A low-temperature activated nanocomposite crosslinking agent, fracturing fluid, preparation method and application

Abstract

This invention discloses a low-temperature activated nanocomposite crosslinking agent, fracturing fluid, preparation method, and application, belonging to the field of fracturing technology. The nanocomposite crosslinking agent comprises the following components in parts by weight: 84-89.5 parts of organoboron-zirconium, 0.5-1 parts of hydrophilic modified graphene oxide, and 10-15 parts of a low-temperature activating agent; the hydrophilic modified graphene oxide is selected from 6-amino-4-hydroxy-2-naphthalenesulfonic acid modified graphene oxide; the low-temperature activating agent is selected from at least one of ammonium chloride, sodium dodecylbenzenesulfonate, sodium hexadecylsulfonate, dodecyl dimethyl benzyl ammonium chloride, octadecyl dimethyl tertiary amine hydrochloride, and bis(octadecyl methyl tertiary amine hydrochloride). The nanocomposite crosslinking agent of this invention, when applied in fracturing fluid, has advantages such as rapid low-temperature crosslinking, excellent temperature and shear resistance, thorough gel breaking, and minimal reservoir damage, making it suitable for fracturing operations in low-temperature, low-permeability oil and gas reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of fracturing technology, specifically relating to a low-temperature activated nanocomposite crosslinking agent, fracturing fluid, preparation method, and application. Background Technology

[0002] Hydraulic fracturing is a technology that improves oil and gas permeability by injecting high-pressure fluids into underground rock formations to create fractures. In the development of deep, ultra-high temperature (~200℃) oil and gas reservoirs, constructing efficient guiding fractures is crucial for economical extraction. However, this technology has long faced the challenge of extreme temperature differences between the low surface temperature and the high reservoir temperature. Especially in winter or high-latitude regions, surface temperatures can drop below -10℃, while the target reservoir temperature can reach as high as 200℃, posing almost contradictory requirements for fracturing fluid performance throughout the entire process. For certain special reservoirs (such as high-pH alkaline environments), traditional guar gum fracturing fluids are unsuitable due to their limited chemical stability. In contrast, synthetic polymer fracturing fluids (such as polyacrylamide) have become a research hotspot and advantageous technology direction for ultra-high temperature well fracturing due to their highly designable molecular structure, good compatibility with formation fluids, low residue after breaking the gel, and ability to achieve low-friction pumping.

[0003] However, polymer fracturing fluids face two interconnected core bottlenecks when dealing with the aforementioned wide temperature range conditions: First, low-temperature crosslinking failure. The reaction between crosslinking sites such as carboxyl groups on the polymer molecular chain and metal crosslinking agents (such as zirconium and titanium) has a high activation energy. At temperatures below 10°C, the reaction kinetics are significantly slowed, preventing the formation of a three-dimensional gel network with sufficient proppant-carrying strength, leading to proppant settling and sand blockage risks. Second, it is difficult to synergistically optimize rheological and temperature resistance properties. To meet the long-term thermal stability requirements of reservoirs at 200°C, it is often necessary to increase the polymer concentration or molecular weight. However, this directly results in excessively high viscosity and strong adhesion of the base fluid at low temperatures, causing a surge in pumping friction and excessively high operating pressure, creating a "cannot be injected" process dilemma.

[0004] Therefore, while current technological research is dedicated to improving the temperature resistance or delayed crosslinking properties of polymers, how to intelligently and synergistically solve a series of contradictions such as "low-temperature pumpability, low-temperature reliable crosslinking, and ultra-high-temperature long-term stability" in the same system remains a significant technological gap.

[0005] Chinese patent CN 104560003 A discloses an organoboron-zirconium crosslinking agent, primarily addressing the issues of high raw material costs (replacing expensive organoboronium complexes with inexpensive inorganic zirconium salts) and cumbersome preparation processes. Its performance target is temperature resistance above 135℃. Chinese patent CN 119331594 A discloses a multi-stage crosslinking organoboron crosslinking agent resistant to 180℃ and its preparation method. This primarily addresses the temperature and shear resistance issues of guar gum fracturing fluids in deep wells at ultra-high temperatures (180℃), and achieves delayed crosslinking through multi-stage chelation to reduce friction. Its focus is on maximizing high-temperature performance, without mentioning or addressing the crosslinking activation problem in low-temperature environments. Chinese patent CN 119242288 A discloses a method for preparing a high-temperature resistant, shear-resistant organoboron-zirconium crosslinking agent with a maximum temperature resistance of 155℃, but the type of fracturing fluid it can be used for is unclear. Chinese Patent Publication No. CN 109971451 A discloses a graphene oxide nano-crosslinking agent for fracturing fluid and its preparation method. It mainly solves the problems of traditional crosslinking agents, such as difficulty in gel breaking, poor flowback, and excessive residue. Through reversible non-covalent crosslinking, it achieves shear dilution and rapid flowback, with a target temperature resistance of 166℃. Chinese Patent Publication No. CN 112251204 A discloses a high-temperature resistant nanocellulose crosslinking agent for fracturing fluid, its preparation method, and its application. The nanocellulose crosslinking agent is a nanofiber powder with surface-modified organic and inorganic boric acid groups. It is used in low amounts and forms a fracturing fluid gel with guar gum under neutral conditions, exhibiting excellent temperature and shear resistance.

[0006] The aforementioned existing technologies each optimize a single dimension, such as reducing costs, improving backflow, delaying crosslinking, and improving high-temperature limits. None of the existing technologies take "effectively activating crosslinking at low temperatures" as their objective or a technical problem that needs to be solved.

