Multiple network fracturing fluid based on biomass and polyethylene oxide and preparation method thereof

Abstract

The present application relates to the technical field of oilfield chemical fracturing fluid, and proposes a multiple network fracturing fluid based on biomass and polyethylene oxide, which comprises biomass polysaccharide, aldehyde-based crosslinking agent A, dieneophile crosslinking agent B, linear polymer C and solvent, and forms a two-fold dynamic covalent network and a one-fold physical entanglement network: the aldehyde-based crosslinking agent A reacts with the amino group of chitosan to form a first-fold pH / temperature response dynamic covalent network, the aldehyde-based crosslinking agent A is furfural and glutaraldehyde, and the aldehyde-based crosslinking agent A reacts with the amino group of chitosan to form a furan ring diene, wherein the molar ratio of the amino group of chitosan to the aldehyde group of furfural is 1:(0.6-0.8); the dieneophile crosslinking agent B forms a second-fold heat response dynamic covalent network through Diels-Alder reaction with the furan ring diene; the molecular chain of the linear polymer C is physically inserted into the double-fold dynamic covalent bond multiple network to form a third-fold physical entanglement network with enhanced flowability, and the three-fold network coordinates with each other to realize the synergistic effect of environmental protection, low input and high output.

Description

Technical Field

[0001] This invention relates to the field of oilfield chemical fracturing fluid technology, specifically to a multi-network fracturing fluid based on biomass and polyethylene oxide and its preparation method, which is particularly suitable for the stimulation of unconventional reservoirs such as shale gas and tight oil and gas. Background Technology

[0002] Hydraulic fracturing, especially slickwater fracturing, is a core technology for developing unconventional resources such as shale oil and gas. Its core requirement is to use extremely low concentrations of drag-reducing agents (usually high molecular weight polymers) to significantly reduce pipeline friction during high-speed pumping, while also having a certain sand-suspending capacity. Currently, commercial fracturing fluids mainly rely on polyacrylamide (PAM)-based synthetic polymers. Although its drag-reducing performance is acceptable, it has three fundamental defects: (1) Non-degradability: The polymer chains are difficult to degrade by microorganisms or chemical action in the reservoir. Long-term retention will block micropores and fractures, causing serious reservoir damage and affecting the final recovery rate; (2) Irreversible shear degradation: Under high-pressure pumping and high shear rates, the polymer backbone is prone to irreversible breakage, resulting in permanent loss of drag-reducing performance; (3) Environmentally unfriendly: The raw materials are derived from petrochemicals, and the residues pose a potential risk to the underground ecosystem.

[0003] In existing technologies, fracturing fluid systems generally follow the conventional approach of "enhancing performance with a high crosslinking agent / functional component ratio." However, limited by a single network structure or a single functional component, it is impossible to simultaneously achieve multiple core performance characteristics. Existing Chinese invention patent CN111171799A provides a temporarily plugging and redirecting fracturing fluid based on dynamic covalent bonds, its preparation method, and its application. This temporarily plugging and redirecting fracturing fluid based on dynamic covalent bonds can effectively avoid the residue problems associated with conventional polymer gels. However, this patented technology only forms a single dynamic network through the DA reaction, while limiting the functional group ratio of dienes to dienophiles to 1:(0.3-1). It ensures the formation of a dense network by increasing the proportion of dienesophiles to achieve the temporarily plugging function, but lacks dynamic response flexibility and flowability control capabilities.

[0004] Therefore, there is an urgent need to develop a multi-network fracturing fluid based on biomass and polyethylene oxide to solve the above problems. Summary of the Invention

[0005] The purpose of this invention is to overcome the limitations of existing technologies, such as the non-degradability and reservoir damage of synthetic polymer fracturing fluids, the limited performance and poor stability of traditional biomass fracturing fluids, and the reliance on high crosslinking agent ratios to improve performance. This invention provides a multi-network fracturing fluid based on chitosan and polyethylene oxide, constructed through ingenious molecular design using a Schiff base / Diels-Alder dual dynamic covalent interpenetrating network, interspersed with linear polymers. This system is not only environmentally friendly but also possesses excellent drag reduction performance, outstanding self-healing capabilities, and controllable rheological behavior, achieving superior overall performance even with low crosslinking agent ratios.

[0006] To achieve the above objectives, this invention provides the following technical solution: a multi-network fracturing fluid based on biomass and polyethylene oxide and its preparation method, comprising: biomass polysaccharide, aldehyde crosslinking agent A, dienophilic crosslinking agent B, linear polymer C, and solvent, which together form two dynamic covalent networks and one physically entangled network.

