Low-viscosity shear strength improving agent, water-based drilling fluid and preparation method and application of low-viscosity shear strength improving agent

Abstract

The invention provides a low-viscosity shear strength improving agent, a water-based drilling fluid and a preparation method and application of the low-viscosity shear strength improving agent and the water-based drilling fluid, the low-viscosity shear strength improving agent is obtained by blending and modifying xanthan gum and tannic acid, and the mass ratio of the xanthan gum to the tannic acid is (15-20): 1. The low-viscosity shear strength improving agent can greatly improve the shear force of the drilling fluid on the premise of slightly improving the viscosity of the drilling fluid, and reduces the risks of well leakage, sticking and the like caused by too high viscosity while ensuring the sand carrying property of the drilling fluid. The low-viscosity shear strength improving agent, the biological film-forming lubricant and the nano cross-linking blocking agent are adopted, fluid in a shaft can be prevented from invading stratum fractures, the compactness of shale is improved, filtrate invasion and clay hydration are reduced, the good shale stabilizing capacity is achieved, the well cleaning capacity is high, the lubricating property is good, and the drilling fluid can be used for drilling a well. And the drilling requirement of a well with the horizontal section larger than 3000 m can be met.

Description

Technical Field

[0001] The present invention belongs to the technical field of drilling in the oil and gas industry, and specifically relates to a low-viscosity thixotropic agent, an aqueous drilling fluid, and a preparation method and application thereof. Background Art

[0002] Horizontal wells are one of the main well types for oil and gas resource development and are playing an increasingly important role. The longer the horizontal section, the larger the single-well drainage area and the higher the production. At the same time, horizontal wells are also one of the means for developing oil and gas reservoirs in ecological environment areas such as forest areas and water sources, and can maximize the exploitation of underground reserves without damaging the environment.

[0003] Major oil and gas fields have successively deployed multiple horizontal wells with horizontal sections exceeding 3000 m, and the longest horizontal section in China has reached 5256 m. For ultra-long horizontal sections, the drilling difficulty of the horizontal section increases geometrically with the increase in the length of the horizontal section, and there are mainly the following problems: First, the open-hole section in the horizontal section is long, the contact area between the drill string and the wellbore is large, the bottom-hole friction is large, the lubricity of the drilling fluid is poor, and it is difficult to reduce friction and resistance; Second, the reservoir continuity is poor, and it is easy to drill into mudstone. The mudstone has poor stability and is prone to collapse, resulting in difficulty in preventing wellbore collapse; Third, the horizontal section is long, and the cuttings at the bottom of the well are difficult to be carried out of the wellbore and are easily deposited by gravity to form a cuttings bed, resulting in an increase in friction and torque, and even sticking accidents; Fourth, there are natural faults in ultra-long horizontal sections, which are prone to serious losses, and the plugging difficulty is extremely high. Moreover, the upper limit of the density at which losses occur is less than the lower limit of the density required for wellbore stability, resulting in a contradiction between collapse and loss; Fifth, the drilling fluid invades the reservoir and blocks the pore throat channels, which is not conducive to reservoir protection.

[0004] At present, the vast majority of ultra-long horizontal section drilling at home and abroad mainly uses oil-based drilling fluids. For example, Well Mahu 029-H in Xinjiang Oilfield of PetroChina (horizontal section length 3038 m), Well Ning 209H71-3 in Southwest Oil and Gas Field (horizontal section length 3100 m), Well Sheng Ye 9-3HF in East China Petroleum Bureau of Sinopec (horizontal section length 4035 m), and the American shale gas well Mercury B 5H (horizontal section length 6340 m) are all completed with oil-based drilling fluids. Oil-based drilling fluids have the advantages of high temperature resistance, salt resistance, good lubricity, etc., but they also face problems such as high preparation and maintenance costs, environmental pollution, and difficult treatment of waste liquid and cuttings, which limit their popularization and application.

