A polyacrylamide-based thickener for fracturing and its preparation method

Abstract

This invention discloses a polyacrylamide-based thickener for fracturing and its preparation method, belonging to the field of thickener technology. The thickener comprises the following raw materials in parts by weight: 88-92 parts of anionic polyacrylamide, 3-7 parts of chitosan, and 3-7 parts of lignin sulfonate. The anionic polyacrylamide is prepared by aqueous solution polymerization of acrylamide monomer and acrylic monomer under the action of an initiator, with an acrylic structural unit content of 5%-9%. This invention, through the synergistic effect of anionic polyacrylamide, chitosan, and lignin sulfonate, solves the problems of poor temperature and salt resistance and insufficient environmental friendliness of traditional polyacrylamide-based thickeners, and has good industrial application value.

Description

Technical Field

[0001] This invention relates to the field of thickener technology, and in particular to a polyacrylamide-based thickener for fracturing and its preparation method. Background Technology

[0002] As oil and gas exploration and development expands into deeper, ultra-deep, and unconventional reservoirs, fracturing operations face increasingly stringent requirements regarding formation temperature (often exceeding 150℃) and formation water salinity. Polyacrylamide (PAM) thickeners are core additives in water-based fracturing fluids, but they exhibit significant shortcomings in high-temperature, high-salt environments: high temperatures accelerate molecular chain thermal motion and hydrolysis, leading to a sharp decrease in viscosity; high concentrations of salt ions (such as Na+)... + Ca 2+ Mg 2+ The polymer chains curl up due to the charge shielding effect, thus losing their thickening ability; Existing technologies mainly improve the temperature and salt resistance of PAM through two approaches: one is to introduce rigid or temperature-resistant monomers (such as 2-acrylamido-2-methylpropanesulfonic acid) into the PAM molecular chain, but the synthesis is complex and costly, and may impair the solubility of the polymer; the other is to add chemical crosslinking agents (such as metal ions and organic crosslinking agents) to form covalent or coordination crosslinking networks. Although this can temporarily increase the viscosity at high temperatures, the crosslinking is irreversible, leading to difficulty in breaking down the gel, more residue, and significant damage to the reservoir.

[0003] In recent years, the development of environmentally friendly oilfield chemicals using bio-based materials has become a trend. Chitosan and lignin sulfonates are widely available, inexpensive, and biodegradable. However, existing technologies mostly use them alone as clay stabilizers, dispersants, or physical fillers. For example, patent CN114891493A uses thiol-chitosan as a chain transfer agent to attach to polymer end groups for clay stabilization. These applications do not involve utilizing them at the molecular level to construct dynamic, reversible crosslinking networks with PAM to fundamentally improve the bulk temperature and salt resistance of the thickener. Therefore, there is an urgent need to develop a fracturing fluid thickener that can fully utilize the characteristics of biomass materials and produce a synergistic effect with PAM, thereby maintaining excellent performance even under extreme conditions.

[0004] Therefore, this invention is proposed to solve the above-mentioned technical problems. Summary of the Invention

[0005] This invention aims to overcome the shortcomings of the prior art and provide a polyacrylamide-based thickener for fracturing based on the synergistic effect of multiple crosslinking of anionic polyacrylamide, chitosan, and lignin sulfonate, and its preparation method.

[0006] The core of this invention lies in the first-ever, innovative use of chitosan and lignin sulfonate simultaneously as crosslinking agents and stabilizers in anionic polyacrylamide. Through specific molecular design and non-covalent interactions, these three components construct an interlocking three-dimensional dynamic network. This network possesses the following synergistic mechanism: 1. Structural synergy: The protonated amino groups (-NH3) of chitosan + ) and the carboxyl group (-COO) of anionic polyacrylamide - ) forms the first heavy ionic bond and crosslinks with hydrogen bonds; the sulfonic acid group (-SO3) of lignin sulfonate - ) and chitosan (-NH3) + This forms a stronger and more salt-resistant second electrostatic crosslink; simultaneously, the phenolic hydroxyl groups of lignin sulfonate form hydrogen bonds with the anionic polyacrylamide chains. These three components constitute a stable “PAM-CS-LS” triangular dynamic crosslinking network, whose thermal and mechanical stability far exceeds that of single or double crosslinking systems.

