Drilling fluid and preparation method thereof

Abstract

The present application relates to the technical field of oil and gas production, and specifically discloses a drilling fluid and a preparation method thereof. A drilling fluid, the raw materials of which include: 30-50 parts of coconut oil heptyl ester derivative; 20-30 parts of glyceryl stearate; 20-30 parts of capric triglyceride; 15-20 parts of octadecyl paraben; 10-20 parts of polystyrene-coated paraffin microcapsules; 5-10 parts of calcium chloride; 1-5 parts of emulsifier; 1-5 parts of lightener; 1-5 parts of fluid loss reducer; 10-20 parts of water. The drilling fluid of the present application has strong low temperature resistance and good rheology, and is suitable for oil and gas drilling work under low temperature conditions.

Description

Technical Field

[0001] The present application relates to the technical field of oil and gas exploitation, and more specifically, to a drilling fluid and a preparation method thereof. Background Art

[0002] When developing unconventional oil and gas resources such as shale gas, since shale layers are sensitive to water, problems such as unstable wellbore walls and easy collapse may occur during drilling. Therefore, drilling fluid needs to be added during drilling to protect the wellbore walls. The principle of drilling fluid protecting the wellbore walls is mainly through the combined effect of the mud skin formed by the drilling fluid and the balanced formation pressure, thereby improving the stability of the wellbore walls.

[0003] In the related art, for example, the application document with publication number CN111518526A discloses a drilling fluid and its preparation method, which comprises the following components in percentage by mass: 20%-42% base oil; 0.5%-2% flow pattern regulator or organic soil; 2%-4% primary emulsifier; 2%-4% auxiliary emulsifier; 0.5%-1% calcium oxide; 1%-3% fluid loss reducer; 18%-38% weighting agent; the balance is an aqueous solution of calcium chloride. The base oil is a C-containing solution obtained by treating vegetable oil. 10 To C 20 Saturated oil of mixed alkanes of straight-chain normal alkanes, wherein C 10 To C 20 The mass percentage of straight chain normal alkanes is greater than 90%. By eliminating the unsaturated double bonds in the vegetable oil, a base oil mainly including C 10 To C 20 The saturated oil of mixed alkanes of straight-chain normal alkanes has low kinematic viscosity and is easily degraded by microorganisms in the environment. At the same time, polycyclic hydrocarbons, aromatic hydrocarbons and other components harmful to the environment and human body are eliminated from the base oil to avoid environmental pollution.

[0004] Regarding the above-mentioned related technologies, in the actual working process, the above-mentioned drilling fluid can improve the stability of the well wall by combining the base oil and the weighting agent. However, under harsh conditions, such as a low temperature environment below -20°C, the above-mentioned drilling fluid has a significant decrease in overall fluidity, an increase in viscosity, and even solidification due to the changes in the freezing point and fluidity of the base oil and the flow type regulator therein. Therefore, under low temperature conditions, it is difficult for the drilling fluid to effectively penetrate into the tiny cracks and pores of the well wall to form a protective film, and it is also impossible to carry and remove drill cuttings in time, which leads to insufficient protection of the well wall and may even cause safety hazards such as well wall collapse. Therefore, the drilling fluid in the related technology has the defect of poor low temperature resistance. Summary of the invention

[0005] In order to improve the low temperature resistance of drilling fluid, the present application provides a drilling fluid and a preparation method thereof.

[0006] The drilling fluid provided in this application adopts the following technical solution:

[0007] A drilling fluid comprising the following raw materials in parts by weight:

[0008] 30-50 parts of coconut oil heptyl ester derivative;

[0009] Glyceryl stearate 20-30 parts;

[0010] Capric triglyceride 20-30 parts;

[0011] 15-20 parts of octadecyl parahydroxybenzoate;

[0012] 10-20 parts of polystyrene-coated paraffin microcapsules;

[0013] Calcium chloride 5-10 parts;

[0014] Emulsifier 1-5 parts;

[0015] 1-5 parts of lightener;

[0016] 1-5 parts of fluid loss reducer;

[0017] 10-20 parts of water.

