Surface treatment method for high-stress-crack-resistant proppant

Abstract

The invention relates to the technical field of proppants, and discloses a surface treatment method of a high-stress-crack-resistant proppant, which comprises the following steps: (1) pretreating a matrix; (2) plasma activation; (3) carrying out titanate bonding pretreatment; (4) preparing an inner layer; (5) preparing an outer layer; (6) post-treatment; through cooperation and collaborative optimization of all the steps, the problems that in the high-stress environment, a proppant is prone to breaking, a coating layer is prone to falling off, and flow conductivity attenuation is fast are successfully solved, quartz sand serves as a base body in the technical scheme, dependence on expensive bauxite raw materials is not needed, the technological process is free of high-temperature and high-energy-consumption steps, all parameters are controllable, repeatability is good, and the proppant is suitable for large-scale production. The production cost and the industrialization difficulty are obviously reduced; besides, due to high stability and long-acting flow conductivity of the proppant, the effective production cycle of an oil and gas well can be prolonged, and a solution with technical advancement and economic feasibility is provided for the oil and gas exploitation industry.

Description

Technical Field

[0001] This invention relates to the field of proppant technology, specifically a surface treatment method for high-stress fracture proppant. Background Technology

[0002] As oil and gas resource exploitation gradually extends to deeper and tighter reservoirs, hydraulic fracturing technology has become a core means of enhancing oil recovery. Proppants, as key materials for maintaining fracture patency, directly determine the durability of the fracturing effect. Currently, commonly used proppants in industry mainly include three types: quartz sand, artificial ceramic proppant, and coated proppant. However, all have unavoidable drawbacks: while quartz sand proppant is widely available and inexpensive, its natural structure is loose, making it prone to breakage under the high closure pressure of deep oil and gas reservoirs, leading to fracture closure and loss of conductivity; artificial ceramic proppant is mostly made from bauxite, which has high strength, but the supply and demand imbalance of bauxite resources and rising prices, coupled with the high density of ceramic proppant, increases the difficulty of pumping fracturing fluid and operational costs.

[0003] Therefore, developing a proppant surface treatment method that is cost-effective and combines high compressive strength with excellent conductivity is key to solving the problem of deep, high-stress oil and gas reservoir development. Summary of the Invention

[0004] To address the problems in the prior art, the present invention provides a surface treatment method for high-stress fracture proppant.

[0005] The technical solution adopted by this invention to solve its technical problem is: a surface treatment method for high-stress fracture proppant, comprising the following steps: (1) Matrix pretreatment: Soak the quartz sand matrix with a particle size of 20-40 mesh in 8-10wt% hydrochloric acid solution for 1-2 hours, wash it with deionized water until neutral, and then dry it at 80-100℃; (2) Plasma activation: The pretreated substrate is placed in a plasma treatment device, the vacuum degree inside the device is maintained at 0.06-0.1MPa, and a mixture of argon and oxygen is introduced. The substrate is treated at a power of 150-300W for 10-15 minutes to introduce hydroxyl and carboxyl active groups on the surface of the substrate. (3) Titanate bond pretreatment: A 5-8 wt% tetrabutyl titanate ethanol solution is uniformly sprayed onto the activated substrate surface using an atomizing spraying method. The spraying amount is 0.4-0.6 mL per gram of substrate. The substrate is left to stand at room temperature for 15-20 min to allow the tetrabutyl titanate to react with the active groups on the substrate surface to form a titanate bond. (4) Inner layer preparation: Prepare 25-35wt% alumina sol, add 50-100nm transition metal oxide powder and 0.5-0.8wt% citric acid dispersant to it, stir evenly and inject into high pressure reactor, put in the substrate treated in step (3), and react at 120-150℃ and 0.3-0.5MPa for 2-4h to form a dense inner layer; (5) Outer layer preparation: 20-30 wt% of pre-prepared silica sol is added to the above-mentioned high-pressure reactor, and the reactor temperature is maintained at 90-120℃ and the pressure at 0.1-0.2 MPa for 1-2 hours to form a porous outer layer; the silica content in the outer layer is 85-95% by mass. (6) Post-treatment: The proppant treated in step (5) is first dried at 80℃ for 1h, then dried at 110℃ for 2h, and calcined for 1-2h; in the later stage of calcination, a mixture of hydrogen and nitrogen gas is introduced to keep it warm for 30min, and then cooled to room temperature.

