Biodegradable ammonium persulfate coating gel breaker for fracturing operation and preparation method of biodegradable ammonium persulfate coating gel breaker

Abstract

The invention provides a biodegradable ammonium persulfate coating gel breaker for fracturing operation and a preparation method of the biodegradable ammonium persulfate coating gel breaker. The preparation method comprises the following steps: determining a mass ratio R of rosin glyceride to ethyl cellulose according to a target reservoir temperature T, and calculating a coating weight gain ratio coefficient K and the total mass of a coating material according to target delayed gel breaking time t; preparing a core material ammonium persulfate into a water phase, and emulsifying in an oil phase to form a reversed-phase microemulsion; a gradient interface deposition method is adopted, a coating solution A prepared from rosin glyceride and part of ethyl cellulose and a coating solution B prepared from the remaining ethyl cellulose are dropwise added in sequence, and a coating material is sequentially deposited on a core material liquid drop interface through pressure reduction to form a compact bottom layer and a slow-release main body layer; according to the invention, the coating layer is biodegradable, chlorine-free and corrosion-free, through the cooperation of the material ratio and the process, the accurate and controllable delay of the gel breaking time at high temperature is realized, the reservoir is effectively protected, and the fracturing effect is improved.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field fracturing and production enhancement technology, and specifically relates to a biodegradable ammonium persulfate-coated breaker for fracturing operations and its preparation method. Background Technology

[0002] In oil and gas field fracturing operations, the breaking effect of the fracturing fluid directly affects the conductivity of the supported fractures and the final fracturing production enhancement effect. Ammonium persulfate, as a highly efficient oxidizing breaking agent, is widely used to degrade thickeners in fracturing fluids, such as guar gum and its derivatives. Its role is to eliminate the damage of polymer residues to the conductivity of supported fractures, ensuring smooth oil and gas production after fracturing. To achieve delayed activation of ammonium persulfate deep within the fracture and avoid premature breaking during fracturing fluid pumping, which would lead to a decrease in fracturing fluid viscosity and a weakened proppant-carrying capacity, it is usually necessary to coat the ammonium persulfate particles to control its release rate.

[0003] However, in current technologies, the coating material for ammonium persulfate breaker generally uses vinylidene chloride-based polymers. These materials are prone to decomposition under the complex high-temperature and high-pressure environment of downhole drilling, releasing highly corrosive chloride ions. The presence of chloride ions causes severe electrochemical corrosion to fracturing tubing, downhole tools, and casing, manifesting as pitting and stress corrosion cracking. This not only significantly shortens equipment lifespan and increases maintenance and replacement costs, but also poses serious operational safety risks, such as tubing leaks and tool failures, and may even lead to fracturing operation failure, resulting in substantial economic losses and safety hazards.

[0004] Furthermore, with the increasing development of unconventional oil and gas resources such as shale gas and tight oil and gas, environmental protection requirements are becoming increasingly stringent. Traditional vinylidene chloride-based coating materials are difficult to biodegrade, and their decomposition products or residual polymers may cause secondary pollution to the formation, which is inconsistent with the industry trend of green development and environmental regulations. At the same time, some traditional coating materials also have problems such as high cost, complex synthesis process, and insufficient precision in controlling the slow-release performance, which limit their large-scale application. Therefore, developing an environmentally friendly, safe, non-corrosive ammonium persulfate coating material with precise slow-release breaking ability has become the key to resolving the contradiction between the efficiency of breaking agents, equipment safety, and environmental protection in current fracturing operations. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a biodegradable ammonium persulfate-coated breaker for fracturing operations and its preparation method. The biodegradable ammonium persulfate-coated breaker of this invention can achieve precise delayed breaker action of ammonium persulfate, while fundamentally eliminating the risk of corrosion to fracturing equipment, avoiding formation contamination, and reducing production costs.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing a biodegradable ammonium persulfate-coated breaker for fracturing operations, characterized by comprising the following steps: (1) Determine the amount of coating material: The coating material includes rosin glycerol ester and ethyl cellulose. The mass ratio R of rosin glycerol ester to ethyl cellulose is determined according to the target reservoir temperature T℃. When T<90℃, R is 0.8-1.2; when T≥90℃, R is 0.3-0.7; where R=W1 / W2, W1 is the mass of rosin glycerol ester; W2 is the mass of ethyl cellulose; Based on the target delayed breakage time t, the required coating weight gain ratio coefficient K is calculated using the following formula, where a is 1.5-2. K=t / a Based on the coating weight gain coefficient K and the mass W3 of ammonium persulfate (W3 is a known quantity), calculate the required total mass W4 of the coating material using the following formula. W4=(K / 100)×W3 Based on the total mass W4 and R of the coating material, calculate the required mass W1 of rosin glycerol ester and the required mass W2 of ethyl cellulose using the following formulas. W1 = [R / (R+1)] × W4 W2 = [1 / (R+1)] × W4 (2) Preparation of coating solution: Preparation of coating solution A: Weigh the rosin glycerol ester (W1 mass) calculated in step (1) and 50% of the ethyl cellulose (W2 mass), add ethyl acetate, stir and dissolve in a water bath at 50-60℃, and prepare a solution with a total concentration of rosin glycerol ester and ethyl cellulose of 8%-15%. Preparation of coating solution B: Dissolve the remaining 50% of W2 mass of ethyl cellulose in anhydrous ethanol to prepare a solution with a concentration of 4%-8%.

