Small-molecular low-adsorption waterproof lock fracturing fluid and preparation method thereof

Abstract

The invention discloses a small-molecule low-adsorption waterproof lock fracturing fluid and a preparation method thereof, and belongs to the technical field of fracturing fluids, the fracturing fluid is composed of a base fluid, a gel breaker solution and a cross-linking agent solution according to a volume ratio of 100: 8-10: 1-2; the base solution is prepared from the following components in percentage by weight: 0.1 to 0.2 percent of micromolecule modified guar gum, 0.2 to 1 percent of polyacrylamide, 0.05 to 0.3 percent of waterproof locking agent, 0.3 to 0.7 percent of degradable fiber and the balance of water; the cross-linking agent solution comprises the following components in percentage by weight: 0.2-0.6% of an organic boron cross-linking agent or 0.2-0.6% of an organic titanium cross-linking agent and the balance of water; the gel breaker solution comprises the following components in percentage by weight: 0.1-0.3% of capsule dressing, 1-2% of capsule core and the balance of water. The fracturing fluid is small in viscosity, good in water locking resistance, high in glue discharging speed and small in damage to strata, and the problem of water locking existing in fracturing fluids in the prior art can be effectively solved.

Description

Technical Field

[0001] The present invention belongs to the technical field of fracturing fluids, and particularly relates to a small molecule low adsorption water-blocking fracturing fluid and a preparation method thereof. Background Art

[0002] The "water-blocking effect" refers to the phenomenon that during processes such as drilling, well completion, and fracturing, after aqueous fluid such as drilling fluid, well completion fluid, and fracturing fluid invades the reservoir, the water saturation of the reservoir increases from the initial water saturation to the irreducible water saturation and then to 100% water saturation, the gas flow resistance increases, and the gas permeability decreases. In essence, due to capillary pressure, an additional skin pressure drop is generated, which is equal to the difference between the non-wetting phase pressure and the wetting phase pressure on both sides of the capillary meniscus, and its magnitude can be determined by the Laplace equation of any curved interface.

[0003] For low-permeability or tight gas reservoirs, the pore throat size for fluid free flow in the reservoir is small, the gas flow channel is narrow, the seepage resistance is large, and the interaction forces between the liquid-solid interface and the liquid-gas interface are large, making the water-blocking effect particularly prominent. Once water-blocking occurs, the permeability damage rate can reach more than 70%, and the gas well production will drop to less than 1 / 3 of the original. Research shows that clay minerals (especially swelling clay minerals) and other authigenic minerals in the reservoir are in a certain salinity. When low-salinity water enters the reservoir, due to the change in salinity, these minerals will expand, disperse, fall off, and migrate, resulting in smaller pores or closed fractures, more micro-pores, and strong water absorption areas, leading to extremely strong water-blocking damage. Currently, most of the fracturing fluid systems used in domestic fields are water-based. When the fracturing fluid breaker fluid cannot be discharged in time, it is easy to generate strong water absorption areas and cause serious water-blocking damage of the fracturing fluid, resulting in no production increase after fracturing, especially for tight gas reservoir fracturing. Summary of the Invention

[0004] In view of the above deficiencies in the prior art, the present invention provides a small molecule low adsorption water-blocking fracturing fluid and a preparation method thereof. The fracturing fluid has a small viscosity, good water-blocking performance, fast gel-breaking speed, and little damage to the formation, and can effectively solve the water-blocking problem existing in the prior art fracturing fluid.

[0005] To achieve the above object, the technical solution adopted by the present invention to solve its technical problems is:

[0006] A small molecule low adsorption water-blocking fracturing fluid is composed of a base fluid, a breaker solution, and a crosslinking agent solution in a volume ratio of 100:8 - 10:1 - 2;

[0007] The base fluid includes the following components in weight percentages: 0.1 - 0.2% of small molecule modified guar gum, 0.2 - 1% of polyacrylamide, 0.05 - 0.3% of water-blocking agent, 0.3 - 0.7% of degradable fiber, and the rest is water;

[0008] The crosslinking agent solution comprises the following components by weight percentage: 0.2 - 0.6% of organic boron crosslinking agent or 0.2 - 0.6% of organic titanium crosslinking agent, and the rest is water;

[0009] The breaker solution comprises the following components by weight percentage: 0.1 - 0.3% of capsule coat, 1 - 2% of capsule core, and the rest is water.

[0010] In the above solution, two components, small molecule modified guar gum and polyacrylamide, are used as thickeners. The small molecule modified guar gum has small molecules, good water solubility, pH stability, and is easy to break gel. Moreover, its thickening ability is strong, which can effectively improve the thickening degree of the fracturing fluid; polyacrylamide has good water solubility and is transparent after dissolution. Using the small molecule modified guar gum and polyacrylamide together can not only reduce the residue content in the fracturing fluid, but also improve the stability and durability of the fracturing fluid, and the fracturing fluid after the combination of the two has good sand-carrying capacity;

[0011] The crosslinking agent can polymerize the small molecule modified guar gum and polyacrylamide to form a gel-like substance, improving the sand-carrying and suspension abilities;

[0012] Degradable fibers are added to the base fluid, which can degrade under the temperature environment of the reservoir, generating an acidic liquid environment, promoting the dissolution of the capsule coat of the breaker, releasing a large amount of the breaker in the capsule, and the breaker quickly breaks the fracturing fluid to form a flowback fluid for discharge, shortening the discharge time and reducing the residue in the reservoir.

