Preparation method of ultra-light ceramsite proppant for long-distance conveying and filling

Abstract

The invention relates to the technical field of oil and gas resource development, in particular to a preparation method of an ultra-light ceramsite proppant for long-distance conveying and filling. The method comprises the steps that after green pellets are prepared, polysilicate precursor sol is sprayed and permeated for several times to conduct surface modification on the green pellets, then pseudo-boehmite sol and alpha-phase nanometer aluminum oxide dispersion liquid are sprayed and added in sequence, mullite crystal nucleus powder is embedded, and finally segmented oxidation presintering and high-temperature sintering are conducted. According to the method, a compact strengthening layer formed by a mullite crystal framework is constructed on the surface layer of a propping agent particle, and a porous light structure is reserved inside, so that low density, high crushing resistance and long-term flow guide stability are effectively considered, and the method is particularly suitable for long-distance conveying and filling operation of low-viscosity slick water fracturing fluid.

Description

A method for preparing ultralight ceramic proppant for long-distance transport and filling Technical Field

[0001] This invention relates to the field of oil and gas resource development technology, and in particular to a method for preparing an ultralight ceramic proppant for long-distance transportation and backfilling. Background Technology

[0002] With the deepening development of unconventional oil and gas resources, low-viscosity slickwater fracturing fluid systems have been widely used due to their low friction and strong proppant carrying capacity. This places demands on proppants to possess good suspension and long-distance transport capabilities in low-viscosity fluids. Ultra-low density ceramic proppants, especially those with a density close to or below 1.30 g / cm³, are particularly important. 3 Products that meet this demand have become a research hotspot. These proppants reduce density by introducing porosity into the matrix, thereby achieving lower settling velocities in low-viscosity fracturing fluids, which is beneficial for achieving uniform sand spreading in long horizontal well sections.

[0003] However, there is an inherent contradiction between low density and high strength and high conductivity. The porous structure introduced to reduce density, especially open or interconnected pores, often becomes a mechanical weakness within the material. Under high closure pressure, stress concentration easily occurs at the edges of these pores, causing proppant particles to break or become embedded. This not only instantly reduces the conductivity of the cracks, but the resulting debris also blocks the pore channels, causing a continuous and rapid decline in conductivity. Therefore, how to effectively improve the mechanical integrity and fracture resistance of the proppant, especially its surface layer, while maintaining low density has become a key technical challenge.

[0004] In existing technologies, the main approaches to improving the performance of low-density ceramic proppant focus on overall formulation optimization and sintering regime adjustments. For example, adjusting the aluminum-silicon raw material ratio and introducing pore-forming agents or sintering aids can regulate the overall microstructure and pore distribution. While these methods can reconcile the relationship between density and strength to some extent, they are often a strategy for overall homogeneity improvement, making it difficult to achieve differentiated design and performance division between the surface and internal structures. Overall densification can improve strength but sacrifices low-density characteristics; while maintaining high overall porosity inevitably weakens its compressive strength.

[0005] Other studies have attempted to treat the proppant particles with surface treatments, such as spraying organic coatings or inorganic films, to improve surface smoothness and prevent adhesion, or to form a dense protective shell. However, the bonding strength between these outer layers and the substrate is limited, making them prone to peeling or cracking under sustained high stress, and the outer layer itself may negatively impact conductivity. Furthermore, some studies have focused on using specific precursors (such as boehmite) or seed crystals (such as mullite nuclei) to promote the overall mullification reaction of the material to improve high-temperature performance, but their applications are mostly limited to bulk gels or preforms, failing to address the issue of directional reinforcement of the surface structure after spheroidization.

[0006] In summary, the existing technology lacks a process method that can precisely control the gradient structure of the ceramsite proppant from the surface to the interior, making it difficult for the product to maintain excellent conveying performance (low density, low settling velocity) while also possessing excellent compressive strength (low breakage rate) and long-term stable flowability. Summary of the Invention

[0007] In view of this, the purpose of this invention is to propose a method for preparing an ultra-lightweight ceramic proppant for long-distance transport and filling, so as to solve the technical problem that existing low-density ceramic proppants, due to their homogenized structure, cannot simultaneously achieve long-distance transport capacity in low-viscosity fracturing fluids and maintain surface integrity, anti-fracture properties, and long-term flow retention under high closure pressure.

[0008] To achieve the above objectives, the present invention provides a method for preparing an ultralight ceramic proppant for long-distance transport and filling, comprising the following steps:

[0009] S1, mix ultrafine kaolin, α-alumina micro powder and soluble starch, add polyvinyl alcohol binder to granulate, dry to obtain green pellets;

[0010] S2, keeping the green ball in a rolling state, spraying the polysilicate precursor sol onto the surface of the green ball in batches, rolling and hot air treatment after each spraying, and continuing to roll after all spraying is completed to obtain surface-modified green balls.

[0011] S3, on the surface-modified green ball, pseudo-boehmite sol and α-phase nano-alumina isopropanol dispersion are sprayed sequentially, and while the surface is still wet, mullite nucleus powder is evenly sprinkled on the surface of the green ball and rolled continuously to embed the mullite nucleus powder into the surface layer. Then it is dried to obtain the modified green ball.

[0012] S4. Modified green pellets are placed in an air atmosphere for segmented oxidation pre-calcination, then heated and sintered, then cooled and sieved to obtain ultra-lightweight ceramic proppant for long-distance conveying and filling.

[0013] Preferably, in step S3, based on 500-540 parts by weight of ultrafine kaolin, the amount of boehmite sol added is 42-58 parts, the amount of α-phase nano-alumina isopropanol dispersion added is 24-36 parts, and the amount of mullite nucleus powder added is 6-10 parts.

[0014] Preferably, the ultrafine kaolin has a mesh size of 5000-7000.

[0015] Preferably, the particle size of the α-alumina micro powder is 0.1-0.3 μm, and the solid content of the α-phase nano-alumina isopropanol dispersion is 18%-22%, wherein the particle size of the α-phase nano-alumina is 20-30 nm.

[0016] Preferably, in step S1, the ratio of the ultrafine kaolin, α-alumina powder and soluble starch by mass is 500-540:400-440:50-70; the polyvinyl alcohol binder is prepared by 18-24 parts of polyvinyl alcohol 1788 and 220-260 parts of deionized water.

[0017] Preferably, the polysilicate precursor sol is prepared by the following method: by weight, 30-42 parts of tetraethoxysilane are added to 55-68 parts of anhydrous ethanol, stirred for 5 min, and then a mixture of 8-12 parts of deionized water and 0.8-1.2 parts of hydrochloric acid with a mass fraction of 36% is added. The mixture is stirred at 22-28°C for 35-50 min to obtain the polysilicate precursor sol.

