Multifunctional gradient coated proppant based on functionalized nanoparticles and method of making

Abstract

This invention discloses a multifunctional gradient-coated proppant based on functionalized nanoparticles and its preparation method. The multifunctional gradient-coated proppant includes a proppant core and a composite coating with a gradient structure covering the surface of the proppant core. The composite coating includes an inner layer connected to the proppant core and an outer layer composed of functionalized nanoparticles connected to the inner layer. The inner layer is composed of a cross-linked resin, and the functionalized nanoparticles are selected from one or more of amino-modified nanoparticles, guanidine-modified nanoparticles, and quaternary ammonium salt-modified nanoparticles. This invention utilizes the dual mechanism of "electrostatic adsorption + hydrogen bond anchoring" or the synergistic effect of "electrostatic + hydrophobic" of the outer functionalized nanoparticles to significantly improve the proppant's resistance to fragmentation while achieving strong capture of formation particles and effective inhibition of gas hydrate formation, making it suitable for oil and gas extraction under complex geological conditions.

Description

Technical Field

[0001] This invention relates to the field of oil and gas extraction technology, and in particular to a multifunctional gradient coating proppant based on functionalized nanoparticles and its preparation method. Background Technology

[0002] Hydraulic fracturing is an indispensable core technology for enhancing production in modern oil and gas field development, especially in the exploitation of unconventional oil and gas reservoirs such as shale and tight sandstone. This technology involves injecting fracturing fluid into the well under high pressure to open and extend fractures in the formation. Subsequently, proppant-carrying fluid containing proppant is pumped in, transporting and laying the proppant within the fractures. When the pumping pressure is released, the formation closure pressure acts on the proppant, which in turn provides support and prevents the fracture from closing. This creates an artificial channel with high conductivity within the formation, significantly improving the seepage environment for oil and gas and resulting in substantial increases in oil and gas well production.

[0003] As a key material in hydraulic fracturing, the performance of proppant directly determines the success or failure of fracturing and the long-term productivity of oil and gas wells. An ideal proppant needs sufficient compressive strength to withstand formation closure pressure without fracturing, while also possessing good permeability to ensure smooth oil and gas flow. Currently, the most widely used proppants in industry are mainly quartz sand and ceramsite.

[0004] However, these two types of conventional proppants have inherent drawbacks:

[0005] 1) Although quartz sand proppant is inexpensive, its compressive strength is generally low and its breakage rate is high, making it difficult to meet the requirements of increasingly deeper wells and higher closure pressures. High-strength, high-quality quartz sand is limited in its source, and long-distance transportation leads to a sharp increase in costs, which is inconsistent with the trend of cost reduction and efficiency improvement in oil fields.

[0006] 2) Ceramsite proppant is made by high-temperature sintering, which has high strength and low density, but its production process is complex, energy-intensive and expensive, which limits its large-scale application.

[0007] As exploration and development expands into deeper, ultra-deeper formations and areas with more complex geological conditions (such as deep water and clay-rich reservoirs), the requirements for proppant performance have gone far beyond compressive strength. In practical applications, even more complex technical challenges arise:

[0008] (I) Migration and Blockage of Formation Particles: Many reservoirs, especially silty mudstone reservoirs, contain a large amount of fine particles such as clay and quartz silt (collectively referred to as "mud and sand" or "particles"). Under the scouring of fracturing fluid and subsequent production fluid flow, these particles are easily detached from the fracture walls, migrate with the fluid, and block the pore throats of the proppant-filled bed, leading to a sharp decline or even complete failure of fracture conductivity. In addition, the breakage of the proppant itself can also generate new particles, exacerbating the blockage problem.

[0009] (ii) Formation of gas hydrates: In low-temperature and high-pressure environments such as deep water and permafrost zones, natural gas and water in the formation are prone to nucleation and growth on the surface of proppant particles, forming solid gas hydrates, which can also block seepage channels and seriously affect oil and gas production.

[0010] To improve proppant performance, existing technologies have proposed improvement schemes, such as enhancing particle strength and functionality through coating or nano-reinforcement. However, these schemes still have many shortcomings and cannot fully address the multiple challenges such as formation particle blockage and gas hydrate formation.

[0011] (1) CN 116285940 A discloses a method to improve compressive strength by adding nano-oxides (such as nano-Al2O3) to a substrate and sintering at high temperature. Although this method can improve mechanical properties, it relies on a high-energy-consuming sintering process, resulting in high production costs. Furthermore, it does not consider the surface modification of nanoparticles and cannot effectively suppress hydrate formation or capture formation particles, which is inconsistent with the current industry's demand for energy conservation, cost reduction, and multifunctionality.

[0012] (2) CN 101531893A discloses a support agent for heat-coated films using phenolic or epoxy resin, which aims to enhance strength and impart certain functions. This method is relatively simple, but the choice of resin is limited, and volatile organic compounds (VOCs) may be released during processing, causing environmental pollution. In addition, its effect on capturing formation particles is limited (lacking electrostatic attraction mechanism), and its hydrophobicity is insufficient (contact angle is usually <100°), making it difficult to effectively suppress the formation of hydrates under low temperature and high pressure.

[0013] (3) CN 102443387A discloses a proppant with a soluble coating, which improves permeability and strength through composite materials. This method can alleviate clogging to some extent, but the solubility of the coating leads to functional degradation under the scouring of formation fluids, and it does not integrate both hydrophobic and electrostatic functions. The processing is complex and costly, and it cannot achieve a comprehensive upgrade of low-cost substrates (such as desert sand). The above problems urgently need to be solved. Summary of the Invention

[0014] To address the aforementioned shortcomings in existing technologies, this invention provides a multifunctional gradient coating proppant based on functionalized nanoparticles and its preparation method. This invention constructs an "inner layer reinforcement - outer layer function" gradient coating structure on the proppant surface, utilizing outer layer functionalized nanoparticles to synergistically achieve strong capture of formation particles and effective suppression of gas hydrate formation.