[0013] Furthermore, the preparation method of the small molecule modified guar gum is as follows: spray the β-mannose aqueous solution onto the guar gum powder under stirring conditions, then irradiate the powder with ultraviolet light and dry it to obtain small molecule guar gum; stir and mix the small molecule guar gum and isopropanol, then add sodium hydroxide solution to the mixture and continue to stir and mix evenly, raise the temperature to 110 - 130 °C and react for 1 - 2 h, and then continue to add propylene oxide in an inert gas environment and stir and react for 1 - 2 h.

[0014] Furthermore, the concentration of the β-mannose aqueous solution is 1 - 10 μ / ml, the ultraviolet irradiation wavelength is 250 - 253 nm, and the irradiation intensity is 20 - 30 μW / cm 2 , the drying temperature is 30 - 80 °C; the mass percentage of isopropanol in the small molecule modified guar gum is 35 - 45%, the mass percentage of sodium hydroxide solution in the small molecule modified guar gum is 15 - 25%, and the mass percentage of propylene oxide in the small molecule modified guar gum is 2 - 5%.

[0015] Furthermore, the molecular weight of the small molecule modified guar gum is 200,000 - 500,000 daltons.

[0016] In the above solution, the small molecule modified guar gum is substituted with hydroxypropyl and alcohol hydroxyl groups. These groups disrupt the intermolecular forces of the guar gum, enabling the fracturing fluid to improve the conductivity of the supported fractures after entering the water-blocked formation, maximizing the promotion of water backflow in the formation and reducing the water-blocking effect. Since the guar gum is prepared into a small molecule structure, on the basis of ensuring the suspension and sand-carrying performance of the fracturing fluid, it can also promote subsequent gel breaking and improve the gel breaking efficiency.

[0017] Further, the water-blocking agent includes nano-silica and a fluorocarbon surfactant, and the mass ratio of nano-silica to the fluorocarbon surfactant is 6 - 8:1.

[0018] In the above solution, the polar end of the fluorocarbon surfactant is adsorbed onto the surface of the highly electronegative nano-silica particles through electrostatic attraction, causing the non-polar end with hydrophobic and oleophobic properties to face outward. When in use, it can not only improve the dispersibility of nano-silica through the principle of like charges repelling each other, avoiding the phenomenon of silica agglomeration, but also form a hydrophobic and oleophobic protective film on the rock surface through the exposed hydrophobic and oleophobic ends, increasing the contact angle of the backflow fracturing fluid on the protective film, significantly reducing the viscous resistance of the fluid on the solid surface, improving the fluidity of the liquid, and thereby reducing the water-blocking effect in the rock.

[0019] Further, the water-blocking agent further includes at least one of methanol, ethylene glycol, n-propanol, isopropanol, n-butanol, and isobutanol.

[0020] In the above solution, an alcohol-based water-blocking agent is also added to assist nano-silica and the fluorocarbon surfactant in improving the water-blocking effect. During the backflow process of the fracturing fluid, it absorbs the residual water in the rock formation, reducing the water-blocking effect.

[0021] Further, the degradable fiber is polylactic acid fiber and / or polycarbonate fiber.

[0022] Further, the molecular weight of the degradable fiber is 7 million - 10 million Daltons, and the length of the degradable fiber is 0.3 - 0.5 mm.

[0023] Further, the capsule coat is an acid-soluble capsule coat.

[0024] In the above solution, the degradable fiber degrades under the storage temperature environment to form an acidic liquid environment. The acidic environment can dissolve the capsule coat of the gel breaker, promoting the release of the core gel breaker. The released gel breaker promotes the formation of a backflow fluid from the fracturing fluid and discharges it.

[0025] Further, the material of the capsule coat is chitosan, and the material of the core includes at least two of ammonium persulfate, potassium persulfate, potassium permanganate, tert-butyl hydroperoxide, and sodium chloride.

[0026] Further, the material of the core includes ammonium persulfate and sodium chloride.

[0027] In the above solution, ammonium persulfate can promote the dissolution and degradation of small molecule modified guar gum, and sodium chloride can promote the dissolution of polyacrylamide. When the two are used together, the flowback effect of the fracturing fluid is improved.

[0028] The preparation method of the above small molecule low adsorption water blocking fracturing fluid includes the following steps:

[0029] (1) Under stirring conditions, add small molecule modified guar gum, polyacrylamide, water blocking agent and degradable fiber to water. After stirring and mixing evenly, adjust the pH value to weak alkalinity to obtain the base fluid;

[0030] (2) Under stirring conditions, add organic boron crosslinking agent or organic titanium crosslinking agent to water. After stirring and mixing evenly, obtain the crosslinking agent;

[0031] (3) Mix the base fluid, crosslinking agent and breaker, and stir and mix evenly to obtain the product.