[0007] 2. Synergistic Functions: Anionic polyacrylamide serves as a thickening framework; chitosan acts as a positively charged crosslinking center, also possessing clay-stabilizing properties; lignin sulfonate not only participates in crosslinking, but its rigid aromatic framework provides physical support for the network, its sulfonic acid groups endow the system with excellent resistance to salt ion shielding, and its phenolic structure can also scavenge free radicals generated at high temperatures and chelate Ca. 2+ Cu 2+ Catalytic metal ions protect polymer chains from oxidative degradation at a chemical level.

[0008] 3. Reversible synergy: The dynamic cross-linked network is mainly formed through intermolecular forces such as electrostatic interaction and hydrogen bonding. It can be partially and reversibly dissociated and recombined at high temperatures. This can buffer thermal stress, maintain the integrity of the network, and ensure that the network can be easily destroyed by conventional breaker agents (such as oxidants and enzymes) after fracturing, thus achieving rapid and thorough debonding.

[0009] To achieve the above objectives, the present invention employs the following technical solution: A polyacrylamide-based thickener for fracturing comprises the following raw materials in parts by weight: 88-92 parts of anionic polyacrylamide, 3-7 parts of chitosan, and 3-7 parts of lignin sulfonate. The anionic polyacrylamide is prepared by aqueous solution polymerization of acrylamide monomer and acrylic monomer under the action of an initiator, with an acrylic structural unit content of 5% to 9%. This specific content ensures optimal synergistic effect with chitosan and lignin sulfonate; too low a content results in insufficient crosslinking points, while too high a content may lead to electrostatic repulsion with lignin sulfonate.

[0010] Preferably, the anionic polyacrylamide is prepared by aqueous solution polymerization of monomers containing acrylamide and acrylic acid at pH 6.0-7.0 and 45-65°C.

[0011] Preferably, the degree of deacetylation of the chitosan is ≥85%.

[0012] Preferably, the lignin sulfonate is sodium lignin sulfonate with a sulfonation degree ≥ 2.0 mmol / g.

[0013] This invention also provides a method for preparing a polyacrylamide-based thickener for fracturing, comprising the following steps: S1. Prepare chitosan acetate solution; S2. Prepare lignin sulfonate solution; S3. Prepare an anionic polyacrylamide solution; S4. Mix the solutions obtained in steps S1, S2 and S3, adjust the pH to 5.5~6.0, and carry out the dynamic cross-linking reaction at room temperature; S5. The crosslinking system obtained in step S4 is dried and pulverized to obtain the polyacrylamide-based thickener for fracturing.

[0014] Preferably, step S1 involves dissolving the chitosan in an aqueous acetic acid solution with a mass concentration of 0.5% to 1.0% to prepare a chitosan solution with a mass concentration of 4% to 6%.

[0015] Preferably, step S2 involves dissolving the lignin sulfonate in deionized water to prepare a lignin sulfonate solution with a mass concentration of 8% to 12%; step S3 involves dissolving the anionic polyacrylamide in deionized water to prepare an anionic polyacrylamide solution with a mass concentration of 14% to 16%.

[0016] Preferably, in step S4, the mixing and reaction of the solution specifically involves placing the polyacrylamide solution in a reaction vessel, adding the chitosan solution first while stirring, mixing evenly, then adding the lignin sulfonate solution, finally adjusting the pH of the system to 5.5~6.0 with alkali solution, and stirring the reaction at 20~30℃ for 2 hours.

[0017] Preferably, the alkaline solution is a sodium hydroxide solution with a mass concentration of 1-2%.

[0018] The beneficial effects of this invention are: This invention exhibits excellent temperature and salt resistance. After being sheared for 2 hours in a high-mineralization brine at 170°C, the viscosity retention rate of this thickener can reach over 90%, far exceeding that of traditional PAM systems and PAM systems modified from single biomass materials. This invention also possesses excellent debinding properties; the dynamic cross-linked network is easily broken, resulting in a low-viscosity debinding solution with minimal residue. Furthermore, the main functional components of this invention are derived from biomass waste, and the preparation process is mild and simple, requiring no expensive monomers or toxic cross-linking agents. Attached Figure Description

[0019] Figure 1 The diagram shows the salt resistance viscosity retention of the thickeners prepared in the examples and comparative examples. Figure 2 The graph shows the free radical scavenging rate of the thickeners prepared in the examples and comparative examples. Detailed Implementation

[0020] 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.

