Multistage well wall plugging reinforced water-based drilling fluid and preparation method thereof

Abstract

The invention provides a multi-stage well wall plugging reinforced water-based drilling fluid and a preparation method thereof. Bentonite lamellas and shear strength improving agent macromolecular chain segments form a space grid structure through hydrogen bonds, electrostatic interaction and the like, efficient carrying of coal rock drilling cuttings is achieved, and the well cleaning degree is improved; the multi-size plugging material comprises nano-scale and micron-scale materials, and the materials are granular, fibrous and sheet-film-shaped, are wide in size distribution and diversified in morphology, and can be used for performing multi-stage plugging on nano-micron pore cracks of the well wall; on the basis, the asphalt plugging filtrate reducer and the polymer filtrate reducer form a compact filter cake to protect the well wall by filling and plugging micron-sized pores in the well wall and a well wall filter cake, so that the filter loss of the drilling fluid to a coal seam is reduced, and the stability of the well wall of the coal seam is promoted. And safe and efficient drilling of the deep coal and rock gas horizontal well is achieved under the multiple synergistic effects of borehole cleaning, multi-stage plugging and well wall strengthening.

Description

Technical Field

[0001] The present invention belongs to the technical field of drilling fluids, and particularly relates to a multi-stage plugging wellbore strengthening water-based drilling fluid and a preparation method thereof. Background Art

[0002] Deep coalbed methane is one of the important alternative energy sources for natural gas exploration and development in China. At present, the first domestic integrated 100-billion-cubic-meter large-scale deep coalbed methane field has been proven in the Ordos Basin. It has a deep burial position (more than 2,000 meters), a large coal seam thickness (about 10 meters), a high gas content, and well-developed cleats and fractures, bringing huge technical challenges to deep coalbed methane development projects such as drilling and fracturing. At present, "long horizontal section drilling + extreme volume fracturing transformation" is the main technology for efficient development of deep coalbed methane. However, the longer the horizontal section traverses in the coal seam, the more conducive it is to the efficient development of coalbed methane. At the same time, the coal seam has well-developed cleats, high brittleness, and a short collapse period, and wellbore instability is likely to occur during the drilling and completion process, resulting in complex accidents such as wellbore abandonment. Therefore, wellbore stability during the drilling and completion process of coalbed methane horizontal wells is a key technology for the efficient development of deep coalbed methane.

[0003] Patent CN103242810A (drilling fluid for protecting coal seams from collapse) discloses a drilling fluid for protecting coal seams from collapse, which is prepared from the following components by weight percentage: 1-2% of clay stabilizer, 0.2-0.5% of viscosifier, 0-0.8% of filtration reducer, 0.01-0.5% of degradation and desorption agent, 0.3-1.2% of pH regulator, and the balance is water. This drilling fluid can adjust the density, viscosity, and degradation time of the drilling fluid by adjusting the formula. This system has no solids and no clay, and has little damage to the coal seam. It shows good cuttings carrying capacity during drilling, can promote wellbore stability, reduce the occurrence of downhole complex accidents, meet the needs of efficient coalbed methane production, and is suitable for the completion operations of horizontal wells, multi-branch horizontal wells, and U-shaped well coal seam drilling.

[0004] Patent CN104046339A (drilling fluid for reducing formation damage and drilling fluid used in coalbed methane development drilling) discloses a drilling fluid for reducing formation damage and a drilling fluid used in coalbed methane development drilling. The drilling fluid for reducing formation damage includes: 100 unit masses of fresh water, 1.5-4.5 unit masses of filtration reducer, 1 unit mass of anti-collapse agent, 2 unit masses of polyhydric alcohol, 1.5 unit masses of inorganic sodium bentonite, 1 unit mass of fluorocarbon surfactant, and 3 unit masses of inorganic base. The invented drilling fluid can effectively reduce the surface tension of reservoir fluids, thereby generating wetting or anti-wetting, and at the same time reduce the capillary resistance of the reservoir, thereby greatly reducing the water lock effect generated during the drilling process. The invented drilling fluid is particularly suitable for use in coalbed methane development drilling.

