Low-molecular scale-inhibiting thickening agent for deep coal and rock gas reservoir fracturing and preparation method of low-molecular scale-inhibiting thickening agent

Abstract

The invention discloses a low-molecular scale-inhibiting thickening agent for deep coal rock gas reservoir fracturing and a preparation method of the thickening agent. The thickening agent comprises 25-40 parts of a hydrophilic monomer; 3-6 parts of a scale inhibition monomer; 3-5 parts of a functional monomer; 50 to 55 parts of deionized water; 0.5 to 0.8 part of an emulsifier; 0.15 part to 0.2 part of an initiator; 1.5 to 3 parts of hydrophobic monomer; and a proper amount of a pH regulator. According to the prepared scale inhibitor, the sand-carrying performance of fracturing fluid is met, meanwhile, groups with the scale inhibition function are innovatively introduced, during gel breaking, macromolecular chains are broken to release the scale inhibition groups, the scale inhibition groups are complexed with calcium and barium ions in formation water, scale deposition is inhibited, the effect and safety of fracturing construction can be further improved, and powerful guarantee is provided for efficient development of oil and gas fields.

Description

Technical Field

[0001] This invention belongs to the field of fracturing in unconventional oil and gas fields, specifically relating to a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs and its preparation method. Background Technology

[0002] The area is rich in deep coalbed methane resources, with reserves of up to 50 × 10⁻⁶ methane in deep coalbed methane formations at depths greater than 1500 meters. 12 m 3 The recoverable resources are 10.01 × 10⁻⁶. 12 m 3 They are mainly distributed in the Junggar Basin, Ordos Basin and Turpan-Santanghu Basin.

[0003] Currently, the efficient development and industrial development of coalbed methane face many technical bottlenecks. To achieve large-scale and efficient development of deep coalbed methane, three major changes in fracturing concepts and technologies are necessary to form the "ultimate volumetric fracturing" technology for deep coal reservoirs, characterized by "ultra-large, ultra-dense, and fully supported volumetric fracture networks." Specifically, this means: a shift in concept from matrix acid fracturing to volumetric fracturing; a shift in technology from "fracturing open" to "fracturing fragments"; and a shift in effect from "creating more fractures" to "creating more effective fractures."

[0004] Meanwhile, the mineralization of coal-rock gas reservoirs is usually high. High mineralization can lead to severe scaling, which can block the pores and fractures of the coal-rock gas reservoir, reduce permeability, affect gas flow, and thus reduce the production of a single well, reduce the final recovery rate, and affect economic benefits.

[0005] Most commercially available scale-inhibiting fracturing fluids involve adding scale inhibitors to the fracturing fluid itself, which not only increases the cost of chemical materials but also affects the performance of the fracturing fluid. Patent CN202410208632 discloses a scale-inhibiting fracturing fluid and its preparation method, which requires adding a composite scale inhibitor containing phosphate compounds to the fracturing fluid during the preparation process to achieve scale inhibition. This increases the complexity of the on-site preparation process, and phosphates have negative impacts on water bodies and the ecological environment. Patent CN202211164218 discloses a method for preparing an integrated scale-inhibiting, drag-reducing, and thickening fracturing emulsion. However, because the product contains a large amount of mineral oil, surfactants, and phosphorus-containing monomers, it is unfriendly to the reservoir and the aquatic environment.

[0006] Therefore, to achieve increased reserves and production of deep coalbed methane and meet the requirements of fracturing technology with "long-range support, large displacement, large fluid volume, and large sand volume," it is necessary to reduce the risk of scaling from the source, decrease the scaling tendency on the fracture surface and in the pores, maintain the conductivity of the fracture, and avoid a rapid decline in permeability due to scaling after fracturing, which would affect the well's production and stable production cycle. This requires the development of an environmentally friendly, fast-dissolving, low-molecular-weight, scale-inhibiting thickener suitable for reservoir stimulation of deep coalbed methane reservoirs. Summary of the Invention

[0007] This invention provides a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs and its preparation method, which addresses the shortcomings described in the prior art.