[0021] Raw material preparation: Anionic PAM (PAM-A): Acrylic acid content 5%. Preparation method: Dissolve 95.2g acrylamide (AM) and 4.8g acrylic acid (AA) in 900g deionized water, adjust pH to 6, add 0.05% (as a percentage of monomer) of water-soluble azo initiator V-50, react at 45℃ for 7 hours, precipitate with ethanol, wash, vacuum dry at 60℃ for 12 hours, pulverize and pass through a 100-mesh sieve to obtain a white powder.

[0022] Anionic PAM (PAM-B): Acrylic acid content 7%. Preparation method: Dissolve 93g AM and 7g AA in 900g water, adjust pH to 6.5, add 0.03% V-50 initiator, react at 50℃ for 6 hours, precipitate with ethanol, wash, vacuum dry at 60℃ for 12 hours, pulverize and pass through a 100-mesh sieve to obtain a white powder.

[0023] Anionic PAM (PAM-C): Acrylic acid content 9%. Preparation method: Dissolve 91g AM and 9g AA in 900g water, adjust pH to 7, add 0.02% V-50 initiator, react at 65℃ for 5 hours, precipitate with ethanol, wash, vacuum dry at 60℃ for 12 hours, pulverize and pass through a 100-mesh sieve to obtain a white powder.

[0024] Chitosan: Deacetylation degree 90%, molecular weight 150,000.

[0025] Sodium lignosulfonate: sulfonation degree 2.2 mmol / g, molecular weight approximately 8000.

[0026] Example 1 This embodiment of a polyacrylamide-based thickener for fracturing includes the following raw materials in parts by weight: 88 parts of anionic polyacrylamide, 3 parts of chitosan, and 3 parts of lignin sulfonate. The preparation method in this embodiment is as follows: S1. Weigh 3g of chitosan and dissolve it in 72g of 0.5% acetic acid aqueous solution. Stir at room temperature for 2 hours to obtain a chitosan solution with a mass fraction of approximately 4%. S2. Weigh 3g of sodium lignosulfonate, dissolve it in 34.5g of deionized water, stir for 0.5 hours to obtain a solution with a mass fraction of approximately 8%; S3. Weigh 88g of PAM-B, dissolve it in about 504g of deionized water, stir for 2 hours to obtain a solution with a mass fraction of about 14%; S4. Place the PAM-B solution obtained in S3 into a 1000ml beaker and stir mechanically (300 rpm). Slowly add all the chitosan solution obtained in S1 over about 10 minutes. After the addition is complete, continue stirring for 30 minutes. Then, slowly add all the sodium lignosulfonate solution obtained in S2. After the addition is complete, stir for 30 minutes. Finally, slowly adjust the pH of the system to 5.5 with 1% NaOH solution and maintain the reaction at 30℃ for 2 hours. S5. Pour the homogeneous sol after reaction into a stainless steel tray and place it in a vacuum drying oven. Dry it at 60℃ and a vacuum of -0.095 MPa for 12 hours. Crush the dried solid block using a universal pulverizer and pass it through an 80-mesh sieve to obtain a polyacrylamide-based fracturing thickener.