[0007] (3) Preparation of the aqueous phase for the core material: The ammonium persulfate with a mass of W3 in step (1) is dissolved in deionized water to serve as the aqueous phase of the core material.

[0008] (4) Construction of reverse microemulsions: Span 80 and n-butanol were added sequentially to cyclohexane or n-hexane and stirred at 40±5℃ to form an oil phase. Then, the aqueous phase of the core material from step (3) was slowly added dropwise to the oil phase at a stirring rate of 2-5 mL / min at a stirring rate of 300-500 rpm to obtain a crude emulsion. The obtained crude emulsion was placed in an ice-water bath and ultrasonically treated until a stable reverse microemulsion with a particle size of 200-400 nm was formed.

[0009] (5) Gradient interface deposition: At a stirring rate of 300-500 rpm and a temperature of 40±5℃, coating solution A was slowly added dropwise to the reverse microemulsion at a rate of 1-2 mL / min. After the addition was completed, stirring was continued at a stirring rate of 300-500 rpm for 1 hour to allow rosin glycerol ester and ethyl cellulose to fully diffuse and accumulate at the interface of the ammonium persulfate core material aqueous droplets. Then, the pressure was reduced to -0.08 MPa to -0.095 MPa at a rate of 2-4 kPa / min and maintained at this pressure for 1.5-3 hours to remove most of the ethyl acetate. As the ethyl acetate evaporated, the rosin glycerol ester and ethyl cellulose dissolved in it would deposit at the interface to form a highly adhesive and dense hydrophobic bottom layer. At this point, the system is still in an emulsion state, but its fluidity is reduced. Then, the pressure is restored to normal at a rate of 2-4 kPa / min, and coating solution B is added dropwise at a rate of 1-2 mL / min while stirring at 300-500 rpm. After the addition is complete, stirring is continued at 300-500 rpm for 0.5 hours. Then, the system is subjected to reduced pressure evaporation again at a rate of 2-4 kPa / min, reducing the pressure to -0.08 MPa to -0.095 MPa, and maintaining this pressure for 1.5-3 hours to remove ethanol and residual solvent. During this process, ethyl cellulose in coating solution B is further deposited and solidified on the already formed bottom layer, constructing a dense main sustained-release layer. Finally, the system becomes a non-flowing viscous paste.

[0010] (6) Solvent-free curing and separation: Add cooled n-hexane to the paste obtained in step (5). The amount of n-hexane added is twice the amount of cyclohexane or n-hexane in step (4) above. Stir vigorously at 500-700 rpm for 30-45 minutes in an ice-water bath to obtain the system to be treated. n-hexane is a non-solvent for rosin glycerol ester and ethyl cellulose, but it is miscible with cyclohexane. This operation completely solidifies the coating layer and promotes the separation of microcapsules from the oil phase.

[0011] (7) Washing and drying: The system to be treated is centrifuged at 4℃ and 8000-10000 rpm for 10 minutes to collect the solid. Then, the solid is washed twice each with hexane, ethanol aqueous solution and deionized water to remove all residual solvent, oil phase and emulsifier. The washed wet material is then freeze-dried under vacuum to obtain biodegradable ammonium persulfate coated degumming agent.

[0012] The aqueous phase of the core material mentioned in step (3) above is a solution with a mass fraction of 30%-40% ammonium persulfate.

[0013] In step (4) above, Span 80 and n-butanol are added to cyclohexane or n-hexane in sequence. First, the volume of cyclohexane or n-hexane is measured to be 4-6 times the volume of the aqueous phase of the core material obtained in step (3), and the mass is converted according to its density and recorded as the mass of cyclohexane or n-hexane. Then, Span 80 and n-butanol are added to cyclohexane or n-hexane in sequence. The amount of Span 80 added is 3%-8% of the cyclohexane or n-hexane, and the amount of n-butanol added is 10%-20% of the mass of Span 80.

[0014] The addition of cooled n-hexane in step (6) above refers to cooling n-hexane at -5±1℃ for 2 hours.

[0015] The vacuum freeze drying described in step (7) above involves pre-freezing at -40±2℃ for 3-4 hours, followed by freezing at -20±2℃ for 24-36 hours.

[0016] The ethanol-water solution mentioned in step (7) above is an ethanol-water solution with a volume ratio of ethanol to water of 1:1.