[0008] The technical solution adopted in this invention is as follows: A low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs is prepared from the following raw materials in parts by weight: 25-40 parts hydrophilic monomer; 3-6 parts scale-inhibiting monomer; 3-5 parts functional monomer; 50-55 parts deionized water; 0.5-0.8 parts emulsifier; 0.15-0.2 parts initiator; 1.5-3 parts hydrophobic monomer; and an appropriate amount of pH adjuster.

[0009] In a preferred embodiment of the present invention, the scale inhibitor monomer is at least one of itaconic acid and epoxysuccinic acid. The scale inhibitor monomer may be itaconic acid alone, epoxysuccinic acid alone, or a mixture of itaconic acid and epoxysuccinic acid. The introduction of itaconic acid or epoxysuccinic acid provides multiple carboxyl groups that can interact with metal ions from different directions to form stable complexes, thereby improving the scale inhibition ability of the polymer.

[0010] In a preferred embodiment of the present invention, the hydrophilic monomer is at least one of acrylic acid and acrylamide. Acrylic acid alone or acrylamide alone may be used, or a mixture of acrylic acid and acrylamide may be used.

[0011] In a preferred embodiment of the present invention, the functional monomer is at least one of sodium 2-acrylamido-2-methylpropanesulfonate and sodium p-styrenesulfonate. Sodium 2-acrylamido-2-methylpropanesulfonate alone, or sodium p-styrenesulfonate alone, or a mixture of sodium 2-acrylamido-2-methylpropanesulfonate and sodium p-styrenesulfonate may be used.

[0012] In a preferred embodiment of the present invention, the hydrophobic monomer is at least one of hexadecyl dimethyl allyl ammonium chloride and octadecyl dimethyl allyl ammonium chloride. Only hexadecyl dimethyl allyl ammonium chloride may be used, or only octadecyl dimethyl allyl ammonium chloride may be used, or a mixture of hexadecyl dimethyl allyl ammonium chloride and octadecyl dimethyl allyl ammonium chloride may be used.

[0013] In a preferred embodiment of the present invention, the emulsifier is one or a mixture of two water-soluble emulsifiers Tween-80 and OP-10; the initiator is a redox initiator or an azo initiator; and the pH adjuster is a 40% (w / w) NaOH aqueous solution.

[0014] In a preferred embodiment of the present invention, the redox initiator is composed of an oxidant and a reducing agent, wherein the oxidant is ammonium persulfate or potassium persulfate, and the reducing agent is sodium sulfite or sodium bisulfite.

[0015] This invention also provides a method for preparing a low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs, comprising the following steps: S1, by weight, weigh 25-40 parts of hydrophilic monomer, 2-3 parts of functional monomer, 1.5-3 parts of hydrophobic monomer, and 2-3 parts of pH adjuster, and dissolve them in 60-70 parts of deionized water. Stir to dissolve and obtain an aqueous phase with a pH of 7-7.5. Stirring and dissolving are carried out in a reactor. The hydrophobic monomer is added after adjusting the pH. S2, continuously introduce nitrogen, an inert gas, into the aqueous phase and maintain the temperature of the aqueous phase at 5-10℃; the nitrogen flow rate is 1-2 mL / min, and the outlet end of the nitrogen tube is placed 0.5 cm below the surface of the aqueous phase. S3, under the protection of nitrogen, an inert gas, add 0.5 to 0.8 parts of emulsifier while stirring at a speed of 200 to 300 rpm, and continuously introduce nitrogen for 15 to 20 minutes; S4, stop the inert gas supply, add 0.02 to 0.05 parts of initiator to the reactor to carry out the polymerization reaction. The polymerization reaction time is 3 to 6 hours to obtain a gel block.