[0005] The aqueous drilling fluid disclosed in the invention patent with the patent number 202210394595.8 and the patent name "An Aqueous Drilling Fluid, a Viscosity Reducing Agent for Aqueous Drilling Fluid, and a Preparation Method" includes a viscosity reducing agent and has the characteristics of temperature resistance and salt resistance, and can reduce the apparent viscosity, plastic viscosity, dynamic shear force, and static shear force of high-temperature and high-density aqueous drilling fluids. However, its lubricity is poor and it is difficult to meet the requirements of drilling in ultra-long horizontal sections above 3000 m. Summary of the Invention

[0006] The object of the present invention is to provide a low-viscosity thixotropic agent which can significantly increase the shear force of drilling fluid on the premise of slightly increasing the viscosity of the drilling fluid, reduce the risks such as lost circulation and differential sticking caused by too high viscosity while ensuring the sand-carrying capacity of the drilling fluid.

[0007] Another object of the present invention is to provide an aqueous drilling fluid which can improve the compactness of shale, reduce filtrate invasion and clay hydration, has excellent shale stabilizing ability, and has strong borehole cleaning ability and good lubricity, and can meet the drilling requirements of wells with a horizontal section exceeding 3000 m.

[0008] Another object of the present invention is to provide a preparation method of an aqueous drilling fluid, the raw materials of which are simple and easy to obtain and convenient for on-site preparation.

[0009] For this reason, the technical solutions provided by the present invention are as follows: A low-viscosity thixotropic agent is obtained by blending and modifying xanthan gum and tannic acid, and the mass ratio of xanthan gum to tannic acid is 15-20:1. An aqueous drilling fluid comprises the following components: low-viscosity thixotropic agent, bentonite, alkalinity regulator, natural polymer filtrate reducer, plugging and wall protection agent, potassium chloride, lubricant, metal anti-wear agent, organic salt weighting agent and water; Calculated by mass percentage based on water: the mass percentage of bentonite is 2-4%, the mass percentage of alkalinity regulator is 0.1-0.3%, the mass percentage of low-viscosity thixotropic agent is 0.2-0.5%, the mass percentage of natural polymer filtrate reducer is 0.5-1%, the mass percentage of plugging and wall protection agent is 2-5%, the mass percentage of potassium chloride is 8-10%, the mass percentage of lubricant is 2-5%, the mass percentage of metal anti-wear agent is 0.1-0.2%, and the mass percentage of organic salt weighting agent is 30-65%.

[0010] The natural polymer filtrate reducer is modified plant gum NAT20 or polyanionic cellulose PAC, and the relative molecular mass range of the polyanionic cellulose PAC is 100,000-200,000.

[0011] The plugging and wall protection agent is nano-silica NSi550 modified by silane coupling agent or ultrafine calcium carbonate, and the mesh number of the ultrafine calcium carbonate is 1250-3000 meshes.

[0012] The lubricant is a biofilm-forming lubricant, a polyhydric alcohol or a fatty acid ester, and the biofilm-forming lubricant is a fatty acid graft-modified glycolipid lubricant HTZ; the metal anti-wear agent is an organic acid metal salt GXJM.

[0013] The organic salt weighting agent is sodium formate, potassium formate or Weigh2.

[0014] A preparation method of a water-based drilling fluid, comprising the following steps: Step 1) Add the formulated amount of bentonite and alkalinity regulator to water, and stir at 10,000 - 120,000 r / min for 8 - 12 h to prepare a bentonite slurry; Step 2) While stirring, sequentially add the formulated amounts of low-viscosity shear enhancer, natural polymer filtration reducer, plugging and wall protection agent, potassium chloride, lubricant, metal anti-wear agent, and organic salt weighting agent to the bentonite-based slurry to obtain a base slurry; Step 3) Add the formulated amounts of potassium chloride and organic salt weighting agent to the base slurry, and mix evenly to obtain the product.

