Nano lignin cross-linking agent for high-temperature-resistant fracturing fluid as well as preparation method and application of nano lignin cross-linking agent

Abstract

The invention discloses a nano lignin cross-linking agent for a high-temperature-resistant fracturing fluid, and the nano lignin cross-linking agent comprises the following components in parts by mass: 0.5-10 parts of nano lignin; 0.5 to 20 parts of a polymer cross-linking agent; the nano lignin cross-linking agent for the high-temperature-resistant fracturing fluid is suitable for the technical field of oil and gas field development, the preparation process is simple, lignin and polyethyleneimine in the raw materials are environment-friendly materials and are more energy-saving and environment-friendly, the influence on the environment is reduced, the demand quantity of a cross-linking agent system for nano materials is small, and the nano lignin cross-linking agent for the high-temperature-resistant fracturing fluid is more environment-friendly. When the fracturing fluid is prepared, the used guanidine gum is low in concentration, and the dosage of the cross-linking agent is small, so that the friction resistance and the cost of the fracturing fluid are reduced.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of oil and gas field development, and particularly relates to a nano-lignin crosslinking agent for high-temperature fracturing fluid and a preparation method and application thereof. BACKGROUND

[0002] As a main technical means for reservoir reconstruction, fracturing has become a basic measure for improving the productivity of oil and gas wells of various types of oil reservoirs at home and abroad. As a main medium for energy transmission in the fracturing process, the performance of fracturing fluid directly affects the productivity of oil and gas wells after the measures. Especially in the high-temperature reservoir environment, new challenges are put forward for the fracturing fluid system. In recent years, modified nano-crosslinking agent materials have achieved good results in improving the temperature resistance of fracturing fluid crosslinking agents.

[0003] CN 105754578 A provides a preparation method of a nano-titanium crosslinking agent: isopropyl alcohol, glycerol and triethanolamine are added into a reactor in a mass ratio of 1:2:(2.5-3) and uniformly mixed, and 3.5-4 parts of titanium ester are added, and an organic titanium is obtained by stirring and reacting at 75-80 DEG C for 2-3 h; wherein the titanium ester is selected from one of tetrabutyl titanate, tetraethyl titanate, isopropyl titanate, diisooctyl titanate and tetra-t-butyl titanate; then, nano-particle materials are added into a dispersant, ultrasonic dispersion is performed for 30-50 min at room temperature by using an ultrasonic cleaning instrument, then the organic titanium is added, and ultrasonic dispersion is continuously performed for 30-50 min, and the nano-titanium crosslinking agent is obtained by drying at 50-90 DEG C for 5 h. The use method is as follows: the nano-titanium crosslinking agent is mixed with a guanidine gum aqueous solution to prepare a fracturing fluid or a profile control water plugging agent; or the nano-titanium crosslinking agent is mixed with a partially hydrolyzed polyacrylamide aqueous solution to prepare a fracturing fluid or a profile control water plugging agent. The gel fracturing fluid formed by the nano-titanium crosslinking agent can be rapidly broken after the addition of a gel breaker, and has the advantages of low residue, small formation damage and the like.

[0004] CN 112251204 A provides a preparation method of a nano-cellulose crosslinking agent for high-temperature fracturing fluid: nano-cellulose is surface modified by using a silane coupling agent to prepare silane-modified nano-cellulose powder; the silane-modified nano-cellulose powder is added into a transparent solution formed by an inorganic boron compound (boric acid, sodium tetraborate, potassium tetraborate or zinc boric acid), isopropyl alcohol and ethylene glycol in a mass ratio of 1:(1-5):(1-5), and a nano-cellulose crosslinking agent is prepared by reacting at a temperature of 70-85 DEG C, and the mass ratio of the silane-modified nano-particle and the transparent solution is 1:(3-6). The method for using the prepared nano-cellulose crosslinking agent to crosslink a guanidine gum and a derivative thereof fracturing fluid is as follows: a guanidine gum solution with a certain concentration is prepared and is completely swelled; the swelled guanidine gum base solution is taken, the pH value of the solution is adjusted to 7-14, the nano-cellulose crosslinking agent is added, and the base solution is stirred until it can be hung; the formed fracturing fluid gel has good temperature resistance and shear resistance, and the temperature resistance can reach 166 DEG C.

