Process for preparing a fracturing fluid thickener for oil fields

Abstract

The application belongs to the technical field of thickening agent preparation, and particularly relates to a preparation method of a fracturing fluid thickening agent for oil fields. The preparation method comprises the following steps: (1) preparing an ethanol solution of N-dodecyl acrylamide; (2) preparing an aqueous phase; (3) removing oxygen; (4) initiating; and (5) post-treatment. The preparation method of the fracturing fluid thickening agent for oil fields uses acrylamide as a main monomer, and introduces functional monomers N,N-dimethyl-N-acrylamide propyl-N-propane sulfonic acid inner salt, 4-acryloyl morpholine and N-dodecyl acrylamide, and performs free radical copolymerization reaction through a potassium persulfate / sodium bisulfite redox initiation system. The obtained product has stable performance, and is particularly suitable for oil and gas well fracturing operations under high-temperature, high-salt and medium-low shear working conditions.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of thickening agent preparation, and particularly relates to a preparation method of a fracturing fluid thickening agent for oil fields. BACKGROUND

[0002] Fracturing refers to a method of forming cracks in an oil layer by using hydraulic action in an oil production process. In oil field development operations, fracturing has become a crucial link in oil field production increase because it can transform oil layer properties, increase the flow area of the formation fluid, reduce resistance, and improve the production capacity of oil wells, and is increasingly widely used in major oil fields. The fracturing fluid used in fracturing operations is a non-homogeneous and unstable chemical system formed by a plurality of additives in a certain ratio, and is usually composed of various functional additives such as thickening agents, dispersants, anti-swelling agents, bactericides, etc.

[0003] The patent CN201911025482.5 discloses a fracturing emulsion polymer and a preparation method thereof. The fracturing emulsion polymer in the patent application can take into account the thickening and drag reduction effects, but its shear viscosity in 600 mg / L brine (6000 salinity) is only 30-40 mPa·s, and the drag reduction rate is only 70-74%, and its thickening performance and drag reduction performance under high salinity conditions need to be further improved.

[0004] The common thickening agents on the domestic and foreign markets mainly include natural high molecular polymer, biological polymer and synthetic high molecular polymer. Compared with natural high molecular polymer, synthetic high molecular polymer has better temperature resistance and shear resistance, strong thickening capacity, good sand suspension capacity and gel breaking property, and is not sensitive to bacteria, and is superior to natural polymer in all aspects. The synthetic high molecular polymer thickening agents at home and abroad mainly include acrylamide polymer, ethylene polymer and cross-linked polymer. The acrylamide polymer thickening agent has become the mainstream product in the thickening agent because of its good performance, but it still has problems such as slow dissolution, poor salt resistance and temperature resistance.

[0005] Therefore, it is necessary to explore a new fracturing fluid thickening agent for oil fields. SUMMARY

[0006] The application aims to provide a preparation method of a fracturing fluid thickening agent for oil fields. The preparation method has strong controllability, and the prepared thickening agent has excellent temperature resistance, salt resistance and shear resistance.

[0007] The preparation method of the fracturing fluid thickening agent for oil fields comprises the following steps:

[0008] (1) ultrasonic treatment of N-dodecyl acrylamide in ethanol until the solid is completely dissolved, then rotary evaporation is performed, and the rotary evaporation is stopped when the mass of the recovered ethanol reaches 30% of the initial mass of ethanol, and the remaining solution is the ethanol solution of N-dodecyl acrylamide;

[0009] (2) preparation of the aqueous phase

[0010] The reaction vessel is charged with water for dissolution, and then acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt and 4-acryloylmorpholine are sequentially added and stirred for 30-33 min until completely dissolved, followed by the addition of the ethanol solution of N-dodecyl acrylamide prepared in step (1) and continued stirring for 5-7 min, addition of water for constant volume, addition of sodium hydroxide to adjust the pH of the reaction system to 7.8-8.2, and finally addition of disodium ethylenediaminetetraacetate and stirring for 5-7 min to prepare the aqueous phase;

[0011] (3) oxygen removal

[0012] Under a 22-25℃ water bath environment, nitrogen gas is continuously introduced into the aqueous phase prepared in step (2) at a flow rate of 0.3 L / min for 30-35 min to remove oxygen, and then the flow rate is reduced to 0.05 L / min to maintain a slightly positive pressure of nitrogen gas protection, and the stirring state is maintained until the reaction is completed;