[0015] The preparation process of the low-viscosity shear enhancer is as follows: Add water to a reaction vessel, then add xanthan gum, and stir while adding at 30 - 35 °C to form a homogeneous colloidal solution. Then, at 30 - 35 °C, add tannic acid and stir for 1 h until completely dissolved; heat up to 60 - 70 °C, and drip a 10% sodium hydroxide solution with a dropping rate of 100 - 120 mL / min into the reaction vessel. Stop dripping the alkali solution after adjusting the pH value of the solution to 9, and continue stirring and reacting at the reaction temperature for 1 - 1.5 h; after the reaction is completed, add ethanol to precipitate, dry for 48 h, and pulverize to obtain it; Among them, the mass ratio of water to xanthan gum is 100:4, and the addition amount of tannic acid is 5% of the mass of xanthan gum.

[0016] An application of a water-based drilling fluid in an ultra-long horizontal section above 3000 m.

[0017] The beneficial effects of the present invention are: The low-viscosity shear enhancer provided by the present invention can greatly increase the shear force of the drilling fluid on the premise of slightly increasing the viscosity of the drilling fluid, and while ensuring the sand-carrying capacity of the drilling fluid, reduce the risks such as lost circulation and sticking caused by too high viscosity.

[0018] The water-based drilling fluid provided by the present invention uses a low-viscosity shear enhancer, a biofilm-forming lubricant, and a nano-crosslinked plugging agent, which can reduce the intrusion of fluid in the wellbore into formation fractures, improve the compactness of shale, reduce filtrate invasion and clay hydration, has excellent shale stability, and has strong wellbore cleaning ability and good lubricity, and can meet the drilling requirements of wells with a horizontal section exceeding 3000 m. Description of the Drawings

[0019] Figure 1 is a bar chart of the lubrication performance test of the drilling fluid in the embodiment of the present invention; Figure 2 is a bar chart of the plugging performance test of the drilling fluid in the embodiment of the present invention. Detailed Embodiments

[0020] The following specific embodiments illustrate the implementation manners of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0021] Now, refer to the accompanying drawings to introduce the exemplary implementation manners of the present invention. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. These embodiments are provided to disclose the present invention in detail and completely, and to fully convey the scope of the present invention to those skilled in the relevant technical field. The terms in the exemplary implementation manners shown in the accompanying drawings are not limitations on the present invention.

[0022] Unless otherwise specified, the terms used herein (including scientific and technical terms) have the ordinary meaning understood by those skilled in the relevant technical field. Additionally, it can be understood that the terms defined in the commonly used dictionary should be understood to have a meaning consistent with the context of their relevant fields, and should not be understood as idealized or overly formal meanings.

[0023] Example 1 This example provides a low-viscosity viscosifier obtained by blending and modifying xanthan gum and tannic acid. The mass ratio of xanthan gum to tannic acid is 15 - 20:1. Preparation process: Slowly add 4 g of xanthan gum to a reaction kettle containing 100 g of water, and stir while adding at 30°C (try to control the addition speed to prevent caking) until a uniform colloidal solution is formed; then, while maintaining the condition of 30°C, add 0.2 g of tannic acid and stir for 1 h until completely dissolved; heat up to 60°C, and dropwise add a sodium hydroxide solution with a mass concentration of 10% to the reaction kettle at a rate of 100 mL / min, adjust the pH value of the solution to 9, stop dropping the alkali solution, and continue stirring and reacting for 1 h while maintaining the reaction temperature; after the reaction is completed, add a large amount of ethanol to precipitate, dry for 48 h, and pulverize to obtain the low-viscosity viscosifier.

[0024] Example 2 This example provides an aqueous drilling fluid, which includes the following components: low-viscosity viscosifier, bentonite, alkalinity regulator, natural polymer filtrate reducer, plugging and wall protection agent, potassium chloride, lubricant, metal anti-wear agent, organic salt weighting agent, and water; Calculated by mass percentage based on water: the mass percentage of bentonite is 2 - 4%, the mass percentage of the alkalinity regulator is 0.1 - 0.3%, the mass percentage of the low-viscosity viscosifier is 0.2 - 0.5%, the mass percentage of the natural polymer filtrate reducer is 0.5 - 1%, the mass percentage of the plugging and wall protection agent is 2 - 5%, the mass percentage of potassium chloride is 8 - 10%, the mass percentage of the lubricant is 2 - 5%, the mass percentage of the metal anti-wear agent is 0.1 - 0.2%, and the mass percentage of the organic salt weighting agent is 30 - 65%.