[0018] By adopting the above technical scheme, the coconut oil heptyl ester derivative, as one of the basic oil phase components of the drilling fluid, has good low-temperature fluidity and stability. The long-chain fatty acid ester group in its molecular structure can maintain a certain flexibility at low temperatures and is not easy to solidify, thereby helping to improve the low-temperature resistance of the drilling fluid. Glyceryl stearate is also an important basic oil phase component with good lubricity and low-temperature stability. In the drilling fluid, it can synergize with the coconut oil heptyl ester derivative to further enhance the fluidity and stability of the drilling fluid. Capric triglyceride has a low freezing point and good biocompatibility, can remain liquid at low temperatures, and provide the necessary fluidity for the drilling fluid. At the same time, it can also interact with other basic oil phase components to form a stable oil film to protect the well wall from damage. Calcium chloride, as an electrolyte component of the drilling fluid, can adjust the ionic strength and osmotic pressure of the drilling fluid, thereby affecting the fluidity and stability of the drilling fluid. Under low temperature conditions, the addition of calcium chloride helps to maintain the fluidity of the drilling fluid and prevent it from solidifying.

[0019] In low temperature environments (especially below -20°C), the brittleness of the polystyrene cladding increases significantly due to its material properties. During the drilling process, it ruptures after friction and collision with the slag, releasing the paraffin inside. Paraffin undergoes a phase change from solid to "pseudo-liquid" at low temperatures. The volume change and microstructural changes caused by this phase change will improve the fluidity of the entire oil complex (drilling fluid) within the local temperature range. This is one of the key mechanisms to improve the fluidity of drilling fluid at low temperatures, and can effectively solve the problem of poor fluidity of drilling fluid at low temperatures, so that the drilling fluid can more effectively penetrate into the tiny cracks and pores on the well wall to form a protective film, thereby better exerting the protective function of the drilling fluid.

[0020] Optionally, the polystyrene-coated paraffin microcapsules are prepared by the following method:

[0021] A. Mix polystyrene emulsion, paraffin wax and deionized water, and treat with ultrasonic cell disruptor for 30-60 min to obtain paraffin oil-in-water emulsion;

[0022] B. Add the paraffin oil-in-water emulsion into a three-necked flask, place in a constant temperature water bath at 60-80° C. and stir for 3-5 hours. During the stirring process, add citric acid dropwise to adjust the pH value of the reaction solution to 3-4. After the reaction is completed, a paraffin microcapsule suspension is obtained;

[0023] C. The paraffin microcapsule suspension is vacuum filtered, washed and dried to obtain polystyrene-coated paraffin microcapsules.

[0024] Optionally, in step A, the mass ratio of polystyrene emulsion, paraffin wax and deionized water is 1:0.6:(3-5).

[0025] Optionally, the raw materials also include 5-10 parts of lithium sulfate powder.

[0026] By adopting the above technical solution, the addition of lithium sulfate powder can adjust the pH value and ionic strength of the drilling fluid. The pH value of the drilling fluid has an important influence on its performance, and a reasonable pH value range can maintain the stability and fluidity of the drilling fluid. And because the solubility of lithium sulfate powder increases with decreasing temperature, the solubility characteristics of lithium sulfate powder enable it to continue to dissolve under low temperature conditions, which is crucial for the fluidity of the drilling fluid under low temperature conditions. As the temperature decreases, the fluidity of many drilling fluids will decrease, the viscosity will increase, and even coagulation will occur. The dissolution of lithium sulfate powder can produce a certain ionic effect, reduce the viscosity of the drilling fluid, and improve its fluidity. This will help to better play the protective function of the drilling fluid.

[0027] Optionally, the coconut oil heptyl ester derivative is preferably coconut oil heptyl ester 165.

[0028] By adopting the above technical solution, in the drilling fluid formula, coconut oil heptyl ester 165 has better synergy with other ester components such as glyceryl stearate and capric triglyceride. This synergy can make the oil phase interact better with the emulsifier during the emulsification process, which helps to form a more stable emulsion system. At low temperatures, this stable emulsion system can better resist the adverse effects of temperature changes, such as preventing emulsion demulsification, thereby improving the low temperature resistance of the drilling fluid.

