An environment-friendly fracturing fluid and its preparation method

Abstract

The present invention relates to the technical field of fracturing fluids, and particularly relates to an environmentally friendly fracturing fluid and a preparation method thereof. The fracturing fluid comprises, by weight: deionized water: 90 - 95 parts, modified nano-cellulose - montmorillonite composite powder: 1 - 3 parts, foaming agent: 0.1 - 0.3 parts, thickening agent: 0.2 - 0.8 parts, crosslinking agent: 0.5 - 1.5 parts. The modified nano-cellulose - montmorillonite composite powder is prepared by activating carboxylated nano-cellulose, crosslinking it with carboxylic acid - polyethylene glycol - carboxylic acid and triethylenetetramine, and then compounding and intercalating and modifying montmorillonite. The environmentally friendly fracturing fluid prepared by the present invention has good foaming ability, excellent sand-carrying ability, good stability at different temperatures, and environmentally friendly raw materials, reducing the risk of environmental pollution. It is suitable for fracturing operations under various reservoir conditions and can improve the oil and gas production efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of fracturing fluids, and particularly to an environmentally friendly fracturing fluid and a preparation method thereof. Background Art

[0002] In the field of oil and gas exploitation, fracturing fluid technology is crucial, and its performance directly affects the exploitation efficiency and cost. Traditional fracturing fluids have many drawbacks, seriously restricting the development of the industry. Some traditional fracturing fluids have low foaming efficiency and are difficult to form a large number of stable foams during construction, unable to effectively carry proppants into formation fractures, resulting in poor fracture support effect, affecting the smooth exploitation and transmission of oil and gas. At the same time, their foam stability ability is poor, the foam half-life is short, and it is easy to break, making it difficult to maintain the open state of fractures, reducing the exploitation efficiency. In terms of sand-carrying performance, traditional fracturing fluids also perform poorly, unable to effectively prevent proppant settlement, resulting in uneven distribution of proppants and affecting the improvement of formation permeability, thereby reducing oil and gas production.

[0003] In addition, with the increasingly strict environmental protection requirements, the components of traditional fracturing fluids may be environmentally unfriendly and pose potential pollution risks. Developing a new type of fracturing fluid with high foaming efficiency, strong foam stability ability, excellent sand-carrying performance, appropriate gel-breaking time and environmental friendliness is imminent, which is of great significance for improving oil and gas exploitation efficiency, reducing costs and protecting the environment. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide an environmentally friendly fracturing fluid and a preparation method thereof to solve the problems of poor sand-carrying ability, short foam half-life and poor foam stability ability at high temperature of existing traditional fracturing fluids.

[0005] Based on the above purpose, the present invention provides an environmentally friendly fracturing fluid, which is characterized by comprising the following raw materials in parts by weight: deionized water: 90 - 95 parts, modified nano-cellulose - montmorillonite composite powder: 1 - 3 parts, foaming agent: 0.1 - 0.3 parts, thickening agent: 0.2 - 0.8 parts, cross-linking agent: 0.5 - 1.5 parts;

[0006] The specific preparation method of the modified nano-cellulose - montmorillonite composite powder is as follows:

[0007] (1) Carboxylated nano-cellulose and carboxylic acid - polyethylene glycol - carboxylic acid are co-dispersed in an activation solution, ultrasonicated for 1 - 3 h, then triethylenetetramine is added, and under nitrogen protection, stirred and reacted at 55 - 65 °C for 5 - 7 h, freeze-dried, washed with n-hexane, and dried to obtain modified nano-cellulose;

[0008] (2) Montmorillonite, cetyltrimethylammonium bromide and deionized water are mixed, stirred at 50 - 70 °C for 1 - 3 h, filtered, washed and dried to obtain intercalation-modified montmorillonite;

[0009] (3) Add the intercalated modified montmorillonite in (2) to deionized water, heat it to 60 - 80 °C and stir for 2 - 4 h. Then add the modified nanocellulose obtained in (1), ultrasonicate for 20 - 30 min, and then stir for 6 - 12 h. After cooling to room temperature, freeze-dry and grind through a 1000 - 1500 mesh sieve to obtain the modified nanocellulose-montmorillonite composite powder.

