Ultralow-density high-strength ceramsite proppant and preparation method thereof

Abstract

The invention provides an ultralow-density high-strength ceramsite proppant and a preparation method of the ultralow-density high-strength ceramsite proppant. The thermal insulation material is prepared from the following raw materials in parts by weight: 40-50 parts of fly ash floating beads, 8-12 parts of expanded perlite microspheres, 10-15 parts of medium-grade bauxite, 5-8 parts of first-grade fly ash, 3-5 parts of boron nitride coated mullite hollow microspheres, 2-4 parts of nano silicon dioxide, 3-5 parts of microcrystalline cellulose, 2-3 parts of potassium feldspar powder, 2-4 parts of borate-starch grafted modified carboxymethyl cellulose and 1-2 parts of short carbon fibers. And 20-40 parts of absolute ethyl alcohol. According to the ceramsite proppant, the overall density can be efficiently regulated and controlled, a high-strength crystal phase can be formed, the particle binding force and the microsphere hardness can be improved, crack propagation can be inhibited, the sintering temperature can be reduced, the structural stability can be guaranteed, powder dispersion is uniform, and the product performance is uniform.

Description

Technical Field

[0001] This invention relates to the field of ceramic proppant technology, specifically to an ultra-low density, high-strength ceramic proppant and its preparation method. Background Technology

[0002] Ceramsite proppant is a core functional material for hydraulic fracturing in oil and gas fields. Its density and mechanical strength directly affect the conductivity of artificial fractures and the long-term production efficiency of oil and gas wells. As oil and gas extraction extends to deep wells and unconventional reservoirs, fracturing operations place dual demands on ceramsite proppants for ultra-low density and high strength. Simultaneously, relying on the resource utilization of industrial solid waste and the low-carbon preparation concept of low-energy sintering, this direction has become an important development area in the field of non-metallic mineral deep processing.

[0003] To achieve ultra-low density in ceramsite proppants, existing technologies often employ methods such as introducing lightweight hollow particles and adding pore-forming agents. To enhance mechanical strength, aluminosilicate materials are added to promote the formation of high-strength crystalline phases; however, the synergistic effect of these two methods is unsatisfactory. Conventional methods of density reduction easily lead to uneven pore structure in the proppant matrix and insufficient interparticle bonding strength, resulting in a significant decrease in mechanical properties such as compressive strength, failing to meet the high closure pressure requirements of deep wells. Furthermore, traditional preparation processes often rely on high sintering temperatures to ensure crystalline phase development and bonding between raw materials, resulting in high energy consumption, which contradicts the industry's requirements for low-carbon manufacturing. Therefore, developing a ceramsite proppant and its preparation method that can achieve synergistic optimization of ultra-low density and high strength performance and is compatible with low-carbon manufacturing requirements has become a pressing technical challenge in this field. Summary of the Invention

[0004] To address the problems existing in the prior art, this invention provides an ultra-low density, high-strength ceramic proppant and its preparation method.

[0005] To achieve the above objectives, the present invention provides the following technical solution: This invention discloses an ultra-low density, high-strength ceramsite proppant, the raw materials for which are prepared by weight include: 40-50 parts fly ash cenospheres, 8-12 parts expanded perlite microspheres, 10-15 parts medium-grade bauxite, 5-8 parts grade I fly ash, 3-5 parts boron nitride-coated mullite hollow microspheres, 2-4 parts nano silica, 3-5 parts microcrystalline cellulose, 2-3 parts potassium feldspar powder, 2-4 parts borate ester-starch grafted modified carboxymethyl cellulose, 1-2 parts short-cut carbon fibers, and 20-40 parts anhydrous ethanol.

[0006] Using the above technical solutions, fly ash cenospheres and expanded perlite microspheres can synergistically reduce the overall density of the proppant; medium-grade bauxite provides alumina, and primary fly ash supplements silica, both of which can participate in the formation of a high-strength crystalline phase; boron nitride-coated mullite hollow microspheres reduce density through their hollow structure, and the boron nitride coating significantly improves the hardness of the microspheres themselves; nano-silica fills the gaps between raw materials, and the silicon-oxygen bonds formed after sintering enhance the bonding strength between particles; microcrystalline cellulose can form pores during sintering, adjusting the proppant density; potassium feldspar powder, as a flux, can lower the sintering temperature of the system, promoting bonding between raw materials and crystal growth; borate ester-starch grafted modified carboxymethyl cellulose can improve the bonding performance between raw materials, ensuring the structural stability of green pellets after sintering; chopped carbon fibers can be dispersed in the matrix, inhibiting crack propagation and improving the mechanical properties of the proppant; anhydrous ethanol can uniformly disperse the various powder components, ensuring the uniformity of proppant performance.

[0007] Preferably, the medium-grade bauxite has an Al2O3 content of 60%-70% and a particle size of 200-300 mesh; the first-grade fly ash has a loss on ignition ≤5% and a SiO2 content ≥40%; the nano-silica is fumed silica with a specific surface area of ​​180-220 m². 2 / g; the length of the short-cut carbon fiber is 0.5-1mm and the diameter is 10-20μm.

[0008] Using the above technical solutions, the Al2O3 content of medium-grade bauxite can fully participate in the formation of high-strength crystalline phases such as mullite and corundum. Its fine particle size allows it to be uniformly dispersed in the system, improving the matrix density and compressive strength. The low loss on ignition of primary fly ash can reduce the interference of impurities on the formation of crystalline phases during sintering. The sufficient SiO2 content can provide raw materials for the formation of mullite crystalline phases, and its spherical structure can fill the gaps between aggregates. The large specific surface area of ​​fumed nano-silica makes it easy to fill the micron-level voids between aggregates, forming a stable neck connection after sintering, increasing the bonding area between particles, and forming a dense film on the surface to improve the resistance to media. The size characteristics of short-cut carbon fibers allow them to form a continuously distributed structure in the proppant matrix, dispersing stress and hindering crack propagation, thus improving the overall mechanical properties.

[0009] Preferably, the raw materials for preparing boron nitride-coated mullite hollow microspheres include, by weight: 20-25 parts of tetraethyl orthosilicate, 8-10 parts of aluminum nitrate, 75-90 parts of anhydrous ethanol, 25-30 parts of deionized water, 10-12 parts of liquid paraffin, 2-3 parts of sorbitan monooleate (Span-80), and 5-10 parts of ammonia water with a mass fraction of 5%-8%.

[0010] Using the above technical solution, tetraethyl orthosilicate and aluminum nitrate provide silicon and aluminum sources, respectively, providing the material basis for the formation of the mullite crystalline phase; anhydrous ethanol and deionized water are used as mixed solvents to dissolve the above raw materials to form a uniform silica-alumina sol system, ensuring the orderly progress of subsequent reactions; liquid paraffin, as the oil phase component, works with Span-80 to form a stable emulsion template, providing morphological support for the hollow structure of mullite hollow microspheres; ammonia water with a mass fraction of 5%-8% can adjust the pH value of the system, promoting the silica-alumina sol to undergo a gel reaction, forming a core-shell microsphere structure, creating conditions for subsequent sintering into spheres.

