Salt-resistant drag reducer for oil well fracturing and preparation process thereof

Abstract

The invention relates to the technical field of chemical materials, and discloses a salt-tolerant drag reducer for oil well fracturing and a preparation process of the salt-tolerant drag reducer for oil well fracturing. 0.5 to 1.2 parts of a nano silicon dioxide grafted cross-linking agent; 0.1 to 0.3 part of an antioxidant; 2-5 parts of an acetate buffer agent; 0.5 to 0.8 part of an anionic surfactant; 0.02 to 0.05 part of a benzalkonium chloride bactericide; 0.2 to 0.5 part of a bentonite suspending agent; and the balance of deionized water. The modified copolymer is prepared from acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid in a mass ratio of 6: 2: 3 through a free radical copolymerization reaction. The modified copolymer is prepared by introducing 2-acrylamido-2-methylpropanesulfonic acid (AMPS), so that the performance of the drag reducer in a high-salinity environment is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of chemical materials, in particular to a salt-resistant drag reducer for oil well fracturing and a preparation process thereof. Background Art

[0002] Hydraulic fracturing technology is an important means to increase the production of oil and gas wells, especially in the development of unconventional oil and gas reservoirs (such as shale oil and gas). Drag reducers, as key additives for slickwater fracturing fluids, reduce energy consumption and optimize fracture network formation by reducing pipeline friction resistance during high-speed pumping. In existing technologies, drag reducers are mainly based on polyacrylamide (PAM) and its derivatives, supplemented by cross-linking agents, surfactants and other additives, and their performance is improved by modifying the molecular structure (such as introducing hydrophilic or ionic groups) or adjusting the formula. These drag reducers show good drag reduction effects in conventional low-salinity environments and are widely used in fracturing operations in shallow oil and gas reservoirs.

[0003] However, as fracturing operations advance into deep, high-salinity formations, existing polyacrylamide drag reducers have exposed significant drawbacks. In high-salinity environments, polyacrylamide molecular chains are prone to curling or precipitation due to the ion shielding effect, resulting in a significant decrease in drag reduction performance, making it difficult to meet the high-salinity reinjection water preparation requirements for deep formations. Although existing technologies have attempted to improve salt tolerance by introducing hydrophilic or ionic groups, the modification effect is limited, and is often accompanied by complex synthesis processes and high costs, making it difficult to maintain stable drag reduction efficiency and viscosity under high-salinity and high-temperature conditions. Summary of the Invention

[0004] In response to the shortcomings of the existing technology, the present invention provides a salt-resistant drag reducer for oil well fracturing and a preparation process thereof, which solves the problem that the polyacrylamide molecular chains of existing polyacrylamide drag reducers are prone to curling or precipitation due to the ion shielding effect in high-salt environments, resulting in a significant decrease in drag reduction performance.

[0005] To achieve the above objectives, the present invention is implemented through the following technical solutions: A salt-resistant drag reducer for oil well fracturing, the drag reducer is composed of the following components in parts by weight: 25–35 parts of modified copolymer; 0.5–1.2 parts of nano-silica graft cross-linking agent; Antioxidants 0.1–0.3 parts; Acetate buffer 2–5 parts; Anionic surfactant 0.5–0.8 parts; Benzalkonium chloride fungicide 0.02–0.05 parts; Bentonite suspension agent 0.2–0.5 parts; The balance is deionized water; The modified copolymer is prepared by free radical copolymerization of acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid in a mass ratio of 6:2:3. The nano-silica grafted crosslinking agent is formed by grafting nano-silica with a zirconium-based crosslinking agent.

[0006] Through the above technical solution, the modified copolymer is prepared using acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS) in a specific ratio. The sulfonic acid group of AMPS maintains the molecular chain extension in a high-salt environment, and the carboxyl group of crotonic acid enhances water solubility and cross-linking ability. Nano-silica grafted zirconium-based cross-linking agent forms a stable complex with the functional groups of the copolymer through zirconium ions, combined with the structural reinforcement effect of nano-silica, to improve shear resistance and temperature resistance. Deionized water is used as a solvent to supplement the formula to ensure the uniformity of the liquid system and the applicability of construction. Antioxidants prevent oxidative degradation, acetate buffers adjust the pH to optimize performance, anionic surfactants promote rapid dispersion, benzalkonium chloride controls microbial contamination, and bentonite improves suspension stability.

[0007] Preferably, the viscosity average molecular weight of the modified copolymer is 15×10 6 –20×10 6 g / mol.

[0008] Preferably, the particle size of the nano-silica is 20-50 nm, and the zirconium-based cross-linking agent is zirconium oxychloride.

[0009] Preferably, the antioxidant is 2,6-di-tert-butyl-4-methylphenol, and the anionic surfactant is sodium lauryl sulfate.

[0010] Preferably, the pH value of the drag reducer is adjusted to 5.5-6.5 by the acetate buffer.