[0015] The purpose of this invention is to provide a multifunctional gradient coating proppant based on functionalized nanoparticles, comprising a proppant core and a composite coating having a gradient structure on the surface of the proppant core. The composite coating comprises an inner layer connected to the proppant core and an outer layer composed of functionalized nanoparticles connected to the inner layer. The inner layer is composed of a cross-linked resin, and the functionalized nanoparticles are selected from one or more of amino-modified nanoparticles, guanidine-modified nanoparticles, and quaternary ammonium salt-modified nanoparticles.

[0016] The composite coating proposed in this invention features an inner layer close to the proppant core, primarily composed of high-strength cross-linked resin to enhance the mechanical strength of the proppant and its adhesion to the core. The outer layer, farther from the proppant core, is rich in functionalized nanoparticles to provide particle trapping and hydrophobic functions. The nanoparticles proposed in this invention are preferably nano-silica.

[0017] Functionalized nanoparticles can be amino-modified, guanidine-modified, or quaternary ammonium salt-modified. Taking quaternary ammonium salt-modified nanoparticles as an example, quaternary ammonium salt silane modifiers are used to modify the nanoparticles. The molecular structure of quaternary ammonium salt silane modifiers (such as dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride) simultaneously contains: a quaternary ammonium salt functional group: providing a permanent positive charge unaffected by environmental pH, used to efficiently capture negatively charged formation particles through strong electrostatic attraction; and a hydrophobic long alkyl chain (such as octadecyl): imparting high hydrophobicity to the proppant surface (water contact angle can reach 108°~150°), inhibiting the nucleation and growth of gas hydrates by changing surface wettability. This "monomer-dual-function" design integrates two core functions through nanoparticle modification, with a simple process and stable performance.

[0018] Amino or guanidine-modified nanoparticles: Nanoparticles are modified using silane modifiers containing amino or guanidine groups (such as N-aminoethyl-γ-aminopropyltrimethoxysilane, AEAPTES). This not only introduces positively charged amino groups, enabling the capture of particles through electrostatic attraction, but more importantly, the amino (-NH2) and imino (-NH-) groups on their surface act as strong hydrogen bond donors, forming a high-density hydrogen bond network with the hydroxyl groups on the surface of silty clay. This synergistic effect of "electrostatic adsorption + hydrogen bond anchoring" significantly enhances the gripping strength of particles, effectively preventing particle desorption and resuspension under high-speed fluid scouring.

[0019] When functionalized nanoparticles are combined into a bifunctional nanoparticle ensemble, this ensemble comprises two or more functionalized nanoparticles chemically bonded to a nanoparticle matrix and a silane modifier (such as a polyaminosilane) containing multiple coordinating / hydrogen-bonding groups. This combination is used to powerfully capture formation particles through electrostatic attraction, hydrogen bond networks, and chelation. This "functional division of labor and synergy" design allows for highly flexible customization of the proppant's performance by adjusting the proportions of two or more nanoparticles.

[0020] The "functionalized nanoparticle" combination scheme proposed in this invention provides a platform-based method for functional customization. By adjusting the ratio of different functional nanoparticles, proppants with optimal performance can be "tailor-made" for oil and gas reservoirs with different geological conditions, offering unparalleled flexibility and broad industrial application prospects compared to existing technologies.

[0021] Preferably, the proppant core is selected from one of quartz sand, ceramsite, desert sand, and slag.

[0022] Preferably, the crosslinking resin is selected from thermosetting epoxy resin and thermosetting phenolic resin.

[0023] Preferably, the amino-modified nanoparticles, guanidine-modified nanoparticles, or quaternary ammonium salt-modified nanoparticles are prepared by the following steps: dispersing nano-SiO2 in ethyl acetate to obtain a SiO2 suspension; preparing a silane modifier / ethyl acetate solution containing amino, guanidine, or quaternary ammonium cationic groups; heating the reaction solvent ethyl acetate to 50℃-70℃ under nitrogen protection and stirring; simultaneously adding the SiO2 suspension and the silane modifier / ethyl acetate solution to ethyl acetate (the SiO2 suspension and the silane modifier / ethyl acetate solution are added simultaneously at the same flow rate); after the addition is complete, maintaining the reaction at 55℃-65℃ for 12–20 h; cooling to room temperature and then filtering, washing, and drying to obtain the amino-modified nanoparticles, guanidine-modified nanoparticles, or quaternary ammonium salt-modified nanoparticles.

[0024] Further preferred, the concentration of SiO2 in the suspension is 0.3-0.4 g / mL, the molar concentration of the silane modifier / ethyl acetate solution containing amino, guanidine or quaternary ammonium cationic groups is 3-5 mmol / L, and the reaction conditions are: 60℃ for 12–18 h.

[0025] Further preferred, the concentration of SiO2 in the suspension is 1 / 30 g / mL, and the molar concentration of the silane modifier / ethyl acetate solution containing amino, guanidine or quaternary ammonium cationic groups is 4 mmol / L.

[0026] The quaternary ammonium salt modified nanoparticles include TPOAC (dimethyloctadecyl[3-trimethoxysilylpropyl]ammonium chloride)-SiO2, (3-trimethoxysilylpropyl)trimethylammonium chloride-SiO2, and trimethyl[3-(triethoxysilyl)propyl]ammonium chloride-SiO2.

[0027] Preferably, the amino-modified nanoparticles are AEAPTES-SiO2, and the guanidine-modified nanoparticles are TMSPG-SiO2 (TMSPG is 1-[3-(trimethoxysilyl)propyl]guanidine).

[0028] Preferably, the mass ratio of the proppant core, inner layer and outer layer is 100:(2-4):(0.1-2.0).

[0029] Further preferred, the mass ratio of the proppant core, inner layer and outer layer is 100:(2-3):(0.2-1.0).

[0030] Preferably, when the functionalized nanoparticles are amino-modified nanoparticles and quaternary ammonium salt-modified nanoparticles, the mass ratio of the two is 1:1-2:1; when the functionalized nanoparticles are amino-modified nanoparticles, quaternary ammonium salt-modified nanoparticles and guanidine-modified nanoparticles, the mass ratio of the three is 1:1:1.