[0029] Optionally, the fluid loss reducer comprises fly ash and methyl methacrylate in a mass ratio of 1:(0.4-0.5).

[0030] By adopting the above technical solution, the fluid loss reducer with the above ratio can more easily penetrate into the tiny cracks and pores of the well wall under low temperature environment. At the same time, the lightening agent can also improve the sand carrying capacity of the drilling fluid, remove the drill cuttings in time, and prevent the well wall from collapsing.

[0031] Optionally, the emulsifier is selected from any one of polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monostearate and polyoxyethylene fatty alcohol ether.

[0032] By adopting the above technical solution, the above emulsifier can better disperse the oil phase and the water phase in this solution evenly to form a stable emulsion. Under low temperature conditions, the addition of the emulsifier can prevent the base oil phase and the water phase from separating and maintain the stability and fluidity of the drilling fluid.

[0033] Optionally, the lightening agent is any one of hollow glass microspheres and silicate micropowder.

[0034] The present application also provides a method for preparing drilling fluid, which adopts the following technical solution:

[0035] A method for preparing a drilling fluid comprises the following steps:

[0036] S1, mixing coconut oil heptyl ester derivative, glyceryl stearate, capric triglyceride, octadecyl paraben and polystyrene-coated paraffin microcapsules according to a ratio to obtain an oil phase mixture;

[0037] S2, mixing calcium chloride, emulsifier, lightener, fluid loss reducer, lithium sulfate powder and water according to a proportion to obtain an aqueous phase mixture;

[0038] S3. Stir and shear the oil phase mixture and the water phase mixture in a high-speed shear emulsifier for 30-60 minutes at a stirring speed of 5000 r / min. After the stirring is completed, an emulsified mixture is obtained. The emulsified mixture is homogenized by a homogenizer to obtain a drilling fluid.

[0039] In summary, this application has the following beneficial effects:

[0040] 1. This application uses coconut oil heptyl ester derivatives and glycerol stearate as the main components of the base oil of the drilling fluid, which has good low-temperature fluidity and stability. The long-chain fatty acid ester group in its molecular structure can maintain a certain flexibility at low temperatures and is not easy to solidify, which helps to improve the low-temperature resistance of the drilling fluid. By introducing polystyrene-coated paraffin microcapsules, the brittleness of the polystyrene shell increases significantly under low-temperature conditions due to its material properties. During the drilling process, it ruptures after friction and collision with the slag, releasing the internal paraffin. Paraffin undergoes a phase change from solid to "pseudo-liquid" at low temperatures. The volume change and microstructural changes caused by this phase change will improve the fluidity of the entire oil complex (drilling fluid) within the local temperature range, thereby effectively solving the problem of poor fluidity of the drilling fluid at low temperatures and improving the low-temperature resistance of the drilling fluid.

[0041] 2. Lithium sulfate powder is also added in this application. The addition of lithium sulfate powder can adjust the pH value and ionic strength of the drilling fluid. The pH value of the drilling fluid has an important influence on its performance. A reasonable pH value range can maintain the stability and fluidity of the drilling fluid. And because the solubility of lithium sulfate powder increases as the temperature decreases, the solubility characteristics of lithium sulfate powder enable it to continue to dissolve under low temperature conditions. This characteristic is crucial for the fluidity of the drilling fluid in a low temperature environment. As the temperature decreases, the fluidity of many drilling fluids will decrease, the viscosity will increase, and even coagulation will occur. The dissolution of lithium sulfate powder can produce a certain ionic effect, reduce the viscosity of the drilling fluid, and improve its fluidity. This will help to better play the protective function of the drilling fluid and improve the low temperature resistance of the drilling fluid.

