Coal rock gas well low damage fracturing fluid and preparation method thereof

Abstract

This invention discloses a low-damage fracturing fluid for coal and rock gas wells and its preparation method, relating to the field of petrochemical technology. In preparing the low-damage fracturing fluid for coal and rock gas wells, the invention first reacts allyl phosphorus dichloride with 4-(dimethylaminomethyl)phenol to obtain a double-bonded ditertiary amine intermediate; reacts the double-bonded ditertiary amine intermediate with a haloalkane to obtain a gemini quaternary ammonium hydrophobic monomer; reacts N,N'-ethylenediamine disuccinic acid with allyl glycidyl ether to obtain a metal chelating monomer; copolymerizes acrylamide, acrylic acid, fumaric acid, the metal chelating monomer, and the gemini quaternary ammonium hydrophobic monomer to obtain modified polyacrylamide; and mixes the modified polyacrylamide with water and additives to obtain the low-damage fracturing fluid for coal and rock gas wells. The low-damage fracturing fluid for coal and rock gas wells prepared by this invention has the advantages of low damage, pore enlargement and permeability enhancement, anti-swelling, temperature and shear resistance, and salt resistance.

Description

Technical Field

[0001] This invention relates to the field of petrochemical technology, specifically to a low-damage fracturing fluid for coal and rock gas wells and its preparation method. Background Technology

[0002] Coalbed methane, also known as coal mine gas, is a clean and combustible gas, composed of over 95% methane. This gas is adsorbed onto coal seams, mudstone, and other materials. my country has abundant coal resources, and its associated coalbed methane resources are also quite rich. Due to its extremely high methane content and the large amount of heat released upon combustion, coalbed methane is being considered as a new type of energy source. With advancements in exploration technology, more and more coalbed methane blocks are being discovered, especially deep coalbed methane below 2000m, which boasts enormous reserves. Deep coalbed methane reservoirs have low permeability but high methane content; with appropriate extraction methods, considerable production can be achieved, effectively alleviating energy shortages.

[0003] However, due to the complex geological conditions of coal seams in my country, most coal reservoirs exhibit characteristics such as low permeability and low pressure. Changes in the sedimentary environment and complex geological tectonic processes lead to significant heterogeneity and anisotropy within the coal seams. Coal seams are sensitive to foreign compounds, and the fracturing fluid after hydraulic fracturing causes severe damage to the coal reservoir. After fracturing fluid is injected into the coal seam, water and additives are lost into the reservoir, altering the physical and chemical properties of the coal seam and having both positive and negative effects on its permeability. The permeability characteristics of the coal seam are a crucial parameter determining the efficiency of coalbed methane extraction. Therefore, developing fracturing fluids with low damage rates is of great significance for the efficient development of coalbed methane. Summary of the Invention

[0004] The purpose of this invention is to provide a low-damage fracturing fluid for coal and rock gas wells and its preparation method, so as to solve the problems existing in the prior art.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] A low-damage fracturing fluid for coal and rock gas wells, wherein the low-damage fracturing fluid for coal and rock gas wells is prepared by mixing modified polyacrylamide with water and additives;

[0007] The modified polyacrylamide is prepared by copolymerizing acrylamide, acrylic acid, fumaric acid, metal chelating monomers, and gemini quaternary ammonium hydrophobic monomers.

[0008] The geminal quaternary ammonium hydrophobic monomer is prepared by reacting a double-bonded ditertiary amine intermediate with a haloalkane.

[0009] The double-bonded ditertiary amine intermediate is prepared by reacting allyl phosphorus dichloride with 4-(dimethylaminomethyl)phenol.

[0010] The metal chelating monomer is prepared by reacting N,N'-ethylenediamine disuccinic acid with allyl glycidyl ether.

[0011] As an optimization, the additives include debonding agents, anti-swelling agents, waterproofing additives, and drag-reducing agents.

[0012] As an optimization, the haloalkanes include chloroalkanes, bromoalkanes, and iodoalkanes, with a carbon chain length of 12 to 22.

[0013] A method for preparing a low-damage fracturing fluid for coal and rock gas wells includes the following preparation steps:

[0014] (1) By mass fraction, under a nitrogen atmosphere, 2-3 parts of 4-(dimethylaminomethyl)phenol and 15-30 parts of diethylene glycol monomethyl ether are mixed evenly. At room temperature, at 200-300 r / min, 1.3-1.96 parts of triethylamine are added dropwise at 4-10 ml / min, followed by 0.04-0.08 parts of hydroquinone. The mixture is then transferred to an ice-water bath to cool to 0-2℃. 3.34-5 parts of 30wt% diethylene glycol monomethyl ether solution of allyl phosphorus dichloride are added dropwise at 1-1.8 ml / min. After the addition is complete, the mixture is reacted at 300-400 r / min for 10-14 h at room temperature. The mixture is then heated to 55-65℃ and reacted for another 4-6 h. The mixture is filtered, and the solvent is removed by rotary evaporation of the filtrate. The filtrate is then extracted, washed, concentrated, and purified with dichloromethane and saturated sodium chloride aqueous solution to obtain a double-bonded ditertiary amine intermediate.

