-linked structure precipitates polyvinyl alcohol fibers.

[0085] The polyvinyl alcohol (PVA) fibers are modified by adding 3% ethylene ester monomers and 1% acrylamide by mass ratio. The modified or unmodified PVA fibers can be heated to a molten state, or hot air can be sprayed into the PVA fiber solution to evaporate the PVA fiber solvent, encapsulate the cement ash, and then solidify. Repeating this process can increase the thickness, such as until the outer diameter of the core reaches 5 mm, i.e., the PVA fiber thickness reaches 0.5 mm.

[0086] Methods for coating the kernel with a water-soluble shell include:

[0087] The core is placed inside a molten, water-soluble shell, which then solidifies to encapsulate the core.

[0088] By incorporating a core and a water-soluble shell in its structural design, this technology enables the cementing backfill material to solidify rapidly upon contact with water, improving cementing speed and quality. Simultaneously, the presence of the water-soluble shell enhances the adaptability of the backfill material to the surrounding formation, reducing the risk of cementing failure.

[0089] The core prepared by this method has a cement ash volume of 50% of the total volume. According to the theory of packing of equal-diameter spheres and the results of indoor experiments, the volume ratio of the core to the pores is 60:40. The calculated mass ratio of cement ash to water is 100:40, which meets the requirements for cement thickening.

[0090] The results of laboratory experiments on the dissolution time of the core prepared by this method in water and the cement thickening time are as follows: In water at 20℃, the dissolution time of the polyvinyl alcohol fiber shell is 6.5h, the cement setting time is about 16h, and the cement stone strength is 17.2MPa after 24h; In water at 50℃, the dissolution time of the polyvinyl alcohol fiber shell is 5.7h, the cement setting time is about 14h, and the cement stone strength is 18.5MPa after 24h; In water at 90℃, the dissolution time of the polyvinyl alcohol fiber shell is 4.5h, the cement setting time is about 10h, and the cement stone strength is 20.2MPa after 24h.

[0091] This invention also proposes an embodiment of a method for cementing a wellbore, wherein the specific steps of the backfilling process under different wellbore requirements are as follows:

[0092] For cement backfilling within wellbore 2, including backfilling of vertical or horizontal sections in large and small inclined wells, please refer to the relevant documentation. Figure 1 and Figure 5 As shown, first assemble drill string 1 of known length and lower it to the bottom of the well. The lowering depth can be determined based on the length of drill string 1. Connect the mud pump to the upper part of drill string 1 and start the pump to circulate and clean the wellbore 2. (See reference...) Figure 2 and Figure 6 This diagram illustrates the process of pumping clean water 4 into the required backfill section and drill string 1 to replace the drilling fluid; based on the wellbore diameter D and the backfill section depth h, the required backfill volume V = πD²h / 4 is calculated, resulting in a core volume of 0.6V. (See reference...) Figure 3 and Figure 7 As shown, clean water 4 is placed in the surface circulation tank, and then the core is poured into clean water 4. Using a pumping method, the water is circulated from the circulation tank through the mud pump and drilling tool 1 to the bottom of the well; see reference. Figure 4 and Figure 8 As shown, drill string 1 is retrieved. After the water-soluble shell dissolves, the cement-ash mixture inside reacts with water to form cement slurry, which then solidifies to form plug section 6. This achieves the purpose of cementing the well and saves time waiting for the cementing truck to prepare the mud, thus reducing drilling costs. The arrows in the above diagram indicate the direction of drilling fluid or water flow.

[0093] For backfilling the annulus 9 outside the casing: First, based on the acoustic amplitude results, it was determined that the cementing had not returned to the specified depth, requiring cement backfilling from outside the casing. Simultaneously, the cement surface depth in annulus 9 was determined. Using the inner diameter D of the upper casing and the outer diameter d of the current casing, the cross-sectional area S of annulus 9 was determined as S = π(D² - d²)h / 4. Combined with the depth h of the uncemented section of annulus 9, the volume V = Sh required to seal annulus 9 was calculated, and the amount of backfill material 5 to be injected was calculated to be 0.6V. (See reference...) Figure 9 As shown, a high-pressure hose 7 is inserted into the annulus 9 outside the wellhead casing, serving as the channel for the core. It is then filled with clean water 4 to replace the drilling fluid 3. (See reference...) Figure 10 As shown, the core is pumped into the annulus 9. The core sinks freely under gravity to the upper surface of the original cement 8, filling the entire annulus 9 section. If the pressure at the filling location is weak, clean water 4 can be injected into the annulus 9 outside the casing, and then the core can be poured into the annulus 9 outside the casing. (See reference...) Figure 11 As shown, the core sinks freely to the cement ash surface under the action of gravity and remains stationary. After the water-soluble shell dissolves, the cement inside reacts with water to form cement slurry, and the whole solidifies to form plug segment 6.

[0094] In summary, this invention enables on-site material storage for immediate use, solves the problem of downtime during backfilling in drilling operations, and reduces the operating costs of cement ash trucks and pump trucks.