[0017] In step (4) of this invention, an inverse microemulsion is constructed by ultrasonically dispersing an aqueous solution of ammonium persulfate in the oil phase into nanodroplets of 200-400 nm. Each droplet is an independent "microreactor," providing a basis for uniform size for subsequent uniform coating. In step (5) of this invention, gradient interface deposition is performed by first enriching rosin glycerol ester dissolved in ethyl acetate and 50% ethyl cellulose at the oil-water interface. After vacuum decompression, the ethyl acetate evaporates. As the ethyl acetate evaporates, the rosin glycerol ester exhibits high adhesion. The rosin glycerol ester and ethyl cellulose are co-deposited at the interface to form a dense, hydrophobic, and highly adhesive bottom layer, which is firmly fixed to the surface of the core material. Then, ethyl cellulose dissolved in ethanol is deposited on the bottom layer. The molecular chains of ethyl cellulose are intertwined to form a main sustained-release skeleton with high mechanical strength. The formula calculation in step (1) can control the amount of ethyl cellulose, thereby regulating the delayed gel breaking time. After adding a large amount of n-hexane in step (6), n-hexane is not a solvent for rosin glycerol ester and ethyl cellulose, but it is miscible with cyclohexane. This step completely solidifies the coating layer and causes the microcapsules to separate from the oil phase. The dissolving power of the entire system for the polymer decreases sharply, forcing the deposited rosin glycerol ester and ethyl cellulose to precipitate, shrink and solidify instantly. This process can lock the formed coating structure, transform it from a gel state to a solid wall, and cause the microcapsules to completely separate from the oil phase, making it easy to collect.

[0018] In this invention, ethyl cellulose serves as the sustained-release skeleton material. Its alcohol solubility and film-forming properties allow it to form a dense polymer film with high mechanical strength. The permeability of this film is temperature-sensitive; as the temperature increases, the movement of polymer chain segments intensifies, and the intermolecular porosity increases, thereby accelerating temperature-triggered release. Rosin glycerol ester's adhesiveness ensures a strong bond between the coating layer and the ammonium persulfate core material, preventing detachment. Simultaneously, its hydrophobicity enhances the initial barrier properties of the coating layer, effectively preventing moisture penetration during the initial construction phase. Furthermore, as a small-molecule plasticizer, it can adjust the flexibility and density of ethyl cellulose. In this invention, the two are compounded at a specific ratio R according to the temperature. When applied to medium-low temperature reservoirs (T < 90℃, R = 0.8-1.2), the composite material's strength lies primarily in the strong sealing and high adhesion of rosin glycerol ester, constructing a denser initial barrier to strictly prevent early release at relatively low construction temperatures. When applied to high-temperature reservoirs, when T... For temperatures ≥90℃ (high-temperature reservoirs), with R set at 0.3-0.7, the composite coating material requires the stability and integrity of the ethyl cellulose skeleton structure. Reducing the proportion of rosin glycerol esters helps to minimize its potential over-plasticization effect at high temperatures, ensuring that the sustained-release network maintains suitable mechanical strength and controllable pore structure in high-temperature environments, thereby achieving stable, long-lasting, and controllable delayed release. Furthermore, based on the diffusion-controlled release theory, the delay time (t) of the breaker mainly depends on the effective diffusion thickness of the coating layer. This invention uses the coating weight gain ratio coefficient K as a direct characterization of thickness and dosage, and establishes its relationship with the target time t as K=t / a, where a is 1.5-2.0.

[0019] Compared with the prior art, the present invention has the following advantages: (1) Environmentally friendly and biodegradable: The biodegradable rosin glycerol ester and ethyl cellulose used in this invention are used as coating materials. Under the temperature, humidity and microbial action of the formation, they can be completely biodegraded into carbon dioxide and water, without the release of toxic chloride ions or the residue of polymers that are difficult to degrade. This not only fundamentally avoids secondary pollution to the formation and protects the original environment of the oil and gas reservoir, but also fully meets the increasingly stringent environmental protection requirements in the development of unconventional resources such as shale gas and tight oil and gas, and conforms to the industry trend of green mining and sustainable development.

[0020] (2) Safe and corrosion-free, improving operational safety: Since the coating material itself does not contain chlorine and does not produce chloride ions or other corrosive substances during degradation, it can completely eliminate the corrosion risks of traditional polyvinylidene chloride coating materials to fracturing equipment (including high-pressure manifolds, packers, wellhead devices, and casings). This significantly reduces the risk of pitting corrosion and stress corrosion cracking in metal equipment, effectively improves the safety and stability of equipment during fracturing operations, extends the service life of equipment, and reduces the cost of equipment maintenance and replacement.

[0021] (3) Precise and Slow Release, Optimized Debriding Effect: This invention establishes a dynamic model of the mass ratio (R) of rosin glycerol ester to ethyl cellulose and the target reservoir temperature (T), as well as a model of the coating weight gain coefficient (K) and the target delay time (t) (K=t / a), thereby controlling the debriding behavior. Combined with the gradient interface deposition preparation method, the prepared debriding agent can not only effectively protect the core material and prevent premature release under high temperature and high shear construction conditions, but also ensure that the debriding agent is released stably and completely within the target time. Ultimately, it achieves stable maintenance of fracturing fluid viscosity during construction and thorough and rapid debriding after construction, optimizes fracture conductivity, and improves fracturing effect.

[0022] In summary, the biodegradable ammonium persulfate-coated breaker and its preparation method provided by this invention solve the problems of equipment corrosion, environmental pollution, and insufficient slow-release precision of existing breaker coating materials. It provides an innovative solution for oil and gas field fracturing operations that combines high performance, high safety, and low cost. Detailed Implementation

[0023] The present invention will be further described below with reference to embodiments. Example 1

[0024] Example 1 describes the preparation of a biodegradable ammonium persulfate-coated breaker suitable for a target reservoir temperature of T=90℃ and requiring a delayed breaker time of t=24 hours. The mass of the ammonium sulfate core material is W3=1000g.