[0011] Preferably, the preparation method of boron nitride-coated mullite hollow microspheres includes the following steps: 1) Mix anhydrous ethanol and deionized water at a volume ratio of 3:1, add tetraethyl orthosilicate and aluminum nitrate, adjust the pH to 3.0-3.5 with 1%-5% dilute nitric acid, and stir magnetically at 300-400 r / min for 20-30 min to obtain a silica-alumina sol. 2) Mix liquid paraffin with Span-80 sorbitan monooleate as the oil phase. Under high-speed shearing of 8000-10000 r / min, the oil phase is dropped into the silica-alumina sol at an oil-water volume ratio of 1:5. Emulsify for 15-20 min to form a W / O type emulsion. 3) Add 5%-8% ammonia water to the mixture obtained in step 2) to adjust the pH to 8.0-8.5, let it stand at 50-60℃ for 12-16 hours, and centrifuge at 8000-10000 r / min for 10-15 minutes to obtain core-shell microspheres; 4) Wash the core-shell microspheres with anhydrous ethanol 3-5 times, then dry them in a forced-air drying oven at 60-80℃ for 8-10 hours, and then transfer them to a muffle furnace. Heat the muffle furnace to 1100-1200℃ at 5-8℃ / min in an air atmosphere and hold for 2-3 hours to obtain mullite hollow microspheres. 5) Place the mullite hollow microspheres in a tube furnace and introduce nitrogen gas as a carrier gas at a rate of 100-200 mL / min. After heating to 800-900℃, introduce a mixture of borane and ammonia gas with a volume ratio of 1:2. The total flow rate of the mixed gas is 150-300 mL / min. Chemical vapor deposition is carried out under normal pressure for 2-3 hours. After cooling, boron nitride-coated mullite hollow microspheres are obtained.

[0012] Using the above technical solution, a uniform silica-alumina sol is formed by adjusting the pH value of the system and cooperating with magnetic stirring to lay the foundation for spheroid formation; the emulsification operation under high-speed shearing promotes the formation of a stable W / O type emulsion between the oil phase and the aqueous phase, providing morphological support for the hollow structure of the microspheres; subsequent pH adjustment, aging, and centrifugation treatment promote the formation of a core-shell microsphere structure from the sol gel, ensuring morphological regularity; after washing and drying to remove impurities, high-temperature sintering transforms the core-shell microspheres into the mullite crystalline phase, forming structurally stable mullite hollow microspheres; a chemical vapor deposition process using nitrogen as the carrier gas forms a boron nitride coating layer on the surface of the mullite hollow microspheres, improving the compressive strength of the microspheres, and finally obtaining boron nitride-coated mullite hollow microspheres that combine a hollow structure and a surface strengthening layer.

[0013] Preferably, the raw materials for preparing borate-starch grafted modified carboxymethyl cellulose, by weight, include: 10-12 parts carboxymethyl cellulose, 3-4 parts soluble starch, 5-6 parts trimethyl borate, 100-120 parts deionized water, 30-40 parts anhydrous ethanol, and 0.1-0.3 parts ammonium persulfate.

[0014] Using the above technical solution, carboxymethyl cellulose serves as the reaction matrix, providing basic bonding properties for the grafted modified product; soluble starch and trimethyl borate serve as grafting monomers, participating in the cross-linking reaction to optimize the product's structure and molding compatibility; deionized water and anhydrous ethanol serve as dissolving media, adapting to the solubility of different raw materials to construct a homogeneous reaction system; ammonium persulfate serves as an initiator, starting the grafting polymerization reaction, promoting the effective combination of each component, ensuring that the modified product forms a stable grafted structure, thereby possessing good bonding strength and temperature resistance.

[0015] Preferably, the preparation method of borate ester-starch grafted modified carboxymethyl cellulose includes the following steps: (1) Dissolve carboxymethyl cellulose with a viscosity of 2000-4000 mPa·s in deionized water and stir at 200-300 r / min for 30-40 min in a water bath at 50-60℃ to obtain an aqueous solution of carboxymethyl cellulose; (2) Dissolve soluble starch and trimethyl borate in all anhydrous ethanol and stir at 200-300 r / min for 15-25 min to obtain a monomer mixture; (3) Add the monomer mixture dropwise to the carboxymethyl cellulose aqueous solution, stir at 200-300 r / min for 10-15 min at 50-60℃, purge with nitrogen to remove oxygen for 20-30 min, add ammonium persulfate, and stir at 200-300 r / min for 4-6 h. (4) Slowly pour the reaction solution obtained in step (3) into anhydrous ethanol for precipitation. After filtration, place the filter cake in a vacuum drying oven and dry it for 10-12 hours under vacuum conditions of -0.08MPa to -0.10MPa and 60-70℃. Then pulverize it to 200-300 mesh to obtain borate ester-starch grafted modified carboxymethyl cellulose.

[0016] Using the above technical solution, carboxymethyl cellulose is fully dissolved under water bath stirring to form a uniform aqueous solution as the grafting reaction matrix; soluble starch and trimethyl borate are dissolved to prepare a monomer mixture, ensuring uniform monomer dispersion and providing a material basis for the grafting reaction; nitrogen purging to remove oxygen can reduce the influence of impurities on the product structure; ammonium persulfate initiates the grafting crosslinking reaction between the monomer and carboxymethyl cellulose to form a stable modified bonding structure; the reaction solution is purified by anhydrous ethanol precipitation, vacuum dried to remove moisture, and pulverized to a specified mesh size, thereby improving the purity and uniformity of the product, giving it good solubility and bonding strength, and making it suitable for the molding requirements of ceramsite pellets.

[0017] This invention also discloses a method for preparing an ultra-low density, high-strength ceramsite proppant, comprising the following steps: S1. Place fly ash cenospheres, expanded perlite microspheres, medium-grade bauxite, grade 1 fly ash, and potassium feldspar powder separately in a forced-air drying oven and dry them at 105-110℃ for 4-6 hours until the moisture content is ≤2%. S2. Add the pretreated material from step S1, along with microcrystalline cellulose, chopped carbon fibers, and nano-silica, to a ball mill. Add anhydrous ethanol, control the ball-to-material ratio at 3:1, and mill at 200-250 r / min for 1.5-2 h to obtain a uniform dry-mixed slurry. S3. Add boron nitride-coated mullite hollow microspheres to the dry mixture slurry and continue ball milling for 30-40 minutes. S4. Prepare a 5%-8% (w / w) aqueous solution of borate ester-starch grafted modified carboxymethyl cellulose with deionized water, slowly add it dropwise to the slurry, stir at 150-200 r / min for 20-30 min, granulate using a disc granulator at a granulation speed of 30-40 r / min for 10-15 min, and obtain ceramsite green pellets with a particle size of 0.8-2.0 mm. S5. Place the raw ceramsite pellets in a forced-air drying oven and dry them at 40-50℃ for 2-3 hours, then raise the temperature to 80-90℃ and dry them for 4-5 hours until the moisture content is ≤1%. S6. Place the dried green pellets in a muffle furnace and heat them to 400-500℃ at 5-8℃ / min, hold for 1-1.5h to allow the microcrystalline cellulose to fully pyrolyze and volatilize; then heat them to 1150-1250℃ at 3-5℃ / min, hold for 2-3h, and then cool to room temperature at 2-3℃ / min to obtain the semi-finished ceramsite. S7. Prepare a suspension of nano-silica with anhydrous ethanol at a mass fraction of 3%-5%. Immerse the semi-finished ceramic aggregate in the suspension for 1-2 minutes, then remove it and dry it in a drying oven at 60-70℃ for 2-3 hours to obtain ultra-low density high-strength ceramic aggregate proppant.