[0011] Preferably, a process for preparing a salt-resistant drag reducer for oil well fracturing comprises the following steps: S1, dissolving acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid in deionized water at a mass ratio of 6:2:3, adding ammonium persulfate initiator, reacting at 60° C. under nitrogen protection for 6 hours, drying and crushing the colloid to obtain a modified copolymer; S2, dispersing nano-silica in ethanol, adding a silane coupling agent and reacting at 70°C for 4 hours, then adding zirconium oxychloride and reacting at 80°C for 3 hours, centrifuging, washing, and drying to obtain a nano-silica graft cross-linking agent; S3. Add deionized water to the reactor, and sequentially add the modified copolymer, nano-silica graft cross-linking agent, antioxidant, acetate buffer, anionic surfactant, benzalkonium chloride and bentonite, and mix well to obtain a drag reducer.

[0012] Preferably, in step S1, the total concentration of acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid is 25-35%, and the amount of ammonium persulfate used is 0.03-0.07% of the weight of the monomers.

[0013] Preferably, in step S2, the silane coupling agent is KH-550, and the amount used is 1-3% of the mass of the nano-silicon dioxide, and the amount used is zirconium oxychloride is 8-12% of the mass of the nano-silicon dioxide.

[0014] Preferably, in step S3, the modified copolymer is stirred for 35-45 minutes, the nano-silica grafted cross-linking agent is stirred for 40-50 minutes, and the stirring speed is 400-600 rpm.

[0015] Preferably, the deionized water in step S3 may be partially or completely replaced by reinjection water with a total dissolved solids content of up to 200,000 mg / L.

[0016] The present invention provides a salt-resistant drag reducer for oil well fracturing and a preparation process thereof. It has the following beneficial effects: 1. The present invention introduces 2-acrylamido-2-methylpropanesulfonic acid (AMPS) to prepare a modified copolymer, which improves the performance of the drag reducer in high-salinity environments. The sulfonic acid group of AMPS effectively prevents the curling of the molecular chain in high-salinity environments, maintains chain extensibility and drag reduction efficiency, and is suitable for fracturing operations in deep high-salinity formations. It is superior to traditional polyacrylamide drag reducers.

[0017] 2. The present invention uses nano-silica grafted zirconium-based cross-linking agents to enhance the stability of the cross-linked network, enabling the drag reducer to maintain excellent viscosity retention in high-temperature and high-shear environments, making it suitable for deep formation fracturing. Nano-silica provides structural support, and zirconium ions form stable complexes with copolymer functional groups, significantly improving shear resistance and temperature resistance.

[0018] 3. The present invention uses anionic surfactants and pH-optimized formula design to achieve rapid dissolution of the drag reducer in high-salinity fluids, significantly shortening the preparation time and improving on-site construction efficiency. The synergistic effect of pH control and surfactants promotes the dispersion of the copolymer and cross-linking agent, meeting the rapid needs of fracturing operations.

[0019] 4. The present invention reduces fresh water consumption and environmental impact by partially or completely replacing deionized water with high-salinity reinjection water. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The present invention is a process flow chart of a process for preparing a salt-resistant drag reducer for oil well fracturing. DETAILED DESCRIPTION

[0021] The following will clearly and completely describe the technical solution of the present invention in conjunction with the accompanying drawings. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0022] Please see the attached Figure 1 The embodiment of the present invention provides a salt-resistant drag reducer for oil well fracturing, the drag reducer comprising the following components in parts by weight: 25–35 parts of modified copolymer; 0.5–1.2 parts of nano-silica graft cross-linking agent; Antioxidants 0.1–0.3 parts; Acetate buffer 2–5 parts; Anionic surfactant 0.5–0.8 parts; Benzalkonium chloride fungicide 0.02–0.05 parts; Bentonite suspension agent 0.2–0.5 parts; The balance is deionized water; The modified copolymer is prepared by free radical copolymerization of acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid in a mass ratio of 6:2:3, and the nano-silica grafted cross-linking agent is formed by grafting nano-silica and a zirconium-based cross-linking agent.

[0023] Specifically, the salt-tolerant drag reducer formulation ensures excellent drag reduction performance and viscosity stability in high-salinity environments (total dissolved solids up to 200,000 mg / L) by precisely optimizing the proportions of each component. The modified copolymer is composed of acrylamide (AM), crotonic acid (CA), and 2-acrylamido-2-methylpropanesulfonic acid (AMPS). The sulfonic acid groups of AMPS enhance the stretchability of the molecular chain in high-salinity environments, while the carboxyl groups of crotonic acid improve water solubility and crosslinking ability. The nano-silica grafted crosslinker forms a stable complex with the sulfonic and carboxyl groups of the copolymer through zirconium ions. Combined with the structural reinforcement of nano-silica, it enhances shear resistance and temperature resistance. Deionized water is used as the solvent, and the formula is supplemented to 100 parts by weight to ensure uniformity and application suitability of the liquid system. Antioxidants prevent oxidative degradation, acetate buffers optimize pH, anionic surfactants promote dispersion, benzalkonium chloride controls microbial contamination, and bentonite enhances suspension stability.