[0025] A method for preparing a biodegradable ammonium persulfate decoction agent for fracturing operations includes the following steps: (1) Determine the amount of coating material: The coating material includes rosin glycerol ester and ethyl cellulose. According to the target reservoir temperature of 90℃, the mass ratio R of rosin glycerol ester to ethyl cellulose is 0.5 when T=90℃; where R=W1 / W2, W1 is the mass of rosin glycerol ester; W2 is the mass of ethyl cellulose. Based on the target delayed debonding time t, the required coating weight gain ratio coefficient K is calculated using the following formula, where a is 2 in this embodiment. K = 24 / 2, which means K = 12 Based on the coating weight gain coefficient K=12 and the mass of ammonium persulfate W3=1000g, calculate the total mass W4 of the required coating material using the following formula. W4 = (K / 100) × W3 That is, W4 = (12 / 100) × 1000, which means W4 = 120g Based on a total coating material mass W4 of 120g and R of 0.5, calculate the required mass of rosin glycerol ester W1 and ethyl cellulose W2 using the following formulas. W1 = [R / (R+1)] × W4 W2 = [1 / (R+1)] × W4 That is, W1 = [0.5 / (0.5+1)] × 120, W1 = 40g W2 = [1 / (0.5+1)] × 120, W2 = 80g (2) Preparation of coating solution: Preparation of coating solution A: Weigh 40g of rosin glycerol ester and 40g of ethyl cellulose (accounting for 50% of W2) calculated in step (1), add ethyl acetate, stir and dissolve in a 55℃ water bath, and prepare a solution with a total concentration of 10% of rosin glycerol ester and ethyl cellulose. Preparation of coating solution B: Dissolve the remaining 40g of ethyl cellulose (accounting for 50% of W2) in anhydrous ethanol to prepare a 5% solution.

[0026] (3) Preparation of the aqueous phase of the core material 1000g of ammonium persulfate was added to an appropriate amount of deionized water and stirred at room temperature until completely dissolved. Then, the volume was adjusted to obtain an aqueous phase of core material with an ammonium persulfate mass fraction of 35%. The density of the 35% ammonium persulfate aqueous solution at room temperature was measured to be 1.22g / mL. The volume was calculated to be 2342mL. The volume = (1000 / 35%) / 1.22, so the calculated volume is 2342mL.

[0027] (4) Construction of reverse microemulsion Measure 11710 mL of cyclohexane (approximately 5 times the volume of the aqueous phase of the core material, 2342 mL), weigh it to be 9200 g, add 460 g of Span 80 (5% of its mass) and 69 g of n-butanol (15% of the mass of Span 80) to the cyclohexane, place the mixture in a 40°C water bath, stir at 400 rpm until the system is homogeneous and transparent, and obtain the oil phase; then stir at 400 rpm, slowly add the aqueous phase of the core material from step (2) to the oil phase at a constant rate of 3 mL / min, and after the addition is complete, obtain the crude emulsion; place the obtained crude emulsion in an ice-water bath, and use ultrasonic treatment, which can be performed using an ultrasonic cell disruptor, until a stable reverse microemulsion with a particle size of 280 ± 40 nm is formed, and the particle size can be sampled and detected using a laser particle size analyzer.

[0028] (5) Gradient interface deposition and solvent evaporation While maintaining the microemulsion system at 40℃ and 300 rpm with gentle stirring, the prepared coating solution A was slowly added dropwise to the reverse microemulsion at a rate of 1.5 mL / min. After the addition was complete, the system was stirred at the same temperature for 1 hour. Then, the system was subjected to reduced pressure evaporation at 2 kPa / min, gradually reducing the pressure to -0.08 MPa and maintaining this pressure for 1.5 hours. This process removed most of the ethyl acetate, allowing the dissolved rosin glycerol ester and ethyl cellulose to co-deposit at the interface of the ammonium persulfate aqueous phase droplets, forming a dense hydrophobic bottom layer. At this point, the system was still an emulsion, but its fluidity was significantly reduced. The system was then restored to atmospheric pressure at a rate of 4 kPa / min. Then, while stirring at 300 rpm, the coating solution B was added dropwise to the system at a rate of 1.5 mL / min. After the addition was complete, the system was stirred for 0.5 hours. Then, the system was subjected to reduced pressure again at 2 kPa / min, gradually reducing the pressure to -0.08 MPa and maintaining this pressure for 1.5 hours to completely remove ethanol and residual solvent. During this process, the ethyl cellulose in coating solution B further deposits and solidifies on top of the underlying layer, forming the main sustained-release layer. The final reaction system transforms into a non-flowing, homogeneous, viscous paste.

[0029] (6) Non-solvent curing and separation: Add 23420 ml of n-hexane (the amount of which is twice that of cyclohexane in step (2)) pre-cooled to -5°C to the paste obtained in step (5), and stir vigorously at 600 rpm for 40 minutes to obtain the system to be treated. During this process, the coating polymer is completely cured, forming solid microcapsules and separating from the oil phase.