[0005] The high-temperature resistant crosslinker systems prepared in the above methods each has characteristics, and can improve the temperature resistance of the fracturing fluid crosslinking system to a certain extent, but still has some deficiencies, for example, the preparation process is complex; the crosslinker grafted with nanomaterials is a non-environmentally friendly material, which may have a certain impact on the environment; and the crosslinker system has a large demand for nanomaterials, the concentration of guar gum or hydroxypropyl guar gum used when the fracturing fluid is prepared is high, the amount of crosslinker used is large, and the application cost is high.

[0006] Lignin as a new environmentally friendly nanomaterial is an organic flocculent fibrous material obtained by chemical treatment and mechanical processing of natural renewable wood; it is non-toxic, odorless, non-polluting and non-radioactive, and has excellent flexibility and dispersibility. Lignin is insoluble in water, weak acid and alkaline solution, and has a neutral pH value, which can improve the corrosion resistance. Lignin has a small specific gravity and a large specific surface area, and has very good application prospects. Polyethyleneimine as a high-temperature resistant crosslinker material is also an environmentally friendly material of food contact grade, and has very broad application prospects in the current situation that countries and regions pay more and more attention to environmental protection and people's environmental protection awareness is getting stronger and stronger.

[0007] Therefore, there is an urgent need for a nanolignin grafted branched low molecular weight polyethyleneimine crosslinker system. SUMMARY

[0008] The purpose of the present application is to overcome the defects of the prior art, and to provide a nanolignin crosslinker for high-temperature resistant fracturing fluid and a preparation method and application thereof.

[0009] To achieve the above-mentioned purpose, the technical scheme adopted by the present application is as follows:

[0010] In a first aspect, a nanolignin crosslinker for high-temperature resistant fracturing fluid comprises the following components by mass fraction:

[0011] 0.5-10 parts of nanolignin;

[0012] 0.5-20 parts of a polymeric crosslinker;

[0013] 70-99 parts of a dispersant.

[0014] Preferably, the nanolignin is an organic flocculent fibrous material obtained by chemical treatment and mechanical processing of natural renewable wood.

[0015] Preferably, the nanolignin has a rod shape, a diameter in the range of 10-50 nm, and a length in the range of 200-500 nm.

[0016] Preferably, the polymeric crosslinker comprises polyethyleneimine, silane coupling agent, polyamide, polyurethane and polyethylene glycol dimethacrylate.

[0017] Preferably, the dispersant includes a surfactant type dispersant, a polymer type dispersant, an organic solvent type dispersant, and an aqueous dispersant.

[0018] In a second aspect, a method for preparing a nano-lignin crosslinking agent for high-temperature resistant fracturing fluid includes the following steps:

[0019] S1: adding a formula amount of nano-lignin into concentrated H2SO4, after a period of reaction, pouring the synthesized mixture into deionized water to form a suspension, removing the supernatant of the suspension by centrifugation, dialyzing the remaining part with water to pH≈7, continuing to stir for a period of time, and then evaporating the solvent to obtain a hydroxylated nano-lignin powder;

[0020] S2: adding the nano-lignin powder obtained in step 1 into a mixed solution of a formula amount of dispersant and a formula amount of polymer crosslinking agent to perform a grafting reaction, thereby obtaining a nano-lignin crosslinking agent.

[0021] Preferably, in the step S1, the concentration of the concentrated H2SO4 is 80% to 95%.

[0022] Preferably, in the step S1, the reaction time is 1 to 2 hours, and the reaction temperature is 45 to 60℃.

[0023] Preferably, in the step S1, the centrifugal speed is 7000 to 9000 revolutions per minute, and the centrifugal time is 5 to 15 minutes.