[0024] The viscosity average molecular weight of the modified copolymer is 15×10 6 –20×10 6 g / mol.

[0025] Specifically, the viscosity average molecular weight of the modified copolymer is in the range of 15×10 6–20×10 6 The molecular weight range of g / mol is precisely achieved by controlling polymerization reaction conditions (e.g., monomer concentration and initiator dosage). This molecular weight range ensures the copolymer exhibits high viscoelasticity and drag reduction properties while avoiding either high molecular weights that result in solubility difficulties or low molecular weights that result in insufficient performance. Molecular weights are determined by gel permeation chromatography (GPC) using polystyrene standards for calibration.

[0026] The particle size of nano-silica is 20–50 nm, and the zirconium-based cross-linking agent is zirconium oxychloride.

[0027] Specifically, nano-silica particles with a size of 20–50 nm are prepared via vapor phase or precipitation methods to ensure high specific surface area and excellent dispersion. Zirconium oxychloride (ZrOCl2·8H2O) is used as a zirconium-based crosslinker and chemically grafted onto the nano-silica surface via a silane coupling agent, forming a stable nanocomposite structure. This structure enhances the mechanical properties of the cross-linked network. The zirconium ions form multi-point coordination with the functional groups of the copolymer, significantly improving the drag reducer's stability in high-salt, high-shear environments.

[0028] The antioxidant is 2,6-di-tert-butyl-4-methylphenol, and the anionic surfactant is sodium lauryl sulfate.

[0029] Specifically, 2,6-di-tert-butyl-4-methylphenol acts as an antioxidant, effectively preventing molecular chain degradation of the drag reducer due to oxidation during storage and use, thereby extending the product's service life. Sodium lauryl sulfate, an anionic surfactant, promotes the rapid dispersion of the copolymer and crosslinker in water by reducing surface tension, shortening the dissolution time to 150–200 seconds (stirring at 700 rpm). These two additives were selected based on their high efficiency, low cost, and compatibility with high-salinity reinjection water. Their addition significantly improves the stability of the formula and construction efficiency.

[0030] The pH of the drag reducer was adjusted to 5.5–6.5 using an oxyacetate buffer.

[0031] Specifically, the pH was adjusted to 5.5–6.5 using an oxyacetate buffer (sodium acetate-acetic acid system), a range that optimizes the water solubility and cross-linking efficiency of the modified copolymer. Too low a pH could cause the copolymer chains to curl, while too high a pH could weaken the coordination ability of the zirconium ion with the functional groups.

[0032] A process for preparing a salt-resistant drag reducer for oil well fracturing comprises the following steps: S1, dissolving acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid in deionized water at a mass ratio of 6:2:3, adding ammonium persulfate initiator, reacting at 60° C. under nitrogen protection for 6 hours, drying and crushing the colloid to obtain a modified copolymer; S2, dispersing nano-silica in ethanol, adding a silane coupling agent and reacting at 70°C for 4 hours, then adding zirconium oxychloride and reacting at 80°C for 3 hours, centrifuging, washing, and drying to obtain a nano-silica graft cross-linking agent; S3. Add deionized water to the reactor, and sequentially add the modified copolymer, nano-silica graft cross-linking agent, antioxidant, acetate buffer, anionic surfactant, benzalkonium chloride and bentonite, and mix well to obtain a drag reducer.

[0033] Specifically, the preparation method ensures the efficient combination of each component through step-by-step synthesis and sequential addition processes to form a uniform and stable drag reducer system. Step S1 uses free radical polymerization to synthesize the modified copolymer, nitrogen protection to prevent oxygen from inhibiting the reaction, and precipitation and drying steps to ensure the purity of the copolymer. Step S2 uses silane coupling agent grafting technology to achieve chemical bonding between nano-silica and zirconium-based cross-linking agents to improve cross-linking efficiency. Step S3 uses high-temperature stirring and pH control processes to promote the formation of a stable network between the copolymer and the cross-linking agent, and the filtration step removes undissolved particles to ensure the purity of the product. The entire process is simple and controllable, suitable for industrial production, and the cost is lower than that of traditional modified drag reducers.

[0034] In step S1, the total concentration of acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid is 25-35%, and the amount of ammonium persulfate used is 0.03-0.07% of the monomer mass.

[0035] Specifically, a total monomer concentration of 25–35% in step S1 ensures the reaction system has an appropriate viscosity and reaction rate. Excessive concentrations may lead to gelation, while too low concentrations reduce polymerization efficiency. The amount of ammonium persulfate used, 0.03–0.07%, was optimized experimentally to balance initiation efficiency and molecular weight distribution. Excessive initiator may result in low-molecular-weight byproducts.

[0036] In step S2, the silane coupling agent is KH-550, and the amount used is 1-3% of the mass of the nano-silicon dioxide, and the amount used is zirconium oxychloride is 8-12% of the mass of the nano-silicon dioxide.

