Lightweight high-strength ceramic proppant based on composite coating and low-temperature densification and preparation method thereof

Abstract

This invention discloses a lightweight, high-strength ceramsite proppant based on composite coating and low-temperature densification, and its preparation method. The coated proppant includes a ceramsite proppant and a coating layer covering the surface of the ceramsite proppant. It relates to the field of fracturing technology in oil and gas engineering. The ceramsite proppant includes bauxite, perlite, a composite sintering aid, B2O3, and AlF3; the composite sintering aid includes MnO2 and TiO2; wherein the weight ratio of bauxite, perlite, composite sintering aid, B2O3, and AlF3 is 80:20:2.5-7.5:3-12:3-12. This invention uses a low-grade bauxite and high-silica raw material system, combined with the introduction of an appropriate amount of sintering aid, effectively reducing the sintering temperature and energy consumption; simultaneously reducing the bulk density of the ceramsite proppant, resulting in a lightweight product.

Description

Technical Field

[0001] This invention relates to the field of fracturing technology in oil and gas engineering, specifically to a lightweight, high-strength ceramic proppant based on composite coating and low-temperature densification, and its preparation method. Background Technology

[0002] Hydraulic fracturing is a key technology for improving the productivity of low-permeability oil and gas reservoirs. Its core lies in enhancing the reservoir's permeability through fracture modification. The proppant plays a crucial role in this process, filling the fractures and preventing them from reopening under closure pressure, thus maintaining the effectiveness of the fracture flow channels.

[0003] Traditional ceramsite proppant is mostly made from high-grade bauxite. Due to its high Al2O3 content, it requires a high sintering temperature to achieve good mechanical properties, which also results in a high density of ceramsite proppant. According to Stokes' settling theory, the settling rate of proppant is positively correlated with its density. In actual fracturing operations, high-density proppant tends to accumulate in fractures, blocking seepage channels and thus reducing the conductivity of the fractures.

[0004] While using high-viscosity fracturing fluids can reduce settling rates, this not only increases construction costs but may also contaminate and damage the reservoir. To reduce proppant density and improve its settling behavior, researchers have recently attempted to prepare lightweight ceramsite proppants using low-grade bauxite combined with high-silica feedstocks. Although this strategy has achieved some success in reducing density and sintering temperature, the prepared proppants often have a high breakage rate, making it difficult to meet the application requirements under high closure pressures.

[0005] To improve the overall performance of lightweight ceramsite, existing studies have focused on strengthening it by adding sintering aids or using resin coatings. However, current fracturing proppant coatings generally use thermosetting or thermoplastic polymers, such as phenolic resins and epoxy resins. Although these coatings can improve the proppant's compressive strength and conductivity to some extent, under high temperature and high closure pressure (>52 MPa), the coating material is prone to softening, creeping deformation, and even clumping together, leading to a rapid decrease in conductivity and affecting the fracturing operation.

[0006] Furthermore, existing coating methods primarily rely on solution encapsulation and room-temperature curing, which makes it difficult to form a dense, strongly adhesive, and uniform coating on the proppant surface, leading to easy peeling or localized failure of the coating. Therefore, there is an urgent need for a novel coating system with high thermal stability, high mechanical strength, and strong adhesion to meet the performance requirements under deep and ultra-deep well fracturing conditions. Summary of the Invention

[0007] The purpose of this invention is to provide a lightweight, high-strength ceramic proppant based on composite coating and low-temperature densification, and its preparation method, and to provide a novel epoxy-expanded perlite inorganic-organic composite coating material with high thermal stability and high rigidity, and its coating method, thereby improving the flow conductivity and service life of the proppant in high-temperature and high-pressure environments.

[0008] Specifically, the present invention provides a lightweight, high-strength ceramsite proppant based on composite coating and low-temperature densification. The coated proppant includes a ceramsite proppant and a coating layer covering the surface of the ceramsite proppant. The ceramsite includes bauxite, perlite, composite sintering aid, B2O3, and AlF3. The composite sintering aid includes MnO2 and TiO2.

[0009] The coating layer consists of expanded perlite and epoxy resin.

[0010] Ceramsite proppant serves as a high-strength substrate, while the coating layer provides buffer protection. The synergistic effect of these two components significantly improves compressive strength, abrasion resistance, and environmental adaptability. In principle, proppants need to operate for extended periods in downhole environments with high pressure (over 100 MPa), high temperature (above 200°C), and corrosive fluids. Traditional proppants are prone to failure due to brittle fracture or chemical corrosion, leading to fracture closure and reduced productivity.