[0025] The natural polymer filtration reducer is modified plant gum NAT20 or polyanionic cellulose PAC, and the relative molecular mass range of the polyanionic cellulose PAC is 100,000 - 200,000.

[0026] The plugging and wall protection agent is nano-silica NSi550 modified by silane coupling agent or ultra-fine calcium carbonate, and the mesh number of the ultra-fine calcium carbonate is 1250 - 3000 meshes.

[0027] The lubricant is a biofilm-forming lubricant, a polymeric polyol or a fatty acid ester, and the biofilm-forming lubricant is a fatty acid graft-modified glycolipid lubricant HTZ; the metal anti-friction agent is an organic acid metal salt GXJM.

[0028] The organic salt weighting agent is sodium formate, potassium formate or Weigh2.

[0029] Principle of the present invention: The natural polymer filtration reducer contains a polysaccharide polymer with an adsorption group and a cyclic structure, has good filtration reduction ability, good environmental protection performance and is easily biodegradable. The plugging and wall protection agent can enter the micro-pores and micro-fractures of the formation, enhance the internal friction of the rock, prevent the fluid in the wellbore from entering the formation, and play a good plugging role. The lubricant can be adsorbed on the surface of clay particles, enhance the hydrophobicity, increase the contact angle, have a good penetration effect on the steel sheet, and facilitate the cutting of the drill bit. The metal anti-friction agent plays an anti-wear role. The organic salt weighting agent can be dissolved in water, reduce the solid phase of the water-based drilling fluid, reduce the friction in the ultra-long horizontal section, and the cation in the organic salt can tighten the clay crystal layer through ion exchange to inhibit the hydration swelling of the clay. The present invention can reduce the intrusion of fluid in the wellbore into the formation fractures, improve the compactness of the shale, reduce the intrusion of filtrate and the clay hydration effect, has excellent shale stability ability, and has strong wellbore cleaning ability and good lubricity.

[0030] Example 3 Based on Example 2, this example provides a water-based drilling fluid, including water and the following substances in mass percentage based on water: The mass percentage of bentonite is 4%, the mass percentage of the alkalinity regulator is 0.1%, the mass percentage of the low-viscosity viscosity increasing agent is 0.2%, the mass percentage of the natural polymer filtration reducer is 0.5%, the mass percentage of the plugging and wall protection agent is 2%, the mass percentage of potassium chloride is 8%, the mass percentage of the lubricant is 2%, the mass percentage of the metal anti-friction agent is 0.1%, and the mass percentage of the organic salt weighting agent is 43.7%.

[0031] Preparation process: Add 16 g of bentonite and 0.4 g of caustic soda to a high-speed stirring cup containing 400 mL of water, stir at 10,000 r / min for 8 - 12 h, and cure at room temperature for 24 h to prepare bentonite slurry; then add 0.8 g of low-viscosity viscosity enhancer, 2 g of natural polymer filtration loss reducer NAT20, 8 g of nano-crosslinked plugging agent NSi550, 8 g of biofilm-forming lubricant HTZ, 0.4 g of metal anti-wear agent GXJM, and 32 g of potassium chloride to the bentonite-based slurry in sequence. After adding each treatment agent, stir at a high speed of 10,000 r / min for 20 min to mix evenly before adding the next treatment agent; finally, add 174.8 g of potassium formate to the high-speed stirring cup and stir at a high speed of 10,000 r / min for 1 - 2 h to mix evenly to prepare a high-performance water-based drilling fluid base slurry system with a density of 1.30 g / cm 3 .

[0032] In this example, the alkalinity regulator is caustic soda, the low-viscosity viscosity enhancer is the low-viscosity viscosity enhancer (number MXSG) prepared in Example 2, the natural polymer filtration loss reducer is the natural polymer filtration loss reducer NAT20, the plugging and hole-protecting agent is the nano-crosslinked plugging agent NSi550, the lubricant is the biofilm-forming lubricant HTZ, the metal anti-wear agent is the metal anti-wear agent GXJM, and the organic salt weighting agent is potassium formate.