[0010] Preferably, in (1), the weight ratio of carboxylated nanocellulose, carboxylic acid-polyethylene glycol-carboxylic acid, activation solution and triethylenetetramine is 0.6 - 1:0.2 - 0.6:15 - 25:0.1 - 0.2. Under the action of ultrasound, the activated carboxylated nanocellulose can be completely dissolved in the activation solution within this weight ratio range.

[0011] Preferably, in (1), the outer diameter of the carboxylated nanocellulose is 4 - 10 nm and the length is 200 nm. Nanocellulose within this size range can better adsorb on the foam surface, enhancing the interfacial film strength of the foam. The shorter and thinner nanocellulose can intertwine with the montmorillonite lamellae to form a more stable three-dimensional network structure, improving the viscosity and temperature and shear resistance of the fracturing fluid. Nanocellulose of appropriate size can interact with thickener molecules through hydrogen bonds and other interactions to further strengthen the network structure and improve the overall performance of the fracturing fluid, meeting the fracturing construction requirements under different reservoir conditions.

[0012] Preferably, in (1), the activation solution is a phosphate buffer system activation solution. In the phosphate buffer system activation solution, the concentration of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride is 0.1 mol / L and the concentration of N-hydroxysuccinimide is 0.05 mol / L. The reaction activity of the activated carboxylated nanocellulose is significantly enhanced, and its compatibility with other materials is improved.

[0013] Preferably, in (2), the weight ratio of montmorillonite, cetyltrimethylammonium bromide and deionized water is 0.8 - 1.2:0.03 - 0.05:3 - 5.

[0014] Preferably, in (3), the weight ratio of intercalated modified montmorillonite, modified nanocellulose and deionized water is 1:0.8 - 1.2:8 - 12. According to this weight ratio, the modified nanocellulose can be completely dissolved in deionized water. During the process of cooling to room temperature after the reaction, the intercalated modified montmorillonite suspended in deionized water does not show obvious sedimentation, indicating that the intercalated modified montmorillonite enters the cross-linked voids of the modified nanocellulose to form a new cross-linked structure, and the modified nanocellulose acts as a skeleton to support the montmorillonite and inhibit the sedimentation of the montmorillonite.

[0015] Preferably, the foaming agent refers to lauryl betaine. The surface of carboxylated nanocellulose is negatively charged. Therefore, the electrical properties of the foaming agent have a significant impact on the initial volume and stability of the foam. Lauryl betaine contains both anionic and cationic groups. When combined with carboxylated nanocellulose, it can provide good foam stability.

[0016] Preferably, the thickener refers to one of xanthan gum, carboxymethyl cellulose or hydrolyzed polyacrylamide.

[0017] Preferably, the crosslinking agent refers to one of triethanolamine borate, zirconium oxychloride or zirconium acetate.

[0018] Furthermore, the present invention also provides a preparation method of the above-mentioned environmentally friendly fracturing fluid, which specifically includes the following steps:

[0019] S1. Add the modified nanocellulose-montmorillonite composite powder into deionized water, and stir at 200-300 rpm for 30-50 min to obtain a dispersion;

[0020] S2. Add the foaming agent, thickener and crosslinking agent into the dispersion obtained in S1, stir at 200-300 rpm for 2-4 h, and then stir at a speed of 5000-10000 rpm on a high-speed emulsifier for 3-6 min to obtain the environmentally friendly fracturing fluid.