[0007] The aldehyde crosslinking agent A reacts with the amino groups of chitosan to form a first-order pH / temperature-responsive dynamic covalent network. The aldehyde crosslinking agent A is furfural and glutaraldehyde. The aldehyde crosslinking agent A reacts with the amino groups of chitosan to form a furan cyclic diene, wherein the molar ratio of chitosan amino groups to furfural aldehyde groups is 1:(0.6-0.8).

[0008] The dienophilic crosslinking agent B and the furan cyclodiene form a second thermally responsive dynamic covalent network through a Diels-Alder reaction;

[0009] The molecular chains of the linear polymer C are physically interwoven within the dual dynamic covalent bond network, forming a third layer of physically entangled network with enhanced fluidity.

[0010] The first and second networks interpenetrate through covalent bonds to form a dual dynamic covalent bond multi-network.

[0011] Preferably, the dienophilic crosslinking agent B is maleic anhydride and / or a maleic anhydride derivative, and the linear polymer C is polyethylene oxide.

[0012] Preferably, the fracturing fluid comprises, by weight, the following parts: 1-8 parts chitosan, 0.5-5 parts aldehyde crosslinking agent A, 0.2-3 parts dienophilic crosslinking agent B, 0.05-3 parts polyethylene oxide, and 0.5-3 parts acidic solvent.

[0013] Preferably, the acidic solvent is an aqueous solution of acetic acid with a concentration of 0.5%-5%.

[0014] Preferably, the polyethylene oxide has a weight-average molecular weight of 500,000 to 8,000,000, and is produced through molecular entanglement and a dual dynamic covalent network.

[0015] Preferably, the method for preparing fracturing fluid based on a multi-network of biomass and polyethylene oxide includes the following steps:

[0016] S1: Dissolve chitosan in a diluted acidic aqueous solution to prepare a homogeneous solution;

[0017] S2: Add polyethylene oxide to the solution from step S1 and stir until fully dispersed or dissolved to obtain a mixed base solution;

[0018] S3: Add aldehyde crosslinking agent A to the mixed base liquid to cause chitosan to undergo a Schiff base reaction with aldehyde crosslinking agent A to form a dynamic crosslinked prepolymer network containing furan rings. The reaction conditions are controlled to ensure that the molar ratio of chitosan amino groups to furfural aldehyde groups is in the range of 1:(0.6-0.8).

[0019] S4: Add dienophilic crosslinking agent B to the reaction system of step S3, so that the dienophilic crosslinking agent B and the furan cyclodienes in the network undergo a Diels-Alder reaction, and finally form a fracturing fluid base fluid with dual dynamic covalent bonds containing a polyoxyethylene physically interwoven network.

[0020] Preferably, in step S3, the reaction temperature is 5℃-60℃ and the reaction time is 0.5-4 hours to control the dynamic equilibrium and network structure of the Schiff base reaction. In step S4, the reaction temperature is 40℃-80℃ and the reaction continues for 1-6 hours to control the rate and depth of the Diels-Alder reaction, thereby finely regulating the rheological properties and self-healing characteristics of the entire multi-network.

[0021] Preferably, the fracturing fluid is used directly or diluted with water to the target concentration and then used as a slickwater fracturing fluid.

[0022] Preferably, the mass concentration of the effective network structure material in the diluted working solution is 0.01%-0.5%.

[0023] Preferably, the fracturing fluid of the present invention is applied to hydraulic fracturing of shale gas or tight oil and gas reservoirs.

[0024] This invention provides a multi-network fracturing fluid based on biomass and polyethylene oxide and its preparation method, which has the following advantages compared with existing technologies:

[0025] This invention uses biomass raw materials such as chitosan as the basic component to replace traditional polyacrylamide, which significantly improves the biodegradability of the fracturing fluid system. In this technical solution, the fracturing fluid can reach a degradation rate of 55% in simulated formation water in 30 days, which is much higher than the less than 5% of conventional PAM, effectively avoiding reservoir blockage and environmental pollution.

[0026] This invention achieves a high-performance balance in fracturing fluid with a low crosslinking agent ratio through the synergistic effect of a triple network. The fracturing fluid of this invention boasts a drag reduction rate of up to 72%, significantly higher than conventional products of the same type. Simultaneously, it exhibits outstanding self-healing capabilities, with a viscosity recovery rate exceeding 80% after shearing, ensuring performance stability under high shear conditions during fracturing. The formed Schiff base pH / temperature responsive dynamic network and DA bond thermal responsive dynamic network enable the fracturing fluid to adapt to changes in the reservoir environment, preventing irreversible degradation.

[0027] This invention employs an interpenetrating network of physically entangled and chemically cross-linked networks, significantly improving sand-carrying performance. It achieves the original settling rate with half the amount of cross-linking agent and maintains excellent sand-carrying capacity even at low cross-linking agent ratios. Furthermore, by controlling reaction conditions (temperature and time), the rheological behavior of the network can be precisely adjusted to meet different pumping requirements, avoiding the loss of fluidity caused by traditional highly cross-linked systems.