[0015] (2) By mass fraction, 3-4 parts of double-bonded ditertiary amine intermediate, 0.008-0.012 parts of hydroquinone, 0.2-0.3 parts of potassium iodide, and 20-30 parts of acetonitrile are mixed evenly. The haloalkane is added at a molar ratio of 1:(2-2.2) of double-bonded ditertiary amine intermediate to haloalkane. The mixture is refluxed and stirred at 300-500 r / min at 80-85℃ for 12-16 h. The solvent is removed by rotary evaporation. After recrystallization and purification, the mixture is vacuum dried at 50-60℃ for 8-10 h to obtain the Gemini quaternary ammonium hydrophobic monomer.

[0016] (3) Dissolve 3-4 parts of N,N'-ethylenediamine disuccinic acid in 40-50 parts of pure water by mass, adjust the pH to 10-11 with 1 mol / L sodium hydroxide solution, add 4.3-5.73 parts of 30 wt% allyl glycidyl ether ethanol solution under nitrogen atmosphere, stir and reflux at 65-70℃ and 300-400 r / min for 10-12 h, cool to 0-4℃ in ice water bath, adjust the pH to 6.5-7 with 1 mol / L hydrochloric acid solution, dialyze at 0-4℃ using a dialysis membrane with a molecular weight cutoff of 300 Da, change the pure water every 3-4 h, dialyze until the conductivity is less than 5 μs / cm, concentrate by rotary evaporation to 10%-20% of the original volume, freeze dry for 24-30 h to obtain the metal chelate monomer;

[0017] (4) By mass, 13-17 parts acrylamide, 6-8 parts acrylic acid, 0.13-0.15 parts fumaric acid, 2.5-2.8 parts emulsifier, and 40-50 parts pure water are mixed evenly, and the pH is adjusted to 7-8. Under a nitrogen atmosphere, 2.1-2.3 parts metal chelating monomer, 1.4-1.8 parts gemini quaternary ammonium hydrophobic monomer, and 0.09-0.12 parts initiator are added. The mixture is reacted at 75-80℃ and 300-400 r / min for 10-12 h. After naturally cooling to room temperature, the mixture is cut into small pieces, washed 3-4 times with anhydrous ethanol, and vacuum dried at 50-60℃ for 10-12 h. The mixture is then pulverized and passed through a 120-mesh sieve to obtain modified polyacrylamide.

[0018] (5) Mix 0.1 to 1 part of modified polyacrylamide and 100 to 110 parts of pure water evenly by mass, stir at 300 to 400 r / min for 30 to 40 min at room temperature, adjust the pH to 6 to 6.5, and obtain low-damage fracturing fluid for coal and rock gas wells.

[0019] As an optimization, the reaction process of the double-bonded ditertiary amine intermediate in step (1) is as follows:

[0020] .

[0021] As an optimization, the reaction process of the gemini quaternary ammonium hydrophobic monomer in step (2) is as follows:

[0022] .

[0023] As an optimization, the reaction process of the metal chelating monomer in step (3) is as follows:

[0024] .

[0025] As an optimization, the emulsifier in step (4) is OP-10, and the initiator is azobisisobutyronitrile or azobisisopropylimidazoline hydrochloride.

[0026] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0027] In preparing a low-damage fracturing fluid for coal and gas wells, this invention first reacts allyl phosphorus dichloride with 4-(dimethylaminomethyl)phenol to obtain a double-bonded ditertiary amine intermediate; then reacts the double-bonded ditertiary amine intermediate with a haloalkane to obtain a gemini quaternary ammonium hydrophobic monomer; finally, reacts N,N'-ethylenediamine disuccinic acid with allyl glycidyl ether to obtain a metal chelating monomer; then copolymerizes acrylamide, acrylic acid, fumaric acid, the metal chelating monomer, and the gemini quaternary ammonium hydrophobic monomer to obtain modified polyacrylamide; and finally, mixes the modified polyacrylamide with water and additives to obtain the low-damage fracturing fluid for coal and gas wells.

[0028] First, allyl phosphorus dichloride is reacted with 4-(dimethylaminomethyl)phenol to prepare a double-bonded ditertiary amine intermediate. The two phosphoryl chloride groups on the allyl phosphorus dichloride undergo a substitution reaction with the phenolic group on the 4-(dimethylaminomethyl)phenol, thereby obtaining a disubstituted product. This yields a double-bonded ditertiary amine intermediate containing two tertiary amine groups and a double bond. The double bond allows it to participate in subsequent copolymerization. The two tertiary amines can form geminal quaternary ammonium salts with haloalkanes, and the presence of two rigid benzene rings improves thermal stability. The double-bonded ditertiary amine intermediate is then reacted with haloalkanes to prepare a geminal quaternary ammonium hydrophobic monomer. The halogen atoms on the haloalkanes undergo a quaternization reaction with the tertiary amine groups on the double-bonded ditertiary amine intermediate, thereby forming… Geminid quaternary ammonium hydrophobic monomers, containing two hydrophobic carbon chains, two quaternary ammonium groups, two benzene rings, and double bonds, significantly enhance local hydrophobicity. The concentrated quaternary ammonium cations can adsorb onto the negatively charged surfaces of coal, rock, and clay, neutralizing their ionic charges. Simultaneously, the hydrophobic carbon chains extend outward, causing the chemical bonds of oxygen-containing functional groups on the coal matrix surface to break and mineral components to detach, resulting in a hydrophobic state. This prevents swelling caused by water intrusion and inhibits permeability damage caused by water-sensitive swelling of clay. Furthermore, the hydrophobic long carbon chains can reduce surface tension, decrease permeability damage caused by water lock in microcracks, and increase the desorption and diffusion channels for coalbed methane.

