Viscosity reducer for drilling fluid as well as preparation method and application of viscosity reducer

Abstract

The invention belongs to the technical field of oilfield chemical assistants, and particularly relates to a drilling fluid viscosity reducer as well as a preparation method and application thereof. The drilling fluid viscosity reducer provided by the invention comprises a quadripolymer, the quadripolymer is obtained by copolymerization of N, N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, and the molar ratio of the N, N-dimethylacrylamide to the 2-acrylamide-2-methylpropanesulfonic acid to the styrene to the maleic anhydride is (0.5-1.5): (1-1.5): (1-2.5): (0.5-1.5). A quadripolymer formed on the basis of a sulfonic group, a carboxyl group and an amide group is competitively adsorbed on the surface of an inferior solid phase in the drilling fluid, and a contact point of a macromolecular polymer and the inferior solid phase is shielded, so that a net rack structure is disassembled, the effect of reducing the viscosity and shearing force of the drilling fluid, especially a high-density drilling fluid, is achieved, and the rheological property of the drilling fluid is improved.

Description

Technical Field

[0001] This invention relates to the field of oilfield chemical additives technology, specifically to a drilling fluid viscosity reducer, its preparation method, and its application. Background Technology

[0002] In recent years, with the continuous deepening of deep and ultra-deep oil and gas exploration and development in my country, and even the drilling and development of wells reaching depths of tens of thousands of meters, high-density water-based drilling fluids have been widely used in deep and ultra-deep well drilling operations to balance formation pressure. High-density water-based drilling fluids generally achieve the designed density requirements by adding premium barite, resulting in a high solid content. Combined with formation mud preparation, increased slag content, and the continuous intrusion of inferior solid phases, this leads to a continuous increase in the system's plastic viscosity and shear stress.

[0003] High-density water-based drilling fluid systems contain high levels of solids such as barite and bentonite, leading to a significant conflict between rheological properties and settling stability. Drilling fluid viscosity reducers, as one of the drilling fluid additives, play a crucial role in the viscosity reduction process. While solids control equipment can effectively remove various solid phases from the drilling fluid, regulating its rheological properties and reducing the amount of viscosity reducer needed, it cannot replace the viscosity reducer itself. With prolonged use, inferior solid phases that enter the water-based drilling fluid cannot be effectively removed by solids control equipment, resulting in thickening of the fluid, difficulty in controlling its rheological properties, ineffective viscosity reduction, and failure to meet drilling requirements. This leads to the consumption of large amounts of drilling fluid and additives, delays drilling time, and severely impacts drilling production and wellbore quality.

[0004] There are many types of drilling fluid viscosity reducers, which can be classified into dispersible drilling fluid viscosity reducers and polymeric drilling fluid viscosity reducers according to their mechanism of action. Currently, the most widely used viscosity reducers both domestically and internationally are those obtained by modifying natural materials (such as tannins, phenolic resins, and lignin), or synthetic low-molecular-weight acrylic polymer viscosity reducers. Natural product-modified viscosity reducers, such as tannins and lignin sulfonates, belong to the dispersible drilling fluid viscosity reducer category. Their effects are only significant at higher pH levels. They mainly achieve viscosity reduction through adsorption hydration, breaking down the clay network structure. However, they cannot eliminate the network structure formed between polymers or between polymers and clay. Typically, they adsorb onto the positively charged edges of clay particles, making them negatively charged and forming a thicker hydration layer. This breaks down the network structure formed by the clay particles, releasing the trapped free water and reducing the viscosity of the system. Simultaneously, they increase the potential of the clay particles, enhancing the electrostatic repulsion between particles and weakening the formation of the network structure. Polymer viscosity reducers work by firmly adsorbing onto clay particles, eliminating the network structure between the polymer and the clay, thus reducing viscosity, but they still cannot break down the network structure between the polymers.