[0021] The beneficial effects of the present invention:

[0022] 1. By introducing the modified nanocellulose-montmorillonite composite powder, the present invention improves the foaming and foam-stabilizing properties of the fracturing fluid. After specific modification treatment of the nanocellulose, its surface properties change, and it can better adsorb on the foam surface. During the modification process, carboxylated nanocellulose crosslinks with carboxylic acid-polyethylene glycol-carboxylic acid and triethylenetetramine to form a crosslinked network structure. The unique lamellar structure of montmorillonite interweaves with the modified nanocellulose and embeds into the network of the modified nanocellulose. This structure enhances the interfacial film strength of the foam, effectively restricts the diffusion of gas in the foam, reduces the drainage rate of the foam, thereby improving the foaming efficiency and foam-stabilizing ability, and ensuring that a large amount of stable foam can be formed during the fracturing construction to provide a good carrier for sand carrying.

[0023] 2. For the environmentally friendly fracturing fluid of the present invention, the structure formed by the modified nanocellulose-montmorillonite composite powder and other components has good stability in different temperature environments. At lower temperatures, it can effectively restrict molecular movement and maintain foam stability; at higher temperatures, the lamellar structure of montmorillonite can support part of the network and slow down the speed of structure destruction, ensuring that the fracturing fluid can stably play its role under different reservoir temperature conditions.

[0024] 3. The environmentally friendly fracturing fluid of the present invention uses deionized water as the main solvent, reducing the use of chemical solvents harmful to the environment. At the same time, the selected raw materials such as modified nano-cellulose-clay composite powder, foaming agent, thickening agent, and cross-linking agent have good biodegradability and environmental friendliness on the basis of meeting the high-performance requirements of the fracturing fluid, reducing the potential pollution risk of the fracturing operation to the formation and the surrounding environment. Brief Description of the Drawings

[0025] Figure 1 It is a graph showing the change of the half-life of the fracturing fluid prepared by using the preparation methods of Examples 1 to 3 and Comparative Examples 1 to 7 at different temperatures. Detailed Description of the Embodiments

[0026] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to specific embodiments.

[0027] Example 1: A specific preparation method of an environmentally friendly fracturing fluid includes the following processes:

[0028] (1) Dispersed 5 g of carboxylated nano-cellulose and 1.67 g of carboxylic acid-polyethylene glycol-carboxylic acid in 125 g of an activation solution containing 0.1 M of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and 0.05 M of N-hydroxysuccinimide (pH = 5.5, phosphate buffer system), ultrasonicated for 1 h, then added 0.83 g of triethylenetetramine, and stirred and reacted at 55 °C for 5 h under nitrogen protection, freeze-dried, washed with n-hexane, and dried to obtain modified nano-cellulose;

[0029] (2) Mixed 5 g of montmorillonite, 0.19 g of cetyltrimethylammonium bromide, and 18.75 g of deionized water, stirred at 50 °C for 1 h, filtered, washed, and dried to obtain intercalated modified montmorillonite;

[0030] (3) Added 4 g of the intercalated modified montmorillonite in (2) to 32 g of deionized water, heated to 60 °C and stirred for 2 h, then added 3.2 g of the modified nano-cellulose obtained in (1), ultrasonicated for 20 min, and then stirred for 6 h. After cooling to room temperature, it was freeze-dried and ground through a 1000-mesh sieve to obtain modified nano-cellulose-montmorillonite composite powder.

[0031] (4) Added 4 g of the modified nano-cellulose-montmorillonite composite powder in (3) to 360 g of deionized water, and stirred at 200 rpm for 3 min to obtain a dispersion;

[0032] (5) Add 0.4 g of lauryl betaine, 0.8 g of xanthan gum, and 2 g of triethanolamine borate to the dispersion obtained in (4), stir at 200 rpm for 2 h, and then stir at 5000 rpm for 3 min on a high-speed emulsifier to obtain an environmentally friendly fracturing fluid.

[0033] Example 2: A specific preparation method of an environmentally friendly fracturing fluid, including the following process:

[0034] (1) Disperse 10 g of carboxylated nanocellulose and 5 g of carboxylic acid-polyethylene glycol-carboxylic acid in 250 g of an activation solution containing 0.1 M of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and 0.05 M of N-hydroxysuccinimide (pH = 5.5, phosphate buffer system), ultrasonically treat for 2 h, then add 1.88 g of triethylenetetramine, and under nitrogen protection, stir and react at 60 °C for 6 h, freeze-dry, wash with n-hexane, and dry to obtain modified nanocellulose.