[0033] Among them, the natural polymer filtrate reducer NAT20 (manufacturer: Beijing Peikang Jiaye Technology Development Co., Ltd.) is a polysaccharide polymer containing adsorption groups and cyclic structures. After graft modification, it has good filtrate reduction ability under high temperature and high salt conditions, good environmental protection performance, and is easily biodegradable. The nano-crosslinked plugging agent NSi550 is prepared by modifying nano-silica with silane coupling agent KH550. Its particle size distribution range is 70 - 80 nm. It is easily dispersed and does not settle in the drilling fluid, and has good hydrophobicity. Under the action of differential pressure, NSi550 can enter the micro-pores and micro-fractures of the formation, enhance the internal friction force of the rock, prevent the fluid in the wellbore from entering the formation, and play a good plugging role. The preparation process of the nano-crosslinked plugging agent NSi550 can refer to the existing technical literature (Liu Bowen, Jiao Long, Yan Chunhua, etc. Modification and performance evaluation of nano-silica by silane coupling agent KH-550 [J]. Applied Chemical Industry, 2023, 52(05): 1367 - 1370). The biofilm-forming lubricant HTZ is a fatty acid graft-modified glycolipid, which can adsorb on the surface of clay particles. Its hydrophobic hydrocarbon long chains are arranged outward to form a lubricating film, enhancing hydrophobicity, increasing the contact angle, having a good penetration effect on steel sheets, facilitating the cutting of the drill bit, and with a temperature resistance up to 220°C. The biofilm-forming lubricant HTZ can refer to the glycolipid lubricant HTZ mentioned in the existing technical literature (Liu Xiongxiong. Research on drilling fluid lubricants based on plant materials [D]. Xi'an Shiyou University, 2021). The metal anti-wear agent GXJM (manufacturer: Chengdu Xiyou Huawei Technology Co., Ltd.). It can adsorb on the surface of the casing and drill pipe through organic negative ions to form a high-strength extreme pressure lubricating film to play an anti-wear role, with a temperature resistance up to 220°C.

[0034] Example 4 This example provides an aqueous drilling fluid. Compared with Example 3, the dosage of the low-viscosity shear enhancer is 0.3% (i.e., 1.2 g), the wall plugging agent is ultrafine calcium carbonate, with a mesh number of 1250 - 3000 meshes; the lubricant is fatty acid ester. The other components and the preparation process are the same.

[0035] Example 5 This example provides an aqueous drilling fluid. Compared with Example 3, the dosage of the low-viscosity shear enhancer is 0.4% (i.e., 1.6 g), and the natural polymer filtrate reducer is polyanionic cellulose PAC, with a relative molecular mass range of 100,000 - 200,000. The other components and the preparation process are the same.

[0036] Example 6 This example provides an aqueous drilling fluid. Compared with Example 3, the dosage of the low-viscosity shear enhancer is 0.5% (i.e., 2.0 g). The other components and the preparation process are the same.

[0037] Comparative Example Replace the low-viscosity shear enhancer MXSG in Example 3 with xanthan gum, and the addition amounts are 0.2% (0.8 g) and 0.3% (1.2 g) respectively. The other components and the preparation process are the same, and Comparative Example 1 and Comparative Example 2 are obtained.

[0038] Performance test: Referring to GB / T 16783.1-2014 Field Testing of Drilling Fluids - Part 1: Water-Based Drilling Fluids, load the drilling fluid systems prepared in Examples 3-6 and Comparative Examples into an aging tank, heat roll and age at 120 °C for 16 h, and examine the rheology of the drilling fluids. The results are shown in Table 1.

[0039] Table 1 Rheology and Filtration Loss of Each Experimental Drilling Fluid System

[0040] Note: AV is the apparent viscosity of the drilling fluid; PV is the plastic viscosity of the drilling fluid; YP is the dynamic shear force of the drilling fluid.