[0007] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0008] The purpose of this invention is to provide a low-temperature activated nanocomposite crosslinking agent, fracturing fluid, preparation method, and application. It solves the technical problems of existing polymer fracturing fluids, such as difficulty in activating the crosslinking reaction at low temperatures, difficulty in low-temperature pumping, and difficulty in maintaining long-term stability at ultra-high temperatures. The low-temperature activated nanocomposite crosslinking agent of this invention can ensure crosslinking during low-temperature pumping and long-term stability at ultra-high temperatures.

[0009] To achieve the above objectives, the present invention provides a low-temperature activated nanocomposite crosslinking agent comprising the following components in parts by weight: 84-89.5 parts of organoboron zirconium, 0.5-1 parts of hydrophilic modified graphene oxide, and 10-15 parts of a low-temperature activation adjuvant; wherein the hydrophilic modified graphene oxide is selected from 6-amino-4-hydroxy-2-naphthalenesulfonic acid modified graphene oxide; and the low-temperature activation adjuvant is selected from at least one of ammonium chloride, sodium dodecylbenzenesulfonate, sodium hexadecylsulfonate, dodecyl dimethyl benzyl ammonium chloride, octadecyl dimethyl tertiary amine hydrochloride, and bis(octadecyl methyl tertiary amine hydrochloride).

[0010] The schematic structural formula of the hydrophilic modified graphene oxide is as follows: ;

[0011] In Equation I, R1 is The connection position of R1 in Equation I is not fixed; similar structural sites can react and connect to R1.

[0012] Preferably, the organoborozzirconium is obtained by reacting zirconium oxychloride in a mixed solution of polyol and water with organic acid, organic alcohol amine and boric acid at 50-60°C.

[0013] More preferably, the polyol is selected from at least one of ethylene glycol, propylene glycol, glycerol, sorbitol and xylitol; or / and, the organic acid is selected from at least one of lactic acid, citric acid, tartaric acid and salicylic acid; or / and, the organic alcohol amine is selected from at least one of diethanolamine and triethanolamine.

[0014] A second objective of this invention is to provide a method for preparing the aforementioned low-temperature activated nanocomposite crosslinking agent, the method comprising:

[0015] A stable and uniform nanocomposite crosslinking agent is prepared by ultrasonication under heating conditions using the organic boron zirconium, the hydrophilic modified graphene oxide, and the low-temperature activating agent in parts by weight; wherein the heating temperature is 40~60℃.

[0016] Preferably, the method for preparing the hydrophilic modified graphene oxide includes: ;

[0017] Graphene oxide was ultrasonically dispersed in water to form a GO dispersion; 6-amino-4-hydroxy-2-naphthalenesulfonic acid was adjusted to neutral or weakly alkaline solution using an inorganic alkaline aqueous solution to form an AS aqueous solution; the AS aqueous solution was continuously stirred and added to the GO dispersion at a rate of 5-10 mL / min, and the reaction was continued under stirring and reflux at 70-90°C; after the reaction was completed, the mixture was centrifuged, and the supernatant was dried to obtain hydrophilic modified graphene oxide.

[0018] More preferably, the mass ratio of graphene oxide to 6-amino-4-hydroxy-2-naphthalenesulfonic acid is 1:5~10; or / and, the pH of the AS aqueous solution is 7~9; or / and, the reflux reaction time is 8~12h; or / and, the centrifugation is performed at 2000~4000r / min for 10~20min; or / and, the drying is carried out at 0℃ under vacuum conditions.

[0019] Preferably, the method for preparing the organoboron zirconium comprises:

[0020] Zirconium oxychloride is dissolved in a mixed solution of water and polyol, and then organic acid, organic alcohol amine and boric acid are added. The mixture is heated to 50-60°C, stirred and kept at the temperature to carry out the reaction, and organoborozzirconium is obtained.

[0021] More preferably, the mass ratio of zirconium oxychloride, organic acid, organic alcohol amine, boric acid, water and polyol is (8~12):(3~5):(10~15):(6~8):(40~58):(15~20); or / and, the reaction time is 5~8h.

[0022] The third objective of this invention is to provide the application of the aforementioned low-temperature activated nanocomposite crosslinking agent in hydraulic fracturing, wherein the nanocomposite crosslinking agent can initiate crosslinking at a low temperature of -5 to 0°C.

[0023] A fourth objective of this invention is to provide a polymer fracturing fluid system containing the aforementioned low-temperature activated nanocomposite crosslinking agent.

[0024] The low-temperature activated nanocomposite crosslinking agent, fracturing fluid, preparation method, and application of the present invention solve the technical problems of existing polymer fracturing fluids, such as difficulty in activating the crosslinking reaction at low temperatures, difficulty in low-temperature pumping, and difficulty in maintaining long-term stability at ultra-high temperatures. These advantages are as follows:

[0025] (1) This invention constructs a ternary composite system of "organo-boron zirconium, hydrophilically modified graphene oxide with sulfonic acid anion groups, and low-temperature activation agent," achieving efficient synergy at the molecular and nanoscale. The core innovation of this system lies in the synergistic effect of sulfonic acid anion modified graphene oxide and specific low-temperature activation agents (such as sodium dodecylbenzene sulfonate and sodium hexadecyl sulfonate). The acidic ions dissociated from it can effectively reduce the activation energy of crosslinking between metal ions and polymers in organo-boron zirconium, thus overcoming the industry problem of difficulty in initiating crosslinking at low temperatures of -5 to 0℃. The low-temperature activation temperature and intensity can be adjusted by controlling the ratio and dosage. The ternary composite system of this invention jointly constructs a "chemical-physical-nano" composite network with high strength, high elasticity, and excellent thermal stability, ultimately enabling the gel to achieve high efficiency at 200℃ and 100s. -1Two hours after the initial shearing, the viscosity remained above 93.8 mPa·s, achieving intelligent response throughout the entire process from cryogenic pumping to long-term stability at ultra-high temperatures;

[0026] (2) From an engineering application perspective, this invention precisely optimizes the rheological path of the fracturing fluid, achieving a "delayed enhancement, easy-to-strength" response. Its viscosity is moderate (<200 mPa·s) during the critical pumping stage, significantly reducing wellbore friction and construction pressure risks. The viscosity increases significantly after entering high-temperature formations, ensuring efficient proppant carrying. This system exhibits good compatibility with hyperbranched polymers, thorough gel breaking, and low residue, greatly reducing damage to reservoir and fracture conductivity. Furthermore, its excellent environmental adaptability and simple preparation process provide an efficient and safe solution for fracturing operations in deep, ultra-high temperature, and low temperature environments. Attached Figure Description

[0027] Figure 1 The infrared characterization spectrum of the hydrophilic modified graphene in Example 1 of this invention is shown.