[0011] The bauxite of this invention provides skeletal strength, perlite reduces density, B2O3 and AlF3 improve sintering efficiency, and the composite sintering aid (MnO2 and TiO2) acts as a flux to reduce energy consumption. Expanded perlite in the coating layer enhances thermal insulation and buffering, while epoxy resin provides toughness and sealing protection. The weight ratio of bauxite, perlite, composite sintering aid, B2O3, and AlF3 is 80:20:2.5-7.5:3-12:3-12. This component ratio ensures component balance, with high bauxite content guaranteeing strength and low perlite content controlling density.

[0012] The weight ratio of expanded perlite to epoxy resin in the coating layer is 0.1-0.5:3-12; this ratio allows the epoxy resin to dominate, forming a tough outer shell, while the expanded perlite fills the micropores.

[0013] Expanded perlite has a particle size of 1-2μm to ensure uniform dispersion and coating adhesion; the coating thickness of 85-170μm can buffer impact, but too thick a coating will cause the coating support to deform greatly, thus reducing the conductivity.

[0014] The film proppant of the present invention can not only effectively improve the breakage resistance and has a low density which is beneficial for pumping, but also effectively improve the temperature resistance.

[0015] Specifically, bauxite and perlite have complementary effects. Bauxite is rich in alumina, providing high hardness and high-temperature resistance, but it has a high density and is prone to brittleness. Perlite, whose main component is silicate, can reduce the overall density and introduce a microporous structure, improving thermal insulation. The 80:20 ratio balances strength and lightweight, and the ratio of bauxite to perlite avoids the brittleness problem of pure bauxite.

[0016] MnO2, as a variable-valence cation compound, can promote lattice distortion in mullite in the raw materials, generating a suitable amount of point defects and reducing sintering defects such as porosity. TiO2 lowers the sintering temperature, enhances mullite growth, and increases sintering density. The 3:1 weight ratio of MnO2 to TiO2 in the composite sintering aid optimizes flowability; excessive MnO2 easily leads to overburning, while excessive TiO2 reduces strength. The configuration of this invention not only shortens the sintering time but also further improves compressive strength.

[0017] B2O3 forms a glassy phase, filling grain boundary pores and enhancing toughness; AlF3 volatilizes to generate a fluorine atmosphere, purifying impurities and reducing uneven sintering shrinkage. Both can promote the formation of finer mullite whiskers, thereby improving the strength of the proppant.

[0018] Compared with existing proppant, it has high sintering temperature, high energy consumption, and high density, which makes it difficult to carry fracturing fluid. This invention not only reduces production energy consumption, but also reduces the sintering temperature of ceramic proppant through multi-component synergy, achieving low-temperature dense sintering.

[0019] Furthermore, the coating layer encapsulates ceramic particles, providing external protection. The expanded perlite, with a particle size of 1-2 μm, ensures uniform dispersion within the epoxy resin, forming a microporous buffer layer. Its lightweight properties absorb impact energy, reducing stress concentration. The precise proportion of expanded perlite avoids excessive strength reduction while providing excellent thermal insulation, making it suitable for high-temperature wells.

[0020] Epoxy resin forms a continuous, flexible film that bonds and seals the expanded perlite surface. The chemical inertness of epoxy resin resists H2S / CO2 corrosion, while its toughness compensates for the brittleness of the expanded perlite, preventing particle breakage during fracturing.

[0021] The ratio of the coating layer to the ceramsite support frame in this invention ensures a strong interfacial bond, and the particle size of the expanded perlite matches the coating thickness to prevent peeling. The coating layer of this invention can effectively reduce the breakage rate to below 5% while maintaining high conductivity.

[0022] Furthermore, this invention proposes a second objective: a method for preparing a lightweight, high-strength ceramsite proppant based on composite coating and low-temperature densification, comprising the following steps:

[0023] Step 1: Prepare ceramic proppant;

[0024] Step 2, prepare the coating solution;

[0025] Step 3: The prepared ceramic proppant is kept at 300℃ for 30 min; preheating at 300℃ can remove adsorbed water on the surface of the ceramic proppant and activate the surface hydroxyl groups (-OH), which react with the epoxy groups of the epoxy resin to form ether bonds (COC), thereby improving the interfacial bonding force.