[0024] Preferably, in the step S1, the continuous stirring time is 10 to 45 minutes.

[0025] Preferably, in the step S2, the reaction temperature of the grafting reaction is 20 to 80℃, and the reaction time is 4 to 24 hours.

[0026] Preferably, in the step S2, the stirring is continuously performed during the grafting reaction, and the stirring rate is 200 to 800 revolutions per minute.

[0027] In a third aspect, the nano-lignin crosslinking agent for high-temperature resistant fracturing fluid is used to form a fracturing fluid system with a guar gum solution, wherein the amount of the nano-lignin crosslinking agent is 0.1 to 0.3 wt%, and the concentration of the hydroxypropyl guar gum solution is 0.1 to 1.0%, thereby preparing a guar gum fracturing fluid.

[0028] In a fourth aspect, the nanolignin crosslinking agent for high-temperature-resistant fracturing fluid is used to form a gel system for profile control and displacement with partially hydrolyzed polyacrylamide, wherein the partially hydrolyzed polyacrylamide has a molecular weight of 12 million and a hydrolysis degree of 25-30%; the nanolignin crosslinking agent accounts for 0.05-0.3 wt.%, the concentration of the partially hydrolyzed polyacrylamide solution is 0.1-0.5%, and the gel system for profile control and displacement is prepared.

[0029] In summary, due to the adoption of the technical solutions described above, the present application has the following beneficial effects:

[0030] (1) The preparation process is simple, and the lignin and polyethyleneimine in the raw materials are both environmentally friendly materials, which are more energy-saving and environmentally friendly and reduce the impact on the environment.

[0031] (2) The nanolignin crosslinking agent formed by the present application can be successfully crosslinked with the fracturing fluid formed by the guanidium gel at a temperature exceeding 160℃, forming a gel that can be completely hung; and the crosslinking time can be controlled by adjusting the preparation conditions of the nanolignin crosslinking agent and the amount of the crosslinking agent.

[0032] (3) The crosslinking agent system of the present application requires less nanomaterials, and the fracturing fluid prepared using the crosslinking agent system has a low guanidium gel concentration and a small amount of crosslinking agent, which are conducive to reducing the friction of the fracturing fluid and the cost of the fracturing fluid.

[0033] (4) Under the same concentration, the nanolignin crosslinking agent formed by the nanolignin and polyethyleneimine can increase the gel strength of the gel system for profile control and displacement by more than 25%, and can significantly improve the shear resistance of the gel. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 is a schematic diagram of the nanolignin crosslinking agent in the embodiments of the present application;

[0035] Figure 2 is a schematic diagram of the gel formation state of the guanidium gel fracturing fluid gel in the embodiments of the present application;

[0036] Figure 3 is a schematic diagram of the gel formation process of the gel system for profile control and displacement of the present application;

[0037] Figure 4 is a comparison of the viscoelasticity data of the gel system formed by the crosslinking agent and the conventional crosslinking agent. DETAILED DESCRIPTION

[0038] The following embodiments are further described to illustrate the specific implementation modes of the nanolignin crosslinking agent for high-temperature-resistant fracturing fluid, the preparation method and application thereof. The nanolignin crosslinking agent for high-temperature-resistant fracturing fluid, the preparation method and application thereof are not limited to the description of the following embodiments.

[0039] Example 1:

[0040] A preparation method of a nano-lignin crosslinking agent for high-temperature resistant fracturing fluid, comprising the following steps:

[0041] S1 add 0.5 parts of nano-lignin to concentrated H2SO4 with a concentration of 80%, react at 45°C for 1h, then pour the synthesized mixture into deionized water to form a suspension, centrifuge the supernatant of the suspension at 7000r / min for 5min, repeatedly dialyze the remaining part with water to pH≈7, continue stirring for 10min, and evaporate the solvent to obtain hydroxylated nano-lignin powder.