[0006] As a further technical solution, the volume ratio of argon to oxygen in step (2) is 3:1.

[0007] As a further technical solution, the atomization pressure of the atomization spraying in step (3) is 0.2-0.3 MPa; The ethanol concentration in the tetrabutyl titanate ethanol solution is 70-80 vol.

[0008] As a further technical solution, in step (4), the alumina sol is prepared by aluminum isopropoxide, nitric acid and deionized water in a molar ratio of 1:(0.1-0.3):(10-20) at 60-80℃.

[0009] As a further technical solution, the transition metal oxide in step (4) is a mixture of zirconium oxide and titanium oxide, with a mass ratio of 1:1-3.

[0010] As a further technical solution, the inner layer in step (4) has an alumina mass ratio of 80-90% and a transition metal oxide doping amount of 3-8wt%.

[0011] As a further technical solution, the silica sol in step (5) is prepared by tetraethyl orthosilicate, ethanol and deionized water in a volume ratio of 1:(3-5):(1-2), and the pH value is adjusted to 3-4 during the preparation process.

[0012] As a further technical solution, the volume ratio of hydrogen to nitrogen in step (6) is 1:4.

[0013] As a further technical solution, the roasting in step (6) is as follows: the temperature is then increased to 500-600℃ at a heating rate of 5-6℃ / min for roasting treatment.

[0014] 10. The method according to claim 1, wherein the inner and outer layers of the high-stress crack proppant constitute a dual-gradient composite metal oxide coating layer, the total thickness of the coating layer is 12-20 μm, the inner layer accounts for 60-70% of the thickness, and the outer layer accounts for 30-40% of the thickness.

[0015] The beneficial effects of this invention are: 1. This invention employs a plasma activation step, using plasma bombardment with a mixture of argon and oxygen gas to introduce abundant hydroxyl and carboxyl active groups onto the surface of the quartz sand matrix. This significantly enhances the surface reactivity of the matrix, creating sufficient reaction sites for subsequent titanate bonding and thus solving the problem of weak interfacial bonding between the matrix and the coating layer in traditional treatments. The titanate bonding pretreatment forms a uniform tetrabutyl titanate ethanol solution film through atomized spraying. The tetrabutyl titanate reacts chemically with the active groups to generate stable titanate bonds, constructing a chemical connection bridge between the matrix and the coating layer, thus avoiding the phenomenon of coating layer peeling under high stress. The doping of transition metal oxides (zirconia and titanium oxide) utilizes their nanoscale particle size effect to refine the inner alumina grains, while their high hardness improves the density and wear resistance of the inner layer, thereby solving the problem of insufficient impact resistance of a single alumina inner layer.

[0016] In this invention, plasma activation and titanate bonding pretreatment work synergistically. The highly active surface provided by the activation step allows for more complete titanate bonding, while the titanate bonds tightly anchor the activated surface to the subsequent coating layer, forming an activation-bonding-coating interface strengthening system. This improves the bonding strength of the coating layer compared to traditional physical coating. The inner layer's transition metal oxide doping and dual-gradient structure complement each other. The high density of the inner layer provides the proppant with core compressive strength, while the porous structure of the outer layer ensures unobstructed flow channels. At the same time, the outer layer can also disperse some stress, reducing breakage caused by stress concentration in the inner layer. In the post-treatment process, stepwise drying and hydrogen-nitrogen mixed calcination work synergistically. Stepwise drying avoids cracks in the coating layer due to rapid moisture evaporation, while mixed gas calcination optimizes the crystal structure of the coating layer and reduces internal defects through hydrogen reduction, further improving the structural stability of the proppant.

[0017] This invention, through the coordinated and synergistic optimization of each step, successfully solves the problems of proppant fragility, coating layer detachment, and rapid attenuation of conductivity under high stress conditions. The prepared proppant has a compressive strength of 83-98 MPa, a fragility resistance exceeding 94.5%, and a conductivity of 162-198 mD·cm, with comprehensive performance far exceeding that of traditional proppants. Furthermore, the technical solution of this invention uses quartz sand as the matrix, eliminating the need for expensive bauxite raw materials. The process avoids high-temperature and high-energy-consumption steps, and all parameters are controllable and highly repeatable, significantly reducing production costs and industrialization difficulties. In addition, the high stability and long-term conductivity of the proppant can extend the effective production cycle of oil and gas wells and improve the recovery rate of deep tight oil and gas reservoirs. This provides the oil and gas extraction industry with a solution that combines technological advancement and economic feasibility, possessing broad application prospects and industry promotion value. Detailed Implementation