[0005] Chinese Patent CN107828016B, authorized on April 3, 2020, discloses a viscosity reducer for drilling fluids. Using acrylic acid, maleic anhydride, 2-acrylamido-2-methylpropanesulfonic acid, and sodium aminotrimethylmylate as raw materials, a low-molecular-weight polymer viscosity reducer is prepared through an aqueous solution polymer compounding method. This reducer can effectively lower the viscosity of freshwater mud, salt mud, and calcium-containing mud, at a density ρ = 1.40 g / cm³. 3 Adding the aforementioned low-molecular-weight polymer viscosity reducer to the drilling fluid achieves a viscosity reduction rate of 13.11–31.15%. US Patent No. US06612280, published on January 28, 1986, discloses a high-temperature drilling fluid comprising a terpolymer formed from maleic anhydride, styrene, and methacrylamide, with a weight-average molecular weight of approximately 500–10,000, which gives the drilling fluid excellent rheological properties at high temperatures.

[0006] High-density drilling fluids have high viscosity due to their high solid content and the infiltration of formation mud, slag, and inferior solid phases. However, the aforementioned natural modified viscosity reducers and ternary polymer viscosity reducers can only reduce viscosity by breaking down the network structure formed by clay particles or the network structure between clay particles and polymers. They are mainly for reducing viscosity of low-density non-weighted mud and cannot effectively reduce viscosity of high-density drilling fluids. Summary of the Invention

[0007] The purpose of this invention is to provide a drilling fluid viscosity reducer that solves the problem of poor viscosity reduction effect of existing drilling fluid viscosity reducers on high-density drilling fluids.

[0008] The second objective of this invention is to provide a method for preparing a drilling fluid viscosity reducer, thereby solving the problem of the complexity of the preparation methods for quaternary polymer viscosity reducers in the prior art.

[0009] The third objective of this invention is to provide an application of a drilling fluid viscosity reducer, which solves the problem that existing technologies cannot effectively reduce the viscosity of high-density drilling fluids.

[0010] To solve the above problems, the technical solution of the drilling fluid viscosity reducer of the present invention is as follows:

[0011] A drilling fluid viscosity reducer includes a quaternary copolymer obtained by copolymerizing N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, wherein the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is (0.5-1.5):(1-1.5):(1-2.5):(0.5-1.5).

[0012] This invention improves upon existing technology and provides a drilling fluid viscosity reducer comprising a quaternary copolymer obtained by copolymerizing N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene, and maleic anhydride. By controlling the proportion of raw materials, the ratio of functional groups such as sulfonic acid groups, carboxyl groups, and amide groups in the quaternary copolymer is controlled. Based on the quaternary copolymer formed by sulfonic acid groups, carboxyl groups, and amide groups, it competitively adsorbs onto the surface of inferior solid phases in the drilling fluid, shielding the contact points between the macromolecular polymer and the inferior solid phase, thereby breaking down the network structure formed between polymers and between polymers and the solid phase. This reduces the viscosity and shear stress of drilling fluids, especially high-density drilling fluids, and improves the rheological properties of the drilling fluid.

[0013] To further enhance the disintegration effect of the quaternary copolymer on the network structure, preferably, the number-average molecular weight of the quaternary copolymer is 6000-8000.

[0014] The technical solution of the drilling fluid viscosity reducer preparation method of the present invention is as follows:

[0015] A method for preparing a drilling fluid viscosity reducer includes the following steps: mixing N,N-dimethylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, styrene and maleic anhydride to form a mixed raw material, and then performing a copolymerization reaction under the action of an initiator to obtain a quaternary copolymer.

[0016] The drilling fluid viscosity reducer preparation method provided by this invention involves copolymerizing four polymeric monomers under the action of an initiator to obtain a quaternary copolymer, which is the drilling fluid viscosity reducer. The quaternary polymer can significantly improve the viscosity reduction rate of the viscosity reducer on drilling fluids, especially high-density drilling fluids. The preparation method of the drilling fluid viscosity reducer provided by this invention is simple, easy to operate, and low in cost, suitable for large-scale production, and can be used in large quantities at drilling sites.

[0017] To further reduce the starting energy of the copolymerization reaction and ensure its smooth progress, preferably, the amount of initiator added is 0.5% to 1% of the mass of the mixed raw materials.

[0018] To further improve the copolymerization efficiency, preferably, the copolymerization reaction temperature is 60-75°C and the copolymerization reaction time is 3-6 hours.