[0026] Step 4: Add the coating solution to the heated ceramsite support in proportion, stir evenly, add the curing agent, and continue stirring for pre-curing;

[0027] Step 5: Add lubricant before the ceramsite support and resin solution show signs of aggregation or local agglomeration, and stir evenly to obtain a coated support sample; before the resin gel point, i.e. before aggregation or local agglomeration, the long-chain alkyl groups are oriented to form a hydrophobic film on the particle surface, blocking the hydrogen bonding of resin adhesion.

[0028] Step 6: Place the film-coated support sample in an oven and cure it at 120°C for 3 hours;

[0029] Step 7: After the sample has solidified, disperse it using a pulverizer and then sieve it through a 30-50 mesh to obtain the film-coated support.

[0030] The preparation of the ceramsite proppant specifically includes the following steps:

[0031] Step 11: Add bauxite, perlite, composite sintering aid, B2O3, AlF3 and deionized water into a ball mill jar in proportion, and ball mill for 6 hours to obtain a uniform slurry; the particle size of bauxite and perlite after ball milling is 1-1.5 μm.

[0032] Step 12: After drying the slurry in an oven, grind and filter it to obtain ceramsite powder;

[0033] Step 13: Add some of the ceramsite powder to the sugar coating machine, spray PVA solution to obtain spherical particles, and sieve out 70-100 mesh particles as mother balls;

[0034] Step 14: After putting the mother ball into a clean sugar coating machine, spray PVA solution, add some ceramsite powder, and sieve out 50-70 mesh ceramsite.

[0035] Step 15: Put 50-70 mesh ceramsite into a clean sugar coating machine, spray PVA solution, add some ceramsite powder, and sieve out 40-50 mesh ceramsite particles.

[0036] Step 16: After drying the 40-50 mesh ceramsite particles in an oven at 120℃, remove the binder in a resistance furnace at 600℃ for 30 minutes to obtain ceramsite support green body.

[0037] Step 17: Put the ceramsite proppant green body into a high-temperature furnace and heat it to 1200-1300℃ for sintering for 90 minutes to obtain ceramsite proppant.

[0038] The preparation of the coating solution specifically includes the following steps:

[0039] Step 21: Add expanded perlite, epoxy resin, and diluent to a polytetrafluoroethylene cup in the specified proportions, stir well, and then sonicate for 60 minutes.

[0040] Step 22: After heating the ceramsite proppant obtained in Step 17 at 300℃ for 30 minutes, add it to the solution prepared in Step 21, stir and disperse for 3 minutes, then add the curing agent, sonicate and stir for 3-5 minutes, remove the sample and place it in a tray, then place it in an oven to cure, thus obtaining the coated proppant. The curing agent is polyetheramine, the diluent is benzyl alcohol, and the lubricant is calcium stearate.

[0041] Expanded perlite is incorporated into the epoxy resin of this invention to form a composite coating. The resin provides a tough encapsulation, while the microporous structure of the perlite absorbs stress impact and reduces overall density. The coating solution is added after the expanded clay aggregate is preheated to 300°C. The high temperature promotes resin penetration into the pores of the expanded clay aggregate surface, forming a dual interfacial bond of mechanical interlocking and chemical bonding. A curing temperature of 120°C is selected, and the addition of calcium stearate inhibits particle adhesion, ensuring uniform coating.

[0042] Therefore, in ceramsite proppant, perlite acts as a pore-forming agent to reduce the proppant density, while its active SiO2 and Al2O3 form a mullite phase to reinforce the framework at 1200℃.

[0043] In the coating layer, ultrasonically dispersed micro-nano-sized expanded perlite serves as a resin reinforcing phase, inhibiting cracking of the coating layer through a crack deflection mechanism. The perlite softens and fills the gaps between bauxite particles during sintering at 1200–1300℃. Therefore, this invention achieves low-temperature densification through liquid-phase sintering. The particle size is increased progressively through three stages of granulation: 70–100 mesh, 50–70 mesh, and 40–50 mesh.

[0044] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0045] This invention discloses a lightweight, high-strength ceramsite proppant based on composite coating and low-temperature densification, and its preparation method. This invention uses a low-grade bauxite and high-silica raw material system, combined with the introduction of an appropriate amount of sintering aid, which effectively reduces the sintering temperature and energy consumption; at the same time, it reduces the bulk density of the ceramsite proppant, thereby obtaining a lightweight, high-strength product.