[0018] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] This invention provides a surface treatment method for high-stress fracture proppant. A six-step continuous process constructs a dual-gradient composite metal oxide coating layer, enhancing the proppant's compressive strength, fracture resistance, and conductivity. This method is suitable for high-stress oil and gas reservoir fracturing operations. The method specifically includes the following steps: (1) Matrix pretreatment: Quartz sand with a particle size of 20 to 40 mesh is selected as the matrix raw material. The quartz sand must meet the requirements of purity ≥99% and impurity content ≤1%, and can be purchased through commercial channels. The quartz sand is soaked in an 8wt% to 10wt% hydrochloric acid solution for 1 to 2 hours to remove surface impurities and the oxide layer. After soaking, the quartz sand is repeatedly washed with deionized water until the washing solution is neutral. The washed quartz sand is then placed in a drying device and dried at a temperature of 80℃ to 100℃, ensuring that the matrix moisture content is ≤0.5%.

[0020] (2) Plasma activation: The pretreated quartz sand matrix is ​​placed inside the plasma treatment equipment. The equipment door is closed and a vacuum is drawn, maintaining the vacuum level between 0.06 MPa and 0.1 MPa. A mixture of argon and oxygen is introduced into the equipment at a fixed volume ratio of 3:1. The plasma generator is turned on, and the matrix is ​​treated for 10 to 15 minutes at a power of 150 W to 300 W. The plasma bombardment introduces hydroxyl and carboxyl active groups onto the matrix surface, enhancing surface reactivity.

[0021] (3) Titanate bonding pretreatment: A 5 wt% to 8 wt% tetrabutyl titanate ethanol solution was prepared, wherein the ethanol concentration was 70 vol% to 80 vol%. This solution was uniformly sprayed onto the activated substrate surface using an atomization spraying method. The atomization pressure was controlled at 0.2 MPa to 0.3 MPa, and the spraying amount was 0.4 mL to 0.6 mL per gram of substrate. After spraying, the substrate was allowed to stand at room temperature for 15 to 20 minutes to allow the tetrabutyl titanate to fully react with the hydroxyl and carboxyl active groups on the substrate surface, forming stable titanate bonds, laying the foundation for subsequent coating layer bonding.

[0022] (4) Inner layer preparation: First, an alumina sol was prepared. Raw materials were weighed according to a molar ratio of aluminum isopropoxide, nitric acid, and deionized water of 1:(0.1-0.3):(10-20). The mixture was stirred at 60℃ to 80℃ for 2 to 3 hours to obtain an alumina sol of 25 wt% to 35 wt%. Transition metal oxide powder of 50 nm to 100 nm, a mixture of zirconium oxide and titanium oxide at a mass ratio of 1:(1-3) and a doping amount of 3 wt% to 8 wt%, was added to the sol. Simultaneously, 0.5 wt% to 0.8 wt% citric acid dispersant was added, and the mixture was stirred at 300 rpm to 500 rpm for 30 to 60 minutes to ensure uniform mixing. The mixed sol is injected into a high-pressure reactor, and a substrate pretreated with titanate bonding is placed inside. The temperature inside the reactor is controlled at 120°C to 150°C, and the pressure at 0.3 MPa to 0.5 MPa. The reaction is carried out for 2 to 4 hours, forming a dense inner layer on the surface of the substrate. The alumina content in the inner layer is 80% to 90% by mass.

[0023] (5) Outer layer preparation: To prepare silica sol, raw materials were weighed according to a volume ratio of tetraethyl orthosilicate, ethanol, and deionized water of 1:(3-5):(1-2). After mixing, the pH was adjusted to 3 to 4, and the mixture was stirred and hydrolyzed at room temperature for 4 to 6 hours to obtain a silica sol of 20 wt% to 30 wt%. This silica sol was pre-injected into the aforementioned high-pressure reactor, and the reactor temperature was maintained at 90°C to 120°C and the pressure at 0.1 MPa to 0.2 MPa. The reaction was continued for 1 to 2 hours, forming a porous outer layer on the surface of the dense inner layer. The silica content in the outer layer was 85% to 95% by mass. The inner and outer layers constituted a dual-gradient composite metal oxide coating layer with a total thickness of 12 μm to 20 μm, of which the inner layer accounted for 60% to 70% and the outer layer accounted for 30% to 40%.