[0029] Secondly, a metal chelating monomer is prepared by reacting N,N'-ethylenediamine disuccinic acid with allyl glycidyl ether. The epoxy group on the allyl glycidyl ether reacts with the secondary amine group on N,N'-ethylenediamine disuccinic acid to obtain a metal chelating monomer with metal chelating groups and polymerizable double bonds. N,N'-ethylenediamine disuccinic acid has a strong metal chelating ability. In fracturing operations, it can capture metal ions from coal, rock, and clay, assisting the geminal quaternary ammonium hydrophobic monomer in fracturing the coal matrix and clay minerals, expanding the pore size and number of pores, achieving the effect of pore enlargement and permeability enhancement, and improving the efficiency of methane gas desorption.

[0030] Finally, modified polyacrylamide was prepared by copolymerizing acrylamide, acrylic acid, fumaric acid, metal chelating monomers, and gemini quaternary ammonium hydrophobic monomers. Acrylamide and acrylic acid served as the backbone of the modified polyacrylamide, exhibiting high average bond energy, stable main chain properties, and strong thickening and drag-reducing properties. Gemini quaternary ammonium hydrophobic monomers and metal chelating monomers served as the main functional side groups on the chain segments. The gemini quaternary ammonium hydrophobic monomers contain two long carbon chains, two quaternary ammonium cations, and two rigid benzene ring groups, which significantly improve temperature and shear resistance. The cationic groups enhance the polymer's salt resistance. The hydrophobic association of the hydrophobic long carbon chains in the fracturing fluid increases the spatial network structure of the fracturing fluid, thereby increasing viscosity and enhancing proppant carrying capacity. After the fracturing fluid breaks down, some chain segments simultaneously possess both hydrophobic and hydrophilic groups, enhancing the surface activity of the fracturing fluid. This effectively reduces the surface tension of the fracturing fluid, facilitating its flowback. The quaternary ammonium salt molecules formed after fracturing can also continue to adhere to the surface of clay minerals, inhibiting their hydration, swelling, and migration, thus reducing the decrease in permeability caused by pore throat blockage. The addition of fumaric acid monomers provides a certain degree of isolation on the main chain, resulting in a comb-like random distribution of hydrophobic side groups. The addition of metal chelating monomers introduces strong chelating groups with very strong metal chelating capabilities. Furthermore, their anions carry a negative charge, combining with positively charged quaternary ammonium cations to form an amphoteric structure, exhibiting a significant "anti-polyelectrolyte effect," effectively improving salt tolerance. Detailed Implementation

[0031] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0032] The raw material information used in all the following examples and comparative examples is as follows:

[0033] Halogenated alkanes: 1-bromohexadecane;

[0034] Emulsifier: OP-10;

[0035] Initiator: Azobisisopropylimidazoline hydrochloride.

[0036] Example 1:

[0037] A method for preparing a low-damage fracturing fluid for coal and rock gas wells, comprising the following preparation steps:

[0038] (1) By mass fraction, under a nitrogen atmosphere, 2 parts of 4-(dimethylaminomethyl)phenol and 15 parts of diethylene glycol monomethyl ether were mixed evenly. At room temperature, at 200 r / min, 1.3 parts of triethylamine were added dropwise at 4 ml / min, followed by 0.04 parts of hydroquinone. The mixture was then transferred to an ice-water bath to cool to 0°C. 3.34 parts of 30 wt% diethylene glycol monomethyl ether solution of allyl phosphorus dichloride were added dropwise at 1 ml / min. After the addition was completed, the mixture was reacted at 300 r / min for 14 h at room temperature. The mixture was then heated to 55°C and reacted for another 6 h. The mixture was filtered, and the solvent was removed by rotary evaporation of the filtrate. The filtrate was then extracted, washed, concentrated, and purified with dichloromethane and saturated sodium chloride aqueous solution to obtain a double-bonded ditertiary amine intermediate.

[0039] (2) By mass fraction, 3 parts of double bond ditertiary amine intermediate, 0.008 parts of hydroquinone, 0.2 parts of potassium iodide and 20 parts of acetonitrile were mixed evenly. The haloalkane was added at a molar ratio of 1:2 of double bond ditertiary amine intermediate to haloalkane. The mixture was stirred under reflux at 300 r / min at 80 °C for 16 h. The solvent was removed by rotary evaporation. After recrystallization and purification, the mixture was dried under vacuum at 50 °C for 10 h to obtain the Gemini quaternary ammonium hydrophobic monomer.