[0013] (4) initiation

[0014] Under stirring, a 5% potassium persulfate aqueous solution is first added to the reaction system of step (3) within 15-18 s, stirred for 1 min, and then a 5% NaHSO3 aqueous solution is added to the reaction system within 15-18 s, reacted at 22-25℃ for 6 h, and then the temperature is increased to 43-45℃ for continued reaction for 2 h to prepare the gel;

[0015] (5) post-treatment

[0016] The gel prepared in step (4) is cut into small pieces, then soaked in deionized water, and finally vacuum dried and crushed to prepare the fracturing fluid thickener for oilfields;

[0017] In step (2), the sum of the mole percentages of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt, 4-acryloylmorpholine and N-dodecyl acrylamide in step (1) is 100%, acrylamide accounts for 82-84 mol%, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt accounts for 10-11 mol%, 4-acryloylmorpholine accounts for 5.4-6.3 mol%, and N-dodecyl acrylamide accounts for 0.5-0.7 mol%.

[0018] The mass ratio of N-dodecyl acrylamide to ethanol in step (1) is 1:10.

[0019] The ultrasonic dissolution temperature in the ultrasonic treatment in step (1) is 39-42℃, the ultrasonic dissolution time is 17-20min, the ultrasonic power is 300W, and the ultrasonic frequency is 40KHz.

[0020] The rotary evaporation temperature in step (1) is 40℃, and the rotary evaporation vacuum degree is -0.09MPa.

[0021] The stirring temperature in step (2) is 22-25℃, and the stirring speed is 200-300r / min.

[0022] The concentration of sodium hydroxide in step (2) is 1mol / L.

[0023] The function of adding disodium ethylenediaminetetraacetate in step (2) is to chelate trace Ca 2+ , Mg 2+ , Fe 2+ , prevent metal ions from inducing radical inactivation, and affect the subsequent crosslinking reaction, and the mass accounts for 0.025% of the total mass of the aqueous phase.

[0024] The mass of the dissolving water and the constant volume water in step (2) is 100%, wherein the dissolving water accounts for 79%, and the constant volume water accounts for 21%.

[0025] The mass of acrylamide, N,N-dimethyl-N-acrylamide propyl-N-propane sulfonate inner salt, 4-acryloyl morpholine in step (2) and N-dodecyl acrylamide in step (1) accounts for 20% of the mass of acrylamide, N,N-dimethyl-N-acrylamide propyl-N-propane sulfonate inner salt, 4-acryloyl morpholine, dissolving water, constant volume water and N-dodecyl acrylamide in step (1) in step (2).

[0026] The stirring speed in step (3) is 200-300r / min.

[0027] In step (4), 0.32 wt % of potassium persulfate and 0.32 wt % of sodium bisulfite are added respectively based on the mass of acrylamide in step (2), and both are dry mass.

[0028] In step (5), the deionized water is replaced every 8h, and the total replacement is 5-7 times.

[0029] In step (5), the vacuum drying temperature is 40℃, the vacuum drying time is 24h, and the powder is crushed to 80 mesh.

[0030] Compared with the prior art, the present application has the following beneficial effects:

[0031] (1) The preparation method of the fracturing fluid thickening agent for oil field, acrylamide is used as the main monomer, and functional monomers N,N-dimethyl-N-acrylamide propyl-N-propane sulfonic acid inner salt, 4-acryloyl morpholine and N-dodecyl acrylamide are introduced, and free radical copolymerization is carried out through potassium persulfate / sodium bisulfite redox initiation system. Among them, the N,N-dimethyl-N-acrylamide propyl-N-propane sulfonic acid inner salt is used as a zwitterionic monomer, the opposite charge centers in the molecule can effectively inhibit the negative influence of salt effect on the solution viscosity through the "counterion effect", and the precipitation problem caused by divalent calcium and magnesium ions can be avoided; the rigid morpholine ring structure of the side chain of the 4-acryloyl morpholine can significantly enhance the rigidity of the polymer main chain, reduce the thermal motion and conformational curling of the molecular chain at high temperature, and thus improve the temperature resistance of the product; the long alkyl chain of N-dodecyl acrylamide as a hydrophobic monomer can form a dynamic and reversible three-dimensional physical crosslinking network in an aqueous solution through intermolecular hydrophobic association, and the network structure gives the product good viscosity increasing ability and unique shear resistance. The synergistic effect of the above four kinds of monomers makes the finally prepared thickening agent have good solubility, high temperature resistance, salt resistance, high viscosity and shear resistance.