[0024] (6) Post-processing: The proppant prepared with the outer layer was removed from the high-pressure reactor and first placed in a drying device to dry at 80°C for 1 hour, then heated to 110°C and dried for 2 hours to remove moisture. The proppant was then placed in a calcination furnace and heated to 500°C to 600°C at a programmed rate of 5°C / min to 6°C / min, and calcined for 1 to 2 hours. During the later stages of calcination, a mixture of hydrogen and nitrogen gas (volume ratio 1:4) was introduced into the furnace, and the mixture was held at this temperature for 30 minutes. Finally, it was allowed to cool naturally to room temperature to obtain the finished high-stress fracture-resistant proppant.

[0025] The processing method provided by this invention strengthens the bonding force between the coating layer and the substrate through the synergistic effect of plasma activation and titanate bonding pretreatment; the dual-gradient coating structure balances the strength and conductivity of the proppant; and the doping of transition metal oxides further enhances the density and wear resistance of the inner layer. This method offers controllable and reproducible processes, solving the problems of traditional proppants being easily broken and exhibiting rapid attenuation of conductivity under high stress environments. Furthermore, it features a short preparation cycle, controllable cost, and is suitable for industrial production.

[0026] To further illustrate the present invention, detailed descriptions are provided below through embodiments and comparative examples.

[0027] Example 1: (1) Matrix pretreatment: Select quartz sand matrix with a particle size of 25 mesh, soak it in 9wt% hydrochloric acid solution for 1.5h, wash it with deionized water until neutral, and dry it at 90℃ to ensure that the water content is ≤0.5%.

[0028] (2) Plasma activation: The pretreated substrate is placed in a plasma treatment device, the vacuum degree inside the device is maintained at 0.08MPa, and a mixture of argon and oxygen gas (volume ratio 3:1) is introduced. The substrate is treated at 220W power for 12min to introduce hydroxyl and carboxyl active groups on the surface of the substrate.

[0029] (3) Pretreatment for titanate bonding: Prepare a 6wt% tetrabutyl titanate ethanol solution with an ethanol concentration of 75 vol%. Spray the solution evenly using atomization spraying with an atomization pressure of 0.25 MPa and a spraying amount of 0.5 mL per gram of substrate. Let it stand at room temperature for 18 min to allow the tetrabutyl titanate to react with the active groups on the substrate surface to form titanate bonds.

[0030] (4) Inner layer preparation: Weigh the raw materials according to the molar ratio of aluminum isopropoxide, nitric acid and deionized water 1:0.2:15, and prepare 30wt% alumina sol at 70℃. Add 80nm transition metal oxide powder (zirconia to titanium oxide mass ratio 1:2) with a doping amount of 5wt%, and add 0.6wt% citric acid dispersant, and stir for 40min until uniform. Inject the mixed sol into a high-pressure reactor, place the substrate treated in step (3) into it, and react at 135℃ and 0.4MPa for 3h to form a dense inner layer with alumina mass ratio of 85%.

[0031] (5) Outer layer preparation: Silica sol was prepared by mixing tetraethyl orthosilicate, ethanol, and deionized water in a volume ratio of 1:4:1.5. The pH was adjusted to 3.5 to obtain a 25wt% silica sol. This sol was added to a high-pressure reactor and reacted for 1.5 h at a temperature of 105℃ and a pressure of 0.15 MPa to form a porous outer layer. The silica content in the outer layer was 90%. The total thickness of the coating layer was 16 μm, with the inner layer accounting for 65% and the outer layer accounting for 35%.

[0032] (6) Post-treatment: The proppant was first dried at 80℃ for 1h, then dried at 110℃ for 2h, and then heated to 550℃ at a heating rate of 5.5℃ / min and calcined for 1.5h. During the later stage of calcination, a mixture of hydrogen and nitrogen gas (volume ratio 1:4) was introduced and kept at the temperature for 30min, and then cooled to room temperature to obtain the finished product.