[0028] Figure 2 The temperature and shear resistance curves of the crosslinking system of the composite nano-crosslinking agent in Example 5 of the present invention are shown. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] It should be noted that: Unless otherwise specified in the examples, conditions should be followed according to standard conditions or the manufacturer's recommendations. Instruments whose manufacturers are not specified are all commercially available products. Raw materials and reagents whose manufacturers are not specified are all commercially available goods or can be prepared using known methods.

[0031] In this invention, all features defined in the form of numerical ranges or percentage ranges, such as numerical values, quantities, contents, and concentrations, are used only for simplicity and convenience. Accordingly, the description of numerical ranges or percentage ranges should be considered as covering and specifically disclosing all possible sub-ranges and individual numerical values ​​(including integers and fractions) within those ranges.

[0032] The features mentioned in this invention can be combined arbitrarily, and all possible combinations should be considered within the scope of this specification, provided that there is no contradiction in the combination of these features. Each feature disclosed in the specification can be replaced by any alternative feature that provides the same, equivalent, or similar purpose. Therefore, unless otherwise specified, the disclosed features are merely general examples of equivalent or similar features.

[0033] In the description of this invention, it should be noted that the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0034] This invention achieves highly efficient synergy at the molecular and nanoscale by constructing a ternary composite system of "organo-boron zirconium, hydrophilically modified graphene oxide with sulfonic acid anionic groups, and low-temperature activation aids." The core innovation of this system lies in the synergistic effect of sulfonic acid anionic modified graphene oxide and specific low-temperature activation aids (such as sodium dodecylbenzenesulfonate and sodium hexadecyl sulfonate). The acidic ions released by these aids effectively reduce the activation energy of crosslinking between metal ions and polymers in organo-boron zirconium, thus overcoming the industry challenge of initiating crosslinking at -5 to 0°C. Furthermore, the low-temperature activation temperature and intensity can be adjusted by controlling the ratio and dosage. The modified graphene oxide, with its two-dimensional sheet structure serving as a nanoframework, contains abundant carboxyl, hydroxyl, and sulfonic acid functional groups on its surface, which can form multiple dynamic coordinations with boron and zirconium central ions, significantly enhancing the density and rigidity of the crosslinked network. This framework, combined with the hydrophobic association effect generated by the long-chain alkyl groups in the activation aids at high temperatures, jointly constructs a "chemophysical nano" composite network possessing high strength, high elasticity, and excellent thermal stability.

[0035] In this invention, the term "acidic ion" should be interpreted broadly based on its functionality, rather than strictly limited to the chemical definition of a protonic acid (H+). + Specifically, the core function of the low-temperature activating agent is to reduce the activation energy of the cross-linking reaction between metal ions in organoborozzirconium and polymer carboxyl groups. Ammonium chloride (NH4Cl) dissociates into ammonium ions (NH4) in aqueous solution. + ), and cationic surfactants (such as quaternary ammonium salts R4N) +Although the quaternary ammonium cations carried by the zirconium oxide do not directly donate protons, as positive charge centers, they can exert an "ion-pair effect" or "salt effect" on the zirconium oxide ions through electrostatic interactions—that is, weakening the binding strength between the zirconium oxide ions and the original ligands through charge repulsion or competitive coordination, thus making it easier for the zirconium oxide ions to bind to the carboxyl groups on the polymer molecular chain, achieving rapid cross-linking at low temperatures. This mechanism has a solid theoretical basis in coordination chemistry. Meanwhile, sulfonic acid anion modification of graphene oxide plays a crucial synergistic amplification role in this process: its surface densely packed sulfonic acid groups (-SO3)... - ) and carboxyl group (-COO) - It carries a strong negative charge and can attract NH4 in the solution through electrostatic adsorption. + Quaternary ammonium cations may accumulate on the surface of two-dimensional nanosheets, forming locally high-concentration positive charge microregions. This accumulation effect greatly enhances the "activation" efficiency of positive ions on zirconium centers, enabling significant low-temperature crosslinking promotion effects to be achieved at a relatively low overall concentration.

[0036] The ternary composite system of this invention is prepared by ultrasonication of organoboron zirconium, hydrophilic modified graphene oxide, and a low-temperature activating agent under heating conditions. The three substances primarily undergo physicochemical interactions, forming a stable nanocomposite structure through non-covalent self-assembly, rather than a destructive chemical reaction. Specifically, under ultrasonic and heating conditions: firstly, the sulfonic acid anions (-SO3-) on the surface of the modified graphene oxide... - ) and cations in low-temperature activating agents (such as NH4) + Electrostatic adsorption occurs between the zirconium ions (or quaternary ammonium salts) and the zirconium ions (Zr ions) in organoborozirconium. 4+ The modified boron zirconium possesses strong empty orbital coordination ability, enabling it to coordinate and complex with the carboxyl and hydroxyl groups on the surface of graphene oxide, anchoring the graphene oxide to the zirconium center. Simultaneously, the long-chain alkyl and hydroxyl groups among the components further stabilize the composite system through hydrogen bonding and hydrophobic interactions. This ultimately forms a nanocomposite crosslinking agent with modified graphene oxide as the framework, organoboron zirconium as crosslinking points, and activating agents as functional units. This process did not damage the core functions of each component (such as the crosslinking ability of boron zirconium and the low-temperature activity of the activating agents); instead, it achieved synergistic performance enhancement. If an irreversible chemical reaction occurred leading to structural destruction, the excellent low-temperature crosslinking and temperature and shear resistance could not be achieved.