[0037] Specifically, in step S2, the KH-550 silane coupling agent (aminopropyltriethoxysilane) is used at a 1–3% concentration to ensure sufficient surface modification of the nano-silica, providing sufficient active sites for reaction with zirconium oxychloride. An 8–12% concentration of zirconium oxychloride optimizes the efficiency of zirconium ion grafting. Excessive amounts may lead to side reactions, while insufficient amounts may reduce crosslinking strength. This parameter range ensures the stability of the crosslinker in high-salt, high-temperature environments.

[0038] In step S3, the modified copolymer is stirred for 35-45 minutes, the nano-silica grafted cross-linking agent is stirred for 40-50 minutes, and the stirring speed is 400-600 rpm.

[0039] Specifically, in step S3, the modified copolymer is stirred for 35–45 minutes and the crosslinker is stirred for 40–50 minutes to ensure complete dissolution of the components and formation of a crosslinked network. A stirring speed of 400–600 rpm balances dispersion efficiency and bubble formation, avoiding system instability caused by excessively high speeds.

[0040] In step S3, the deionized water may be partially or completely replaced by reinjection water with a total dissolved solids content of up to 200,000 mg / L.

[0041] Specifically, in step S3, reinjection water containing up to 200,000 mg / L of total dissolved solids (TDS) can be used to partially or completely replace deionized water, demonstrating the formulation's adaptability to high-salinity environments. High concentrations of salts (such as NaCl and CaCl2) in the reinjection water do not significantly affect the copolymer's solubility or cross-linking efficiency, thanks to the salt-tolerance properties of the AMPS sulfonic acid groups and the stabilizing effect of the nano-silica grafted cross-linker.

[0042] The following is an introduction with reference to specific embodiments: Example 1 (highest data) Formula composition (parts by weight): Modified copolymer: 35 parts Nano-silica graft cross-linking agent: 1.2 parts Antioxidant (2,6-di-tert-butyl-4-methylphenol): 0.3 parts Acetate buffer (sodium acetate-acetic acid): 5 parts Anionic surfactant (sodium lauryl sulfate): 0.8 parts Benzalkonium chloride fungicide: 0.05 parts Bentonite suspension agent: 0.5 parts Deionized water: balance (make up to 100 parts) Preparation method: S1: Acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid (mass ratio 6:2:3) were dissolved in deionized water (total monomer concentration 35%), and ammonium persulfate (0.07% of monomer mass) was added as an initiator. The mixture was reacted at 65°C for 7 hours under nitrogen protection, precipitated with ethanol, dried at 50°C, and crushed to obtain a product with a viscosity average molecular weight of about 20×10 6 g / mol modified copolymer.

[0043] S2: Disperse nanosilica (particle size 50 nm) in ethanol, add KH-550 silane coupling agent (3% of the mass of nanosilica), react at 75°C for 5 hours, then add zirconium oxychloride (12% of the mass of nanosilica), react at 85°C for 4 hours, centrifuge, wash, and dry to obtain a nanosilica graft crosslinker.

[0044] S3: Add deionized water to the reactor, add 35 parts of the modified copolymer, and stir at 600 rpm for 45 minutes until completely dissolved; add 1.2 parts of nano-silica grafted crosslinking agent and stir at 600 rpm for 50 minutes; add 0.3 parts of antioxidant, 5 parts of acetate buffer (adjust pH to 6.5), 0.8 parts of anionic surfactant, and 0.05 parts of benzalkonium chloride in sequence and stir for 15 minutes; add 0.5 parts of bentonite and stir at 600 rpm for 40 minutes to obtain a salt-resistant drag reducer.

[0045] Performance testing: Test conditions: 200,000mg / LTDS reinjection water, 90°C, shear rate 170s -1 , the friction meter flow rate is 5m / s.

[0046] Results: Drag reduction rate was 80.1%, dissolution time was 150 seconds (700 rpm), viscosity retention rate was 69.8% after 60 minutes, and viscosity was 27.2 mPa·s.

[0047] Comparative Example 1: Formula: Commercially available polyacrylamide (PAM, molecular weight 15×10 6 g / mol) to replace the modified copolymer, the cross-linking agent is 1.2 parts of a conventional zirconium-based cross-linking agent (without nano-silica grafting), and the remaining components and amounts are the same as in Example 1.

[0048] Preparation method: Prepare according to step S3 of Example 1 (omit S1 and S2, and directly purchase PAM and zirconium-based cross-linking agent).

[0049] Performance test: drag reduction rate 52.4%, dissolution time 450 seconds, viscosity retention rate 38.5% after 60 minutes, viscosity 12.3mPa·s.

[0050] Note: Comparative Example 1 uses a traditional PAM drag reducer, which lacks the salt-tolerant group of AMPS and the structural reinforcement of nano-silica, resulting in curling of the molecular chain in a high-salt environment. The drag reduction performance and viscosity stability are significantly lower than those of Example 1.

