Cationic flocculant as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of oilfield chemistry, and relates to a cationic flocculant as well as a preparation method and application thereof. The preparation method of the cationic flocculant provided by the invention comprises the following steps: (1) dissolving dimethylaminoethyl methacrylate and a cationic acrylamide monomer in ethanol, and introducing nitrogen to remove oxygen; (2) heating to a first reaction temperature, adding an initiator, and reacting to obtain a prepolymer solution A; and (3) adding alkyl chloride into the prepolymer solution A, heating to a second reaction temperature, and continuing the reaction to obtain the oil-based drilling fluid flocculant after the reaction is finished. The prepared oil-based drilling fluid flocculating agent has the following advantages: (1) the oil-based drilling fluid flocculating agent can act on an oil-based flocculating agent, a small-particle-size inferior solid phase can be combined and flocculated into large particles, the content of the inferior solid phase is reduced, and the flocculating effect on barite is almost avoided; and (2) the rheological property of the drilling fluid can be optimized, the viscosity and shear reduction effect is achieved, and the stability of the drilling fluid is not affected.

Description

Technical Field

[0001] The invention belongs to the technical field of oilfield chemistry, and relates to a cationic flocculant and a preparation method and application thereof, and in particular to a cationic flocculant and a preparation method thereof and application thereof in removing low-density inferior solid phase in oil-based drilling fluid. Background Art

[0002] Oil-based drilling fluids are widely used due to their excellent inhibitory, lubricating and anti-pollution properties. However, after repeated use, the content of low-density inferior solids in oil-based drilling fluids will become higher and higher. According to statistics, the content of low-density inferior solids (by volume) will increase by 4%-6% after each well is drilled in Sichuan and Chongqing. The accumulation of these low-density inferior solids will not only increase the viscosity and shear force of the drilling fluid, and deteriorate the rheological properties of the drilling fluid, but also slow down the mechanical drilling speed due to increased internal friction, reduce drilling efficiency, and thus increase drilling costs. Therefore, effectively reducing the content of low-density inferior solids in drilling fluids is of great significance for stabilizing the performance of drilling fluids.

[0003] Since conventional solid control systems have poor removal capabilities for low-density, low-quality solids smaller than 10 μm, in order to improve the removal of low-quality solids, it is necessary to flocculate small-size low-quality solids into large particles so that they can meet the processing conditions of solid control equipment and be removed, thereby improving drilling fluid performance and reducing the occurrence of downhole complications. Therefore, it is necessary to develop oil-soluble flocculants to address the problem of removing low-quality solids in oil-based drilling fluids.

[0004] The Chinese invention patent application with publication number CN109652026A discloses a method for preparing a flocculating flow type regulator for oil-based drilling fluid. The method comprises the following steps: (1) adding nano activated carbon by weight percentage into a beaker equipped with a stirrer and a thermometer; (2) adding 1-40% of low viscosity synthetic base oil into the reactor of step (1), stirring at 50-90°C for 3-6 hours; (3) adding 1-5% of emulsifier into the reactor of step (1), stirring for 3-6 hours; (4) adding 1-10% of wetting agent into the reactor of step (1), stirring for 3-5 hours, and the obtained suspension is the flocculating flow type regulator for oil-based drilling fluid. This method can reduce the content of finely dispersed particles in oil-based drilling fluids, but the flocculation effect is relatively limited, and it also has a flocculation effect on useful solid phases such as barite and organic soil in the oil-based drilling fluid, which limits its application in high-density oil-based drilling fluids.

[0005] The Chinese invention patent application with publication number CN113292731A discloses a flocculant for removing rock cuttings from oil-based drilling fluid, and its preparation method and application. The method comprises the following steps: (1) dissolving a hydrophilic monomer, a cationic monomer and glycidyl methacrylate in a first solvent to obtain a solution a; (2) adding a first initiator to the solution a for polymerization reaction to obtain a prepolymer A; (3) dissolving a dispersant in a second solvent to obtain a solution b; (4) adding an oil-soluble monomer, acrylamide and 2-mercaptoethylamine hydrochloride to the solution b for mixing, and then adding a second initiator for polymerization reaction to obtain a prepolymer B; (5) dissolving prepolymer A and prepolymer B in a third solvent for graft polymerization reaction to obtain a flocculant. The method can reduce the content of finely dispersed particles in oil-based drilling fluid. However, the preparation process of the method is complicated and the cost is high, which limits its application in oil-based drilling fluid.

