Crosslinking agent, preparation method thereof and viscosity-breaking fracturing fluid

Abstract

The application provides a crosslinking agent formed by mixed components; the mixed components include metal salts, organic ligands, surfactants and solvents; the metal salts include zirconium salts and aluminum salts; the organic ligands include alkanolamines, alpha-hydroxycarboxylic acids and polyhydroxyl compounds. Compared with the prior art, the application utilizes zirconium salts, aluminum salts and organic ligands such as alcohol amine and organic acid to form an organic aluminum zirconium crosslinking agent through a coordination complex reaction, which can effectively reduce the amount of a suspended emulsion polymer thickening agent, achieve ultra-low concentration (0.2% of the suspended emulsion polymer thickening agent) crosslinking, has a delayed crosslinking characteristic, can effectively reduce the pipeline construction frictional resistance, can also improve the viscosity of a variable viscosity fracturing fluid system under high temperature and high shear, has a wide viscosity adjustment window, has strong adaptability, meets the requirements of deep shale oil and gas online mixing and large-scale sanding, and has a good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of oilfield chemical technology, and particularly relates to a crosslinking agent, its preparation method, and a viscosity-modifying fracturing fluid. Background Technology

[0002] Variable viscosity fracturing fluids offer advantages such as simultaneously reducing drag and carrying proppant, as well as ease of online preparation, making them the preferred fluid for shale oil and gas reservoir development. Currently, variable viscosity fracturing fluids are composed of various additives, including suspension emulsion polymer thickeners, clay stabilizers, and drainage aids. The dosage of the suspension emulsion thickener is 0.05%–0.8%, the viscosity is 1–80 mPa·s, and the temperature resistance is 140℃.

[0003] As exploration and development progresses to greater depths, reservoirs are buried at increasingly deeper levels (4000–5000 m vertically), with higher temperatures (140–160℃) and higher closure stresses (95–109 MPa). To achieve better oil and gas production, it is necessary to further increase the drilling displacement (18–22 m³ / s). 3 The viscosity of variable viscosity fracturing fluid systems is limited by factors such as viscosity ( / min) and overall proppant-to-liquid ratio (10%-13%). However, existing systems have a narrow viscosity adjustment range, require high amounts of thickener in the suspension emulsion, and exhibit poor temperature and shear resistance. For high-temperature, deep shale oil and gas reservoirs with strong heterogeneity, large reservoir stress differences, and underdeveloped fractures, these systems struggle to meet the demands for large-scale proppant addition, thus affecting effective conductivity. Therefore, it is necessary to take measures to improve the viscosity and proppant carrying capacity of variable viscosity fracturing fluid systems under high temperature and high shear conditions.

[0004] Crosslinking agents, as key additives for improving fracturing fluid performance, have become the preferred choice for research on variable viscosity fracturing fluid systems. The addition of crosslinking agents causes the suspended emulsion polymer to crosslink into a network gel, transforming the variable viscosity fracturing fluid from a self-crosslinked type to a crosslinked type, significantly improving the system's temperature and shear resistance. However, existing crosslinking agents for variable viscosity fracturing fluids are generally synthesized from zirconium or boron salts, resulting in relatively short crosslinking times, leading to high pipeline friction. Furthermore, the ligand types are limited, and the number of crosslinking sites is small, making it impossible to achieve low-concentration crosslinking. Therefore, how to control the crosslinking time through crosslinking agents to reduce pipeline friction, while simultaneously increasing the crosslinking density to achieve low-concentration thickener crosslinking, improve the system's temperature and shear resistance, enhance proppant strength, and ultimately meet the requirements for reservoir stimulation, is one of the problems that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0005] In view of this, the technical problem to be solved by the present invention is to provide a crosslinking agent, its preparation method and a variable viscosity fracturing fluid. The crosslinking agent applied to the variable viscosity fracturing fluid can achieve low-concentration crosslinking, reduce construction costs, and at the same time has the characteristic of delaying crosslinking, which can reduce pipeline friction and improve the temperature and shear resistance of the system, thus meeting the requirements of large-scale sand addition for deep shale oil and gas.

[0006] This invention provides a crosslinking agent formed from a mixture of components; said mixture includes a metal salt, an organic ligand, a surfactant, and a solvent;

[0007] The metal salts include zirconium salts and aluminum salts;

[0008] The organic ligands include alkanolamines, α-hydroxycarboxylic acids, and polyhydroxy compounds.

[0009] Preferably, the mixed components include:

[0010]

[0011] Preferably, the mass ratio of the zirconium salt to the aluminum salt is (3-5):1;

[0012] The mass ratio of the alkanolamine, α-hydroxycarboxylic acid and polyhydroxy compound is (8-12):(12-15):(5-8).