[0016] As a preferred embodiment of the present invention, after obtaining the glue block, step S5 is performed to cut, dry, and crush the glue block obtained by polymerization, and sieve 80-mesh thickener particles to obtain a low molecular weight scale inhibitor thickener suitable for fracturing deep coal and gas reservoirs, which can be used after packaging.

[0017] The present invention also provides a fracturing fluid containing the prepared low-molecular-weight scale inhibitor thickener.

[0018] Beneficial effects of this invention: (1) The thickener has a simple molecular structure, good temperature and salt resistance, and low molecular weight, which can make the fracturing fluid have good fluidity and permeability, and can penetrate into the micro pores and cracks of coal and rock, reduce damage to coal and rock reservoirs, and thus improve coal and rock gas recovery rate.

[0019] (2) It can improve the gel breaking performance of low molecular weight fracturing fluid, and can break the gel quickly. The viscosity of the broken gel fluid is less than 2 mPa·s, which facilitates rapid flowback, reduces the residence time of fracturing fluid in the reservoir, and reduces damage to the reservoir. At the same time, after the thickener breaks the gel, it can prevent calcium, magnesium, barium and other ions from combining with sulfate and carbonate ions to form scale (such as CaCO3, BaSO4) through chelation or dispersion. This reduces the risk of scaling from the source, prevents scaling on the fracture surface and in the pores, maintains the conductivity of the fracture, and avoids the rapid decline in permeability caused by scaling after fracturing, which affects the well's production and stable production cycle.

[0020] (3) Despite its low molecular structure, it has low viscosity, high elasticity and strong sand carrying capacity, which can effectively carry the proppant to the depth of the fracture, form a stable supporting fracture, improve the flow capacity of the fracture, and thus provide a good channel for the production of coal and rock gas.

[0021] (4) The excellent salt resistance of low molecular weight fracturing fluids allows for the reuse of flowback fluids, which helps reduce environmental pollution during oil and gas development and achieve green development. Detailed Implementation

[0022] The following detailed embodiments clearly and completely describe the technical solution of the present invention. It should be understood that the preferred embodiments described herein are for illustration and explanation only and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments based on the following content of the present invention. In this invention, unless otherwise expressly specified and limited, the technical terms used in this application should have the ordinary meaning understood by those skilled in the art.

[0023] Example 1: A low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs is prepared using a four-necked flask as a reactor. It is composed of 31 parts acrylamide, 4 parts itaconic acid, 2 parts sodium 2-acrylamido-2-methylpropanesulfonate, 1 part sodium p-styrenesulfonate, 59 parts deionized water, 0.5 parts emulsifier, 2.5 parts hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, 0.15 parts redox initiator, and an appropriate amount of pH adjuster.

[0024] The preparation method is as follows: (1) Add 31 parts of acrylamide, 4 parts of itaconic acid, 2 parts of sodium 2-acrylamide-2-methylpropanesulfonate, 1 part of sodium p-styrenesulfonate and 59 parts of deionized water to a four-necked flask in sequence. Turn on the stirring and continuously introduce nitrogen gas at a flow rate of 2 mL / min. Place the outlet end of the nitrogen gas tube at 0.5 cm above the liquid level in the four-necked flask. (2) Add an appropriate amount of 40% NaOH solution to the liquid to adjust the pH to 7-7.5, and keep the system temperature below 10℃; (3) After stirring for 30 minutes, slowly add 0.5 parts of emulsifier Tween-80 and 2.5 parts of hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, continue stirring until uniform and continue to purge with nitrogen for 15-20 minutes; (4) Stop the nitrogen flow and slowly add 0.15 parts of redox initiator to the four-necked flask, wherein the molar ratio of potassium persulfate to sodium bisulfite is 2:1. When the reactants begin to polymerize, react for 3 to 6 hours to obtain the colloid of low molecular weight scale inhibitor thickener. (5) Take out the colloid, dry it at 95°C for 3 hours, and crush it through an 80-mesh standard sieve to obtain a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0025] Example 2: A low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs is prepared using a four-necked flask as a reactor. It is composed of 31 parts acrylamide, 4 parts epoxy succinic acid, 2 parts sodium 2-acrylamido-2-methylpropanesulfonate, 1 part sodium p-styrenesulfonate, 59 parts deionized water, 0.5 parts emulsifier, 2.5 parts hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, 0.15 parts redox initiator, and an appropriate amount of pH adjuster.