[0005] Patent CN106467726A (A Clean Polymeric Film Drilling Fluid for Coalbed Methane and Its Preparation Method) provides a clean polymeric film drilling fluid for coalbed methane and its preparation method. By weight, it includes the following components: 80 - 100 parts of clear water, 3 - 5 parts of film-forming agent, 1 - 3 parts of film-fixing agent, 1 - 3 parts of biodegradable strong film agent, 0.5 - 1.5 parts of coalbed cleaner, and 1 - 3 parts of biodegradable emulsion filtrate reducer. The clean polymeric film drilling fluid for coalbed methane of the present invention has unique rheological properties and filtration loss and wall-building properties, can effectively control the filtrate from entering the coalbed, has a strong ability to inhibit the collapse and sloughing of the coalbed, gives full play to the anti-collapse technical advantages of the combination of various treatment agents, has excellent performance, is easy to maintain and process, has fast gel-breaking and no residue, and causes little damage to the coalbed methane reservoir.

[0006] Patent CN111117579B (An Anti-Collapse Drilling Fluid for Coal Seams and Its Preparation Method) provides an anti-collapse drilling fluid for coal seams and its preparation method for drilling in coal-bearing strata. The anti-collapse drilling fluid includes 100 parts of water, 1.0 - 6 parts of bentonite, 0.1 - 0.3 parts of sodium carbonate, 0.25 - 7.5 parts of filtrate reducer, 0.5 - 3 parts of polyamine inhibitor, 2 - 8 parts of sulfonated material, 1 - 5 parts of asphalt material, 1 - 4 parts of plugging agent, 1 - 6 parts of lubricant, and 0.5 - 5 parts of shielding temporary plugging agent. Among them, the pH value of the anti-collapse drilling fluid is 9 - 10, and the density is 1.10 - 2.00 g / cm3. The drilling fluid has the characteristics of good rheological properties, lubricity, strong inhibition and plugging ability, and a wide applicable density range, meeting the requirements of wellbore anti-collapse in formations with fracture development and low strength within 180 °C, especially suitable for anti-collapse in drilling of coal-mudstone alternating strata.

[0007] The drilling fluids in the above-mentioned prior art are developed for solving the anti-collapse technical problems encountered in some coal seam sections during the drilling of other target formations or the development of shallow coalbed methane (burial depth less than 1000 meters). During the drilling of deep coalbed gas (burial depth > 1500 meters), the horizontal section almost needs to pass through the coal seam completely (the coal seam encounter rate needs to be maintained above 90%), and the wellbore passing through the coal seam is about 1000 meters or even 2000 meters long. The requirement for the wellbore stability ability of the drilling fluid is extremely high. At this time, the drilling fluid technologies described in the above documents are difficult to meet the drilling requirements of deep coalbed gas horizontal wells. Summary of the Invention The purpose of the present invention is to provide a multi-stage plugging wellbore strengthening water-based drilling fluid to overcome the above-mentioned technical problems existing in the prior art.

[0008] Another purpose of the present invention is to provide a preparation method of the multi-stage plugging wellbore strengthening water-based drilling fluid to guide the preparation and construction of the drilling fluid at the drilling site and maximize the performance advantages of the drilling fluid.

[0009] Therefore, the technical solution provided by the present invention is as follows: A multi-stage plugging wellbore strengthening water-based drilling fluid, comprising the following components in parts by weight: 100 parts of water, 2-8 parts of bentonite, 0.1-0.3 part of a viscosity increasing agent, 0.5-2 parts of a strongly adsorbing coal-rock nano plugging agent, 0.5-3 parts of a coal-rock wellbore strengthening agent, 2-5 parts of an asphalt-based plugging and filtrate reducing agent, 1-3 parts of a lubricant, 6-15 parts of an organic salt, 0.1-0.5 part of a polymer filtrate reducing agent, and 1-6 parts of a multi-size plugging material. The bentonite is a mixture of ordinary sodium-based bentonite and salt-resistant bentonite, and the weight ratio of the two is 2:1.

[0010] The viscosity increasing agent is gelatin, guar gum or xanthan gum, and the asphalt-based plugging and filtrate reducing agent is white asphalt, emulsified asphalt or sulfonated asphalt.

[0011] The polymer filtrate reducing agent is polyanionic cellulose PAC-LV, starch, carboxymethyl cellulose or acrylamide-based copolymer NAT20.

[0012] The lubricant is graphite, extreme pressure lubricant, liquid polyalcohol or solid polyalcohol.

[0013] The organic salt is a mixture of potassium formate, sodium formate, ammonium formate and cesium formate compounded according to a weight ratio of 3:2:2:4.