[0031] Further optimization is achieved when the functionalized nanoparticles are amino-modified nanoparticles and quaternary ammonium salt-modified nanoparticles, with a mass ratio of 1.5:1.

[0032] This invention also protects the preparation method of the multifunctional gradient coating support based on functionalized nanoparticles, comprising the following steps:

[0033] S1. Pretreatment of proppant core: The proppant core is washed with water and then with alcohol in sequence, and then dried for later use;

[0034] S2. Inner coating preparation: Heat the dried support core to 160℃-180℃, spray in the crosslinking resin solution, stir, and then spray in the curing agent solution;

[0035] S3. Coating semi-curing: After the curing agent solution is sprayed, continue stirring to make the inner coating reach a semi-cured state.

[0036] S4. Preparation of outer coating: The functionalized nanoparticle suspension is sprayed onto the surface of the semi-cured proppant core in several stages to form an outer layer. After the last spraying is completed, the mixture is stirred and then cured to obtain the multifunctional gradient film proppant.

[0037] During preparation, a "multi-coating-simultaneous curing" process is employed, introducing nanoparticles during the semi-curing stage of the inner resin layer to form a stable gradient structure. This invention utilizes the dual mechanism of "electrostatic adsorption + hydrogen bond anchoring" or the synergistic effect of "electrostatic + hydrophobic" of the outer functionalized nanoparticles to significantly enhance the proppant's resistance to breakage while effectively capturing formation particles and inhibiting gas hydrate formation, making it suitable for oil and gas extraction under complex geological conditions.

[0038] Preferably, step S1 is as follows: add 1-2 times the mass of deionized water to the proppant core, mechanically stir at 400-600 rpm for 10-20 min, and then filter. Repeat this washing step several times. Then, add anhydrous ethanol of the same mass as the proppant core, mechanically stir at 500-700 rpm at room temperature for 10-20 min, and then filter. Repeat this washing step several times. Place the obtained clean proppant core in an oven at 100℃-120℃ and dry for 2-4 hours.

[0039] Preferably, the crosslinking resin solution in step S2 is an ethyl acetate solution of crosslinking resin, the crosslinking resin accounts for 2%-4% of the mass of the support core, the concentration of the crosslinking resin solution is 0.1-0.3 g / mL, the curing agent is an aliphatic amine curing agent, the curing agent accounts for 1%-2% of the mass of the support core, and the concentration of the curing agent solution is 10-20 vol.

[0040] Further preferably, the curing agent accounts for 1.4%-1.5% of the mass of the proppant core, and the concentration of the curing agent solution is 15-18 vol.

[0041] Further preferred, the curing agent accounts for 1.45% of the mass of the proppant core, and the concentration of the curing agent solution is 16 vol.

[0042] In the composite coating proposed in this invention, the inner layer thickness is 3-10 μm and the outer layer thickness is 50-200 nm.

[0043] Further preferred, the crosslinking resin is an epoxy resin, and the crosslinking resin solution is prepared by the following steps: dissolving the crosslinking resin in ethyl acetate to obtain a crosslinking resin solution with a concentration of 2 / 15-4 / 15.

[0044] Further optimization shows that the cross-linked resin accounts for 2.25% of the mass of the proppant core, and the inner layer thickness is approximately 5 μm.

[0045] Preferably, the functionalized nanoparticle suspension in step S4 is prepared by dispersing functionalized nanoparticles in anhydrous ethanol, wherein the mass ratio of functionalized nanoparticles to anhydrous ethanol is 1:30-1:60, and the functionalized nanoparticles account for 0.1%-2.0% of the mass of the proppant core.

[0046] Further optimization shows that the functionalized nanoparticles account for 0.5%-1.0% of the mass of the proppant core.

[0047] Preferably, the curing conditions in step S4 are: curing at 90℃-120℃ for 60-120 min.

[0048] In step S4, to maintain the sand temperature, the coating can be applied in several coats, each approximately 20 mL, with an interval of 60-120 seconds between coats. After the final coat, continue stirring for 30-60 seconds, then quickly remove the coating and allow it to cure.

[0049] Compared with the prior art, the present invention has the following advantages:

[0050] 1. This invention, through a platform-based design approach, provides a new, efficient, low-cost, and highly adaptable solution to address the critical challenge of the long-term decline in the conductivity of artificial fractures in hydraulic fracturing.

[0051] 2. This invention constructs an innovative "inner layer enhancement - outer layer function" gradient coating structure on the surface of the proppant, and utilizes functionalized nanoparticles in the outer layer to synergistically achieve strong capture of formation particles and effective suppression of gas hydrate formation.

[0052] 3. The amino or guanidine-modified nanoparticles used in this invention can capture formation microparticles through a dual synergistic mechanism of "electrostatic adsorption + hydrogen bond anchoring". In addition to utilizing positive charge to electrostatically attract negatively charged microparticles, the amino / guanidine groups on the surface act as strong hydrogen bond donors, forming a high-density hydrogen bond network with the hydroxyl groups on the surface of silty clay. This chemical anchoring significantly enhances the binding force between the microparticles and the proppant, effectively preventing the desorption and resuspension of microparticles under high-speed fluid scouring, and its inhibitory effect is significantly better than that of a single electrostatic adsorption mechanism.

[0053] 4. The "semi-curing bonding-co-curing molding" process proposed in this invention forms an integrated gradient interpenetrating structure of inner and outer layers, which endows the functional coating with excellent anti-peeling performance and ensures that its function of inhibiting particle migration and hydrate formation will not fail prematurely under long-term high-intensity scouring, thus significantly extending the effective life of the fracturing crack.

[0054] 5. This invention achieves high-density enrichment of functional sites on the proppant surface by directly grafting functional groups onto nanoparticles with high specific surface area. Compared with existing technologies that introduce functional groups into sol-gel emulsions, this invention significantly improves the capture efficiency of formation particles and the inhibition ability of hydrates, solving the pain points of existing technologies where functional layers are "showy but impractical" and inefficient. Attached Figure Description

[0055] Figure 1This is a schematic diagram of the preparation process of the multifunctional gradient coating support agent in Embodiment 2 of the present invention;

[0056] Figure 2 This is a schematic diagram of the structure of the multifunctional gradient coating support proposed in this invention;

[0057] Figure 3 This is a schematic diagram illustrating the mechanism of the multifunctional gradient coating proppant proposed in this invention under pressure.