[0019] To facilitate further copolymerization of monomers, the copolymerization reaction is preferably carried out under alkaline conditions, wherein the alkaline conditions are pH = 7 to 8.

[0020] The technical solution for the application of the drilling fluid viscosity reducer of the present invention is as follows:

[0021] The application of the drilling fluid viscosity reducer of the present invention involves adding the drilling fluid viscosity reducer to the drilling fluid, wherein the mass ratio of the drilling fluid viscosity reducer to the volume of the drilling fluid is (1-2) g: 100 ml.

[0022] The application of the drilling fluid viscosity reducer of the present invention, by adding 1% to 2% of the drilling fluid viscosity reducer to the drilling fluid, can greatly reduce the viscosity and shear force of the drilling fluid, especially high-density drilling fluid, and has a significant viscosity reduction effect on the drilling fluid. This allows for the regulation of the rheological properties of the drilling fluid to meet different drilling requirements and enable drilling production to proceed smoothly.

[0023] To make the viscosity-reducing effect of the drilling fluid viscosity reducer more significant, preferably, the drilling fluid is a water-based drilling fluid, which is composed of water and, by mass fraction, 3.5%–4.5% bentonite, 0.15%–0.25% Na₂CO₃, 0.3%–0.8% KOH, 0.3%–0.8% sodium carboxymethyl cellulose, 0.5%–1% coating agent, 5%–8% KCl, 0.3%–0.8% ammonium salt, 2%–5% sulfonated phenolic resin, 1%–3% sulfonated lignite resin, 2%–5% sulfonated asphalt, 0.1%–0.5% CaCl₂, 0.5%–1.5% extreme pressure lubricant, and barite.

[0024] To further improve the viscosity reduction rate, preferably, the density of the water-based drilling fluid is 2.0–2.5 g / cm³. 3 . Detailed Implementation

[0025] The technical concept of the drilling fluid viscosity reducer provided by this invention is as follows:

[0026] The drilling fluid viscosity reducer provided by the present invention comprises a quaternary copolymer, wherein the quaternary copolymer is obtained by copolymerization of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, and the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is (0.5-1.5):(1-1.5):(1-2.5):(0.5-1.5).

[0027] The drilling fluid viscosity reducer provided by this invention regulates the proportions of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene, and maleic anhydride, thereby controlling the proportions of functional groups such as sulfonic acid groups, carboxyl groups, and amide groups in the quaternary copolymer. Based on the quaternary copolymer formed by sulfonic acid groups, carboxyl groups, and amide groups, it competitively adsorbs onto the surface of inferior solid phases in the drilling fluid, shielding the contact points between the macromolecular polymer and the inferior solid phase. This breaks down the network structure formed between polymers or between the polymer and the solid phase, thereby reducing the viscosity and shear stress of drilling fluids, especially high-density drilling fluids, and improving the rheological properties of the drilling fluid.

[0028] In a specific embodiment, the softening point of the quaternary copolymer is 150-200°C, and the melting point is 300-350°C.

[0029] A method for preparing a drilling fluid viscosity reducer includes the following steps: mixing N,N-dimethylacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, styrene and maleic anhydride to form a mixed raw material, and then performing a copolymerization reaction under the action of an initiator to obtain a quaternary copolymer.

[0030] In a specific embodiment, N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride are mixed and placed in a constant temperature water bath at 60-75°C for copolymerization.

[0031] In a specific embodiment, the alkaline conditions are achieved by adjusting the pH to 7-8 using NaOH.

[0032] In a specific embodiment, nitrogen gas is purged before the initiator is added to remove oxygen. The nitrogen purging time is 0.8 to 1.5 hours, which allows the copolymerization reaction to proceed completely in a nitrogen environment. The initiator is ammonium persulfate.

[0033] In a specific embodiment, after the copolymerization reaction is completed, the mixture is cooled to room temperature, ethanol is added, and the reaction product is precipitated. After drying and pulverizing, the reaction product becomes a drilling fluid viscosity reducer.