[0042] S2 add the hydroxylated nano-lignin powder to a mixed solution of 70 parts of dispersant and 0.5 parts of polymer crosslinking agent for grafting reaction (reaction temperature is 20°C, stirring rate is 200r / min, and reaction time is 4h), to obtain a nano-lignin crosslinking agent.

[0043] Further, the nano-lignin is an organic flocculent fibrous material obtained by chemical treatment and mechanical processing of natural renewable wood.

[0044] Further, the nano-lignin is rod-shaped, with a diameter ranging from 10nm and a length ranging from 200nm.

[0045] Further, the polymer crosslinking agent includes polyethyleneimine and silane coupling agent.

[0046] Further, the dispersant includes surfactant type dispersant and polymer type dispersant.

[0047] Example 2:

[0048] A preparation method of a nano-lignin crosslinking agent for high-temperature resistant fracturing fluid, comprising the following steps:

[0049] S1 add 5 parts of nano-lignin to concentrated H2SO4 with a concentration of 90%, react at 55°C for 1.5h, then pour the synthesized mixture into deionized water to form a suspension, centrifuge the supernatant of the suspension at 8000r / min for 10min, repeatedly dialyze the remaining part with water to pH≈7, continue stirring for 30min, and evaporate the solvent to obtain hydroxylated nano-lignin powder.

[0050] S2 add the hydroxylated nano-lignin powder to a mixed solution of 88 parts of dispersant and 10 parts of polymer crosslinking agent for grafting reaction (reaction temperature is 50°C, stirring rate is 500r / min, and reaction time is 14h), to obtain a nano-lignin crosslinking agent.

[0051] Further, the nano-lignin is an organic flocculent fibrous material obtained by chemical treatment and mechanical processing of natural renewable wood.

[0052] Further, the nano-lignin is in the shape of a rod, with a diameter ranging from 30 nm and a length ranging from 400 nm.

[0053] Further, the polymer crosslinking agent includes polyethyleneimine, silane coupling agent, polyamide, polyurethane, and polyethylene glycol dimethacrylate.

[0054] Further, the dispersant includes a surfactant type dispersant, a polymer type dispersant, an organic solvent type dispersant, and an aqueous dispersant.

[0055] Example 3:

[0056] A method for preparing a nano-lignin crosslinking agent for a high-temperature resistant fracturing fluid includes the following steps:

[0057] S1: 10 parts of nano-lignin are added to concentrated H2SO4 with a concentration of 95%, and reacted at 60°C for 2 hours. Then, the synthesized mixture is poured into deionized water to form a suspension. The supernatant of the suspension is removed by centrifugation at 9000 rpm for 15 minutes. The remaining part is repeatedly dialyzed with water until pH≈7. After continuous stirring for 45 minutes, the solvent is evaporated to obtain a hydroxylated nano-lignin powder.

[0058] S2: The hydroxylated nano-lignin powder is added to a mixed solution of 99 parts of a dispersant and 20 parts of a polymer crosslinking agent for grafting reaction (reaction temperature is 80°C, stirring rate is 800 rpm, and reaction time is 24 hours), to obtain a nano-lignin crosslinking agent.

[0059] Further, the nano-lignin is an organic flocculent fibrous material obtained by chemical treatment and mechanical processing of natural renewable wood.

[0060] Further, the nano-lignin is in the shape of a rod, with a diameter ranging from 50 nm and a length ranging from 500 nm.

[0061] Further, the polymer crosslinking agent includes polyethyleneimine, silane coupling agent, polyamide, polyurethane, and polyethylene glycol dimethacrylate.

[0062] The polyethyleneimine is branched low molecular weight polyethyleneimine, which can be crosslinked with the amide group in the polyacrylamide or guanidium glue and other polymers through transamidation reaction; the silane coupling agent can form siloxane bond with the hydroxyl group in the polyacrylamide or guanidium glue and other polymers, thereby establishing cross-linking connection between molecules; the polyamide is a kind of polymer with amide group, which can react with the corresponding functional groups in the polyacrylamide or guanidium glue to form cross-linking structure; the polyurethane is a kind of multifunctional polymer, which can react with the active functional groups (such as hydroxyl, amine group, etc.) in the polyacrylamide or guanidium glue to form cross-linking network; the polyethylene glycol dimethacrylate is a kind of bifunctional polymer, which can react with the hydroxyl group in the polyacrylamide and guanidium glue and other polymers to form cross-linking network.