[0026] The preparation method is as follows: (1) Add 31 parts of acrylamide, 4 parts of epoxy succinic acid, 2 parts of sodium 2-acrylamide-2-methylpropanesulfonate, 1 part of sodium p-styrenesulfonate and 59 parts of deionized water to a four-necked flask in sequence. Turn on the stirring and continuously introduce nitrogen gas at a flow rate of 2 mL / min. Place the outlet end of the nitrogen gas tube at 0.5 cm above the liquid level in the four-necked flask. (2) Add an appropriate amount of 40% NaOH solution to the liquid to adjust the pH to 7-7.5, and keep the system temperature below 10℃; (3) After stirring for 30 minutes, slowly add 0.5 parts of emulsifier Tween-80 and 2.5 parts of hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, continue stirring until uniform and continue to purge with nitrogen for 15-20 minutes; (4) Stop the nitrogen flow and slowly add 0.15 parts of redox initiator to the four-necked flask, wherein the molar ratio of potassium persulfate to sodium bisulfite is 2:1. When the reactants begin to polymerize, react for 3 to 6 hours to obtain the colloid of low molecular weight scale inhibitor thickener. (5) Take out the colloid, dry it at 95°C for 3 hours, and crush it through an 80-mesh standard sieve to obtain a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0027] Example 3: A low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs is prepared from 25 parts acrylamide, 7 parts acrylic acid, 3 parts epoxy succinic acid, 2.5 parts sodium 2-acrylamido-2-methylpropanesulfonate, 1 part sodium p-styrenesulfonate, 59 parts deionized water, 0.5 parts emulsifier OP-10, 2 parts hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, 0.15 parts redox initiator, and an appropriate amount of pH adjuster.

[0028] The preparation method is as follows: (1) Add 25 parts of acrylamide, 7 parts of acrylic acid, 3 parts of epoxy succinic acid, 2.5 parts of sodium 2-acrylamide-2-methylpropanesulfonate, 1 part of sodium p-styrenesulfonate and 59 parts of deionized water to a four-necked flask in sequence. Turn on the stirring and continuously introduce nitrogen gas at a flow rate of 2 mL / min. Place the outlet end of the nitrogen gas tube at 0.5 cm above the liquid level in the four-necked flask. (2) Add an appropriate amount of pH adjuster (40% NaOH solution by mass) to the above liquid to adjust the pH to 7-9, and keep the system temperature below 10℃; (3) After stirring for 30 minutes, slowly add 0.5 parts of emulsifier OP-10 and 2 parts of hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, and continue stirring until homogeneous; (4) Slowly add 0.15 parts of redox initiator to a four-necked flask, wherein the molar ratio of potassium persulfate to sodium bisulfite is 2:1. When the reactants begin to polymerize, react for 3 to 6 hours to obtain the colloid of low molecular weight scale inhibitor thickener. (5) Take out the colloid, dry it at 95°C for 3 hours, and crush it through an 80-mesh standard sieve to obtain a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0029] Example 4: A low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs is prepared from 25 parts acrylamide, 7 parts acrylic acid, 3 parts itaconic acid, 2.5 parts sodium 2-acrylamido-2-methylpropanesulfonate, 1 part sodium p-styrenesulfonate, 59 parts deionized water, 0.5 parts emulsifier OP-10, 2 parts hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, 0.15 parts redox initiator, and an appropriate amount of pH adjuster.