[0028] This invention achieves a synergistic effect of "low input and high output" by coordinating three networks to outperform conventional polyacrylamide fracturing fluids in key indicators such as drag reduction, sand suspension, and degradation. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0030] Figure 1 This is a partial diagram of the suspended sand experiment of the present invention. Detailed Implementation

[0031] To make the technical problems, technical solutions, and beneficial effects of the present invention clearer, the present invention will be further described in detail with reference to the embodiments and accompanying drawings. It should be understood that the specific embodiments described herein are merely for explaining the present invention and are not intended to limit the present invention. The technical solutions of the present invention will be described in detail below with reference to the embodiments and accompanying drawings, but the scope of protection is not limited thereto.

[0032] This invention provides a technical solution:

[0033] The technical solution of this invention proposes a dual dynamic covalent bond multi-network fracturing fluid comprising, by mass parts: 1-8 parts chitosan, 0.5-5 parts aldehyde crosslinking agent A, 0.2-3 parts dienophilic crosslinking agent B, 0.05-3 parts polyethylene oxide, and 0.5-3 parts acidic solvent. Wherein, aldehyde crosslinking agent A is furfural and / or glutaraldehyde, dienophilic crosslinking agent B is maleic anhydride or its derivative, and linear polymer C is polyethylene oxide. These components together construct a composite structure combining two dynamic covalent networks and one physically entangled network, including:

[0034] The first-order Schiff base dynamic covalent network utilizes the Schiff base condensation reaction between the primary amino groups on the chitosan chain and the aldehyde groups in furfural and glutaraldehyde. The primary amino groups on the chitosan chain react with glutaraldehyde to form dynamic imine bonds, creating a dynamic covalent cross-linked network. Simultaneously, the primary amino groups on the chitosan chain react with furfural aldehyde groups to form furan rings and dynamic imine bonds. The resulting highly reactive furan ring dienes act as side groups and cross-linking points, anchoring them to the network backbone. This first-order Schiff base dynamic covalent network exhibits pH and thermal reversibility, ensuring that the molar ratio of chitosan amino groups to furfural aldehyde groups is controlled within a low range of 1:(0.6-0.8).

[0035] The second DA dynamic covalent network: Maleic anhydride is added as a dienophile to the first dynamic covalent network. Under heating conditions, the maleic anhydride undergoes a highly efficient Diels-Alder cycloaddition reaction with the furan ring diene introduced in the first network, forming another covalent cross-linking site. This DA bond can undergo a reverse reaction at higher temperatures. This network and the first network achieve interpenetration of chemical bonds at the molecular level through shared reaction sites, forming a double covalent network framework.

[0036] The third layer of physical entanglement network: Simultaneously, linear polyethylene oxide (PEO) molecular chains are dissolved in the above system. The long linear chains of PEO are physically interwoven and entangled between the rigid double chemical network. This effectively reduces the internal flow resistance of the system. By replacing the chemical crosslinking points in conventional systems with physical entanglement, the suspension capacity is maintained at a 30%-50% reduction in the amount of aldehyde crosslinking agent A. Furthermore, PEO itself has excellent biodegradability, further enhancing the environmental friendliness of the entire system.

[0037] In this invention, within the range of the above-mentioned materials by mass fraction, a dynamic balance is achieved through the synergistic effect of a triple network, thus avoiding loss of fluidity caused by a high cross-linking ratio.

[0038] In some embodiments, the weight-average molecular weight of polyethylene oxide is 500,000 to 8,000,000. As a flow enhancement unit, it further optimizes the shear resistance and drag reduction performance of the system through molecular entanglement and synergy of dual dynamic covalent networks.

[0039] In some embodiments, the following steps are included:

[0040] S1: Dissolve chitosan in a diluted acidic aqueous solution to prepare a homogeneous solution;

[0041] S2: Add polyethylene oxide to the solution from step S1 and stir until fully dispersed or dissolved to obtain a mixed base solution;

[0042] S3: Add aldehyde crosslinking agent A to the mixed base liquid and react at 5℃-60℃ for 0.5-4 hours to allow chitosan and aldehyde crosslinking agent A to undergo a Schiff base reaction to form a dynamic crosslinked prepolymer network containing furan rings. By controlling the reaction conditions, ensure that the molar ratio of chitosan amino groups to furfural aldehyde groups is in the range of 1:(0.6-0.8).