[0018] Using the above technical solution, pretreatment drying removes excess moisture from the material, avoiding agglomeration or structural defects caused by residual moisture during subsequent processing; anhydrous ethanol is added during ball milling and relevant parameters are controlled to ensure thorough and uniform mixing of all raw material components; boron nitride-coated mullite hollow microspheres are uniformly dispersed through secondary ball milling, ensuring the consistency of the proppant's internal structure; modified carboxymethyl cellulose aqueous solution is used as a binder, combined with stirring and disc granulation, to coagulate the slurry into ceramsite green pellets with uniform particle size and qualified strength; gradient drying gradually removes moisture from the green pellets, reducing cracking caused by rapid moisture evaporation during drying and ensuring the integrity of the green pellets; programmed sintering, through segmented temperature control, allows the microcrystalline cellulose to fully combust, forming uniform pores, while simultaneously promoting crystal phase transformation and sintering densification; controlled cooling rate reduces internal stress and improves the strength of the ceramsite matrix; surface treatment with nano-silica suspension fills the pores on the surface of the ceramsite, forming a dense surface layer, further improving the proppant's resistance to media and overall structural stability, ultimately obtaining a ceramsite proppant with both ultra-low density and high strength.

[0019] Preferably, in step S4, the humidity of the granulation environment of the disc granulator is controlled at 40%-60%.

[0020] By employing the above technical solution, the borate ester-starch grafted modified carboxymethyl cellulose aqueous solution can fully exert its binding effect, promoting effective agglomeration of slurry particles. This avoids both excessive humidity, which can lead to clumping and uneven particle size of the ceramsite green pellets, and excessive humidity, which can cause rapid moisture loss from the slurry, resulting in weak adhesion and easy collapse of the green pellets. This humidity range ensures the stability of the green pellet molding structure, laying the foundation for subsequent gradient drying and sintering, reducing the probability of green pellet cracking, improving the green pellet molding rate and the structural integrity of the finished product, and indirectly maintaining the mechanical properties and density stability of the proppant.

[0021] Preferably, in step S7, the suspension is prepared by stirring at 300-400 r / min.

[0022] By employing the above technical solution, stirring at 300-400 r / min during the preparation of the nano-silica suspension can prevent the nano-silica from agglomerating due to its small particle size and large specific surface area, ensuring that the nanoparticles are uniformly dispersed in anhydrous ethanol and guaranteeing a uniform and stable suspension concentration. This avoids localized particle accumulation or insufficient concentration in the suspension, ensuring that the surface of the ceramsite semi-finished product can uniformly adsorb nano-silica when immersed, forming a continuous and dense surface layer. At the same time, stirring can reduce the generation of air bubbles in the suspension, preventing residual air bubbles from causing pore defects on the surface layer, improving the bonding strength between the surface layer and the ceramsite matrix, and further enhancing the proppant's resistance to media and the overall structural strength.

[0023] The beneficial effects of this invention are as follows: Fly ash cenospheres and expanded perlite microspheres can synergistically reduce the overall density of the proppant; medium-grade bauxite provides alumina, and primary fly ash supplements silica, both of which can participate in the formation of a high-strength crystalline phase; boron nitride-coated mullite hollow microspheres reduce density through their hollow structure, and the boron nitride coating significantly improves the hardness of the microspheres themselves; nano-silica fills the gaps between raw materials, and the silicon-oxygen bonds formed after sintering enhance the bonding strength between particles; microcrystalline cellulose can form pores during sintering, adjusting the proppant density; potassium feldspar powder can lower the sintering temperature of the system, promoting bonding between raw materials and crystal growth; borate ester-starch grafted modified carboxymethyl cellulose can improve the bonding performance between raw materials, ensuring the structural stability of green pellets after sintering; chopped carbon fibers can be dispersed in the matrix, inhibiting crack propagation and improving the mechanical properties of the proppant; anhydrous ethanol can uniformly disperse the various powder components, ensuring the uniformity of proppant performance. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0025] The specific information on the raw materials used in the embodiments of the present invention is shown in Table 1.

[0026] Table 1 Components Specification source fly ash cenospheres Particle size 40-80μm Lingshou County Tianlong Mineral Products Processing Plant Expanded perlite microspheres 2-7mm Xinyang Tianlong Insulation Building Materials Co., Ltd. medium-grade bauxite <![CDATA[The Al2O3 content is 60%-70%, and the particle size is 200-300 mesh]]> Shijiazhuang Tourmaline Mineral Products Co., Ltd. Grade I fly ash <![CDATA[Loss on ignition ≤ 5%, SiO2 content ≥ 40%]]> Henan Jingkai Environmental Protection Technology Co., Ltd. Tetraethyl orthosilicate 99% purity Guangzhou Yuanda New Materials Co., Ltd., CAS: 11099-06-2 Aluminum nitrate 99% purity Sichuan Huanan Inorganic Salts Co., Ltd., CAS: 13473-90-0 Anhydrous ethanol Analytical Pure Sinopharm Chemical Reagent Co., Ltd. Liquid paraffin 99% purity Nanjing Bermuda Biotechnology Co., Ltd., CAS: 8042-47-5 Span-80 99.9% purity Suzhou Senfida Chemical Co., Ltd., CAS: 1338-43-8 ammonia The quality fraction is 5%-8%. Sinopharm Chemical Reagent Co., Ltd. Nano silica <![CDATA[Fumed nano-silica, specific surface area is 180 - 220m 2 / g]]> Evonik Specialty Chemicals (Shanghai) Co., Ltd. microcrystalline cellulose 99% purity Hebei Lihua Biotechnology Co., Ltd., CAS: 9004-34-6 Potassium feldspar powder 99% purity Hubei Baidu Chemical Co., Ltd., CAS: 68476-25-5 Carboxymethyl cellulose 99% purity Wuhan Kemike Biomedical Technology Co., Ltd., CAS: 9000-11-7 Soluble starch 99% purity Hubei Chengfeng Chemical Co., Ltd., CAS: 9005-84-9 Trimethyl borate 99% purity Shandong Xinheng Chemical Co., Ltd., CAS: 121-43-7 ammonium persulfate 98% purity Shanghai Jieshikai Biotechnology Co., Ltd., CAS: 7727-54-0 Short-cut carbon fiber Length is 0.5-1mm, diameter is 10-20μm Forsmann Technology (Beijing) Co., Ltd., CAS: 308063-67-4

[0027] Example 1: This embodiment discloses an ultra-low density, high-strength ceramic proppant, the raw materials for which, by weight, are: 40 parts fly ash cenospheres, 8 parts expanded perlite microspheres, 10 parts medium-grade bauxite, 5 parts first-grade fly ash, 3 parts boron nitride-coated mullite hollow microspheres, 2 parts nano silica, 3 parts microcrystalline cellulose, 2 parts potassium feldspar powder, 2 parts borate ester-starch grafted modified carboxymethyl cellulose, 1 part short-cut carbon fiber, and 20 parts anhydrous ethanol.

[0028] The medium-grade bauxite has an Al2O3 content of 60% and a particle size of 200 mesh; the first-grade fly ash has a loss on ignition of ≤5% and a SiO2 content of ≥40%; the nano-silica is fumed silica with a specific surface area of ​​180 m². 2 / g; the length of the short-cut carbon fiber is 0.5mm and the diameter is 10μm.

[0029] The raw materials for preparing boron nitride-coated mullite hollow microspheres, by weight, include: 20 parts of tetraethyl orthosilicate, 8 parts of aluminum nitrate, 75 parts of anhydrous ethanol, 25 parts of deionized water, 10 parts of liquid paraffin, 2 parts of Span-80, and 5 parts of ammonia water with a mass fraction of 5%.