[0063] Further, the dispersant includes a surfactant type dispersant, a polymer type dispersant, an organic solvent type dispersant, and a water-based dispersant.

[0064] The surfactant type dispersant, such as dodecyl polyoxyethylene ether, sodium dodecyl sulfate, dodecyl trimethyl ammonium bromide, etc., the surfactant can form an adsorption layer on the surface of the nanolignin material, improve its compatibility with the solvent, and reduce the agglomeration and deposition; the polymer type dispersant, such as polyvinyl alcohol, polyethylene glycol, etc., which can interact with the surface of the nanolignin material to form a protective film, preventing the agglomeration and deposition of the nanolignin material; the organic solvent type dispersant, such as ethanol, isopropyl alcohol, acetone, cyclohexane, dichloromethane, etc., which has good solubility and dispersion performance; the water-based dispersant, such as water-soluble polymer, carboxylated polyethylene glycol, etc.; suitable for water-based system, which can be used for dispersing nanolignin.

[0065] Example 4:

[0066] A preparation method of a nanolignin crosslinking agent for high-temperature resistant fracturing fluid, comprising the following steps:

[0067] 10 parts of nanolignin was added to 90 parts of concentrated H2SO4 with a concentration of 64 wt.%, and reacted at 45℃ for 1.5h, then the synthesized mixture was poured into 1000mL deionized water to form a suspension, the supernatant of the suspension was removed by centrifugation at 8000r / min for 10min, and the remaining part was repeatedly dialyzed with water until pH≈7, and the hydroxylated nanolignin was obtained in the water suspension. Continue to stir to evaporate the solvent to obtain hydroxylated nanolignin powder.

[0068] 15 parts of hydroxylated nanolignin powder was added to a mixed solution of 75 parts of dispersant and 10 parts of branched polyethyleneimine, and a grafting reaction was carried out at 60℃, 600r / min for 10h to obtain a nanolignin crosslinking agent.

[0069] Further, the nano-lignin is rod-shaped, with a diameter of 10 nm and a length of 200 nm.

[0070] Further, the surface of the lignin is treated by hydroxylation with sulfuric acid to obtain a mixed solution.

[0071] Further, the nano-lignin is an organic flocculent fiber material obtained by chemical treatment and mechanical processing of natural renewable wood.

[0072] Further, the polyethyleneimine is a branched low molecular weight polyethyleneimine.

[0073] Further, the polyethyleneimine is a B-PEI.

[0074] Further, the dispersant is cyclohexane and dichloromethane.

[0075] Example 5:

[0076] In the grafting reaction, 15 parts of the hydroxylated nano-lignin powder in Example 4 is added to a mixed solution of 70 parts of the dispersant and 15 parts of the silane coupling agent, and the reaction is carried out at 60°C and 600 rpm for 10 hours to obtain a nano-lignin crosslinking agent.

[0077] Further, the nano-lignin can form a silicon ether bond with the hydroxyl group in the polyacrylamide or guan gum, thereby establishing a crosslinking connection between molecules.

[0078] Further, the dispersant is ethanol.

[0079] Example 6:

[0080] A method for preparing a nano-lignin crosslinking agent for a high-temperature resistant fracturing fluid includes the following steps:

[0081] In the grafting reaction, 15 parts of the hydroxylated nano-lignin powder in Example 4 is added to a mixed solution of 80 parts of the dispersant and 5 parts of the polyamide, and the reaction is carried out at 80°C and 600 rpm for 10 hours to obtain a nano-lignin crosslinking agent.

[0082] Further, the polyamide is a polymer with an amide group, which can undergo a condensation reaction with the corresponding functional groups in the polyacrylamide or guan gum to form a crosslinking structure.