[0014] The multi-size plugging material is a mixture of ultrafine calcium carbonate, attapulgite and graphite oxide sheets, with a weight ratio of 4:2:0.5-1, and the ultrafine calcium carbonate has a mesh number of 800-1200 meshes.

[0015] The coal-rock wellbore strengthening agent is composed of the following components in parts by weight: 100 parts of water, 1-4 parts of chitosan quaternary ammonium salt, 1-2.5 parts of polyvinyl alcohol, and 0.2-1.2 parts of polyethyleneimine.

[0016] A preparation method of a multi-stage plugging wellbore strengthening water-based drilling fluid, comprising the following steps: Step 1) Add water to the bentonite in the formula amount, then add sodium hydroxide to adjust the pH to 9-10, and stir well for more than 12 hours to make the bentonite hydrated and dispersed evenly to obtain a bentonite slurry; Step 2) Add the viscosity increasing agent and the polymer filtrate reducing agent in the formula amount to water while stirring, and then stir for more than 3 hours to make them fully dissolved, and prepare a solution for standby; Step 3) Under stirring conditions, add the standby solution in Step 2) to the bentonite slurry, continue stirring and sequentially add the strongly adsorbing coal-rock nano plugging agent, coal-rock wellbore strengthening agent, asphalt-based plugging and filtrate reducing agent, lubricant, multi-size plugging material and organic salt in the formula amount, and continue stirring for more than 6 hours after adding; Step 4) Measure the pH and density. If the pH is lower than 9, add sodium hydroxide to adjust the pH to 9 - 10, and then add barite to adjust the density to between 1.30 - 1.50 g / cm 3 to obtain the product.

[0017] The preparation method of the strong adsorption coal - rock nano - plugging agent described in step 3) is as follows: 1) Add 1 - 3 parts by weight of nano - silica to 100 parts by weight of ethanol under ultrasonic dispersion conditions, and then stir with a paddle at a speed of 60 r / min for 30 - 50 minutes; among them, the particle size distribution of the nano - silica is between 50 - 200 nm; 2) Take 9.5 parts by weight of N,N - dimethylformamide liquid as a solvent, add 2 - 4 parts by weight of aminopropyltriethoxysilane, 1 - 2.5 parts by weight of succinic anhydride and 0.01 - 0.02 parts by weight of catalyst 4 - dimethylaminopyridine, and stir to obtain a mixed liquid; 3) Add the mixed liquid to 1), magnetically stir at 60 °C for 12 h, and then obtain the strong adsorption coal - rock nano - plugging agent by centrifugation.

[0018] The beneficial effects of the present invention are as follows: The multi - stage plugging well - wall strengthening water - based drilling fluid provided by the present invention forms a spatial network structure through the hydrogen bond and electrostatic interaction between bentonite lamellae and the high - molecular chain segments of the viscosifier, realizing the efficient carrying of coal - rock cuttings and improving the wellbore cleaning degree; the multi - size plugging materials include nano - and micron - sized materials, and these materials are in the forms of granular, fibrous and sheet - film - like, with a wide size distribution and diverse morphologies, which can perform multi - stage plugging on the nano - and micro - sized pores and fractures of the wellbore. On this basis, the asphalt - type filtration loss reducer and polymer filtration loss reducer form a dense filter cake by filling the micron - sized pores on the wellbore wall and the wellbore filter cake to protect the wellbore wall and reduce the filtration loss of the drilling fluid to the coal seam. The less the seepage amount of the drilling fluid into the coal seam, the more stable the coal seam. At the same time, the coal - rock well - wall strengthening agent and the strong adsorption nano - plugging agent further improve the adsorption and residence ability of the plugging materials in the pores and fractures of coal - rock.

[0019] Among them, the hydroxyl groups, tertiary amine groups, and quaternary ammonium groups in the coal-rock wellbore strengthening agent form multiple adsorption sites through hydrogen bonds, electrostatic forces, and van der Waals forces, improving the adsorption and adhesion to the pores and fissures of coal-rock and the nano-micro plugging materials therein. On the other hand, the polar groups such as carboxyl groups and tertiary amine groups on the strongly adsorbed nano plugging agent improve the adsorption and residence ability of the nano plugging agent in the pores and fissures of coal-rock through hydrogen bonds and the van der Waals forces between the nano particles and the pores and fissures of coal-rock, thereby further synergistically improving the plugging effect and playing the role of stabilizing and strengthening the coal-rock wellbore; as the length of the horizontal section increases, the lubricant can form a lubricating film on the wellbore and the surface of the drill string, preventing the occurrence of drag pressure and sticking of the drill string, and improving the drilling efficiency of the horizontal section; at the same time, the potassium ions and ammonium ions in the organic salt can inhibit the hydration and dispersion of clay minerals in coal-rock and coal-rock cuttings, which is beneficial to maintaining the stability of the coal-rock structure, and the organic salt in the system can make the drilling fluid have good rheological properties, improve the degassing property of the drilling fluid, and ensure the stability of the drilling fluid performance.