[0042] 3. The fluid loss reducer of the present application uses fly ash and methyl methacrylate. The fluid loss reducer of the above components can more easily penetrate into the tiny cracks and pores of the well wall under low temperature environment. At the same time, the lightening agent can also improve the sand carrying capacity of the drilling fluid, remove the drill cuttings in time, and prevent the well wall from collapsing, so that the drilling fluid maintains good working performance. DETAILED DESCRIPTION

[0043] The present application is further described in detail below with reference to the embodiments.

[0044] Preparation Example of Polystyrene-coated Paraffin Microcapsules

[0045] Preparation Example 1

[0046] Polystyrene-coated paraffin microcapsules are prepared by the following method:

[0047] A. Mix polystyrene emulsion, paraffin wax and deionized water in a mass ratio of 1:0.6:3, and treat with an ultrasonic cell disruptor for 30 minutes to obtain a paraffin oil-in-water emulsion;

[0048] B. Add the paraffin oil-in-water emulsion into a three-necked flask, place in a constant temperature water bath at 60° C. and stir for 5 h. During the stirring process, add citric acid dropwise to adjust the pH value of the reaction solution to 3.5. After the reaction is completed, a paraffin microcapsule suspension is obtained;

[0049] C. The paraffin microcapsule suspension was vacuum filtered, washed with anhydrous ethanol and deionized water for three times respectively, and then dried in an oven at 40° C. for 12 h to obtain polystyrene-coated paraffin microcapsules.

[0050] Preparation Example 2

[0051] Polystyrene-coated paraffin microcapsules are prepared by the following method:

[0052] A. Mix polystyrene emulsion, paraffin wax and deionized water in a mass ratio of 1:0.6:4, and treat with an ultrasonic cell disruptor for 50 minutes to obtain a paraffin oil-in-water emulsion;

[0053] B. Add the paraffin oil-in-water emulsion into a three-necked flask, place in a constant temperature water bath at 70°C and stir for 4 hours. During the stirring process, add citric acid dropwise to adjust the pH value of the reaction solution to 3.5. After the reaction is completed, a paraffin microcapsule suspension is obtained;

[0054] C. The paraffin microcapsule suspension was vacuum filtered, washed with anhydrous ethanol and deionized water for three times respectively, and then dried in an oven at 40° C. for 12 h to obtain polystyrene-coated paraffin microcapsules.

[0055] Preparation Example 3

[0056] Polystyrene-coated paraffin microcapsules are prepared by the following method:

[0057] A. Mix polystyrene emulsion, paraffin wax and deionized water in a mass ratio of 1:0.6:5, and treat with an ultrasonic cell disruptor for 60 minutes to obtain a paraffin oil-in-water emulsion;

[0058] B. Add the paraffin oil-in-water emulsion into a three-necked flask, place in a constant temperature water bath at 80°C and stir for 3 hours. During the stirring process, add citric acid dropwise to adjust the pH value of the reaction solution to 3.5. After the reaction is completed, a paraffin microcapsule suspension is obtained;

[0059] C. The paraffin microcapsule suspension was vacuum filtered, washed with anhydrous ethanol and deionized water for three times respectively, and then dried in an oven at 40° C. for 12 h to obtain polystyrene-coated paraffin microcapsules.

[0060] Preparation Example 4

[0061] Polyurethane-coated paraffin microcapsules are prepared by the following method:

[0062] A. Mix polyurethane emulsion, paraffin wax and deionized water in a mass ratio of 1:0.4:5, and treat with an ultrasonic cell disruptor for 60 minutes to obtain a paraffin oil-in-water emulsion;

[0063] B. Add the paraffin oil-in-water emulsion into a three-necked flask, place in a constant temperature water bath at 70°C and stir for 4 hours. During the stirring process, add citric acid dropwise to adjust the pH value of the reaction solution to 3.5. After the reaction is completed, a paraffin microcapsule suspension is obtained;

[0064] C. The paraffin microcapsule suspension was vacuum filtered, washed three times with anhydrous ethanol and deionized water respectively, and then dried in an oven at 40°C for 12 h to obtain polyurethane-coated paraffin microcapsules.