[0006] The Chinese invention patent application with publication number CN112029033A discloses a nano-micron inferior solid phase polymer flocculant for oil-based drilling fluid and a preparation method thereof. The method uses two monomers, cationic diallyl ammonium chloride and methacrylate, to prepare a flocculant through free radical copolymerization. The flocculant can selectively flocculate inferior solid phases with negatively charged surfaces, while almost not flocculating useful solid phases such as barite with no charge on the surface. However, since a portion of uncharged particles exist in the inferior solid phase, this will limit the flocculation effect of the flocculant. Summary of the invention

[0007] The purpose of the present invention is to provide a cationic flocculant for removing low-density inferior solid phases in oil-based drilling fluids and a preparation method thereof, so as to solve the problem that on-site solid control equipment has insufficient removal capacity for small-particle, low-density inferior solid phases in oil-based drilling fluids, and that existing flocculants also produce flocculation effects on useful solid phases in the drilling fluid during the flocculation of inferior solid phases.

[0008] The technical solutions adopted are:

[0009] A method for preparing a cationic flocculant, comprising:

[0010] (1) dissolving dimethylaminoethyl methacrylate and cationic acrylamide monomers in ethanol and passing nitrogen gas to remove oxygen;

[0011] (2) After heating to the first reaction temperature, an initiator is added, and after the reaction is completed, a prepolymer solution A is obtained;

[0012] (3) adding alkyl chloride to the prepolymer solution A, raising the temperature to a second reaction temperature to continue the reaction, and obtaining an oil-based drilling fluid flocculant after the reaction is completed.

[0013] In the preparation method of the above cationic flocculant, the cationic acrylamide monomer includes one or two of methacryloyloxyethyl ammonium chloride and acryloyloxyethyl trimethyl ammonium chloride.

[0014] In the above-mentioned method for preparing the cationic flocculant, the first reaction temperature is 45-80° C., and the reaction time is 8-24 hours.

[0015] In the above-mentioned method for preparing the cationic flocculant, the first reaction temperature is 55-70° C., and the reaction time is 8-16 hours.

[0016] In the preparation method of the above cationic flocculant, the initiator is one or more of azobisisobutyronitrile, dimethyl azobisisobutyrate, benzoyl peroxide, and tert-butyl benzoyl peroxide.

[0017] In the preparation method of the above-mentioned cationic flocculant, the general structural formula of the alkyl chloride is as follows:

[0018]

[0019] Here, n is an integer from 10 to 22.

[0020] In the above-mentioned method for preparing a cationic flocculant, the second reaction temperature is 30-60° C., and the reaction time is 2-10 hours.

[0021] In the above-mentioned method for preparing the cationic flocculant, the second reaction temperature is 45-55° C., and the reaction time is 3-6 hours.

[0022] The preparation method of the cationic flocculant is based on 100 parts by weight of ethanol, wherein the dimethylaminoethyl methacrylate is 60-95 parts by weight, the cationic acrylamide monomer is 5-30 parts by weight, the alkyl chloride is 20-60 parts by weight, and the initiator is 0.001-3 parts by weight.

[0023] The preparation method of the cationic flocculant is based on 100 parts by weight of ethanol, wherein the dimethylaminoethyl methacrylate is 75-80 parts by weight, the cationic acrylamide monomer is 10-20 parts by weight, the alkyl chloride is 35-45 parts by weight, and the initiator is 0.1-1 parts by weight.

[0024] On the other hand, the present invention provides a cationic flocculant, which is prepared by the above-mentioned preparation method.