[0032] The beneficial effects produced by the present invention are as follows:

[0033] 1. The fracturing fluid of the present invention contains two stabilizers, small molecule modified guar gum and polyacrylamide. The two act synergistically with each other, making the fracturing fluid have fluid characteristics, and can achieve the performance of full suspension, low damage and low dosage. Its viscosity is small, it is easy to pump, and it has strong sand carrying capacity; due to the small amount of guar gum used, the residue in the formation can be reduced, and the damage to the formation can be reduced.

[0034] 2. A capsule breaker is added to the fracturing fluid of the present invention. After the capsule dissolves, a large amount of breaker is released, which can quickly break the gel and flow back, reducing the damage to the formation.

[0035] 3. A water blocking agent is added to the fracturing fluid of the present invention. In addition, the small molecule modified guar gum used in the present invention itself contains hydrophobic and oleophobic groups and also has water blocking performance. The small molecule modified guar gum and the water blocking agent can form a hydrophobic and oleophobic layer in the rock formation to promote the liquid flowback. An adsorbent is also added to the water blocking agent, which can further adsorb the residual water in the rock formation and improve the water blocking effect. Specific Embodiments

[0036] In order to make the purpose, technical solutions and advantages of the present invention clearer, the following examples are used to further describe the present invention in detail. It should be understood that the specific examples described here are only used to explain the present invention, and are not used to limit the present invention. That is, the described examples are only a part of the embodiments of the present invention, rather than all the embodiments.

[0037] Accordingly, the following detailed description of the provided embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative efforts fall within the scope of protection of the present invention.

[0038] It should be noted that relational terms such as "first" and "second" are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprising", "including" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or also includes elements inherent to such process, method, article or device. Without further limitation, an element defined by the statement "comprising one..." does not exclude the existence of additional identical elements in the process, method, article or device comprising the element.

[0039] The features and performance of the present invention will be further described in detail below in conjunction with embodiments.

[0040] Example 1

[0041] A small molecule low adsorption waterproof locking fracturing fluid is composed of a base fluid, a breaker solution and a crosslinker solution according to a volume ratio of 100:10:2;

[0042] The base fluid includes the following components by weight percentage: 0.1% of small molecule modified guar gum, 1% of polyacrylamide, 0.2% of waterproof locking agent, 0.5% of polylactic acid fiber degradable fiber with a molecular weight of 7 million daltons and a length of 0.4 mm, and the rest is water;

[0043] The preparation method of the small molecule modified guar gum is as follows: spray a β-mannose aqueous solution with a concentration of 5 μ / ml onto the guar gum powder under stirring at 500 r / min, the spray particle size is 20 microns, and then use a wavelength of 250 nm and 30 μW / cm 2The guar gum powder is irradiated with ultraviolet light for 1 h and then dried at 70 °C to obtain small molecule guar gum. Among them, the mass-volume ratio of the guar gum powder to the β-mannose aqueous solution is 1:1; the small molecule guar gum and isopropanol are stirred and mixed, and then a 40 wt% sodium hydroxide solution is added to the mixture and stirred evenly, and then the temperature is raised to 120 °C and reacted for 2 h. Then, propylene oxide is continuously added in an inert gas environment and stirred at 60 °C for 2 h. After the reaction, it is neutralized with hydrochloric acid solution, washed with ethanol, and dried to obtain small molecule modified guar gum. Among them, the mass-volume ratio of small molecule guar gum, isopropanol, sodium hydroxide solution and propylene oxide is 1 kg:2 L:0.5 L:1 kg; the molecular weight of the small molecule modified guar gum is 300,000 daltons;

[0044] The water-proofing agent is methanol, nano-silica and fluorocarbon surfactant, which is a mixture made by mixing nano-silica and fluorocarbon surfactant according to a mass ratio of 7:1; the mass ratio of methanol to the nano-silica / fluorocarbon surfactant mixture is 1:1;

[0045] The crosslinking agent solution includes the following components by weight percentage: 0.4% of organic titanium crosslinking agent, and the rest is water; the preparation method of the organic titanium crosslinking agent is: at room temperature, tetra-isopropyl titanate, sodium gluconate, glycerol, and triethanolamine are added to the flask in sequence according to the mass ratio of 2.5:0.05:10:5, and then the flask is placed in a 60 °C constant temperature device, and the materials in the flask are stirred under closed conditions for 60 min. Then, pure water with the total weight of the solution is added to the flask and stirred at 60 °C for 60 min to obtain it;

[0046] The gel breaker solution includes the following components by weight percentage: 0.2% of capsule coat, 2% of capsule core, and the rest is water; the material of the capsule coat is chitosan, and the material of the capsule core includes ammonium persulfate and sodium chloride, and the mass ratio of ammonium persulfate to sodium chloride is 1:1; the gel breaker is prepared by in-situ polymerization.

[0047] The preparation method of the above-mentioned small molecule low adsorption water-proofing fracturing fluid includes the following steps:

[0048] (1) Under stirring conditions, small molecule modified guar gum, polyacrylamide, water-proofing agent and degradable fiber are added to water, and after stirring evenly, the pH value is adjusted to 10 to obtain the base fluid;

[0049] (2) Under stirring conditions, an organic titanium crosslinking agent is added to water, and after stirring evenly, the crosslinking agent is obtained;

[0050] (3) The base fluid, crosslinking agent and gel breaker are mixed and stirred evenly to obtain it.