[0020] The drilling fluid of the present invention has a low filtration loss (the filtration loss at high temperature and high pressure < 7 mL, and the filtration loss at low temperature and low pressure < 2 mL), excellent rheological properties, strong cuttings carrying capacity, good degassing performance, and high borehole cleaning degree in the horizontal section. Specific embodiments

[0021] The following specific embodiments illustrate the implementation manners of the present invention, and those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification.

[0022] The exemplary implementation manners of the present invention are now introduced. 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 art. The terms used in the exemplary implementation manners are not limitations to the present invention.

[0023] Unless otherwise specified, the terms (including scientific and technical terms) used herein have the ordinary meaning understood by those skilled in the art. In addition, 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 related fields, and should not be understood as idealized or overly formal meanings.

[0024] Example 1 The present invention provides a multi-stage plugging wellbore strengthening water-based drilling fluid, which includes the following components in parts by weight: 100 parts of water, 2 - 8 parts of bentonite, 0.1 - 0.3 parts of thixotropic agent, 0.5 - 2 parts of strongly adsorbed coal-rock nano plugging agent, 0.5 - 3 parts of coal-rock wellbore strengthening agent, 2 - 5 parts of asphalt-based plugging and filtration loss reducing agent, 1 - 3 parts of lubricant, 6 - 15 parts of organic salt, 0.1 - 0.5 parts of polymer filtration loss reducing agent, and 1 - 6 parts of multi-size plugging material. The bentonite is a mixture of ordinary sodium-based bentonite and salt-resistant bentonite, and the weight ratio of the two is 2:1.

[0025] The viscosifier is gelatin, guar gum or xanthan gum, and the asphalt-based plugging and filtrate loss reducer is white asphalt, emulsified asphalt or sulfonated asphalt.

[0026] The polymer filtrate loss reducer is polyanionic cellulose PAC-LV, starch, carboxymethyl cellulose or acrylamide-based copolymer NAT20.

[0027] The lubricant is graphite, extreme pressure lubricant, liquid polyalcohol or solid polyalcohol.

[0028] The organic salt is a mixture obtained by compounding potassium formate, sodium formate, ammonium formate and cesium formate according to a weight ratio of 3:2:2:4.

[0029] The multi-size plugging material is a mixture of ultrafine calcium carbonate, attapulgite and graphite oxide sheets, with a weight ratio of 4:2:0.5-1. The mesh number of the ultrafine calcium carbonate is 800-1200 meshes.

[0030] The coal-rock shaft wall strengthening agent is composed of the following components in parts by weight: 100 parts of water, 1-4 parts of chitosan quaternary ammonium salt, 1-2.5 parts of polyvinyl alcohol, and 0.2-1.2 parts of polyethyleneimine.

[0031] Example 2 On the basis of Example 1, this example provides a multi-stage plugging shaft wall strengthening water-based drilling fluid, which includes the following components in parts by weight: 100 parts of water, 8 parts of bentonite, 0.1 part of viscosifier, 0.5 part of strongly adsorbed coal-rock nano plugging agent, 3 parts of coal-rock shaft wall strengthening agent, 3 parts of asphalt-based plugging and filtrate loss reducer, 3 parts of lubricant, 15 parts of organic salt, 0.1 part of polymer filtrate loss reducer, and 1 part of multi-size plugging material. Preparation process: Step 1) Add water to the bentonite in the formula amount, and then add sodium hydroxide to adjust the pH to 9-10. Stir well for more than 12 hours to make the bentonite hydrate and disperse evenly to obtain a bentonite slurry. Step 2) Add the viscosifier and polymer filtrate loss reducer in the formula amount to water while stirring, and then stir for more than 3 hours to make them fully dissolve, and prepare a solution for standby. Step 3) Under stirring conditions, add the standby solution in Step 2 to the bentonite slurry, continue stirring and sequentially add the strongly adsorbed coal-rock nano plugging agent, coal-rock shaft wall strengthening agent, asphalt-based plugging and filtrate loss reducer, lubricant, multi-size plugging material and organic salt in the formula amount. After adding, continue stirring for more than 6 hours. Step 4) Measure the pH and density. If the pH is lower than 9, add sodium hydroxide to adjust the pH to 10, and then add barite to adjust the density to 1.40 g / cm 3 between them to obtain.