[0013] Preferably, the alkanolamine is selected from one or more of diethanolamine, triethanolamine, and triisopropanolamine;

[0014] The α-hydroxycarboxylic acid is selected from one or more of citric acid, lactic acid, glycolic acid and tartaric acid;

[0015] The polyhydroxy compound is selected from polyols and / or polyhydroxycarboxylates;

[0016] The polyol is selected from one or more of sorbitol, xylitol, mannitol, dipentaerythritol, ethylene glycol, and glycerol;

[0017] The polyhydroxycarboxylate is selected from sodium gluconate and / or sodium citrate.

[0018] Preferably, the organic ligand further includes a small molecule aldehyde; the mass ratio of the small molecule aldehyde to the alkanolamine is (5-8):(8-12).

[0019] Preferably, the small molecule aldehyde is selected from one or more of formaldehyde, acetaldehyde, and glyoxal.

[0020] Preferably, the aluminum salt is selected from one or more of aluminum trichloride, aluminum sulfate, and aluminum citrate;

[0021] The zirconium salt is selected from one or more of zirconium oxychloride, zirconium nitrate, zirconium sulfate and zirconium chloride;

[0022] The surfactant is selected from one or more of sodium dodecylbenzene sulfonate, alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, sodium dodecyl sulfate and propylene glycol block polyether;

[0023] The solvent is a mixture of water and alcohol; the mass ratio of water to alcohol is (2-5):1.

[0024] Preferably, it further includes a pH adjuster; the pH adjuster makes the pH of the mixture 4 to 7; the pH adjuster is selected from one or more of sodium carbonate, sodium bicarbonate and sodium hydroxide.

[0025] The present invention also provides a method for preparing a crosslinking agent, comprising the following steps:

[0026] A metal salt, surfactant, and solvent are mixed, heated to react, and then an organic ligand is added to continue the reaction, yielding a crosslinking agent.

[0027] The present invention also provides a variable viscosity fracturing fluid, comprising the crosslinking agent described above.

[0028] This invention provides a crosslinking agent formed from a mixture of components, including a metal salt, an organic ligand, a surfactant, and a solvent. The metal salt includes zirconium and aluminum salts; the organic ligands include alkanolamines, α-hydroxycarboxylic acids, and polyhydroxy compounds. Compared with existing technologies, this invention utilizes zirconium salts, aluminum salts, and organic ligands such as alkanolamines and organic acids to form an organoaluminum-zirconium crosslinking agent through coordination complexation reactions. This agent can effectively reduce the amount of thickener used in suspension emulsion polymers, achieving ultra-low concentration (0.2% suspension emulsion polymer thickener) crosslinking. Furthermore, it possesses delayed crosslinking properties, effectively reducing pipeline construction friction. Simultaneously, it can improve the viscosity of variable viscosity fracturing fluid systems under high temperature and high shear conditions, resulting in a wide viscosity adjustment window and strong adaptability. This meets the requirements for online blending and large-scale sand addition in deep shale oil and gas, demonstrating promising application prospects. Attached Figure Description

[0029] Figure 1 The figure shows the test results of the high temperature and shear resistance of the crosslinking agent prepared in Example 1 of the present invention;

[0030] Figure 2 The figure shows the test results of the high temperature and shear resistance of the crosslinking agent prepared in Example 2 of the present invention;

[0031] Figure 3 The figure shows the test results of the high temperature and shear resistance of the crosslinking agent prepared in Example 3 of this invention;

[0032] Figure 4 Photographs showing the crosslinking performance test of the crosslinking agent prepared in Example 2 of this invention with 0.2% suspension emulsion polymer thickener;

[0033] Figure 5 The images show the crosslinking performance test results of the crosslinking agent prepared in Comparative Example 1 of this invention with 0.2% and 0.4% suspension emulsion polymer thickener. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0035] The present invention provides a crosslinking agent formed from a mixture of components; the mixture includes a metal salt, an organic ligand, a surfactant, and a solvent; the metal salt includes zirconium salt and aluminum salt; the organic ligand includes alkanolamine, α-hydroxycarboxylic acid, and polyhydroxy compound.