[0035] (2) Mix 10 g of montmorillonite, 0.4 g of cetyltrimethylammonium bromide, and 40 g of deionized water, stir at 60 °C for 2 h, filter, wash, and dry to obtain intercalated modified montmorillonite.

[0036] (3) Add 6 g of the intercalated modified montmorillonite in (2) to 60 g of deionized water, heat to 70 °C and stir for 3 h, then add 6 g of the modified nanocellulose obtained in (1), ultrasonically treat for 25 min, then stir for 9 h, cool to room temperature, freeze-dry, and grind through a 1200-mesh sieve to obtain modified nanocellulose-montmorillonite composite powder.

[0037] (4) Add 8 g of the modified nanocellulose-montmorillonite composite powder in (3) to 370 g of deionized water, stir at 250 rpm for 40 min to obtain a dispersion.

[0038] (5) Add 0.8 g of lauryl betaine, 2 g of xanthan gum, and 4 g of triethanolamine borate to the dispersion obtained in (4), stir at 250 rpm for 3 h, and then stir at 7500 rpm for 5 min on a high-speed emulsifier to obtain an environmentally friendly fracturing fluid.

[0039] Example 3: A specific preparation method of an environmentally friendly fracturing fluid, including the following process:

[0040] (1) Disperse 15 g of carboxylated nanocellulose and 9 g of carboxylic acid-polyethylene glycol-carboxylic acid in 375 g of an activation solution containing 0.1 M of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and 0.05 M of N-hydroxysuccinimide (pH = 5.5, phosphate buffer system), ultrasonically treat for 3 h, then add 3 g of triethylenetetramine, and under nitrogen protection, stir and react at 65 °C for 7 h, freeze-dry, wash with n-hexane, and dry to obtain modified nanocellulose;

[0041] (2) Mix 12 g of montmorillonite, 0.5 g of cetyltrimethylammonium bromide, and 50 g of deionized water, stir at 70 °C for 3 h, filter, wash, and dry to obtain intercalated modified montmorillonite;

[0042] (3) Add 10 g of the intercalated modified montmorillonite in (2) to 120 g of deionized water, heat to 80 °C and stir for 4 h, then add 12 g of the modified nanocellulose obtained in (1), ultrasonically treat for 30 min, then stir for 12 h, cool to room temperature, freeze-dry, and grind through a 1500-mesh sieve to obtain modified nanocellulose-montmorillonite composite powder.

[0043] (4) Add 12 g of the modified nanocellulose-montmorillonite composite powder in (3) to 380 g of deionized water, stir at 300 rpm for 50 min to obtain a dispersion;

[0044] (5) Add 1.2 g of lauryl betaine, 3.2 g of xanthan gum, and 6 g of triethanolamine borate to the dispersion obtained in (4), stir at 300 rpm for 4 h, and then stir at 10000 rpm for 6 min on a high-speed emulsifier to obtain an environmentally friendly fracturing fluid.

[0045] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the carboxylated nanocellulose is not activated by EDC / NHS. The specific preparation method of an environmentally friendly fracturing fluid includes the following process:

[0046] (1) Disperse 10 g of carboxylated nanocellulose and 5 g of carboxylic acid-polyethylene glycol-carboxylic acid in 250 g of deionized water with pH = 5.5 and phosphate buffer, ultrasonically treat for 2 h, then add 1.88 g of triethylenetetramine, and under nitrogen protection, stir and react at 60 °C for 6 h, freeze-dry, wash with n-hexane, and dry to obtain modified nanocellulose;

[0047] (2) Mix 10 g of montmorillonite, 0.4 g of cetyltrimethylammonium bromide, and 40 g of deionized water, stir at 60 °C for 2 h, filter, wash, and dry to obtain intercalated modified montmorillonite;

[0048] (3) Add 6 g of the intercalated modified montmorillonite in (2) to 60 g of deionized water, heat to 70 °C and stir for 3 h, then add 6 g of the modified nanocellulose obtained in (1), sonicate for 25 min, then stir for 9 h. After cooling to room temperature, freeze-dry and grind through a 1200-mesh sieve to obtain the modified nanocellulose-montmorillonite composite powder.