[0065] Preparation Example 5

[0066] Methyl methacrylate-coated paraffin microcapsules are prepared by the following method:

[0067] A. Mix methyl methacrylate emulsion, paraffin wax and deionized water in a mass ratio of 1:0.4:5, and treat with an ultrasonic cell disruptor for 60 minutes to obtain a paraffin oil-in-water emulsion;

[0068] B. Add the paraffin oil-in-water emulsion into a three-necked flask, place in a constant temperature water bath at 70°C and stir for 4 hours. During the stirring process, add citric acid dropwise to adjust the pH value of the reaction solution to 3.5. After the reaction is completed, a paraffin microcapsule suspension is obtained;

[0069] C. The paraffin microcapsule suspension was vacuum filtered, washed three times with anhydrous ethanol and deionized water respectively, and then dried in an oven at 40°C for 12 h to obtain methyl methacrylate-coated paraffin microcapsules.

[0070] Example

[0071] Example 1

[0072] A drilling fluid, the raw material components and dosage of which are shown in Table 1, wherein: the coconut oil heptyl ester derivative is coconut oil heptyl ester 165; the polystyrene-coated paraffin microcapsules are the polystyrene-coated paraffin microcapsules prepared in Preparation Example 1; the emulsifier is polyoxyethylene sorbitan monolaurate; the fluid loss reducer includes 0.7 kg of fly ash and 0.3 kg of methyl methacrylate; and the lightening agent is hollow glass microspheres.

[0073] The drilling fluid was prepared by the following method:

[0074] S1, mixing coconut oil heptyl ester derivative, glyceryl stearate, capric triglyceride, octadecyl paraben and polystyrene-coated paraffin microcapsules according to a ratio to obtain an oil phase mixture;

[0075] S2, mixing calcium chloride, emulsifier, lightener, fluid loss reducer and water according to a proportion to obtain an aqueous phase mixture;

[0076] S3. The oil phase mixture and the water phase mixture are stirred and sheared in a high-speed shear emulsifier for 30 minutes at a stirring speed of 5000 r / min. After the stirring is completed, an emulsified mixture is obtained. The emulsified mixture is homogenized by a homogenizer to obtain a drilling fluid.

[0077] Example 2

[0078] A drilling fluid, the raw material components and dosage of which are shown in Table 1, wherein: the coconut oil heptyl ester derivative is coconut oil heptyl ester 165; the polystyrene-coated paraffin microcapsules are the polystyrene-coated paraffin microcapsules prepared in Preparation Example 2; the emulsifier is polyoxyethylene sorbitan monostearate; the fluid loss reducer includes 2.1 kg of fly ash and 0.9 kg of methyl methacrylate; and the lightening agent is hollow glass microspheres.

[0079] The drilling fluid was prepared by the following method:

[0080] S1, mixing coconut oil heptyl ester derivative, glyceryl stearate, capric triglyceride, octadecyl paraben and polystyrene-coated paraffin microcapsules according to a ratio to obtain an oil phase mixture;

[0081] S2, mixing calcium chloride, emulsifier, lightener, fluid loss reducer and water according to a proportion to obtain an aqueous phase mixture;

[0082] S3. Stir and shear the oil phase mixture and the water phase mixture in a high-speed shear emulsifier for 50 minutes at a stirring speed of 5000 r / min. After the stirring is completed, an emulsified mixture is obtained. The emulsified mixture is homogenized by a homogenizer to obtain a drilling fluid.

[0083] Example 3

[0084] A drilling fluid, the raw material components and dosage of which are shown in Table 1, wherein: the coconut oil heptyl ester derivative is coconut oil heptyl ester 165; the polystyrene-coated paraffin microcapsules are the polystyrene-coated paraffin microcapsules prepared in Preparation Example 3; the emulsifier is polyoxyethylene fatty alcohol ether; the fluid loss reducer includes 3.5 kg of fly ash and 1.5 kg of methyl methacrylate; and the lightening agent is hollow glass microspheres.