[0083] Further, the dispersant is polyethylene glycol.

[0084] Example 7:

[0085] A method for preparing a nano-lignin crosslinking agent for a high-temperature resistant fracturing fluid includes the following steps:

[0086] The hydroxylated nanolignin powder in Example 4 was added to a mixed solution of 80 parts of dispersant and 5 parts of polyurethane, and a grafting reaction was carried out at 80°C and 600 rpm for 10 h to obtain a nanolignin crosslinking agent.

[0087] Further, the polyurethane is a multifunctional polymer that can react with active functional groups (such as hydroxyl groups, amine groups, etc.) in polyacrylamide or guanidine gum to form a crosslinked network.

[0088] Further, the dispersant is sodium dodecyl sulfate.

[0089] Example 8:

[0090] A method for preparing a nanolignin crosslinking agent for a high-temperature resistant fracturing fluid, comprising the following steps:

[0091] The hydroxylated nanolignin powder in Example 4 was added to a mixed solution of 75 parts of dispersant and 10 parts of polyethylene glycol dimethacrylate, and a grafting reaction was carried out at 60°C and 600 rpm for 10 h to obtain a nanolignin crosslinking agent.

[0092] Further, the polyethylene glycol dimethacrylate is a bifunctional polymer that can undergo radical polymerization with hydroxyl groups in polymers such as polyacrylamide and guanidine gum to form a crosslinked network.

[0093] Further, the dispersant is sodium dodecyl sulfate.

[0094] Example 9:

[0095] 5 g of nanolignin was added to 100 mL of concentrated H2SO4 with a concentration of 64 wt.%, and reacted at 45°C for 1.5 h, then the synthesized mixture was poured into 1000 mL of deionized water to form a suspension. The supernatant of the suspension was removed by centrifugation at 8000 rpm for 10 min. The remaining part was repeatedly dialyzed with deionized water until pH≈7, and hydroxylated nanolignin was obtained in an aqueous suspension. The solid content of the nanolignin was determined by drying and weighing method. 2 g of the modified nanolignin powder was added to a 20 mL transparent solution of polyethyleneimine with a molecular weight of 600 daltons and a dispersant cyclohexane, dichloromethane (2:8) to form a transparent solution, and reacted at 40-60°C to prepare a nanolignin crosslinking agent. The micro-morphology of the nanolignin crosslinking agent is as shown in Figure 1 .

[0096] Example 10:

[0097] The nanolignin crosslinking agent of Example 9 was used to form a fracturing fluid system with a guanidine gum solution. The amount of nanolignin crosslinking agent was 0.2 wt.%, the concentration of hydroxypropyl guanidine gum solution was 0.3%, and a guanidine gum fracturing fluid was prepared.

[0098] Under laboratory conditions, 500 mL of 0.3 wt.% hydroxypropyl guar gum solution was prepared and aged for 1 hour to make it fully swollen; 100 mL of the swollen guar gum base solution was taken and 0.2 g of the nanocellulose crosslinking agent prepared in Example 5 was added, and after stirring uniformly, it was placed in a constant temperature oven at 160 °C. After 120 s, the fracturing fluid could be hung, and thus a 0.3 wt.% guar gum fracturing fluid gel was obtained.

[0099] The gelation state of 0.2 wt.% nanocellulose crosslinking agent and 0.4 wt.% hydroxypropyl guar gum was as shown in Figure 2 (a); the gelation state of 0.2 wt.% conventional organic zirconium crosslinking agent and 0.5 wt.% hydroxypropyl guar gum was as shown in Figure 2 (b); and the gelation state of 0.3 wt.% conventional organic zirconium crosslinking agent and 0.4 wt.% hydroxypropyl guar gum was as shown in Figure 2 (c). There was no difference in the appearance of the guar gum fracturing fluid gels of the three components. It was confirmed that under the premise of forming the same crosslinked guar gum fracturing fluid with the same hanging state, the nanocellulose crosslinking agent could significantly reduce the use concentration of guar gum and crosslinking agent compared with the conventional crosslinking agent.