[0049] (4) Add 8 g of the modified nanocellulose-montmorillonite composite powder in (3) to 370 g of deionized water and stir at 250 rpm for 40 min to obtain a dispersion.

[0050] (5) Add 0.8 g of lauryl betaine, 2 g of xanthan gum, and 4 g of triethanolamine borate to the dispersion obtained in (4), stir at 250 rpm for 3 h, and then stir at a speed of 7500 rpm on a high-speed emulsifier for 5 min to obtain an environmentally friendly fracturing fluid.

[0051] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that when preparing the modified nanocellulose, carboxylic acid-polyethylene glycol-carboxylic acid is not added.

[0052] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the carboxylated nanocellulose is directly compounded with the intercalated modified montmorillonite without modification.

[0053] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that montmorillonite is not added during the preparation process.

[0054] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that the modified nanocellulose-montmorillonite composite powder is not added.

[0055] Comparative Example 6: The difference between Comparative Example 6 and Example 2 is that the modified nanocellulose-montmorillonite composite powder is reduced to 3 g.

[0056] Comparative Example 7: The difference between Comparative Example 7 and Example 2 is that the modified nanocellulose-montmorillonite composite powder is reduced to 5 g.

[0057] Performance test:

[0058] 1. Foaming efficiency test: Take 100 ml of the dispersion before stirring on a high-speed emulsifier from Examples 1 - 3 and Comparative Examples 1 - 7, transfer it to a high-speed emulsifier and stir quickly at 8000 r / min for 5 min to prepare foam. Then quickly pour the obtained foam into a 500 mL graduated cylinder and read the volume of the foam liquid. Foaming efficiency: , V0 —— initial liquid volume, V —— volume of the formed foam. The experimental results are shown in Table 1.

[0059] 2. Half-life test: Take 100 ml of the dispersion before stirring with a high-speed emulsifier from Examples 1 to 3 and Comparative Examples 1 to 7, transfer it to the high-speed emulsifier, and quickly stir it at 8000 r / min for 5 min to prepare foam. Then quickly pour the obtained foam into a 500 mL graduated cylinder, and record the time when 50 ml of liquid precipitates from the foam in the graduated cylinder, which is the half-life of the fracturing fluid. The experimental results are shown in Table 1.

[0060] 3. Apparent viscosity test: Use an NXS-11B type rotational viscometer to measure the apparent viscosity of the fracturing fluids prepared in Examples 1 to 3 and Comparative Examples 1 to 7 at room temperature. The experimental results are shown in Table 1.

[0061] 4. Proppant-carrying performance test: Take the fracturing fluids prepared in Examples 1 to 3 and Comparative Examples 1 to 7, fill a 10 cm stoppered graduated cylinder, put ceramsite or glass beads with a diameter of 0.4 - 0.8 cm into the stoppered graduated cylinder, and measure the time it takes for the ceramsite or glass beads to reach the bottom of the graduated cylinder, which is the settling time. The experimental results are shown in Table 1.

[0062] 5. Water breaker time test: At room temperature, mix the fracturing fluids prepared in Examples 1 to 3 and Comparative Examples 1 to 7 with deionized water in a volume ratio of 1:3, shake well and then observe the breaker phenomenon, record the breaker time. The experimental results are shown in Table 1.

[0063] 6. Stability test at different temperatures: Record the change of the half-life of the fracturing fluids prepared in Examples 1 to 3 and Comparative Examples 1 to 7 with temperature at different temperatures. The experimental results are as Figure 1 shown.