[0041] As can be seen from Table 1, for the drilling fluids after heat roll aging at 120 °C for 16 h, with the increase in the addition amount of the low-viscosity shear enhancer MXSG, the dynamic shear force, dynamic-plastic ratio, and static shear force (initial and final shear) increase accordingly. This shows that the low-viscosity shear enhancer MXSG can effectively improve the rheology of the drilling fluid and has good shear enhancing ability. In addition, the shear enhancing ability of the low-viscosity shear enhancer MXSG is comparable to that of xanthan gum, but the viscosity of the drilling fluid increases slowly, and the dynamic-plastic ratio is high, which is more conducive to wellbore cleaning and preventing the hazards caused by excessive viscosity, and improves the drilling speed.

[0042] Example 7 This example provides a water-based drilling fluid. Compared with Example 3, the addition amount of the biofilm-forming lubricant HTZ is changed to 3% (12 g). The other components and the preparation process are the same.

[0043] Example 8 This example provides a water-based drilling fluid. Compared with Example 3, the addition amount of the biofilm-forming lubricant HTZ is changed to 4% (16 g). The other components and the preparation process are the same.

[0044] Example 9 This example provides a water-based drilling fluid. Compared with Example 3, the addition amount of the biofilm-forming lubricant HTZ is changed to 5% (20 g). The other components and the preparation process are the same.

[0045] Example 10 This example provides a water-based drilling fluid. Compared with Example 3, the biofilm-forming lubricant HTZ is replaced with a polyol lubricant, and the addition amount is 2% (8 g). The other components and the preparation process are the same.

[0046] Example 11 This embodiment provides an aqueous drilling fluid. Compared with Embodiment 3, the biofilm-forming lubricant HTZ is replaced with a polyol lubricant, and the addition amount is 5% (20 g). The other components and the preparation process are the same.

[0047] Lubricity test: Referring to GB / T 16783.1-2014 Field Testing of Drilling Fluids - Part 1: Aqueous Drilling Fluids, the drilling fluid systems prepared in Embodiment 3 and Embodiments 7-11 are filled into an aging tank and thermally rolled and aged at 120 °C for 16 h. The extreme pressure lubrication coefficient of the aged drilling fluid and the friction coefficient of the formed mud cake are tested. The results are as Figure 1 shown.

[0048] It can be Figure 1 seen that for the drilling fluid thermally rolled and aged at 120 °C for 16 h, as the addition amount of the biofilm-forming lubricant HTZ increases, the extreme pressure lubrication coefficient and the mud cake friction coefficient gradually decrease, indicating that the high-performance aqueous drilling fluid system has a good lubrication effect. The extreme pressure lubrication coefficient and the mud cake friction coefficient of the drilling fluid using the biofilm-forming lubricant HTZ are both smaller than those of the drilling fluid using the same amount of polyol as the lubricant. The extreme pressure lubrication coefficient of this drilling fluid system < 0.1, and the mud cake friction coefficient < 0.1, which can effectively reduce the friction between the drill string and the wellbore during the drilling of ultra-long horizontal sections.

[0049] Embodiment 12 This embodiment provides an aqueous drilling fluid. Compared with Embodiment 3, the addition amount of potassium chloride is changed to 9% (36 g), and the other components and the preparation process are the same.

[0050] Embodiment 13 This embodiment provides an aqueous drilling fluid. Compared with Embodiment 3, the addition amount of potassium chloride is changed to 10% (40 g), and the other components and the preparation process are the same.

[0051] Embodiment 14 This embodiment provides an aqueous drilling fluid. Compared with Embodiment 3, potassium chloride is replaced with sodium chloride, and the addition amount is 8% (32 g), and the other components and the preparation process are the same.

[0052] Embodiment 15 This embodiment provides an aqueous drilling fluid. Compared with Embodiment 3, potassium chloride is replaced with sodium chloride, and the addition amount is 10% (40 g), and the other components and the preparation process are the same.