[0030] The preparation method is as follows: (1) Add 25 parts of acrylamide, 7 parts of acrylic acid, 3 parts of itaconic acid, 2.5 parts of sodium 2-acrylamide-2-methylpropanesulfonate, 1 part of sodium p-styrenesulfonate and 59 parts of deionized water to a four-necked flask in sequence. Turn on the stirring and continuously introduce nitrogen gas at a flow rate of 2 mL / min. Place the outlet end of the nitrogen gas tube 0.5 cm above the liquid level in the four-necked flask. (2) Add an appropriate amount of pH adjuster (40% NaOH solution by mass) to the above liquid to adjust the pH to 7-9, and keep the system temperature below 10℃; (3) After stirring for 30 minutes, slowly add 0.5 parts of emulsifier OP-10 and 2 parts of hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, and continue stirring until homogeneous; (4) Slowly add 0.15 parts of redox initiator to a four-necked flask, wherein the molar ratio of potassium persulfate to sodium bisulfite is 2:1. When the reactants begin to polymerize, react for 3 to 6 hours to obtain the colloid of low molecular weight scale inhibitor thickener. (5) Take out the colloid, dry it at 95°C for 3 hours, and crush it through an 80-mesh standard sieve to obtain a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0031] Example 5: A low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs is prepared from 25 parts acrylamide, 4 parts acrylic acid, 3 parts itaconic acid, 3 parts epoxy succinic acid, 2.5 parts sodium 2-acrylamido-2-methylpropanesulfonate, 1 part sodium p-styrenesulfonate, 59 parts deionized water, 0.5 parts emulsifier OP-10, 2 parts hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, 0.15 parts redox initiator, and an appropriate amount of pH adjuster.

[0032] The preparation method is as follows: (1) Add 25 parts of acrylamide, 4 parts of acrylic acid, 3 parts of itaconic acid, 3 parts of epoxy succinic acid, 2.5 parts of sodium 2-acrylamide-2-methylpropanesulfonate, 1 part of sodium p-styrenesulfonate and 59 parts of deionized water to a four-necked flask in sequence. Turn on the stirring and continuously introduce nitrogen gas at a flow rate of 2 mL / min. Place the outlet end of the nitrogen gas tube at 0.5 cm above the liquid level in the four-necked flask. (2) Add an appropriate amount of pH adjuster (40% NaOH solution by mass) to the above liquid to adjust the pH to 7-9, and keep the system temperature below 10℃; (3) After stirring for 30 minutes, slowly add 0.5 parts of emulsifier OP-10 and 2 parts of hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, and continue stirring until homogeneous; (4) Slowly add 0.15 parts of redox initiator to a four-necked flask, wherein the molar ratio of potassium persulfate to sodium bisulfite is 2:1. When the reactants begin to polymerize, react for 3 to 6 hours to obtain the colloid of low molecular weight scale inhibitor thickener. (5) Take out the colloid, dry it at 95°C for 3 hours, and crush it through an 80-mesh standard sieve to obtain a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0033] Example 6: A low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs is prepared from 25 parts acrylamide, 4 parts acrylic acid, 3 parts itaconic acid, 3 parts epoxy succinic acid, 2.5 parts sodium 2-acrylamido-2-methylpropanesulfonate, 1 part sodium p-styrenesulfonate, 59 parts deionized water, 0.5 parts emulsifier OP-10, 2 parts hydrophobic monomer octadecyl dimethyl allyl ammonium chloride, 0.15 parts redox initiator, and an appropriate amount of pH adjuster.