[0085] The drilling fluid was prepared by the following method:

[0086] S1, mixing coconut oil heptyl ester derivative, glyceryl stearate, capric triglyceride, octadecyl paraben and polystyrene-coated paraffin microcapsules according to a ratio to obtain an oil phase mixture;

[0087] S2, mixing calcium chloride, emulsifier, lightener, fluid loss reducer and water according to a proportion to obtain an aqueous phase mixture;

[0088] S3. Stir and shear the oil phase mixture and the water phase mixture in a high-speed shear emulsifier for 60 minutes at a stirring speed of 5000 r / min. After the stirring is completed, an emulsified mixture is obtained. The emulsified mixture is homogenized by a homogenizer to obtain a drilling fluid.

[0089] Table 1 Raw material components and dosage of drilling fluid in Examples 1-3 (kg)

[0090]

[0091] Example 4

[0092] A drilling fluid, which is different from Example 3 in that 5 kg of lithium sulfate powder is further added to the raw material of the drilling fluid of this example. When preparing the drilling fluid, the lithium sulfate powder is added in step S2 for mixing.

[0093] Example 5

[0094] A drilling fluid, which is different from Example 3 in that 8 kg of lithium sulfate powder is further added to the raw material of the drilling fluid of this example. When preparing the drilling fluid, the lithium sulfate powder is added in step S2 for mixing.

[0095] Example 6

[0096] A drilling fluid, which is different from Example 3 in that 10 kg of lithium sulfate powder is further added to the raw material of the drilling fluid of this example. When preparing the drilling fluid, the lithium sulfate powder is added in step S2 for mixing.

[0097] Example 7

[0098] A drilling fluid, which is different from Example 3 in that the coconut oil heptyl ester derivative in this example is coconut oil heptyl ester 140.

[0099] Comparative Example

[0100] Comparative Example 1

[0101] The drilling fluid was prepared according to Example A in the application document with publication number CN111518526A and invention name “Drilling fluid and preparation method thereof”.

[0102] Comparative Example 2

[0103] A drilling fluid, which is different from Example 3 in that the polyurethane-coated paraffin microcapsules prepared in Preparation Example 4 are used in equal amounts to replace the polystyrene-coated paraffin microcapsules in this comparative example.

[0104] Comparative Example 3

[0105] A drilling fluid, which is different from Example 3 in that an equal amount of methyl methacrylate-coated paraffin microcapsules prepared in Preparation Example 5 are used to replace polystyrene-coated paraffin microcapsules in this comparative example.

[0106] Performance testing

[0107] The drilling fluids prepared in Examples 1-7 and Comparative Examples 1-3 were aged at -30°C and -50°C for 24 hours, respectively, and their properties were tested. The test results are shown in Table 2.

[0108] Table 2 Test results of Examples 1-7 and Comparative Examples 1-3

[0109]

[0110] After aging for 24 hours at -30°C and -50°C, the drilling fluids of Examples 1-7 did not solidify and maintained good fluidity. In terms of kinematic viscosity, the kinematic viscosities of the drilling fluids of Examples 1 to 3 at -30°C and -50°C were relatively low, indicating that they had good fluidity and good low-temperature resistance, and were suitable for use in low-temperature environments. With the increase in the amount of polystyrene-coated paraffin microcapsules, the kinematic viscosity of the drilling fluid did not increase significantly, indicating that the addition amount of the microcapsules had a good promoting effect on the fluidity of the drilling fluid within the range provided in this application.

[0111] After adding lithium sulfate powder to the drilling fluid in Examples 4 to 6, the kinematic viscosity of the drilling fluid is further reduced, especially at -50°C, where the reduction is particularly significant. This indicates that the addition of lithium sulfate powder effectively improves the fluidity of the drilling fluid at low temperatures, thereby further enhancing the low temperature resistance of the drilling fluid.

[0112] In Example 7, although other types of coconut oil heptyl ester derivatives (coconut oil heptyl ester 140) were used and the kinematic viscosity was reduced, the performance of the drilling fluid remained within an acceptable range and no solidification occurred, which also shows that coconut oil heptyl ester 165 has better low temperature resistance than coconut oil heptyl ester 140 as a drilling fluid raw material.