[0037] The low-temperature activating agent of this invention can be anionic compounds (anionic surfactants: sodium dodecyl sulfonate and sodium dodecylbenzene sulfonate) or / and cationic compounds (including inorganic ammonium chloride and cationic surfactants). Firstly, the main function of both types of low-temperature activators is to induce cross-linking at low temperatures; secondly, to stabilize and disperse the composite system. In traditional colloid and interface chemistry, anionic and cationic compounds often precipitate or flocculate and become ineffective when directly mixed in an aqueous phase due to their charge neutralization effect. However, in the ternary composite system of "organoboron zirconium—sulfonic acid modified graphene oxide—low-temperature activating agent" constructed in this invention, the two can coexist synergistically and perform their functions. The core mechanism lies in:

[0038] First, the carrier isolation effect is key—the 6-amino-4-hydroxy-2-naphthalenesulfonic acid modified graphene oxide used is rich in negatively charged groups such as sulfonic acid groups and carboxyl groups, which have extremely strong ion adsorption capacity; when cationic compounds (such as dodecyl dimethyl benzyl ammonium chloride or octadecyl dimethyl tertiary amine hydrochloride) are added, their positively charged head groups will preferentially adsorb onto the negatively charged surface of the modified graphene oxide through electrostatic interaction, forming a stable nanocomposite structure, thus being anchored on the solid support, avoiding direct contact with free anionic compounds in the bulk phase;

[0039] Secondly, micro-region isolation and competitive complexation provide dual protection: the two-dimensional sheet structure of graphene oxide provides a huge specific surface area, which can realize the physical partitioning of compounds with different charges at the nanoscale; at the same time, the zirconium ions in organoborozirconium have strong coordination ability and will preferentially complex with the sulfonate ions of anionic compounds (such as sodium dodecylbenzenesulfonate), further reducing the probability of free collision between anions and cations in the aqueous phase.

[0040] Therefore, the low-temperature activation adjuvant in this invention can be a single component or a combination of anionic and cationic compounds. Moreover, through the above-mentioned carrier isolation and micro-area isolation mechanism, even if anionic and cationic compounds are used in combination, no precipitation or flocculation will occur in the traditional sense. Instead, a stable system can be obtained through synergy.

[0041] The following embodiments provide a detailed description of the low-temperature activated nanocomposite crosslinking agent, fracturing fluid, preparation method, and application provided by the present invention.

[0042] The sources of several main experimental materials used in the following examples and comparative examples are as follows:

[0043] Zirconium oxychloride (purchased from Guangtong New Materials), boric acid (purchased from Haode Boron Industry); graphene oxide (purchased from Juguang Evonik), sodium dodecylbenzene sulfonate (purchased from Xinjiang Xinhuan), sodium hexadecyl sulfonate, and octadecyl dimethyl tertiary amine hydrochloride (purchased from Qihe Chemical).

[0044] Example 1

[0045] A low-temperature activated nanocomposite crosslinking agent, the preparation method of which includes:

[0046] (1) Preparation of organoboron zirconium

[0047] Weigh 348g of deionized water and 90g of ethylene glycol into a three-necked flask. While stirring, add 48g of zirconium oxychloride into the flask until completely dissolved. Then, add 18g of lactic acid, 60g of diethanolamine and 36g of boric acid in sequence. Heat to 50°C, stir and maintain the temperature, and react for 8 hours to obtain organoborozirconium.

[0048] (2) Preparation of hydrophilic modified graphene oxide

[0049] 1.5 g of graphene oxide (GO) was weighed and ultrasonically dispersed in 200 g of deionized water for 2 h to form a GO dispersion. 7.5 g of 6-amino-4-hydroxy-2-naphthalenesulfonic acid (AS) was neutralized with 1 mol / L sodium hydroxide solution to neutralize (pH=7.0) to form an AS aqueous solution. The AS aqueous solution was continuously stirred and added to the GO dispersion at 5 mL / min. The mixture was stirred and refluxed at 70 °C for 12 h. The reaction mixture was centrifuged at 2000 r / min for 20 min, and the supernatant was dried to obtain hydrophilic modified graphene oxide.

[0050] (3) Preparation of nanocomposite crosslinking agent

[0051] Under stirring, 168g of organoboron zirconium, 2g of hydrophilic modified graphene oxide and 30g of sodium hexadecyl sulfonate were added sequentially to a flask, heated to 45°C in a water bath, and sonicated for 10 minutes at this temperature to obtain a stable and uniform nanocomposite crosslinking agent.

[0052] like Figure 1 The image shown is the infrared characterization spectrum of hydrophilic modified graphene. Infrared spectroscopy analysis indicates that at 3405 cm⁻¹... -1 The broad and strong absorption peak at 1711 cm⁻¹ is attributed to the stretching vibration of the hydroxyl group (-OH). -1 The strong peak at 1620 cm⁻¹ corresponds to the stretching vibration of C=O in the carboxyl group. -1 The absorption indicates the stretching vibration of the benzene ring skeleton C=C, 1191 cm⁻¹ -1 With 1046cm -1 The two strong absorptions originate from the S=O asymmetric stretching and SO stretching vibrations in the sulfonic acid group, respectively, at 849 cm⁻¹. -1 The nearby peaks suggest out-of-plane bending vibrations of the benzene ring CH, further supporting the aromatic structure; in addition, the 1401 cm⁻¹ peak... -1 The absorption at 586 cm⁻¹ reflects the -CH₂- / -CH₃ bending vibration. -1 and 704cm-1 The absorption in the isofinite region indirectly reflects the molecular skeleton and substitution mode. These characteristic peaks collectively confirm that the molecular structure contains target functional groups such as hydroxyl, benzene ring, sulfonic acid group, and carboxyl group, consistent with the structural characteristics of the target product.