[0032] In this embodiment, the viscosifier is gelatin, the asphalt-based plugging fluid loss reducer is white asphalt, the lubricant is graphite, and the polymer fluid loss reducer is polyanionic cellulose PAC-LV.

[0033] The preparation process of the strong adsorption coal and rock nano plugging agent is as follows: (1) Add 1.5 parts by weight of nano-silica (particle size distribution between 50 - 100 nm) to 100 parts by weight of ethanol under ultrasonic dispersion conditions, and then stir with a paddle at a speed of 60 r / min for 30 minutes; (2) Take 9.5 parts by weight of N,N-dimethylformamide liquid as a solvent, add 2 parts by weight of aminopropyltriethoxysilane, 2 parts by weight of succinic anhydride, and 0.01 part by weight of catalyst 4-dimethylaminopyridine; (3) Add the mixed liquid in (2) to (1), magnetically stir at 60 °C for 12 h, and then obtain the strong adsorption coal and rock nano plugging agent QA-1 by centrifugation.

[0034] Polar groups such as carboxyl groups and tertiary amine groups on the strong adsorption coal and rock nano plugging agent can form hydrogen bonds with oxygen-containing active functional groups such as hydroxyl groups, carbonyl groups, and carboxyl groups on the surface of coal and rock. At the same time, the van der Waals forces between the nano particles and the surface of coal and rock can improve the adsorption and residence ability of the strong adsorption nano plugging agent in the pores and fractures of coal and rock, thereby further improving the plugging effect.

[0035] The preparation process of the coal and rock wellbore strengthening agent is as follows: In 100 parts by weight of water, add 2.5 parts by weight of chitosan quaternary ammonium salt (molecular weight 45000 - 70000, quaternization degree 60%) under stirring conditions, then continue to add 1.5 parts by weight of polyvinyl alcohol (molecular weight 15000 - 30000, alcoholysis degree 82%), and finally add 0.4 parts by weight of polyethyleneimine (molecular weight 500 - 65000), and continue to stir for 0.5 h to obtain the coal and rock wellbore strengthening agent MQ-1.

[0036] Chitosan quaternary ammonium salt, polyvinyl alcohol, and polyethyleneimine all have strongly adsorptive functional groups. There are a large number of hydroxyl groups on the molecular chain of polyvinyl alcohol, hydroxyl groups, tertiary amine groups, and cationic quaternary ammonium groups on chitosan quaternary ammonium salt, and a large number of tertiary amine groups on the molecular chain of polyethyleneimine. They can synergistically enhance the adsorption and adhesion of the coal and rock wellbore strengthening agent to coal and rock through interactions such as hydrogen bonds, electrostatic forces, and van der Waals forces, and improve the wellbore stability of the coal and rock section.

[0037] Multi-size plugging materials can be obtained by physical mixing. Mix 1200-mesh ultrafine calcium carbonate, attapulgite, and graphite oxide sheets in a weight ratio of 4:2:1 to obtain MF-1.

[0038] The multi-size plugging materials contain micron-sized ultrafine calcium carbonate particles, rod-shaped attapulgite fibers, and graphite oxide sheets with a nano-scale film thickness. The material sizes range from microns to nanometers, and the material morphologies include granular, fibrous, and sheet-like. The size distribution is wide, and the morphologies are diverse, enabling multi-level plugging of nano-micron pores and fractures in coal rock.

[0039] Example 3 Based on Example 1, this example provides a multi-level plugging wellbore strengthening water-based drilling fluid, which includes the following components in parts by weight: 100 parts of water, 4.5 parts of bentonite, 0.15 part of a viscosity-increasing agent, 2 parts of a strongly adsorbing coal rock nano plugging agent, 2 parts of a coal rock wellbore strengthening agent, 5 parts of an asphalt-based plugging and filtration loss reducing agent, 2 parts of a lubricant, 10 parts of an organic salt, 0.3 part of a polymer filtration loss reducing agent, and 3 parts of multi-size plugging materials. The preparation method is the same as that in Example 2. The difference is that barite is added to adjust the density to 1.50 g / cm 3 .