[0064] Table 1

[0065]

[0066] Data analysis:

[0067] According to Table 1 and Figure 1It can be seen from the data in [Example 1 - 3] that the fracturing fluids prepared by the preparation method of the present invention have balanced performance, high foaming efficiency, strong foam stability, excellent sand - carrying performance, and appropriate gel - breaking time. Especially, they show good foam stability at different temperatures. Among them, the performance of Example 2 is the best. In terms of foaming efficiency, compared with Comparative Example 1, the carboxylated nanocellulose in Example 2 was activated by EDC / NHS, which enhanced the reactivity of nanocellulose, improved the interaction with the foaming agent lauryl betaine, adsorbed more effectively on the foam surface, reduced the surface tension, promoted the formation of foam, and thus increased the foaming efficiency. In Comparative Example 2, without the addition of carboxylic acid - polyethylene glycol - carboxylic acid, the carboxylated nanocellulose could not form a firm network structure. At the same time, in the absence of polyethylene glycol, the formation and stability of the interfacial film were affected, the strength of the interfacial film decreased, and it could not effectively resist external interference, so the foam was easy to break, and it was difficult to form a large number of stable foams, thus reducing the foaming efficiency. In Comparative Example 3, the carboxylated nanocellulose was not modified, and its surface properties were not conducive to synergistic action with the foaming agent, resulting in a lower foaming efficiency. In Comparative Example 4, without the addition of montmorillonite, the composite structure formed by montmorillonite and nanocellulose promoted foam stability. The lack of montmorillonite would cause the foaming efficiency to decline. In Comparative Example 5, without the addition of modified nanocellulose - montmorillonite composite powder, the key foam - stabilizing component was lost, and the foaming efficiency decreased significantly. In Comparative Examples 6 and 7, with the reduction of the composite powder, the number of sites providing stable foam decreased, and the foaming efficiency was also affected.

[0068] In terms of the half - life, in Comparative Example 1, the unactivated carboxylated nanocellulose led to weak interaction with other components, unstable foam interfacial film, and short half - life. In Comparative Example 2, without carboxylic acid - polyethylene glycol - carboxylic acid, a stable network structure could not be formed to maintain foam stability, and the half - life was shortened. In Comparative Example 3, the unmodified nanocellulose could not effectively synergize with other components to stabilize the foam, and the half - life was relatively short. In Comparative Example 4, without montmorillonite, the structure formed by the interweaving of montmorillonite layers and nanocellulose was crucial for foam stability. The lack of it would reduce foam stability and shorten the half - life. In Comparative Example 5, without the composite powder, stable foam could hardly be formed, and the half - life was extremely short. In Comparative Examples 6 and 7, with the reduction of the composite powder, the ability to stabilize the foam decreased, and the half - life was also shortened accordingly. In Example 2, the proportion of each component was appropriate, and the modified nanocellulose - montmorillonite composite powder, foaming agent, thickener, and cross - linker cooperated with each other to form a stable foam structure, resulting in a longer half - life.

[0069] In terms of apparent viscosity, in Example 2, carboxylated nanocellulose with appropriate size interacted with thickener molecules through hydrogen bonds and other interactions, strengthening the network structure. At the same time, the three-dimensional network structure formed by the modified nanocellulose-montmorillonite composite powder also contributed to the increase in viscosity. In Comparative Example 1, the interaction between unactivated nanocellulose and the thickener was weak, and the viscosity could not be effectively increased. In Comparative Example 2, the lack of carboxylic acid-polyethylene glycol-carboxylic acid affected the formation of the composite structure, thereby affecting the viscosity increase. In Comparative Example 3, the unmodified nanocellulose could not participate well in the formation of a high-viscosity network structure. In Comparative Example 4, the lack of montmorillonite led to an incomplete network structure and a low viscosity. In Comparative Example 5, there was no composite powder, and almost no effective thickening structure was formed, resulting in an extremely low viscosity. In Comparative Examples 6 and 7, the amount of composite powder was insufficient, and the thickening effect was not as good as that in Example 2.