[0046] While ensuring lightweight properties, the sintering aid also improves the density and mechanical strength of the ceramsite to a certain extent, achieving a synergistic optimization of low density and high strength.

[0047] Furthermore, by coating the surface of the ceramsite with a curable composite resin film, this invention not only further reduces the overall density but also improves its resistance to breakage. Compared to traditional coating proppant which is prone to plastic deformation under high closure pressure, the resin used in this invention incorporates inorganic fillers, giving the coating layer higher rigidity and thermal stability. This effectively suppresses problems such as coating deformation and clumping under high pressure, achieving a harmonious balance of low energy consumption, low density, and high strength, and has promising prospects for engineering applications. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments, but this does not limit the invention to the scope of the embodiments described. Experimental methods in the following embodiments that do not specify specific conditions were performed according to conventional methods and conditions, or as selected according to the product instructions.

[0049] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0050] Example 1

[0051] A lightweight, high-strength ceramsite proppant based on composite coating and low-temperature densification, wherein the coated proppant comprises a ceramsite proppant and a coating layer covering the surface of the ceramsite proppant;

[0052] The proppant for the expanded clay aggregate includes bauxite, perlite, a composite sintering aid, B2O3, and AlF3; the composite sintering aid includes MnO2 and TiO2; the weight ratio of bauxite, perlite, composite sintering aid, B2O3, and AlF3 is 80:20:2.5-7.5:3-12:3-12. The coating layer includes expanded perlite and epoxy resin; the weight ratio of expanded perlite to epoxy resin in the coating layer is 0.1-0.5:3-12. The weight ratio of MnO2 to TiO2 in the composite sintering aid is 3:1. The particle size of the expanded perlite is 1-2 μm, and the thickness of the coating layer is 85-170 μm.

[0053] Among them, the chemical composition of bauxite, perlite / expanded perlite was analyzed by X-ray fluorescence (XRF), and the results are shown in Table 1.

[0054] Table 1 Raw material composition (wt%)

[0055]

[0056] A method for preparing a lightweight, high-strength ceramsite proppant based on composite coating and low-temperature densification includes the following steps:

[0057] Step 1: Prepare ceramic proppant;

[0058] Step 11: Place bauxite, perlite, composite sintering aid, B2O3, AlF3, and deionized water into a ball mill jar in a certain proportion, and ball mill for 6 hours to obtain a uniform slurry; the weight parts of bauxite, perlite, composite sintering aid, B2O3, AlF3, and deionized water are 76 parts, 19 parts, 4.8 parts, 3 parts, 3 parts, and 120 parts, respectively.

[0059] Step 12: After drying the slurry in an oven for 24 hours, grind it into a fine powder using a mortar and pestle, and then sieve the powder through a 350-mesh sieve to obtain ceramsite powder.

[0060] Step 13: Add 40 parts of ceramsite powder to a sugar coating machine and spray with 1.5wt% PVA solution to obtain spherical particles. Use 50, 70 and 100 mesh sieves to separate 70-100 mesh particles as mother balls.

[0061] Step 14: After putting the mother ball into a clean sugar coating machine, spray PVA solution, add some ceramsite powder, and sieve out 50-70 mesh ceramsite.

[0062] Step 15: Put 50-70 mesh ceramsite into a clean sugar coating machine, spray PVA solution, add some ceramsite powder, and sieve out 40-50 mesh ceramsite particles.

[0063] Step 16: After drying the 40-50 mesh ceramsite particles in an oven at 120℃, remove the binder in a resistance furnace at 600℃ for 30 minutes to obtain ceramsite support green body.

[0064] Step 17: The ceramsite proppant green body is put into a high-temperature furnace and heated to 1000℃ at a rate of 5℃ / min, and then heated to the target temperature of 1250℃ at a rate of 3℃ / min. The green body is sintered for 90 min to obtain the ceramsite proppant.

[0065] Step 2, prepare the coating solution;

[0066] Step 21: Add expanded perlite, epoxy resin, and benzyl alcohol to a polytetrafluoroethylene cup in the specified proportions, stir evenly, and sonicate for 60 minutes; the weight parts of expanded perlite, epoxy resin, and benzyl alcohol are 0.6 parts, 10 parts, and 0.1 parts, respectively.