[0018] Preferably, in step S1, the moisture content of the green pellets is 8.8%-10.5%.

[0019] Preferably, in step S2, the green pellets are continuously rolled at 42-48°C; the polysilicate precursor sol is sprayed onto the surface of the green pellets in 4-8 applications; after each application, the pellets are rolled for 2-4 minutes and hot air at 50-60°C is introduced for 1.5-3 minutes; after all the pellets are sprayed, they are rolled for another 6-10 minutes.

[0020] Preferably, the pseudoboehmite sol is prepared by the following method: 190-215 parts by weight of deionized water are heated to 82-88°C, and 18-23 parts by weight of aluminum isopropoxide are added within 25-35 minutes under continuous stirring and reflux conditions, and the reaction is maintained at 82-88°C for 50-70 minutes; then an acidification solution consisting of 18-22 parts by weight of deionized water and 0.8-1.2 parts by weight of 36% hydrochloric acid is added, and the mixture is kept at this temperature for 75-105 minutes, so that the hydrolysis product is converted into pseudoboehmite sol through peptization; then the mixture is cooled and concentrated by volume reduction at 67-74°C to obtain 72-88 parts by weight of pseudoboehmite sol.

[0021] Preferably, the mullite nucleus powder is prepared by the following method: 110-130 parts by mass of anhydrous ethanol are added to a container, 26-34 parts of aluminum isopropoxide are added at 57-62°C, and after stirring until clear, 8-13 parts of tetraethoxysilane are added. Then, a mixture of 16-22 parts of deionized water and 0.8-1.2 parts of 36% hydrochloric acid is added dropwise over 20 minutes, and stirring is continued at 57-62°C for 1.5-2.5 hours to obtain a precursor sol. The precursor sol is allowed to stand for 10-14 hours, then dried at 105-115°C for 8-12 hours, and then heated to 1270-1320°C at a rate of 2.8-3.2°C / min and held at that temperature for 1.5-2.5 hours. After cooling, the mixture is ball-milled for 3-5 hours and passed through a 325-400 mesh sieve to obtain the mullite nucleus powder.

[0022] Preferably, in step S3, the pseudo-boehmite sol is first sprayed onto the surface of the green pellet in 3-5 applications, with rolling for 3-5 minutes after each application. After all the spraying is completed, the pellet is treated with hot air at 55-65℃ for 8-12 minutes. Then, the α-phase nano-alumina isopropanol dispersion is sprayed onto the surface of the green pellet in 2-4 applications, with rolling for 4-6 minutes after each application. Next, the mullite nucleus powder is evenly sprinkled onto the surface of the green pellet and the pellet is rolled for another 4-6 minutes. Finally, the pellet is dried at 105-115℃ for 1.5-2.5 hours to obtain the modified green pellet.

[0023] Preferably, in step S4, the segmented oxidation pre-firing and sintering specifically involve: heating to 330-375℃ at 1.8-2.3℃ / min and holding for 20-40min in an air atmosphere, then heating to 620-690℃ at 1.2-1.7℃ / min and holding for 45-80min, followed by heating to 860-940℃ at 1.8-2.3℃ / min and holding for 20-40min; then heating to 1365-1398℃ at 3.8-4.3℃ / min and holding for 40-60min, and finally cooling to 550-650℃ at no more than 5℃ / min and then cooling with the furnace.

[0024] The beneficial effects of this invention are:

[0025] This invention achieves a gradient structure of dense outer layer and porous inner layer in ceramsite proppant through a process combining post-granulation outer layer modification with segmented oxidation pre-calcination. The multi-stage spraying of polysilicate precursor sol onto the surface of the green pellets selectively enriches the outer layer of the particles, laying the compositional foundation for subsequent preferential reaction and densification of the outer layer.

[0026] In the surface modification step, the first sprayed pseudo-boehmite sol can quickly adhere to the surface of the green pellets and form a highly reactive aluminum source network, which plays a role in the initial fixation and activation of the surface layer; the subsequently sprayed α-phase nano-alumina isopropanol dispersion can effectively fill and bridge, inhibit the excessive shrinkage and cracking of the surface material at high temperature, thereby significantly improving the continuity, integrity and toughness of the surface sintered body.

[0027] Embedding mullite nuclei on this moist and active surface layer guides the preferential and concentrated nucleation and growth of the outer mullite phase, prompting the surface layer to form a high-strength, high-stability mullite crystal framework first during sintering. The inner layer, not deeply affected by the aforementioned modified substances, primarily relies on the loss of soluble starch to form closed pores, thus retaining its low-density characteristics.

[0028] Furthermore, the specific segmented oxidation pre-calcination process allows the organic matter to fully and gradually decompose and escape before the main framework is sintered at high temperature, while simultaneously promoting the early stabilization and strengthening of the outer inorganic reaction network. This not only improves the repeatability of the preparation process and the consistency of the product, but also ultimately achieves a balance between low density, high strength, and high conductivity in the ultralight ceramic proppant prepared by this invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0030] Raw materials and sources: Ultrafine kaolin was sourced from Shanghai Maclean Biochemical Technology Co., Ltd., K812213, with a specification of 6000 mesh; α-alumina micro powder was sourced from Shanghai Maclean Biochemical Technology Co., Ltd., A810836, with a purity of 99.99%, α-phase crystal form, and a particle size of 0.2 μm; α-phase nano-alumina isopropanol dispersion was sourced from Shanghai Maclean Biochemical Technology Co., Ltd., A801482, with a particle size of 20-30 nm and a solid content of 20.5%; polyvinyl alcohol 1788 was sourced from Shanghai Maclean Biochemical Technology Co., Ltd., P815723; and soluble starch was sourced from Shanghai Maclean Biochemical Technology Co., Ltd., S741671.

[0031] Example 1:

[0032] Step 1: Add 120g of anhydrous ethanol to a reaction vessel, add 30g of aluminum isopropoxide at 60℃, stir until clear, then add 10g of tetraethoxysilane, and then add a mixture of 18g of deionized water and 1g of 36% hydrochloric acid dropwise over 20min. Continue stirring at 60℃ for 2h to obtain a uniformly dispersed aluminum-silicon precursor sol. After the obtained sol is allowed to stand and age for 12h, it is dried at 110℃ for 10h, then heated to 1300℃ at 3℃ / min and held for 2h. After cooling, it is ball-milled for 4h and passed through a 325-mesh sieve to obtain mullite nucleus powder.