[0027] Example 2 This embodiment of a polyacrylamide-based thickener for fracturing includes the following raw materials in parts by weight: 90 parts of anionic polyacrylamide, 5 parts of chitosan, and 5 parts of lignin sulfonate. The preparation method in this embodiment is as follows: S1. Weigh 5g of chitosan and dissolve it in 95g of 1% acetic acid aqueous solution. Stir at room temperature for 2 hours to obtain a chitosan solution with a mass fraction of approximately 5%. S2. Weigh 5g of sodium lignosulfonate, dissolve it in 45g of deionized water, stir for 0.5 hours to obtain a solution with a mass fraction of approximately 10%. S3. Weigh 90g of PAM-B, dissolve it in about 510g of deionized water, stir for 2 hours to obtain a solution with a mass fraction of about 15%. S4. Place the PAM-B solution obtained in S3 into a 1000ml beaker and stir mechanically (300 rpm). Slowly add all the chitosan solution obtained in S1 over about 10 minutes. After the addition is complete, continue stirring for 30 minutes. Then slowly add all the sodium lignosulfonate solution obtained in S2. After the addition is complete, stir for 30 minutes. Finally, slowly adjust the pH of the system to 5.8 with 1% NaOH solution and maintain the reaction at 20℃ for 2 hours. S5. Pour the homogeneous sol after reaction into a stainless steel tray and place it in a vacuum drying oven. Dry it at 60℃ and a vacuum of -0.095 MPa for 12 hours. Crush the dried solid block using a universal pulverizer and pass it through an 80-mesh sieve to obtain a polyacrylamide-based fracturing thickener.

[0028] Example 3 This embodiment of a polyacrylamide-based thickener for fracturing includes the following raw materials in parts by weight: 92 parts of anionic polyacrylamide, 7 parts of chitosan, and 7 parts of lignin sulfonate. The preparation method in this embodiment is as follows: S1. Weigh 7g of chitosan and dissolve it in 109g of 1% acetic acid aqueous solution. Stir at room temperature for 2 hours to obtain a chitosan solution with a mass fraction of approximately 6%. S2. Weigh 7g of sodium lignosulfonate, dissolve it in 51g of deionized water, stir for 0.5 hours to obtain a solution with a mass fraction of approximately 12%; S3. Weigh 92g of PAM-B, dissolve it in about 479g of deionized water, stir for 2 hours to obtain a solution with a mass fraction of about 16%; S4. Place the PAM-B solution obtained in S3 into a 1000ml beaker and stir mechanically (300 rpm). Slowly add all the chitosan solution obtained in S1 over about 10 minutes. After the addition is complete, continue stirring for 30 minutes. Then, slowly add all the sodium lignosulfonate solution obtained in S2. After the addition is complete, stir for 30 minutes. Finally, slowly adjust the pH of the system to 6.0 with 2% NaOH solution and maintain the reaction at 25℃ for 2 hours. S5. Pour the homogeneous sol after reaction into a stainless steel tray and place it in a vacuum drying oven. Dry it at 70℃ and a vacuum of -0.095 MPa for 10 hours. Crush the dried solid block using a universal pulverizer and pass it through a 100-mesh sieve to obtain a polyacrylamide-based fracturing thickener.

[0029] Example 4 The difference between this embodiment and Embodiment 3 is that the anionic polyacrylamide used is anionic PAM-A.

[0030] Example 5 The difference between this embodiment and Embodiment 3 is that the anionic polyacrylamide used is anionic PAM-C.

[0031] Comparative Example 1 The difference between this comparative example and Example 2 is that sodium lignosulfonate is not added, and in step S4, an equal mass of deionized water is used instead of sodium lignosulfonate solution.

[0032] Comparative Example 2 The difference between this comparative example and Example 2 is that chitosan is not added, and in step S4, an equal mass of deionized water is used instead of the chitosan solution.

[0033] Comparative Example 3 The difference between this comparative example and Example 2 is that the polyacrylamide used in this comparative example is an anionic polyacrylamide with an acrylic acid structural unit content of approximately 10%. The preparation method of this polymer is as follows: 90g of acrylamide (AM) and 10g of acrylic acid (AA) are dissolved in 900g of deionized water, the pH is adjusted to 6.5 with dilute sodium hydroxide solution, and initiator V-50, accounting for 0.03% of the total monomer mass, is added. The mixture is reacted at 50°C for 6 hours. After the reaction, the mixture is precipitated with ethanol, washed, vacuum dried at 60°C for 12 hours, pulverized, and passed through a 100-mesh sieve to obtain a white powder (except for the acrylic acid ratio, the other preparation conditions are completely consistent with the PAM-B used in Example 2).