[0036] According to the present invention, the mixed components preferably include:

[0037]

[0038] The crosslinking agent provided by this invention uses zirconium and aluminum as central ions. By complementing the properties of the two central ions, the crosslinking and temperature resistance properties of the system are balanced, and the temperature resistance properties of the system are maximized while meeting the requirements for crosslinking at room temperature. In this invention, the mass ratio of zirconium salt to aluminum salt is preferably (3-5):1, more preferably (3-4.5):1, even more preferably (3-4):1, and most preferably (3.3-4):1; in the embodiments provided by this invention, the mass ratio of zirconium salt to aluminum salt is specifically 10:3, 12:3, or 15:4; the zirconium salt can be any zirconium salt known to those skilled in the art, and there are no special limitations. In this invention, one or more of zirconium oxychloride, zirconium nitrate, zirconium sulfate, and zirconium chloride are preferred; the aluminum salt can be any aluminum salt known to those skilled in the art, and there are no special limitations. In this invention, one or more of aluminum trichloride, aluminum sulfate, and aluminum citrate are preferred; in the crosslinking agent provided by this invention, the content of the metal salt in the mixed component is preferably 8-25 parts by weight, more preferably 10-20 parts by weight, and even more preferably 13-19 parts by weight; in the embodiments provided by this invention, the content of the metal salt in the mixed component is specifically 13 parts by weight, 15 parts by weight, or 19 parts by weight.

[0039] According to the present invention, the content of the organic ligand in the mixed component is preferably 20-45 parts by weight, more preferably 25-40 parts by weight; in the embodiments provided by the present invention, the content of the organic ligand in the mixed component is specifically 25 parts by weight, 35 parts by weight, or 40 parts by weight; the organic ligand in the present invention includes an alkanolamine, an α-hydroxycarboxylic acid, and a polyhydroxy compound; the mass ratio of the alkanolamine, α-hydroxycarboxylic acid, and the polyhydroxy compound is preferably (8-12):(12-15):(5-8); in the embodiments provided by the present invention, the mass ratio of the alkanolamine, α-hydroxycarboxylic acid, and the polyhydroxy compound is specifically 8:12:5, 10:12:8, or 12:15. 5; wherein, alkanolamine ligands can emulsify and thicken the solution, increasing the temperature resistance of the crosslinking gel; the number of carbon atoms in the alkyl group of the alkanolamine is preferably 1 to 5, more preferably 2 to 4, and even more preferably 2 or 3; in this invention, the alkanolamine preferably includes, but is not limited to, one or more of diethanolamine, triethanolamine, and triisopropanolamine; α-hydroxycarboxylic acid ligands can improve the temperature resistance and stability of the crosslinking agent; the α-hydroxycarboxylic acid preferably includes, but is not limited to, one or more of citric acid, lactic acid, glycolic acid, and tartaric acid; the polyhydroxy compound is preferably a polyol and / or a polyhydroxycarboxylate; the polyol preferably includes, but is not limited to, sorbitol, xylitol, mannitol, and dipentaerythritol. One or more of alcohols, ethylene glycol, and glycerol; polyol ligands can increase the solubility of ligands and solutions, promote the formation of central ion-ligand coordination bonds, increase the effective content of crosslinking agents, achieve low-concentration crosslinking while improving the temperature resistance of the system, and the alcohol ligands containing branched dipentaerythritol can increase the molecular weight of the crosslinking agent, while the branched structure has a delayed crosslinking function, better control the viscosity of the system, and effectively reduce the friction of pipeline construction; the polyhydroxycarboxylate preferably includes, but is not limited to, sodium gluconate and / or sodium citrate; the carboxylate ligand has a good affinity for the crosslinking agent atoms, which can improve the stability of the crosslinking agent; in this invention, the organic ligand preferably also includes small molecule aldehydes; the small The preferred mass ratio of molecular aldehyde to alkanolamine is (5-8):(8-12), more preferably (5-8):(10-12); the small molecular aldehyde preferably includes, but is not limited to, one or more of formaldehyde, acetaldehyde, and glyoxal; the small molecular aldehyde ligand has multiple strongly coordinating oxygen and nitrogen atoms, which can bind to the central ion at multiple sites, thereby increasing the crosslinking density points of the crosslinking agent, thus achieving low-concentration crosslinking, and significantly improving the crosslinking performance and temperature resistance of the system. Furthermore, the carbonyl group of the small molecular aldehyde and the carboxyl group of the α-hydroxycarboxylic acid have a synergistic effect, making the oxygen of the carboxyl group and carbonyl group into an electron-rich nucleophile, which complexes with the central ion (electrophile) that can provide empty orbitals, making the crosslinking agent product more stable.

[0040] According to the present invention, the content of the surfactant in the mixed components is preferably 1.5 to 2.5 parts by weight, more preferably 2 parts by weight; the surfactant can be any surfactant known to those skilled in the art, and there are no special limitations. In the present invention, it is preferred to include, but not limited to, one or more of sodium dodecylbenzenesulfonate, alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, sodium dodecyl sulfate and propylene glycol block polyether.