[0033] Step 2: Heat 200g of deionized water to 85℃, add 20g of aluminum isopropoxide within 30min under continuous stirring and reflux, and maintain the reaction at 85℃ for 1h; then add an acidification solution consisting of 20g of deionized water and 1g of 36% hydrochloric acid, and continue to keep warm for 90min, then cool and concentrate at 70℃ to obtain 80g of pseudoboehmite sol;

[0034] Step 3: Add 520g of ultrafine kaolin, 420g of α-alumina micro powder, and 60g of soluble starch to a mixer and dry mix for 30 minutes; separately, add 20g of polyvinyl alcohol 1788 to 240g of deionized water, heat to 90℃ and keep warm for 30 minutes until completely dissolved, then cool to 40℃ to obtain a binder liquid; uniformly spray the binder liquid into the mixed powder, add 50g of deionized water, and granulate in a conventional disc granulator to obtain green pellets with a particle size of 0.35-0.65mm; dry the obtained green pellets at 55℃ for 40 minutes to reduce the total moisture content to 9.5%.

[0035] Step 4: Add 36g of tetraethoxysilane to 60g of anhydrous ethanol, stir for 5 minutes, then add a mixture of 10g of deionized water and 1g of 36% hydrochloric acid. Continue stirring at 25°C for 40 minutes to obtain a polysilicate precursor sol. Keep the green pellets with a water content of 9.5% obtained in Step 3 at 45°C and keep them rolling. Spray the above polysilicate precursor sol onto the surface of the green pellets in 6 portions, with 18g sprayed on each of the first 5 portions and 17g sprayed on the 6th portion. After each spraying, roll for 3 minutes and pass hot air at 55°C for 2 minutes. After all the pellets have been sprayed, roll for another 8 minutes.

[0036] Step 5: On the green balls with surface adhesion obtained in Step 4, first spray 50g of pseudo-boehmite sol in 4 portions, rolling for 4 minutes after each spray. After all spraying is completed, treat with hot air at 60℃ for 10 minutes. Then, spray 30g of α-phase nano-alumina isopropanol dispersion in 3 portions, rolling for 5 minutes after each spray. While the surface is still wet, evenly sprinkle 8g of mullite nucleus powder on the surface of the green balls and continue rolling for 5 minutes to embed the mullite nuclei into the surface. Finally, dry at 110℃ for 2 hours to obtain modified green balls.

[0037] Step 6: Place the modified green pellets from Step 5 in an air atmosphere, heat them to 350℃ at 2℃ / min and hold for 30 min, then heat them to 650℃ at 1.5℃ / min and hold for 60 min, then heat them to 900℃ at 2℃ / min and hold for 30 min; continue heating them to 1380℃ in an air atmosphere at 4℃ / min and hold for 50 min, then cool them to 600℃ at no more than 5℃ / min and cool them with the furnace; sieve them to 40-70 mesh and remove particles with abnormal morphology to obtain ultra-lightweight ceramic proppant for long-distance transport and filling of shale oil and gas horizontal wells and low-viscosity fracturing fluid systems.

[0038] Example 2:

[0039] Step 1: Add 110g of anhydrous ethanol to a reaction vessel, add 26g of aluminum isopropoxide at 58℃, stir until clear, then add 8g of tetraethoxysilane, and then add a mixture of 16g ​​of deionized water and 0.8g of 36% hydrochloric acid dropwise over 20min. Continue stirring at 58℃ for 1.5h to obtain a uniformly dispersed aluminum-silicon precursor sol. After the obtained sol is allowed to stand and age for 10h, it is dried at 105℃ for 8h, then heated to 1270℃ at 2.8℃ / min and held for 1.5h. After cooling, it is ball-milled for 3h and passed through a 350-mesh sieve to obtain mullite nucleus powder.

[0040] Step 2: Heat 190g of deionized water to 82℃, add 18g of aluminum isopropoxide over 25min under continuous stirring and reflux, and maintain the reaction at 82℃ for 50min; then add an acidification solution consisting of 18g of deionized water and 0.8g of 36% hydrochloric acid, and continue to keep warm for 75min, then cool and concentrate at 67℃ to obtain 72g of pseudoboehmite sol;

[0041] Step 3: Add 500g of ultrafine kaolin, 440g of α-alumina micro powder, and 50g of soluble starch to a mixer and dry mix for 25 minutes; separately, add 18g of polyvinyl alcohol 1788 to 220g of deionized water, heat to 88℃ and keep warm for 25 minutes until completely dissolved, then cool to 36℃ to obtain a binder liquid; uniformly spray the binder liquid into the mixed powder, add 40g of deionized water, and granulate in a conventional disc granulator to obtain green pellets with a particle size of 0.35-0.65mm; dry the obtained green pellets at 50℃ for 35 minutes to reduce the total moisture content to 8.8%;

[0042] Step 4: Add 30g of tetraethoxysilane to 55g of anhydrous ethanol, stir for 5 minutes, then add a mixture of 8g of deionized water and 0.8g of 36% hydrochloric acid. Continue stirring at 22°C for 35 minutes to obtain a polysilicate precursor sol. Keep the green pellets with a water content of 8.8% obtained in Step 3 at 42°C and keep them rolling. Spray the above polysilicate precursor sol onto the surface of the green pellets in four separate applications, with the amounts sprayed in the four applications being 23g, 23g, 24g, and 23.8g respectively. After each application, roll for 2 minutes and pass hot air at 50°C for 1.5 minutes. After all the pellets have been sprayed, roll for another 6 minutes.

[0043] Step 5: On the green balls with surface adhesion obtained in Step 4, 42g of pseudo-boehmite sol is sprayed in three portions, with rolling for 3 minutes after each spray. After all the spraying is completed, the balls are treated with hot air at 55℃ for 8 minutes. Then, 24g of α-phase nano-alumina isopropanol dispersion is sprayed in two portions, with rolling for 4 minutes after each spray. While the surface is still wet, 6g of mullite nucleus powder is evenly sprinkled on the surface of the green balls and the balls are rolled for another 4 minutes to embed the mullite nuclei into the surface. Finally, the balls are dried at 105℃ for 1.5 hours to obtain the modified green balls.

[0044] Step 6: Place the modified green pellets from Step 5 in an air atmosphere, heat them to 330℃ at 1.8℃ / min and hold for 20 min, then heat them to 620℃ at 1.2℃ / min and hold for 45 min, then heat them to 860℃ at 1.8℃ / min and hold for 20 min; continue heating them in an air atmosphere to 1365℃ at 3.8℃ / min and hold for 40 min, then cool them to 550℃ at no more than 5℃ / min and cool them in the furnace; sieve them to 40-70 mesh and remove particles with abnormal morphology to obtain ultra-lightweight ceramic proppant for long-distance conveying and filling.