[0032] (2) The preparation method of the fracturing fluid thickening agent for oil field, through free radical copolymerization, the characteristics of each functional monomer are effectively integrated on the same polymer molecular chain. The rigid side ring of 4-acryloyl morpholine and the large side group of N,N-dimethyl-N-acrylamide propyl-N-propane sulfonic acid inner salt together construct the rigid skeleton of the polymer, which greatly enhances the thermal stability of the material, so that it is not easy to chemically degrade or physically collapse in a high temperature environment. The presence of N,N-dimethyl-N-acrylamide propyl-N-propane sulfonic acid inner salt enables the polymer solution to maintain or even improve its apparent viscosity in a high salinity environment. The introduction of N-dodecyl acrylamide is due to its hydrophobic association property, which can form a huge dynamic physical crosslinking network in water, thereby providing high initial viscosity; the network structure will temporarily dissociate when encountering high-speed shear, and will quickly re-associate once the shear effect is eliminated, thereby showing excellent shear resistance.

[0033] (3) The preparation method of the fracturing fluid thickening agent for oil field, the process parameters are clear, the operation is controllable, and the scale-up preparation is easy to realize, and the performance of the obtained product is stable, and it is especially suitable for oil and gas well fracturing operation under high temperature, high salt and medium-low shear conditions. DETAILED DESCRIPTION

[0034] Example 1

[0035] The preparation method of the fracturing fluid thickening agent for oil field in this embodiment 1 is composed of the following steps:

[0036] (1) ultrasonic treatment of N-dodecyl acrylamide in ethanol until the solid is completely dissolved, then rotary evaporation is performed, and the rotary evaporation is stopped when the mass of the recovered ethanol reaches 30% of the initial mass of ethanol, and the remaining solution is the ethanol solution of N-dodecyl acrylamide;

[0037] (2) preparation of an aqueous phase

[0038] The reaction vessel is charged with water for dissolution, and then acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt, and 4-acryloylmorpholine are sequentially added and stirred for 31 min until completely dissolved, followed by the addition of the ethanol solution of N-dodecyl acrylamide prepared in step (1) and continued stirring for 6 min, addition of water for constant volume, addition of sodium hydroxide to adjust the pH of the reaction system to 8.0, and finally addition of disodium ethylenediaminetetraacetate and stirring for 6 min to prepare the aqueous phase;

[0039] (3) oxygen removal

[0040] Under a 23℃ water bath environment, nitrogen gas is continuously introduced into the aqueous phase prepared in step (2) at a flow rate of 0.3 L / min for 33 min to remove oxygen, and then the flow rate is reduced to 0.05 L / min to maintain a slight positive pressure of nitrogen gas protection, and the stirring state is maintained until the reaction is completed;

[0041] (4) initiation

[0042] Under stirring, a 5% potassium persulfate aqueous solution is first added to the reaction system of step (3) within 16 s, stirred for 1 min, and then a 5% NaHSO3 aqueous solution is added to the reaction system within 16 s, reacted at 23℃ for 6 h, and then the temperature is increased to 44℃ and reacted for another 2 h to prepare the gel;

[0043] (5) post-treatment

[0044] The gel prepared in step (4) is cut into small pieces, then soaked in deionized water, and finally vacuum dried and crushed to prepare the fracturing fluid thickener for oilfields;

[0045] In step (2), the sum of the mole percentages of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt, 4-acryloylmorpholine, and N-dodecyl acrylamide in step (1) is 100%, acrylamide accounts for 83 mol%, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt accounts for 10.5 mol%, 4-acryloylmorpholine accounts for 6 mol%, and N-dodecyl acrylamide accounts for 0.5 mol%.

[0046] The mass ratio of N-dodecyl acrylamide to ethanol in step (1) is 1:10.