[0070] In terms of sand-carrying performance, that is, the sedimentation time, in Example 2, the stable three-dimensional network structure formed by the modified nanocellulose-montmorillonite composite powder and the increased viscosity of the system by the thickener could effectively hinder the sedimentation of ceramsite or glass beads. In Comparative Examples 1-3, due to insufficient modification or activation of nanocellulose, a good network structure could not be formed to hinder sedimentation. In Comparative Example 4, the lack of montmorillonite led to an imperfect network structure, a decrease in sand-carrying capacity, and a shortened sedimentation time. In Comparative Example 5, there was no composite powder, and almost no sand-carrying capacity, resulting in an extremely short sedimentation time. In Comparative Examples 6 and 7, the amount of composite powder decreased, the sand-carrying performance weakened, and the sedimentation time was shorter than that in Example 2.

[0071] In terms of gel-breaking time, although the modified nanocellulose produced a cross-linked structure, which would prolong the gel-breaking time of the fracturing fluid, in the examples, carboxylic acid-polyethylene glycol-carboxylic acid, as a flexible cross-linking agent, reacted with the carboxyl groups of nanocellulose and the amino groups of triethylenetetramine to form a cross-linked network. Carboxylic acid-polyethylene glycol-carboxylic acid provided a large steric hindrance, enabling the degree of cross-linking to be controlled. The formed network provided both stability and allowed reversible dissociation under the action of water or shear force, thus avoiding the difficulty of gel-breaking caused by excessive cross-linking. At the same time, the lamellar structure of montmorillonite was dispersed in the nanocellulose network, forming a physical support framework, reducing the dependence on chemical cross-linking. During gel-breaking, the penetration of water molecules between the montmorillonite layers could accelerate the disintegration of the network without completely breaking chemical bonds. On the basis of ensuring the high foaming efficiency and sand-carrying performance of the fracturing fluid, the gel-breaking time was controlled within a reasonable range. In Comparative Example 2, the key component of carboxylic acid-polyethylene glycol-carboxylic acid was not added to the modified nanocellulose, and the degree of cross-linking of nanocellulose could not be well controlled, resulting in a relatively dense cross-linking and difficulty in gel-breaking of the fracturing fluid.

[0072] In terms of stability at different temperatures, for the fracturing fluid prepared with the addition of montmorillonite, at lower temperatures (20~40°C), the half-life is generally stable or even increases. At higher temperatures (40~90°C), the decrease in the half-life is relatively slower compared to the fracturing fluid without the addition of montmorillonite. This may be because montmorillonite has a special lamellar structure. After adding montmorillonite, it can intertwine with modified nanocellulose to form a stable three-dimensional network structure at lower temperatures. This network structure is like a framework that can effectively restrict the movement of liquid molecules, reduce the drainage rate of foam, thus making the foam more stable and maintaining or even increasing the half-life. At the same time, although high temperature will affect the stability of the system to a certain extent, the network structure formed by montmorillonite and modified nanocellulose can still maintain integrity to a certain extent. The lamellar structure of montmorillonite has good thermal stability and can support part of the network to prevent it from completely collapsing. However, for the system without the addition of montmorillonite, there is a lack of such a support structure at high temperatures, and the network structure quickly collapses, resulting in a significant decrease in stability. Finally, it can be seen from Example 2 and Comparative Examples 6~7 that the addition amount of the composite powder also directly affects the stability of the fracturing fluid at different temperatures. This may be because in Example 2, the addition amount of the modified nanocellulose-montmorillonite composite powder is appropriate, and nanocellulose and montmorillonite are fully intertwined. In a low-temperature environment, this composite structure can effectively restrict molecular thermal motion, reduce foam drainage and gas diffusion. The network formed by montmorillonite lamellae and nanocellulose, like a "molecular fence", fixes the liquid and gas in a specific area to maintain the stability of the foam. In Comparative Examples 6~7, the addition amount of the composite powder is insufficient, the network structure is sparse, and it cannot effectively restrict molecular motion, resulting in a decrease in stability.