[0034] An application of a drilling fluid viscosity reducer, wherein the drilling fluid viscosity reducer is added to the drilling fluid, and the mass ratio of the drilling fluid viscosity reducer to the volume of the drilling fluid is (1-2) g: 100 ml.

[0035] In a specific embodiment, the water-based drilling fluid is composed of water and bentonite with a mass fraction of 3.5%–4.5%, Na2CO3 with a mass fraction of 0.15%–0.25%, KOH with a mass fraction of 0.3%–0.8%, CMC-LV with a mass fraction of 0.3%–0.8%, FA367 with a mass fraction of 0.5%–1%, KCl with a mass fraction of 5%–8%, ammonium salt with a mass fraction of 0.3%–0.8%, SMP-1 with a mass fraction of 2%–5%, SPNH with a mass fraction of 1%–3%, sulfonated asphalt with a mass fraction of 2%–5%, CaCl2 with a mass fraction of 0.1%–0.5%, extreme pressure lubricant with a mass fraction of 0.5%–1.5%, and barite.

[0036] The embodiments of the present invention will be further described below with reference to specific examples. Unless otherwise specified, the chemical reagents involved in the following examples are all commercially available conventional products.

[0037] I. Specific Embodiments of the Drilling Fluid Viscosity Reducer of the Present Invention

[0038] Example 1

[0039] The drilling fluid viscosity reducer provided in this embodiment includes a quaternary copolymer, which is obtained by copolymerizing N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, wherein the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is 0.51:1.01:1.92:0.51.

[0040] Example 2

[0041] The drilling fluid viscosity reducer provided in this embodiment includes a quaternary copolymer, which is obtained by copolymerizing N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, wherein the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is 0.59:1.05:1.92:0.54.

[0042] Example 3

[0043] The drilling fluid viscosity reducer provided in this embodiment includes a quaternary copolymer, which is obtained by copolymerizing N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, wherein the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is 0.54:1.03:1.92:0.58.

[0044] Example 4

[0045] The drilling fluid viscosity reducer provided in this embodiment includes a quaternary copolymer, which is obtained by copolymerizing N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, wherein the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is 1:1.01:1.92:1.01.

[0046] Example 5

[0047] The drilling fluid viscosity reducer provided in this embodiment includes a quaternary copolymer, which is obtained by copolymerizing N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, wherein the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is 1:1.06:2.08:1.07.

[0048] II. Specific Embodiments of the Preparation Method of the Drilling Fluid Viscosity Reducer of the Present Invention

[0049] Example 6

[0050] The method for preparing the drilling fluid viscosity reducer provided in this embodiment includes the following steps:

[0051] According to the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene, and maleic anhydride described in Example 1, 3g of N,N-dimethylacrylamide (molecular weight 99.13, the same below), 12.6g of 2-acrylamide-2-methylpropanesulfonic acid (207.25), 12.0g of styrene (104.15), and 3g of maleic anhydride (98.06) were weighed and added to a three-necked flask. After stirring evenly, NaOH was added to adjust the pH to 7.5. The three-necked flask was placed in a constant temperature water bath at 70°C, and nitrogen gas was purged into the three-necked flask for 0.8h. Then, 0.1g of ammonium persulfate initiator was added, and the reaction was carried out at a constant temperature with stirring for 3h. After the reaction was completed, the mixture was cooled to room temperature, and the reaction product was precipitated in anhydrous ethanol. The reaction product was dried under normal pressure and pulverized to obtain a drilling fluid viscosity reducer.

[0052] Example 7

[0053] The method for preparing the drilling fluid viscosity reducer provided in this embodiment includes the following steps:

[0054] According to the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene, and maleic anhydride described in Example 2, 3.5 g of N,N-dimethylacrylamide, 13.0 g of 2-acrylamide-2-methylpropanesulfonic acid, 12.0 g of styrene, and 3.2 g of maleic anhydride were weighed and added to a three-necked flask. After stirring evenly, NaOH was added to adjust the pH to 7.5. The three-necked flask was placed in a constant temperature water bath at 65°C, and nitrogen gas was purged into the three-necked flask for 1 hour. Then, 0.12 g of ammonium persulfate initiator was added, and the reaction was carried out at a constant temperature with stirring for 4 hours. After the reaction was completed, the mixture was cooled to room temperature, and the reaction product was precipitated in anhydrous ethanol. The reaction product was dried under normal pressure and pulverized to obtain a drilling fluid viscosity reducer.