[0025] In yet another aspect, the present invention also provides the use of the above cationic flocculant in removing low-density inferior solid phase in oil-based drilling fluid.

[0026] The oil-based drilling fluid flocculant obtained by the preparation method of the present invention has the following advantages:

[0027] (1) It can act on oil-based flocculants and can flocculate small-sized inferior solid phases into large particles, reduce the content of inferior solid phases, and has almost no flocculation effect on barite;

[0028] (2) It can optimize the rheological properties of drilling fluid and has the effect of reducing viscosity and shear, but does not affect the stability of drilling fluid. DETAILED DESCRIPTION

[0029] In order to fully understand the purpose, features and effects of the present invention, the present invention is described in detail through the following specific embodiments. Except for the following contents, the process method of the present invention adopts conventional methods or devices in the art. Unless otherwise specified, the following terms have the meanings commonly understood by those skilled in the art.

[0030] When a numerical range is disclosed herein, the above range is considered to be continuous and includes the minimum and maximum values ​​of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values ​​of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein should be understood to include any and all subranges included therein.

[0031] Specifically, the method for preparing the cationic flocculant provided by the present invention comprises:

[0032] (1) dissolving dimethylaminoethyl methacrylate and cationic acrylamide monomers in ethanol and passing nitrogen gas to remove oxygen;

[0033] (2) After heating to the first reaction temperature, an initiator is added, and after the reaction is completed, a prepolymer solution A is obtained;

[0034] (3) adding alkyl chloride to the prepolymer solution A, raising the temperature to a second reaction temperature to continue the reaction, and obtaining an oil-based drilling fluid flocculant after the reaction is completed.

[0035] The cationic acrylamide monomer is used in the flocculant to provide an adsorption group to improve the flocculant molecule and the harmful solid phase. In some preferred embodiments, the cationic acrylamide monomer includes one or both of methacryloyloxyethyl ammonium chloride and acryloyloxyethyl trimethyl ammonium chloride.

[0036] In step (2), an initiator is added to cause redox free radical polymerization of dimethylaminoethyl methacrylate and cationic acrylamide monomers to obtain a prepolymer solution A.

[0037] In some preferred embodiments, the first reaction temperature is 45-80°C, and the reaction time is 8-24h; further preferably, the first reaction temperature is 55-70°C, and the reaction time is 8-16h.

[0038] In some preferred embodiments, the initiator is one or more of azobisisobutyronitrile, dimethyl azobisisobutyrate, benzoyl peroxide, and tert-butyl benzoyl peroxide.

[0039] In step (3), the halogen in the alkyl chloride undergoes an atomic substitution reaction with the prepolymer A.

[0040] In some preferred embodiments, the general structural formula of the alkyl chloride is as follows:

[0041]

[0042] Here, n is an integer from 10 to 22.

[0043] Alkyl chloride as a functional monomer in flocculant can improve the dispersion of flocculant in drilling fluid on one hand, and can also serve as an adsorption group to enhance the flocculation effect of flocculant on harmful solid phase on the other hand. Among them, n in alkyl chloride has a greater impact on the dispersion of polymer. Too small or too large will lead to poor dispersibility of flocculant in drilling fluid.

[0044] In some preferred embodiments, the second reaction temperature is 30-60°C, and the reaction time is 2-10 hours; further preferably, the second reaction temperature is 45-55°C, and the reaction time is 3-6 hours.

[0045] In some preferred embodiments, based on 100 parts by weight of ethanol, the dimethylaminoethyl methacrylate is 60-95 parts by weight; the cationic acrylamide monomer is 5-30 parts by weight; the alkyl chloride is 20-60 parts by weight; and the initiator is 0.001-3 parts by weight.

[0046] In practice, too much or too little dimethylaminoethyl methacrylate, cationic acrylamide monomer and alkyl chloride will lead to poor flocculation performance of the flocculant. Too much initiator will lead to too fast initiation and poor flocculant performance.