[0034] The preparation method is as follows: (1) Add 25 parts of acrylamide, 4 parts of acrylic acid, 3 parts of itaconic acid, 3 parts of epoxy succinic acid, 2.5 parts of sodium 2-acrylamide-2-methylpropanesulfonate, 1 part of sodium p-styrenesulfonate and 59 parts of deionized water to a four-necked flask in sequence. Turn on the stirring and continuously introduce nitrogen gas at a flow rate of 2 mL / min. Place the outlet end of the nitrogen gas tube at 0.5 cm above the liquid level in the four-necked flask. (2) Add an appropriate amount of pH adjuster (40% NaOH solution by mass) to the above liquid to adjust the pH to 7-9, and keep the system temperature below 10℃; (3) After stirring for 30 minutes, slowly add 0.5 parts of emulsifier OP-10 and 2 parts of hydrophobic monomer octadecyl dimethyl allyl ammonium chloride, and continue stirring until homogeneous; (4) Slowly add 0.15 parts of redox initiator to a four-necked flask, wherein the molar ratio of potassium persulfate to sodium bisulfite is 2:1. When the reactants begin to polymerize, react for 3 to 6 hours to obtain the colloid of low molecular weight scale inhibitor thickener. (5) Take out the colloid, dry it at 95°C for 3 hours, and crush it through an 80-mesh standard sieve to obtain a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0035] Example 7: A low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs is prepared from 25 parts acrylamide, 4 parts acrylic acid, 3 parts itaconic acid, 3 parts epoxy succinic acid, 2.5 parts sodium 2-acrylamido-2-methylpropanesulfonate, 1 part sodium p-styrenesulfonate, 59 parts deionized water, 0.5 parts emulsifier OP-10, 1 part hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, 1 part hydrophobic monomer octadecyl dimethyl allyl ammonium chloride, 0.15 parts redox initiator, and an appropriate amount of pH adjuster.

[0036] The preparation method is as follows: (1) Add 25 parts of acrylamide, 4 parts of acrylic acid, 3 parts of itaconic acid, 3 parts of epoxy succinic acid, 2.5 parts of sodium 2-acrylamide-2-methylpropanesulfonate, 1 part of sodium p-styrenesulfonate and 59 parts of deionized water to a four-necked flask in sequence. Turn on the stirring and continuously introduce nitrogen gas at a flow rate of 2 mL / min. Place the outlet end of the nitrogen gas tube at 0.5 cm above the liquid level in the four-necked flask. (2) Add an appropriate amount of pH adjuster (40% NaOH solution by mass) to the above liquid to adjust the pH to 7-9, and keep the system temperature below 10℃; (3) After stirring for 30 minutes, slowly add 0.5 parts of emulsifier OP-10, 1 part of hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, and 1 part of hydrophobic monomer octadecyl dimethyl allyl ammonium chloride, and continue stirring until uniform; (4) Slowly add 0.15 parts of redox initiator to a four-necked flask, wherein the molar ratio of potassium persulfate to sodium bisulfite is 2:1. When the reactants begin to polymerize, react for 3 to 6 hours to obtain the colloid of low molecular weight scale inhibitor thickener. (5) Take out the colloid, dry it at 95°C for 3 hours, and crush it through an 80-mesh standard sieve to obtain a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0037] Example 8: A low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs is prepared from 25 parts acrylamide, 10 parts acrylic acid, 2.5 parts sodium 2-acrylamido-2-methylpropanesulfonate, 1 part sodium p-styrenesulfonate, 59 parts deionized water, 0.5 parts emulsifier OP-10, 1 part hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, 1 part hydrophobic monomer octadecyl dimethyl allyl ammonium chloride, 0.15 parts redox initiator, and an appropriate amount of pH adjuster.