[0100] Example 11:

[0101] 5 g of nanolignin was added to 100 mL of concentrated H2SO4 with a concentration of 64 wt.% and reacted at 45 °C for 1.5 h, and then the synthesized mixture was poured into 1000 mL of deionized water to form a suspension. The supernatant of the suspension was removed by centrifugation at 8000 rpm for 10 min. The remaining part was repeatedly dialyzed with water until pH ≈ 7 to obtain hydroxylated nanolignin in water suspension. The solid content of the nanolignin was determined by drying and weighing method. 2 g of the modified nanolignin powder was added to a 20 mL transparent solution of polyethyleneimine with a molecular weight of 1800 and a dispersant cyclohexane, dichloromethane (4:6) to form a 20 mL transparent solution, and reacted at 40-60 °C to prepare a nanolignin crosslinking agent.

[0102] Example 12:

[0103] The nanolignin crosslinking agent of Example 11 was used to form a gel system for profile control and displacement with partially hydrolyzed polyacrylamide. The partially hydrolyzed polyacrylamide had a molecular weight of 12 million and a hydrolysis degree of 25-30%; the amount of nanolignin crosslinking agent was 0.2 wt.%, and the concentration of partially hydrolyzed polyacrylamide solution was 0.4 wt.% to prepare a gel system for profile control and displacement.

[0104] The gel system configuration method is as follows: under laboratory conditions, a 500 mL partially hydrolyzed polyacrylamide solution with a concentration of 3 wt.% is prepared and aged for 24 h; a nano-lignin crosslinking agent is prepared into a mother liquor with a concentration of 6 wt.%; 19 mL of deionized water is added to a 30 mL sample bottle, 3 mL of the nano-lignin crosslinking agent mother liquor is added thereto, stirred uniformly, 8 mL of the partially hydrolyzed polyacrylamide solution is added, and the mixture is fully stirred and placed in an 80°C constant-temperature oven to observe the gelation process and gelation effect. Gelation starts after 4-6 h, and the final gelation strength can reach H level (slight deformation: only the gel surface is slightly deformed under the action of inverted gravity) according to the coding method, as shown in Figure 3 .

[0105] Comparing the viscoelastic modulus of the mature gel formed by 0.2 wt.% nano-lignin crosslinking agent and 0.4 wt.% partially hydrolyzed polyacrylamide with the mature gel formed by 0.2 wt.% polyethyleneimine and 0.4 wt.% partially hydrolyzed polyacrylamide, it is found that under the same concentration, the nano-lignin crosslinking agent formed by nano-lignin and polyethyleneimine increases the strength (elastic modulus) of the mature gel from 4.5 Pa to 6.4 Pa, and significantly prolongs the linear viscoelastic range of the mature gel, that is, the shear resistance is enhanced, as shown in Figure 4 .

[0106] Therefore, compared with the existing system, the nano-lignin grafted polymer crosslinking agent system of the application has the following beneficial effects:

[0107] (1) The preparation process is simple, and the lignin and polyethyleneimine in the raw materials are both environmentally friendly materials, which are more energy-saving and environmentally friendly, and reduce the impact on the environment.

[0108] (2) The high-temperature-resistant nano-lignin crosslinking agent formed by the application can be successfully crosslinked with the guar gum fracturing fluid at more than 160°C to form a gel that can be completely hung; and the crosslinking time can be controlled by adjusting the preparation conditions of the nano-lignin crosslinking agent and the amount of the crosslinking agent.

[0109] (3) The crosslinking agent system of the application has a small demand for nano-materials, and the concentration of guar gum used when preparing the fracturing fluid is low (reduced by more than 20%), and the amount of the crosslinking agent is small (reduced by more than 30%), which is conducive to reducing the friction of the fracturing fluid and the cost of the fracturing fluid.