[0047] More preferably, based on 100 parts by weight of ethanol, the dimethylaminoethyl methacrylate is 75-80 parts by weight; the cationic acrylamide monomer is 10-20 parts by weight; the alkyl chloride is 35-45 parts by weight; and the initiator is 0.1-1 parts by weight.

[0048] On the other hand, the present invention also provides a cationic flocculant, which is prepared by the above-mentioned preparation method.

[0049] The cationic flocculant of the present invention is a transparent amber viscous liquid, which can selectively flocculate inferior solid phases with negatively charged surfaces to form large and dense flocs, making them easy to be removed by solid control equipment, while almost not flocculating useful solid phases such as barite with no charge on the surface; and can reduce the viscosity and shear force of oil-based drilling fluids, thereby improving the utilization rate of oil-based drilling fluids.

[0050] In yet another aspect, the present invention also provides the use of the above cationic flocculant in removing low-density inferior solid phase in oil-based drilling fluid.

[0051] The cationic flocculant of the present invention has excellent solubility in oil-based drilling fluids. On this basis, it has the characteristic of selectively flocculating inferior solid phases in oil-based drilling fluids, while the flocculation effect on useful solid phases such as barite and organic soil is relatively weak, which can significantly improve the removal effect of cuttings in oil-based drilling fluids.

[0052] The present invention is further described below by way of examples, but the present invention is not limited to the scope of the examples. The experimental methods in the following examples without specifying specific conditions are carried out according to conventional methods and conditions. The raw materials used in the following examples are all conventionally purchased.

[0053] Embodiment 1:

[0054] A cationic oil-based drilling fluid flocculant was prepared by solution polymerization.

[0055] 75 g of dimethylaminoethyl methacrylate and 10 g of methacryloyloxyethyl ammonium chloride were dissolved in 100 g of ethanol, and nitrogen was purged for 20 min to remove oxygen. After heating to 60° C., 0.5 g of azobisisobutyronitrile was added, and the reaction was completed for 12 h to obtain a prepolymer solution A;

[0056] 35 g of alkyl chloride 18 (carbon number 18) was added to the prepolymer solution A, and the temperature was raised to 50° C. to continue the reaction for 4 hours. After the reaction was completed, a transparent amber viscous liquid was obtained, which was the oil-based drilling fluid flocculant S1.

[0057] Embodiment 2:

[0058] A cationic oil-based drilling fluid flocculant was prepared by solution polymerization.

[0059] 75 g of dimethylaminoethyl methacrylate and 10 g of acryloyloxyethyl trimethylammonium chloride were dissolved in 100 g of ethanol, and nitrogen was purged for 20 min to remove oxygen. After heating to 60° C., 0.5 g of azobisisobutyronitrile was added, and the reaction was completed for 12 h to obtain a prepolymer solution A;

[0060] Add 35 g of alkyl chloride 18 (carbon number 18) to the prepolymer solution A, raise the temperature to 50° C. and continue the reaction for 4 hours. After the reaction is completed, a transparent amber viscous liquid is obtained, which is the oil-based drilling fluid flocculant S2.

[0061] Embodiment 3:

[0062] A cationic oil-based drilling fluid flocculant was prepared by solution polymerization.

[0063] 75 g of dimethylaminoethyl methacrylate and 10 g of methacryloyloxyethyl ammonium chloride were dissolved in 100 g of ethanol, and nitrogen was purged for 20 min to remove oxygen. After heating to 60° C., 0.1 g of azobisisobutyronitrile was added, and the reaction was completed for 12 h to obtain a prepolymer solution A.

[0064] 35 g of alkyl chloride 12 (carbon number 12) was added to the prepolymer solution A, and the temperature was raised to 50° C. to continue the reaction for 4 hours. After the reaction was completed, a transparent amber viscous liquid was obtained, which was the oil-based drilling fluid flocculant S3.

[0065] Embodiment 4:

[0066] A cationic oil-based drilling fluid flocculant was prepared by solution polymerization.