[0030] The preparation method of boron nitride-coated mullite hollow microspheres includes the following steps: 1) Mix anhydrous ethanol and deionized water at a volume ratio of 3:1, add tetraethyl orthosilicate and aluminum nitrate, adjust the pH to 3.0 with 1% dilute nitric acid, and stir magnetically at 300 r / min for 20 min to obtain a silica-alumina sol. 2) Liquid paraffin was mixed with Span-80 sorbitan monooleate as the oil phase. The oil phase was added dropwise to the silica-alumina sol under high-speed shear at 8000 r / min with an oil-to-water volume ratio of 1:5. The mixture was emulsified for 15 min to form a W / O type emulsion. 3) Add 5% ammonia water to the solution obtained in step 2) to adjust the pH to 8.0, let it stand at 50℃ for 12 hours, and centrifuge at 8000 r / min for 10 minutes to obtain core-shell microspheres; 4) The core-shell microspheres were washed three times with anhydrous ethanol, then dried in a 60°C oven for 8 hours, and then transferred to a muffle furnace. The temperature was increased to 1100°C at 5°C / min under air atmosphere and held for 2 hours to obtain mullite hollow microspheres. 5) Place the mullite hollow microspheres in a tube furnace and introduce nitrogen gas as a carrier gas at a rate of 100 mL / min. After heating to 800℃, introduce a mixture of borane and ammonia gas with a volume ratio of 1:2. The total flow rate of the mixed gas is 150 mL / min. Chemical vapor deposition is carried out under normal pressure for 2 hours. After cooling, boron nitride-coated mullite hollow microspheres are obtained.

[0031] The raw materials for preparing borate-starch grafted modified carboxymethyl cellulose, by weight, include: 10 parts carboxymethyl cellulose, 3 parts soluble starch, 5 parts trimethyl borate, 100 parts deionized water, 30 parts anhydrous ethanol, and 0.1 parts ammonium persulfate.

[0032] The preparation method of borate ester-starch grafted modified carboxymethyl cellulose includes the following steps: (1) Dissolve carboxymethyl cellulose with a viscosity of 2000 mPa·s in deionized water and stir at 200 r / min for 30 min in a water bath at 50℃ to obtain an aqueous solution of carboxymethyl cellulose. (2) Dissolve soluble starch and trimethyl borate in all anhydrous ethanol and stir at 200 r / min for 15 min to obtain a monomer mixture; (3) Add the monomer mixture dropwise to the carboxymethyl cellulose aqueous solution, stir at 200 r / min for 10 min at 50 °C, purge with nitrogen to remove oxygen for 20 min, add ammonium persulfate, and stir at 200 r / min for 4 h. (4) The reaction solution obtained in step (3) is slowly poured into anhydrous ethanol for precipitation. After filtration, the filter cake is placed in a vacuum drying oven and dried for 10 hours under vacuum of -0.08 MPa and 60°C. It is then pulverized to 200 mesh to obtain borate ester-starch grafted modified carboxymethyl cellulose.

[0033] This embodiment also discloses a method for preparing an ultra-low density, high-strength ceramic proppant, comprising the following steps: S1. Place fly ash cenospheres, expanded perlite microspheres, medium-grade bauxite, grade 1 fly ash, and potassium feldspar powder separately in a forced-air drying oven and dry at 105℃ for 4 hours until the moisture content is ≤2%. S2. Add the pretreated material from step S1, along with microcrystalline cellulose, chopped carbon fibers, and nano-silica, to a ball mill. Add anhydrous ethanol, control the ball-to-material ratio at 3:1, and mill at 200 r / min for 1.5 h to obtain a uniform dry-mixed slurry. S3. Add boron nitride-coated mullite hollow microspheres to the dry mixture slurry and continue ball milling for 30 minutes. S4. Prepare a 5% (w / w) aqueous solution of borate ester-starch grafted modified carboxymethyl cellulose with deionized water, and slowly add it dropwise to the slurry. Stir at 150 r / min for 20 min, and granulate using a disc granulator. The humidity of the granulation environment is controlled at 40%, the granulation speed is 30 r / min, and the granulation time is 10 min to obtain ceramsite green pellets with a particle size of 0.8 mm. S5. Place the raw ceramsite pellets in a forced-air drying oven and dry them at 40℃ for 2 hours, then raise the temperature to 80℃ and dry them for 4 hours until the moisture content is ≤1%. S6. Place the dried green pellets in a muffle furnace and heat them to 400℃ at 5℃ / min, hold for 1 hour to allow the microcrystalline cellulose to fully pyrolyze and volatilize; then heat them to 1150℃ at 3℃ / min, hold for 2 hours, and then cool them to room temperature at 2℃ / min to obtain the semi-finished ceramsite. S7. Stir at 300 r / min, prepare a 3% (w / w) suspension of nano-silica with anhydrous ethanol, immerse the semi-finished ceramic aggregate in the suspension for 1 min, remove it and dry it in a 60℃ drying oven for 2 h to obtain ultra-low density high strength ceramic aggregate proppant.

[0034] Example 2: This embodiment discloses an ultra-low density, high-strength ceramic proppant, the raw materials for which, by weight, are: 50 parts fly ash cenospheres, 12 parts expanded perlite microspheres, 15 parts medium-grade bauxite, 8 parts first-grade fly ash, 5 parts boron nitride-coated mullite hollow microspheres, 4 parts nano silica, 5 parts microcrystalline cellulose, 3 parts potassium feldspar powder, 4 parts borate ester-starch grafted modified carboxymethyl cellulose, 2 parts short-cut carbon fibers, and 40 parts anhydrous ethanol.

[0035] The medium-grade bauxite has an Al2O3 content of 70% and a particle size of 300 mesh; the first-grade fly ash has a loss on ignition of ≤5% and a SiO2 content of ≥40%; the nano-silica is fumed silica with a specific surface area of ​​220 m². 2 / g; the length of the short-cut carbon fiber is 1mm and the diameter is 20μm.

[0036] The raw materials for preparing boron nitride-coated mullite hollow microspheres, by weight, include: 25 parts of tetraethyl orthosilicate, 10 parts of aluminum nitrate, 90 parts of anhydrous ethanol, 30 parts of deionized water, 12 parts of liquid paraffin, 3 parts of Span-80, and 10 parts of ammonia water with a mass fraction of 8%.

[0037] The preparation method of boron nitride-coated mullite hollow microspheres includes the following steps: 1) Mix anhydrous ethanol and deionized water at a volume ratio of 3:1, add tetraethyl orthosilicate and aluminum nitrate, adjust the pH to 3.5 with 5% dilute nitric acid, and stir magnetically at 400 r / min for 30 min to obtain a silica-alumina sol. 2) Liquid paraffin was mixed with Span-80 sorbitan monooleate as the oil phase. Under high-speed shearing at 10,000 r / min, the oil phase was dropped into the silica-alumina sol at an oil-to-water volume ratio of 1:5. The mixture was emulsified for 20 min to form a W / O type emulsion. 3) Add 8% ammonia solution to the mixture obtained in step 2) to adjust the pH to 8.5, let it stand at 60℃ for 16 hours, and centrifuge at 10000 r / min for 15 minutes to obtain core-shell microspheres; 4) The core-shell microspheres were washed 5 times with anhydrous ethanol, then dried in an 80°C oven for 10 hours, and then transferred to a muffle furnace. The temperature was increased to 1200°C at 8°C / min under air atmosphere and held for 3 hours to obtain mullite hollow microspheres. 5) Place the mullite hollow microspheres in a tube furnace and introduce nitrogen gas as a carrier gas at a rate of 200 mL / min. After heating to 900℃, introduce a mixture of borane and ammonia gas with a volume ratio of 1:2. The total flow rate of the mixed gas is 300 mL / min. Chemical vapor deposition is carried out for 3 hours under normal pressure. After cooling, boron nitride-coated mullite hollow microspheres are obtained.