[0053] Example 2

[0054] A low-temperature activated nanocomposite crosslinking agent, the preparation method of which includes:

[0055] (1) Preparation of organoboron zirconium

[0056] Weigh 312g of deionized water and 90g of glycerol into a three-necked flask. While stirring, add 48g of zirconium oxychloride into the flask until completely dissolved. Then, add 24g of lactic acid, 72g of diethanolamine and 36g of boric acid in sequence. Heat to 55°C, stir and maintain the temperature for 6 hours to obtain organoborozirconium.

[0057] (2) Preparation of hydrophilic modified graphene oxide

[0058] 1.5 g of graphene oxide (GO) was weighed and ultrasonically dispersed in 200 g of deionized water for 1.5 h to form a GO dispersion. 9 g of 6-amino-4-hydroxy-2-naphthalenesulfonic acid (AS) was neutralized with 1 mol / L sodium hydroxide solution to neutralize (pH=8.0) to form an AS aqueous solution. The AS aqueous solution was continuously stirred and added to the GO dispersion at 8 mL / min. The mixture was stirred and refluxed at 80 °C for 10 h. The reaction mixture was centrifuged at 2000 r / min for 20 min, and the supernatant was dried to obtain hydrophilic modified graphene oxide.

[0059] (3) Preparation of nanocomposite crosslinking agent

[0060] Under stirring, 172.4g of organoboron zirconium, 1.6g of hydrophilic modified graphene oxide and 26g of sodium dodecylbenzenesulfonate were added sequentially to a flask, heated to 40°C in a water bath, and sonicated at this temperature for 20 minutes to obtain a stable and uniform nanocomposite crosslinking agent.

[0061] Example 3

[0062] A low-temperature activated nanocomposite crosslinking agent, the preparation method of which includes:

[0063] (1) Preparation of organoboron zirconium

[0064] Weigh 288 g of deionized water and 96 g of sorbitol into a three-necked flask. While stirring, add 60 g of zirconium oxychloride into the flask until completely dissolved. Then, add 30 g of citric acid, 78 g of diethanolamine and 48 g of boric acid in sequence. Heat to 60 °C, stir and maintain the temperature for 5 h to obtain organoborozirconium.

[0065] (2) Preparation of hydrophilic modified graphene oxide

[0066] 1.3 g of graphene oxide (GO) was weighed and ultrasonically dispersed in 250 g of deionized water for 1 h to form a GO dispersion. 10.4 g of 6-amino-4-hydroxy-2-naphthalenesulfonic acid (AS) was neutralized with 1 mol / L sodium hydroxide solution to neutralize (pH=9.0) to form an AS aqueous solution. The AS aqueous solution was continuously stirred and added to the GO dispersion at 10 mL / min. The mixture was stirred and refluxed at 90 °C for 8 h. The reaction mixture was centrifuged at 3000 r / min for 15 min, and the supernatant was dried to obtain hydrophilic modified graphene oxide.

[0067] (3) Preparation of nanocomposite crosslinking agent

[0068] Under stirring, 176g of organoboron zirconium, 1.2g of hydrophilic modified graphene oxide and 22.8g of octadecyl dimethyl tertiary amine hydrochloride were added sequentially to a flask, heated to 50°C in a water bath, and sonicated for 15 minutes at this temperature to obtain a stable and uniform nanocomposite crosslinking agent.

[0069] Example 4

[0070] A low-temperature activated nanocomposite crosslinking agent, the preparation method of which includes:

[0071] (1) Preparation of organoboron zirconium

[0072] Weigh 240g of deionized water and 120g of glycerol into a three-necked flask. While stirring, add 72g of zirconium oxychloride into the flask until completely dissolved. Then add 30g of citric acid, 90g of triethanolamine and 48g of boric acid in sequence. Heat to 55°C, stir and maintain the temperature for 7 hours to obtain organoborozirconium.

[0073] (2) Preparation of hydrophilic modified graphene oxide

[0074] 1 g of graphene oxide (GO) was weighed and ultrasonically dispersed in 250 g of deionized water for 1.5 h to form a GO dispersion. 10 g of 6-amino-4-hydroxy-2-naphthalenesulfonic acid (AS) was neutralized with 1 mol / L sodium hydroxide solution to neutralize (pH=8.5) to form an AS aqueous solution. The AS aqueous solution was continuously stirred and added to the GO dispersion at 10 mL / min. The mixture was stirred and refluxed at 80 °C for 10 h. The reaction mixture was centrifuged at 4000 r / min for 10 min, and the supernatant was dried to obtain hydrophilic modified graphene oxide.

[0075] (3) Preparation of nanocomposite crosslinking agent

[0076] Under stirring, 178.6 g of organoboron zirconium, 1.4 g of hydrophilic modified graphene oxide and 20 g of sodium dodecylbenzenesulfonate were added sequentially to a flask, heated to 60 °C in a water bath, and sonicated for 10 min at this temperature to obtain a stable and uniform nanocomposite crosslinking agent.

[0077] Example 5

[0078] A low-temperature activated nanocomposite crosslinking agent, the preparation method of which includes:

[0079] (1) Preparation of organoboron zirconium

[0080] Weigh 300g of deionized water and 102g of ethylene glycol into a three-necked flask. While stirring, add 66g of zirconium oxychloride into the flask until completely dissolved. Then, add 18g of lactic acid, 72g of triethanolamine and 42g of boric acid in sequence. Heat to 60°C, stir and maintain the temperature for 6 hours to obtain organoborozirconium.