[0055] Example 8

[0056] The method for preparing the drilling fluid viscosity reducer provided in this embodiment includes the following steps:

[0057] According to the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene, and maleic anhydride described in Example 3, 3.2 g of N,N-dimethylacrylamide, 12.8 g of 2-acrylamide-2-methylpropanesulfonic acid, 12.0 g of styrene, and 3.4 g of maleic anhydride were weighed and added to a three-necked flask. After stirring evenly, NaOH was added to adjust the pH to 7.5. The three-necked flask was placed in a constant temperature water bath at 65°C, and nitrogen gas was purged into the three-necked flask for 1.5 h. Then, 0.15 g of ammonium persulfate initiator was added, and the reaction was carried out at a constant temperature with stirring for 4.5 h. After the reaction was completed, the mixture was cooled to room temperature, and the reaction product was precipitated by adding anhydrous ethanol. The reaction product was dried under normal pressure and pulverized to obtain a drilling fluid viscosity reducer.

[0058] Example 9

[0059] According to the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene, and maleic anhydride described in Example 4, 6g of N,N-dimethylacrylamide (molecular weight 99.13, the same below), 12.6g of 2-acrylamide-2-methylpropanesulfonic acid (207.25), 12.0g of styrene (104.15), and 6g of maleic anhydride (98.06) were weighed and added to a three-necked flask. After stirring evenly, NaOH was added to adjust the pH to 7.8. The three-necked flask was placed in a constant temperature water bath at 70°C, and nitrogen gas was purged into the three-necked flask for 0.8h. Then, 0.1g of ammonium persulfate initiator was added, and the reaction was carried out at a constant temperature with stirring for 5h. After the reaction was completed, the mixture was cooled to room temperature, and the reaction product was precipitated in anhydrous ethanol. The reaction product was dried under normal pressure and pulverized to obtain a drilling fluid viscosity reducer.

[0060] Example 10

[0061] According to the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene, and maleic anhydride described in Example 5, 6g of N,N-dimethylacrylamide (molecular weight 99.13, the same below), 13.2g of 2-acrylamide-2-methylpropanesulfonic acid (207.25), 13.0g of styrene (104.15), and 6.3g of maleic anhydride (98.06) were weighed and added to a three-necked flask. After stirring evenly, NaOH was added to adjust the pH to 7.8. The three-necked flask was placed in a constant temperature water bath at 70°C, and nitrogen gas was purged into the three-necked flask for 0.8h. Then, 0.1g of ammonium persulfate initiator was added, and the reaction was carried out at a constant temperature with stirring for 5h. After the reaction was completed, the mixture was cooled to room temperature, and the reaction product was precipitated in anhydrous ethanol. The reaction product was dried under normal pressure and pulverized to obtain a drilling fluid viscosity reducer.

[0062] III. Specific Examples of the Application of the Drilling Fluid Viscosity Reducer of the Present Invention

[0063] Example 11

[0064] The drilling fluid viscosity reducer provided in this embodiment is applied by adding the drilling fluid viscosity reducer of Example 1 to a water-based drilling fluid. The mass ratio of the drilling fluid viscosity reducer to the volume of the drilling fluid is 1.5g:100ml. The water-based drilling fluid is composed of water and the following components by mass fraction: 4% bentonite, 0.2% Na2CO3, 0.5% KOH, 0.5% CMC-LV, 0.8% FA367, 6% KCl, 0.5% ammonium salt, 3% SMP-1, 2% SPNH, 3% sulfonated asphalt, 0.3% CaCl2, 1% extreme pressure lubricant, and barite. The barite increases the density of the water-based drilling fluid to 2.5g / cm³. 3 .