[0113] The drilling fluid of Comparative Example 1 has partially solidified at -30°C, but completely solidified at -50°C. This shows that its low temperature resistance is poor and cannot meet the requirements of low temperature drilling. In addition, its density at low temperature is significantly higher, and it is easy to completely block the well wall at low temperature, making it difficult for oil and gas to seep out, so its working performance is poor.

[0114] Although the drilling fluids of Comparative Examples 2 and 3 did not completely solidify, they partially solidified at -50°C and had high kinematic viscosity, indicating that their fluidity was poor. Since the packaging materials of polyurethane-coated paraffin microcapsules and methyl methacrylate-coated paraffin microcapsules are difficult to be destroyed at low temperatures, the paraffin phase change material cannot be released, resulting in poor fluidity of the drilling fluid at low temperatures.

[0115] This specific embodiment is merely an explanation of the present application and is not a limitation of the present application. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed, but such modifications are protected by patent law as long as they are within the scope of the claims of the present application.

Claims

1. A drilling fluid, characterized in that: The invention comprises the following raw materials in parts by weight: 30-50 parts of coconut oil heptyl ester derivative; Glyceryl stearate 20-30 parts; Capric triglyceride 20-30 parts; 15-20 parts of octadecyl parahydroxybenzoate; 10-20 parts of polystyrene-coated paraffin microcapsules; Calcium chloride 5-10 parts; Emulsifier 1-5 parts; 1-5 parts of lightener; 1-5 parts of fluid loss reducer; 10-20 parts of water.

2. A drilling fluid according to claim 1, characterized in that: The polystyrene-coated paraffin microcapsules are prepared by the following method: A. Mix polystyrene emulsion, paraffin wax and deionized water, and treat with ultrasonic cell disruptor for 30-60 min to obtain paraffin oil-in-water emulsion; B. Add the paraffin oil-in-water emulsion into a three-necked flask, place in a constant temperature water bath at 60-80° C. and stir for 3-5 hours. During the stirring process, add citric acid dropwise to adjust the pH value of the reaction solution to 3-4. After the reaction is completed, a paraffin microcapsule suspension is obtained; C. The paraffin microcapsule suspension is vacuum filtered, washed and dried to obtain polystyrene-coated paraffin microcapsules.

3. A drilling fluid according to claim 2, characterized in that: In step A, the mass ratio of polystyrene emulsion, paraffin wax and deionized water is 1:0.6:(3-5).

4. A drilling fluid according to claim 1, characterized in that: The raw materials also include 5-10 parts of lithium sulfate powder.

5. A drilling fluid according to claim 1, characterized in that: The coconut oil heptyl ester derivative is coconut oil heptyl ester 165.

6. A drilling fluid according to claim 1, characterized in that: The fluid loss reducer comprises fly ash and methyl methacrylate in a mass ratio of 1:(0.4-0.5).

7. A drilling fluid according to claim 1, characterized in that: The emulsifier is selected from any one of polyoxyethylene sorbitan monolaurate, polyoxyethylene sorbitan monostearate and polyoxyethylene fatty alcohol ether.

8. A drilling fluid according to claim 1, characterized in that: The lightening agent is any one of hollow glass microspheres and silicate micropowder.

9. A method for preparing a drilling fluid according to any one of claims 4 to 8, characterized in that: The steps include: S1, mixing coconut oil heptyl ester derivative, glyceryl stearate, capric triglyceride, octadecyl paraben and polystyrene-coated paraffin microcapsules according to a ratio to obtain an oil phase mixture; S2, mixing calcium chloride, emulsifier, lightener, fluid loss reducer, lithium sulfate powder and water according to a proportion to obtain an aqueous phase mixture; S3. Stir and shear the oil phase mixture and the water phase mixture in a high-speed shear emulsifier for 30-60 minutes at a stirring speed of 5000 r / min. After the stirring is completed, an emulsified mixture is obtained. The emulsified mixture is homogenized by a homogenizer to obtain a drilling fluid.

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