[0051] Example 2 (minimum data) Formula composition (parts by weight): Modified copolymer: 25 parts Nano-silica graft cross-linking agent: 0.5 parts Antioxidant (2,6-di-tert-butyl-4-methylphenol): 0.1 part Acetate buffer (sodium acetate-acetic acid): 2 parts Anionic surfactant (sodium lauryl sulfate): 0.5 parts Benzalkonium chloride fungicide: 0.02 parts Bentonite suspension agent: 0.2 parts Deionized water: balance (make up to 100 parts) Preparation method: S1: Acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid (mass ratio 6:2:3) were dissolved in deionized water (total monomer concentration 25%), and ammonium persulfate (0.03% of monomer mass) was added as an initiator. The mixture was reacted at 55°C for 5 hours under nitrogen protection, precipitated with ethanol, dried at 50°C, and crushed to obtain a product with a viscosity average molecular weight of about 15×10 6 g / mol modified copolymer.

[0052] S2: Nano-silica (particle size 20 nm) was dispersed in ethanol, KH-550 silane coupling agent (1% of the mass of nano-silica) was added, and the mixture was reacted at 65°C for 3 hours. Zirconium oxychloride (8% of the mass of nano-silica) was then added, and the mixture was reacted at 75°C for 2 hours. The mixture was centrifuged, washed, and dried to obtain a nano-silica graft cross-linking agent.

[0053] S3: Add deionized water to the reactor, add 25 parts of modified copolymer, stir at 400 rpm for 35 minutes until completely dissolved; add 0.5 parts of nano-silica grafted crosslinking agent, stir at 400 rpm for 40 minutes; add 0.1 parts of antioxidant, 2 parts of acetate buffer (adjust pH to 5.5), 0.5 parts of anionic surfactant, 0.02 parts of benzalkonium chloride in sequence, stir for 15 minutes; add 0.2 parts of bentonite, stir at 400 rpm for 30 minutes to obtain a salt-resistant drag reducer.

[0054] Performance testing: Test conditions: 200,000mg / LTDS reinjection water, 90°C, shear rate 170s -1 , the flow velocity of the turbulent rheometer is 5m / s.

[0055] Results: Drag reduction rate 75.3%, dissolution time 195 seconds (700 rpm), viscosity retention rate 65.2% after 60 minutes, viscosity 23.8 mPa·s.

[0056] Comparative Example 2: Formula: replace the modified copolymer with 25 parts of a copolymer of acrylamide and crotonic acid (mass ratio 6:2, without AMPS), and 0.5 parts of nano-silica grafted crosslinking agent. The remaining components and amounts are the same as those in Example 2.

[0057] Preparation method: In step S1, AMPS is replaced with an equal amount of acrylamide, and the reaction conditions are the same as in Example 2; steps S2 and S3 are the same as in Example 2.

[0058] Performance test: drag reduction rate 60.2%, dissolution time 300 seconds, viscosity retention rate 50.1% after 60 minutes, viscosity 15.7mPa·s.

[0059] Note: Comparative Example 2 removes AMPS, resulting in a lack of salt-resistant sulfonic acid groups in the copolymer. The molecular chain is prone to curling in a high-salt environment, and the drag reduction performance and viscosity stability are significantly lower than those of Example 2, and the dissolution time is prolonged.

[0060] Example 3 (intermediate data) Formula composition (parts by weight): Modified copolymer: 30 parts Nano-silica graft cross-linking agent: 0.85 parts Antioxidant (2,6-di-tert-butyl-4-methylphenol): 0.2 parts Acetate buffer (sodium acetate-acetic acid): 3.5 parts Anionic surfactant (sodium lauryl sulfate): 0.65 parts Benzalkonium chloride fungicide: 0.035 parts Bentonite suspension agent: 0.35 parts Deionized water: balance (make up to 100 parts) Preparation method: S1: Acrylamide, crotonic acid, and 2-acrylamido-2-methylpropanesulfonic acid (mass ratio 6:2:3) were dissolved in deionized water (total monomer concentration 30%), and ammonium persulfate (0.05% of the monomer mass) was added as an initiator. The mixture was reacted at 60°C under nitrogen for 6 hours, precipitated with ethanol, dried at 50°C, and pulverized to obtain a modified copolymer with a viscosity-average molecular weight of approximately 17.5×10^6 g / mol.

[0061] S2: Disperse nanosilica (particle size 35 nm) in ethanol, add KH-550 silane coupling agent (2% of the mass of nanosilica), react at 70°C for 4 hours, then add zirconium oxychloride (10% of the mass of nanosilica), react at 80°C for 3 hours, centrifuge, wash, and dry to obtain a nanosilica graft crosslinker.

[0062] S3: Add deionized water to the reactor, add 30 parts of modified copolymer, stir at 500 rpm for 40 minutes until completely dissolved; add 0.85 parts of nano-silica grafted crosslinking agent, stir at 500 rpm for 45 minutes; add 0.2 parts of antioxidant, 3.5 parts of acetate buffer (adjust pH to 6.0), 0.65 parts of anionic surfactant, 0.035 parts of benzalkonium chloride in sequence, stir for 15 minutes; add 0.35 parts of bentonite, stir at 500 rpm for 35 minutes to obtain a salt-resistant drag reducer.