[0040] (3) Dissolve 3 parts of N,N'-ethylenediamine disuccinic acid in 40 parts of pure water by mass, adjust the pH to 10 with 1 mol / L sodium hydroxide solution, add 4.3 parts of 30 wt% allyl glycidyl ether ethanol solution under nitrogen atmosphere, stir and reflux at 65℃ and 300 r / min for 12 h, cool to 0℃ in ice water bath, adjust the pH to 6.5 with 1 mol / L hydrochloric acid solution, dialyze at 0℃ using a dialysis membrane with a molecular weight cutoff of 300 Da, change the pure water every 3 h, dialyze until the conductivity is less than 5 μs / cm, concentrate to 10% of the original volume by rotary evaporation, freeze dry for 24 h to obtain the metal chelate monomer;

[0041] (4) By mass, 13 parts acrylamide, 6 parts acrylic acid, 0.13 parts fumaric acid, 2.5 parts emulsifier, and 40 parts pure water were mixed evenly, and the pH was adjusted to 7. Under a nitrogen atmosphere, 2.1 parts metal chelating monomer, 1.4 parts gemini quaternary ammonium hydrophobic monomer, and 0.09 parts initiator were added. The mixture was reacted at 75°C and 300 r / min for 12 h. After naturally cooling to room temperature, the mixture was cut into pieces, washed three times with anhydrous ethanol, dried under vacuum at 50°C for 12 h, pulverized and ground into powder, and passed through a 120-mesh sieve to obtain modified polyacrylamide.

[0042] (5) Mix 0.5 parts of modified polyacrylamide and 100 parts of pure water evenly by mass, stir at 300 r / min for 40 min at room temperature, adjust pH to 6, and prepare low-damage fracturing fluid for coal and rock gas wells.

[0043] Example 2:

[0044] A method for preparing a low-damage fracturing fluid for coal and rock gas wells, comprising the following preparation steps:

[0045] (1) By mass, under a nitrogen atmosphere, 2.5 parts of 4-(dimethylaminomethyl)phenol and 22 parts of diethylene glycol monomethyl ether were mixed evenly. At room temperature, at 250 r / min, 1.63 parts of triethylamine were added dropwise at 7 ml / min, followed by 0.06 parts of hydroquinone. The mixture was then transferred to an ice-water bath to cool to 1°C. 4.17 parts of 30 wt% diethylene glycol monomethyl ether solution of allyl phosphorus dichloride were added dropwise at 1.4 ml / min. After the addition was completed, the mixture was reacted at 350 r / min for 12 h at room temperature. The mixture was then heated to 60°C and reacted for another 5 h. The mixture was filtered, and the solvent was removed by rotary evaporation of the filtrate. The filtrate was then extracted, washed, concentrated, and purified with dichloromethane and saturated sodium chloride aqueous solution to obtain a double-bonded ditertiary amine intermediate.

[0046] (2) By mass fraction, 3.5 parts of double bond ditertiary amine intermediate, 0.01 parts of hydroquinone, 0.25 parts of potassium iodide and 25 parts of acetonitrile were mixed evenly. The haloalkane was added at a molar ratio of 1:2.1 of double bond ditertiary amine intermediate and haloalkane. The mixture was stirred under reflux at 80°C and 400 r / min for 14 h. The solvent was removed by rotary evaporation. After recrystallization and purification, the mixture was dried under vacuum at 55°C for 9 h to obtain the Gemini quaternary ammonium hydrophobic monomer.

[0047] (3) Dissolve 3.5 parts of N,N'-ethylenediamine disuccinic acid in 45 parts of pure water by mass, adjust the pH to 10.5 with 1 mol / L sodium hydroxide solution, add 5.01 parts of 30 wt% allyl glycidyl ether ethanol solution under nitrogen atmosphere, stir and reflux at 350 r / min at 65℃ for 11 h, cool to 2℃ in an ice water bath, adjust the pH to 6.5 with 1 mol / L hydrochloric acid solution, dialyze at 2℃ using a dialysis membrane with a molecular weight cutoff of 300 Da, change the pure water every 3.5 h, dialyze until the conductivity is less than 5 μs / cm, concentrate to 15% of the original volume by rotary evaporation, freeze dry for 27 h to obtain the metal chelate monomer;

[0048] (4) By mass, 15 parts acrylamide, 7 parts acrylic acid, 0.14 parts fumaric acid, 2.6 parts emulsifier, and 45 parts pure water were mixed evenly, and the pH was adjusted to 7.5. Under a nitrogen atmosphere, 2.2 parts metal chelating monomer, 1.6 parts gemini quaternary ammonium hydrophobic monomer, and 0.105 parts initiator were added. The mixture was reacted at 80°C and 350 r / min for 11 h. After naturally cooling to room temperature, the mixture was cut into pieces, washed three times with anhydrous ethanol, and vacuum dried at 55°C for 11 h. The mixture was then pulverized, ground into powder, and passed through a 120-mesh sieve to obtain modified polyacrylamide.

[0049] (5) Mix 0.6 parts of modified polyacrylamide and 105 parts of pure water evenly by mass, stir at 350 r / min for 35 min at room temperature, adjust pH to 6, and prepare low-damage fracturing fluid for coal and rock gas wells.

[0050] Example 3:

[0051] A method for preparing a low-damage fracturing fluid for coal and rock gas wells, comprising the following preparation steps:

[0052] (1) By mass fraction, under a nitrogen atmosphere, 3 parts of 4-(dimethylaminomethyl)phenol and 30 parts of diethylene glycol monomethyl ether were mixed evenly. At room temperature, at 300 r / min, 1.96 parts of triethylamine were added dropwise at 10 ml / min, and then 0.08 parts of hydroquinone were added and mixed evenly. The mixture was then transferred to an ice-water bath to cool to 2°C. 5 parts of 30 wt% diethylene glycol monomethyl ether solution of allyl phosphorus dichloride were added dropwise at 1.8 ml / min. After the addition was completed, the mixture was reacted at 400 r / min for 10 h at room temperature. The mixture was then heated to 65°C and reacted for another 4 h. The mixture was filtered, and the solvent was removed by rotary evaporation of the filtrate. The filtrate was extracted, washed, concentrated and purified with dichloromethane and saturated sodium chloride aqueous solution to obtain a double-bonded ditertiary amine intermediate.