[0041] According to the present invention, the content of the solvent in the mixed components is preferably 40-65 parts by weight, more preferably 45-60 parts by weight, and even more preferably 50-60 parts by weight; the solvent is preferably a water-alcohol solvent; the mass ratio of the water to the alcohol solvent is preferably (2-5):1, more preferably (3-5):1, even more preferably (3.5-4.5):1, and most preferably 4:1.

[0042] According to the present invention, the mixed component preferably further includes a pH adjuster; the pH adjuster makes the pH of the mixed component 4-7, more preferably 4.5-6.5, and even more preferably 5-6; in the present invention, preferably, the content of the pH adjuster in the mixed component is 2-6 parts by weight, more preferably 2-5 parts by weight, and even more preferably 2-4 parts by weight; the pH adjuster preferably includes, but is not limited to, one or more of sodium carbonate, sodium bicarbonate and sodium hydroxide.

[0043] This invention utilizes zirconium salts, aluminum salts, and organic ligands such as alkanolamines and organic acids to form an organoaluminum-zirconium crosslinking agent through coordination complexation reactions. This agent can effectively reduce the amount of suspension emulsion polymer thickener used, achieving ultra-low concentration (0.2% suspension emulsion polymer thickener) crosslinking. It also has the characteristic of delaying crosslinking, which can effectively reduce pipeline construction friction. At the same time, it can also improve the viscosity of the variable viscosity fracturing fluid system under high temperature and high shear, making its viscosity adjustment window wide and its adaptability strong. It meets the requirements of online mixing and large-scale sand addition of deep shale oil and gas, and has good application prospects.

[0044] The present invention also provides a method for preparing the above-mentioned crosslinking agent, comprising the following steps: mixing a metal salt, a surfactant and a solvent, then heating and reacting, and then adding an organic ligand to continue the reaction to obtain the crosslinking agent.

[0045] In this invention, there are no special restrictions on the source of any raw materials; commercially available materials are acceptable. The metal salts, surfactants, solvents, and organic ligands are all as described above and will not be repeated here.

[0046] In this invention, it is preferable to first deoxygenate the solvent; the method of deoxygenation can be any method known to those skilled in the art and is not particularly limited. In this invention, it is preferable to deoxygenate by introducing a protective gas under stirring conditions; the stirring speed is preferably 230-430 r / min, more preferably 280-380 r / min, and even more preferably 330-350 r / min; the protective atmosphere can be any protective gas known to those skilled in the art and is not particularly limited. In this invention, nitrogen is preferred; the time for introducing the protective gas is preferably 10-40 min, more preferably 20-30 min.

[0047] Then, the metal salt and surfactant are added and mixed, and the mixture is heated to react. In this invention, more preferably, the surfactant is added first and stirred, followed by the addition of the metal salt. The stirring speed is preferably 230–430 r / min, more preferably 280–380 r / min, and even more preferably 330–350 r / min. The stirring time is preferably 1–5 min, more preferably 2–4 min, and even more preferably 3 min. The mixing time is preferably 5–15 min, more preferably 7–13 min, even more preferably 8–12 min, even more preferably 9–11 min, and most preferably 10 min. The mixing is preferably carried out under stirring conditions. The stirring speed is preferably 230–430 r / min. The mixing speed is preferably 280–380 r / min, and even more preferably 330–350 r / min; the mixing temperature is preferably 20°C–40°C, more preferably 25°C–35°C, and even more preferably 30°C; the heating reaction temperature is preferably 50°C–100°C, more preferably 60°C–90°C, even more preferably 70°C–80°C, and most preferably 75°C; the heating reaction time is preferably 0.3–0.8 h, more preferably 0.4–0.6 h, and even more preferably 0.5 h; the heating reaction rate is preferably 0.2–1.4°C / min, more preferably 0.4–1.2°C / min, even more preferably 0.6–1.0°C / min, and most preferably 0.7–0.9°C / min.

[0048] An organic ligand is then added to continue the reaction, yielding a crosslinking agent. In this invention, a pH adjuster is preferably added after the organic ligand. The type and amount of the pH adjuster are the same as described above and will not be repeated here. The reaction time is preferably 0.5 to 4 hours, more preferably 1 to 3 hours, and even more preferably 1.5 to 2 hours.

[0049] In this invention, the timing of adding the organic ligand can be adjusted according to the properties of the organic ligand. For example, if the organic ligand includes a low-temperature organic ligand, it can be added before the heating reaction; if the organic ligand includes a high-temperature organic ligand, it can be added after the heating reaction.