[0045] Example 3:

[0046] Step 1: Add 130g of anhydrous ethanol to a reaction vessel, add 34g of aluminum isopropoxide at 62℃, stir until clear, then add 13g of tetraethoxysilane, and then add a mixture of 22g of deionized water and 1.2g of 36% hydrochloric acid dropwise over 20min. Continue stirring at 62℃ for 2.5h to obtain a uniformly dispersed aluminum-silicon precursor sol. After the obtained sol is allowed to stand and age for 14h, it is dried at 115℃ for 12h, then heated to 1320℃ at 3.2℃ / min and held for 2.5h. After cooling, it is ball-milled for 5h and passed through a 400-mesh sieve to obtain mullite nucleus powder.

[0047] Step 2: Heat 215g of deionized water to 88℃, add 23g of aluminum isopropoxide over 35min under continuous stirring and reflux, and maintain the reaction at 88℃ for 70min; then add an acidification solution consisting of 22g of deionized water and 1.2g of 36% hydrochloric acid, and continue to keep warm for 105min, then cool and concentrate at 74℃ to obtain 88g of pseudoboehmite sol;

[0048] Step 3: Add 540g of ultrafine kaolin, 400g of α-alumina micro powder, and 70g of soluble starch to a mixer and dry mix for 35 minutes; separately, add 24g of polyvinyl alcohol 1788 to 260g of deionized water, heat to 92℃ and keep warm for 35 minutes until completely dissolved, then cool to 44℃ to obtain a binder liquid; uniformly spray the binder liquid into the mixed powder, add 60g of deionized water, and granulate in a conventional disc granulator to obtain green pellets with a particle size of 0.35-0.65mm; dry the obtained green pellets at 60℃ for 50 minutes to reduce the total moisture content to 10.5%;

[0049] Step 4: Add 42g of tetraethoxysilane to 68g of anhydrous ethanol, stir for 5 minutes, then add a mixture of 12g of deionized water and 1.2g of 36% hydrochloric acid. Continue stirring at 28°C for 50 minutes to obtain a polysilicate precursor sol. Keep the green pellets with a water content of 10.5% obtained in Step 3 at 48°C and keep them rolling. Spray the above polysilicate precursor sol onto the surface of the green pellets in 8 portions, with 15g sprayed on each of the first 7 portions and 18.2g sprayed on the 8th portion. After each spraying, roll for 4 minutes and pass hot air at 60°C for 3 minutes. After all the pellets have been sprayed, roll for another 10 minutes.

[0050] Step 5: On the green balls with surface adhesion obtained in Step 4, 58g of pseudo-boehmite sol is sprayed in 5 portions, with rolling for 5 minutes after each spray. After all spraying is completed, the balls are treated with hot air at 65℃ for 12 minutes. Then, 36g of α-phase nano-alumina isopropanol dispersion is sprayed in 4 portions, with rolling for 6 minutes after each spray. While the surface is still wet, 10g of mullite nucleus powder is evenly sprinkled on the surface of the green balls and the balls are rolled for another 6 minutes to embed the mullite nuclei into the surface. Finally, the balls are dried at 115℃ for 2.5 hours to obtain the modified green balls.

[0051] Step 6: Place the modified green pellets from Step 5 in an air atmosphere, heat them to 375℃ at 2.3℃ / min and hold for 40 min, then heat them to 690℃ at 1.7℃ / min and hold for 80 min, then heat them to 940℃ at 2.3℃ / min and hold for 40 min; continue heating them in an air atmosphere to 1398℃ at 4.3℃ / min and hold for 60 min, then cool them to 650℃ at no more than 5℃ / min and cool them in the furnace; sieve them to 40-70 mesh and remove particles with abnormal morphology to obtain ultra-lightweight ceramic proppant for long-distance conveying and filling.

[0052] Example 4:

[0053] Step 1: Add 118g of anhydrous ethanol to a reaction vessel, add 28g of aluminum isopropoxide at 57℃, stir until clear, then add 9g of tetraethoxysilane, and then add a mixture of 17g of deionized water and 0.9g of 36% hydrochloric acid dropwise over 20min. Continue stirring at 57℃ for 1.8h to obtain a uniformly dispersed aluminum-silicon precursor sol. After the obtained sol is allowed to stand and age for 11h, it is dried at 108℃ for 9h, then heated to 1285℃ at 2.9℃ / min and held for 1.8h. After cooling, it is ball-milled for 3.5h and passed through a 350-mesh sieve to obtain mullite nucleus powder.

[0054] Step 2: Heat 205g of deionized water to 84℃, add 19g of aluminum isopropoxide over 28min under continuous stirring and reflux, and maintain the reaction at 84℃ for 55min; then add an acidification solution consisting of 19g of deionized water and 0.9g of 36% hydrochloric acid, and continue to keep warm for 80min, then cool and concentrate at 69℃ to obtain 78g of pseudoboehmite sol;

[0055] Step 3: Add 510g of ultrafine kaolin, 430g of α-alumina micro powder, and 55g of soluble starch to a mixer and dry mix for 28 minutes; separately, add 19g of polyvinyl alcohol 1788 to 230g of deionized water, heat to 89℃ and keep warm for 28 minutes until completely dissolved, then cool to 38℃ to obtain a binder liquid; uniformly spray the binder liquid into the mixed powder, add 45g of deionized water, and granulate in a conventional disc granulator to obtain green pellets with a particle size of 0.35-0.65mm; dry the obtained green pellets at 54℃ for 36 minutes to reduce the total moisture content to 9.1%;

[0056] Step 4: Add 34g of tetraethoxysilane to 58g of anhydrous ethanol, stir for 5 minutes, then add a mixture of 9g of deionized water and 0.9g of 36% hydrochloric acid. Continue stirring at 24°C for 38 minutes to obtain a polysilicate precursor sol. Maintain the green pellets with a water content of 9.1% obtained in Step 3 at 44°C and keep them rolling. Spray the above polysilicate precursor sol onto the surface of the green pellets in 5 portions, with the amounts sprayed in the 5 portions being 20g, 20g, 20g, 20g and 21.9g respectively. After each spraying, roll for 3 minutes and pass hot air at 52°C for 2 minutes. After all the pellets have been sprayed, roll for another 7 minutes.

[0057] Step 5: On the green balls with surface adhesion obtained in Step 4, 48g of pseudo-boehmite sol is sprayed in 4 portions, with rolling for 4 minutes after each spray. After all the spraying is completed, the balls are treated with hot air at 58℃ for 9 minutes. Then, 28g of α-phase nano-alumina isopropanol dispersion is sprayed in 3 portions, with rolling for 4 minutes after each spray. While the surface is still wet, 7g of mullite nucleus powder is evenly sprinkled on the surface of the green balls and the balls are rolled for another 5 minutes to embed the mullite nuclei into the surface. Finally, the balls are dried at 108℃ for 1.8 hours to obtain the modified green balls.