[0063] Performance testing: Test conditions: 200,000mg / LTDS reinjection water, 90°C, shear rate 170s -1, the flow velocity of the turbulent rheometer is 5m / s.

[0064] Results: Drag reduction rate was 78.2%, dissolution time was 170 seconds (700 rpm), viscosity retention rate after 60 minutes was 67.1%, and viscosity was 25.6 mPa·s.

[0065] Comparative Example 3: Formula: 30 parts of the modified copolymer, 0.85 parts of a conventional zirconium-based crosslinking agent (without nano-silica grafting) replacing the nano-silica grafted crosslinking agent, and the remaining components and amounts are the same as in Example 3.

[0066] Preparation method: Steps S1 and S3 are the same as in Example 3, S2 is omitted, and a conventional zirconium-based cross-linking agent is directly used.

[0067] Performance test: drag reduction rate 68.7%, dissolution time 220 seconds, viscosity retention rate 55.3% after 60 minutes, viscosity 18.9mPa·s.

[0068] Note: Comparative Example 3 removes the nano-silica grafting, resulting in a decrease in the stability of the cross-linked network, a large viscosity loss under high salt and high shear environments, and lower drag reduction performance and dissolution rate than Example 3.

[0069] Example 4 (reinjection water replacement) Formula composition (parts by weight): Modified copolymer: 30 parts Nano-silica graft cross-linking agent: 0.85 parts Antioxidant (2,6-di-tert-butyl-4-methylphenol): 0.2 parts Acetate buffer (sodium acetate-acetic acid): 3.5 parts Anionic surfactant (sodium lauryl sulfate): 0.65 parts Benzalkonium chloride fungicide: 0.035 parts Bentonite suspension agent: 0.35 parts Reinjection water (TDS 200,000 mg / L): balance (to 100 parts) Preparation method: S1: Same as Example 3, prepared with a viscosity-average molecular weight of about 17.5×10 6 g / mol modified copolymer.

[0070] S2: Same as Example 3, prepare a nano-silica graft cross-linking agent.

[0071] S3: Add reinjection water with a TDS of 200,000 mg / L to the reactor, add 30 parts of the modified copolymer, and stir at 500 rpm for 40 minutes until completely dissolved; add 0.85 parts of nano-silica grafted crosslinking agent and stir at 500 rpm for 45 minutes; add 0.2 parts of antioxidant, 3.5 parts of acetate buffer (adjust pH to 6.0), 0.65 parts of anionic surfactant, and 0.035 parts of benzalkonium chloride in sequence and stir for 15 minutes; add 0.35 parts of bentonite and stir at 500 rpm for 35 minutes to obtain a salt-resistant drag reducer.

[0072] Performance testing: Test conditions: 200,000mg / LTDS reinjection water, 90°C, shear rate 170s -1 , the flow velocity of the turbulent rheometer is 5m / s.

[0073] Results: Drag reduction rate 77.8%, dissolution time 175 seconds (700 rpm), viscosity retention rate 66.8% after 60 minutes, viscosity 25.2 mPa·s.

[0074] Comparative Example 4: Recipe: Same as Example 4, except that deionized water is used instead of reinjection water. The remaining components and amounts are the same.

[0075] Preparation method: Same as Example 4, except that deionized water is used in S3 instead of reinjection water.

[0076] Performance test: drag reduction rate 78.5%, dissolution time 168 seconds, viscosity retention rate 67.4% after 60 minutes, viscosity 25.8mPa·s.

[0077] Note: Comparative Example 4 is prepared using deionized water, and its performance is slightly better than that of Example 4, but it consumes a large amount of fresh water, increasing the water resource cost by about 70%, and lacks the environmental protection advantage of Example 4 (reinjection water utilization).

[0078] Table 1: Comparison of properties of different embodiments and comparative examples Table character explanation: sample: Definition: Refers to the drag reducer samples tested, including Examples 1-4 (formulations and processes of the present invention) and Comparative Examples 1-4 (formulations compared to prior art or with key innovations removed).

[0079] Significance: Example 1 (highest data): Using the upper limit values ​​of the components and process parameters in the claims (such as 35 parts of modified copolymer, 65°C, 7 hours), represents the formulation with the best performance.

[0080] Example 2 (lowest data): using lower limit values ​​(eg, 25 parts modified copolymer, 55°C, 5 hours), represents the lowest cost formulation.

[0081] Example 3 (intermediate data): using intermediate values ​​(eg, 30 parts of modified copolymer, 60° C., 6 hours) to balance performance and cost.

[0082] Example 4 (reinjection water replacement): Based on the median value, 200,000 mg / LTDS reinjection water was used to replace deionized water, highlighting environmental protection.

[0083] Comparative Example 1: using traditional polyacrylamide (PAM) and conventional zirconium-based crosslinking agent, representing the prior art.

[0084] Comparative Example 2: The AMPS monomer was removed to verify its salt tolerance.