[0050] The crosslinking agent prepared in this invention is generally a pale yellow transparent liquid that can form a gel with the suspension emulsion polymer thickener to increase the system's temperature and shear resistance, thereby facilitating sand carrying. It also has certain delayed crosslinking characteristics, with the crosslinking occurring in the wellbore, effectively reducing friction. At the same time, it can achieve low-concentration crosslinking, reducing liquid costs.

[0051] The present invention also provides a variable viscosity fracturing fluid, comprising the crosslinking agent described above.

[0052] The volume of the crosslinking agent is preferably 0.1% to 1% of the volume of the viscous fracturing fluid, more preferably 0.1% to 0.5%, even more preferably 0.2% to 0.4%, and most preferably 0.3%.

[0053] The crosslinking agent in the variable viscosity fracturing fluid provided by this invention crosslinks under neutral conditions. Under neutral conditions, the crosslinking agent exists as a coordination bond type complex. Only under this environment can it combine with carboxyl groups, amide groups and other groups on the polymer chain to form a more stable crosslinked gel.

[0054] According to the present invention, the variable viscosity fracturing fluid preferably further includes a fracturing fluid base fluid; the fracturing fluid base fluid includes water, an emulsion polymer thickener, a drainage aid, and a clay stabilizer.

[0055] The emulsion polymer thickener is preferably a suspension emulsion polymer thickener; the solid content of the suspension emulsion polymer thickener is preferably 20%–50%, more preferably 30%–45%, even more preferably 35%–45%, and most preferably 40%; the emulsion polymer thickener preferably comprises polymer powder, oil phase, emulsifier, thickening stabilizer, and water; the polymer powder is preferably a quaternary copolymer of acrylic acid, acrylamide, N,N-dimethylacrylamide, and 2-acrylamide-2-methylpropanesulfonic acid; the oil phase is preferably white oil; the emulsifier is preferably Tween, more preferably Tween 81; and the thickening stabilizer is preferably one or more of sodium bentonite, xanthan gum, and sodium carboxymethyl cellulose.

[0056] The extrusion aid can be any extrusion aid known to those skilled in the art and is not particularly limited. In this invention, it is preferred to include, but not limited to, one or more of alkyl ethoxy compounds, alkylphenol polyoxyethylene ethers, and alkyl glycoside surfactants.

[0057] The clay stabilizer can be any clay stabilizer known to those skilled in the art, and there are no special limitations. In this invention, it is preferred to include, but is not limited to, two compound formulations of KCl, NH4Cl, tetramethylammonium chloride, benzyltrimethylammonium chloride, and diallyldimethylammonium chloride.

[0058] According to the present invention, the volume ratio of water, emulsion polymer thickener, drainage aid and clay stabilizer in the fracturing fluid base is preferably 1000:(5-20):(0.5-5):(0.5-3), more preferably 1000:(10-15):(2-4):(1-2), and even more preferably 1000:12:3:1.

[0059] The fracturing fluid base fluid can be prepared according to methods well known to those skilled in the art, and there are no special limitations. In this invention, it is preferred to mix water, emulsion polymer thickener, drainage aid and clay stabilizer to obtain fracturing fluid base fluid; the mixing is preferably carried out under stirring conditions; the mixing time is preferably 20s to 90s, more preferably 30 to 80s, and even more preferably 60s.

[0060] The crosslinking agent provided by this invention can form a gel with the suspension emulsion thickener, with a wide viscosity adjustment window, which helps to enhance the temperature resistance of the system and is beneficial for sand carrying. The crosslinking agent series products increase the crosslinking density point, which can effectively reduce the amount of suspension emulsion polymer thickener used to achieve ultra-low concentration (0.2% suspension emulsion polymer thickener) crosslinking. It also has the characteristic of delaying crosslinking, which can effectively reduce pipeline construction friction and reduce construction pressure.

[0061] The present invention also provides a method for preparing the above-mentioned variable viscosity fracturing fluid, comprising: mixing a fracturing fluid base fluid with a crosslinking agent to obtain a variable viscosity fracturing fluid; the fracturing fluid base fluid and the crosslinking agent are the same as described above, and will not be repeated here.

[0062] In the embodiments provided by this invention, the preparation method of the variable viscosity fracturing fluid is as follows: 1000 mL of tap water is poured into a mixing mixer, and the speed of the mixer is adjusted until the liquid vortex can be seen at the top of the mixing mixer blade. Then, 12 mL of suspension emulsion polymer thickener is quickly added to a stirring cup, followed by 3 mL of drainage aid and 1 mL of clay stabilizer. The mixture is stirred for 60 seconds to obtain the fracturing fluid base. 100 mL of the above fracturing fluid base is taken, 0.3 mL of crosslinking agent is added, and the mixture is stirred evenly to obtain the variable viscosity fracturing fluid.