[0043] S4: Add dienophilic crosslinking agent B to the reaction system of step S3, and continue the reaction at 40℃-80℃ for 1-6 hours to allow the dienophilic crosslinking agent B to undergo a Diels-Alder reaction with the furan ring in the network, ultimately forming a fracturing fluid base fluid with dual dynamic covalent bonds containing a polyoxyethylene physically interwoven network.

[0044] Example 1:

[0045] This embodiment uses a mixed crosslinking agent of furfural and glutaraldehyde, with chitosan amino groups and furfural aldehyde groups in a ratio of 1:0.6.

[0046] Weigh 3.0 g of chitosan (degree of deacetylation ≥ 95%) and add it to a 1.0 wt% acetic acid solution prepared by 150 mL of deionized water and 3.0 mL of glacial acetic acid. Stir mechanically at 25 °C for 4 hours to obtain a clear and viscous chitosan solution.

[0047] Add 0.8 g of polyethylene oxide (PEO, Mw ≈ 1 million) to the above solution and stir slowly in a 40°C water bath for 3 hours to ensure that the PEO is completely dissolved and uniformly dispersed to obtain a mixed base solution.

[0048] Add 1.5 g of furfural and 0.5 g of glutaraldehyde (50% aqueous solution) to the mixed base solution, and stir the mixture at 200 rpm for 2.5 hours in a constant temperature water bath at 50℃ to construct a Schiff base network. It was observed that the viscosity of the system increased significantly and a pale yellow, elastic hydrogel-like prepolymer was formed, forming the first dynamic covalent network.

[0049] The reaction system temperature was raised to 65°C, and 1.0 g of maleic anhydride powder was quickly added. The temperature was maintained, and the reaction was continued with stirring for 4 hours. During the reaction, the system transformed from a highly elastic gel into a denser, tougher, and smoother elastomer, indicating that the DA reaction was successful and the dual dynamic covalent network was completed. The resulting product is the multi-network fracturing fluid base fluid.

[0050] The final product obtained after the reaction exhibits the following characteristics: the resulting base liquid possesses both high initial viscoelasticity and good dynamic properties. Due to the cross-linking effect of glutaraldehyde, the network is more stable; at the same time, the retained furan ring ensures the DA reaction to proceed, allowing the material to be reversibly repaired after strong shearing through the DA bond.

[0051] Example 2:

[0052] This embodiment uses a comparative preparation without adding linear polymer C, employing a dual chemical network with chitosan amino:furfural aldehyde ratio of 1:0.6.

[0053] First, prepare a chitosan solution by weighing 3.0 g of chitosan (degree of deacetylation ≥ 95%) and adding it to a 1.0 wt% acetic acid solution prepared from 150 mL of deionized water and 3.0 mL of glacial acetic acid. Stir mechanically at 25 °C for 4 hours to obtain a clear and viscous solution.

[0054] Subsequently, a Schiff base network was constructed by adding 1.5 g of furfural and 0.5 g of glutaraldehyde (50% aqueous solution) to the above solution and stirring at 200 rpm for 2.5 hours in a constant temperature water bath at 50°C to form a pre-crosslinked gel.

[0055] Then, a DA network was constructed, the reaction system temperature was raised to 65°C, 1.0 g of maleic anhydride powder was quickly added, and the temperature was maintained while stirring for 4 hours.

[0056] The final product obtained after the reaction has the following characteristics: it is a dark yellow hard gel block with high brittleness and poor toughness. It is difficult to disperse in water and requires strong stirring and extended time to dilute into a heterogeneous test solution.

[0057] Example 3:

[0058] This embodiment uses a low-aldehyde crosslinking agent A, wherein the ratio of chitosan amino groups to furfural aldehyde groups is 1:0.4.

[0059] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0060] Add 0.8 g of polyethylene oxide (Mw ≈ 3 million) and stir at 40°C for 3 hours.

[0061] Add 0.8 g of furfural and react at 50 °C for 2.5 hours.

[0062] Heat to 65°C, add 1.0 g of maleic anhydride, and react for 4 hours.

[0063] The final product obtained after the reaction has the following characteristics: the resulting base liquid has a low viscosity, exhibiting a viscous solution rather than a strong gel state. The formed network has a low cross-linking density and weak mechanical strength.

[0064] Example 4:

[0065] This embodiment uses a high-aldehyde crosslinking agent A, wherein the ratio of chitosan amino groups to furfural aldehyde groups is 1:2.

[0066] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0067] Add 0.8 g of polyethylene oxide (Mw ≈ 3 million) and stir at 40°C for 3 hours.

[0068] Add 4.0 g of furfural and react at 50 °C for 2.5 hours.

[0069] Heat to 65°C, add 1.0 g of maleic anhydride, and react for 4 hours.