[0053] (2) By mass fraction, 4 parts of double bond ditertiary amine intermediate, 0.012 parts of hydroquinone, 0.3 parts of potassium iodide and 30 parts of acetonitrile were mixed evenly. The haloalkane was added at a molar ratio of 1:2.2 of double bond ditertiary amine intermediate and haloalkane. The mixture was stirred under reflux at 85°C and 500 r / min for 12 h. The solvent was removed by rotary evaporation. After recrystallization and purification, the mixture was dried under vacuum at 60°C for 8 h to obtain the Gemini quaternary ammonium hydrophobic monomer.

[0054] (3) Dissolve 4 parts of N,N'-ethylenediamine disuccinic acid in 50 parts of pure water by mass, adjust the pH to 11 with 1 mol / L sodium hydroxide solution, add 5.73 parts of 30 wt% allyl glycidyl ether ethanol solution under nitrogen atmosphere, stir and reflux at 70℃ and 400 r / min for 10 h, cool to 4℃ in ice water bath, adjust the pH to 7 with 1 mol / L hydrochloric acid solution, dialyze at 4℃ using a dialysis membrane with a molecular weight cutoff of 300 Da, change the pure water every 4 h, dialyze until the conductivity is less than 5 μs / cm, concentrate to 20% of the original volume by rotary evaporation, freeze dry for 30 h to obtain the metal chelate monomer;

[0055] (4) By mass, 17 parts acrylamide, 8 parts acrylic acid, 0.15 parts fumaric acid, 2.8 parts emulsifier, and 50 parts pure water were mixed evenly, and the pH was adjusted to 8. Under a nitrogen atmosphere, 2.3 parts metal chelating monomer, 1.8 parts gemini quaternary ammonium hydrophobic monomer, and 0.12 parts initiator were added. The mixture was reacted at 80°C and 400 r / min for 10 h. After naturally cooling to room temperature, the mixture was cut into pieces, washed 4 times with anhydrous ethanol, dried under vacuum at 60°C for 10 h, pulverized and ground into powder, and passed through a 120-mesh sieve to obtain modified polyacrylamide.

[0056] (5) Mix 0.7 parts of modified polyacrylamide and 110 parts of pure water evenly by mass, stir at 400 r / min for 30 min at room temperature, adjust pH to 6.5, and prepare low-damage fracturing fluid for coal and rock gas wells.

[0057] Comparative Example 1:

[0058] The difference between the preparation method of the low-damage fracturing fluid for coal and gas wells in Comparative Example 1 and Example 2 lies in step (2). Step (2) is modified as follows: 3.5 parts by mass of the double-bonded ditertiary amine intermediate, 0.01 parts of hydroquinone, 0.25 parts of potassium iodide, and 25 parts of acetonitrile are mixed evenly. 1-Bromopropane is added at a molar ratio of 1:2.1 between the double-bonded ditertiary amine intermediate and 1-bromopropane. The mixture is refluxed and stirred at 80°C and 400 r / min for 14 h. The solvent is removed by rotary evaporation, and after recrystallization and purification, it is vacuum dried at 55°C for 9 h to obtain the gemini quaternary ammonium hydrophobic monomer. The remaining steps are the same as in Example 2.

[0059] Comparative Example 2:

[0060] The preparation method of the low-damage fracturing fluid for coal and rock gas wells in Comparative Example 2 differs from that in Example 2 in that steps (1) and (2) are omitted, and step (4) is modified as follows: 15 parts acrylamide, 7 parts acrylic acid, 0.14 parts fumaric acid, 2.6 parts emulsifier, and 45 parts pure water are mixed evenly by mass, the pH is adjusted to 7.5, 2.2 parts metal chelating monomer and 0.105 parts initiator are added under a nitrogen atmosphere, and the mixture is reacted at 80°C and 350 r / min for 11 h. After natural cooling to room temperature, the mixture is cut into small pieces, washed three times with anhydrous ethanol, vacuum dried at 55°C for 11 h, pulverized, and passed through a 120-mesh sieve to obtain modified polyacrylamide. The remaining steps are the same as in Example 2.

[0061] Comparative Example 3:

[0062] The preparation method of the low-damage fracturing fluid for coal and rock gas wells in Comparative Example 3 differs from that in Example 2 in that step (3) is omitted, and step (4) is modified as follows: 15 parts acrylamide, 7 parts acrylic acid, 0.14 parts fumaric acid, 2.6 parts emulsifier, and 45 parts pure water are mixed evenly by mass, the pH is adjusted to 7.5, and 1.6 parts gemini quaternary ammonium hydrophobic monomer and 0.105 parts initiator are added under a nitrogen atmosphere. The mixture is reacted at 80°C and 350 r / min for 11 h, naturally cooled to room temperature, cut into small pieces, washed three times with anhydrous ethanol, vacuum dried at 55°C for 11 h, pulverized, and passed through a 120-mesh sieve to obtain modified polyacrylamide. The remaining steps are the same as in Example 2.