[0058] Explanation of reference numerals in the attached figures: 1. Support core; 2. Inner layer; 3. Outer layer. Detailed Implementation

[0059] The following embodiments are further illustrations of the present invention, but not limitations thereof.

[0060] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention. Unless otherwise specified, the experimental materials and reagents used herein are commercially available products conventionally available in this technical field. The curing agent (A875174 amine curing agent, phenolic modified, amine value: 450~500 mgKOH / g, viscosity (25℃): 1100~1300 mpa.s) used in the following examples or comparative examples was purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0061] like Figure 2 As shown, a multifunctional gradient coating support based on functionalized nanoparticles includes a support core 1 and a composite coating with a gradient structure covering the surface of the support core 1. The composite coating includes an inner layer 2 connected to the support core and an outer layer 3 composed of functionalized nanoparticles connected to the inner layer. The inner layer 2 is composed of a cross-linked resin, and the functionalized nanoparticles are selected from one or more of amino-modified nanoparticles, guanidine-modified nanoparticles, and quaternary ammonium salt-modified nanoparticles. The amino-modified nanoparticles, guanidine-modified nanoparticles, and quaternary ammonium salt-modified nanoparticles are chemically bonded to a nanoparticle matrix and a silane modifier containing multiple coordinating / hydrogen-bonding groups of amino, guanidine, or quaternary ammonium. Figure 2 The paper demonstrates a gradient structure of "support core - inner resin layer - outer functionalized nanoparticles".

[0062] Figure 3 This is a schematic diagram of the mechanism of a multifunctional gradient coating proppant under pressure, showing how the outer nanoparticles disperse stress through breakage and deformation while capturing microparticles.

[0063] Amino-modified nanoparticles, guanidine-modified nanoparticles, or quaternary ammonium salt-modified nanoparticles are prepared by the following steps: nano-SiO2 is dispersed in ethyl acetate to obtain a SiO2 suspension; a silane modifier / ethyl acetate solution containing amino, guanidine, or quaternary ammonium cationic groups is prepared; ethyl acetate is heated to 50℃-70℃ under nitrogen protection and stirred; the SiO2 suspension and the silane modifier / ethyl acetate solution are simultaneously added dropwise to ethyl acetate at the same flow rate; after the addition is complete, the reaction is maintained at 55℃-65℃ for 10–20 h; after cooling to room temperature, the mixture is filtered, washed, and dried to obtain amino-modified nanoparticles, guanidine-modified nanoparticles, or quaternary ammonium salt-modified nanoparticles.

[0064] In the following examples or comparative examples, the concentration of SiO2 in the suspension is 0.3-0.4 g / mL, the molar concentration of the silane modifier / ethyl acetate solution containing amino, guanidine or quaternary ammonium cationic groups is 3-5 mmol / L, and the reaction conditions are: 60℃ for 12-18 h.

[0065] In the following examples or comparative examples, the concentration of SiO2 in the suspension is 1 / 30 g / mL, and the molar concentration of the silane modifier / ethyl acetate solution containing amino, guanidine or quaternary ammonium cationic groups is 4 mmol / L.

[0066] The preparation method of a multifunctional gradient coating proppant based on functionalized nanoparticles includes the following steps (taking quartz sand as the proppant core and TPOAC-SiO2 as the functionalized nanoparticles as an example):

[0067] (1) Pretreatment of quartz sand substrate

[0068] Weigh 200 g of 20 / 40 mesh quartz sand, add 1.5 times its weight of deionized water, stir at 400–600 r / min for 10–20 min, filter, and repeat 3 times. Then add an equal weight of anhydrous ethanol, stir at 500–700 r / min for 10–20 min, filter, and repeat 3 times. Place the quartz sand in an oven at 100℃–120℃ to dry for 2–4 h, and cool for later use.

[0069] (2) Epoxy resin inner layer coating and semi-curing

[0070] Weigh 2%–4% of epoxy resin E-51 by weight of quartz sand and dissolve it in ethyl acetate to obtain a resin dilution with a concentration of 0.1–0.3 g / mL. Weigh 1.4%–1.5% of aliphatic amine curing agent by weight of quartz sand and dissolve it in anhydrous ethanol to obtain a curing agent dilution with a concentration of 15–18 vol%. Add the dried quartz sand to a stainless steel coated autoclave and heat it to 160℃–180℃, stirring at 200–500 r / min. Use a pneumatic spray gun (nitrogen 0.5 MPa) to evenly spray the resin dilution for 1–3 min, and continue to stir for 60–90 s to coat the resin. Then spray the curing agent dilution in the same way and stir at 150℃–190℃ for 60–90 s to partially cross-link the resin, increase its viscosity, but not completely cure it, forming an adhesive semi-cured inner layer.

[0071] (3) Functionalized nano-SiO2 outer layer spraying and final curing

[0072] Weigh 0.1-2.0 wt% of TPOAC-SiO2 from quartz sand. Disperse TPOAC-SiO2 with anhydrous ethanol at a mass ratio of 1:30–1:60 in anhydrous ethanol and sonicate for 10–15 min to obtain a suspension. During the semi-curing stage of the inner resin, keep the temperature and stirring speed of the coating vessel constant, change the spray gun, and spray the nanoparticle suspension onto the tumbling coated sand in batches, about 20 mL each time, with an interval of 60–120 s to allow ethanol evaporation. After completion, tumble for another 30–60 s to promote contact and embedding of nanoparticles with the resin layer. Remove the coated sand, spread it flat or put it into a heat-resistant container, and place it in an oven at 90℃–130℃ for 60–150 min to allow the epoxy resin to fully cross-link and cure. Cool and sieve out agglomerated particles to obtain a gradient coating support agent consisting of "quartz sand core – epoxy resin inner layer – functionalized nano-SiO2 outer layer". Replacing TPOAC-SiO2 with AEAPTES-SiO2 can produce AEAPTES functionalized gradient coating support.