[0070] The final product obtained after the reaction has the following characteristics: the product reacts rapidly and forms an over-crosslinked, dense solid gel that is extremely difficult to disperse in water. It requires high temperature (60°C) to assist stirring in forming a partially swollen suspension, and it loses the dilutionability and fluidity required as a fracturing fluid.

[0071] Example 5:

[0072] This embodiment uses furfural as crosslinking agent A, wherein the ratio of chitosan amino groups to furfural aldehyde groups is 1:0.8.

[0073] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0074] Add 0.8 g of polyethylene oxide (Mw ≈ 3 million) and stir at 40°C for 3 hours.

[0075] Add 2.0 g of furfural and react at 50 °C for 2.5 hours.

[0076] Heat to 65°C, add 1.0 g of maleic anhydride, and react for 4 hours.

[0077] The final product obtained after the reaction has the following characteristics: the resulting base liquid has both high initial viscoelasticity and good dynamic properties.

[0078] Examples 6-10 below were all conducted using furfural and glutaraldehyde as aldehyde crosslinking agents A, with a chitosan amino group to furfural aldehyde group ratio of 1:0.8.

[0079] Example 6:

[0080] In this embodiment, a Schiff base network reaction is used at low temperature and for a short time.

[0081] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0082] Add 0.8 g of polyethylene oxide (Mw ≈ 3 million) and stir at 40°C for 3 hours.

[0083] Add 1.5 g of furfural and 0.5 g of glutaraldehyde, and react at 5 °C for 0.5 hours. The viscosity of the system increases slightly, forming a pale yellow, thin gel prepolymer.

[0084] Heat to 65°C, add 1.0 g of maleic anhydride, and react for 4 hours.

[0085] The final product obtained after the reaction has the characteristics of being a soft gel that is easy to pump.

[0086] Example 7:

[0087] This embodiment uses a Schiff base network reaction with medium temperature and time, and a DA network reaction with high temperature and long time.

[0088] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0089] Add 0.8 g of polyethylene oxide (Mw ≈ 3 million) and stir at 40°C for 3 hours.

[0090] Add 1.5 g of furfural and 0.5 g of glutaraldehyde, and react at 30 °C for 2 hours. The viscosity of the system increases significantly, forming a uniform elastic prepolymer.

[0091] Heat to 80°C, add 1.0 g of maleic anhydride, and react for 6 hours.

[0092] The final product obtained after the reaction has the characteristics of forming a dense and tough gel with a slippery surface.

[0093] Example 8:

[0094] This embodiment employs a Schiff base network reaction at high temperature and short duration, and a DA network reaction at medium temperature and short duration.

[0095] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0096] Add 0.8 g of polyethylene oxide (Mw ≈ 3 million) and stir at 40°C for 3 hours.

[0097] Add 1.5 g of furfural and 0.5 g of glutaraldehyde, and react at 60 °C for 1 hour. The system rapidly gels, forming a pale yellow elastic prepolymer.

[0098] Maintain the temperature at 60°C, add 1.0 g of maleic anhydride, and react for 1 hour.

[0099] The final product obtained after the reaction has the characteristic of forming a dense gel.

[0100] Example 9:

[0101] This embodiment employs a Schiff base network reaction with ultra-high temperature and ultra-long duration, and a DA network reaction with ultra-high temperature and ultra-long duration.

[0102] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0103] Add 0.8 g of polyethylene oxide (Mw ≈ 3 million) and stir at 40°C for 3 hours.

[0104] Add 1.5 g of furfural and 0.5 g of glutaraldehyde, and react at 70 °C for 5 hours. The system rapidly thickens and then separates into layers: the upper layer is a pale yellow clear liquid, and the lower layer is a dense, hard mass.

[0105] The temperature was raised to 90°C, and 1.0 g of maleic anhydride was added. The reaction was allowed to proceed for 7 hours. The lower hard layer showed no significant change, and the upper clear liquid did not gel.

[0106] The final product obtained after the reaction has the following characteristics: severe stratification, and it is impossible to form a uniform gel.

[0107] Example 10:

[0108] This embodiment employs a Schiff base network reaction at ultra-low temperature and ultra-short time, and a DA network reaction at ultra-low temperature and ultra-short time.

[0109] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0110] Add 0.8 g of polyethylene oxide (Mw ≈ 3 million) and stir at 40°C for 3 hours.

[0111] Add 1.5 g of furfural and 0.5 g of glutaraldehyde, and react at 2 °C for 0.3 hours. The viscosity of the system did not change significantly and remained a low-viscosity solution.

[0112] The temperature was raised to 35°C, and 1.0 g of maleic anhydride was added. The reaction was allowed to proceed for 0.8 hours. The viscosity of the system increased slightly, but it remained inelastic.

[0113] The final product obtained after the reaction is a low-viscosity solution.