[0040] In this example, the viscosity-increasing agent is guar gum, the asphalt-based plugging and filtration loss reducing agent is emulsified asphalt, the lubricant is liquid polyalcohol, the polymer filtration loss reducing agent is carboxymethyl cellulose, and the multi-size plugging material is MF-1 in Example 2.

[0041] The preparation process of the strongly adsorbing coal rock nano plugging agent is as follows: (1) Add 2.5 parts by weight of nano-silica (with a particle size distribution between 150 - 200 nm) to 100 parts by weight of ethanol under ultrasonic dispersion conditions, and then stir with a paddle at a speed of 60 r / min for 50 minutes; (2) Take 9.5 parts by weight of N,N-dimethylformamide liquid as a solvent, and add 4 parts by weight of aminopropyltriethoxysilane, 1.5 parts by weight of succinic anhydride, and 0.02 part by weight of the catalyst 4-dimethylaminopyridine; (3) Add the mixed liquid in (2) to (1) and stir magnetically at 60 °C for 12 h, and then obtain the strongly adsorbing coal rock nano plugging agent QA-2 by centrifugation.

[0042] The preparation process of the coal rock wellbore strengthening agent is as follows: Under gentle stirring conditions, 3.0 parts by weight of quaternary ammonium chitosan (with a molecular weight of 45,000 - 70,000 and a quaternization degree of 70%) was slowly added to 100 parts by weight of water, and then 2.5 parts by weight of polyvinyl alcohol (with a molecular weight of 15,000 - 30,000 and a degree of alcoholysis of 87%) was continuously added. Finally, 0.6 parts by weight of polyethyleneimine (with a molecular weight of 500 - 65,000) was added, and stirring was continued for 1 h to obtain the coal-rock shaft wall strengthening agent MQ-2.

[0043] Example 4 On the basis of Example 1, this example provides a multi-stage plugging shaft wall strengthening water-based drilling fluid, which includes the following components in parts by weight: 100 parts of water, 2 parts of bentonite, 0.3 part of viscosifier, 1.5 parts of strongly adsorbed coal-rock nano plugging agent, 0.5 part of coal-rock shaft wall strengthening agent, 2 parts of asphalt-based plugging and filtrate loss reducing agent, 1 part of lubricant, 6 parts of organic salt, 0.5 part of polymer filtrate loss reducing agent, and 6 parts of multi-size plugging material. The preparation method is the same as that of Example 2. The difference is that barite is added to adjust the density to 1.30 g / cm 3 .

[0044] In this example, the viscosifier is xanthan gum, the asphalt-based plugging and filtrate loss reducing agent is sulfonated asphalt, the lubricant is solid polyalcohol, and the polymer filtrate loss reducing agent is acrylamide copolymer NAT20. The strongly adsorbed coal-rock nano plugging agent is the same as that in Example 2, and the coal-rock shaft wall strengthening agent is the same as that in Example 3. The multi-size plugging material can be obtained by physical mixing, and MF-2 is obtained by compounding and mixing 800-mesh ultrafine calcium carbonate, attapulgite, and graphite oxide sheets in a weight ratio of 4:2:0.5.

[0045] Comparative Example 1 The drilling fluid was prepared according to the method of Example 2. The difference is that DQA-1 was used instead of the strongly adsorbed coal-rock nano plugging agent QA-1 and the multi-size plugging material MF-1, that is, the addition amount of DQA-1 was 5.5 parts by weight, and QA-1 and MF-1 were not used, and the drilling fluid DM-1 was obtained.

[0046] Among them, DQA-1 was prepared according to the preparation method of the strongly adsorbed coal-rock nano plugging agent in Example 2. The difference is that the amount of N,N-dimethylformamide liquid was 4.5 parts by weight, and the amount of succinic anhydride was 0.3 part, and it was obtained after magnetic stirring at 30 °C for 12 h.

[0047] Comparative Example 2 The drilling fluid was prepared according to the method of Example 2. The difference is that the polymer mixture DMQ-1 and DMQ-2 in equal parts by weight were used instead of MQ-1, and the drilling fluids DM-2 and DM-3 were obtained respectively.

[0048] DMQ-1 was prepared according to the preparation method of the coal and rock shaft wall strengthening agent in Example 2, except that chitosan quaternary ammonium salt was not added.

[0049] DMQ-2 was prepared according to the preparation method of the coal and rock shaft wall strengthening agent in Example 3, except that polyvinyl alcohol was not added.