[0067] Step 22: After the ceramic proppant obtained in step 17 is kept at 300℃ for 30 minutes, it is added to the solution prepared in step 21, stirred and dispersed for 3 minutes, then the curing agent is added, and the mixture is ultrasonicated and stirred for 3-5 minutes. The sample is then taken out, placed in a tray, and then placed in an oven for curing to obtain the film proppant.

[0068] Step 3: Incubate the prepared ceramic proppant at 300℃ for 30 min;

[0069] Step 4: Add the coating solution to the heated ceramsite support in proportion and stir continuously for 3 minutes. Then add 20-25 wt% of polyetheramine and continue stirring for 3 minutes. Continue stirring to pre-cur the mixture. The weight parts of the coating solution and the ceramsite support are 5.4 parts and 60 parts, respectively.

[0070] Step 5: Add calcium stearate before the ceramsite support and resin solution show signs of aggregation or localized agglomeration, and stir evenly to prevent adhesion, to obtain a coated support sample; the weight of calcium stearate is 2-3 parts.

[0071] Step 6: Place the film-coated support sample in an oven and cure it at 120°C for 3 hours;

[0072] Step 7: After the sample has solidified, disperse it using a pulverizer and then sieve it through a 30-50 mesh to obtain the film-coated support.

[0073] In this process, bauxite and perlite are placed in a ball mill jar, and the particle size after ball milling is 1-1.5 μm. The curing agent is polyetheramine, the diluent is benzyl alcohol, and the lubricant is calcium stearate.

[0074] Example 2

[0075] A method for preparing a low-density ceramsite proppant, based on Example 1, involves step 11 in which bauxite, perlite, and deionized water are placed in a ball mill jar in a certain proportion and ball milled for 6 hours to obtain a uniform slurry; the weight parts of bauxite, perlite, and deionized water are 76 parts, 19 parts, and 120 parts, respectively.

[0076] Example 3

[0077] A method for preparing a low-density ceramsite proppant, based on Example 1, in step 11, bauxite, perlite, B2O3 and deionized water are placed in a ball mill jar in proportion and ball milled for 6 hours to obtain a uniform slurry; the weight parts of bauxite, perlite, B2O3 and deionized water are 76 parts, 19 parts, 3 parts and 120 parts, respectively.

[0078] Example 4

[0079] A method for preparing a low-density ceramsite proppant, based on Example 1, in step 11, bauxite, perlite, AlF3 and deionized water are placed in a ball mill jar in proportion and ball milled for 6 hours to obtain a uniform slurry; the weight parts of bauxite, perlite, AlF3 and deionized water are 76 parts, 19 parts, 3 parts and 120 parts, respectively.

[0080] Example 5

[0081] A method for preparing a low-density ceramsite proppant, based on Example 1, wherein in step 4, the weight parts of the coating solution and the ceramsite proppant are 0 parts and 60 parts, respectively.

[0082] Example 6

[0083] A method for preparing a low-density ceramic proppant, based on Example 1, wherein in step 4, the weight parts of the coating solution and the ceramic proppant are 1.8 parts and 60 parts, respectively.

[0084] Example 7

[0085] A method for preparing a low-density ceramsite proppant, based on Example 1, wherein in step 4, the weight parts of the coating solution and the ceramsite proppant are 7.2 parts and 60 parts, respectively.

[0086] Example 8

[0087] A method for preparing a low-density ceramic proppant, based on Example 1, wherein the weight ratio of MnO2 to TiO2 in the composite sintering aid is 1:1.

[0088] Example 9

[0089] A method for preparing a low-density ceramic proppant, based on Example 1, wherein the thickness of the coating layer is 50 μm.

[0090] Example 10

[0091] A method for preparing a low-density ceramic proppant, based on Example 1, wherein the target temperature in step 17 is 1200℃.

[0092] Example 11

[0093] A method for preparing a low-density ceramic proppant, based on Example 1, wherein the target temperature in step 17 is 1300℃.

[0094] Example 12

[0095] The proppant prepared in Examples 1-11 were subjected to performance tests on bulk density, apparent density, and breakage rate. The experimental results are shown in Table 2.

[0096] Table 2

[0097]

[0098] The proppant of Examples 1 and 7 were obtained under the preparation method and composition of the present invention. Examples 1 and 7 have the best performance. Examples 10 and 11 were sintered at different target temperatures and also achieved good performance.