[0030] (7) Washing and drying The system to be treated obtained in step (6) was centrifuged at 4℃ and 8000-10000 rpm for 10 minutes to collect the solid. The system was then centrifuged at 4℃ and 9000 rpm for 10 minutes. The supernatant was discarded and the solid precipitate was collected. The solid precipitate was then washed by centrifugation with hexane, ethanol-water (1:1, v / v) mixture and deionized water, respectively, twice each.

[0031] The washed wet solids were subjected to vacuum freeze-drying: first, they were pre-frozen at -40°C for 3 hours, and then freeze-dried at -20°C and a vacuum of less than 10 Pa for 30 hours to obtain the biodegradable ammonium persulfate-coated degumming agent of Example 1.

[0032] Comparative Experiment A still targets T=90℃ and t=24 hours, but R=1.5 instead of R=0.5 in Example 1. Other parameters (a=2) are exactly the same as in Example 1. Calculate K=t / a=24 / 2=12; W4=(12 / 100)×1000=120g; W1=[1.5 / (1.5+1)]×120=72g; W2=[1 / (1.5+1)]×120=48g; The preparation method of Control Experiment A is exactly the same as in Example 1, except that 72g of rosin glycerol ester and 48g of ethyl cellulose (still allocated to coating solutions A and B in a 1:1 ratio) are weighed according to the above calculations, and the rest of the steps are exactly the same as in Example 1, to obtain control sample A.

[0033] Control Experiment B: The objective of this control experiment is the same as that of Example 1, which is T=90℃ and t=24 hours. The steps (1) of Control Experiment B are the same, T=90℃, but t=24h is required. R=0.5, a=2, and the calculated W1=40g and W2=80g. The difference from Example 1 is that in step (2), the coating solution is prepared by dissolving all the coating materials (40g rosin glycerol ester + 80g ethyl cellulose) in ethyl acetate at one time to prepare a solution with a total concentration of about 10%. Step (3) is the same as that of Example 1. Step (4) is different from that of Example 1. The coating solution is added to the control sample B at one time. This single coating solution is added dropwise and stirred. Then, only one long-term vacuum evaporation is performed for a total time of 3 hours to remove the solvent. The subsequent steps (5)-(7) are the same as those of Example 1 to obtain the control sample B.

[0034] Comparative Experiment C: This comparative experiment is the same as Example 1 except that steps (1)-(2) are the same. After the crude emulsion is formed in step (3), it is placed in an ice-water bath and treated with higher energy ultrasound to form a stable reverse microemulsion with a particle size of 120±30nm. The subsequent steps are the same as in Example 1 to obtain control sample C.

[0035] According to the "SY / T 6380-2008 Test Method for Performance of Fracturing Breaker", the effective content, release time (90% release), fracturing fluid viscosity retention rate after 2 hours of shearing, high temperature resistance (release time change rate after 24 hours of accelerated aging at 120℃), and core permeability damage rate of Example 1 and control samples A, B and C were tested. The test results are shown in Table 1 below.

[0036]

[0037] Table 1 above shows the performance of Example 1 and control samples A, B, and C. As can be seen from Table 1, the effective content in Example 1 is very high, reaching 95.2%, and the release time is 24.5 hours, which matches the design target of 24 hours. The high viscosity retention rate after shearing of 78.5% indicates that the dense bottom layer and slow-release main layer structure formed by its gradient coating can effectively resist the high-speed shearing during fracturing fluid operation and prevent premature release of the breaker. At the same time, the extremely low high-temperature resistance change rate of +2.1% and core damage rate of 8.5% indicate that Example 1 of the present invention has reliable performance in high-temperature reservoirs and causes little damage to the reservoir matrix, which is beneficial to protecting oil and gas production capacity.

[0038] As shown in Table 1 above, the effective content of control sample A is 90%, and the release time (90% release) is 14.2 hours. At a high temperature of 90℃, the excessive proportion of rosin glycerol esters leads to excessive softening of the coating layer and insufficient structural strength, thus causing early burst release or excessively rapid release of the breaker, failing to achieve the 24-hour delay target. After 2 hours of shearing, the viscosity retention rate of the fracturing fluid is only 45.9%, mainly due to severe early release and rapid breaker breaking. The high-temperature resistance performance (the rate of change in release time after 24 hours of accelerated aging at 120℃) is +18.7%, mainly due to the softening of the coating layer and significant performance degradation. The core permeability damage rate is 17.4%, indicating incomplete breaker breaking, excessive residue, and high damage. Compared with the control sample B, which was a single coating and did not use the stepwise deposition gradient structure of the present invention, all coating materials were formed at one time. Although the release time was acceptable at 20.3 hours, the release was uneven and there was a sudden release in the early stage, resulting in a viscosity retention rate of only 50.3% after shearing. The release was uneven and lower than that of Example 1. Its high temperature resistance (the rate of change of release time after 24 hours of accelerated aging at 120°C) was +15.5%, mainly due to its insufficient structural stability. The core permeability damage rate was also poor at 16.6%, with uneven gel breaking and moderate damage.

[0039] Compared with sample C, the core material aqueous phase droplet size (120nm) is much smaller than that of Example 1 (280nm), resulting in a significant increase in total surface area. With the total amount of coating material remaining unchanged, the coating layer becomes thinner, and the release time is advanced to 16.8 hours. After 2 hours of shearing, the fracturing fluid viscosity retention rate is 54%. The thinner coating layer is more easily penetrated under shearing. The high temperature resistance (the rate of change in release time after 24 hours of accelerated aging at 120℃) is +12.3%. The thinner layer is more sensitive to heat, resulting in lower performance than Example 1. The core permeability damage rate is 13.3%, which is slightly higher due to the thinner coating layer.