[0051] Example 2

[0052] A small molecule low-adsorption waterproof and locking fracturing fluid is composed of a base fluid, a breaker solution and a crosslinker solution in a volume ratio of 100:8:1;

[0053] The base fluid comprises the following components in weight percentages: 0.2% of small molecule modified guar gum, 0.2% of polyacrylamide, 0.3% of waterproof locking agent, 0.7% of polycarbonate degradable fiber with a molecular weight of 10 million Dalton and a length of 0.5 mm, and the balance is water;

[0054] The preparation method of the small molecule modified guar gum is as follows: under the stirring condition of 500 r / min, spray the β-mannose aqueous solution with a concentration of 5 μ / ml onto the guar gum powder, the spray particle size is 20 microns, and then irradiate the powder with ultraviolet light with a wavelength of 250 nm and a power of 30 μW / cm 2 for 1 h, and then dry at 70 °C to obtain small molecule guar gum, wherein the mass-volume ratio of the guar gum powder to the β-mannose aqueous solution is 1:1; stir and mix the small molecule guar gum and isopropanol, then add a 40 wt% sodium hydroxide solution to the mixture and continue to stir and mix evenly, then raise the temperature to 120 °C and react for 2 h, then continue to add propylene oxide in an inert gas environment, stir and react at 60 °C for 2 h, after the reaction is completed, neutralize with hydrochloric acid solution, wash with ethanol, and dry to obtain the small molecule modified guar gum, wherein the mass-volume ratio of the small molecule guar gum, isopropanol, sodium hydroxide solution and propylene oxide is 1 kg:2 L:0.5 L:1 kg; the molecular weight of the small molecule modified guar gum is 200,000 Dalton;

[0055] The waterproof locking agent is methanol, nano-silica and fluorocarbon surfactant, a mixture made by mixing nano-silica and fluorocarbon surfactant in a mass ratio of 7:1; the mass ratio of methanol to the nano-silica / fluorocarbon surfactant mixture is 1:1;

[0056] The crosslinker solution comprises the following components in weight percentages: 0.2% of organic titanium crosslinker, and the balance is water; the preparation method of the organic titanium crosslinker is as follows: at room temperature, add tetra-isopropyl titanate, sodium gluconate, glycerol and triethanolamine to the flask in a mass ratio of 2.5:0.05:10:5 in sequence, then place the flask in a 60 °C constant temperature device, stir the materials in the flask under closed conditions for 60 min, then add pure water with a total weight of the solution to the flask, and stir at 60 °C for 60 min to obtain;

[0057] The breaker solution comprises the following components in weight percentages: 0.1% of capsule coat, 2% of capsule core, and the balance is water; the material of the capsule coat is chitosan, the material of the capsule core comprises potassium persulfate and sodium chloride, and the mass ratio of ammonium persulfate and sodium chloride is 1:1; the breaker is prepared and formed by in-situ polymerization method.

[0058] The preparation method of the above-mentioned small molecule low-adsorption waterproof plugging fracturing fluid comprises the following steps:

[0059] (1) Under stirring conditions, add small molecule modified guar gum, polyacrylamide, waterproof plugging agent and degradable fiber to water, adjust the pH value to 10 after stirring and mixing evenly to obtain the base fluid;

[0060] (2) Under stirring conditions, add organic titanium crosslinking agent to water, and obtain the crosslinking agent after stirring and mixing evenly;

[0061] (3) Mix the base fluid, crosslinking agent and breaker, and stir and mix evenly to obtain the product.

[0062] Example 3

[0063] A small molecule low-adsorption waterproof plugging fracturing fluid is composed of a base fluid, a breaker solution and a crosslinking agent solution according to a volume ratio of 100:10:1;

[0064] The base fluid comprises the following components by weight percentage: 0.2% of small molecule modified guar gum, 1% of polyacrylamide, 0.6% of waterproof plugging agent, 0.3% of polylactic acid fiber degradable fiber with a molecular weight of 8 million Dalton and a length of 0.3 mm, and the rest is water;

[0065] The preparation method of the small molecule modified guar gum is as follows: spray the β-mannose aqueous solution with a concentration of 5 μ / ml on the guar gum powder under stirring conditions of 500 r / min, the spray particle size is 20 microns, and then irradiate the powder with ultraviolet light with a wavelength of 250 nm and a power of 30 μW / cm 2 for 1 h, and then dry at 70 °C to obtain small molecule guar gum, wherein the mass-volume ratio of the guar gum powder to the β-mannose aqueous solution is 1:1; stir and mix the small molecule guar gum and isopropanol, then add 40 wt% sodium hydroxide solution to the mixture and continue to stir and mix evenly, then raise the temperature to 120 °C and react for 2 h, then continue to add propylene oxide in an inert gas environment, stir and react at 60 °C for 2 h, after the reaction is completed, neutralize with hydrochloric acid solution, wash with ethanol, and dry to obtain the small molecule modified guar gum, wherein the mass-volume ratio of the small molecule guar gum, isopropanol, sodium hydroxide solution and propylene oxide is 1 kg:2 L:0.5 L:1 kg; the molecular weight of the small molecule modified guar gum is 400,000 Dalton;