[0038] The preparation method is as follows: (1) Add 25 parts of acrylamide, 10 parts of acrylic acid, 2.5 parts of sodium 2-acrylamide-2-methylpropanesulfonate, 1 part of sodium p-styrenesulfonate and 59 parts of deionized water to a four-necked flask in sequence. Turn on the stirring and continuously introduce nitrogen gas at a flow rate of 2 mL / min. Place the outlet end of the nitrogen gas tube at 0.5 cm above the liquid level in the four-necked flask. (2) Add an appropriate amount of pH adjuster (40% NaOH solution by mass) to the above liquid to adjust the pH to 7-9, and keep the system temperature below 10℃; (3) After stirring for 30 minutes, slowly add 0.5 parts of emulsifier OP-10, 1 part of hydrophobic monomer hexadecyl dimethyl allyl ammonium chloride, and 1 part of hydrophobic monomer octadecyl dimethyl allyl ammonium chloride, and continue stirring until uniform; (4) Slowly add 0.15 parts of redox initiator to a four-necked flask, wherein the molar ratio of potassium persulfate to sodium bisulfite is 2:1. When the reactants begin to polymerize, react for 3 to 6 hours to obtain the colloid of low molecular weight scale inhibitor thickener. (5) Take out the colloid, dry it at 95°C for 3 hours, and crush it through an 80-mesh standard sieve to obtain a low-molecular-weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0039] Referring to the preparation method of synthetic polymer fracturing fluid in SY / T7627-2021 "Technical Requirements for Water-Based Fracturing Fluids", the dissolution time of the thickeners prepared in Examples 1 to 8 of this invention was determined. The specific operation is as follows: At room temperature, 0.15 g (0.3%) of the low molecular weight scale inhibitor thickener prepared in Examples 1 to 8 was weighed and added to 5 portions of 500 mL deionized water. Under the condition of stirring rate of 3000 ± 90 r / min, the time when the dry powder completely disappeared in the water was recorded. The time from the addition of the dry powder to the disappearance of the dry powder is the dissolution time of the low molecular weight scale inhibitor thickener for fracturing deep coal and gas reservoirs.

[0040] The dissolution time of the thickeners prepared in Examples 1 to 8 is shown in Table 1.

[0041] Table 1. Dissolution time of thickener Table 1 shows that the low-molecular-weight scale inhibitor thickener prepared by the present invention has excellent solubility in water and can meet the requirements of on-site rapid mixing for online mixing and fracturing operations of dry powder in coal and rock gas reservoirs.

[0042] To further illustrate the salt resistance of the thickener of the present invention, the viscosity of the thickeners prepared in Examples 1 to 8 of the present invention was measured. The specific operation is as follows: The viscosity of the thickeners prepared in Examples 1-8 in deionized water and standard saline solution at 0.3% concentration for 30 minutes is shown in Table 2.

[0043] Table 2 Viscosity of Thickener Table 2 shows that at room temperature, the viscosity of the aqueous solution of 0.3% thickener in deionized water is ≥40 mPa·s; in standard brine (refer to SY / T 5107-2016 Performance Evaluation Method of Water-based Fracturing Fluid), the viscosity retention rate of the aqueous solution of 0.3% thickener is above 90%. This indicates that the low-molecular-weight scale-inhibiting thickener prepared by this invention has excellent salt resistance and can be used for the reuse of flowback fluids with high mineralization and high calcium and magnesium ion content.

[0044] To further illustrate the scale inhibition performance of the thickener provided by the present invention, the viscosity of the thickeners prepared in Examples 1 to 8 of the present invention was measured. The specific operation is as follows: The thickeners prepared in Examples 1-8 were formulated into 0.3% fracturing fluids, and 0.03% ammonium persulfate was used to break the gel at 70°C for 2 hours. After the gelled solution was stirred evenly, it was used as a scale inhibitor. The scale inhibition efficiency of the gelled solution was evaluated. The scale inhibition efficiency of CaCO3 scale, CaSO4 scale, and BaSO4 scale was tested according to the standard of "Technical Specification for Corrosion and Scale Inhibitors for Oilfield Water Treatment" (Q / SY 17126—2019). The results are shown in Table 3.