[0114] Comparative Example 1:

[0115] This comparative example was conducted without the addition of polyethylene oxide and under conditions with a high proportion of glutaraldehyde.

[0116] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution.

[0117] Without adding polyethylene oxide, 4.0 g of glutaraldehyde was directly added dropwise to the above solution. The amount of aldehyde crosslinking agent used was twice that in Example 1 of this invention.

[0118] The mixture was stirred in a 50°C constant temperature water bath for 2.5 hours and then allowed to cool naturally to room temperature to obtain a hard gel.

[0119] The final product obtained after the reaction has the following characteristics: it is a hard gel with poor flowability and requires vigorous stirring.

[0120] Comparative Example 2:

[0121] This comparative example does not include linear polymer C, but uses a high-affinity diene crosslinking agent B.

[0122] Weigh 3.0 g of chitosan and dissolve it in 150 mL of 1.0 wt% acetic acid solution. Add 2.0 g of furfural and react at 50 °C for 2.5 hours to form a prepolymer containing a furan ring.

[0123] Add 1.5 grams of maleic anhydride to the prepolymer.

[0124] The reaction was carried out at 65°C for 4 hours to obtain a dense gel.

[0125] The final product obtained after the reaction is a dense, rigid gel with poor pumpability.

[0126] Comparative Example 3:

[0127] This comparative example does not use biomass-free chitosan and increases the proportion of linear polymer C used.

[0128] Weigh 0.8 g of polyethylene oxide (Mw≈1 million) and dissolve it in 150 mL of 1.0 wt% acetic acid solution. Stir at 40°C for 3 hours.

[0129] Without adding chitosan, adding 4.0 grams of furfural results in an aldehyde crosslinking agent dosage that is twice that of Example 1 of this invention.

[0130] Add 1.0 g of maleic anhydride and react at 65 °C for 4 hours to obtain the synthetic polymer gel.

[0131] The final product obtained after the reaction has the following characteristics: it is a thick gel with slow flowability.

[0132] Comparative Example 4:

[0133] This comparative example does not use aldehyde crosslinking agent A and dienophilic crosslinking agent B, but only uses a high proportion of linear polymer C.

[0134] Weigh 3.0 g of chitosan and dissolve it in 150 ml of 1.0 wt% acetic acid solution.

[0135] Furfural and maleic anhydride are not added, thus lacking the dual dynamic covalent network, but 4.0 g of polyethylene oxide is added.

[0136] Stirring at room temperature for 3 hours yields an aqueous polymer solution.

[0137] The final product obtained after the reaction is characterized as a viscous aqueous solution with no gelation.

[0138] To further verify the feedback from the obtained products in actual production, the fracturing fluid base fluids obtained in the examples and comparative examples were diluted with standard hard water to a concentration of 0.1% (w / v) for performance evaluation. The concentration ratio of standard hard water added for dilution was adjusted according to actual usage needs to produce different effects.

[0139] The testing environment includes:

[0140] (1) Drag reduction performance test: In the indoor annular friction test circuit, the flow velocity is 3m / s.

[0141] (2) Self-healing performance test: Apply a high shear rate (1000 s⁻¹) to the sample for 5 minutes. After shearing is stopped, allow the sample to stand for 15 minutes and then test it. Calculate the ratio of viscosity to initial viscosity.

[0142] (3) Slurry suspension test: Take 400 mL of solution and place it in a graduated cylinder. Add 80 mL of 20-40 mesh quartz sand, shake well and let stand. After 1.5 hours, observe the proportion of proppant sedimentation volume and calculate the sedimentation rate. The lower the sedimentation rate, the better the slurry suspension performance.

[0143] (4) Degradation test: The fracturing fluid gel was placed in simulated formation water rich in minerals at 60°C, and samples were taken after 30 days to calculate the mass loss rate.

[0144] The performance of the products obtained in the above experiments was summarized and compared. The performance comparison test results are shown in Table 1 below.

[0145] Table 1: Comparison of the performance of each experimental product

[0146]

[0147] By comparing the raw material ratios and performance data of the examples and comparative examples, the existing highly crosslinked system (taking Comparative Example 1 as an example) requires the addition of 4.0g of glutaraldehyde as a chemical crosslinking agent to ensure the basic network strength; while Example 1 of the technical solution of the present invention only adds 1.5g of furfural and 0.5g of glutaraldehyde as aldehyde crosslinking agent A, reducing the amount by 50%. Example 1 introduces 0.8g of polyethylene oxide, whose linear molecular chains are physically interwoven in a dual dynamic covalent network, forming reversible physical entanglement points. These entanglement points replace approximately 25% of the chemical crosslinking points in the conventional system. Comparative Example 1 uses twice the amount of crosslinking agent as Example 1, but its suspended sedimentation rate is 8%, while that of Example 1 is less than 10%, indicating that their suspended sedimentation capabilities are essentially the same. Example 1 is superior to Comparative Example 1 in terms of drag reduction, viscosity recovery, and suspended sedimentation performance. Therefore, by replacing some chemical crosslinking points with the physical entanglement of polyethylene oxide, this invention can maintain the same suspended sedimentation capability as the conventional highly crosslinked system even with a 50% reduction in the amount of aldehyde crosslinking agent A, and it also exhibits better flowability, drag reduction, and self-healing properties.