[0066] The waterproof plugging agent is methanol, nano-silica and fluorocarbon surfactant, a mixture made by mixing nano-silica and fluorocarbon surfactant according to a mass ratio of 6:1; the mass ratio of methanol to the nano-silica / fluorocarbon surfactant mixture is 1:1;

[0067] The crosslinking agent solution comprises the following components in weight percentages: 0.3% of an organic titanium crosslinking agent, and the rest is water. The preparation method of the organic titanium crosslinking agent is as follows: at normal temperature, tetra-isopropyl titanate, sodium gluconate, glycerol, and triethanolamine are sequentially added to a flask according to the mass ratio of 2.5:0.05:10:5, and then the flask is placed in a constant temperature device at 60 °C. The materials in the flask are stirred under airtight conditions for 60 min, and then pure water with the total weight of the solution is added to the flask, and the mixture is stirred at 60 °C for 60 min to obtain the product;

[0068] The breaker solution comprises the following components in weight percentages: 0.1% of capsule coat, 1% of capsule core, and the rest is water. The material of the capsule coat is chitosan, and the material of the capsule core includes ammonium persulfate and sodium chloride, and the mass ratio of ammonium persulfate to sodium chloride is 1:1. The breaker is prepared by in-situ polymerization.

[0069] The preparation method of the above-mentioned small molecule low adsorption water-blocking fracturing fluid comprises the following steps:

[0070] (1) Under stirring conditions, a small molecule modified guar gum, polyacrylamide, a water-blocking agent, and a degradable fiber are added to water. After stirring and mixing evenly, the pH value is adjusted to 10 to obtain a base fluid;

[0071] (2) Under stirring conditions, an organic titanium crosslinking agent is added to water. After stirring and mixing evenly, a crosslinking agent is obtained;

[0072] (3) The base fluid, the crosslinking agent, and the breaker are mixed and stirred evenly to obtain the product.

[0073] Example 4

[0074] A small molecule low adsorption water-blocking fracturing fluid is composed of a base fluid, a breaker solution, and a crosslinking agent solution according to a volume ratio of 100:8:2;

[0075] The base fluid comprises the following components in weight percentages: 0.2% of a small molecule modified guar gum, 0.5% of polyacrylamide, 0.3% of a water-blocking agent, 0.5% of a polycarbonate degradable fiber with a molecular weight of 9 million Daltons and a length of 0.5 mm, and the rest is water;

[0076] The preparation method of the small molecule modified guar gum is as follows: under stirring conditions at 500 r / min, an aqueous solution of β-mannose with a concentration of 5 μ / ml is sprayed onto the guar gum powder, the spray particle size is 20 microns, and then it is irradiated with light with a wavelength of 250 nm and a power density of 30 μW / cm 2The guar gum powder is irradiated with ultraviolet light for 1 h and then dried at 70 °C to obtain small-molecule guar gum. Among them, the mass-volume ratio of the guar gum powder to the β-mannose aqueous solution is 1:1. The small-molecule guar gum and isopropanol are stirred and mixed, and then a 40 wt% sodium hydroxide solution is added to the mixture and stirred evenly. Then, the temperature is raised to 120 °C and the reaction is carried out for 2 h. Then, propylene oxide is continuously added in an inert gas environment and stirred at 60 °C for 2 h. After the reaction, it is neutralized with hydrochloric acid solution, washed with ethanol, and dried to obtain small-molecule modified guar gum. Among them, the mass-volume ratio of small-molecule guar gum, isopropanol, sodium hydroxide solution, and propylene oxide is 1 kg:2 L:0.5 L:1 kg. The molecular weight of the small-molecule modified guar gum is 500,000 Daltons.

[0077] The water-proofing and plugging agent is methanol, nano-silica, and fluorocarbon surfactant, which is a mixture made by mixing nano-silica and fluorocarbon surfactant according to a mass ratio of 7:1. The mass ratio of methanol to the nano-silica / fluorocarbon surfactant mixture is 1:1.

[0078] The crosslinking agent solution includes the following components in weight percentage: 0.5% of organic titanium crosslinking agent, and the rest is water. The preparation method of the organic titanium crosslinking agent is as follows: at room temperature, tetra-isopropyl titanate, sodium gluconate, glycerol, and triethanolamine are added to the flask in sequence according to the mass ratio of 2.5:0.05:10:5. Then, the flask is placed in a 60 °C constant temperature device, and the materials in the flask are stirred under airtight conditions for 60 min. Then, pure water with the total weight of the solution is added to the flask and stirred at 60 °C for 60 min to obtain it.

[0079] The gel breaker solution includes the following components in weight percentage: 0.3% of capsule coat, 1% of capsule core, and the rest is water. The material of the capsule coat is chitosan, and the material of the capsule core includes ammonium persulfate and sodium chloride, and the mass ratio of ammonium persulfate to sodium chloride is 1:1. The gel breaker is prepared by in-situ polymerization.