[0038] The raw materials for preparing borate-starch grafted modified carboxymethyl cellulose, by weight, include: 12 parts carboxymethyl cellulose, 4 parts soluble starch, 6 parts trimethyl borate, 120 parts deionized water, 40 parts anhydrous ethanol, and 0.3 parts ammonium persulfate.

[0039] The preparation method of borate ester-starch grafted modified carboxymethyl cellulose includes the following steps: (1) Dissolve carboxymethyl cellulose with a viscosity of 4000 mPa·s in deionized water and stir at 300 r / min for 40 min in a water bath at 60℃ to obtain an aqueous solution of carboxymethyl cellulose. (2) Dissolve soluble starch and trimethyl borate in all anhydrous ethanol and stir at 300 r / min for 25 min to obtain a monomer mixture; (3) Add the monomer mixture dropwise to the carboxymethyl cellulose aqueous solution, stir at 300 r / min for 15 min at 60 °C, purge with nitrogen gas to remove oxygen for 30 min, add ammonium persulfate, and stir at 300 r / min for 6 h. (4) The reaction solution obtained in step (3) is slowly poured into anhydrous ethanol for precipitation. After filtration, the filter cake is placed in a vacuum drying oven and dried for 12 hours under vacuum of -0.10 MPa and 70°C. It is then pulverized to 300 mesh to obtain borate ester-starch grafted modified carboxymethyl cellulose.

[0040] This embodiment also discloses a method for preparing an ultra-low density, high-strength ceramic proppant, comprising the following steps: S1. Place fly ash cenospheres, expanded perlite microspheres, medium-grade bauxite, grade 1 fly ash, and potassium feldspar powder separately in a forced-air drying oven and dry at 110℃ for 6 hours until the moisture content is ≤2%. S2. Add the pretreated material from step S1, along with microcrystalline cellulose, chopped carbon fibers, and nano-silica, to a ball mill. Add anhydrous ethanol, control the ball-to-material ratio to be 3:1, and the ball milling speed to be 250 r / min. Mill for 2 hours to obtain a uniform dry-mixed slurry. S3. Add boron nitride-coated mullite hollow microspheres to the dry-mixed slurry and continue ball milling for 40 minutes. S4. Prepare an 8% (w / w) aqueous solution of borate ester-starch grafted modified carboxymethyl cellulose with deionized water, slowly add it dropwise to the slurry, stir at 200 r / min for 30 min, granulate using a disc granulator, control the humidity of the granulation environment at 60%, the granulation speed at 40 r / min, and the granulation time at 15 min to obtain ceramsite green pellets with a particle size of 2.0 mm. S5. Place the raw ceramsite pellets in a forced-air drying oven and dry them at 50℃ for 3 hours, then raise the temperature to 90℃ and dry them for 5 hours until the moisture content is ≤1%. S6. Place the dried green pellets in a muffle furnace, heat to 500℃ at 8℃ / min, and hold for 1.5h to allow the microcrystalline cellulose to fully pyrolyze and volatilize; then heat to 1250℃ at 5℃ / min and hold for 3h, and then cool to room temperature at 3℃ / min to obtain the ceramsite semi-finished product. S7. Stir at 400 r / min, prepare a 5% (w / w) suspension of nano-silica with anhydrous ethanol, immerse the semi-finished ceramic aggregate in the suspension for 2 min, remove it and dry it in a 70℃ drying oven for 3 h to obtain ultra-low density high strength ceramic aggregate proppant.

[0041] Example 3: This embodiment discloses an ultra-low density, high-strength ceramic proppant, the raw materials for which, by weight, are: 45 parts fly ash cenospheres, 10 parts expanded perlite microspheres, 12 parts medium-grade bauxite, 7 parts first-grade fly ash, 4 parts boron nitride-coated mullite hollow microspheres, 3 parts nano silica, 4 parts microcrystalline cellulose, 2.5 parts potassium feldspar powder, 3 parts borate ester-starch grafted modified carboxymethyl cellulose, 1.5 parts short-cut carbon fibers, and 30 parts anhydrous ethanol.

[0042] The medium-grade bauxite has an Al2O3 content of 65% and a particle size of 250 mesh; the first-grade fly ash has a loss on ignition ≤5% and a SiO2 content ≥40%; the nano-silica is fumed silica with a specific surface area of ​​200 m². 2 / g; the length of the short-cut carbon fiber is 0.7mm and the diameter is 15μm.

[0043] The raw materials for preparing boron nitride-coated mullite hollow microspheres, by weight, include: 22 parts of tetraethyl orthosilicate, 9 parts of aluminum nitrate, 82 parts of anhydrous ethanol, 27 parts of deionized water, 11 parts of liquid paraffin, 2.5 parts of Span-80, and 7 parts of ammonia water with a mass fraction of 6.5%.

[0044] The preparation method of boron nitride-coated mullite hollow microspheres includes the following steps: 1) Mix anhydrous ethanol and deionized water at a volume ratio of 3:1, add tetraethyl orthosilicate and aluminum nitrate, adjust the pH to 3.2 with 3% dilute nitric acid, and stir magnetically at 350 r / min for 25 min to obtain a silica-alumina sol. 2) Liquid paraffin was mixed with Span-80 sorbitan monooleate as the oil phase. The oil phase was added dropwise to the silica-alumina sol under high-speed shear at 9000 r / min with an oil-to-water volume ratio of 1:5. The mixture was emulsified for 17 min to form a W / O type emulsion. 3) Add 6.5% ammonia water to the mixture obtained in step 2) to adjust the pH to 8.2, let it stand at 55℃ for 14 hours, and centrifuge at 9000 r / min for 12 minutes to obtain core-shell microspheres; 4) The core-shell microspheres were washed four times with anhydrous ethanol, then dried in a 70°C oven for 9 hours, and then transferred to a muffle furnace. The temperature was increased to 1150°C at 6°C / min under air atmosphere and held for 2.5 hours to obtain mullite hollow microspheres. 5) The mullite hollow microspheres were placed in a tube furnace, and nitrogen gas was introduced at a rate of 150 mL / min as the carrier gas. After the temperature was raised to 850℃, a mixture of borane and ammonia gas with a volume ratio of 1:2 was introduced. The total flow rate of the mixed gas was 220 mL / min. Chemical vapor deposition was carried out under normal pressure for 2.5 h. After cooling, boron nitride-coated mullite hollow microspheres were obtained.

[0045] The raw materials for preparing borate-starch grafted modified carboxymethyl cellulose, by weight, include: 11 parts carboxymethyl cellulose, 3.5 parts soluble starch, 5.5 parts trimethyl borate, 110 parts deionized water, 35 parts anhydrous ethanol, and 0.2 parts ammonium persulfate.