[0058] Step 6: Place the modified green pellets from Step 5 in an air atmosphere, heat them to 345℃ at 2.0℃ / min and hold for 25 min, then heat them to 640℃ at 1.4℃ / min and hold for 50 min, then heat them to 890℃ at 2.0℃ / min and hold for 25 min; continue heating them in an air atmosphere to 1372℃ at 4.0℃ / min and hold for 45 min, then cool them to 580℃ at no more than 5℃ / min and cool them in the furnace; sieve them to 40-70 mesh and remove particles with abnormal morphology to obtain ultra-lightweight ceramic proppant for long-distance conveying and filling.

[0059] Example 5:

[0060] Step 1: Add 125g of anhydrous ethanol to a reaction vessel, add 32g of aluminum isopropoxide at 61℃, stir until clear, then add 11g of tetraethoxysilane, and then add a mixture of 20g of deionized water and 1.1g of 36% hydrochloric acid dropwise over 20min. Continue stirring at 61℃ for 2.2h to obtain a uniformly dispersed aluminum-silicon precursor sol. After the obtained sol is allowed to stand and age for 13h, it is dried at 112℃ for 11h, then heated to 1310℃ at 3.1℃ / min and held for 2.2h. After cooling, it is ball-milled for 4.5h and passed through a 400-mesh sieve to obtain mullite nucleus powder.

[0061] Step 2: Heat 210g of deionized water to 87℃, add 22g of aluminum isopropoxide over 32min under continuous stirring and reflux, and maintain the reaction at 87℃ for 65min; then add an acidification solution consisting of 21g of deionized water and 1.1g of 36% hydrochloric acid, and continue to keep warm for 95min, then cool and concentrate at 72℃ to obtain 85g of pseudoboehmite sol;

[0062] Step 3: Add 535g of ultrafine kaolin, 405g of α-alumina micro powder, and 65g of soluble starch to a mixer and dry mix for 32 minutes; separately, add 23g of polyvinyl alcohol 1788 to 250g of deionized water, heat to 91℃ and keep warm for 32 minutes until completely dissolved, then cool to 42℃ to obtain a binder liquid; uniformly spray the binder liquid into the mixed powder, add 55g of deionized water, and granulate in a conventional disc granulator to obtain green pellets with a particle size of 0.35-0.65mm; dry the obtained green pellets at 58℃ for 48 minutes to reduce the total moisture content to 10.1%;

[0063] Step 4: Add 39g of tetraethoxysilane to 64g of anhydrous ethanol, stir for 5 minutes, then add a mixture of 11g of deionized water and 1.1g of 36% hydrochloric acid. Continue stirring at 27°C for 45 minutes to obtain a polysilicate precursor sol. Keep the green pellets with a water content of 10.1% obtained in Step 3 at 47°C and keep them rolling. Spray the above polysilicate precursor sol onto the surface of the green pellets in 7 portions, with 16g sprayed on each of the first 6 portions and 19.1g sprayed on the 7th portion. After each spraying, roll for 3.5 minutes and pass hot air at 57°C for 2.5 minutes. After all the pellets have been sprayed, roll for another 9 minutes.

[0064] Step 5: On the green balls with surface adhesion obtained in Step 4, 55g of pseudo-boehmite sol was sprayed in 4 portions, with rolling for 4.5min after each spray. After all the spraying was completed, the balls were treated with hot air at 62℃ for 11min. Then, 34g of α-phase nano-alumina isopropanol dispersion was sprayed in 3 portions, with rolling for 5.5min after each spray. While the surface was still wet, 9g of mullite nucleus powder was evenly sprinkled on the surface of the green balls and the balls were rolled for another 5.5min to embed the mullite nuclei into the surface. Finally, the balls were dried at 112℃ for 2.2h to obtain the modified green balls.

[0065] Step 6: Place the modified green pellets from Step 5 in an air atmosphere, heat them to 360℃ at 2.2℃ / min and hold for 35 min, then heat them to 670℃ at 1.6℃ / min and hold for 70 min, then heat them to 920℃ at 2.2℃ / min and hold for 35 min; continue heating them to 1390℃ in an air atmosphere at 4.2℃ / min and hold for 55 min, then cool them to 620℃ at no more than 5℃ / min and cool them in the furnace; sieve them to 40-70 mesh and remove particles with abnormal morphology to obtain ultra-lightweight ceramic proppant for long-distance conveying and filling.

[0066] Comparative Example 1:

[0067] The difference from Example 1 is that the polysilicate precursor sol in step 4 is replaced by spraying it into the mixed powder in 6 batches along with the binder in step 3 before granulation, and step 4 is no longer performed; in order to keep the total moisture content of the green pellets at 9.5% at the end of step 3, 50g of deionized water is no longer added in step 3, and drying is continued at 55°C until the total moisture content drops to 9.5%; the other conditions are the same as in Example 1.

[0068] The specific steps are as follows:

[0069] Steps 1 and 2 are the same as in Example 1;

[0070] Step 3: Add 520g of ultrafine kaolin, 420g of α-alumina micro powder, and 60g of soluble starch to a mixer and dry mix for 30 minutes; separately, add 20g of polyvinyl alcohol 1788 to 240g of deionized water, heat to 90℃ and keep warm for 30 minutes until completely dissolved, then cool to 40℃ to obtain a binder solution; spray the polysilicate precursor sol from Step 4 and the binder solution together into the mixed powder in 6 portions, and granulate in a conventional disc granulator to obtain green pellets with a particle size of 0.35-0.65mm; dry the obtained green pellets at 55℃ until the total moisture content drops to 9.5%;

[0071] Step 4: Do not proceed further;

[0072] Steps 5 and 6 are the same as in Example 1.

[0073] Comparative Example 2:

[0074] The difference from Example 1 is that in step 4, the polysilicate precursor sol is sprayed onto the surface of the green pellets continuously in one go, instead of being sprayed in 6 separate times, and hot air at 55°C is not introduced between each spraying; to ensure that the total amount of hot air treatment is consistent, hot air at 55°C is introduced uniformly for 12 minutes after all spraying is completed; the other conditions are the same as in Example 1.