[0065] Example 12

[0066] The application of the drilling fluid viscosity reducer provided in this embodiment differs from that in Example 11 in that the drilling fluid viscosity reducer of Example 2 is added to the water-based drilling fluid, and the mass ratio of the drilling fluid viscosity reducer to the volume of the drilling fluid is 1.5g:100ml.

[0067] Example 13

[0068] The application of the drilling fluid viscosity reducer provided in this embodiment differs from that in Example 11 in that the drilling fluid viscosity reducer of Example 3 is added to the water-based drilling fluid, and the mass ratio of the drilling fluid viscosity reducer to the volume of the drilling fluid is 1.5g:100ml.

[0069] Example 14

[0070] The drilling fluid viscosity reducer provided in this embodiment is applied by adding the drilling fluid viscosity reducer of Example 4 to a water-based drilling fluid. The mass ratio of the drilling fluid viscosity reducer to the volume of the drilling fluid is 1g:100ml. The water-based drilling fluid is composed of water and, by mass fraction, 4.5% bentonite, 0.25% Na2CO3, 0.4% KOH, 0.4% CMC-LV, 0.75% FA367, 7% KCl, 0.6% ammonium salt, 4% SMP-1, 3% SPNH, 2% sulfonated asphalt, 0.2% CaCl2, 1.2% extreme pressure lubricant, and barite. The barite increases the density of the water-based drilling fluid to 2.3g / cm³. 3 .

[0071] IV. Comparative Example

[0072] Comparative Example 1

[0073] The drilling fluid viscosity reducer provided in this comparative example includes a terpolymer obtained by copolymerizing 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, wherein the molar ratio of 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is 1.01:1.92:0.51.

[0074] The preparation method of the drilling fluid viscosity reducer provided in this comparative example is as follows:

[0075] According to the above molar ratio of 2-acrylamido-2-methylpropanesulfonic acid, styrene, and maleic anhydride, 12.6 g of 2-acrylamido-2-methylpropanesulfonic acid (molecular weight 207.25, the same below), 12.0 g of styrene (104.15), and 3 g of maleic anhydride (98.06) were weighed and added to a three-necked flask. After stirring evenly, NaOH was added to adjust the pH to 7.5. The three-necked flask was placed in a constant temperature water bath at 70°C, and nitrogen gas was purged into the three-necked flask for 0.8 h. Then, 0.1 g of ammonium persulfate initiator was added, and the reaction was carried out at a constant temperature with stirring for 3 h. After the reaction was completed, the mixture was cooled to room temperature, and the reaction product was precipitated in anhydrous ethanol. The reaction product was dried under normal pressure and pulverized to obtain a drilling fluid viscosity reducer.

[0076] Comparative Example 2

[0077] The application of the drilling fluid viscosity reducer provided in this comparative example differs from that in Example 11 in that the drilling fluid viscosity reducer of Comparative Example 1 is added to the water-based drilling fluid, and the mass ratio of the drilling fluid viscosity reducer to the volume of the drilling fluid is 1.5g:100ml.

[0078] V. Experimental Examples

[0079] Experimental Example 1

[0080] This experiment tested the physicochemical properties of the drilling fluid viscosity reducer provided by the present invention. The number average molecular weight of the quaternary copolymer viscosity reducer in Example 1 was measured to be 6800, the softening point was 160℃, and the melting point was 320℃. The number average molecular weight of the quaternary copolymer viscosity reducer in Example 2 was measured to be 6850, the softening point was 162℃, and the melting point was 325℃.

[0081] Experiment Example 2

[0082] The rheological properties of the drilling fluid after aging at 120℃ were tested using the rheological test method in GB / T 16783.1-2014 Petroleum and Natural Gas Industry Drilling Fluid Field Testing Part 1: Water-based Drilling Fluids. The rheological property test included apparent viscosity (AV), plastic viscosity (PV), and dynamic shear force (YP). The rheological property test results of the drilling fluid before and after the addition of the drilling fluid viscosity reducer provided by this invention are shown in Table 1. The blank control group consisted of drilling fluid without the addition of a viscosity reducer. The drilling fluid was the water-based drilling fluid from Example 11, which was composed of water and the following components by mass fraction: 4% bentonite, 0.2% Na₂CO₃, 0.5% KOH, 0.5% CMC-LV, 0.8% FA367, 6% KCl, 0.5% ammonium salt, 3% SMP-1, 2% SPNH, 3% sulfonated asphalt, 0.3% CaCl₂, 1% extreme pressure lubricant, and barite. The barite increased the density of the water-based drilling fluid to 2.5 g / cm³. 3 .