[0081] (2) Preparation of hydrophilic modified graphene oxide

[0082] 1.5 g of graphene oxide (GO) was weighed and ultrasonically dispersed in 250 g of deionized water for 2 h to form a GO dispersion. 12 g of 6-amino-4-hydroxy-2-naphthalenesulfonic acid (AS) was neutralized with 1 mol / L sodium hydroxide solution to neutralize (pH=7.5) to form an AS aqueous solution. The AS aqueous solution was continuously stirred and added to the GO dispersion at 8 mL / min. The mixture was stirred and refluxed at 75 °C for 10 h. The reaction mixture was centrifuged at 3000 r / min for 20 min, and the supernatant was dried to obtain hydrophilic modified graphene oxide.

[0083] (3) Preparation of nanocomposite crosslinking agent

[0084] Under stirring, 178g of organoboron zirconium, 2g of hydrophilic modified graphene oxide and 20g of octadecyl dimethyl tertiary amine hydrochloride were added sequentially to a flask, heated to 50°C in a water bath, and sonicated for 15 minutes at this temperature to obtain a stable and uniform nanocomposite crosslinking agent.

[0085] Example 6

[0086] Following the preparation method of Example 5, simply replacing the low-temperature activating agent with 25g of ammonium chloride can also yield a stable and uniform nanocomposite crosslinking agent.

[0087] Example 7

[0088] Following the preparation method of Example 5, by simply replacing the low-temperature activating agent with 18g of dodecyl dimethyl benzyl ammonium chloride and 2.5g of sodium dodecyl sulfonate, a stable and uniform nanocomposite crosslinking agent can also be obtained.

[0089] Example 8

[0090] Following the preparation method of Example 5, by simply replacing the low-temperature activating agent with 16.8g of ammonium chloride and 3.2g of sodium dodecylbenzenesulfonate, a stable and uniform nanocomposite crosslinking agent can also be obtained.

[0091] Comparative Example 1

[0092] Compared to Example 5, the system does not contain organoboron zirconium. Specifically, under stirring, 178g of deionized water, 2g of hydrophilic modified graphene oxide, and 20g of octadecyl dimethyl tertiary amine hydrochloride were added sequentially to a flask. The mixture was heated in a water bath to 50°C and sonicated at this temperature for 15 minutes until a homogeneous liquid was obtained.

[0093] Comparative Example 2

[0094] Compared to Example 5, the system does not contain hydrophilic modified graphene oxide. Specifically, 178g of organoboron zirconium, 20g of octadecyl dimethyl tertiary amine hydrochloride, and 2g of deionized water were added sequentially to a flask under stirring. The mixture was then heated to 50°C in a water bath and sonicated for 15 minutes until a homogeneous liquid was obtained.

[0095] Comparative Example 3

[0096] Compared to Example 5, the system does not contain the low-temperature activating agent (octadecyl dimethyl tertiary amine hydrochloride). Specifically, 178g of organoboron zirconium, 2g of hydrophilic modified graphene oxide, and 20g of deionized water are added sequentially to a flask under stirring. The mixture is then heated to 50°C in a water bath and sonicated at this temperature for 15 minutes until a homogeneous liquid is obtained.

[0097] Comparative Example 4

[0098] Compared to Example 5, this system does not contain hydrophilic modified graphene oxide or the low-temperature activating agent (octadecyl dimethyl tertiary amine hydrochloride). Specifically, 178 g of organoborozzirconium and 22 g of deionized water are added sequentially to a flask with stirring, and the mixture is stirred at room temperature for 15 minutes until a homogeneous liquid is obtained.

[0099] Comparative Example 5

[0100] Compared to Example 5, the system does not contain hydrophilic modified graphene oxide and organoboron zirconium. Specifically, 180g of deionized water and 20g of low-temperature activating agent (octadecyl dimethyl tertiary amine hydrochloride) are added sequentially to a flask under stirring, and the mixture is stirred at room temperature for 15 minutes until a homogeneous liquid is obtained.

[0101] Comparative Example 6

[0102] Compared to Example 5, the system does not contain organoborozzirconium or low-temperature activating agents. Specifically, 198g of deionized water and 2g of hydrophilic modified graphene oxide are added sequentially to a flask under stirring. The mixture is then heated to 50°C in a water bath and sonicated for 15 minutes until a uniformly dispersed liquid is achieved.

[0103] Comparative Example 7

[0104] Compared to Example 5, the hydrophilic modified graphene oxide was replaced with unmodified graphene oxide in the system. Specifically, 178g of deionized water, 2g of unmodified graphene oxide, and 20g of octadecyl dimethyl tertiary amine hydrochloride were added sequentially to a flask under stirring. The mixture was heated to 50°C in a water bath and sonicated at that temperature for 15 minutes until a homogeneous liquid was obtained.

[0105] This invention primarily provides a nanocomposite crosslinking agent that can form weak crosslinks at low temperatures, while also achieving high-temperature and high-shear resistance at relatively low thickener concentrations, as well as a high-temperature resistant polymer fracturing fluid system (comprising a base fluid and a crosslinking agent). The performance of the nanocomposite crosslinking agents prepared in various embodiments and comparative examples of this invention is tested, as detailed below:

[0106] 1. Evaluation of low-temperature crosslinking performance

[0107] (1) Preparation of base solution: Prepare an aqueous solution of hyperbranched thickener (thickener WZ946, Beijing Xitao New Materials Co., Ltd., https: / / www.xitao.com / products_32 / 157.html) with a mass fraction of 0.4%, and keep it in a refrigerator until the solution temperature is -5~0℃. The viscosity of the base solution was tested with a six-speed rotational viscometer and found to be 65.2 mPa·s (170s). -1 The base liquid is non-slip and has good fluidity, meeting the requirements for on-site pumping.