[0063] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a crosslinking agent, its preparation method, and a variable viscosity fracturing fluid.

[0064] All reagents used in the following examples are commercially available.

[0065] Example 1

[0066] The crosslinking agent comprises, by weight, 10 parts zirconium salt, 3 parts aluminum salt, 25 parts organic ligand (8 parts alkanolamine, 12 parts α-hydroxycarboxylic acid, 5 parts polyhydroxycarboxylic acid), 2 parts surfactant, 4 parts pH adjuster, and 50 parts solvent (10 parts organic solvent and 40 parts water); wherein the zirconium salt is zirconium oxychloride octahydrate, the aluminum salt is aluminum trichloride, the alkanolamine is diethanolamine, the α-hydroxycarboxylic acid is lactic acid, the polyhydroxycarboxylic acid is sodium citrate, the surfactant is sodium dodecyl sulfate, the organic solvent is glycerol, and the pH adjuster is sodium hydroxide.

[0067] The preparation method of the crosslinking agent is as follows: set the temperature to 30℃ and the rotation speed to 330r / min, purify the solvent with nitrogen for 30min, add the surfactant to the solvent, stir and mix for 3min, then add zirconium salt and aluminum salt, keep the temperature and rotation speed constant and continue stirring for 10min until dissolved to obtain a mixture; raise the temperature of the mixture to 75℃ at a heating rate of 0.8±0.1℃ / min, continue stirring for 0.5h, add high-temperature organic ligand to continue the reaction, after mixing, adjust the pH of the mixture to between 5.0 and 6.0 with a pH adjuster, continue the reaction for 1.5h to obtain the crosslinking agent.

[0068] Example 2

[0069] The crosslinking agent comprises, by weight, 12 parts zirconium salt, 3 parts aluminum salt, 35 parts organic ligand (10 parts alkanolamine, 12 parts α-hydroxycarboxylic acid, 8 parts polyol, 5 parts small molecule aldehyde), 2 parts surfactant, 4 parts pH adjuster, and 50 parts solvent (10 parts organic solvent and 40 parts water); wherein the zirconium salt is zirconium oxychloride octahydrate, the aluminum salt is aluminum trichloride, the alkanolamine is triethanolamine, the α-hydroxycarboxylic acid is lactic acid, the polyol is ethylene glycol and dipentaerythritol (mass ratio of 3:5), the small molecule aldehyde is formaldehyde, the surfactant is sodium dodecyl sulfate, the organic solvent is glycerol, and the pH adjuster is sodium hydroxide.

[0070] The preparation method of the crosslinking agent is as follows: set the temperature to 30℃ and the rotation speed to 330r / min, purify the solvent with nitrogen for 30min, add the surfactant to the solvent, stir and mix for 3min, then add zirconium salt and aluminum salt, keep the temperature and rotation speed constant and continue stirring for 10min until dissolved to obtain a mixture; raise the temperature of the mixture to 75℃ at a heating rate of 0.8±0.1℃ / min, continue stirring for 0.5h, add high-temperature organic ligand to continue the reaction, after mixing, adjust the pH of the mixture to between 5.0 and 6.0 with a pH adjuster, continue the reaction for 2h to obtain the crosslinking agent.

[0071] Example 3

[0072] The crosslinking agent comprises, by weight, 15 parts zirconium salt, 4 parts aluminum salt, 40 parts organic ligand (12 parts alkanolamine, 15 parts α-hydroxycarboxylic acid, 5 parts polyhydroxycarboxylic acid salt, 8 parts small molecule aldehyde), 2 parts surfactant, 3 parts pH adjuster, and 60 parts solvent (15 parts organic solvent and 45 parts water); wherein the zirconium salt is zirconium oxychloride octahydrate, the aluminum salt is aluminum trichloride, the alkanolamine is triethanolamine, the α-hydroxycarboxylic acid is lactic acid, the polyhydroxycarboxylic acid salt is sodium citrate, the small molecule aldehyde is formaldehyde, the surfactant is sodium dodecyl sulfate, the organic solvent is glycerol, and the pH adjuster is sodium hydroxide.

[0073] The preparation method of the crosslinking agent is as follows: set the temperature to 30℃ and the rotation speed to 330r / min, purify the solvent with nitrogen for 30min, add the surfactant to the solvent, stir and mix for 3min, then add zirconium salt and aluminum salt, keep the temperature and rotation speed constant and continue stirring for 10min until dissolved to obtain a mixture; raise the temperature of the mixture to 75℃ at a heating rate of 0.8±0.1℃ / min, continue stirring for 0.5h, add high-temperature organic ligand to continue the reaction, after mixing, adjust the pH of the mixture to between 6 and 0 with a pH adjuster, continue the reaction for 2h to obtain the crosslinking agent.