[0034] Comparative Example 4 like Figure 1 and Figure 2 As shown, the difference between this comparative example and Example 2 is that the polyacrylamide used in this comparative example is an anionic polyacrylamide with an acrylic acid structural unit content of approximately 4%. The preparation method of this polymer is as follows: 96g of acrylamide (AM) and 4g of acrylic acid (AA) are dissolved in 900g of deionized water, the pH is adjusted to 6.5 with dilute sodium hydroxide solution, and initiator V-50, accounting for 0.03% of the total monomer mass, is added. The mixture is reacted at 50°C for 6 hours. After the reaction, the mixture is precipitated with ethanol, washed, vacuum dried at 60°C for 12 hours, pulverized, and passed through a 100-mesh sieve to obtain a white powder (except for the acrylic acid ratio, the other preparation conditions are completely consistent with the PAM-B used in Example 2).

[0035] Actual test The thickener powders obtained in the examples and comparative examples were dissolved in the standard test base solution at an addition of 0.5 wt%. After thorough stirring and dissolution, the following performance tests were conducted. The standard test base solution was simulated formation water with the following composition: NaCl 20000 mg / L, CaCl 2 5000 mg / L, MgCl 2 1000 mg / L, and pH adjusted to 7.0 with NaHCO 3.

[0036] 1. Initial viscosity test: The apparent viscosity V0 of the sample was measured at 25°C using a Brookfield DV2T rotational viscometer equipped with a ULA ultra-low viscosity adapter. Each sample was tested in parallel three times and the average value was taken.

[0037] 2. Temperature and shear resistance test: A HAAKE MARS 60 advanced rotational rheometer was used, set at 170℃, for 170 seconds. -1 The sample was continuously tested at the shear rate for 4 hours, and the viscosity V1 data was recorded.

[0038] 3. Breakage Performance Test: Prepare a 0.5 wt% thickener test solution, add 0.1% (by weight) ammonium persulfate (APS) breaker, shake rapidly, and place in a 90°C constant temperature water bath. Take out approximately 10 mL of sample every 5 minutes, rapidly cool to 25°C, and measure its viscosity using the Brookfield viscometer described above. Record the time required for the viscosity to decrease to below 5 mPa·s as the breaking time. If no breakage occurs within 120 minutes, record it as >120 min.

[0039] 4. Free radical scavenging ability test: The DPPH (1,1-diphenyl-2-picrylhydrazine) free radical scavenging method was used for evaluation. A 0.1 mmol / L DPPH-anhydrous ethanol solution was prepared. 2.0 mL of 0.1% (w / v) thickener test solution was mixed with 2.0 mL of DPPH solution in a 10 mL centrifuge tube, vortexed for 10 seconds, and allowed to stand at room temperature in the dark for 30 minutes. Using anhydrous ethanol as a reference, the absorbance A1 of the mixture was measured at 517 nm using a UV-Vis spectrophotometer. 2.0 mL of deionized water was used as a blank control instead of the sample solution, and the absorbance A2 was measured using the same method. The free radical scavenging rate was calculated as: Scavenging rate (%) = [(A2-A1) / A2]*100%. Each sample was tested in triplicate, and the average value was taken.

[0040] 5. Salt resistance test: Salt resistance: According to SY / T 5107-2016 standard, CaCl2 was added, and the mixture was stirred at a constant temperature of 60℃ for 4 hours. The viscosity V2 of the treated sample was measured, and the viscosity retention rate data was recorded. Viscosity retention rate (%) = (V2 / V0)*100%; The above results are shown in Table 1.