[0108] (2) Take 100 g of the base liquid at -5~0℃ above into a 250 mL beaker, and add 0.5 g of crosslinking agent sample (Examples 1~8, Comparative Examples 1~7 and commercially available organic crosslinking agent - organic zirconium crosslinking agent JSCFR-2, purchased from Dongying Xingjia New Materials Co., Ltd.; organic boron crosslinking agent BSA-601, purchased from Beijing Shida Aode Technology Co., Ltd.). Stir continuously with a glass rod at -5~0℃, observe and record the crosslinking properties of the fracturing fluid at different crosslinking times, evaluate its crosslinking strength, and the test results are shown in Table 1.

[0109] Table 1. Delayed crosslinking properties of different crosslinking systems at low temperatures (-5~0℃)

[0110] .

[0111] As shown in Table 1, all examples achieved effective crosslinking from "weak" to "strong" within 240 to 600 seconds in base fluids below 0°C. In contrast, all comparative examples and the two commercially available organozirconium crosslinking agents showed "no crosslinking" within 20 minutes. This qualitatively demonstrates the synergistic effect of the organoborozirconium and low-temperature activating agent of this invention, successfully overcoming the industry challenge of existing fracturing fluids failing to crosslink at low temperatures. Furthermore, by adjusting the type (such as sodium hexadecyl sulfonate or octadecyl dimethyl tertiary amine hydrochloride) and concentration of the low-temperature activating agent, the crosslinking time can be controllably delayed within the range of 60 to 600 seconds, meeting the pumping time requirements for field operations in oil and gas reservoirs with well temperatures of 60–200°C.

[0112] In the ternary composite system of "organoboron zirconium-sulfonic acid modified graphene oxide-low-temperature activation adjuvant" constructed in this invention, anionic and cationic compounds can coexist synergistically and perform their functions. The feasibility of this synergistic mechanism has been verified through experiments. As shown in Table 1, Examples 7 and 8, which contain a mixture of anionic and cationic compounds, not only did not show precipitation and stratification at low temperatures, but also successfully achieved low-temperature activation and cross-linking of fracturing fluid. In contrast, the comparative examples, lacking any key component, could not cross-link at all. Thus, it can be seen that in the specific ternary composite system of this invention, the carrier anchoring effect of modified graphene oxide and the competitive complexation with zirconium ions successfully break the traditional technical prejudice that anionic and cationic compounds cannot coexist, and realize the synergistic application of the two.

[0113] As shown in Table 1, Example 5, using octadecyl dimethyl tertiary amine hydrochloride (a typical cationic surfactant) as a low-temperature activation adjuvant, successfully achieved effective cross-linking of the fracturing fluid at -5 to 0°C. In contrast, Comparative Example 3, with the exact same formulation but lacking this adjuvant, failed to achieve cross-linking at all. This stark contrast directly demonstrates that even without relying on protic acids, the synergistic effect of organic cations and modified graphene oxide alone can achieve the low-temperature activation function undertaken by "acidic ions." Therefore, ammonium chloride and the cationic surfactant in this system do indeed play a broad "acidic ion" role by providing positive charge centers.

[0114] 2. Evaluation of temperature and shear resistance

[0115] The temperature and shear resistance of the above crosslinked system were tested according to the test method described in industry standard SY / T 7627-2021 "Technical Requirements for Water-Based Fracturing Fluids". The test temperature was 200℃ and the shear rate was 100s. -1 The test results are shown in Table 2. Figure 2 The temperature and shear resistance curves of the crosslinking system of the composite nano crosslinking agent in Example 5 are shown.

[0116] Table 2 Comparison of temperature and shear resistance of different systems

[0117] .

[0118] As can be seen from Table 2, the temperature and shear resistance of the nanocomposite crosslinking agent of this invention (200℃, 100 s) is... -1 The 2h example demonstrates an optimized design for fracturing operations. From a process perspective, initial shear viscosity directly affects operational feasibility and safety risks: commercially available organozzirconium or organoboron crosslinking agents (JSCFR-2, BSA-601) produce ultra-high viscosity exceeding 430 mPa·s within the first 5 minutes. This leads to a sharp increase in the flow friction of the fracturing fluid within the wellbore, significantly increasing the construction pump pressure, equipment load, and sand blockage risk. Especially in low-temperature environments, combined with high-viscosity base fluids, pumping becomes exceptionally difficult. In contrast, the viscosity curve of Example 5 of this invention exhibits a "delayed enhancement, easy at first, strong later" response characteristic. It maintains a moderate viscosity (<200 mPa·s) during the critical pumping stage, significantly reducing wellbore friction and construction pressure risks. Only after the fluid enters the formation and is heated does the viscosity rise to a peak of 188 mPa·s (33 min), achieving the process goal of "easy pumping on the surface and high sand carrying capacity underground." The final viscosity of the nanocomposite crosslinking agent in Example 5, at 95.3 mPa·s, was 2.4 times that of Example 2 (without hydrophilic modified graphene oxide), quantitatively demonstrating the crucial role of the two-dimensional sheet structure of hydrophilic modified graphene oxide in supporting the network framework under high-temperature shear and preventing molecular chain breakage. Meanwhile, the final viscosity of the nanocomposite crosslinking agent in Example 5 was 1.2 times that of Example 3 (without low-temperature activation agent), indicating that the low-temperature activation agent may have provided additional dynamic physical crosslinking points through hydrophobic association, enhancing network elasticity and resilience.

[0119] According to industry standards, fracturing fluid systems for deep / ultra-deep wells must maintain viscosity stability under shear stress for 2 hours at reservoir temperature to preserve proppant carrying capacity and ensure smooth operation. Therefore, the temperature and shear resistance of this invention demonstrates that its system can meet the long-term thermal stability requirements of reservoirs at 200℃.