[0047] The ultrasonic dissolution temperature in step (1) is 41℃, the ultrasonic dissolution time is 18 min, the ultrasonic power is 300 W, and the ultrasonic frequency is 40 KHz.

[0048] The rotary evaporation temperature in step (1) is 40℃, and the rotary evaporation vacuum degree is -0.09 MPa.

[0049] The stirring temperature in step (2) is 23℃, and the stirring speed is 250 r / min.

[0050] The concentration of sodium hydroxide in step (2) is 1 mol / L.

[0051] The function of adding disodium ethylenediaminetetraacetate in step (2) is to chelate trace Ca 2+ , Mg 2+ , Fe 2+ , to prevent metal ions from inducing radical deactivation and affecting subsequent crosslinking reactions, and the mass of the added disodium ethylenediaminetetraacetate accounts for 0.025% of the total mass of the aqueous phase.

[0052] The mass of the dissolving water and the constant volume water in step (2) is 100%, wherein the dissolving water accounts for 79% and the constant volume water accounts for 21%.

[0053] The mass of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt, 4-acryloyl morpholine, and N-dodecyl acrylamide in step (1) in step (2) accounts for 20% of the mass of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt, 4-acryloyl morpholine, dissolving water, constant volume water, and N-dodecyl acrylamide in step (1) in step (2).

[0054] The stirring speed in step (3) is 250 r / min.

[0055] In step (4), 0.32 wt % of potassium persulfate and 0.32 wt % of sodium bisulfite are added, respectively, based on the mass of acrylamide in step (2), and both are in dry mass.

[0056] In step (5), the deionized water is replaced every 8 h, and the total number of replacements is 6.

[0057] In step (5), the vacuum drying temperature is 40℃, the vacuum drying time is 24 h, and the powder is crushed to pass through an 80-mesh sieve.

[0058] Example 2

[0059] The preparation method of the fracturing fluid thickening agent for oil field in the embodiment 2 is composed of the following steps:

[0060] (1) N-dodecyl acrylamide is added into ethanol for ultrasonic treatment until the solid is completely dissolved, then rotary evaporation is carried out, and when the mass of recovered ethanol reaches 30% of the initial mass of ethanol, rotary evaporation is stopped, and the remaining solution is N-dodecyl acrylamide ethanol solution;

[0061] (2) Preparation of water phase

[0062] Dissolved water is added into the reaction container, then acrylamide, N, N-dimethyl-N-acrylamide propyl-N-propane sulfonic acid inner salt and 4-acryloyl morpholine are sequentially added and stirred for 33 min until completely dissolved, then the N-dodecyl acrylamide ethanol solution prepared in step (1) is added and stirred for 5 min, water for constant volume is added, sodium hydroxide is added to adjust the pH value of the reaction system to 8.2, and finally ethylenediaminetetraacetic acid disodium salt is added and stirred for 7 min to prepare the water phase;

[0063] (3) Oxygen removal

[0064] Under the water bath environment at 22℃, nitrogen gas is continuously introduced into the water phase prepared in step (2) at a flow rate of 0.3 L / min for 35 min to remove oxygen, then the flow rate is reduced to 0.05 L / min to maintain a slight positive pressure nitrogen protection, and the stirring state is maintained until the reaction is completed;

[0065] (4) Initiation

[0066] Under the stirring state, first, the mass concentration of 5% potassium persulfate aqueous solution is added to the reaction system of step (3) within 18 s, and stirred for 1 min, then the mass concentration of 5% NaHSO3 aqueous solution is added to the reaction system within 18 s, first, the reaction is carried out at 22℃ for 6 h, then the temperature is increased to 43℃ and the reaction is continued for 2 h to prepare the gel;

[0067] (5) Post-treatment

[0068] The gel prepared in step (4) is cut into small pieces, then soaked in deionized water, and finally vacuum dried and crushed to prepare the fracturing fluid thickening agent for oil field;

[0069] Wherein:

[0070] The sum of the mole percentages of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate inner salt, 4-acryloylmorpholine, and N-dodecyl acrylamide in step (1) in step (2) is 100%, acrylamide accounts for 82 mol%, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate inner salt accounts for 11 mol%, 4-acryloylmorpholine accounts for 6.3 mol%, and N-dodecyl acrylamide accounts for 0.7 mol%.