[0074] Comparative Example 1

[0075] The crosslinking agent comprises, by weight, 12 parts zirconium salt, 3 parts aluminum salt, 35 parts organic ligand (20 parts alkanolamine, 15 parts α-hydroxycarboxylic acid), 2 parts surfactant, 4 parts pH adjuster, and 50 parts solvent (10 parts organic solvent and 40 parts water); wherein the zirconium salt is zirconium oxychloride octahydrate, the aluminum salt is aluminum trichloride, the alkanolamine is triethanolamine, the α-hydroxycarboxylic acid is lactic acid, the surfactant is sodium dodecyl sulfate, the organic solvent is glycerol, and the pH adjuster is sodium hydroxide.

[0076] The preparation method of the crosslinking agent is as follows: set the temperature to 30℃ and the rotation speed to 330r / min, purify the solvent with nitrogen for 30min, add the surfactant to the solvent, stir and mix for 3min, then add zirconium salt and aluminum salt, keep the temperature and rotation speed constant and continue stirring for 10min until dissolved to obtain a mixture; raise the temperature of the mixture to 75℃ at a heating rate of 0.8±0.1℃ / min, continue stirring for 0.5h, continue to add high-temperature organic ligands to react, after mixing, adjust the pH of the mixture to between 5.0 and 6.0 with a pH adjuster, continue the reaction for 2h to obtain the crosslinking agent.

[0077] Performance testing

[0078] Preparation of base liquid: Measure 1000 mL of tap water and pour it into the Wu Yin mixer. Adjust the speed of the Wu Yin mixer until the liquid vortex can be seen at the top of the central shaft of the mixer blade. Then, measure and add 12 mL of suspension emulsion polymer thickener (self-made, prepared according to Example 2 of CN 114456795 A), 3 mL of drainage aid (self-made, 5 parts by weight of perfluoroalkyl polyoxyethylene ether CF16400 from Jinan Qifu New Material Technology Co., Ltd., 5 parts by weight of alkylphenol polyoxyethylene ether OP-107 from Jiangsu Haian Petrochemical Plant, 3 parts by weight of cocoyl alkyl glycoside APG from Jiangsu Haian Petrochemical Plant, and 85 parts by weight of water), and 1 mL of clay stabilizer (a compound of KCl and tetramethylammonium chloride from Beijing Baofengchun Petroleum Technology Co., Ltd.). After 1 min, stop stirring to obtain the base liquid.

[0079] Take 100 mL of the above base liquid, add 0.3 mL of the crosslinking agent prepared in Examples 1 to 3 respectively, and stir evenly to obtain different viscous fracturing fluids.

[0080] The high-temperature and shear resistance of the above-mentioned variable viscosity fracturing fluid was tested using an RS300 high-temperature and high-pressure rheometer. The heating rate was controlled at 3.0℃ / min ± 0.2℃ / min, the experimental temperature was 180℃, and the shear rate was 170s. -1 The shearing time was 90 min; the test results showed that at 180℃, the shearing time was 170 s. -1 After shearing for 90 min, the rheological viscosities of the systems corresponding to the crosslinking agents prepared in Examples 1-3 were 138 mPa·s, 200 mPa·s, and 175 mPa·s, respectively. The test results... Figures 1-3 As shown, all of them meet the viscosity requirement of ≥50 mPa·s in the industry standard SY / T 7627—2021 "Technical Requirements for Water-Based Fracturing Fluids". It can also be seen that the crosslinking agent prepared in Example 2 has superior performance.

[0081] Crosslinked gels were prepared using the crosslinking agent prepared in Example 2, 0.2% suspension emulsion polymer thickener, and water. The crosslinking performance was tested using 0.3% of the crosslinking agent. The test results were... Figure 4 As shown, a hangable gel can be formed. Comparative Example 1, without the addition of polyols or small molecule aldehyde ligands, prepared a crosslinking agent with an amount of 0.3%, and prepared crosslinked gels with 0.2% and 0.4% suspension emulsion polymer thickener, respectively. The test results... Figure 5 As shown, it can crosslink but not stick with 0.2% suspension emulsion polymer, and can form sticky gel with 0.4% suspension emulsion polymer. Both have white particle incompatibility, which is very damaging to the reservoir.