[0073] Those of ordinary skill in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the present invention is limited to these examples; within the concept of the present invention, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above. For the sake of brevity, they are not provided in detail.

Claims

1. An environmentally friendly fracturing fluid, characterized in that: The invention comprises the following raw materials in parts by weight: deionized water: 90-95 parts, modified nanocellulose-montmorillonite composite powder: 1-3 parts, foaming agent: 0.1-0.3 parts, thickener: 0.2-0.8 parts, crosslinking agent: 0.5-1.5 parts; The specific preparation method of the modified nanocellulose-montmorillonite composite powder is as follows: (1) Dispersing carboxylated nanocellulose and carboxylic acid-polyethylene glycol-carboxylic acid in a phosphate buffer system activation solution, ultrasonically treating for 1 to 3 hours, adding triethylenetetramine, stirring and reacting at 55 to 65°C for 5 to 7 hours under nitrogen protection, freeze-drying, washing, and drying to obtain modified nanocellulose; (2) Mixing montmorillonite, hexadecyltrimethylammonium bromide and deionized water, stirring at 50-70°C for 1-3 hours, filtering, washing and drying to obtain intercalated modified montmorillonite; (3) adding the intercalated modified montmorillonite prepared in (2) into deionized water, heating to 60-80°C and stirring for 2-4 hours, then adding the modified nanocellulose prepared in (1), ultrasonicating for 20-30 minutes, stirring for 6-12 hours, cooling to room temperature, freeze-drying, grinding through a 1000-1500 mesh sieve, and obtaining a modified nanocellulose-montmorillonite composite powder; The concentration of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride in the phosphate buffer system activation solution is 0.1 mol / L, the concentration of N-hydroxysuccinimide is 0.05 mol / L, and the pH is 5.

5.

2. The environmentally friendly fracturing fluid according to claim 1, characterized in that: The weight ratio of carboxylated nanocellulose, carboxylic acid-polyethylene glycol-carboxylic acid, activation solution and triethylenetetramine in (1) is 0.6~1:0.2~0.6:15~25:0.1~0.

2.

3. The environmentally friendly fracturing fluid according to claim 1, characterized in that: The outer diameter of the carboxylated nanocellulose in (1) is 4-10 nm and the length is 200 nm.

4. The environmentally friendly fracturing fluid according to claim 1, characterized in that: The weight ratio of montmorillonite, hexadecyltrimethylammonium bromide and deionized water in (2) is 0.8-1.2:0.03-0.05:3-5.

5. The environmentally friendly fracturing fluid according to claim 1, characterized in that: The weight ratio of the intercalated modified montmorillonite, modified nanocellulose and deionized water in (3) is 1:0.8~1.2:8~12.

6. The environmentally friendly fracturing fluid according to claim 1, characterized in that: The foaming agent refers to lauryl betaine.

7. The environmentally friendly fracturing fluid according to claim 1, characterized in that: The thickener is one of xanthan gum, carboxymethyl cellulose or hydrolyzed polyacrylamide.

8. The environmentally friendly fracturing fluid according to claim 1, characterized in that: The cross-linking agent is one of triethanolamine borate, zirconium oxychloride or zirconium acetate.

9. The method for preparing the environmentally friendly fracturing fluid according to any one of claims 1 to 8, characterized in that: The following steps are involved: S1. Add the modified nanocellulose-montmorillonite composite powder to deionized water and stir at 200-300 rpm for 30-50 min to obtain a dispersion; S2. Add a foaming agent, a thickener, and a cross-linking agent to the dispersion obtained in S1, stir at 200-300 rpm for 2-4 hours, and then stir at a speed of 5000-10000 rpm for 3-6 minutes on a high-speed emulsifier to obtain an environmentally friendly fracturing fluid.

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