[0067] 75 g of dimethylaminoethyl methacrylate and 10 g of methacryloyloxyethyl ammonium chloride were dissolved in 100 g of ethanol, and nitrogen was purged for 20 min to remove oxygen. After heating to 60° C., 0.1 g of benzoyl peroxide was added, and the reaction was completed for 12 h to obtain a prepolymer solution A;

[0068] Add 35 g of alkyl chloride 18 (carbon number 18) to the prepolymer solution A, raise the temperature to 50° C. and continue the reaction for 4 hours. After the reaction is completed, a transparent amber viscous liquid is obtained, which is the oil-based drilling fluid flocculant S4.

[0069] Embodiment 5:

[0070] A cationic oil-based drilling fluid flocculant was prepared by solution polymerization.

[0071] 65 g of dimethylaminoethyl methacrylate and 30 g of methacryloyloxyethyl ammonium chloride were dissolved in 100 g of ethanol, and nitrogen was purged for 20 min to remove oxygen. After heating to 70° C., 1.5 g of azobisisobutyronitrile was added, and the reaction was completed for 12 h to obtain a prepolymer solution A.

[0072] Add 20 g of alkyl chloride 12 (carbon number 12) to the prepolymer solution A, raise the temperature to 50°C and continue the reaction for 8 hours. After the reaction is completed, a transparent amber viscous liquid is obtained, which is the oil-based drilling fluid flocculant S5.

[0073] Comparative Example 1:

[0074] The method of Example 1 was followed, except that methacryloyloxyethyl ammonium chloride was not used in step (1). Other conditions were the same as those of Example 1, to obtain flocculant D1.

[0075] Comparative Example 2:

[0076] The method of Example 1 was followed, except that no alkyl chloride was added in step (2). Other conditions were the same as those of Example 1, to obtain flocculant D2.

[0077] The flocculants prepared in Examples 1-5 and Comparative Examples 1-2 were used to treat oil-based drilling fluids to evaluate the performance of the flocculants provided by the present invention for removing rock cuttings from oil-based drilling fluids. In the following test examples,

[0078] The median solid phase particle size of the drilling fluid was tested using a focused beam reflectometer (Mettler-Toledo, ParticleTrack G600B);

[0079] The plastic viscosity (mPa·s), dynamic shear force (Pa), and initial and final shear force (Pa) of the drilling fluid were measured using a six-speed viscometer according to the method specified in GB / T 16783.2-2012;

[0080] The drilling fluid density, the low-density solid content and the barite solid content of the drilling fluid are measured according to the method specified in GB / T16783.2-2012. Unless otherwise specified, the low-density solid content and the barite solid content refer to the volume content.

[0081] The manufacturer of the six-speed viscometer is Qingdao Tongchun Petroleum Instrument Co., Ltd., and the model is ZNN-D6B;

[0082] The manufacturer of the low-speed mixer is Qingdao Tongchun Petroleum Instrument Co., Ltd., model D90;

[0083] The manufacturer of the centrifuge is Hunan Xiangyi Laboratory Instrument Development Co., Ltd., model TG16-WS.

[0084] Oil-based drilling fluid sample S0:

[0085] The oil-based drilling fluid in this test example is the drilling fluid returned to the shore from the offshore platform. It is an oil-based drilling fluid sample obtained by processing solid control equipment such as a vibrating screen, a desilter, a desander, and a centrifuge. The following tests are performed on the sample.

[0086] 1. Solid phase particle size test after adding flocculant to drilling fluid sample

[0087] 0.1 g of the flocculants prepared in Examples 1-5 and Comparative Examples 1-2 of the present invention were added to 100 mL of the oil-based drilling fluid sample, and a low-speed stirrer was used to stir for 5 min at 500 r / min to obtain the drilling fluid after flocculation treatment, and the median solid phase particle size of the drilling fluid was tested using a focused beam reflection meter. S0 was used as a blank control group, without adding a flocculant, and other test conditions were exactly the same as those of the other test groups. The test results are shown in Table 1.