[0148] In Example 3, the molar ratio of chitosan amino to furfural aldehyde was 1:0.4, which is lower than the optimal range of 1:(0.6-0.8) specified in this invention. The amount of aldehyde crosslinking agent A used was only 40% of that in Example 1. Due to the insufficient number of chemical crosslinking points, the physical entanglement of polyethylene oxide could not fully compensate for the defects, resulting in a suspended sedimentation rate greater than 40% and a drag reduction rate of only 58%, leading to a significant performance degradation. Therefore, when the molar ratio of aldehyde crosslinking agent A is controlled within the range of 1:(0.6-0.8), the physical entanglement of polyethylene oxide can better synergize with the chemical crosslinking network, achieving the dual goals of "low crosslinking agent usage + high performance".

[0149] The data from the complete embodiments clearly show that only when the molar ratio of Embodiments 1 and 5 of the present invention falls within the optimal range of 1:0.6-0.8 can the comprehensive optimal performance of drag reduction rate greater than 70%, viscosity recovery rate greater than or equal to 75%, suspended sedimentation rate less than or equal to 10%, and 30-day degradation rate greater than or equal to 50% be achieved. In contrast, Comparative Examples 1-4 either rely on a high crosslinking agent ratio to ensure a single performance or have incomplete performance due to a low crosslinking ratio. The comparison between Embodiments 1 and 4 directly proves that the conventional high crosslinking approach will lead to system failure. The present invention achieves high performance through triple network synergy and low crosslinking ratio.

[0150] When the reaction temperature is between 5℃ and 60℃ and the reaction time is between 0.5 and 4 hours, the chitosan amino group and the aldehyde crosslinking agent A can undergo a complete and controllable Schiff base reaction. The reaction can form a uniform prepolymer network containing furan rings, while avoiding the ring-opening degradation of furan rings and the breaking of chitosan glycosidic bonds caused by excessively high temperatures, as well as the insufficient crosslinking caused by excessively low temperatures and short reaction times. The products of Examples 6 (5℃, 0.5h), 7 (30℃, 2h), and 8 (60℃, 1h) all formed effective dynamic networks, and their core properties such as drag reduction rate, viscosity recovery rate, and suspended sand settling rate all met the basic requirements for fracturing fluid use, confirming the effectiveness of this parameter range. When the parameters are outside this range, Example 9 (70℃, 5h) shows system stratification and network degradation failure, and Example 10 (3℃, 0.3h) fails to form an effective prepolymer, directly proving the necessity of the upper and lower limits of the parameters.

[0151] The temperature range of 40℃-80℃ and the time range of 1-6h ensure efficient DA reaction between the dienophilic crosslinking agent B and the furan ring, forming a second dynamic covalent network interpenetrating with the Schiff base network. Within this parameter range, the reaction can promote the DA cycloaddition reaction through heating, while avoiding the reversible dissociation of DA bonds caused by temperatures exceeding 80℃ and the problem of insufficient network interpenetration when the time is less than 6h. The product performance data of Examples 6-8 show that a balance between network strength and dynamic responsiveness can be achieved within this parameter range. However, the excessive temperature and time in Example 9 (90℃, 7h) caused DA bond dissociation and network collapse, and the low temperature and short time in Example 10 (35℃, 0.8h) failed to complete effective crosslinking, further verifying the irreplaceable nature of this parameter range.

[0152] The performance data of Examples 6-8 were all lower than the optimal parameter combination, confirming that this parameter is the best choice to balance reaction efficiency, network performance, and environmental friendliness. It also demonstrates that the parameter range of reaction conditions can achieve the technical goal of "adjustable performance," making it suitable for fracturing operations at different reservoir temperatures. By adjusting the reaction conditions (temperature and time), the rheological behavior of the network can be finely controlled to meet different pumping requirements, avoiding the loss of fluidity caused by traditional highly cross-linked systems.

[0153] The linear chain physical interpenetration of polyethylene oxide is the main factor that enables the system to obtain excellent fluidity, high drag reduction rate and pumpability. Without polyethylene oxide, a simple dual chemical network would form a solid with extremely poor fluidity, which cannot meet the requirements of fracturing fluid construction.