[0063] Comparative Example 4:

[0064] The difference between the preparation method of the low-damage fracturing fluid for coal and gas wells in Comparative Example 4 and Example 2 lies in step (4). Step (4) is modified as follows: 15 parts acrylamide, 7 parts acrylic acid, 2.6 parts emulsifier, and 45 parts pure water are mixed evenly by mass, and the pH is adjusted to 7.5. Under a nitrogen atmosphere, 2.2 parts metal chelating monomer, 1.6 parts gemini quaternary ammonium hydrophobic monomer, and 0.105 parts initiator are added. The mixture is reacted at 80°C and 350 r / min for 11 h, naturally cooled to room temperature, cut into small pieces, washed three times with anhydrous ethanol, vacuum dried at 55°C for 11 h, pulverized, and passed through a 120-mesh sieve to obtain modified polyacrylamide. The remaining steps are the same as in Example 2.

[0065] Test Example 1:

[0066] Salt resistance, temperature resistance, and shear resistance tests: The apparent viscosity, temperature resistance viscosity retention rate, and salt resistance viscosity retention rate of the prepared low-damage fracturing fluid for coal and rock gas wells were tested to evaluate its temperature resistance and salt resistance. The specific test methods are as follows:

[0067] Apparent viscosity: 500 ml of the prepared low-damage fracturing fluid for coal and rock gas wells was measured in a beaker and its viscosity at room temperature and shear rate of 170 s⁻¹ was determined using a ZNND six-speed rotational viscometer. -1 The apparent viscosity of each sample was measured in parallel five times, and the average value was recorded.

[0068] Temperature and shear resistance: The prepared low-damage fracturing fluid for coal and rock gas wells was heated at 90℃ for 2 hours, and its resistance at a shear rate of 170 s⁻¹ was measured using a ZNND six-speed rotational viscometer. -1 The viscosity was measured and the viscosity retention rate was calculated and recorded as the temperature-resistant viscosity retention rate. Each group was tested in parallel for 5 times, and the average value was recorded.

[0069] Salt tolerance: A standard saline solution with a mineralization of 85,000 mg / L was prepared by mixing 5.5% sodium chloride, 2.0% potassium chloride, 0.45% magnesium chloride, and 0.55% calcium chloride. The pure water in step (5) was replaced with the standard saline solution to prepare the sample. The saline solution was used to measure the salt tolerance at room temperature and a shear rate of 170 s⁻¹. -1 The viscosity at which the salt resistance is measured is recorded as the salt resistance viscosity. Each group is tested in parallel 5 times, and the average value is recorded.

[0070] The results are shown in Table 1.

[0071] Table 1

[0072] ;

[0073] A comparison of the experimental data from Examples 1-3 and Comparative Examples 1-4 in Table 1 reveals that the low-damage fracturing fluid for coal and rock gas wells prepared in this invention has good temperature resistance, shear resistance, and salt resistance.

[0074] By comparing the data in the table, the apparent viscosity data of Examples 1-3 show that the viscosity increases with the increase of the amount of modified polyacrylamide added, exhibiting the characteristic of becoming viscous. The proportion can be flexibly adjusted according to the needs on the construction site. At the same time, since the modified polyacrylamide chain contains both quaternary ammonium cations and carboxyl anions provided by metal chelate monomers, the zwitterionic structure gives it the "anti-polyelectrolyte effect". In high-salt water, the viscosity does not decrease but increases, showing good salt resistance.

[0075] By comparing the data in the table, the data in Comparative Example 1 shows that longer hydrophobic carbon chains have a hydrophobic association effect, which can effectively improve the apparent viscosity and temperature resistance of fracturing fluids.

[0076] By comparing the data in the table, the data in Comparative Example 2 shows that the rigid benzene ring in the Gemini quaternary ammonium hydrophobic monomer can effectively improve the temperature resistance. At the same time, when the quaternary ammonium cation is lost, the "anti-polyelectrolyte effect" no longer exists, and the salt resistance decreases.

[0077] By comparing the data in the table, the data in Comparative Example 3 shows that the metal chelating monomer provides a large number of anions that combine with quaternary ammonium cations to provide an "anti-polyelectrolyte effect". When it is not added, the number of anions decreases, the "anti-polyelectrolyte effect" decreases, and the salt resistance property brought by the metal chelating monomer itself is lost.

[0078] Test Example 2:

[0079] Coal core permeability damage rate and swelling prevention performance test: The matrix permeability damage rate and swelling prevention rate of the prepared low-damage fracturing fluid for coal gas wells are tested to evaluate its damage to coal core permeability and its swelling prevention performance. The specific test method is as follows:

[0080] Matrix permeability damage rate test: The matrix permeability damage rate of the prepared low-damage fracturing fluid for coal gas wells was tested according to the procedure for determining the matrix permeability damage of coal core in standard NB / T 10034-2016. The filtrate collection method of the prepared low-damage fracturing fluid for coal gas wells was carried out according to the filtrate collection method of plant gum fracturing fluid in the standard. Ammonium persulfate was used as the breaker. The permeability damage rate η was calculated and recorded according to the formula in the standard. d Each group was tested in parallel 5 times, and the average value was recorded.