[0099] The coated proppant of this invention lowers the sintering temperature to 1200-1300°C through B2O3, AlF3, and a composite sintering aid (MnO2-TiO2), promoting grain growth and pore closure, thereby reducing density and increasing strength. The coated proppant prepared by this invention has a bulk density of 1.314-1.452 g / cm³, an apparent density of 2.40-2.532 g / cm³, a breakage rate of 1.44-2.85% at 69 MPa, and a breakage rate of 3.98%-6.72% at 86 MPa.

[0100] In Example 1, composite sintering aids (MnO2 and TiO2) are used as fluxes to reduce energy consumption, while B2O3 and AlF3 are used as fluxes. During sintering, B2O3 and AlF3 form a low-melting-point glassy phase (approximately 1000-1200℃), filling the pores inside the ceramic particles, promoting interparticle diffusion sintering, and lowering the densification temperature to 1200-1300℃. Therefore, the lowest bulk density of Example 1 is 1.314 g / cm³, and the apparent density is 2.40 g / cm³.

[0101] The bulk density of Example 2 is 1.564 g / cm³. Example 2 has no additives, which means that a higher sintering temperature is required to achieve densification and increase porosity. In addition, the perlite is not fully activated, which weakens the lightweight effect.

[0102] The bulk density of Example 3 was 1.514 g / cm³, and the additives in Example 3 included only B2O3; the bulk density of Example 4 was 1.547 g / cm³, and the additives in Example 4 included only AlF3.

[0103] Therefore, it can be seen that the additives in Example 1 work synergistically to reduce porosity and increase densification, thus improving strength, i.e., a breakage rate of 1.44% at 69 MPa. In contrast, with a single additive or without additives, the pores cannot be effectively filled. Therefore, the bulk density of the film-coated proppant in Example 1 is significantly lower than that of the film-coated proppants in Examples 2, 3, and 4.

[0104] Although Examples 3 and 4 performed slightly better than Example 2, Example 1 further promoted the formation of the low-temperature glass phase under the synergistic effect of the quaternary additives. Therefore, the performance of Examples 3 and 4 was inferior to that of Example 1.

[0105] Example 5 is an example of Example 1 without a film coating. Its bulk density is lower due to the lack of film coating, at 1.683 g / cm³. 3 The strength is lower, and the breakage rate rises to 15% at 69 MPa. Without the coating layer, the surface of the ceramsite is exposed, making it prone to microcracks under pressure; the lack of expanded perlite weakens the lightweight buffer, thus the surface of the ceramsite is exposed, and the breakage rate rises to 15% at 69 MPa, which is much higher than in Example 1.

[0106] In Example 6, the weight parts of the coating solution and the ceramsite proppant were 1.8 parts and 60 parts, respectively. Uneven coating resulted in a breakage rate of 10.11% at 86 MPa. In Example 7, the weight parts of the coating solution and the ceramsite proppant were 5.4 parts and 60 parts, respectively. Example 7 is within the scope of protection of this invention. Therefore, when the coating amount in Example 7 is appropriate, the epoxy resin is uniformly coated, improving compressive strength and impact resistance.

[0107] In Example 8, the composite sintering aid contains MnO2:TiO2 in a ratio of 1:1. MnO2 promotes diffusion on the surface of Al2O3 grains, while TiO2 refines grains and inhibits abnormal growth. The 3:1 ratio of the two optimizes the grain boundary structure, reduces grain boundary defects, and improves flexural strength.

[0108] In Example 9, the coating thickness was 50 μm, with expanded perlite (1-2 μm) used as a lightweight filler to reduce the coating density. Epoxy resin provided bonding strength, coating microcracks on the surface of the ceramic particles and reducing stress concentration. A coating thickness of 85-170 μm can balance lightweight and protective effects. However, in Example 9, due to incomplete coverage, the coating was easily peeled off under pressure, resulting in decreased strength. 1250℃ was the optimal temperature, at which the glass phase was adequate, filling pores without over-melting, and ensuring uniform grain growth. At temperatures of 1200℃ and 1300℃, i.e., Examples 10 and 11, excessive glass phase weakened grain boundaries, slightly increased bulk density, and slightly increased breakage rate.