[0095] Cementing backfilling uses readily available materials, has a simple operating principle, is easy to operate on-site, and the backfill section length can be adjusted to meet on-site construction needs.

[0096] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A cementing backfill material, characterized in that, It includes a core and a water-soluble shell covering the core. The core comprises quartz sand particles of a cement ash mixture, wherein the mass ratio of the cement ash mixture to the quartz sand particles is 10:1 to 20:

1. The water-soluble shell has a water dissolution rate of 4-6 hours at room temperature. The water-soluble shell is a polyvinyl alcohol fiber coated film or a modified polyvinyl alcohol fiber film.

2. The cementing backfill material according to claim 1, characterized in that, The thickness of the water-soluble shell is greater than or equal to 0.5 mm.

3. The cementing backfill material according to claim 1, characterized in that, The diameter of the quartz sand particles is 0.5mm-3mm.

4. The cementing backfill material according to claim 1, characterized in that, The cement ash mixture comprises 100 parts by weight of cement dry ash powder, 1-3 parts by weight of water loss reducing agent powder, 1-4 parts by weight of setting accelerator powder, 0.5-2.5 parts by weight of dispersant powder, and 1-4 parts by weight of expansion agent powder.

5. The cementing backfill material according to claim 4, characterized in that, The water loss reducing agent is an AMPS-type water loss reducing agent.

6. The cementing backfill material according to claim 4, characterized in that, The coagulant is an inorganic salt coagulant.

7. The cementing backfill material according to claim 4, characterized in that, The dispersant is a sulfonate dispersant.

8. The cementing backfill material according to claim 4, characterized in that, The expanding agent is a calcium magnesium aluminum oxide expanding agent.

9. A cementing backfill material according to claim 4, characterized in that, The strength grade of the cement dry ash powder is G or above.

10. A cementing backfill material according to claim 1, characterized in that, The core is spherical or ellipsoidal.

11. A method for preparing cementing backfill material, characterized in that, include: Prepare the core and water-soluble shell as described in any one of claims 1-10; The water-soluble shell is used to cover the core.

12. A method for preparing cementing backfill material according to claim 11, characterized in that, The method for preparing the kernel includes: A mixture of quartz sand particles and cement ash is prepared, and the quartz sand particles are coated with the cement ash mixture and compacted to form a solid core.

13. A method for preparing cementing backfill material according to claim 12, characterized in that, The method for preparing quartz sand particles includes: Quartz sand particles with a diameter of 0.5 mm to 3 mm were obtained by screening.

14. A method for preparing cementing backfill material according to claim 12, characterized in that, The method for preparing the cement ash mixture includes: Add 1%-3% of water loss reducing agent powder, 1%-4% of setting accelerator powder, 0.5%-2.5% of dispersant powder, and 1%-4% of expansion agent powder to the cement dry ash powder in a mass ratio of 1%-3% to 4% of the cement dry ash powder, and stir to mix evenly.

15. A method for preparing cementing backfill material according to claim 11, characterized in that, The method for preparing the water-soluble shell includes: Preparation of polyvinyl alcohol fibers: Vinyl acetate is prepared by the ethylene process. Vinyl acetate is polymerized in methanol solution to produce polyvinyl acetate. It is then alcoholyzed under the action of alkali to form polyvinyl alcohol. After washing with water, it is purified by dehydration and then dissolved in water to form a polyvinyl alcohol solution. Prepare a 40% polyvinyl alcohol solution, add 3% boric acid (by mass) to the polyvinyl alcohol solution to form a cross-linked structure with the polyvinyl alcohol; Polyvinyl alcohol fibers are produced by boric acid gel spinning and high-ratio stretching of a solution having the cross-linked structure.

16. A method for preparing cementing backfill material according to claim 15, characterized in that, The method for preparing the water-soluble shell further includes: The polyvinyl alcohol fiber was modified by adding 3% ethylene ester monomers and 1% acrylamide by mass ratio to the polyvinyl alcohol fiber.

17. A method for preparing cementing backfill material according to claim 10, characterized in that, The method of covering the core with a water-soluble shell includes: The core is placed inside molten polyvinyl alcohol fiber or modified polyvinyl alcohol fiber, and a coating film is formed to encapsulate the core through solidification.

18. A method for cementing a wellbore, characterized in that, The method includes: The cementing backfill material described in any one of claims 1-10 is transported to the well section to be backfilled. After being dissolved by the water-soluble shell, the core solidifies upon contact with water, and the whole solidifies in the well section to be backfilled to form a plug cementing section.

19. A method for cementing a wellbore according to claim 18, characterized in that, The method for delivering the material to the location of the well section requiring backfilling includes: The dissolution rate of the water-soluble crust is obtained, and the thickness of the water-soluble crust is obtained based on the dissolution rate, the temperature and depth of the backfill section; the backfill material is delivered to the section to be backfilled by a pumping drill bit.

20. A method for cementing a wellbore according to claim 18, characterized in that, The method for delivering the material to the location of the well section requiring backfilling also includes: By inserting a high-pressure hose into the annulus; The backfill material is delivered to the annulus to be backfilled using a high-pressure pump and a high-pressure hose.

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