[0046] The preparation method of borate ester-starch grafted modified carboxymethyl cellulose includes the following steps: (1) Dissolve carboxymethyl cellulose with a viscosity of 3000 mPa·s in deionized water and stir at 250 r / min for 35 min in a water bath at 55℃ to obtain an aqueous solution of carboxymethyl cellulose; (2) Dissolve soluble starch and trimethyl borate in all anhydrous ethanol and stir at 250 r / min for 20 min to obtain a monomer mixture; (3) Add the monomer mixture dropwise to the carboxymethyl cellulose aqueous solution, stir at 250 r / min for 12 min at 55 °C, purge with nitrogen to remove oxygen for 25 min, add ammonium persulfate, and stir at 250 r / min for 5 h. (4) The reaction solution obtained in step (3) is slowly poured into anhydrous ethanol for precipitation. After filtration, the filter cake is placed in a vacuum drying oven and dried for 11 hours under vacuum of -0.09 MPa and 65°C. It is then pulverized to 250 mesh to obtain borate ester-starch grafted modified carboxymethyl cellulose.

[0047] This embodiment also discloses a method for preparing an ultra-low density, high-strength ceramic proppant, comprising the following steps: S1. Place fly ash cenospheres, expanded perlite microspheres, medium-grade bauxite, grade 1 fly ash, and potassium feldspar powder separately in a forced-air drying oven and dry at 108℃ for 5 hours until the moisture content is ≤2%. S2. Add the pretreated material from step S1, along with microcrystalline cellulose, chopped carbon fibers, and nano-silica, to a ball mill. Add anhydrous ethanol, control the ball-to-material ratio to be 3:1, and the ball milling speed to be 225 r / min. Mill for 2 hours to obtain a uniform dry-mixed slurry. S3. Add boron nitride-coated mullite hollow microspheres to the dry-mixed slurry and continue ball milling for 35 minutes. S4. Prepare a 6.5% (w / w) aqueous solution of borate ester-starch grafted modified carboxymethyl cellulose with deionized water, slowly add it dropwise to the slurry, stir at 175 r / min for 25 min, granulate using a disc granulator, control the humidity of the granulation environment at 50%, the granulation speed at 35 r / min, and the granulation time at 12 min to obtain ceramsite green pellets with a particle size of 1.4 mm. S5. Place the raw ceramsite pellets in a forced-air drying oven and dry them at 45℃ for 2.5 hours, then raise the temperature to 85℃ and dry them for another 4.5 hours until the moisture content is ≤1%. S6. Place the dried green pellets in a muffle furnace and heat them to 450°C at 6°C / min, hold for 1.5 hours to allow the microcrystalline cellulose to fully pyrolyze and volatilize; then heat them to 1200°C at 4°C / min, hold for 2.5 hours, and then cool them to room temperature at 3°C / min to obtain the ceramsite semi-finished product. S7. Stir at 350 r / min, prepare a 4% (w / w) suspension of nano-silica with anhydrous ethanol, immerse the semi-finished ceramic aggregate in the suspension for 2 min, remove it and dry it in a 65℃ drying oven for 2.5 h to obtain ultra-low density high strength ceramic aggregate proppant.

[0048] Comparative Example 1: An ultra-low density, high-strength ceramic proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that boron nitride-coated mullite hollow microspheres are not added.

[0049] Comparative Example 2: An ultra-low density high-strength ceramic proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that borate ester-starch grafted modified carboxymethyl cellulose is not added, but is replaced with commercially available carboxymethyl cellulose in equal amounts.

[0050] Comparative Example 3: An ultra-low density high-strength ceramic proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that no short-cut carbon fibers are added.

[0051] Comparative Example 4: An ultra-low density, high-strength ceramic proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that expanded perlite microspheres are not added.

[0052] Comparative Example 5: An ultra-low density, high-strength ceramic proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that microcrystalline cellulose is not added.

[0053] Comparative Example 6: An ultra-low density, high-strength ceramic particle proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that no nano-silica is added.

[0054] Comparative Example 7: An ultra-low density high-strength ceramic proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that the sintering temperature is changed to 1000℃.

[0055] Comparative Example 8: An ultra-low density high-strength ceramic proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that the sintering temperature is changed to 1350℃.

[0056] Comparative Example 9: An ultra-low density high-strength ceramic proppant and its preparation method are disclosed. The only difference between this proppant and Example 3 is that anhydrous ethanol is replaced with an equal amount of deionized water as the dispersion medium.

[0057] The density, mechanical properties, acid and alkali resistance, and formability of the ceramsite proppants obtained in Examples 1-3 and Comparative Examples 1-9 were tested. The testing methods and standards for each property are as follows: (a) Density testing: Apparent density testing: According to the apparent density test method in GB / T 17431.2-2010. Take 50g of the proppant sample dried to constant weight, place it in a 100mL density bottle, add anhydrous ethanol to the mark, and record the volume V1; remove the sample and add anhydrous ethanol again to the mark, recording the volume V2. The true sample volume V = V1 - V2. The apparent density is calculated using the formula: ρ = m / V, where m is the sample mass in g / cm³. 3The test result is the arithmetic mean of three measurements.

[0058] Bulk density test: According to the bulk density test method in SY / T 5108-2014. Slowly add the proppant sample to a 100mL standard graduated cylinder, gently tap the cylinder wall until the volume no longer changes, and record the bulk volume V. The bulk density calculation formula is: ρb=m / V, where m is the sample mass in g / cm³. 3 The test result is the arithmetic mean of three measurements.

[0059] (II) Mechanical property testing: Compressive strength testing: The single-particle compressive strength test method is based on SY / T 5108-2014. An electronic universal testing machine is used, selecting proppant single particles with a particle size of 1.0-1.2 mm, and conducting a uniaxial compression test at a loading rate of 1 mm / min. The maximum load F at the point of particle breakage is recorded. The compressive strength calculation formula is: P = 4F / (πd 2 ), where d is the particle diameter, and the arithmetic mean of 50 samples is taken, with the unit being MPa.

[0060] Breakage resistance test: The breakage resistance test method is based on SY / T 5108-2014. Take 50g of proppant sample with a particle size of 0.8-2.0mm, place it in a breakage resistance tester, and vibrate it under 69MPa pressure for 30min. After completion, pass it through a 0.15mm standard sieve. The breakage resistance calculation formula is: η=(m1 / m0)×100%, where m1 is the mass of the sample passing through the sieve, and m0 is the total sample mass, expressed as a percentage. The test result is the arithmetic mean of three measurements.

[0061] (III) Testing of resistance to media: Acid resistance test: According to the acid solubility test method in SY / T 5108-2014. Take 20g of the proppant sample, place it in a 10wt% hydrochloric acid solution, and immerse it in an 80℃ constant temperature water bath for 24 hours. After removal, rinse with deionized water until neutral, and dry to constant weight. The acid resistance calculation formula is: R a =(m2 / m3)×100%, where m2 is the mass after soaking and m3 is the mass before soaking, and the unit is %. The test result is the arithmetic mean of three measurements.

[0062] Alkali resistance test: Refer to the acid resistance test method, replace the medium with a 10wt% sodium hydroxide solution, and perform the test and calculation according to the same steps. The unit is %.

[0063] (iv) Molding performance testing: Green pellet forming rate test: Take all the material after granulation, and count the mass of green pellets (m4) with a particle size in the range of 0.8-2.0 mm and the total mass of granulated material (m5). The green pellet forming rate is calculated as follows: ηf = (m4 / m5) × 100%, in percent.