[0075] The specific steps are as follows:

[0076] Steps 1 to 3 are the same as in Example 1;

[0077] Step 4: Add 36g of tetraethoxysilane to 60g of anhydrous ethanol, stir for 5 minutes, then add a mixture of 10g of deionized water and 1g of 36% hydrochloric acid. Continue stirring at 25°C for 40 minutes to obtain polysilicate precursor sol. Keep the green pellets with a water content of 9.5% obtained in Step 3 at 45°C and keep them rolling. Spray 117g of polysilicate precursor sol onto the surface of the green pellets in one continuous spray over 6 minutes. After spraying, keep rolling for 8 minutes and pass hot air at 55°C for 12 minutes.

[0078] Steps 5 and 6 are the same as in Example 1.

[0079] Comparative Example 3:

[0080] The difference from Example 1 is that in step 5, 50g of boehmite sol and 30g of α-phase nano alumina isopropanol dispersion are pre-mixed evenly before being sprayed, instead of spraying boehmite sol first and then spraying α-phase nano alumina isopropanol dispersion; the other conditions are the same as in Example 1.

[0081] The specific steps are as follows:

[0082] Steps 1 to 4 are the same as in Example 1;

[0083] Step 5: On the green balls with surface adhesion obtained in Step 4, first mix 50g of pseudo-boehmite sol and 30g of α-phase nano-alumina isopropanol dispersion evenly; spray the resulting mixture onto the surface of the green balls in 7 spraying intervals according to the spraying rhythm of Example 1, with 12.5g sprayed each time for the first 4 times, and rolling for 4 minutes after each spraying. After the first 4 sprayings are completed, treat with hot air at 60℃ for 10 minutes; then spray the remaining mixture in 3 spraying intervals, with 10g sprayed each time, and rolling for 5 minutes after each spraying; while the surface is still wet, evenly sprinkle 8g of mullite nucleus powder onto the surface of the green balls and continue rolling for 5 minutes to embed the mullite nuclei into the surface layer; finally, dry at 110℃ for 2 hours to obtain modified green balls;

[0084] Step 6 is the same as in Example 1.

[0085] Comparative Example 4:

[0086] The difference from Example 1 is that the 8g of mullite nucleus powder in step 5 is replaced by adding it to the mixer together with 520g of ultrafine kaolin, 420g of α-alumina micro powder and 60g of soluble starch in step 3 for dry mixing. In step 5, the mullite nucleus powder is no longer sprinkled and embedded in the surface while it is still wet. The other conditions are the same as in Example 1.

[0087] The specific steps are as follows:

[0088] Steps 1 and 2 are the same as in Example 1;

[0089] Step 3: Add 520g of ultrafine kaolin, 420g of α-alumina micro powder, 60g of soluble starch, and 8g of mullite nucleus powder to a mixer and dry mix for 30 minutes; separately, add 20g of polyvinyl alcohol 1788 to 240g of deionized water, heat to 90℃ and keep warm for 30 minutes until completely dissolved, then cool to 40℃ to obtain a binder liquid; uniformly spray the binder liquid into the mixed powder, add 50g of deionized water, and granulate in a conventional disc granulator to obtain green pellets with a particle size of 0.35-0.65mm; dry the obtained green pellets at 55℃ for 40 minutes to reduce the total moisture content to 9.5%;

[0090] Step 4 is the same as in Example 1;

[0091] Step 5: On the green balls with surface adhesion obtained in Step 4, 50g of pseudo-boehmite sol is sprayed in 4 portions, with rolling for 4 minutes after each spray. After all the spraying is completed, the balls are treated with hot air at 60℃ for 10 minutes. Then, 30g of α-phase nano-alumina isopropanol dispersion is sprayed in 3 portions, with rolling for 5 minutes after each spray. Finally, the balls are dried at 110℃ for 2 hours to obtain the modified green balls.

[0092] Step 6 is the same as in Example 1.

[0093] Comparative Example 5:

[0094] The difference from Example 1 is that in step 5, instead of adding 30g of α-phase nano alumina isopropanol dispersion, 6.15g of α-alumina micro powder and 23.85g of isopropanol dispersion are added, and the spraying is carried out according to the number of spraying times and rolling time of α-phase nano alumina isopropanol dispersion in Example 1; the other conditions are the same as in Example 1.

[0095] The specific steps are as follows:

[0096] Steps 1 to 4 are the same as in Example 1;

[0097] Step 5: On the green balls with surface adhesion obtained in Step 4, 50g of pseudo-boehmite sol is sprayed in 4 portions, with rolling for 4 minutes after each spray. After all the spraying is completed, the balls are treated with hot air at 60℃ for 10 minutes. Then, 6.15g of α-alumina micro powder is added to 23.85g of isopropanol to prepare a dispersion, and this dispersion is sprayed in 3 portions, with rolling for 5 minutes after each spray. While the surface is still moist, 8g of mullite nucleus powder is evenly sprinkled on the surface of the green balls, and rolling continues for 5 minutes to embed the mullite nuclei into the surface. Finally, the balls are dried at 110℃ for 2 hours to obtain the modified green balls.

[0098] Step 6 is the same as in Example 1.

[0099] Comparative Example 6:

[0100] The difference from Example 1 is that in step 5, 8g of mullite nucleus powder is replaced with 8g of α-alumina micro powder, and the same spreading and rolling method as in Example 1 is used while the surface is still wet; the other conditions are the same as in Example 1.

[0101] The specific steps are as follows:

[0102] Steps 1 to 4 are the same as in Example 1;

[0103] Step 5: On the green balls with surface adhesion obtained in Step 4, first spray 50g of pseudo-boehmite sol in 4 portions, rolling for 4 minutes after each spray. After all spraying is completed, treat with hot air at 60℃ for 10 minutes. Then, spray 30g of α-phase nano-alumina isopropanol dispersion in 3 portions, rolling for 5 minutes after each spray. While the surface is still wet, evenly sprinkle 8g of α-alumina micro powder on the surface of the green balls and continue rolling for 5 minutes. Finally, dry at 110℃ for 2 hours to obtain modified green balls.

[0104] Step 6 is the same as in Example 1.

[0105] Performance testing:

[0106] Sample Preparation and Numbering: The calcined samples obtained in Examples 1-5 and Comparative Examples 1-6 were sieved to obtain 40-70 mesh particles. Broken, agglomerated, and deformed particles were removed. Surface powder was rapidly washed off with anhydrous ethanol, and the samples were dried at 105℃ for 2 hours, then cooled to room temperature in a desiccator for later use. Unless otherwise specified, all tests were conducted using Examples 1-5 and Comparative Examples 1-6 as the test subjects. Each sample was tested independently in triplicate, and the arithmetic mean was taken. The density of the sand layer on the samples used for the flow-guiding capacity test was uniformly controlled at 9.76 kg / m² based on the effective area of ​​the flow-guiding pool. 2 The base liquid for the sedimentation test of low-viscosity slickwater was prepared according to the evaluation method of shale gas slickwater, and all samples were ensured to use the same batch of base liquid.