[0073] Example 1

[0074] like Figure 1 As shown, a method for preparing a multifunctional gradient coating support based on functionalized nanoparticles includes the following steps:

[0075] S1. Pretreatment of quartz sand substrate: Take 200 g of 20 mesh quartz sand, add 1.5 times the mass of deionized water, mechanically stir at 500 rpm for 15 min and then filter; repeat this washing step three times; then, add an equal mass of anhydrous ethanol to the quartz sand, mechanically stir at 600 rpm at room temperature for 15 min and then filter; repeat this washing step three times; place the obtained clean quartz sand in an oven at 105℃ to dry for 3 hours, and cool for later use.

[0076] S2. Inner Coating Preparation and Spraying:

[0077] Preparation of resin solution: Dissolve 6 g of epoxy resin (E-51 type) in 30 mL of ethyl acetate to obtain a resin solution with a concentration of 0.2 g / mL.

[0078] Preparation of curing agent solution: Take 2.9 g of fatty amine curing agent and dilute it in anhydrous ethanol to obtain a curing agent solution with a concentration of 16 vol%.

[0079] The dried quartz sand was placed in a stainless steel autoclave and heated to 170°C, and then mechanically tumbled and stirred at 300 rpm.

[0080] Using a high-pressure atomizing spray gun (0.5 MPa, nitrogen carrier gas), spray the diluted epoxy resin solution at a stable rate of 10–12 mL per minute, and apply evenly for 1–3 minutes. Continue tumbling for 75 seconds to coat the resin, and then spray the curing agent solution in the same manner.

[0081] S3. Semi-cured coating: After spraying, roll at 170℃ for 75 seconds to allow the resin to partially cross-link and increase viscosity, but not to fully cure, so that the first coating (inner layer) reaches a semi-cured state. At this time, the coating has high viscosity but still has permeability.

[0082] S4. Outer coating preparation, spraying and final curing:

[0083] Preparation of nanoparticle suspension: Weigh 1.0 g AEAPTES-SiO2 (i.e., 0.5 wt% loading), disperse it in 45 mL anhydrous ethanol, and sonicate for 15 min to obtain the suspension.

[0084] AEAPTES-SiO2 was prepared by the following steps: 2.0 g of nano-SiO2 was weighed and dispersed in 60 mL of ethyl acetate, and sonicated for 15 min to obtain a suspension, which was then placed in a constant-pressure dropping funnel A. An AEAPTES / ethyl acetate solution with a concentration of 4 mmol / L was prepared and placed in a dropping funnel B. 30 mL of ethyl acetate was added to a 250 mL three-necked flask, a magnetic stir bar was placed, and a condenser and two dropping funnels were connected. The mixture was heated to approximately 60 °C under nitrogen protection and stirred. The dropping rates of A and B were adjusted so that the SiO2 suspension and the AEAPTES solution were added simultaneously at approximately the same flow rate for 1.5 h to maintain a low instantaneous concentration of AEAPTES and reduce self-condensation. After the addition was complete, the reaction was maintained at 60 °C for 15 h. After cooling to room temperature, the mixture was filtered, and the solid was washed at least twice with anhydrous ethanol, dried at 100 °C for 2 h, and then ground to obtain AEAPTES-SiO2.

[0085] During the semi-curing stage of the inner resin, the temperature and stirring speed of the coating vessel are kept constant. The spray gun is changed, and the nanoparticle suspension is sprayed onto the tumbling coating sand in batches, about 20 mL each time, with a 90-second interval to allow ethanol evaporation. After completion, the sand is tumbled for another 45 seconds to promote contact and embedding of the nanoparticles into the resin layer. The coating sand is then removed, spread flat or placed in a heat-resistant container, and kept in a 105℃ oven for 90 minutes to allow the epoxy resin to fully cross-link and cure. After cooling and sieving to remove agglomerated particles, a gradient coating support agent consisting of "quartz sand core – epoxy resin inner layer – functionalized nano-SiO2 outer layer" is obtained, with an inner layer thickness of 5 μm and an outer layer thickness of approximately 100-200 nm.

[0086] Example 2

[0087] A method for preparing a multifunctional gradient coating support based on functionalized nanoparticles includes the following steps:

[0088] S1. Quartz Sand Substrate Pretreatment: Take 200 g of 20-mesh quartz sand, add 1.5 times the mass of quartz sand in deionized water, mechanically stir at 500 rpm for 15 min, and then filter; repeat this washing step three times; add an equal mass of anhydrous ethanol to the quartz sand, mechanically stir at 600 rpm at room temperature for 15 min, and then filter; repeat this washing step three times. Place the obtained clean quartz sand in an oven at 105℃ to dry for 3 hours, and then cool for later use.

[0089] S2. Inner Coating Preparation and Spraying:

[0090] Preparation of resin solution: Dissolve 4.9 g of epoxy resin (E-51 type) in ethyl acetate to obtain a resin solution with a concentration of 0.2 g / mL.

[0091] Preparation of curing agent solution: Take 2.9 g of fatty amine curing agent and dilute it in anhydrous ethanol to obtain a curing agent solution with a concentration of 16 vol%.

[0092] The dried quartz sand was placed in a stainless steel autoclave and heated to 170°C, and then mechanically tumbled and stirred at 300 rpm.

[0093] Using a high-pressure atomizing spray gun (0.5 MPa, nitrogen carrier gas), spray the diluted epoxy resin solution at a stable rate of 10–12 mL per minute, and apply evenly for 1–3 minutes. Continue tumbling for 75 seconds to coat the resin, and then spray the curing agent solution in the same manner.

[0094] S3. Coating semi-curing: After spraying, roll at 170℃ for 75 seconds to allow the resin to partially cross-link and increase viscosity, but not to completely cure, so that the first coating (inner layer) reaches a semi-cured state. At this time, the coating surface has high viscosity but still has permeability, which prepares for the subsequent coating bonding.