[0064] Finished product qualification rate test: Take all samples after sintering, and count the mass m6 of finished products with particle size between 0.8-2.0mm and no obvious cracks or deformation, and the total mass m7 of green pellets. The finished product qualification rate is calculated using the formula: ηq=(m6 / m7)×100%, in percentages (%).

[0065] The results are shown in Tables 2 and 3.

[0066] Table 2. Results of density and mechanical property tests Group <![CDATA[Apparent density (g / cm 3 ).]]> <![CDATA[Bulk density (g / cm 3 )]]> Compressive strength (MPa) Breakage resistance (%) Example 1 1.45 0.82 90 0.9 Example 2 1.38 0.79 98 0.7 Example 3 1.32 0.75 110 0.5 Comparative Example 1 1.31 0.74 75 1.6 Comparative Example 2 1.33 0.76 88 0.9 Comparative Example 3 1.32 0.75 82 1.4 Comparative Example 4 1.50 0.86 95 0.6 Comparative Example 5 1.20 0.68 68 2.1 Comparative Example 6 1.32 0.75 96 0.8 Comparative Example 7 1.35 0.77 70 1.9 Comparative Example 8 1.42 0.80 92 1.2 Comparative Example 9 1.34 0.77 85 1.3 Table 3. Test results of acid and alkali resistance and molding performance Group Acid resistance rate (%) Alkali resistance rate (%) Green pellet forming rate (%) Finished product pass rate (%) Example 1 97.2 97.8 96.5 94.2 Example 2 98.5 98.9 97.8 96.5 Example 3 99.1 99.5 98.8 97.5 Comparative Example 1 98.9 99.3 98.6 97.3 Comparative Example 2 97.5 98.2 92.8 89.5 Comparative Example 3 99.0 99.4 98.7 97.4 Comparative Example 4 98.8 99.2 98.5 97.2 Comparative Example 5 98.7 99.1 98.4 97.0 Comparative Example 6 95.5 96.2 98.6 97.3 Comparative Example 7 96.8 97.5 97.9 95.8 Comparative Example 8 97.0 97.8 97.6 94.5 Comparative Example 9 97.3 98.0 94.2 91.0 Using Example 3 as the control group, the performance differences and causes of Comparative Examples 1-9 are analyzed as follows: Comparative Example 1 (mullite hollow microspheres without boron nitride coating): Compared to Example 3, the compressive strength decreased from 110 MPa to 75 MPa, a decrease of 31.8%; the breakage resistance increased from 0.5% to 1.6%, an increase of 220%. This is because the lack of micron-level micro-support units and hierarchical reinforcing skeletons meant the proppant matrix lacked local compressive load-bearing structures, making crack propagation easier within the matrix. Since the porosity and aggregate composition remained unchanged, the density did not change significantly.

[0067] Comparative Example 2 (replacing borate-starch grafted modified carboxymethyl cellulose with ordinary carboxymethyl cellulose in equal amounts): Compared to Example 3, the compressive strength decreased from 110 MPa to 88 MPa, a decrease of 20%; the breakage rate increased from 0.5% to 0.9%, an increase of 80%; the green pellet formation rate decreased from 98.8% to 92.8%, a decrease of 6.1%; and the finished product qualification rate decreased from 97.5% to 89.5%, a decrease of 8.2%. This is because ordinary carboxymethyl cellulose lacks dynamic cross-linking characteristics, and its adhesion and temperature resistance are far lower than those of the modified binder. The green pellet strength is insufficient, and microcracks are easily generated during drying and sintering, leading to a decrease in matrix density.

[0068] Comparative Example 3 (without chopped carbon fibers): Compared to Example 3, the compressive strength decreased from 110 MPa to 82 MPa, a decrease of 25.5%; the breakage rate increased from 0.5% to 1.4%, an increase of 180%. This is because the lack of a three-dimensional network crack-blocking structure allows microcracks generated in the matrix to easily propagate into macrocracks. Since carbon fibers do not participate in density, media resistance, or the molding process, other properties remained largely unchanged.

[0069] Comparative Example 4 (without expanded perlite microspheres): Compared to Example 3, the apparent density increased from 1.32 g / cm³. 3 Increased to 1.50 g / cm³ 3 The increase was 13.6%; the bulk density increased from 0.75 g / cm³. 3 Increased to 0.86 g / cm³ 3 The increase was 14.7%; the compressive strength decreased from 110MPa to 95MPa, a decrease of 13.6%. The reason is the lack of lightweight aggregate, which increases the overall aggregate density of the proppant. At the same time, the porous structure of expanded perlite microspheres can help disperse stress, and the lack of these components reduces the compressive strength.

[0070] Comparative Example 5 (without microcrystalline cellulose): Compared to Example 3, the compressive strength decreased from 110 MPa to 68 MPa, a decrease of 38.2%; the breakage rate increased from 0.5% to 2.1%, an increase of 320%; and the apparent density increased from 1.32 g / cm³. 3 Reduced to 1.20 g / cm³ 3 The decrease was 9.1%. The reason is that although the density decreased due to the absence of a pore-forming agent, the lack of a uniform closed-cell structure led to stress concentration between aggregate particles. After sintering, the matrix density was uneven, and local stress cracking was prone to occur, resulting in a significant decrease in mechanical properties.

[0071] Comparative Example 6 (without nano-silica): Compared to Example 3, the compressive strength decreased from 110 MPa to 96 MPa, a decrease of 12.7%; the breakage resistance increased from 0.5% to 0.8%, an increase of 60%; the acid resistance decreased from 99.1% to 95.5%, a decrease of 3.6%; and the alkali resistance decreased from 99.5% to 96.2%, a decrease of 3.3%. This is because the proppant surface lacks a dense silica coating, resulting in increased surface porosity. Furthermore, the role of nano-silica in filling the gaps between aggregates is lost, leading to a thinner sintering neck between particles and a decrease in overall strength.

[0072] Comparative Example 7 (lower sintering temperature): Compared to Example 3, the compressive strength decreased from 110 MPa to 70 MPa, a decrease of 36.4%; the fracture resistance increased from 0.5% to 1.9%, an increase of 280%. The reason is that the sintering temperature did not reach the optimal temperature range for the formation of the mullite-corundum composite crystal phase, resulting in incomplete crystal phase development, insufficient glass phase formation, poor adhesion between aggregate particles, and low matrix density.

[0073] Comparative Example 8 (higher sintering temperature): Compared to Example 3, the apparent density increased from 1.32 g / cm³. 3 Increased to 1.42 g / cm³ 3 The increase was 7.6%; the bulk density increased from 0.75 g / cm³. 3 Increased to 0.80 g / cm³ 3The increase was 6.7%; the compressive strength decreased from 110MPa to 92MPa, a decrease of 16.4%. The reason is that the excessively high sintering temperature caused the internal pores of the proppant to shrink excessively or even close, resulting in an increase in density; at the same time, the crystal phase grew excessively, the interlocking of the needle-like mullite crystals decreased, and the brittleness of the matrix increased.

[0074] Comparative Example 9 (dispersion medium replaced with deionized water): Compared to Example 3, the compressive strength decreased from 110 MPa to 85 MPa, a decrease of 22.7%; the breakage resistance increased from 0.5% to 1.3%, an increase of 160%; the green pellet formation rate decreased from 98.8% to 94.2%, a decrease of 4.7%; and the finished product qualification rate decreased from 97.5% to 91.0%, a decrease of 6.7%. This is because aqueous dispersion leads to agglomeration of powder components, especially boron nitride-coated mullite hollow microspheres and nano-silica, etc. Agglomeration creates localized weak areas, resulting in uneven performance after sintering.