[0053] Inhibiting property test: Referring to the evaluation method for the inhibition of shale by drilling fluids in NB / T 10121-2018, the drilling fluids and cuttings prepared in Example 3 and Examples 12 to 15 were put into an aging tank together, and heat-rolled at 120°C for 16 h. The mass of the recovered cuttings was weighed, and the primary recovery rate of the cuttings was calculated. Then, the recovered cuttings and fresh water were put into the aging tank, and heat-rolled at 120°C for 16 h. The mass of the recovered cuttings was weighed, and the secondary recovery rate of the cuttings was calculated. The results are shown in Table 2.

[0054] Table 2 Inhibition of each drilling fluid system

[0055] As can be seen from Table 2, for the drilling fluids after heat-rolling and aging at 120°C for 16 h, as the potassium chloride dosage increases, both the primary recovery rate and the secondary recovery rate of the cuttings gradually increase. The primary cuttings recovery rate > 80%, and the secondary recovery rate > 15%, indicating that the high-performance water-based drilling fluid system has good inhibition. Compared with the drilling fluid with the same amount of sodium chloride added, the inhibition of this drilling fluid system is better, which can effectively prevent the hydration and dispersion of mud shale.

[0056] Example 16 This example provides a water-based drilling fluid. Compared with Example 3, the dosage of the nano-crosslinked plugging agent NSi550 was changed to 3% (12 g), and the other components and the preparation process were the same.

[0057] Example 17 This example provides a water-based drilling fluid. Compared with Example 3, the dosage of the nano-crosslinked plugging agent NSi550 was changed to 4% (16 g), and the other components and the preparation process were the same.

[0058] Example 18 This example provides a water-based drilling fluid. Compared with Example 3, the dosage of the nano-crosslinked plugging agent NSi550 was changed to 5% (120 g), and the other components and the preparation process were the same.

[0059] Comparative example This example provides a water-based drilling fluid. Compared with Example 3, the nano-crosslinked plugging agent NSi550 was replaced with emulsified asphalt, and the dosages were 2% and 5% (8 g and 20 g), and the other components and the preparation process were the same, obtaining Comparative Example 3 and Comparative Example 4.

[0060] Plugging property test The drilling fluid systems prepared in Example 3 and Examples 16 to 18 and Comparative Example 3 and Comparative Example 4 were put into an aging tank, and heat-rolled and aged at 120°C for 16 h. Then, the medium-pressure filtration loss of the drilling fluid was measured. After the measurement, the mud cake was retained, the liquid in the drilling fluid cup was replaced with fresh water, and the medium-pressure filtration loss was measured again with the obtained mud cake. The water penetration amount in 30 min was recorded. The results are as Figure 2As shown

[0061] It can be seen from Figure 2 that for the drilling fluid after heat-rolling aging at 120°C for 16 h, as the dosage of the nano-crosslinked plugging agent NSi550 increases, the filtration loss and water permeability gradually decrease, indicating that the high-performance water-based drilling fluid system has strong plugging performance. Compared with the drilling fluid with the same amount of emulsified asphalt added, the plugging performance of this drilling fluid system is stronger, which can effectively reduce the formation collapse caused by the fluid entering the shale through pores and fractures.

[0062] Field implementation The water-based drilling fluid prepared in Example 3 of the present invention was implemented in Well Jingx in Wushenqi, Inner Mongolia. Well Jingx is located in Talaiwusu Gacha, Sulide Sumu, Wushenqi, Ordos City, Inner Mongolia Autonomous Region. It is a three-opening horizontal well with the target formation being the Shihezi Formation. The designed length of the horizontal section is 5000 m, the actual drilled length is 5256 m, the drilling period is 62.17 days, and the average ROP is 9.13 m / h. The performance of the drilling fluid in the horizontal section is shown in Table 3.

[0063] Table 3 Performance of the drilling fluid in the field implementation of the ultra-long horizontal section of this drilling fluid

[0064] During the drilling process, the use of this water-based drilling fluid reduced the friction during tripping, ensuring wellbore cleaning and wellbore stability. After Well Jingx was completed, the maximum pulling friction was 38 t and the maximum running friction was 25 t, and there was no downhole complication, which proved that this drilling fluid system has good lubricity and anti-collapse performance and is applicable to ultra-long horizontal section drilling.