[0033] Example 2: (1) Matrix pretreatment: Select quartz sand matrix with a particle size of 20 mesh, soak it in 8wt% hydrochloric acid solution for 1 hour, wash it with deionized water until neutral, and dry it at 80℃ to ensure that the water content is ≤0.5%.

[0034] (2) Plasma activation: The pretreated substrate is placed in a plasma treatment device, the vacuum degree inside the device is maintained at 0.06MPa, and a mixture of argon and oxygen (volume ratio 3:1) is introduced. The substrate is treated at 150W power for 10min to introduce hydroxyl and carboxyl active groups on the substrate surface.

[0035] (3) Pretreatment for titanate bonding: Prepare a 5wt% tetrabutyl titanate ethanol solution with an ethanol concentration of 70 vol%. Spray the solution evenly using atomization spraying with a pressure of 0.2 MPa and a spraying amount of 0.4 mL per gram of substrate. Let it stand at room temperature for 15 min to allow the tetrabutyl titanate to react with the active groups on the substrate surface to form titanate bonds.

[0036] (4) Inner layer preparation: Weigh the raw materials according to the molar ratio of aluminum isopropoxide, nitric acid and deionized water 1:0.1:10, and prepare 25wt% alumina sol at 60℃. Add 50nm transition metal oxide powder (zirconia to titanium oxide mass ratio 1:1) with a doping amount of 3wt%, and add 0.5wt% citric acid dispersant, and stir for 30min until uniform. Inject the mixed sol into a high-pressure reactor, place the substrate treated in step (3) into it, and react at 120℃ and 0.3MPa for 2h to form a dense inner layer with alumina mass ratio of 80%.

[0037] (5) Preparation of the outer layer: A silica sol was prepared by mixing tetraethyl orthosilicate, ethanol and deionized water in a volume ratio of 1:3:1. The pH was adjusted to 3 to obtain a 20 wt% silica sol. This sol was added to a high-pressure reactor and reacted for 1 h at a temperature of 90 °C and a pressure of 0.1 MPa to form a porous outer layer. The silica content of the outer layer was 85%. The total thickness of the coating layer was 12 μm, with the inner layer accounting for 60% and the outer layer accounting for 40%.

[0038] (6) Post-treatment: The proppant is first dried at 80℃ for 1 hour, then dried at 110℃ for 2 hours, and then heated to 500℃ at a heating rate of 5℃ / min and calcined for 1 hour. During the later stage of calcination, a mixture of hydrogen and nitrogen gas (volume ratio 1:4) is introduced and kept at the temperature for 30 minutes, and then cooled to room temperature to obtain the finished product.

[0039] Example 3: (1) Matrix pretreatment: Select quartz sand matrix with a particle size of 40 mesh, soak it in 10wt% hydrochloric acid solution for 2 hours, wash it with deionized water until neutral, and dry it at 100℃ to ensure that the water content is ≤0.5%.

[0040] (2) Plasma activation: The pretreated substrate is placed in a plasma treatment device, the vacuum degree inside the device is maintained at 0.1MPa, and a mixture of argon and oxygen gas (volume ratio 3:1) is introduced. The substrate is treated at 300W power for 15min to introduce hydroxyl and carboxyl active groups on the surface of the substrate.

[0041] (3) Pretreatment for titanate bonding: Prepare an 8wt% tetrabutyl titanate ethanol solution with an ethanol concentration of 80 vol%. Spray the solution evenly using atomization spraying with a pressure of 0.3 MPa and a spraying amount of 0.6 mL per gram of substrate. Let it stand at room temperature for 20 min to allow the tetrabutyl titanate to react with the active groups on the substrate surface to form titanate bonds.

[0042] (4) Inner layer preparation: Weigh the raw materials according to the molar ratio of aluminum isopropoxide, nitric acid and deionized water 1:0.3:20, and prepare 35wt% alumina sol at 80℃. Add 100nm of transition metal oxide powder (zirconia to titanium oxide mass ratio 1:3) with a doping amount of 8wt%, and add 0.8wt% citric acid dispersant, and stir for 60min until uniform. Inject the mixed sol into a high-pressure reactor, place the substrate treated in step (3) into it, and react at 150℃ and 0.5MPa for 4h to form a dense inner layer with alumina mass ratio of 90%.