[0110] (4) Under the same concentration, the nano-lignin crosslinking agent formed by nano-lignin and polyethyleneimine increases the gelation strength (elastic modulus) of the gel system for profile control by more than 25%, and can significantly improve the shear resistance of the gel.

[0111] The above is further detailed description of the present application in combination with specific preferred embodiments, and cannot be deemed as limitation of the specific implementation of the present application to these descriptions. For those skilled in the art to which the present application belongs, without departing from the concept of the present application, a number of simple deductions or substitutions can be made, and all should be deemed as falling within the protection scope of the present application.

Claims

1. A nano-lignin crosslinking agent for high-temperature resistant fracturing fluid, characterized in that, By mass fraction, comprising the following components: 0.5-10 parts of nanolignin; 0.5-20 parts of polymer crosslinking agent; 70-99 parts of dispersant.

2. The nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 1, characterized in that: The nanolignin is an organic flocculent fibrous material obtained by chemical treatment and mechanical processing of natural renewable wood.

3. The nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 1, characterized in that: The nanolignin is rod-shaped, with a diameter ranging from 10 to 50 nm and a length ranging from 200 to 500 nm.

4. The nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 1, characterized in that: The polymer crosslinking agent includes polyethyleneimine, silane coupling agent, polyamide, polyurethane, and polyethylene glycol dimethacrylate.

5. The nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 1, characterized in that: The dispersant includes surfactant-type dispersant, polymer-type dispersant, organic solvent-type dispersant, and water-based dispersant.

6. A method for preparing a nano-lignin crosslinking agent for high-temperature resistant fracturing fluid, characterized in that, The method comprises the following steps: S1: adding the nanolignin in the formula amount to concentrated H2SO4, after a period of reaction, pouring the synthesized mixture into deionized water to form a suspension, removing the supernatant of the suspension by centrifugation, dialyzing the remaining part with water to pH≈7, continuing to stir for a period of time, and evaporating the solvent to obtain a hydroxylated nanolignin powder; S2: adding the nanolignin powder obtained in step 1 to a mixed solution of the dispersant and the polymer crosslinking agent in the formula amount to perform grafting reaction, obtaining a nanolignin crosslinking agent.

7. The method for preparing nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 6, characterized in that, In the step S1, the concentration of concentrated H2SO4 is 80%-95%.

8. The method for preparing nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 6, characterized in that, In the step S1, the reaction time is 1-2 hours, and the reaction temperature is 45-60℃.

9. The method for preparing nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 6, characterized in that, In the step S1, the centrifugal speed is 7000-9000 rpm, and the centrifugal time is 5-15 min.

10. The method for preparing nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 6, characterized in that, In the step S1, the continuous stirring time is 10-45 min.

11. The method for preparing nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 6, characterized in that, In the step S2, the grafting reaction temperature is 20-80℃, and the reaction time is 4-24 h.

12. The method for preparing nano-lignin crosslinking agent for high-temperature resistant fracturing fluid according to claim 6, characterized in that, In the step S2, stirring is continuously performed during the grafting reaction, and the stirring rate is 200-800 rpm.

13. A nano-lignin crosslinker for use in a high-temperature fracturing fluid as claimed in any one of claims 1 to 5, the nano-lignin crosslinker for use in forming a fracturing fluid system with a guan gum solution, wherein, The nanolignin crosslinking agent is used in an amount of 0.1-0.3 wt.%, the hydroxypropyl guanidine gum solution has a concentration of 0.1-1.0%, and a guanidine gum fracturing fluid is prepared.

14. A nano-lignin crosslinker for high-temperature fracturing fluid according to any one of claims 1-5, wherein the nano-lignin crosslinker is used to form a gel system with partially hydrolyzed polyacrylamide for profile control and flooding. The partially hydrolyzed polyacrylamide has a molecular weight of 12 million and a hydrolysis degree of 25-30%, the nanolignin crosslinking agent is used in an amount of 0.05-0.3 wt.%, and a partially hydrolyzed polyacrylamide solution has a concentration of 0.1-0.5%, and a gel system for profile control and flooding is prepared.

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