[0080] The preparation method of the above small-molecule low-adsorption water-proofing and plugging fracturing fluid includes the following steps:

[0081] (1) Under stirring conditions, small-molecule modified guar gum, polyacrylamide, water-proofing and plugging agent, and degradable fiber are added to water. After stirring evenly, the pH value is adjusted to 10 to obtain the base fluid.

[0082] (2) Under stirring conditions, an organic titanium crosslinking agent is added to water. After stirring evenly, the crosslinking agent is obtained.

[0083] (3) The base fluid, crosslinking agent, and gel breaker are mixed and stirred evenly to obtain it.

[0084] Comparative Example 1

[0085] A small molecule low adsorption waterproof and locking fracturing fluid is composed of a base fluid, a breaker solution and a crosslinker solution according to a volume ratio of 100:10:2;

[0086] The base fluid comprises the following components in weight percentages: 0.5% of small molecule modified guar gum, 0.2% of waterproof and locking agent, 0.5% of polylactic acid fiber degradable fiber with a molecular weight of 7 million Daltons and a length of 0.4 mm, and the rest is water;

[0087] The preparation method of the small molecule modified guar gum is as follows: spray a β-mannose aqueous solution with a concentration of 5 μ / ml onto the guar gum powder under the stirring condition of 500 r / min, the spray particle size is 20 microns, and then irradiate the powder with ultraviolet light with a wavelength of 250 nm and a power of 30 μW / cm 2 for 1 h, and then dry at 70 °C to obtain small molecule guar gum, wherein the mass-volume ratio of the guar gum powder to the β-mannose aqueous solution is 1:1; stir and mix the small molecule guar gum and isopropanol, then add a 40 wt% sodium hydroxide solution to the mixture and continue to stir and mix evenly, then raise the temperature to 120 °C and react for 2 h, then continue to add propylene oxide in an inert gas environment, stir and react at 60 °C for 2 h, after the reaction is completed, neutralize with hydrochloric acid solution, wash with ethanol, and dry to obtain the small molecule modified guar gum, wherein the mass-volume ratio of the small molecule guar gum, isopropanol, sodium hydroxide solution and propylene oxide is 1 kg:2 L:0.5 L:1 kg; the molecular weight of the small molecule modified guar gum is 300,000 Daltons;

[0088] The waterproof and locking agent is methanol, nano-silica and fluorocarbon surfactant, a mixture made by mixing nano-silica and fluorocarbon surfactant according to a mass ratio of 7:1; the mass ratio of methanol to the nano-silica / fluorocarbon surfactant mixture is 1:1;

[0089] The crosslinker solution comprises the following components in weight percentages: 0.4% of organic titanium crosslinker, and the rest is water;

[0090] The breaker solution comprises the following components in weight percentages: 0.2% of capsule coat, 2% of capsule core, and the rest is water; the material of the capsule coat is chitosan, the material of the capsule core comprises ammonium persulfate and sodium chloride, and the mass ratio of ammonium persulfate to sodium chloride is 1:1; the breaker is prepared and formed by in-situ polymerization.

[0091] The preparation method of the above small molecule low adsorption waterproof and locking fracturing fluid is the same as that of Example 1.

[0092] Comparative Example 2

[0093] A small molecule low adsorption waterproof and locking fracturing fluid is composed of a base fluid, a breaker solution and a crosslinker solution according to a volume ratio of 100:10:2;

[0094] The base fluid comprises the following components by weight percentage: 0.1% of small molecule modified guar gum, 1% of polyacrylamide, 0.2% of waterproofing agent, 0.5% of polylactic acid fiber degradable fiber with a molecular weight of 7 million Daltons and a length of 0.4 mm, and the balance is water;

[0095] The preparation method of the small molecule modified guar gum is as follows: spray the β-mannose aqueous solution with a concentration of 5 μ / ml onto the guar gum powder under the stirring condition of 500 r / min, the spray particle size is 20 microns, and then irradiate the powder with ultraviolet light with a wavelength of 250 nm and a power of 30 μW / cm 2 for 1 h, and then dry at 70 °C to obtain small molecule guar gum, wherein the mass-volume ratio of the guar gum powder to the β-mannose aqueous solution is 1:1; stir and mix the small molecule guar gum and isopropanol, then add a 40 wt% sodium hydroxide solution to the mixture and continue to stir and mix evenly, then raise the temperature to 120 °C and react for 2 h, then continue to add propylene oxide in an inert gas environment and stir and react at 60 °C for 2 h. After the reaction is completed, neutralize with hydrochloric acid solution, wash with ethanol, and dry to obtain small molecule modified guar gum, wherein the mass-volume ratio of the small molecule guar gum, isopropanol, sodium hydroxide solution and propylene oxide is 1 kg:2 L:0.5 L:1 kg; the molecular weight of the small molecule modified guar gum is 300,000 Daltons;

[0096] The waterproofing agent is methanol;

[0097] The crosslinking agent solution comprises the following components by weight percentage: 0.4% of organic titanium crosslinking agent, and the balance is water; the preparation method of the organic titanium crosslinking agent is as follows: at room temperature, add tetra-isopropyl titanate, sodium gluconate, glycerol, and triethanolamine to the flask in sequence according to the mass ratio of 2.5:0.05:10:5, then put the flask into a 60 °C constant temperature device, stir the materials in the flask under closed conditions for 60 min, then add pure water with a weight equal to the total weight of the solution to the flask, and stir at 60 °C for 60 min to obtain;

[0098] The breaker solution comprises the following components by weight percentage: 0.2% of capsule coat, 2% of capsule core, and the balance is water; the material of the capsule coat is chitosan, the material of the capsule core includes ammonium persulfate and sodium chloride, and the mass ratio of ammonium persulfate to sodium chloride is 1:1; the breaker is prepared by in-situ polymerization.