[0045] Table 3 Scale inhibition efficiency of thickeners Comparative experiments in Examples 1, 7, and 8 show that a single carboxyl group (−COOH) has a certain complexing effect on metal ions (such as calcium and barium ions), but its complexing ability is limited. In contrast, epoxy succinic acid, besides having a carboxyl group that can complex metal ions, allows multiple carboxyl groups to interact with metal ions from different directions to form stable complexes. The epoxy groups may increase the spatial structural advantage, resulting in stronger scale inhibition ability than a single carboxyl group.

[0046] In the above examples, the decolloid containing a large amount of -COOH, -SO3H and -CONH2 has a significant scale inhibition effect on calcium and barium scale, with scale inhibition efficiencies of 96%, 98% and 80% for CaCO3, CaSO4 and BaSO4 scale, respectively; the scale inhibition performance is excellent.

[0047] The scale inhibitor prepared by this invention not only meets the proppant-carrying performance of fracturing fluid, but also innovatively introduces scale-inhibiting functional groups: when the gel breaks, the polymer chain breaks and releases scale-inhibiting groups, which complex with calcium and magnesium ions in the formation water to inhibit scale deposition. This can further improve the effectiveness and safety of fracturing operations and provide a strong guarantee for the efficient development of oil and gas fields.

[0048] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the technical solution of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the technical solution of the present invention are also considered.

Claims

1. A low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs, characterized in that: It is prepared from the following parts by weight of raw materials: 25-40 parts hydrophilic monomer; 3-6 parts scale inhibitor; 3-5 parts functional monomer; 50-55 parts deionized water; 0.5-0.8 parts emulsifier; 0.15-0.2 parts initiator; 1.5-3 parts hydrophobic monomer; appropriate amount of pH adjuster.

2. The low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs according to claim 1, characterized in that: The scale inhibitor monomer is at least one of itaconic acid and epoxysuccinic acid.

3. The low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs according to claim 2, characterized in that: The hydrophilic monomer is at least one of acrylic acid and acrylamide.

4. The low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs according to claim 3, characterized in that: The functional monomer is at least one of sodium 2-acrylamide-2-methylpropanesulfonate and sodium p-styrenesulfonate.

5. The low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs according to claim 3, characterized in that: The hydrophobic monomer is at least one of hexadecyl dimethyl allyl ammonium chloride and octadecyl dimethyl allyl ammonium chloride.

6. The low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs according to claim 3, characterized in that: The emulsifier is one or a mixture of two water-soluble emulsifiers, Tween-80 and OP-10; the initiator is a redox initiator or an azo initiator; and the pH adjuster is a 40% (w / w) NaOH aqueous solution.

7. The low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs according to claim 6, characterized in that: The redox initiator consists of an oxidant and a reducing agent, wherein the oxidant is ammonium persulfate or potassium persulfate, and the reducing agent is sodium sulfite or sodium bisulfite.

8. A method for preparing a low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs as described in any one of claims 1-7, characterized in that, Includes the following steps: S1, by weight, weigh 25 to 40 parts of hydrophilic monomer, 3 to 6 parts of scale inhibitor monomer, 3 to 5 parts of functional monomer, 1.5 to 3 parts of hydrophobic monomer, and an appropriate amount of pH adjuster, and dissolve them in 50 to 55 parts of deionized water. Stir to dissolve and obtain an aqueous phase with a pH value of 7-7.

5. S2, introduce inert gas into the aqueous phase and maintain the temperature of the aqueous phase at 5-10℃; S3, under the protection of an inert gas, stir and add 0.5 to 0.8 parts of emulsifier; S4, stop the inert gas supply, add 0.15 to 0.2 parts of initiator to carry out the polymerization reaction, and the polymerization reaction time is 3 to 6 hours.

9. The preparation method of the low-molecular-weight scale-inhibiting thickener for fracturing deep coal and gas reservoirs according to claim 8, characterized in that, It also includes S5, which involves cutting, drying, and crushing the polymerized block, and then sieving 80-mesh thickener particles to obtain a low-molecular-weight scale inhibitor thickener suitable for fracturing deep coal and gas reservoirs.

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