[0081] Swelling resistance test: The swelling resistance of the prepared low-damage fracturing fluid for coal and rock gas wells was tested according to the swelling resistance determination method in standard SY / T 5971-2016. The centrifugation method was used, and the swelling resistance was calculated and recorded according to the formula in the standard. Each group was tested in parallel for 5 times, and the average value was recorded.

[0082] The sodium-based bentonite used in the anti-swelling rate test was purchased from Dongzheng Chemical Co., Ltd. in Shangnan County, Shaanxi Province, and meets the requirements of the standard.

[0083] The results are shown in Table 2.

[0084] Table 2

[0085] ;

[0086] A comparison of the experimental data from Examples 1-3 and Comparative Examples 1-4 in Table 2 reveals that the low-damage fracturing fluid for coal and rock gas wells prepared in this invention exhibits low damage, pore enlargement and permeability enhancement, and good anti-swelling properties.

[0087] By comparing the data in the table, the permeability damage rate of Examples 1-3 is negative, indicating that after treatment with low-damage fracturing fluid for coal and rock gas wells, the permeability of the coal and rock matrix does not decrease but increases, indicating that low-damage fracturing fluid for coal and rock gas wells has the performance of expanding pores and increasing permeability.

[0088] By comparing the data in the table, the data in Comparative Example 1 shows that the long carbon chain of the Gemini quaternary ammonium hydrophobic monomer has the ability to reduce surface tension and provide hydrophobic effect. It can effectively reduce water lock damage and the decrease in permeability caused by adsorption. Furthermore, the hydrophobic properties can effectively improve the anti-swelling performance.

[0089] By comparing the data in the table, the data in Comparative Example 2 shows that the addition of the Gemini quaternary ammonium hydrophobic monomer, in synergy with the metal chelate monomer, improves pore-expanding and permeability enhancement, reduces permeability damage to the coal and rock matrix, and effectively inhibits the water-sensitive swelling of bentonite.

[0090] By comparing the data in the table, the data in Comparative Example 3 shows that the strong metal chelating ability of the metal chelating monomer can capture metal cations in the coal matrix and clay minerals, causing them to break down. At the same time, the improved hydrophobicity provided by the gemini quaternary ammonium hydrophobic monomer can effectively reduce the damage to the permeability of the coal matrix and effectively inhibit the water-sensitive swelling of bentonite.

[0091] By comparing the data in the table, the data in Comparative Example 4 shows that the addition of fumaric acid promotes the random distribution of hydrophobic side groups into a comb-like shape, reduces the permeability damage to the coal matrix, and improves the anti-swelling performance.

[0092] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-damage fracturing fluid for coal rock gas wells, characterized in that, The coal rock gas well low-damage fracturing fluid is prepared by mixing modified polyacrylamide, water and additives; The modified polyacrylamide is prepared by copolymerization of acrylamide, acrylic acid, fumaric acid, metal chelating monomer and gemini quaternary ammonium hydrophobic monomer; The gemini quaternary ammonium hydrophobic monomer is prepared by reaction of double bond di-tert-amine intermediate and halogenated alkane; The double bond di-tert-amine intermediate is prepared by reaction of allyl phosphine dichloride and 4-(dimethylaminomethyl) phenol; The metal chelating monomer is prepared by reaction of N,N'-ethylenediamine disuccinic acid and allyl glycidyl ether; The halogenated alkane includes chloroalkane, bromoalkane and iodoalkane, and the carbon chain length is 12-22; The double bond di-tert-amine intermediate is prepared by mixing 2-3 parts of 4-(dimethylaminomethyl) phenol and 15-30 parts of diethylene glycol monomethyl ether under nitrogen atmosphere, stirring at room temperature, adding 1.3-1.96 parts of triethylamine dropwise, then adding 0.04-0.08 parts of hydroquinone, cooling, adding 3.34-5 parts of 30wt% allyl phosphine dichloride diethylene glycol monomethyl ether solution dropwise, stirring at room temperature after dropwise addition, continuing the reaction by heating, filtering, purifying the filtrate, and then drying; The gemini quaternary ammonium hydrophobic monomer is prepared by mixing 3-4 parts of double bond di-tert-amine intermediate, 0.008-0.012 parts of hydroquinone, 0.2-0.3 parts of potassium iodide and 20-30 parts of acetonitrile, adding halogenated alkane according to the molar ratio of double bond di-tert-amine intermediate to halogenated alkane being 1:(2-2.2), refluxing and stirring, purifying, and then drying; The metal chelating monomer is prepared by dissolving 3-4 parts of N,N'-ethylenediamine disuccinic acid in 40-50 parts of pure water, adjusting pH, adding 4.3-5.73 parts of 30wt% allyl glycidyl ether ethanol solution under nitrogen atmosphere, stirring and refluxing, cooling, adjusting pH, separating and purifying, and then drying; The modified polyacrylamide is prepared by mixing 13-17 parts of acrylamide, 6-8 parts of acrylic acid, 0.13-0.15 parts of fumaric acid, 2.5-2.8 parts of emulsifier and 40-50 parts of pure water, adjusting pH, adding 2.1-2.3 parts of metal chelating monomer, 1.4-1.8 parts of gemini quaternary ammonium hydrophobic monomer and 0.09-0.12 parts of initiator under nitrogen atmosphere, stirring, cooling, shearing, washing, drying, grinding and sieving.