[0041] Table 1

[0042] As can be seen from the table, compared with Example 2 and Comparative Example 2, the lack of chitosan reduces the high-temperature viscosity and salt resistance of the system. This is because the protonated amino groups (-NH3) of chitosan... + ) and the carboxyl group (-COO) of anionic polyacrylamide -The first heavy ion crosslinking network formed is the structural basis of the entire system. The absence of this network prevents the system from constructing an effective spatial structure to resist high-temperature shear and salt ion attack, resulting in a simple mixture of linear polymers and severely degraded performance. Comparing Example 2 and Comparative Example 1, it is evident that the salt resistance and antioxidant properties of the system drop sharply after the absence of lignin sulfonate. This reveals the dual function of lignin sulfonate: First, the active groups such as phenolic hydroxyl groups in its molecule can effectively scavenge free radicals generated at high temperatures, inhibiting the oxidative degradation of the PAM backbone from the chemical source (chemical stabilization mechanism); Second, its sulfonic acid groups form stronger electrostatic crosslinks with chitosan amino groups, and with its excellent resistance to calcium and magnesium ions, it shields the PAM carboxyl network from the damage caused by metal ions. Compared with Example 2, Comparative Example 3, although the initial viscosity of Comparative Example 3 is acceptable (178 mPa·s), its high-temperature viscosity retention rate is extremely low and its debonding is extremely difficult (>120). The PAM chain exhibits poor salt resistance due to its excessively high carboxyl content, which leads to a dual negative effect: firstly, in high-mineralization environments, the PAM chains undergo severe curling due to a strong charge shielding effect, resulting in a loss of self-adhesiveness; secondly, it generates electrostatic repulsion with the similarly negatively charged lignin sulfonate, disrupting the uniformity and synergy of the "chitosan-PAM-lignin" network, forming a defective aggregated structure, which in turn makes it difficult for the degumming agent to penetrate and function. Comparative Example 4 exhibits rapid degumming because its network crosslinking points are insufficient, and its initial viscosity and high-temperature viscosity are significantly lower than those of the embodiments of this invention. This indicates that while an excessively low carboxyl content is beneficial for salt resistance, it results in insufficient active sites for crosslinking with chitosan, making it impossible to construct a sufficiently dense and robust network framework, thus causing the overall adhesiveness and high-temperature structural stability of the system to fail to meet high-performance requirements.

[0043] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A polyacrylamide-based thickener for fracturing, characterized in that, The raw materials include the following parts by weight: 88-92 parts of anionic polyacrylamide, 3-7 parts of chitosan, and 3-7 parts of lignin sulfonate; The anionic polyacrylamide is prepared by aqueous solution polymerization of acrylamide monomer and acrylic acid monomer under the action of an initiator, and its acrylic acid structural unit content is 5%~9%.

2. The polyacrylamide-based thickener for fracturing according to claim 1, characterized in that, The anionic polyacrylamide is prepared by aqueous solution polymerization of monomers containing acrylamide and acrylic acid at pH 6.0-7.0 and 45-65°C.

3. The polyacrylamide-based thickener for fracturing according to claim 1, characterized in that, The degree of deacetylation of the chitosan is ≥85%.

4. The polyacrylamide-based thickener for fracturing according to claim 1, characterized in that, The lignin sulfonate is sodium lignin sulfonate with a sulfonation degree ≥ 2.0 mmol / g.

5. A method for preparing a polyacrylamide-based thickener for fracturing as described in claim 1, characterized in that, Includes the following steps: S1. Prepare chitosan acetate solution; S2. Prepare lignin sulfonate solution; S3. Prepare an anionic polyacrylamide solution; S4. Mix the solutions obtained in steps S1, S2 and S3, adjust the pH to 5.5~6.0, and carry out the dynamic cross-linking reaction at room temperature; S5. The crosslinking system obtained in step S4 is dried and pulverized to obtain the polyacrylamide-based thickener for fracturing.

6. The preparation method according to claim 5, characterized in that, Step S1 involves dissolving the chitosan in an aqueous acetic acid solution with a mass concentration of 0.5% to 1.0% to prepare a chitosan solution with a mass concentration of 4% to 6%.

7. The preparation method according to claim 5, characterized in that, Step S2 involves dissolving the lignin sulfonate in deionized water to prepare a lignin sulfonate solution with a mass concentration of 8% to 12%; Step S3 involves dissolving the anionic polyacrylamide in deionized water to prepare an anionic polyacrylamide solution with a mass concentration of 14% to 16%.

8. The preparation method according to claim 5, characterized in that, In step S4, the mixing and reaction of the solution specifically involves placing the anionic polyacrylamide solution in a reaction vessel, adding the chitosan solution first while stirring, mixing evenly, then adding the lignin sulfonate solution, and finally adjusting the pH of the system to 5.5-6.0 with alkali solution, and stirring the reaction at 20-30°C for 2 hours.

9. The preparation method according to claim 8, characterized in that, The alkaline solution is a sodium hydroxide solution with a mass concentration of 1-2%.

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