[0107] Bulk density and apparent porosity: Bulk density was determined according to SY / T 5108-2014, and apparent porosity was determined according to GB / T 25995-2010. 50.0 g of each sample was used for bulk density testing. 30.0 g of intact particles were vacuumed for 30 min under a vacuum degree not exceeding 10 Pa, then saturated with water. The dry mass, saturated mass, and suspended mass were measured, and the apparent porosity was calculated.

[0108] Crushing rate at 52 MPa: Tested according to SY / T 5108-2014. Weigh 5.0 g of each sample and put it into the crushing test cylinder. The sample thickness is controlled to be consistent according to the standard. The closing pressure is increased to 52 MPa within 30 s and maintained for 2 min. After unloading, the sample is sieved with a 70 mesh test sieve. The crushing rate is calculated as the percentage of the mass of the sample below the sieve relative to the initial mass of the sample.

[0109] Short-term flow carrying capacity: Tested according to SY / T 6302-2019. Each sample was tested based on a sand-covered surface density of 9.76 kg / m². 2 The sample was filled into a flow-guiding tank, using a 2.0% potassium chloride aqueous solution as the flow medium, and the test temperature was controlled at 60℃. After filling, the sample was first equilibrated at a closed pressure of 35 MPa for 30 min, and then the pressure difference was measured at 35 MPa, 52 MPa, and 70 MPa, respectively, with flow rates of 5.0 mL / min, 10.0 mL / min, and 15.0 mL / min, and the flow capacity was calculated. The results were recorded as the flow capacity at 35 MPa, 52 MPa, and 70 MPa, respectively.

[0110] Long-term conductivity retention rate: Tested according to NB / T 14023-2017. Each sample was tested based on a sand-covered surface density of 9.76 kg / m². 2 The sample was filled into a long-term flow conduction test device, with a 2.0% potassium chloride aqueous solution as the flow medium. The test temperature was controlled at 90℃, the closing pressure was kept constant at 52MPa, and the liquid was continuously passed through for 24h. The flow conduction capacity was recorded at 1h and 24h respectively, and the flow conduction retention rate was calculated according to the ratio of the flow conduction capacity at 24h to the flow conduction capacity at 1h.

[0111] Settling velocity and complete settling time in low-viscosity slickwater: The sand-carrying base solution was prepared according to NB / T 14003.1-2015, and the state of the base solution was pre-inspected according to SY / T 5185-2024. Specifically, the slickwater base solution was prepared using 0.08% (w / w) of drag-reducing agent and 0.10% (w / w) of potassium chloride, allowed to stand at 25℃ for 30 minutes to defoam, and its settling velocity was controlled within 170 seconds. -1The apparent viscosity at 25℃ was 3.5±0.5mPa·s. Then, 1000mL was added to a transparent sedimentation column with an inner diameter of 50mm. 20.0g of sample was weighed each time and added from the center of the liquid surface. The time required for the leading edge of the particle group to descend 50cm was recorded, and the average sedimentation velocity was calculated. At the same time, the time required for all particles to settle to the bottom of the column was recorded as the complete sedimentation time.

[0112] Table 1 Performance Test Results

[0113] Index Bulk Density (g / cm3) Apparent Porosity (%) Breakage Rate at 52MPa (%) Flow Conductivity at 35MPa (μm2·cm) Flow Conductivity at 52MPa (μm2·cm) Flow Conductivity at 70MPa (μm2·cm) Flow Conductivity Retention Rate at 52MPa and 24h (%) Average Settling Velocity (cm / s) Complete Settling Time (s) Example 1 1.25 6.8 2.84 4.63 2.72 0.38 7.2 0.80 80.4 Example 2 1.29 7.4 3.94 1.12 8.81 7.18 2.7 0.90 72.9 Example 3 1.22 7.6 3.24 5.93 1.31 8.98 5.1 0.74 86.2 Example 4 1.26 7.0 3.54 2.83 0.11 8.38 4.6 0.86 77.6 Example 51.237.23.045.131.719.585.80.7684.3 Comparative Example 11.309.16.735.723.513.174.60.9865.2 Comparative Example 21.259.67.136.924.013.373.40.8278.5 Comparative Example 31.278.85.837. 625.314.777.80.8972.1 Comparative Example 41.278.45.438.226.015.278.90.8973.0 Comparative Example 51.268.35.138.926.615.879.70.8874.1 Comparative Example 61.268.14.839.127.116.180.50.8675.7 surface

[0114] Data Analysis:

[0115] As can be seen from the data in Table 1, the ultralight ceramic proppant prepared by this invention maintains a low bulk density and slow settling characteristics while still exhibiting a low breakage rate, high conductivity, and good long-term conductivity retention, achieving a balance between long-distance transport and filling and stable support under high closure pressure. This may be because the polysilicate precursor sol, after being sprayed onto the green pellet surface in stages, preferentially accumulates in the outer layer. The pseudoboehmite sol first provides surface fixation and reactivity, while the α-phase nano-alumina isopropanol dispersion then provides shrinkage constraint. Subsequently, the mullite nucleus powder embedded in the surface further promotes the preferential growth of the outer mullite layer. Combined with segmented oxidation pre-calcination, the outer reaction layer is stabilized first, and the inner closed pores are preserved, thus achieving both transport capacity in low-density, low-viscosity slickwater and continuous conductivity.

[0116] As can be seen from the data in Table 1 for Example 1, Comparative Example 1, and Comparative Example 2, when the polysilicate precursor sol is not localized on the surface of the green pellets after granulation, or when it is applied continuously in a single spraying even though it is located on the surface of the green pellets, the breakage rate of the samples increases, and the conductivity and long-term conductivity retention decrease. In the former case, the silicon source is more likely to enter the main framework, weakening the preferential growth of mullite and the initial densification of the outer layer; in the latter case, it is not conducive to the establishment of outer layer continuity and edge integrity.

[0117] As can be seen from the data in Example 1 and Comparative Example 3 in Table 1, pre-mixing the pseudoboehmite sol with the α-phase nano-alumina isopropanol dispersion before spraying causes a simultaneous decrease in both the conductivity under high closure pressure and the long-term conductivity retention. The main reason for this is likely that the pseudoboehmite sol originally serves to fix the surface layer and enhance reactivity, while the α-phase nano-alumina isopropanol dispersion primarily serves to constrain shrinkage and improve surface continuity. Only when both are introduced into the surface layer in a predetermined order can a stable outer reaction system be formed.