[0040] Example 2 Example 2 describes the preparation of a biodegradable ammonium persulfate coating breaker suitable for a target reservoir temperature of T=70℃, requiring a delayed gel breaking time of t=18 hours, with an ammonium sulfate core material mass of W3=1000g.

[0041] A method for preparing a biodegradable ammonium persulfate breaker for fracturing operations includes the following steps: (1) Determine the amount of coating material: target reservoir temperature T=70℃ (<90℃), R value is 0.8, that is, the mass ratio of rosin glycerol ester to ethyl cellulose R=0.8.

[0042] The target delayed breaking time is t=18 hours, and a is 1.5.

[0043] Calculate the coating weight gain ratio coefficient K: K = t / a = 18 / 1.5 = 12 Given that the mass of the ammonium persulfate core material is W3 = 1000g Calculate the total mass W4 of the coating material required: W4 = (K / 100) × W3 = (12 / 100) × 1000 = 120g Calculate the mass of rosin glycerol esters W1 and the mass of ethyl cellulose W2: W1=[R / (R+1)]×W4=[0.8 / (0.8+1)]×120= 53.4g W2=[1 / (R+1)]×W4=[1 / (0.8+1)]×120=66.6g (2) Preparation of coating solution Preparation of coating solution A: Weigh 53.3g of rosin glycerol ester and 33.3g of ethyl cellulose (accounting for 50% of W2), add an appropriate amount of ethyl acetate, stir and dissolve in a water bath at 55℃ to prepare a solution with a total concentration of 10%.

[0044] Preparation of coating solution B: Dissolve the remaining 33.3g of ethyl cellulose in anhydrous ethanol to prepare a 5% solution.

[0045] (3) Preparation of the aqueous phase of the core material 1000g of ammonium persulfate was dissolved in deionized water to prepare a 35% aqueous solution, and the volume was measured to be approximately 2342mL.

[0046] (4) Construction of reverse microemulsions: 9368 mL of cyclohexane (4 times the volume of the aqueous phase of the core material) was weighed and weighed to be 7360 g. 8% of its mass of Span 80 (588.8 g) and 20% of its mass of n-butanol (117.8 g) were added. The mixture was stirred evenly at 40 °C to obtain an oil phase. The aqueous phase of the core material in step (3) was added dropwise to the oil phase at a rate of 5 mL / min under stirring at 450 rpm to obtain a crude emulsion. The emulsion was then ultrasonically treated under an ice-water bath to form a stable reverse microemulsion with a particle size of 300 ± 40 nm.

[0047] (5) Gradient interface deposition and solvent evaporation: At a stirring rate of 450 rpm and a temperature of 40 ± 5 °C, the prepared coating solution A was slowly added dropwise to the reverse microemulsion at a rate of 2 mL / min. After the addition was complete, stirring was continued under these conditions for 1 hour. Then, the system was subjected to reduced pressure evaporation at 4 kPa / min, gradually reducing the pressure to -0.08 MPa and maintaining this pressure for 1.5 hours. This process removed most of the ethyl acetate, allowing the dissolved rosin glycerol ester and ethyl cellulose to co-deposit at the interface of the ammonium persulfate aqueous phase droplets, forming a dense hydrophobic bottom layer. At this point, the system was still an emulsion, but its fluidity was significantly reduced. The system was then restored to atmospheric pressure at a rate of 4 kPa / min. Then, while stirring at 300 rpm, coating solution B was added dropwise to the system at a rate of 2 mL / min. After the addition was complete, stirring was continued for 0.5 hours. Then, the system was subjected to reduced pressure again at a rate of 4 kPa / min, gradually reducing the pressure to -0.08 MPa and maintaining this pressure for 1.5 hours to completely remove ethanol and residual solvent. During this process, the ethyl cellulose in coating solution B further deposits and solidifies on top of the underlying layer, forming the main sustained-release layer. The final reaction system transforms into a non-flowing, homogeneous, viscous paste.

[0048] (6) Non-solvent curing and separation: Add 18736 ml of n-hexane (the amount is twice that of cyclohexane in step (2)) pre-cooled to -5°C to the paste obtained in step (5), and stir vigorously at 600 rpm for 40 minutes to obtain the system to be treated. During this process, the coating polymer is completely cured, forming solid microcapsules and separating from the oil phase.

[0049] (7) Washing and drying The system to be treated obtained in step (6) was centrifuged at 4°C and 10,000 rpm for 10 minutes to collect the solid, the supernatant was discarded, and the solid precipitate was collected. Then, the solid precipitate was washed by centrifugation with hexane, ethanol-water (1:1, v / v) mixture and deionized water in sequence, each washing twice. After washing, the wet material was pre-frozen at -40°C for 3 hours and then freeze-dried at -20°C and <10Pa vacuum for 30 hours to obtain the second example.