[0120] 3. Evaluation of Debonding Performance

[0121] The fracturing fluid's gel breaking performance at 200℃ was tested according to the test method described in industry standard SY / T 7627-2021 "Technical Requirements for Water-Based Fracturing Fluids". The formulation was: 0.4% hyperbranched thickener WZ946 + 0.5% crosslinking agent (Examples 1~8) + 0.06% gel breaking agent (ammonium persulfate).

[0122] As shown in Table 3, the ultra-high temperature fracturing fluid can completely break down the gel within 8 hours (the viscosity of the breaking fluid is less than 5 mPa·s), and the residue of the breaking fluid is <100 mg / L, which meets the industry technical requirements. The breaking fluid is a homogeneous liquid with good compatibility for high-temperature breaking.

[0123] Table 3. Test results of the performance of different systems of gel breaking solution

[0124] .

[0125] Therefore, the nanocomposite crosslinking agent of the present invention, through the ternary synergy of organic boron zirconium, hydrophilic modified graphene oxide and low-temperature activating agent, not only achieves high strength and thermal stability (viscosity retention rate of over 50% after 2 hours) at the material level, but also optimizes the viscosity evolution path at the engineering level, directly solving the on-site construction problems of high friction and high pump pressure, and ensuring the safety and efficiency of deep / ultra-high temperature reservoir fracturing operations.

[0126] Although the present invention has been described in detail through the preferred embodiments above, it should be understood that the above description should not be considered as a limitation of the present invention. Various modifications and substitutions to the present invention will be apparent to those skilled in the art after reading the above description. Therefore, the scope of protection of the present invention should be defined by the appended claims.

Claims

1. A low-temperature activated nanocomposite crosslinking agent, characterized in that, The nanocomposite crosslinking agent contains the following components in parts by weight: 84-89.5 parts of organoboron zirconium, 0.5-1 parts of hydrophilic modified graphene oxide, and 10-15 parts of low-temperature activation agent; The hydrophilic modified graphene oxide is selected from 6-amino-4-hydroxy-2-naphthalenesulfonic acid modified graphene oxide; The low-temperature activation adjuvant is selected from at least one of ammonium chloride, sodium dodecylbenzenesulfonate, sodium hexadecylsulfonate, dodecyl dimethyl benzyl ammonium chloride, octadecyl dimethyl tertiary amine hydrochloride, and bis(octadecyl methyl tertiary amine hydrochloride). The organoborozirconium is obtained by reacting zirconium oxychloride in a mixed solution of polyol and water with organic acid, organic alcohol amine and boric acid at 50-60°C. The preparation method of the low-temperature activated nanocomposite crosslinking agent includes: A stable and uniform nanocomposite crosslinking agent is prepared by ultrasonication of the organoboron zirconium, the hydrophilic modified graphene oxide, and the low-temperature activating agent under heating conditions in parts by weight; wherein the heating temperature is 40~60℃.

2. The low-temperature activated nanocomposite crosslinking agent according to claim 1, characterized in that, The polyol is selected from at least one of ethylene glycol, propylene glycol, glycerol, sorbitol, and xylitol; Or / and, the organic acid is selected from at least one of lactic acid, citric acid, tartaric acid and salicylic acid; Or / and, the organic alcohol amine is selected from at least one of diethanolamine and triethanolamine.

3. The preparation method of the low-temperature activated nanocomposite crosslinking agent as described in claim 1 or 2, characterized in that, The method includes: A stable and uniform nanocomposite crosslinking agent is prepared by ultrasonication of the organoboron zirconium, the hydrophilic modified graphene oxide, and the low-temperature activating agent under heating conditions in parts by weight; wherein the heating temperature is 40~60℃.

4. The preparation method according to claim 3, characterized in that, The method for preparing the hydrophilic modified graphene oxide includes: Graphene oxide was ultrasonically dispersed in water to form a GO dispersion; 6-amino-4-hydroxy-2-naphthalenesulfonic acid was adjusted to neutral or weakly alkaline with an inorganic alkaline aqueous solution to form an AS aqueous solution. The AS aqueous solution was continuously stirred and added to the GO dispersion at a rate of 5-10 mL / min, and the mixture was stirred and refluxed at 70-90°C. After the reaction is complete, the mixture is centrifuged, the supernatant is dried, and hydrophilic modified graphene oxide is obtained.

5. The preparation method according to claim 4, characterized in that, The mass ratio of graphene oxide to 6-amino-4-hydroxy-2-naphthalenesulfonic acid is 1:5~10; Or / and, the pH of the AS aqueous solution is 7-9; Or / and, the reflux reaction, for 8-12 hours; Or / and, the centrifugation is performed at 2000~4000 r / min for 10~20 min; Or / and, the drying is carried out under vacuum conditions at 0°C.

6. The preparation method according to claim 3, characterized in that, The method for preparing the organoborozzirconium comprises: Zirconium oxychloride is dissolved in a mixed solution of water and polyol, and then organic acid, organic alcohol amine and boric acid are added. The mixture is heated to 50-60°C, stirred and kept at the temperature to carry out the reaction, and organoborozzirconium is obtained.

7. The preparation method according to claim 6, characterized in that, The mass ratio of zirconium oxychloride, organic acid, organic alcohol amine, boric acid, water and polyol is (8~12):(3~5):(10~15):(6~8):(40~58):(15~20). Or / and, the reaction time is 5-8 hours.

8. The application of the low-temperature activated nanocomposite crosslinking agent as described in claim 1 or 2 in hydraulic fracturing, characterized in that, The nanocomposite crosslinking agent can initiate crosslinking at low temperatures of -5 to 0°C.

9. A polymer fracturing fluid system containing the low-temperature activated nanocomposite crosslinking agent as described in claim 1 or 2.

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