[0099] The preparation method of the above-mentioned small molecule low adsorption waterproof fracturing fluid is the same as that of Example 1.

[0100] Comparative Example 3

[0101] A small molecule low adsorption waterproof fracturing fluid is composed of a base fluid, a breaker solution and a crosslinking agent solution according to a volume ratio of 100:10:2;

[0102] The base fluid comprises components in the following weight percentages: 3% of polyacrylamide, 0.2% of a water-blocking agent, 0.5% of polylactic acid fiber degradable fibers with a molecular weight of 7 million Daltons and a length of 0.4 mm, and the balance is water;

[0103] The water-blocking agent is methanol, nano-silica and a fluorocarbon surfactant, which is a mixture prepared by mixing nano-silica and the fluorocarbon surfactant in a mass ratio of 7:1; the mass ratio of methanol to the nano-silica / fluorocarbon surfactant mixture is 1:1;

[0104] The crosslinking agent solution comprises components in the following weight percentages: 0.4% of an organic titanium crosslinking agent, and the balance is water; the preparation method of the organic titanium crosslinking agent is as follows: at room temperature, tetra-isopropyl titanate, sodium gluconate, glycerol and triethanolamine are sequentially added into a flask according to a mass ratio of 2.5:0.05:10:5, then the flask is placed in a constant temperature device at 60°C, and the materials in the flask are stirred under airtight conditions for 60 min. Then, pure water accounting for the total weight of the solution is added into the flask, and the mixture is stirred at 60°C for 60 min to obtain the product;

[0105] The breaker solution comprises components in the following weight percentages: 0.2% of a capsule coat, 2% of a capsule core, and the balance is water; the material of the capsule coat is chitosan, and the material of the capsule core comprises ammonium persulfate and sodium chloride, and the mass ratio of ammonium persulfate to sodium chloride is 1:1; the breaker is prepared by in-situ polymerization.

[0106] The preparation method of the above-mentioned small molecule low-adsorption water-blocking fracturing fluid comprises the following steps:

[0107] (1) Under stirring conditions, polyacrylamide, the water-blocking agent and the degradable fibers are added to water, and after stirring and mixing evenly, the pH value is adjusted to 10 to obtain the base fluid;

[0108] (2) Under stirring conditions, an organic boron crosslinking agent or an organic titanium crosslinking agent is added to water, and after stirring and mixing evenly, the crosslinking agent is obtained;

[0109] (3) The base fluid, the crosslinking agent and the breaker are mixed and stirred evenly to obtain the product.

[0110] The base fluid in the above scheme further comprises conventional additive components such as an antifoaming agent and a bactericide. The usage amounts of the above components are the conventional usage amounts in the prior art. The above components have no special influence on the performance of the fracturing fluid and do not belong to the protection scope of the present invention. Therefore, for the consideration of saving space, they will not be elaborated.

[0111] Experimental Example

[0112] I. Low-temperature dissolution performance test

[0113] Taking the products in Example 1 and Comparative Examples 1-3 as examples, water at 5°C was added to the products in an environment of 5°C, and stirred at a speed of 300 r / min to prepare a fracturing fluid with a mass concentration of 1%. The viscosity of the fracturing fluid was measured every 1 minute. The specific results are shown in Table 1.

[0114] Table 1: Viscosity test results

[0115]

[0116]

[0117] It can be seen from the data in the above table that the dissolution rate of the fracturing fluid in this application is relatively fast, which can meet the technical requirements of on-site liquid preparation and construction.

[0118] II. Temperature and shear resistance performance test

[0119] The 1% aqueous solution of the fracturing fluid in Example 1 and Comparative Examples 1-3 was subjected to a shear resistance test at 120°C using a rheometer, and the shear rate was 170 s -1 , and the heating rate was 3°C / min. After heating to 120°C and shearing for 1 h, the temperature and shear resistance performance of the 1% aqueous solution of the fracturing fluid was observed.

[0120] Through the shear results, it was found that the viscosity of the 1% aqueous solution of the fracturing fluid in Example 1 was greater than 50 mPa·s after shearing for 1 h, generally between 58-60 mPa·s, and had good temperature and shear resistance performance;

[0121] The viscosity of the 1% aqueous solution of the fracturing fluid in Comparative Example 1 was greater than 50 mPa·s after shearing for 1 h, generally between 51-53 mPa·s, and also had good temperature and shear resistance performance;

[0122] The viscosity of the 1% aqueous solution of the fracturing fluid in Comparative Example 2 was greater than 50 mPa·s after shearing for 1 h, generally between 56-58 mPa·s, and also had good temperature and shear resistance performance;

[0123] The viscosity of the 1% aqueous solution of the fracturing fluid in Comparative Example 3 was greater than 50 mPa·s after shearing for 1 h, generally between 50-51 mPa·s, and also had good temperature and shear resistance performance.