[0050] In control experiment D, T=70℃, t=18 hours, R=0.2 instead of R=0.8 in Example 2. Other parameters were exactly the same as in Example 2. Calculations were: K=t / a=18 / 1.5=12; W4=(12 / 100)×1000=120g; W1=[0.2 / (0.2+1)]×120=20g; W2=[1 / (0.2+1)]×120=100g. The preparation method of control experiment D differed from Example 2 except that 20g of rosin glycerol ester and 100g of ethyl cellulose (still allocated to coating solutions A and B in a 1:1 ratio) were weighed according to the above calculations. The remaining steps were completely consistent with Example 2, and control sample D was obtained.

[0051] In control experiment E, step (1) was the same as in Example 2, except that steps (2) and (5) were changed. All coating materials (26.67g rosin glycerol ester + 33.33g ethyl cellulose) were dissolved in ethyl acetate at once to prepare a 10% solution. In the interface deposition step, only this single coating solution was added dropwise, followed by a long-term vacuum evaporation for a total of 3 hours. The remaining steps were completely consistent with Example 2, and control sample E was obtained.

[0052] Control Experiment F was prepared in the same manner as in Example 2, with steps (1) to (5) being exactly the same, except for step (6). The added hexane was not pre-cooled, and solidification and separation were performed using hexane at room temperature (about 25°C). The stirring rate and time remained unchanged. The remaining washing and drying steps were the same, and control sample F was obtained.

[0053] According to the "SY / T 6380-2008 Test Method for Performance of Fracturing Breaker", the effective content, release time (90% release), fracturing fluid viscosity retention rate after 2 hours of shearing, high temperature resistance (release time change rate after 24 hours of accelerated aging at 120℃) and core permeability damage rate of this Example 2 and control samples D, E and F were tested. The test results are shown in Table 2 below.

[0054]

[0055] Table 2 shows the performance tests of control sample D, control sample E, and control sample F in Example 2 of the present invention. As can be seen from Table 2, the effective content of the sample in Example 2 of the present invention is 94.3%, the release time (90% release) is 18.8 hours, the viscosity retention rate of the fracturing fluid after 2 hours of shearing is 80.5%, the high temperature resistance (the rate of change of release time after 24 hours of accelerated aging at 120℃) is +2.4%, and the core permeability damage rate is 7.5%. The preparation method of the present invention can achieve precise delayed gel breaking of 18 hours, and all performance indicators are excellent.

[0056] The effective content of control sample D was 86.9%, and the release time (90% release) was 30 hours, indicating a severe delay in release. After 2 hours of shearing, the viscosity retention rate of the fracturing fluid was 97.9%, indicating very little release, which was not ideal. The high-temperature resistance (the rate of change in release time after 24 hours of accelerated aging at 120℃) was -18.3%, mainly because the coating layer was too rigid, causing it to crack under elevated temperatures and accelerating release. The core permeability damage rate was 40.4%, mainly due to incomplete gel breaking and excessive residue. At 70℃, the excessively low rosin glycerol ester ratio (R=0.3) resulted in excessive rigidity and insufficient flexibility of the coating layer, leading to poor adhesion at the core material interface. Furthermore, the already weak polymer chain mobility at low temperatures further affected the compactness and slow-release function of the coating layer, resulting in extremely delayed release, incomplete gel breaking, and reservoir damage.

[0057] Compared with the control sample E, which was a single coating and did not use the stepwise deposition gradient structure of the present invention, all coating materials were formed at one time. As shown in Table 2, its effective content was 85%, and the release time (90% release) was 20 hours. However, the release was uneven. The initial sudden release caused the fracturing fluid viscosity retention rate after 2 hours of shearing to be 50.5%, which was significantly lower than that of Example 2. The high temperature resistance (the rate of change of release time after 24 hours of accelerated aging at 120℃) was +17.5%, and the core permeability damage rate was 17.6%. This was mainly due to its insufficient structural stability, uneven gel breaking, and moderate damage.

[0058] The effective content of control sample F was 87%, and the release time (90% release) was 12 hours, which was significantly shorter than that of Example 2. Pre-temperature n-hexane, as a non-solvent, caused a more intense and rapid precipitation and solidification process of the polymer coating layer, resulting in a more loose and porous coating layer structure. The excessively rapid solidification affected the tight adhesion between the coating layer and the core material, ultimately leading to a decrease in the product's sustained-release performance. After 2 hours of shearing, the fracturing fluid viscosity retention rate of control sample F was 53%, the high-temperature resistance (the rate of change in release time after 24 hours of accelerated aging at 120°C) was +16.3%, and the core permeability damage rate was 20.3%. All key performance indicators were worse than those of Example 2.

[0059] In summary, the biodegradable ammonium persulfate-coated breaker and its preparation method provided by this invention solve the problems of equipment corrosion, environmental pollution, and insufficient slow-release precision of the coating materials in the prior art.