[0050] Comparative Example 3 The drilling fluid was prepared according to the method of Example 3, except that the coal and rock shaft wall strengthening agent MQ-2 was used instead of the strongly adsorbed coal and rock plugging agent QA-2 and the multi-size plugging material MF-1, that is, the addition amount of the coal and rock shaft wall strengthening agent MQ-2 was 9.5 parts by weight, and QA-2 and MF-1 were not used, and the drilling fluid DM-4 was obtained.

[0051] Comparative Example 4 The drilling fluid was prepared according to the method of Example 3, except that the strongly adsorbed coal and rock plugging agent QA-2 with an equal weight was used instead of the coal and rock shaft wall strengthening agent MQ-2, that is, MQ-2 was not used, and the drilling fluid DM-5 was obtained.

[0052] The performance tests of the drilling fluids prepared in Examples 2-4 and Comparative Examples 1-4 were carried out, and the experimental results are shown in Table 1.

[0053] I. 6-rotation reading value and filtration loss of the drilling fluid The 6-rotation reading value, high-temperature and high-pressure filtration loss (100 °C), and low-temperature and low-pressure filtration loss of the drilling fluid were measured according to the measurement method described in GBT 16783.1-2014 "Petroleum and natural gas industry - Field testing of drilling fluids - Part 1: Water-based drilling fluids".

[0054] II. Recovery rate The measurement process of the coal and rock rolling recovery rate of the drilling fluid is as follows: Take 20 g of coal and rock cuttings (passed through a 6-10 mesh sieve), put them into 350 mL of the above-mentioned drilling fluid respectively, then heat roll them in an aging tank at 100 °C for 16 h, pass the cuttings in the drilling fluid after heat rolling through a 40-mesh sieve, dry the cuttings remaining on the sieve and weigh their mass (m1), and the rolling recovery rate (R) can be calculated according to the following formula:

[0055] III. Coal and rock swelling amount The measurement process of the coal and rock swelling amount of the drilling fluid is as follows: First, press 5 g of coal and rock powder under 10 MPa for 5 min to obtain a coal and rock sheet, then use the Qingdao Tongchun CPZ-2 type dual-channel linear dilatometer to measure the swelling amount of the coal and rock sheet. Place the pressed coal and rock sheet in the sample cell of the instrument and add 20 mL of the above-mentioned drilling fluid to it in portions to completely immerse the coal and rock sheet, and then continuously record the swelling height of the coal and rock sheet for 24 h to obtain the coal and rock swelling amount.

[0056] Table 1 Test Results of Drilling Fluid Performance

[0057] As can be seen from Table 1, the multi-stage plugging and wellbore strengthening water-based drilling fluid provided by the present invention has a low filtration loss. At 100 °C and a differential pressure of 3.5 MPa, the filtration loss is < 7 mL, and at a differential pressure of 0.69 MPa, the filtration loss is < 2 mL. It has excellent rheological properties, strong cuttings carrying capacity, good degassing performance, and a high degree of borehole cleanliness in the horizontal section.

[0058] During the drilling construction of the horizontal section of Well Mi 172H in the deep coalbed methane of the Ordos Basin, the coalbed rock in the horizontal section maintained wellbore stability. The cuttings returned by the shale shaker were angular and uniform in size. No downhole complex situations such as coal seam collapse, caving, and pipe sticking occurred, ensuring the safe and efficient completion of the well. The total depth of the well is 4,006 meters, the drilling period is 40.79 days, and the length of the horizontal section is 1,314 meters. For the first time, a one-trip drilling in the horizontal section of coalbed methane was achieved, setting a record for the shortest drilling period of a three-opening coalbed methane horizontal well.

[0059] In summary, the drilling fluid of the present invention solves the problem of wellbore stability in drilling through hundreds or even thousands of meters of wellbore in coal seams, prevents the collapse and caving of coal seams, and realizes the safe and rapid drilling of deep coalbed methane horizontal wells.

[0060] 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 multi-stage plugging wellbore-strengthened water-based drilling fluid, characterized in that: It comprises the following components in parts by weight: 100 parts of water, 2 - 8 parts of bentonite, 0.1 - 0.3 part of a viscosity increasing agent, 0.5 - 2 parts of a strong adsorption coal rock nano plugging agent, 0.5 - 3 parts of a coal rock wellbore strengthening agent, 2 - 5 parts of an asphalt-based plugging and filtrate loss reducing agent, 1 - 3 parts of a lubricant, 6 - 15 parts of an organic salt, 0.1 - 0.5 part of a polymer filtrate loss reducing agent, and 1 - 6 parts of a multi-size plugging material.