[0043] (5) Preparation of the outer layer: A silica sol was prepared by mixing tetraethyl orthosilicate, ethanol and deionized water in a volume ratio of 1:5:2. The pH was adjusted to 4 to obtain a 30 wt% silica sol. This sol was added to a high-pressure reactor and reacted for 2 hours at a temperature of 120 °C and a pressure of 0.2 MPa to form a porous outer layer. The silica content of the outer layer was 95%. The total thickness of the coating layer was 20 μm, with the inner layer accounting for 70% and the outer layer accounting for 30%.

[0044] (6) Post-treatment: The proppant is first dried at 80℃ for 1h, then dried at 110℃ for 2h, and then heated to 600℃ at a heating rate of 6℃ / min and calcined for 2h. During the later stage of calcination, a mixture of hydrogen and nitrogen gas (volume ratio 1:4) is introduced and kept at the temperature for 30min. The mixture is then cooled to room temperature to obtain the finished product.

[0045] Comparative Example 1: (1) Substrate pretreatment: exactly the same as in Example 1. (2) Plasma activation step omitted, titanate ester bonding pretreatment was performed directly. (3) Titanate ester bonding pretreatment: exactly the same as in Example 1. (4) Inner layer preparation: exactly the same as in Example 1. (5) Outer layer preparation: exactly the same as in Example 1. (6) Post-treatment: exactly the same as in Example 1.

[0046] Comparative Example 2: (1) Substrate pretreatment: exactly the same as in Example 1. (2) Plasma activation: exactly the same as in Example 1. (3) The titanate bonding pretreatment step was omitted, and the plasma-activated substrate was directly used for inner layer preparation. (4) Inner layer preparation: exactly the same as in Example 1. (5) Outer layer preparation: exactly the same as in Example 1. (6) Post-treatment: exactly the same as in Example 1.

[0047] Comparative Example 3: (1) Matrix pretreatment: exactly the same as in Example 1. (2) Plasma activation: exactly the same as in Example 1. (3) Titanate bonding pretreatment: exactly the same as in Example 1. (4) Inner layer preparation: raw materials were weighed according to the molar ratio of aluminum isopropoxide, nitric acid and deionized water of 1:0.2:15, and 30wt% alumina sol was prepared at 70°C. Only 0.6wt% citric acid dispersant was added, and no transition metal oxide powder was added. The mixture was stirred for 40 min until homogeneous. The subsequent high-pressure reaction conditions were exactly the same as in Example 1, and the mass percentage of alumina in the inner layer was 100%. (5) Outer layer preparation: exactly the same as in Example 1. (6) Post-treatment: exactly the same as in Example 1.

[0048] Comparative Example 4: (1) Matrix pretreatment: exactly the same as in Example 1. (2) Plasma activation: exactly the same as in Example 1. (3) Titanate bonding pretreatment: exactly the same as in Example 1. (4) Inner layer preparation: exactly the same as in Example 1. (5) The outer layer preparation step was omitted, and the support after the inner layer preparation was directly subjected to post-treatment. (6) Post-treatment: exactly the same as in Example 1. The coating layer is a single inner layer with a thickness of 10 μm.

[0049] Experimental verification: Experiment 1: Compressive strength test: Referring to GB / T19189-2022, 50g of proppant samples from each example and comparative example were placed in the pressure cylinder of a standard fracturing proppant compressive strength testing device. Using a displacement control method, pressure was applied at a rate of 0.5mm / min. The pressure value at which the sample breakage rate reached 5% was recorded as the compressive strength. The results are as follows: Table 1

[0050] As can be seen from Table 1, the compressive strength of Examples 1-3 is all above 83 MPa. Among them, Example 3 has the best performance with a compressive strength of 98 MPa due to the use of higher plasma power, longer reaction time and higher transition metal oxide doping.

[0051] Experiment 2: Breakage Resistance Test Referring to GB / T19189-2022, 50g of each sample was placed in a breakage resistance test vessel, and 100mL of deionized water was added. The vessel was then kept under a pressure of 69MPa for 5 minutes. The samples were then removed and sieved through a 20-mesh standard sieve. The mass of the unbroken particles remaining on the sieve was weighed, and the breakage resistance was calculated (breakage resistance = mass of particles remaining on the sieve / total sample mass × 100%). The results are as follows: Table 2

[0052] As can be seen from Table 2, the breakage resistance of Examples 1-3 is all above 94.5%, indicating that they have excellent structural stability under high pressure.