2. The low-damage fracturing fluid for coal rock gas well according to claim 1, characterized in that, The additives include gel breaking additives, anti-swelling additives, water lock prevention additives and drag reduction agents.

3. The method for preparing the low-damage fracturing fluid for coal rock gas well according to claim 1, characterized in that, The preparation steps include: (1) mixing 4-(dimethylaminomethyl) phenol and diethylene glycol monomethyl ether under nitrogen atmosphere, stirring and adding triethylamine dropwise at room temperature, mixing with hydroquinone, cooling, adding allyl phosphine dichloride solution dropwise, stirring at room temperature after dropwise addition, continuing the reaction by heating, filtering, purifying the filtrate, and then preparing double bond di-tert-amine intermediate; (2) mixing double bond di-tert-amine intermediate, hydroquinone, potassium iodide and acetonitrile, adding halogenated alkane, refluxing and stirring, purifying, and then drying to prepare gemini quaternary ammonium hydrophobic monomer; (3) Dissolve N,N'-ethylenediamine disuccinic acid in pure water, adjust pH, add allyl glycidyl ether solution under nitrogen atmosphere, stir and reflux, cool, adjust pH, separate, purify and dry to obtain metal chelating monomer; (4) Mix acrylamide, acrylic acid, fumaric acid, emulsifier and pure water, adjust pH, add metal chelating monomer, gemini quaternary ammonium hydrophobic monomer and initiator under nitrogen atmosphere, stir and react, cool, cut, wash, dry and grind to obtain modified polyacrylamide; (5) Mix modified polyacrylamide and pure water, stir at room temperature, adjust pH to obtain low-damage fracturing fluid for coal and rock gas well.

4. The method according to claim 3, characterized in that, In step (1), the double bond di-tert-butylamine intermediate is prepared by mixing 2-3 parts of 4-(dimethylaminomethyl) phenol and 15-30 parts of diethylene glycol monomethyl ether under nitrogen atmosphere, stirring at room temperature, adding 1.3-1.96 parts of triethylamine at a rate of 4-10 ml / min, then adding 0.04-0.08 parts of hydroquinone, mixing uniformly, cooling in an ice water bath to 0-2℃, adding 3.34-5 parts of 30wt% allyl phosphine dichloride solution in diethylene glycol monomethyl ether at a rate of 1-1.8 ml / min, after the addition is completed, stirring at room temperature for 10-14 h, heating to 55-65℃, continuing to react for 4-6 h, filtering, removing the solvent from the filtrate by rotary evaporation, and extracting, washing, concentrating and purifying to obtain the double bond di-tert-butylamine intermediate.

5. The method according to claim 3, characterized in that, In step (2), the gemini quaternary ammonium hydrophobic monomer is prepared by mixing 3-4 parts of double bond di-tert-butylamine intermediate, 0.008-0.012 parts of hydroquinone, 0.2-0.3 parts of potassium iodide and 20-30 parts of acetonitrile, adding halogenated alkane according to the molar ratio of double bond di-tert-butylamine intermediate to halogenated alkane being 1:(2-2.2), stirring and refluxing at 80-85℃ for 12-16 h, removing the solvent by rotary evaporation, and purifying by recrystallization and vacuum drying at 50-60℃ for 8-10 h to obtain the gemini quaternary ammonium hydrophobic monomer.

6. The method for preparing a low-damage fracturing fluid for coal and rock gas wells according to claim 3, characterized in that, In step (3), the metal chelating monomer is prepared by dissolving 3-4 parts of N,N'-ethylenediamine disuccinic acid in 40-50 parts of pure water, adjusting pH to 10-11, adding 4.3-5.73 parts of 30wt% allyl glycidyl ether ethanol solution under nitrogen atmosphere, stirring and refluxing at 65-70℃ for 10-12 h, cooling in an ice water bath to 0-4℃, adjusting pH to 6.5-7 with hydrochloric acid solution, separating and purifying, and freeze-drying for 24-30 h to obtain the metal chelating monomer.

7. The method according to claim 3, characterized in that, In step (4), the modified polyacrylamide is prepared by mixing 13-17 parts of acrylamide, 6-8 parts of acrylic acid, 0.13-0.15 parts of fumaric acid, 2.5-2.8 parts of emulsifier and 40-50 parts of pure water, adjusting pH to 7-8, adding 2.1-2.3 parts of metal chelating monomer, 1.4-1.8 parts of gemini quaternary ammonium hydrophobic monomer and 0.09-0.12 parts of initiator under nitrogen atmosphere, stirring and reacting at 75-80℃ for 10-12 h, naturally cooling to room temperature, cutting, washing, drying, grinding and sieving through a 120 mesh sieve to obtain the modified polyacrylamide.

8. The method according to claim 3, characterized in that, The emulsifier in step (4) is OP-10, and the initiator is azobisdimethyl isobutyronitrile or azobisdimethyl isopropyl imidazoline hydrochloride.

9. The method according to claim 3, characterized in that, The low-damage fracturing fluid for coal rock gas wells in step (5) is prepared by uniformly mixing 0.1-1 parts of modified polyacrylamide and 100-110 parts of pure water, stirring for 30-40 min at room temperature, and adjusting pH to 6-6.5.