[0154] This invention uses a mixture of glutaraldehyde and furfural. Glutaraldehyde rapidly forms a denser Schiff base network, while furfural introduces furan rings for dynamic crosslinking with DA (dimethylaminopropionate). This further enhances the initial mechanical strength and sand-carrying capacity of the material, making it suitable for fracturing slugs with higher sand-carrying requirements, while still maintaining the core characteristic of a low crosslinking agent ratio.

[0155] The above description is a further detailed explanation of the present invention in conjunction with specific preferred embodiments. For those skilled in the art, several simple deductions or substitutions can be made without departing from the present invention, and all such deductions or substitutions should be considered as falling within the scope of patent protection determined by the submitted claims.

Claims

1. A multi-network fracturing fluid based on biomass and polyethylene oxide, characterized in that, include: Biomass polysaccharides, aldehyde crosslinking agent A, dienophilic crosslinking agent B, linear polymer C, and solvent together form two dynamic covalent networks and one physically entangled network: The aldehyde crosslinking agent A reacts with the amino groups of chitosan to form a first-order pH / temperature-responsive dynamic covalent network. The aldehyde crosslinking agent A is furfural and glutaraldehyde. The aldehyde crosslinking agent A reacts with the amino groups of chitosan to form a furan cyclic diene, wherein the molar ratio of chitosan amino groups to furfural aldehyde groups is 1:(0.6-0.8). The dienophilic crosslinking agent B and the furan cyclodiene form a second thermally responsive dynamic covalent network through a Diels-Alder reaction; The molecular chains of the linear polymer C are physically interwoven within the dual dynamic covalent bond network, forming a third layer of physically entangled network with enhanced fluidity. The first and second networks interpenetrate through covalent bonds to form a dual dynamic covalent bond multi-network.

2. The fracturing fluid according to claim 1, characterized in that: The dienophilic crosslinking agent B is maleic anhydride and / or maleic anhydride derivatives, and the linear polymer C is polyethylene oxide.

3. The fracturing fluid according to claim 2, characterized in that: The fracturing fluid comprises, by weight, the following parts: 1-8 parts chitosan, 0.5-5 parts aldehyde crosslinking agent A, 0.2-3 parts dienophilic crosslinking agent B, 0.05-3 parts polyethylene oxide, and 0.5-3 parts acidic solvent.

4. The fracturing fluid according to claim 3, characterized in that: The acidic solvent is an aqueous solution of acetic acid with a concentration of 0.5%-5%.

5. The fracturing fluid according to claim 2, characterized in that: The polyethylene oxide has a weight-average molecular weight of 500,000 to 8,000,000, and is produced through molecular entanglement and a dual dynamic covalent network.

6. The method for preparing fracturing fluid based on a multi-network of biomass and polyethylene oxide according to claim 2, characterized in that: Includes the following steps: S1: Dissolve chitosan in a diluted acidic aqueous solution to prepare a homogeneous solution; S2: Add polyethylene oxide to the solution from step S1 and stir until fully dispersed or dissolved to obtain a mixed base solution; S3: Add aldehyde crosslinking agent A to the mixed base liquid to cause chitosan to undergo a Schiff base reaction with aldehyde crosslinking agent A to form a dynamic crosslinked prepolymer network containing furan rings. The reaction conditions are controlled to ensure that the molar ratio of chitosan amino groups to furfural aldehyde groups is in the range of 1:(0.6-0.8). S4: Add dienophilic crosslinking agent B to the reaction system of step S3, so that the dienophilic crosslinking agent B and the furan cyclodienes in the network undergo a Diels-Alder reaction, and finally form a fracturing fluid base fluid with dual dynamic covalent bonds containing a polyoxyethylene physically interwoven network.

7. The method for preparing fracturing fluid according to claim 6, characterized in that: In step S3, the reaction temperature is 5℃-60℃ and the reaction time is 0.5-4 hours to control the dynamic equilibrium and network structure of the Schiff base reaction. In step S4, the reaction temperature is 40℃-80℃ and the reaction continues for 1-6 hours to control the rate and depth of the Diels-Alder reaction, thereby regulating the rheological properties and self-healing characteristics of the entire multi-network.

8. The fracturing fluid according to any one of claims 1-5, characterized in that: The fracturing fluid can be used directly or diluted with water to the target concentration and used as a slickwater fracturing fluid.

9. The fracturing fluid according to any one of claims 1-5, characterized in that: The effective network structure material in the diluted working solution has a mass concentration of 0.01%-0.5%.

10. The application of the fracturing fluid according to any one of claims 1-5 in hydraulic fracturing of shale gas or tight oil and gas reservoirs.

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