[0065] The above examples are only illustrative of the present invention and do not constitute a limitation on the protection scope of the present invention. Any design identical or similar to the present invention falls within the protection scope of the present invention.

Claims

1. A low-viscosity viscosifier, characterized in that: It is obtained by blending and modifying xanthan gum and tannic acid, and the mass ratio of xanthan gum to tannic acid is 15 - 20:

1.

2. A water-based drilling fluid, characterized in that: It includes the following components: the low-viscosity viscosifier described in claim 1, bentonite, alkalinity regulator, natural polymer filtrate reducer, plugging and wall protection agent, potassium chloride, lubricant, metal anti-wear agent, organic salt weighting agent and water; Calculated by mass percentage based on water: the mass percentage of bentonite is 2 - 4%, the mass percentage of alkalinity regulator is 0.1 - 0.3%, the mass percentage of low-viscosity viscosifier is 0.2 - 0.5%, the mass percentage of natural polymer filtrate reducer is 0.5 - 1%, the mass percentage of plugging and wall protection agent is 2 - 5%, the mass percentage of potassium chloride is 8 - 10%, the mass percentage of lubricant is 2 - 5%, the mass percentage of metal anti-wear agent is 0.1 - 0.2%, and the mass percentage of organic salt weighting agent is 30 - 65%.

3. The water-based drilling fluid according to claim 2, wherein: The natural polymer filtrate reducer is modified plant gum NAT20 or polyanionic cellulose PAC, and the relative molecular mass range of the polyanionic cellulose PAC is 100,000 - 200,000.

4. A water-based drilling fluid according to claim 2, characterized in that: The plugging and wall protection agent is nano-silica NSi550 modified by silane coupling agent or ultra-fine calcium carbonate, and the mesh number of the ultra-fine calcium carbonate is 1250 - 3000 mesh.

5. A water-based drilling fluid according to claim 2, characterized in that: The lubricant is biofilm-forming lubricant, polymeric polyol or fatty acid ester, and the biofilm-forming lubricant is fatty acid graft-modified glycolipid lubricant HTZ; the metal anti-wear agent is organic acid metal salt GXJM.

6. The water-based drilling fluid according to claim 2, characterized in that: The organic salt weighting agent is sodium formate, potassium formate or Weigh2.

7. The preparation method of an aqueous drilling fluid according to claim 2, characterized in that: It includes the following steps: Step 1) Add the formulated amount of bentonite and alkalinity regulator to water, and stir at 10,000 - 120,000 r / min for 8 - 12 h to prepare bentonite slurry; Step 2) While stirring, sequentially add the formulated amounts of low-viscosity viscosifier, natural polymer filtrate reducer, plugging and wall protection agent, potassium chloride, lubricant, metal anti-wear agent, organic salt weighting agent to the bentonite-based slurry to obtain the base slurry; Step 3) Add the formulated amounts of potassium chloride and organic salt weighting agent to the base slurry and mix evenly to obtain it.

8. A preparation method of an aqueous drilling fluid according to claim 7, characterized in that: The preparation process of the low-viscosity viscosifier is as follows: Add water to the reaction vessel, then add xanthan gum, and stir while adding at 30 - 35 °C to form a homogeneous colloidal solution. Then, at 30 - 35 °C, add tannic acid and stir for 1 h until completely dissolved; heat up to 60 - 70 °C, dropwise add a 10% sodium hydroxide solution to the reaction vessel at a rate of 100 - 120 mL / min, stop dropping the alkali solution after adjusting the pH value of the solution to 9, and keep stirring and reacting at the reaction temperature for 1 - 1.5 h; after the reaction is completed, add ethanol to precipitate, dry for 48 h, and pulverize to obtain it; Among them, the mass ratio of water to xanthan gum is 100:4, and the addition amount of tannic acid is 5% of the mass of xanthan gum.

9. Application of a water-based drilling fluid according to claim 2 in an ultra-long horizontal section above 3000 m.

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