[0124] III. Water-blocking performance test

[0125] A simulated core was used, and the water-blocking performance of the fracturing fluid was tested according to SY / T 6540-2002. The specific test results are shown in Table 2.

[0126] Table 2: Water-blocking performance

[0127]

[0128]

[0129] As can be seen from the results in the above table, the fracturing fluid in Example 1 has good water-blocking performance and less damage to the formation.

[0130] Comparing Comparative Example 1 with Example 1, in Comparative Example 1, the addition of polyacrylamide was cancelled and the use of small molecule modified guar gum was increased. It can be seen that the dissolution performance of the prepared fracturing fluid decreased slightly.

[0131] Comparing Comparative Example 2 with Example 1, the water-blocking agent in Comparative Example 2 is only methanol. From the results, it can be seen that the water-blocking performance of the fracturing fluid in Comparative Example 2 becomes worse, proving that the use of nano-silica and fluorocarbon surfactant in the water-blocking agent has a great influence on the water-blocking performance.

[0132] Comparing Comparative Example 3 with Example 1, in Comparative Example 3, the use of small molecule modified guar gum was cancelled. From the results, it can be seen that the water-blocking performance of the fracturing fluid in Comparative Example 3 becomes worse, proving that the hydrophobic and oleophobic groups contained in the small molecule modified guar gum directly affect the water-blocking performance of the fracturing fluid.

Claims

1. A small molecule low adsorption waterproof locking fracturing fluid, characterized in that, It is composed of a base liquid, a breaker solution, and a crosslinking agent solution in a volume ratio of 100:8 - 10:1 - 2; The base liquid includes the following components by weight percentage: 0.1 - 0.2% of small molecule modified guar gum, 0.2 - 1% of polyacrylamide, 0.05 - 0.3% of water - proofing agent, 0.3 - 0.7% of degradable fiber, and the rest is water; The crosslinking agent solution includes the following components by weight percentage: 0.2 - 0.6% of organic boron crosslinking agent or 0.2 - 0.6% of organic titanium crosslinking agent, and the rest is water; The breaker solution includes the following components by weight percentage: 0.1 - 0.3% of capsule coat, 1 - 2% of capsule core, and the rest is water.

2. The small molecule low adsorption waterproof locking fracturing fluid according to claim 1, characterized in that The small molecule modified guar gum is prepared by the following method: Spraying β - mannose aqueous solution onto guar gum powder under stirring conditions, then irradiating the powder with ultraviolet light and drying to obtain small molecule guar gum; Stirring and mixing the small molecule guar gum and isopropyl alcohol, then adding sodium hydroxide solution to the mixture and continuing to stir and mix evenly, then heating to 110 - 130 °C and reacting for 1 - 2 h, and then continuing to add propylene oxide in an inert gas environment and stirring and reacting for 1 - 2 h.

3. The low adsorption and water-blocking small molecule fracturing fluid according to claim 1, wherein, The molecular weight of the small molecule modified guar gum is 200,000 - 500,000 Daltons.

4. The small molecule low adsorption waterproof fracturing fluid according to claim 1, wherein The water - proofing agent includes nano - silica and fluorocarbon surfactant, and the mass ratio of nano - silica to fluorocarbon surfactant is 6 - 8:

1.

5. The small molecule low adsorption waterproof gelling fracturing fluid according to claim 4, wherein The water - proofing agent also includes at least one of methanol, ethylene glycol, n - propanol, isopropyl alcohol, n - butanol, and isobutanol.

6. The small molecule low adsorption waterproof plugging fracturing fluid according to claim 1, wherein The degradable fiber is polylactic acid fiber and / or polycarbonate fiber.

7. The small molecule low adsorption waterproof locking fracturing fluid according to claim 6, wherein, The molecular weight of the degradable fiber is 7,000,000 - 10,000,000 Daltons, and the length of the degradable fiber is 0.3 - 0.5 mm.

8. The small molecule low adsorption waterproof locking fracturing fluid according to claim 1, wherein The capsule coat is an acid - soluble capsule coat.

9. The small molecule low adsorption waterproof plugging fracturing fluid according to claim 8, wherein, The material of the capsule coat is chitosan, and the material of the capsule core includes at least two of ammonium persulfate, potassium persulfate, potassium permanganate, tert - butyl hydroperoxide, and sodium chloride.

10. The preparation method of the small molecule low adsorption waterproof fracturing fluid according to any one of claims 1-9, characterized in that, It includes the following steps: (1) Under stirring conditions, adding small molecule modified guar gum, polyacrylamide, water - proofing agent, and degradable fiber to water, adjusting the pH value to 9 - 10 after stirring and mixing evenly to obtain the base liquid; (2) Under stirring conditions, adding organic boron crosslinking agent or organic titanium crosslinking agent to water, and obtaining the crosslinking agent after stirring and mixing evenly; (3) Mixing the base liquid, crosslinking agent, and breaker, and stirring and mixing evenly to obtain it.

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