[0109] Therefore, the present invention achieves low-temperature densification at 1200-1300℃ through a quaternary additive system of B2O3-AlF3-MnO2-TiO2, and the expanded perlite-epoxy resin coating layer improves the surface integrity.

[0110] Experimental data show that the optimal coated proppant is obtained through synergistic effects of additives, appropriate coating amount, and sintering temperature of 1250℃. The coated proppant obtained by this invention has a bulk density of 1.314-1.452 g / cm³, an apparent density of 2.40-2.532 g / cm³, a breakage rate of ≤2.85% at 69 MPa, and a breakage rate of ≤6.72% at 86 MPa, meeting the lightweight and high-strength requirements of shale gas extraction.

[0111] The above description is merely some specific embodiments of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a light weight high strength coated proppant based on composite coating and low temperature densification, characterized by, Includes the following steps: Step 1: Add bauxite, perlite, composite sintering aid, B2O3, AlF3, and deionized water to a ball mill jar in a specific ratio. Mill for 6 hours to obtain a uniform slurry. Dry the slurry in an oven, then grind and filter to obtain ceramsite powder. Add a portion of the ceramsite powder to a coating machine, spray with PVA solution to obtain spherical particles. Sieve off particles of 70-100 mesh as master spheres. Place the master spheres into a clean coating machine, spray with PVA solution, and then add a portion of the ceramsite powder. 50-70 mesh ceramsite was sieved out; the 50-70 mesh ceramsite was fed into a clean sugar coating machine, sprayed with PVA solution, and then some ceramsite powder was added, and 40-50 mesh ceramsite particles were sieved out; the 40-50 mesh ceramsite particles were dried in an oven at 120℃, and then debinded in a resistance furnace at 600℃ for 30 minutes to obtain ceramsite proppant green body; the ceramsite proppant green body was put into a high-temperature furnace and heated to 1200-1300℃ for sintering for 90 minutes to obtain ceramsite proppant; Step 2, prepare the coating solution; Step 3: Incubate the prepared ceramic proppant at 300℃ for 30 min; Step 4: Add the coating solution to the heated ceramsite support in proportion, stir evenly, add the curing agent, and continue stirring for pre-curing; Step 5: Before the ceramsite proppant and resin solution agglomerate or locally agglomerate, add lubricant and stir evenly to obtain a coated proppant sample. Step 6: Place the film-coated support sample in an oven and cure it at 120°C for 3 hours; Step 7: After the sample has solidified, disperse it using a pulverizer and then sieve it through a 30-50 mesh screen to obtain the film-coated support agent; The coating support includes a ceramic proppant and a coating layer covering the surface of the ceramic proppant; The composite sintering aid includes MnO2 and TiO2; wherein the weight ratio of bauxite, perlite, composite sintering aid, B2O3, and AlF3 is 80:20:2.5-7.5:3-12:3-12; the coating layer includes expanded perlite and epoxy resin; the weight ratio of expanded perlite to epoxy resin in the coating layer is 0.1-0.5:3-12; the weight ratio of MnO2 to TiO2 in the composite sintering aid is 3:1; the particle size of expanded perlite is 1-2 μm; and the thickness of the coating layer is 85-170 μm.

2. The method for preparing a lightweight, high-strength coated support based on composite coating and low-temperature densification according to claim 1, characterized in that, The preparation of the coating solution specifically includes the following steps: Step 21: Add expanded perlite, epoxy resin, and diluent to a polytetrafluoroethylene cup in the specified proportions, stir well, and then sonicate for 60 minutes. Step 22: After the obtained ceramic proppant is kept at 300℃ for 30 minutes, it is added to the solution prepared in step 21, stirred and dispersed for 3 minutes, then the curing agent is added, and the mixture is ultrasonicated and stirred for 3-5 minutes. The sample is then taken out, placed in a tray, and then placed in an oven for curing to obtain the film proppant.

3. The method for preparing a lightweight, high-strength coated support based on composite coating and low-temperature densification according to claim 1, characterized in that, Bauxite and perlite were placed in a ball mill jar, and the particle size after ball milling was 1-1.5 μm.

4. The method for preparing a lightweight, high-strength coated support based on composite coating and low-temperature densification according to claim 1, characterized in that, The weight ratio of the coating solution to the ceramic proppant is 5-8:

60.

5. The coating support prepared by any one of the preparation methods according to claims 1-4.

Images