[0085] Comparative Example 3: Removing the nano-silica grafting to verify its structural strengthening effect.

[0086] Comparative Example 4: Deionized water was used to replace the reinjection water to verify the applicability of the reinjection water process.

[0087] Technical background: The examples reflect the innovations of the present invention (AMPS modification, nano-silica grafted cross-linking agent, and water reinjection process), while the comparative examples simulate the existing technology or remove the innovations to highlight the superiority of the present invention.

[0088] Drag reduction rate (%): Definition: The percentage by which a drag reducer reduces frictional resistance under turbulent flow conditions (flow rate 5 m / s, turbulent rheometer). The calculation formula is: (frictional resistance without drag reducer - frictional resistance with drag reducer) / frictional resistance without drag reducer × 100%.

[0089] Test conditions: 200,000mg / LTDS reinjection water, 90°C, shear rate 170s -1 .

[0090] Significance: Drag reduction rate is the core indicator for measuring the performance of drag reducers. A high drag reduction rate means lower pumping energy consumption and better crack expansion effect.

[0091] Examples 1-4: The drag reduction rate is 75.3-80.1%, which is much higher than 52.4-68.7% of Comparative Examples 1-3, thanks to the salt tolerance of the AMPS sulfonic acid group and the network stability of the nano-silica grafted crosslinker.

[0092] Comparative Example 1: The molecular chain of traditional PAM curls in a high-salt environment, and the drag reduction rate is only 52.4%.

[0093] Comparative Example 2: The absence of AMPS resulted in decreased salt resistance and a drag reduction rate of 60.2%.

[0094] Comparative Example 3: No nano-grafting was used to reduce the cross-linking strength, and the drag reduction rate was 68.7%.

[0095] Comparative Example 4: The drag reduction rate is 78.5%, which is slightly higher than that of Example 4 (77.8%) due to the low salinity of deionized water, but has no environmental advantage.

[0096] Comparative analysis: Example 1 (80.1%) has the best performance because the high dosage of modified copolymer and cross-linking agent enhances molecular chain extension and network stability; Example 2 (75.3%), although using the lowest dosage, is still significantly better than the comparative example; Example 4 is close to Comparative Example 4, indicating that reinjection water does not significantly affect the drag reduction performance.

[0097] Dissolution time (seconds): Definition: The time required for the drag reducer to completely dissolve in 200,000 mg / LTDS reinjection water under stirring at 700 rpm, confirmed by visual observation of the absence of particles or by using a turbidimeter.

[0098] Significance: Shorten the dissolution time, improve on-site preparation efficiency, and reduce construction waiting time.

[0099] Examples 1-4: Dissolution time 150-195 seconds, optimized dispersibility thanks to anionic surfactant (sodium lauryl sulfate) and pH 5.5-6.5.

[0100] Comparative Example 1: The dissolution time of PAM is 450 seconds. Due to its single molecular structure, its dispersibility is poor.

[0101] Comparative Example 2: No AMPS was used to reduce water solubility, and the dissolution time was 300 seconds.

[0102] Comparative Example 3: No nanografting slightly affects dispersion, and the dissolution time is 220 seconds.

[0103] Comparative Example 4: The dissolution time was 168 seconds, slightly better than Example 4 (175 seconds) due to less ion interference in deionized water.

[0104] Comparative analysis: Example 1 (150 seconds) dissolves the fastest due to the high dosage of surfactant and buffer; Example 2 (195 seconds) is slightly slower but still much better than the comparative example; Example 4 is close to the comparative example 4, indicating that the effect of reinjection water on dissolution is limited.

[0105] Viscosity retention after 60 minutes (%): Definition: Drag reducer at 90℃ and shear rate 170s -1 The percentage of the viscosity after 60 minutes to the initial viscosity is calculated as follows: (viscosity after 60 minutes / initial viscosity)×100%, and is measured using a rotational viscometer.

[0106] Significance: Viscosity retention reflects the stability of the drag reducer in high shear, high temperature, and high salt environments. A high retention rate ensures long-term fracturing performance.

[0107] Examples 1–4: Viscosity retention of 65.2–69.8%, attributed to the salt tolerance of AMPS and the network enhancement of the nano-silica grafted cross-linker.

[0108] Comparative Example 1: The PAM retention rate was only 38.5% because the molecular chain was easily broken.

[0109] Comparative Example 2: No AMPS, retention rate 50.1%, insufficient salt tolerance.

[0110] Comparative Example 3: No nanografting, the retention rate is 55.3%, and the cross-linking network is weak.

[0111] Comparative Example 4: The retention rate was 67.4%, slightly better than Example 4 (66.8%), because deionized water reduced ion interference.

[0112] Comparative analysis: Example 1 (69.8%) has the highest retention rate due to the enhanced network of high-dose cross-linking agent; Example 2 (65.2%) has the lowest but is still better than the comparative example; Example 4 is close to the comparative example 4, verifying the stability of the water reinjection process.

[0113] Viscosity (mPa·s): Definition: Drag reducer at 90℃ and shear rate 170s -1 The apparent viscosity of the mixture was measured using a rotational viscometer.