[0118] As can be seen from the data in Table 1 for Example 1 and Comparative Example 4, when mullite nucleus powder is added to the main powder in advance, instead of being sprinkled and embedded in the surface while it is still wet, the sample's resistance to breakage and its conductivity stability under high closure pressure both decrease. This may be because the mullite nucleus powder that enters the main framework is more easily dispersed in the inner layer and is less likely to concentrate its effect on the outer reaction layer. Therefore, it cannot fully promote the preferential growth of mullite in the outer layer, nor is it conducive to establishing the continuity and edge integrity of the outer layer.

[0119] As can be seen from the data in Table 1 for Examples 1, 5, and 6, simply replacing the α-phase nano-alumina isopropanol dispersion with α-alumina micro-powder dispersion, or replacing the surface mullite nucleus powder with α-alumina micro-powder, both reduce the conductivity and long-term conductivity retention. The former indicates that α-alumina micro-powder cannot achieve the same effect as the α-phase nano-alumina isopropanol dispersion in terms of surface shrinkage constraint and improved surface continuity; the latter indicates that α-alumina micro-powder also cannot replace the preferential growth-promoting effect of the surface mullite nucleus powder on the outer mullite layer.

[0120] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing an ultralight ceramic proppant for long-distance transport and filling, characterized in that, Includes the following steps: S1, mix ultrafine kaolin, α-alumina micro powder and soluble starch, add polyvinyl alcohol binder to granulate, dry to obtain green pellets; S2, Keeping the green pellets in a rolling state, the polysilicate precursor sol is sprayed onto the surface of the green pellets in stages. After each spraying, the pellets are rolled and hot air is introduced. After all the pellets are sprayed, the rolling continues to obtain surface-modified green pellets. S3, On the surface-modified green pellets, pseudo-boehmite sol and α-phase nano-alumina isopropanol dispersion are sprayed sequentially. While the surface is still wet, mullite nucleus powder is evenly sprinkled on the surface of the green pellets and the rolling continues to allow the mullite nucleus powder to embed into the surface layer. The pellets are then dried to obtain modified green pellets. S4, The modified green pellets are placed in an air atmosphere for segmented oxidation pre-calcination, then heated and sintered. After cooling and sieving, ultra-lightweight ceramic support for long-distance conveying and filling is obtained.

2. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, Based on 500-540 parts by weight of ultrafine kaolin, in step S3, the amount of pseudo-boehmite sol added is 42-58 parts, the amount of α-phase nano-alumina isopropanol dispersion added is 24-36 parts, and the amount of mullite nucleus powder added is 6-10 parts.

3. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, The ultrafine kaolin has a mesh size of 5000-7000 mesh, the α-alumina micro powder has a particle size of 0.1-0.3 μm, and the solid content of the α-phase nano-alumina isopropanol dispersion is 18%-22%, wherein the particle size of the α-phase nano-alumina is 20-30 nm.

4. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, In step S1, the ratio of ultrafine kaolin, α-alumina powder and soluble starch by mass is 500-540:400-440:50-70; the polyvinyl alcohol binder is prepared by 18-24 parts of polyvinyl alcohol 1788 and 220-260 parts of deionized water.

5. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, The polysilicate precursor sol is prepared by the following method: by weight, 30-42 parts of tetraethoxysilane are added to 55-68 parts of anhydrous ethanol, and after stirring for 5 min, a mixture consisting of 8-12 parts of deionized water and 0.8-1.2 parts of hydrochloric acid with a mass fraction of 36% is added, and stirring is continued at 22-28℃ for 35-50 min to obtain the polysilicate precursor sol.

6. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, In step S2, the green pellets are continuously rolled at 42-48℃; the polysilicate precursor sol is sprayed onto the surface of the green pellets in 4-8 applications; after each application, the pellets are rolled for 2-4 minutes and hot air at 50-60℃ is introduced for 1.5-3 minutes; after all the pellets are sprayed, they are rolled for another 6-10 minutes.

7. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, The pseudoboehmite sol is prepared by the following method: 190-215 parts by weight of deionized water are heated to 82-88°C, and 18-23 parts by weight of aluminum isopropoxide are added within 25-35 minutes under continuous stirring and reflux conditions, and the reaction is maintained at 82-88°C for 50-70 minutes; then an acidification solution consisting of 18-22 parts by weight of deionized water and 0.8-1.2 parts by weight of 36% hydrochloric acid is added, and the mixture is kept at this temperature for 75-105 minutes, so that the hydrolysis product is converted into pseudoboehmite sol through peptization; then the mixture is cooled and concentrated by volume reduction at 67-74°C to obtain 72-88 parts by weight of pseudoboehmite sol.

8. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, The mullite nucleus powder is prepared by the following method: 110-130 parts by mass of anhydrous ethanol are added to a container, 26-34 parts of aluminum isopropoxide are added at 57-62℃, and after stirring until clear, 8-13 parts of tetraethoxysilane are added. Then, a mixture of 16-22 parts of deionized water and 0.8-1.2 parts of 36% hydrochloric acid is added dropwise over 20 min, and stirring is continued at 57-62℃ for 1.5-2.5 h to obtain a precursor sol. The precursor sol is allowed to stand for 10-14 h, then dried at 105-115℃ for 8-12 h, and then heated to 1270-1320℃ at 2.8-3.2℃ / min and held for 1.5-2.5 h. After cooling, it is ball-milled for 3-5 h and passed through a 325-400 mesh sieve to obtain mullite nucleus powder.

9. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, In step S3, the pseudo-boehmite sol is first sprayed onto the surface of the green ball in 3-5 applications. After each application, the ball is rolled for 3-5 minutes. After all the balls are sprayed, they are treated with hot air at 55-65℃ for 8-12 minutes. Subsequently, the α-phase nano-alumina isopropanol dispersion was sprayed in 2-4 portions, with rolling for 4-6 minutes after each spray; then, mullite nucleus powder was evenly sprinkled on the surface of the green pellets and rolled for another 4-6 minutes; then, the pellets were dried at 105-115℃ for 1.5-2.5 hours to obtain the modified green pellets.

10. The method for preparing ultralight ceramic proppant for long-distance conveying and filling according to claim 1, characterized in that, In step S4, the segmented oxidation pre-firing and sintering specifically involve: in an air atmosphere, heating to 330-375℃ at 1.8-2.3℃ / min and holding for 20-40min, then heating to 620-690℃ at 1.2-1.7℃ / min and holding for 45-80min, followed by heating to 860-940℃ at 1.8-2.3℃ / min and holding for 20-40min; then heating to 1365-1398℃ at 3.8-4.3℃ / min and holding for 40-60min, and finally cooling to 550-650℃ at no more than 5℃ / min and then cooling with the furnace.