[0082] Compared with CN 110003877A "A Crosslinking Agent, Viscosity-Reducing Agent and Preparation Method for High-Mineralization Clean Viscous Slippery Water" and CN 115287054A "Organic Aluminum Crosslinking Agent and its Preparation Method and Application in Continuously Mixed Fracturing Fluids", the crosslinking agent prepared in this embodiment is simpler in process. Furthermore, the addition of small molecule alkanolamines and aldehydes to the ligands, which possess multiple strongly coordinated oxygen and nitrogen atoms, allows for multi-site binding with the central ion, significantly improving the crosslinking performance (crosslinking with 0.2% suspension emulsion polymer thickener is possible) and temperature resistance (viscosity > 100 mPa·s after continuous shearing at 180°C for 90 min with 1.2% suspension emulsion polymer thickener), can meet the large-scale sand addition requirements of deep shale oil and gas.

[0083] The crosslinking agent provided by this invention has two complementary central ions, which effectively balance the crosslinking and temperature resistance of the system, maximizing the temperature resistance of the system while meeting the requirements of room temperature crosslinking. The ligands are selected as small molecules with low steric hindrance and multiple binding sites, enabling multi-head crosslinking. Furthermore, by changing the type of ligand and the reaction temperature, reaction time, pH, and other conditions, a series of crosslinking agent products can be formed, allowing them to form a gel with the suspension emulsion polymer thickener. This gel has a wide viscosity adjustment window and a temperature resistance up to 180℃, which is beneficial for sand carrying. It can effectively reduce the amount of suspension emulsion polymer thickener used, achieving ultra-low concentration (0.2% suspension emulsion polymer thickener) crosslinking. It also has a delayed crosslinking characteristic, which can effectively reduce pipeline construction friction and reduce construction pressure.

[0084] The preferred embodiments of the present invention disclosed above are merely for illustrating the present invention, and the present invention is not limited thereto. Those skilled in the art will understand that, within the scope of the present invention's concept, modifications can be made to the technical solutions of the present invention, or some technical features can be combined in any other way. Such modifications or combinations do not cause the essence of the corresponding technical solutions to depart from the spirit and scope of the various embodiments of the present invention, and should be considered as the content disclosed in the present invention, all of which fall within the protection scope of the present invention.

Claims

1. A crosslinking agent, characterized in that, Formed from a mixture of components; The mixed components include: 5-25 parts by weight of metal salt; Organic ligands, 20-50 parts by weight; 1-3 parts by weight of surfactant; Solvent 35~65 parts by weight; The metal salts include zirconium salts and aluminum salts; The organic ligands include alkanolamines, α-hydroxycarboxylic acids, and polyhydroxy compounds; The mass ratio of zirconium salt to aluminum salt is (3~5):1; The aluminum salt is selected from one or more of aluminum trichloride, aluminum sulfate, and aluminum citrate; The zirconium salt is selected from one or more of zirconium oxychloride, zirconium nitrate, zirconium sulfate and zirconium chloride; The mass ratio of the alkanolamine, α-hydroxycarboxylic acid and polyhydroxy compound is (8~12):(12~15):(5~8); The alkanolamine is selected from one or more of diethanolamine, triethanolamine, and triisopropanolamine; The α-hydroxycarboxylic acid is selected from one or more of citric acid, lactic acid, glycolic acid and tartaric acid; The polyhydroxy compound is selected from polyols and / or polyhydroxycarboxylates; The polyol is selected from one or more of sorbitol, xylitol, mannitol, dipentaerythritol, ethylene glycol, and glycerol; The polyhydroxycarboxylate is selected from sodium gluconate and / or sodium citrate; The surfactant is selected from one or more of sodium dodecylbenzene sulfonate, alkylphenol polyoxyethylene ether, fatty alcohol polyoxyethylene ether, sodium dodecyl sulfate and propylene glycol block polyether; The solvent is a mixture of water and alcohol; the mass ratio of water to alcohol is (2~5):

1.

2. The crosslinking agent according to claim 1, characterized in that, The organic ligand also includes a small molecule aldehyde; the mass ratio of the small molecule aldehyde to the alkanolamine is (5~8):(8~12).

3. The crosslinking agent according to claim 2, characterized in that, The small molecule aldehyde is selected from one or more of formaldehyde, acetaldehyde, and glyoxal.

4. The crosslinking agent according to claim 1, characterized in that, It also includes a pH adjuster; the pH adjuster adjusts the pH of the mixture to 4 to 7; the pH adjuster is selected from one or more of sodium carbonate, sodium bicarbonate and sodium hydroxide.

5. A method for preparing the crosslinking agent according to claim 1, characterized in that, Includes the following steps: A metal salt, surfactant, and solvent are mixed, heated to react, and then an organic ligand is added to continue the reaction, yielding a crosslinking agent.

6. A variable viscosity fracturing fluid, characterized in that, It includes the crosslinking agent according to any one of claims 1 to 4 or the crosslinking agent prepared by the preparation method according to claim 5.

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