[0114] Significance: High viscosity supports suspended sand particles and maintains cracks open, suitable for high-salt fracturing fluids.

[0115] Examples 1-4: The viscosity is 23.8-27.2 mPa·s, which is much higher than the 12.3-18.9 mPa·s of Comparative Examples 1-3, because AMPS and nanografting improve the molecular chain rigidity and cross-linking strength.

[0116] Comparative Example 1: PAM viscosity is 12.3 mPa·s due to molecular chain contraction in a high salt environment.

[0117] Comparative Example 2: No AMPS, viscosity 15.7 mPa·s, poor salt resistance.

[0118] Comparative Example 3: No nanografting, viscosity 18.9 mPa·s, insufficient network stability.

[0119] Comparative Example 4: The viscosity was 25.8 mPa·s, slightly higher than that of Example 4 (25.2 mPa·s), because deionized water facilitated molecular chain extension.

[0120] Comparative analysis: Example 1 (27.2 mPa·s) has the highest viscosity due to the high-dose component; Example 2 (23.8 mPa·s) has the lowest viscosity but is still better than the comparative example; Example 4 is close to the comparative example 4, indicating that the reinjection water does not significantly affect the viscosity.

[0121] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A salt-resistant drag reducer for oil well fracturing, characterized in that: The drag reducer is composed of the following components in parts by weight: 25–35 parts of modified copolymer; 0.5–1.2 parts of nano-silica graft cross-linking agent; Antioxidants 0.1–0.3 parts; Acetate buffer 2–5 parts; Anionic surfactant 0.5–0.8 parts; Benzalkonium chloride fungicide 0.02–0.05 parts; Bentonite suspension agent 0.2–0.5 parts; The balance is deionized water; The modified copolymer is prepared by free radical copolymerization of acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid in a mass ratio of 6:2:

3. The nano-silica grafted crosslinking agent is formed by grafting nano-silica with a zirconium-based crosslinking agent.

2. The salt-resistant drag reducer for oil well fracturing according to claim 1, characterized in that: The viscosity average molecular weight of the modified copolymer is 15×10 6 –20×10 6 g / mol.

3. The salt-resistant drag reducer for oil well fracturing according to claim 1, characterized in that: The particle size of the nano-silicon dioxide is 20-50 nm, and the zirconium-based cross-linking agent is zirconium oxychloride.

4. The salt-resistant drag reducer for oil well fracturing according to claim 1, characterized in that: The antioxidant is 2,6-di-tert-butyl-4-methylphenol, and the anionic surfactant is sodium lauryl sulfate.

5. The salt-resistant drag reducer for oil well fracturing according to claim 1, characterized in that: The pH value of the drag reducer is adjusted to 5.5-6.5 by the acetate buffer.

6. A process for preparing a salt-resistant drag reducer for oil well fracturing, characterized in that: The salt-resistant drag reducer for oil well fracturing according to any one of claims 1 to 5 comprises the following steps: S1, acrylamide, crotonic acid and 2-acrylamido-2-methylpropanesulfonic acid were dissolved in deionized water in a mass ratio of 6:2:3, ammonium persulfate initiator was added, and the mixture was reacted at 60 ° C under nitrogen protection for 6 hours. The colloid was dried and crushed to obtain a modified copolymer; S2, dispersing nano-silica in ethanol, adding a silane coupling agent, reacting at 70 ° C for 4 hours, then adding zirconium oxychloride, reacting at 80 ° C for 3 hours, centrifuging, washing, and drying to obtain a nano-silica graft cross-linking agent; S3. Add deionized water to the reactor, and sequentially add the modified copolymer, nano-silica graft cross-linking agent, antioxidant, acetate buffer, anionic surfactant, benzalkonium chloride and bentonite, and mix well to obtain a drag reducer.

7. The process for preparing a salt-resistant drag reducer for oil well fracturing according to claim 6, characterized in that: In step S1, the total concentration of acrylamide, crotonic acid, and 2-acrylamido-2-methylpropanesulfonic acid is 25-35%, and the amount of ammonium persulfate used is 0.03-0.07% of the monomer mass.

8. The process for preparing a salt-resistant drag reducer for oil well fracturing according to claim 6, characterized in that: In step S2, the silane coupling agent is KH-550, and the amount used is 1-3% of the mass of the nano-silicon dioxide, and the amount used is zirconium oxychloride is 8-12% of the mass of the nano-silicon dioxide.

9. The process for preparing a salt-resistant drag reducer for oil well fracturing according to claim 6, characterized in that: In step S3, the modified copolymer is stirred for 35-45 minutes, the nano-silica grafted cross-linking agent is stirred for 40-50 minutes, and the stirring speed is 400-600 rpm.

10. The process for preparing a salt-resistant drag reducer for oil well fracturing according to claim 6, characterized in that: The deionized water in step S3 may be partially or completely replaced by reinjection water with a total dissolved solids content of up to 200,000 mg / L.

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