[0071] The mass ratio of N-dodecyl acrylamide to ethanol in step (1) is 1:10.

[0072] The ultrasonic dissolution temperature in the ultrasonic treatment in step (1) is 39℃, the ultrasonic dissolution time is 20 min, the ultrasonic power is 300 W, and the ultrasonic frequency is 40 KHz.

[0073] The rotary evaporation temperature in step (1) is 40℃, and the rotary evaporation vacuum degree is -0.09 MPa.

[0074] The stirring temperature in step (2) is 22℃, and the stirring speed is 300 r / min.

[0075] The concentration of sodium hydroxide in step (2) is 1 mol / L.

[0076] The function of adding disodium ethylenediaminetetraacetate in step (2) is to chelate trace Ca 2+ , Mg 2+ , Fe 2+ , prevent metal ions from inducing radical deactivation, and affect the subsequent cross-linking reaction, and the mass accounts for 0.025% of the total mass of the aqueous phase.

[0077] The sum of the mass of the dissolving water and the constant volume water in step (2) is 100%, wherein the dissolving water accounts for 79% and the constant volume water accounts for 21%.

[0078] The sum of the mass of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate inner salt, 4-acryloylmorpholine, and N-dodecyl acrylamide in step (1) in step (2) accounts for 20% of the sum of the mass of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate inner salt, 4-acryloylmorpholine, dissolving water, constant volume water, and N-dodecyl acrylamide in step (1) in step (2).

[0079] The stirring speed in step (3) is 300 r / min.

[0080] In step (4), 0.32 wt % of potassium persulfate and 0.32 wt % of sodium bisulfite are added based on the dry mass of acrylamide in step (2), both in terms of dry mass.

[0081] The deionized water was replaced every 8 hours during the soaking in step (5), and the total number of replacements was 7.

[0082] The vacuum drying temperature in step (5) was 40°C, the vacuum drying time was 24 hours, and the powder was sieved through an 80-mesh screen.

[0083] Example 3

[0084] The preparation method of the fracturing fluid thickener for oil fields described in this example 3 is composed of the following steps:

[0085] (1) N-dodecyl acrylamide was added to ethanol and ultrasonically treated until the solid was completely dissolved, and then rotary evaporation was performed. When the mass of recovered ethanol reached 30% of the initial mass of ethanol, rotary evaporation was stopped, and the remaining solution was the ethanol solution of N-dodecyl acrylamide;

[0086] (2) Preparation of the aqueous phase

[0087] The reaction vessel was charged with water for dissolution, and then acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonic acid inner salt, and 4-acryloylmorpholine were added in sequence and stirred for 30 minutes until completely dissolved. Subsequently, the ethanol solution of N-dodecyl acrylamide prepared in step (1) was added and stirred for 7 minutes, and water for constant volume was added. Sodium hydroxide was added to adjust the pH of the reaction system to 7.8, and finally, disodium ethylenediaminetetraacetate was added and stirred for 5 minutes to prepare the aqueous phase.

[0088] (3) Oxygen removal

[0089] Under a 25°C water bath environment, nitrogen gas was continuously introduced into the aqueous phase prepared in step (2) at a flow rate of 0.3 L / min for 30 minutes to remove oxygen, and then the flow rate was reduced to 0.05 L / min to maintain a slight positive pressure of nitrogen gas protection, and the stirring state was maintained until the reaction was completed.

[0090] (4) Initiation

[0091] Under stirring, a 5% potassium persulfate aqueous solution was first added to the reaction system of step (3) within 15 seconds, stirred for 1 minute, and then a 5% NaHSO3 aqueous solution was added to the reaction system within 15 seconds. The reaction was carried out at 25°C for 6 hours, and then the temperature was increased to 45°C and the reaction was continued for 2 hours to prepare the gel.

[0092] (5) Post-treatment

[0093] The gel prepared in step (4) was cut into small pieces, then soaked in deionized water, and finally vacuum dried and pulverized to prepare the fracturing fluid thickener for oil fields.

[0094] The sum of the mole percentages of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate inner salt, 4-acryloylmorpholine, and N-dodecyl acrylamide in step (1) in step (2) is 100%, the mole percentage of acrylamide is 84%, the mole percentage of N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate inner salt is 10%, the mole percentage of 4-acryloylmorpholine is 5.4%, and the mole percentage of N-dodecyl acrylamide is 0.6%.