[0088] Table 1 Effect of flocculants on the particle size of inferior solid phase in oil-based drilling fluid

[0089]

[0090] As can be seen from Table 1, after adding the flocculant provided by the present invention to the oil-based drilling fluid sample, the median solid phase particle size of the drilling fluid is greatly increased from 11 μm to more than 65 μm. Among them, Examples 1-3 have the best effect, and the median solid phase particle size of the drilling fluid after flocculation treatment is increased to more than 100 μm. This shows that the flocculant provided by the present invention has an obvious flocculation effect on the finely dispersed particles in the oil-based drilling fluid sample.

[0091] 2. Test of flocculated solid phase components after adding flocculant to drilling fluid

[0092] 0.1 g of the flocculants prepared in Examples 1-5 and Comparative Examples 1-2 of the present invention were added to 100 mL of the oil-based drilling fluid sample, and stirred at 500 r / min for 5 min using a low-speed stirrer to obtain the drilling fluid after flocculation treatment. The drilling fluid was sieved (200 mesh standard sieve), and the flocculants retained on the sieve surface were washed with n-hexane, and then dried at 50° C. The flocculated solid phase was weighed as the sieve residue mass. The element test of the flocculated solid phase was performed, and the oxide (SiO 2 , CaO, BaO, Al 2 O 3 , Fe 2 O 3 The mass percentage of barite (BaSO 4 ), calcium oxide (CaO) and the mass percentage of the inferior solid phase (wherein, the mass percentage of the inferior solid phase = 100% - the mass percentage of barite - the mass percentage of calcium oxide). S0 was used as a blank control group, without adding flocculant, and the other test conditions were exactly the same as those of the other test groups. The test results are shown in Table 2.

[0093] Table 2 Screening mass and flocculation solid phase composition

[0094]

[0095] As can be seen from Table 2, after adding the flocculant provided by the present invention to the oil-based drilling fluid sample, the median solid phase particle size of the drilling fluid is greatly improved, and the mass of the sieve residue after the drilling fluid passes through the 200-mesh standard sieve increases significantly, from 0.31g to more than 5.5g, among which Examples 1-3 have the best effect, and the mass of the sieve residue after flocculation treatment is more than 9.5g. In addition, from the composition of the sieve residue, with the addition of the flocculant, the inferior solid phase in the sieve residue increases significantly, and the barite decreases significantly, indicating that the flocculant provided by the present invention has the effect of selective flocculation, has a strong flocculation ability for inferior solid phases, and a weak flocculation ability for barite.

[0096] 3. Rheological properties test of drilling fluid samples after adding flocculants

[0097] 0.1 g of flocculants prepared in Examples 1-5 and Comparative Examples 1-2 were added to 100 mL of oil-based drilling fluid samples, and stirred at 500 r / min for 5 min using a low-speed stirrer to obtain flocculated drilling fluid. Centrifuge at 3000 rpm for 5 minutes and then take the upper layer of liquid to test its rheological parameters. S0 was used as a blank control group, without adding flocculants, and other test conditions were exactly the same as those of other test groups. The test results are shown in Table 3.

[0098] Table 3 Rheological properties test of centrifuge liquid after flocculation

[0099]

[0100] It can be seen from Table 3 that after adding the flocculant provided by the present invention to the oil-based drilling fluid sample and centrifuging it, the plastic viscosity, dynamic shear force, and initial / final shear of the drilling fluid are significantly reduced. This is because after the inferior solid phase is removed, the solid phase in the drilling fluid is reduced, which reduces the friction between the drilling fluid particles, which is manifested as a decrease in viscosity.

[0101] 3.Drilling fluid sample after adding flocculant centrifuge liquid density and inferior solid content test

[0102] 0.1 g of flocculants prepared in Examples 1-5 and Comparative Examples 1-2 were added to 100 mL of oil-based drilling fluid samples, and stirred at 500 r / min for 5 min using a low-speed stirrer to obtain flocculated drilling fluid. The upper layer of liquid was taken to test its density and inferior solid content after centrifugation at 3000 rpm for 5 minutes. S0 was used as a blank control group, without adding flocculants, and other test conditions were exactly the same as those of other test groups. The test results are shown in Table 4.