[0053] Experiment 3: Flow guiding capacity test: Referring to SY / T5108-2014, each sample was evenly spread on the support mesh surface of the flow guiding chamber to form a 2 cm thick proppant filling layer. Under a closing pressure of 35 MPa and a temperature of 25 °C, deionized water was introduced. After stabilization, the flow rate and pressure difference through the filling layer were measured. The conductivity was calculated using Darcy's law (conductivity = permeability × thickness), and the results are as follows: Table 3

[0054] As can be seen from Table 3, the flow conduction capacity of Examples 1-3 is all above 162 mD·cm, demonstrating the advantages of the dual-gradient coating structure.

[0055] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A surface treatment method for a high-stress fracture proppant, characterized in that, Includes the following steps: (1) Matrix pretreatment: Soak the quartz sand matrix with a particle size of 20-40 mesh in 8-10wt% hydrochloric acid solution for 1-2 hours, wash it with deionized water until neutral, and then dry it at 80-100℃; (2) Plasma activation: The pretreated substrate is placed in a plasma treatment device, the vacuum degree inside the device is maintained at 0.06-0.1MPa, and a mixture of argon and oxygen is introduced. The substrate is treated at a power of 150-300W for 10-15 minutes to introduce hydroxyl and carboxyl active groups on the surface of the substrate. (3) Titanate bond pretreatment: A 5-8 wt% tetrabutyl titanate ethanol solution is uniformly sprayed onto the activated substrate surface using an atomizing spraying method. The spraying amount is 0.4-0.6 mL per gram of substrate. The substrate is left to stand at room temperature for 15-20 min to allow the tetrabutyl titanate to react with the active groups on the substrate surface to form a titanate bond. (4) Inner layer preparation: Prepare 25-35wt% alumina sol, add 50-100nm transition metal oxide powder and 0.5-0.8wt% citric acid dispersant to it, stir evenly and inject into high pressure reactor, put in the substrate treated in step (3), and react at 120-150℃ and 0.3-0.5MPa for 2-4h to form a dense inner layer; (5) Outer layer preparation: 20-30 wt% of pre-prepared silica sol is added to the above-mentioned high-pressure reactor, and the reactor temperature is maintained at 90-120℃ and the pressure at 0.1-0.2 MPa for 1-2 hours to form a porous outer layer; the silica content in the outer layer is 85-95% by mass. (6) Post-treatment: The proppant treated in step (5) is first dried at 80℃ for 1h, then dried at 110℃ for 2h, and calcined for 1-2h; in the later stage of calcination, a mixture of hydrogen and nitrogen gas is introduced to keep it warm for 30min, and then cooled to room temperature.

2. The method according to claim 1, characterized in that, In step (2), the volume ratio of argon to oxygen is 3:

1.

3. The method according to claim 1, characterized in that, The atomization pressure of the atomization spraying in step (3) is 0.2-0.3 MPa; The ethanol concentration in the tetrabutyl titanate ethanol solution is 70-80 vol.

4. The method according to claim 1, characterized in that, In step (4), the alumina sol is prepared by aluminum isopropoxide, nitric acid and deionized water in a molar ratio of 1:(0.1-0.3):(10-20) at 60-80℃.

5. The method according to claim 1, characterized in that, The transition metal oxide mentioned in step (4) is a mixture of zirconium oxide and titanium oxide, with a mass ratio of 1:1-3.

6. The method according to claim 1, characterized in that, In step (4), the inner layer contains 80-90% alumina by mass and 3-8 wt% transition metal oxide doping.

7. The method according to claim 1, characterized in that, The silica sol mentioned in step (5) is prepared by tetraethyl orthosilicate, ethanol and deionized water in a volume ratio of 1:(3-5):(1-2), and the pH value is adjusted to 3-4 during the preparation process.

8. The method according to claim 1, characterized in that, In step (6), the volume ratio of hydrogen to nitrogen is 1:

4.

9. The method according to claim 1, characterized in that, In step (6), the roasting process is as follows: the temperature is then increased to 500-600℃ at a rate of 5-6℃ / min for roasting treatment.

10. The method according to claim 1, characterized in that, The inner and outer layers of the high-stress fracture proppant form a dual-gradient composite metal oxide coating layer with a total coating thickness of 12-20 μm, with the inner layer accounting for 60-70% of the thickness and the outer layer accounting for 30-40%.

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