[0095] The mass ratio of N-dodecyl acrylamide to ethanol in step (1) is 1:10.

[0096] The ultrasonic dissolution temperature in step (1) is 42°C, the ultrasonic dissolution time is 17 min, the ultrasonic power is 300 W, and the ultrasonic frequency is 40 KHz.

[0097] The rotary evaporation temperature in step (1) is 40°C, and the rotary evaporation vacuum degree is -0.09 MPa.

[0098] The stirring temperature in step (2) is 25°C, and the stirring speed is 200 r / min.

[0099] The concentration of sodium hydroxide in step (2) is 1 mol / L.

[0100] The function of adding disodium ethylenediaminetetraacetate in step (2) is to chelate trace amounts of Ca 2+ , Mg 2+ , and Fe 2+ , to prevent metal ions from inducing radical deactivation and affecting subsequent crosslinking reactions, and the mass of the added disodium ethylenediaminetetraacetate accounts for 0.025% of the total mass of the aqueous phase.

[0101] The sum of the masses of the dissolving water and the constant-volume water in step (2) is 100%, wherein the mass of the dissolving water accounts for 79%, and the mass of the constant-volume water accounts for 21%.

[0102] The sum of the masses of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate inner salt, 4-acryloylmorpholine, and N-dodecyl acrylamide in step (1) accounts for 20% of the sum of the masses of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate inner salt, 4-acryloylmorpholine, dissolving water, constant-volume water, and N-dodecyl acrylamide in step (1) in step (2).

[0103] The stirring speed in step (3) is 200 r / min.

[0104] In step (4), 0.32 wt % of potassium persulfate and 0.32 wt % of sodium bisulfite are added based on the mass of acrylamide in step (2), and both are dry masses.

[0105] The deionized water was replaced every 8 hours during the soaking in step (5), and the total number of replacements was 5.

[0106] The vacuum drying temperature in step (5) was 40°C, the vacuum drying time was 24 hours, and the powder was sieved through an 80-mesh screen.

[0107] Comparative Example 1

[0108] The preparation method of the fracturing fluid thickener for oilfields described in Comparative Example 1 was the same as that of Example 1, and the only difference was that no monomer N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate was added when preparing the aqueous phase in step (2).

[0109] The sum of the mole percentages of acrylamide, 4-acryloylmorpholine, and N-dodecyl acrylamide in step (1) was 100%, acrylamide accounted for 93.5 mol%, 4-acryloylmorpholine accounted for 6 mol%, and N-dodecyl acrylamide accounted for 0.5 mol%.

[0110] Comparative Example 2

[0111] The preparation method of the fracturing fluid thickener for oilfields described in Comparative Example 2 was the same as that of Example 1, and the only difference was that no monomer 4-acryloylmorpholine was added when preparing the aqueous phase in step (2). The sum of the mole percentages of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate, and N-dodecyl acrylamide in step (1) was 100%, acrylamide accounted for 89 mol%, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate accounted for 10.5 mol%, and N-dodecyl acrylamide accounted for 0.5 mol%.

[0112] Comparative Example 3

[0113] The preparation method of the fracturing fluid thickener for oilfields described in Comparative Example 3 was the same as that of Example 1, and the only difference was that step (1) was omitted and no ethanol solution of N-dodecyl acrylamide was added in step (2). The sum of the mole percentages of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate, and 4-acryloylmorpholine was 100%, acrylamide accounted for 83.5 mol%, N,N-dimethyl-N-acrylamidopropyl-N-propane sulfonate accounted for 10.5 mol%, and 4-acryloylmorpholine accounted for 6 mol%.

[0114] The fracturing fluid thickeners for oilfields prepared in Examples 1-3 and Comparative Examples 1-3 were prepared into 0.5 wt% solutions, and the apparent viscosities of the solutions were tested at 25°C, 180 s -1 The results are shown in Table 1.

[0115] Table 1 apparent viscosity at room temperature

[0116]

[0117] From Table 1, it can be seen that the oil field fracturing fluid thickening agent prepared in Examples 1-3 has good viscosity increasing effect compared with Comparative Examples 1-3.