[0095] S4. Outer Coating Preparation, Spraying, and Final Curing: TPOAC-SiO2 was prepared by the following steps: 2.0 g of SiO2 was weighed and dispersed in 60 mL of ethyl acetate, and sonicated for 15 min to obtain a suspension (placed in constant pressure dropping funnel A). A 4 mmol / L TPOAC / ethyl acetate dilute solution was prepared (placed in constant pressure dropping funnel B). 30 mL of ethyl acetate was added to a 250 mL three-necked flask, a magnetic stir bar was placed, and a condenser and dropping funnels A and B were connected. The mixture was heated to approximately 60 °C under nitrogen protection and stirred. The dropping rates of dropping funnels A and B were adjusted so that the SiO2 suspension and TPOAC solution were added simultaneously at approximately the same flow rate. The addition was carried out for 1.5 h to maintain a low instantaneous concentration of TPOAC and reduce self-condensation. After the addition was complete, the reaction was maintained at 60 °C for 15 h. After cooling to room temperature, the solid was filtered, washed with anhydrous ethanol at least twice, dried at 100 °C for 2 h, and then ground to obtain TPOAC-SiO2.

[0096] Preparation of nanoparticle suspension: Weigh 0.6 g of AEAPTES-SiO2 and 0.4 g of TPOAC-SiO2 obtained in Example 1 (total mass 1.0 g, i.e. 0.5 wt%) and disperse them together in 45 mL of anhydrous ethanol to obtain a suspension.

[0097] During the semi-curing stage of the inner resin, the temperature and stirring speed of the coating vessel are kept constant. The spray gun is changed, and the nanoparticle suspension is sprayed onto the tumbling coating sand in batches, about 20 mL each time, with a 90-second interval to allow ethanol evaporation. After completion, the sand is tumbled for another 45 seconds to promote contact and embedding of the nanoparticles into the resin layer. The coating sand is then removed, spread flat or placed in a heat-resistant container, and kept in a 105℃ oven for 90 minutes to allow the epoxy resin to fully cross-link and cure. After cooling and sieving to remove agglomerated particles, a gradient coating support agent consisting of "quartz sand core – epoxy resin inner layer – functionalized nano-SiO2 outer layer" is obtained, with an inner layer thickness of 5 μm and an outer layer thickness of approximately 100-200 nm.

[0098] Comparative Example 1

[0099] Same as Example 2, except that in step S4, the nanoparticle suspension is prepared by dispersing 1.0 g of TPOAC-SiO2 in anhydrous ethanol for spraying, with a mass ratio of TPOAC-SiO2 to anhydrous ethanol of 1:45, and sonicating for 12 min to obtain the suspension.

[0100] Comparative Example 2

[0101] Same as Example 2, except that in step S4, the nanoparticle suspension is prepared by weighing 2.0 g of AEAPTES-SiO2 obtained in Example 1 and dispersing it in anhydrous ethanol, with a mass ratio of AEAPTES-SiO2 to anhydrous ethanol of 1:45, and sonicating for 12 min to obtain the suspension.

[0102] Comparative Example 3

[0103] Take 200 g of 20 mesh quartz sand and pretreat it according to step S1 of Example 2 (washing with water, washing with alcohol, and drying). Do not perform any coating treatment and use it directly as the original sand control sample.

[0104] Comparative Example 4

[0105] Similar to Example 2, except that step S4 is not performed. That is, after the epoxy resin inner layer is coated, the coated sand is directly taken out and placed in an oven at 105°C for 90 min to allow the epoxy resin to fully crosslink and cure. After cooling, a pure resin coating support agent (without nanoparticle outer layer) is obtained.

[0106] Example 3

[0107] Same as Example 2, except that:

[0108] S1. Pretreatment of the proppant core: Take 200 g of 40-mesh quartz sand, add deionized water at a mass equal to that of the quartz sand, and mechanically stir at 400 rpm for 20 min, then filter; repeat this washing step three times; add anhydrous ethanol at a mass equal to that of the quartz sand, and mechanically stir at 500 rpm at room temperature for 20 min, then filter; repeat this washing step three times. Place the obtained clean quartz sand in an oven at 100℃ and dry for 4 hours.

[0109] S2. Inner Coating Preparation and Spraying:

[0110] Preparation of resin solution: Dissolve 4 g of epoxy resin (E-51 type) in ethyl acetate to obtain a resin solution with a concentration of 0.1 g / mL.

[0111] Preparation of curing agent solution: Take 2.8 g of fatty amine curing agent and dilute it in anhydrous ethanol to obtain a curing agent dilution with a concentration of 15 vol%.

[0112] The dried quartz sand was placed in a stainless steel autoclave and heated to 160°C, and then mechanically tumbled and stirred at 500 rpm.

[0113] Using a high-pressure atomizing spray gun (0.5 MPa, nitrogen carrier gas), spray the diluted epoxy resin solution at a stable rate of 10-12 mL per minute, spray evenly for 1 minute, and continue to roll for 60 seconds to coat it; then spray the hardener dilution in the same way.

[0114] S3. Semi-cured coating: After spraying, roll at 160℃ for 90 seconds to allow the resin to partially cross-link and increase in viscosity, but not to completely cure, forming a semi-cured inner layer with adhesive properties. At this time, the coating surface has high viscosity but still has permeability, preparing for subsequent coating bonding.

[0115] S4. Outer coating preparation, spraying and final curing:

[0116] Preparation of nanoparticle suspension: 1 g of TPOAC-SiO2 obtained in Example 2 and 1 g of AEAPTES-SiO2 obtained in Example 1 were dispersed in anhydrous ethanol. The mass ratio of TPOAC-SiO2 and AEAPTES-SiO2 to anhydrous ethanol was 1:30. The suspension was obtained by sonication for 10 min.