[0083] Table 1. Rheological property test results of drilling fluid

[0084] AV / mPa·s PV / mPa·s YP / Pa Example 11 52.0 32.0 20.0 Example 12 50.0 30.0 20.0 Example 13 54.0 33.0 21.0 Example 14 52.5 30.0 22.5 Blank control group 86.0 58.0 28.0 Comparative Example 2 75.0 50.0 25.0

[0085] As shown in Table 1, compared with the water-based drilling fluid without the addition of drilling fluid viscosity reducer (blank control group), the drilling fluid with the drilling fluid viscosity reducer provided by this invention has an apparent viscosity reduction of more than 35%, a plastic viscosity reduction of more than 40%, and a dynamic shear force reduction of more than 25%. Compared with the water-based drilling fluid with the introduction of terpolymer (Comparative Example 2), the drilling fluid with the drilling fluid viscosity reducer provided by this invention also shows a significant reduction in apparent viscosity, plastic viscosity, and shear force. This indicates that the drilling fluid viscosity reducer of this invention can significantly reduce the viscosity of high-density drilling fluids, exhibiting a significant viscosity-reducing effect, which is beneficial for solving the problem of difficult-to-control rheological properties of high-density drilling fluids.

[0086] Finally, it should be noted that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A drilling fluid viscosity reducer, characterized in that, The product includes a quaternary copolymer obtained by copolymerizing N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride, wherein the molar ratio of N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride is (0.5-1.5):(1-1.5):(1-2.5):(0.5-1.5).

2. The drilling fluid viscosity reducer as described in claim 1, characterized in that, The number-average molecular weight of the quaternary copolymer is 6000 to 8000.

3. A method for preparing a drilling fluid viscosity reducer as described in claim 1 or 2, characterized in that, Includes the following steps: N,N-dimethylacrylamide, 2-acrylamide-2-methylpropanesulfonic acid, styrene and maleic anhydride are mixed to form a mixed raw material, which is then copolymerized under the action of an initiator to obtain a quaternary copolymer.

4. The method for preparing the drilling fluid viscosity reducer as described in claim 3, characterized in that, The amount of the initiator added is 0.5% to 1% of the mass of the mixed raw materials.

5. The method for preparing the drilling fluid viscosity reducer as described in claim 3, characterized in that, The copolymerization reaction temperature is 60–75°C, and the copolymerization reaction time is 3–6 hours.

6. The method for preparing the drilling fluid viscosity reducer as described in claim 3, characterized in that, The copolymerization reaction is carried out under alkaline conditions, wherein the alkaline conditions are pH = 7 to 8.

7. The application of a drilling fluid viscosity reducer as described in claim 1 or 2, characterized in that, The drilling fluid viscosity reducer is added to the drilling fluid, wherein the mass ratio of the drilling fluid viscosity reducer to the volume of the drilling fluid is (1-2) g: 100 ml.

8. The application of the drilling fluid viscosity reducer as described in claim 7, characterized in that, The drilling fluid is a water-based drilling fluid, which is composed of water and, by mass fraction, 3.5%–4.5% bentonite, 0.15%–0.25% Na₂CO₃, 0.3%–0.8% KOH, 0.3%–0.8% sodium carboxymethyl cellulose, 0.5%–1% coating agent, 5%–8% KCl, 0.3%–0.8% ammonium salt, 2%–5% sulfonated phenolic resin, 1%–3% sulfonated lignite resin, 2%–5% sulfonated bitumen, 0.1%–0.5% CaCl₂, 0.5%–1.5% extreme pressure lubricant, and barite.

9. The application of the drilling fluid viscosity reducer as described in claim 8, characterized in that, The density of the water-based drilling fluid is 2.0–2.5 g / cm³. 3 .