[0118] The oil field fracturing fluid thickening agent prepared in Examples 1-3 and Comparative Examples 1-3 was prepared into a 0.5wt% solution, and the apparent viscosity thereof was tested at 120℃, 180s -1 and 150℃, 180s -1 ; the results are shown in Table 2.

[0119] Table 2 apparent viscosity at high temperature

[0120]

[0121] From Table 2, it can be seen that the oil field fracturing fluid thickening agent prepared in Examples 1-3 still has good viscosity at high temperature compared with Comparative Examples 1-3, which proves that it has good temperature resistance.

[0122] The oil field fracturing fluid thickening agent prepared in Examples 1-3 and Comparative Examples 1-3 was prepared into a 0.5wt% solution, and the apparent viscosity thereof was tested after shearing at 25℃, 180s -1 for 2.5h; the results are shown in Table 3.

[0123] Table 3 apparent viscosity after shearing at 180s -1 for 2.5h

[0124]

[0125] From Table 3, it can be seen that the oil field fracturing fluid thickening agent prepared in Examples 1-3 has good shear resistance compared with Comparative Examples 1-3.

[0126] The oil field fracturing fluid thickening agent prepared in Examples 1-3 and Comparative Examples 1-3 was prepared into a 0.5wt% solution, and NaCl was added to the system until the concentration of NaCl was 1.0g / L, and the apparent viscosity thereof was tested at 25℃, 180s -1 ; the results are shown in Table 4.

[0127] Table 4 results of salt resistance test

[0128]

[0129] From Table 4, it can be seen that the oil field fracturing fluid thickening agent prepared in Examples 1-3 has good salt resistance compared with Comparative Examples 1-3.

[0130] The oilfield fracturing fluid thickening agents prepared in Examples 1-3 and Comparative Examples 1-3 were formulated into 0.5wt% aqueous solution, stirred until completely dissolved, defoamed by standing, and zirconium citrate crosslinking agent was added into the base fluid, the amount of addition was 0.4% based on the mass of the base fluid, the crosslinking reaction was carried out at 90℃ for 2h using a stirrer at a speed of 400r / min, the gel strength of the formed gel was tested, and the results are shown in Table 5.

[0131] Ammonium persulfate was added into the formed gel, the amount of addition of ammonium persulfate was 0.03wt%, the gel breaking was carried out at 90℃ for 8h, the gel breaking fluid was cooled to 25℃, and the apparent viscosity of the gel breaking fluid was tested at 25℃, 180s -1 Table 5 Application effect of oilfield fracturing fluid thickening agent

[0132] 50mL of the gel breaking fluid was centrifuged at 3000r / min for 35min, the supernatant was poured out, the gel breaking container was washed with water and then poured into the centrifuge tube again, centrifuged at 3000r / min for 35min, and the supernatant was poured out again, the centrifuge tube was placed in an oven and dried to constant weight, and the residue content of the fracturing fluid was obtained by dividing the residue mass by the fracturing fluid dosage, and the results are shown in Table 5.

[0133] Table 5 Application effect of oilfield fracturing fluid thickening agent

[0134]

[0135] The oilfield fracturing fluid thickening agents prepared in Examples 1-3 can be well crosslinked compared with Comparative Examples 1-3, the formed gel has high strength, and can be quickly broken, and the residue content after gel breaking is small, which can prove that the damage to the formation is small.

[0136] The above is only the preferred embodiment of the present application, and is not intended to limit the other forms of the present application, and any skilled person in the art can modify or change the above disclosed technical content into equivalent embodiments with equivalent changes. However, any simple modification, equivalent change and modification of the above embodiments made according to the technical essence of the present application without departing from the technical solution content of the present application still belongs to the protection scope of the present application.