[0117] During the semi-curing stage of the inner resin, the temperature and stirring speed of the coating vessel are kept constant. The spray gun is changed, and the nanoparticle suspension is sprayed onto the tumbling coated sand in several applications, approximately 20 mL each time, with a 60-second interval to allow for ethanol evaporation. After each application, the sand is tumbled for another 60 seconds to promote contact and embedding of the nanoparticles into the resin layer. The coated sand is then removed, spread out, or placed in a heat-resistant container and kept in a 90℃ oven for 120 minutes to allow the epoxy resin to fully cross-link and cure. After cooling and sieving to remove agglomerated particles, a gradient coating support agent consisting of "quartz sand core – epoxy resin inner layer – functionalized nano-SiO2 outer layer" is obtained.

[0118] Example 4

[0119] Same as Example 2, except that:

[0120] S1. Pretreatment of the proppant core: Take 200 g of 40-mesh quartz sand, add deionized water at a mass equal to that of the quartz sand, and mechanically stir at 400 rpm for 20 min, then filter; repeat this washing step three times; add anhydrous ethanol at a mass equal to that of the quartz sand, and mechanically stir at 500 rpm at room temperature for 20 min, then filter; repeat this washing step three times. Place the obtained clean quartz sand in an oven at 100℃ and dry for 4 hours.

[0121] S2. Inner Coating Preparation and Spraying:

[0122] Preparation of resin solution: Dissolve 4 g of epoxy resin (E-51 type) in ethyl acetate to obtain a resin solution with a concentration of 0.3 g / mL.

[0123] Preparation of curing agent solution: Take 2.8 g of fatty amine curing agent and dilute it in anhydrous ethanol to obtain a curing agent dilution with a concentration of 18 vol%.

[0124] The dried quartz sand was placed in a stainless steel autoclave and heated to 180°C, and then mechanically tumbled and stirred at 200 rpm.

[0125] Using a high-pressure atomizing spray gun (0.5 MPa, nitrogen carrier gas), spray the diluted epoxy resin solution at a stable rate of 10-12 mL per minute, spray evenly for 3 minutes, and continue to roll for 90 seconds to coat it; then spray the hardener dilution in the same way.

[0126] S3. Semi-cured coating: After spraying, roll at 180℃ for 60 seconds to allow the resin to partially cross-link and increase in viscosity, but not to fully cure, forming a semi-cured inner layer with adhesive properties. At this time, the coating surface has high viscosity but is still permeable, preparing for subsequent coating bonding.

[0127] S4. Outer coating preparation, spraying and final curing:

[0128] Preparation of nanoparticle suspension: 2 g of TPOAC-SiO2 obtained in Example 2 and 4 g of AEAPTES-SiO2 obtained in Example 1 were dispersed in anhydrous ethanol, with a mass ratio of TPOAC-SiO2 and AEAPTES-SiO2 to anhydrous ethanol of 1:60. The suspension was obtained by sonication for 15 min.

[0129] During the semi-curing stage of the inner resin, the temperature and stirring speed of the coating vessel are kept constant. The spray gun is changed, and the nanoparticle suspension is sprayed onto the tumbling coating sand in several applications, approximately 20 mL each time, with a 120-second interval to allow for ethanol evaporation. After each application, the sand is tumbled for another 30 seconds to promote contact and embedding of the nanoparticles into the resin layer. The coating sand is then removed, spread out flat, or placed in a heat-resistant container and kept in a 120°C oven for 60 minutes to allow the epoxy resin to fully cross-link and cure. After cooling and sieving to remove agglomerated particles, a gradient coating support agent consisting of "quartz sand core – epoxy resin inner layer – functionalized nano-SiO2 outer layer" is obtained.

[0130] To verify the performance of the proppant prepared in this invention, a particle transport evaluation device and a contact angle testing device were built. The above-mentioned examples and comparative examples were tested, and the results are shown in Table 1 below:

[0131] Table 1. Performance test results of each group of proppant

[0132] Results analysis:

[0133] 1. Coating reinforcement effect and breakage rate: The breakage rate of Comparative Example 3 (raw sand) was as high as 30.6% at 52 MPa, while the breakage rate of all coated samples (Examples 1-4 and Comparative Examples 1, 2, and 4) was reduced to 7.2%-7.5%. This indicates that the epoxy resin inner layer has a significant and stable effect on improving the mechanical strength of the proppant, and the introduction of a small amount of nanoparticles in the outer layer does not weaken the mechanical properties.

[0134] 2. Enhanced effect of surface functionalization on particle capture: Comparative Example 4 (pure resin coating) showed an initial effluent turbidity as high as 280 NTU and an equilibrium turbidity of 480 NTU, indicating that the inert organic surface does not have the ability to actively retain silt particles. The capture effect was significantly improved after introducing functionalized nanoparticles. Example 1 (0.5% AEAPTES-SiO2 used alone) showed an initial turbidity reduction to 110 NTU, indicating that the dual mechanism of "electrostatic adsorption + hydrogen bond anchoring" provided by amino groups can effectively capture and fix negatively charged particles. Comparative Example 1 (0.5% TPOAC-SiO2 used alone) showed an initial turbidity of 155 NTU, which was also improved, but the hydrophobic barrier formed by the long-chain octadecyl groups to some extent hindered the full contact between fine particles in the aqueous phase and the positive potential point. Therefore, the capture efficiency relying solely on electrostatic adsorption was weaker than that of the AEAPTES system.

[0135] 3. Synergistic effect of AEAPTES-SiO2 and TPOAC-SiO2 (1+1>2): Example 2 uses a mixture of 0.3% AEAPTES-SiO2 and 0.2% TPOAC-SiO2 (total amount is the same as 0.5% in Comparative Example 1 and Example 1), and its performance surpasses that of the two single-component comparative examples.

[0136] (a) Particle capture superior to any single component: The initial turbidity in Example 2 decreased to 95 NTU (superior to 110 NTU in Example 1 and 155 NTU in Comparative Example 1); the equilibrium turbidity was 270 NTU (superior to 300 NTU in Example 1 and 385 NTU in Comparative Example 1). Despite the reduced absolute amount of AEAPTES in Example 2, the capture effect was actually better. The mechanism was analyzed as follows: the long alkyl chain of TPOAC forms local hydrophobic microregions on the surface of the proppant, which breaks down the hydration repulsion barrier when fine particles approach the interface; once the fine particles easily penetrate the hydration layer and approach the surface, they are strongly anchored by the adjacent high-density AEAPTES sites through electrostatic attraction and hydrogen bonding. This relay capture mechanism of "TPOAC reducing the access barrier - AEAPTES implementing chemical anchoring" makes the total capture efficiency surpass that of any single-component system, showing a significant synergistic effect.