2. The multi-stage plugging wellbore strengthening water-based drilling fluid according to claim 1, characterized in that: The bentonite is a mixture of ordinary sodium-based bentonite and salt-resistant bentonite, and the weight ratio of the two is 2:

1.

3. The water-based drilling fluid with multi-stage plugging and wellbore strengthening according to claim 1, characterized in that: The viscosity increasing agent is gelatin, guar gum or xanthan gum, and the asphalt-based plugging and filtrate loss reducing agent is white asphalt, emulsified asphalt or sulfonated asphalt.

4. A multi-stage plugging wellbore strengthened water-based drilling fluid according to claim 1, characterized in that: The polymer filtrate loss reducing agent is polyanionic cellulose PAC-LV, starch, carboxymethyl cellulose or acrylamide copolymer NAT20.

5. A multi-stage plugging wellbore strengthened water-based drilling fluid according to claim 1, characterized in that: The lubricant is graphite, extreme pressure lubricant, liquid polyalcohol or solid polyalcohol.

6. The multi-stage plugging wellbore strengthening water-based drilling fluid according to claim 1, wherein: The organic salt is a mixture obtained by compounding potassium formate, sodium formate, ammonium formate and cesium formate according to a weight ratio of 3:2:2:

4.

7. The water-based drilling fluid with multi-stage plugging and wellbore strengthening according to claim 1, characterized in that: The multi-size plugging material is a mixture of ultrafine calcium carbonate, attapulgite and graphite oxide sheets, with a weight ratio of 4:2:0.5 - 1, and the ultrafine calcium carbonate has a mesh number of 800 - 1200 meshes.

8. A multi-stage plugging wellbore strengthened water-based drilling fluid according to claim 1, characterized in that: The coal rock wellbore strengthening agent is composed of the following components in parts by weight: 100 parts of water, 1 - 4 parts of chitosan quaternary ammonium salt, 1 - 2.5 parts of polyvinyl alcohol, and 0.2 - 1.2 parts of polyethyleneimine.

9. The preparation method of a multi-stage plugging wellbore strengthened water-based drilling fluid according to claim 1, characterized in that: It includes the following steps: Step 1) Add water to the bentonite in the formula amount, then add sodium hydroxide to adjust the pH to 9 - 10, and stir well for more than 12 h to make the bentonite hydrate and disperse evenly to obtain a bentonite slurry. Step 2) Add the viscosity increasing agent and the polymer filtrate loss reducing agent in the formula amount to water while stirring, and then stir for more than 3 h to make them fully dissolve, and prepare a solution for standby. Step 3) Under stirring conditions, add the standby solution in Step 2) to the bentonite slurry, continue stirring and sequentially add the strong adsorption coal rock nano plugging agent, coal rock wellbore strengthening agent, asphalt-based plugging and filtrate loss reducing agent, lubricant, multi-size plugging material and organic salt in the formula amount, and continue stirring for more than 6 h after adding. Step 4) Measure the pH and density. If the pH is lower than 9, add sodium hydroxide to adjust the pH to 9 - 10, and then add barite to adjust the density to between 1.30 - 1.50 g / cm 3 , and that's it.

10. The preparation method of a multi-stage plugging wellbore strengthening water-based drilling fluid according to claim 9, characterized in that: The preparation method of the strong adsorption coal rock nano plugging agent in Step 3) is as follows: 1) Add 1 - 3 parts by weight of nano-silica to 100 parts by weight of ethanol under ultrasonic dispersion conditions, and then stir with a stirring paddle at a speed of 60 r / min for 30 - 50 minutes; among them, the particle size distribution of the nano-silica is between 50 - 200 nm. 2) Take 9.5 parts by weight of N,N-dimethylformamide liquid as a solvent, add 2 - 4 parts by weight of aminopropyltriethoxysilane, 1 - 2.5 parts by weight of succinic anhydride and 0.01 - 0.02 parts by weight of catalyst 4-dimethylaminopyridine, and stir to obtain a mixed liquid. 3) Add the mixed liquid to 1), stir magnetically at 60 °C for 12 h, and then obtain the strong adsorption coal rock nano plugging agent by centrifugation.

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