Claims

1. A method for preparing a thickener for fracturing fluid used in oilfields, characterized in that: It consists of the following steps: (1) Add N-dodecylacrylamide to ethanol and sonicate until the solid is completely dissolved. Then, rotary evaporate. Stop rotary evaporation when the recovered ethanol mass reaches 30% of the initial ethanol mass. The remaining solution is the ethanol solution of N-dodecylacrylamide. (2) Preparation of aqueous phase Add water for dissolving in the reaction vessel, then add acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propanesulfonic acid inner salt and 4-acryloylmorpholine in sequence and stir for 30-33 min until completely dissolved. Then add the ethanol solution of N-dodecylacrylamide prepared in step (1) and continue stirring for 5-7 min. Add water to make up the volume, add sodium hydroxide to adjust the pH of the reaction system to 7.8-8.2, and finally add disodium ethylenediaminetetraacetate and stir for 5-7 min to prepare an aqueous phase. (3) Deoxygenation In a water bath at 22-25℃, nitrogen gas is continuously passed through the aqueous phase prepared in step (2) at a flow rate of 0.3 L / min for 30-35 min to remove oxygen. Then the flow rate is reduced to 0.05 L / min to maintain a slight positive pressure nitrogen protection and the mixture is stirred until the reaction is complete. (4) Trigger Under stirring, a 5% potassium persulfate aqueous solution was first added to the reaction system of step (3) within 15-18s, stirred for 1min, and then a 5% NaHSO3 aqueous solution was added to the reaction system within 15-18s. The reaction was carried out at 22-25℃ for 6h, and then the temperature was raised to 43-45℃ to continue the reaction for 2h to obtain a gel. (5) Post-processing The gel prepared in step (4) is cut into small pieces, then soaked in deionized water, and finally vacuum dried and pulverized to prepare a thickener for fracturing fluid in oil fields. Wherein: the sum of the molar percentages of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propanesulfonic acid inner salt, 4-acryloylmorpholine, and N-dodecylacrylamide in step (1) is 100%, acrylamide accounts for 82-84 mol%, N,N-dimethyl-N-acrylamidopropyl-N-propanesulfonic acid inner salt accounts for 10-11 mol%, 4-acryloylmorpholine accounts for 5.4-6.3 mol%, and N-dodecylacrylamide accounts for 0.5-0.7 mol.

2. The method for preparing the oilfield fracturing fluid thickener according to claim 1, characterized in that: In step (1), the mass ratio of N-dodecylacrylamide to ethanol is 1:10; The ultrasonic dissolution temperature during the ultrasonic treatment in step (1) is 39-42℃, the ultrasonic dissolution time is 17-20min, the ultrasonic power is 300W, and the ultrasonic frequency is 40KHz. In step (1), the rotary evaporation temperature is 40℃ and the rotary evaporation vacuum degree is -0.09MPa.

3. The method for preparing the oilfield fracturing fluid thickener according to claim 1, characterized in that: In step (2), the stirring temperature is 22-25℃ and the stirring speed is 200-300 r / min; In step (2), the concentration of sodium hydroxide is 1 mol / L.

4. The method for preparing the oilfield fracturing fluid thickener according to claim 1, characterized in that: In step (2), the mass of disodium ethylenediaminetetraacetate added accounts for 0.025% of the final total mass of the aqueous phase; In step (2), the total mass of the water used for dissolving and the water used for volume adjustment is 100%, of which the water used for dissolving accounts for 79% and the water used for volume adjustment accounts for 21%.

5. The method for preparing the oilfield fracturing fluid thickener according to claim 1, characterized in that: The total mass of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propanesulfonic acid inner salt, 4-acryloylmorpholine, and N-dodecylacrylamide in step (2) accounts for 20% of the total mass of acrylamide, N,N-dimethyl-N-acrylamidopropyl-N-propanesulfonic acid inner salt, 4-acryloylmorpholine, water for dissolution, water for volume adjustment, and N-dodecylacrylamide in step (1).

6. The method for preparing the oilfield fracturing fluid thickener according to claim 1, characterized in that: In step (3), the stirring speed is 200-300 r / min.

7. The method for preparing the oilfield fracturing fluid thickener according to claim 1, characterized in that: In step (4), based on the mass of acrylamide in step (2), 0.32 wt% of potassium persulfate and 0.32 wt% of sodium bisulfite are added respectively, all on a dry mass basis.

8. The method for preparing the oilfield fracturing fluid thickener according to claim 1, characterized in that: In step (5), the deionized water is changed every 8 hours during soaking, for a total of 5-7 times.

9. The method for preparing the oilfield fracturing fluid thickener according to claim 1, characterized in that: In step (5), the vacuum drying temperature is 40℃, the vacuum drying time is 24h, and the powder is pulverized through an 80-mesh sieve.

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