[0137] (b) Hydrophobicity far superior to AEAPTES single component: The water contact angle of Example 2 reached 138°, significantly better than Example 1 (108°), and close to that of the pure TPOAC system (Comparative Example 1, 142°). This indicates that the introduction of a small amount of TPOAC successfully endowed the composite interface with high hydrophobicity, greatly changing the wettability of the proppant surface, which plays a key role in inhibiting the nucleation and growth of gas hydrates on the proppant surface under high pressure and low temperature.

[0138] (c) Overcoming the performance bottleneck of a single component: Observing Comparative Example 2 (1.0% AEAPTES-SiO2), when the amount of AEAPTES was simply doubled, its initial turbidity (115 NTU) was worse than that of Example 2 (95 NTU), and the contact angle (106°) did not improve. This proves that simply increasing the concentration of a single functional particle cannot simultaneously achieve "efficient capture" and "strong hydrophobicity," and may reduce the effective action sites due to local aggregation. Only the gradient compound system of Example 2 achieved a perfect synergy of multiple mechanisms.

[0139] 4. Impact of Loading Amount and Optimal Range: Comparing Example 2 (total load 0.5%, best overall), Example 3 (total load 1.0%, performance decreased), and Example 4 (total load 3.0%, performance significantly deteriorated), it is clear that excessive nanoparticle loading leads to increased particle agglomeration during spraying. This not only obscures effective positive potential points but also disrupts the uniformity of the outer layer morphology, resulting in a decrease in capture efficiency and hydrophobicity instead of an increase. This "excessive failure" rule confirms that the 0.1%~2.0% loading range defined in this invention has a solid experimental basis and technical rationality, and that the area around 0.5% is the optimal loading range.

[0140] The above description of the embodiments is only for the purpose of helping to understand the technical solution and core idea of ​​the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made to the present invention without departing from the principle of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A multifunctional gradient coated proppant based on functionalized nanoparticles, characterized in that, The invention includes a proppant core and a composite coating having a gradient structure on the surface of the proppant core. The composite coating includes an inner layer connected to the proppant core and an outer layer composed of functionalized nanoparticles connected to the inner layer. The inner layer is composed of a cross-linked resin, and the functionalized nanoparticles are selected from one or more of amino-modified nanoparticles, guanidine-modified nanoparticles, and quaternary ammonium salt-modified nanoparticles.

2. The multifunctional gradient coating support agent according to claim 1, characterized in that, The proppant core is selected from one of the following: quartz sand, ceramsite, desert sand, and slag.

3. The multifunctional gradient coating support agent according to claim 1 or 2, characterized in that, The crosslinking resin is selected from one of thermosetting epoxy resin and thermosetting phenolic resin.

4. The multifunctional gradient coating support agent according to claim 1 or 2, characterized in that, The amino-modified nanoparticles, guanidine-modified nanoparticles, or quaternary ammonium salt-modified nanoparticles are prepared by the following steps: nano-SiO2 is dispersed in ethyl acetate to obtain a SiO2 suspension; a silane modifier / ethyl acetate solution containing amino, guanidine, or quaternary ammonium cationic groups is prepared; ethyl acetate is heated to 50℃-70℃ and stirred under nitrogen protection; the SiO2 suspension and the silane modifier / ethyl acetate solution are simultaneously added dropwise to ethyl acetate; after the addition is complete, the reaction is maintained at 55℃-65℃ for 10-20 hours; after cooling to room temperature, the mixture is filtered, washed, and dried to obtain the amino-modified nanoparticles, guanidine-modified nanoparticles, or quaternary ammonium salt-modified nanoparticles.

5. The multifunctional gradient coating support agent according to claim 1 or 2, characterized in that, The mass ratio of the proppant core, inner layer and outer layer is 100:(2-4):(0.1-2.0).

6. The method for preparing the multifunctional gradient coating support based on functionalized nanoparticles as described in claim 1, characterized in that, Includes the following steps: S1. Pretreatment of proppant core: The proppant core is washed with water and then with alcohol in sequence, and then dried for later use; S2. Inner coating preparation: Heat the dried support core to 160℃-180℃, spray in the crosslinking resin solution, stir, and then spray in the curing agent solution; S3, Semi-cured coating: After the curing agent solution is sprayed, continue stirring to cause partial cross-linking of the resin and increase in viscosity, but without complete curing, forming a semi-cured inner layer with adhesive properties; S4. Preparation of outer coating: The functionalized nanoparticle suspension is sprayed onto the surface of the semi-cured inner layer in stages to form the outer layer, thereby constructing a gradient composite coating. After the last spraying is completed, stirring is continued, and then it is cured to obtain the multifunctional gradient film support agent.

7. The preparation method according to claim 6, characterized in that, The crosslinking resin solution mentioned in step S2 is an ethyl acetate solution of crosslinking resin, the crosslinking resin accounts for 2%-4% of the mass of the support core, the concentration of the crosslinking resin solution is 0.1-0.3 g / mL, the curing agent is an aliphatic amine curing agent, the curing agent accounts for 1%-2% of the mass of the support core, and the concentration of the curing agent solution is 10-20 vol.

8. The preparation method according to claim 6, characterized in that, The functionalized nanoparticle suspension described in step S4 is prepared by dispersing functionalized nanoparticles in anhydrous ethanol, wherein the mass ratio of functionalized nanoparticles to anhydrous ethanol is 1:30-1:60, and the functionalized nanoparticles account for 0.1%-2.0% of the mass of the proppant core.

9. The preparation method according to claim 6, characterized in that, The curing conditions described in step S4 are: curing at 90℃-120℃ for 60-120 min.

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