Claims

1. A method for preparing a biodegradable ammonium persulfate-coated breaker for fracturing operations, characterized in that, Includes the following steps: (1) Determine the amount of coating material: The coating material includes rosin glycerol ester and ethyl cellulose. The mass ratio R of rosin glycerol ester to ethyl cellulose is determined according to the target reservoir temperature T℃. When T<90℃, R is 0.8-1.2; when T≥90℃, R is 0.3-0.7; where R=W1 / W2, W1 is the mass of rosin glycerol ester; W2 is the mass of ethyl cellulose; Based on the target delayed breakage time t, the required coating weight gain ratio coefficient K is calculated using the following formula, where a is 1.5-2. K=t / a Based on the coating weight gain coefficient K and the mass W3 of ammonium persulfate (W3 is a known quantity), calculate the required total mass W4 of the coating material using the following formula. W4 = (K / 100) × W3 Based on the total mass W4 and R of the coating material, calculate the required mass W1 of rosin glycerol ester and the required mass W2 of ethyl cellulose using the following formulas. W1 = [R / (R+1)] × W4 W2 = [1 / (R+1)] × W4 (2) Preparation of coating solution: Prepare coating solution A by weighing rosin glycerol ester of mass W1 calculated in step (1) and ethyl cellulose of mass W2, then adding ethyl acetate and stirring in a water bath at 50-60℃ to dissolve and prepare a solution with a total concentration of rosin glycerol ester and ethyl cellulose of 8%-15%. To prepare coating solution B, dissolve the remaining 50% of the W2 mass of ethyl cellulose in anhydrous ethanol to prepare a solution with a concentration of 4%-8%. (3) Preparation of the aqueous phase of the core material: Dissolve the ammonium persulfate with a mass of W3 in step (1) in deionized water to serve as the aqueous phase of the core material; (4) Constructing a reverse microemulsion: Span 80 and n-butanol were added to cyclohexane or n-hexane in sequence and stirred and mixed evenly at 40±5℃ to prepare an oil phase; then, the aqueous phase of the core material in step (3) was added dropwise to the oil phase at a stirring rate of 2-5 mL / min under a stirring rate of 300-500 rpm to prepare a crude emulsion; then the crude emulsion was placed in an ice-water bath and subjected to ultrasonic treatment until a stable reverse microemulsion with a particle size of 200-400 nm was formed. (5) Gradient interface deposition: At a stirring rate of 300-500 rpm and a temperature of 40±5℃, coating solution A is added dropwise to the reverse microemulsion at a rate of 1-2 mL / min. After the addition is complete, stirring is continued at a stirring rate of 300-500 rpm for 1 hour. Then, the pressure is reduced to -0.08 MPa to -0.095 MPa at a rate of 2-4 kPa / min, and maintained at this pressure for 1.5-3 hours. Then, the pressure is restored to normal at a rate of 2-4 kPa / min, and coating solution B is added dropwise at a stirring rate of 1-2 mL / min at a stirring rate of 300-500 rpm. After the addition is complete, stirring is continued at a stirring rate of 300-500 rpm for 0.5-1 hour. Then, the pressure was reduced again at a rate of 2-4 kPa / min to -0.08 MPa to -0.095 MPa, and maintained at this pressure for 1.5-3 hours to obtain a paste. (6) Non-solvent curing and separation: Add cooled n-hexane to the paste obtained in step (5). The amount of n-hexane added is twice the amount of cyclohexane or n-hexane in step (4) above. Stir vigorously at a rate of 500-700 rpm for 30-45 minutes in an ice-water bath to obtain the system to be treated. (7) Washing and drying: The system to be treated is centrifuged at 4℃ and 8000-10000 rpm for 10-12 minutes, and then washed twice each with hexane, ethanol aqueous solution and deionized water. The washed wet material is then freeze-dried under vacuum to obtain a biodegradable ammonium persulfate coating degumming agent.

2. The method for preparing a biodegradable ammonium persulfate-coated breaker for fracturing operations according to claim 1, characterized in that: The aqueous phase of the core material in step (3) is a solution with a mass fraction of 30%-40% ammonium persulfate.

3. The method for preparing a biodegradable ammonium persulfate-coated breaker for fracturing operations according to claim 1, characterized in that: In step (4), Span 80 and n-butanol are added to cyclohexane or n-hexane in sequence. First, the volume of cyclohexane or n-hexane is measured to be 4-6 times the volume of the aqueous phase of the core material obtained in step (3), and the mass is converted according to its density and recorded as the mass of cyclohexane or n-hexane. Then, Span 80 and n-butanol are added to cyclohexane or n-hexane in sequence. The amount of Span 80 added is 3%-8% of the cyclohexane or n-hexane, and the amount of n-butanol added is 10%-20% of the mass of Span 80.

4. The method for preparing a biodegradable ammonium persulfate-coated breaker for fracturing operations according to claim 1, characterized in that: The addition of cooled n-hexane in step (6) refers to n-hexane cooled at -5±1℃ for 2-4 hours.

5. The method for preparing a biodegradable ammonium persulfate-coated breaker for fracturing operations according to claim 1, characterized in that: The vacuum freeze drying in step (7) involves pre-freezing at -40±2℃ for 3-4 hours, followed by freezing at -20±2℃ for 24-36 hours.

6. The method for preparing a biodegradable ammonium persulfate-coated breaker for fracturing operations according to claim 1, characterized in that: The ethanol-water solution mentioned in step (7) above is an ethanol-water solution with a volume ratio of ethanol to water of 1:

1.

7. A biodegradable ammonium persulfate-coated breaker, characterized in that: The biodegradable ammonium persulfate coated breaker for fracturing operations is prepared according to any one of claims 1-6.

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