[0103] Table 4 Test of centrifuge density and inferior solid content after flocculation

[0104]

[0105] It can be seen from Table 3 that after adding the flocculant provided by the present invention to the oil-based drilling fluid sample and centrifuging it, the density of the drilling fluid and the inferior solid phase are significantly reduced. The density of the centrifuged fluid is 1.51 g / cm 3 Reduced to 1.22g / cm 3 , the inferior solid phase content was reduced from 15.5% to 5.6%, which indicates that the flocculant provided by the present invention has an obvious flocculation and removal effect on the inferior solid phase in the oil-based drilling fluid sample.

[0106] The above description includes examples of one or more embodiments. Of course, it will be appreciated by those skilled in the art that the various embodiments may be further combined and arranged. Therefore, the embodiments described herein are intended to cover all such changes, modifications and variations that fall within the scope of protection of the appended claims. In addition, with respect to the term "comprising" used in the specification or claims, the word is covered in a manner similar to the term "including", as explained by "including" used as a transitional word in the claims. In addition, any term "or" used in the specification of the claims is intended to mean "non-exclusive or".

[0107] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the concept and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A method for preparing a cationic flocculant, characterized in that: include: (1) dissolving dimethylaminoethyl methacrylate and cationic acrylamide monomers in ethanol and passing nitrogen gas to remove oxygen; (2) After heating to the first reaction temperature, an initiator is added, and after the reaction is completed, a prepolymer solution A is obtained; (3) adding alkyl chloride to the prepolymer solution A, raising the temperature to a second reaction temperature to continue the reaction, and obtaining an oil-based drilling fluid flocculant after the reaction is completed.

2. The method for preparing a cationic flocculant according to claim 1, characterized in that: The cationic acrylamide monomer includes one or both of methacryloyloxyethyl ammonium chloride and acryloyloxyethyl trimethyl ammonium chloride.

3. The method for preparing a cationic flocculant according to claim 1, characterized in that: The first reaction temperature is 45-80° C., and the reaction time is 8-24 h.

4. The method for preparing a cationic flocculant according to claim 1, characterized in that: The first reaction temperature is 55-70° C., and the reaction time is 8-16 hours.

5. The method for preparing a cationic flocculant according to claim 1, characterized in that: The initiator is one or more of azobisisobutyronitrile, dimethyl azobisisobutyrate, benzoyl peroxide, and tert-butyl benzoyl peroxide.

6. The method for preparing a cationic flocculant according to claim 1, characterized in that: The general structural formula of the alkyl chloride is shown below: Here, n is an integer from 10 to 22.

7. The method for preparing a cationic flocculant according to claim 1, characterized in that: The second reaction temperature is 30-60° C., and the reaction time is 2-10 h.

8. The method for preparing a cationic flocculant according to claim 1, characterized in that: The second reaction temperature is 45-55°C, and the reaction time is 3-6h.

9. The method for preparing a cationic flocculant according to claim 1, characterized in that: Based on 100 parts by weight of ethanol, the dimethylaminoethyl methacrylate is 60-95 parts by weight; the cationic acrylamide monomer is 5-30 parts by weight; the alkyl chloride is 20-60 parts by weight; and the initiator is 0.001-3 parts by weight.

10. The method for preparing a cationic flocculant according to claim 1, characterized in that: Based on 100 parts by weight of ethanol, the dimethylaminoethyl methacrylate is 75-80 parts by weight; the cationic acrylamide monomer is 10-20 parts by weight; the alkyl chloride is 35-45 parts by weight; and the initiator is 0.1-1 parts by weight.

11. A cationic flocculant, characterized in that: The preparation method is described in any one of claims 1 to 10.

12. Use of the cationic flocculant according to claim 11 in removing low-density inferior solid phase in oil-based drilling fluid.

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