[0075] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A low-density, high-strength ceramic proppant, characterized in that, The raw materials for its preparation, by weight, include: 40-50 parts fly ash cenospheres, 8-12 parts expanded perlite microspheres, 10-15 parts medium-grade bauxite, 5-8 parts grade I fly ash, 3-5 parts boron nitride-coated mullite hollow microspheres, 2-4 parts nano silica, 3-5 parts microcrystalline cellulose, 2-3 parts potassium feldspar powder, 2-4 parts borate ester-starch grafted modified carboxymethyl cellulose, 1-2 parts short-cut carbon fibers, and 20-40 parts anhydrous ethanol.

2. The ultra-low density high-strength ceramic proppant according to claim 1, characterized in that, The medium-grade bauxite has an Al2O3 content of 60%-70% and a particle size of 200-300 mesh; the first-grade fly ash has a loss on ignition ≤5% and a SiO2 content ≥40%; the nano-silica is fumed silica with a specific surface area of ​​180-220 m². 2 / g; the length of the short-cut carbon fiber is 0.5-1mm and the diameter is 10-20μm.

3. The ultra-low density high-strength ceramic proppant according to claim 1, characterized in that, The raw materials for preparing boron nitride-coated mullite hollow microspheres, by weight, include: 20-25 parts of tetraethyl orthosilicate, 8-10 parts of aluminum nitrate, 75-90 parts of anhydrous ethanol, 25-30 parts of deionized water, 10-12 parts of liquid paraffin, 2-3 parts of sorbitan monooleate, and 5-10 parts of ammonia water with a mass fraction of 5%-8%.

4. The ultra-low density high-strength ceramsite proppant according to claim 3, characterized in that, The preparation method of boron nitride-coated mullite hollow microspheres includes the following steps: 1) Mix anhydrous ethanol and deionized water at a volume ratio of 3:1, add tetraethyl orthosilicate and aluminum nitrate, adjust the pH to 3.0-3.5 with 1%-5% dilute nitric acid, and stir magnetically at 300-400 r / min for 20-30 min to obtain a silica-alumina sol. 2) Mix liquid paraffin with dehydrated sorbitan monooleate as the oil phase. Under high-speed shearing of 8000-10000 r / min, the oil phase is dropped into the silica-alumina sol with an oil-to-water volume ratio of 1:

5. Emulsify for 15-20 min to form a W / O type emulsion. 3) Add 5%-8% ammonia water to the mixture obtained in step 2) to adjust the pH to 8.0-8.5, let it stand at 50-60℃ for 12-16 hours, and centrifuge at 8000-10000 r / min for 10-15 minutes to obtain core-shell microspheres; 4) Wash the core-shell microspheres with anhydrous ethanol 3-5 times, then dry them in a forced-air drying oven at 60-80℃ for 8-10 hours, and then transfer them to a muffle furnace. Heat the muffle furnace to 1100-1200℃ at 5-8℃ / min in an air atmosphere and hold for 2-3 hours to obtain mullite hollow microspheres. 5) Place the mullite hollow microspheres in a tube furnace and introduce nitrogen gas as a carrier gas at a rate of 100-200 mL / min. After heating to 800-900℃, introduce a mixture of borane and ammonia gas with a volume ratio of 1:

2. The total flow rate of the mixed gas is 150-300 mL / min. Chemical vapor deposition is carried out under normal pressure for 2-3 hours. After cooling, boron nitride-coated mullite hollow microspheres are obtained.

5. The ultra-low density high-strength ceramic proppant according to claim 1, characterized in that, The raw materials for preparing borate-starch grafted modified carboxymethyl cellulose, by weight, include: 10-12 parts carboxymethyl cellulose, 3-4 parts soluble starch, 5-6 parts trimethyl borate, 100-120 parts deionized water, 30-40 parts anhydrous ethanol, and 0.1-0.3 parts ammonium persulfate.

6. The ultra-low density high-strength ceramsite proppant according to claim 5, characterized in that, The preparation method of borate ester-starch grafted modified carboxymethyl cellulose includes the following steps: (1) Dissolve carboxymethyl cellulose in deionized water and stir at 200-300 r / min for 30-40 min in a water bath at 50-60℃ to obtain an aqueous solution of carboxymethyl cellulose; (2) Dissolve soluble starch and trimethyl borate in all anhydrous ethanol and stir at 200-300 r / min for 15-25 min to obtain a monomer mixture; (3) Add the monomer mixture dropwise to the carboxymethyl cellulose aqueous solution, stir at 200-300 r / min for 10-15 min at 50-60℃, purge with nitrogen to remove oxygen for 20-30 min, add ammonium persulfate, and stir at 200-300 r / min for 4-6 h. (4) Slowly pour the reaction solution obtained in step (3) into anhydrous ethanol for precipitation. After filtration, place the filter cake in a vacuum drying oven and dry it for 10-12 hours under vacuum conditions of -0.08MPa to -0.10MPa and 60-70℃. Then pulverize it to 200-300 mesh to obtain borate ester-starch grafted modified carboxymethyl cellulose.

7. A method for preparing an ultra-low density, high-strength ceramsite proppant according to any one of claims 1-6, characterized in that, Includes the following steps: S1. Place fly ash cenospheres, expanded perlite microspheres, medium-grade bauxite, grade 1 fly ash, and potassium feldspar powder separately in a forced-air drying oven and dry them at 105-110℃ for 4-6 hours until the moisture content is ≤2%. S2. Add the pretreated material from step S1, along with microcrystalline cellulose, chopped carbon fibers, and nano-silica, to a ball mill. Add anhydrous ethanol, control the ball-to-material ratio at 3:1, and mill at 200-250 r / min for 1.5-2 h to obtain a uniform dry-mixed slurry. S3. Add boron nitride-coated mullite hollow microspheres to the dry mixture slurry and continue ball milling for 30-40 minutes. S4. Prepare a 5%-8% (w / w) aqueous solution of borate ester-starch grafted modified carboxymethyl cellulose with deionized water, slowly add it dropwise to the slurry, stir at 150-200 r / min for 20-30 min, granulate using a disc granulator at a granulation speed of 30-40 r / min for 10-15 min, and obtain ceramsite green pellets with a particle size of 0.8-2.0 mm. S5. Place the raw ceramsite pellets in a forced-air drying oven and dry them at 40-50℃ for 2-3 hours, then raise the temperature to 80-90℃ and dry them for 4-5 hours until the moisture content is ≤1%. S6. Place the dried green pellets in a muffle furnace and heat them to 400-500℃ at 5-8℃ / min, and hold for 1-1.5h; then heat them to 1150-1250℃ at 3-5℃ / min, and hold for 2-3h; then cool them to room temperature at 2-3℃ / min to obtain the semi-finished ceramsite. S7. Prepare a suspension of nano-silica with anhydrous ethanol at a mass fraction of 3%-5%. Immerse the semi-finished ceramic aggregate in the suspension for 1-2 minutes, then remove it and dry it in a drying oven at 60-70℃ for 2-3 hours to obtain ultra-low density high-strength ceramic aggregate proppant.

8. The method for preparing ultra-low density high-strength ceramsite proppant according to claim 7, characterized in that, In step S4, the humidity of the granulation environment of the disc granulator is controlled at 40%-60%.

9. The method for preparing ultra-low density high-strength ceramic proppant according to claim 7, characterized in that, In step S7, the suspension is prepared by stirring at 300-400 r / min.