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

Abstract

The application provides a multi-stage plugging well wall reinforced water-based drilling fluid and a preparation method thereof. The space net structure is formed by the hydrogen bond and electrostatic interaction between the bentonite sheet layer and the high molecular chain segment of the viscosity increasing agent, efficient carrying of coal rock drilling cuttings is realized, and the wellbore cleaning degree is improved; the multi-size plugging material contains nano and micro materials, and the materials have particle, fibrous and sheet membrane shapes, the size distribution is wide, the appearance is various, the nano-micron pore cracks of the well wall can be multi-stage plugged, on the basis of which, the asphalt plugging and filtration reducing agent and the polymer filtration reducing agent fill the micron-sized pores on the well wall and the well wall filter cake, form a dense filter cake to protect the well wall, reduce the filtration loss of the drilling fluid to the coal seam, and promote the stability of the coal seam well wall. Under the multiple synergistic effects of wellbore cleaning, multi-stage plugging and well wall reinforcement, safe and efficient drilling of the deep coal rock gas horizontal well is realized.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of drilling fluid, and particularly relates to a multi-stage plugging well wall reinforced water-based drilling fluid and a preparation method thereof. BACKGROUND

[0002] Deep coal rock gas is one of important replacement energies for natural gas exploration and development in China. At present, a first whole deep coal rock gas field with a capacity of 100 billion cubic meters has been proven in Ordos Basin. The deep coal rock gas field is characterized by deep burial (more than 2000 meters), large coal seam thickness (about 10 meters), high gas content and developed cleat and fracture, which brings great technical challenges to drilling and fracturing for deep coal rock gas development engineering. At present, long horizontal section drilling + limit volume fracturing reconstruction is the main technology for efficient development of deep coal rock gas. However, the longer the horizontal section penetrates in the coal seam, the more favorable for efficient development of coal rock gas. Meanwhile, the coal seam cleat is developed, the brittleness is strong, and the collapse period is short, so the well wall is easy to be unstable in the process of drilling and completion, resulting in complex accidents such as wellbore abandonment. Therefore, the well wall stability in the process of drilling and completion of coal rock gas horizontal well is a key technology for efficient development of deep coal rock gas.

[0003] Patent CN103242810A (drilling fluid for protecting coal seam from collapse) discloses a drilling fluid for protecting coal seam from collapse, which is prepared from the following components in percentage by weight: clay stabilizer 1-2%, tackifier 0.2-0.5%, fluid loss reducer 0-0.8%, degradation desorption agent 0.01-0.5%, pH value regulator 0.3-1.2%, and the balance is water. The drilling fluid can adjust the density, viscosity and degradation time of the drilling fluid by adjusting the formula, and the system has no solid phase and no clay, and has small damage to the coal seam. The drilling fluid has good rock carrying capacity in the drilling process, can promote the well wall stability, reduce the occurrence of downhole complex accidents, meet the needs of efficient exploitation of coal seam gas, and is suitable for completion operation of coal seam drilling of horizontal well, multi-lateral horizontal well and U-shaped well.

[0004] Patent CN104046339A (drilling fluid for reducing formation damage and drilling fluid used in coal seam gas development drilling) discloses a drilling fluid for reducing formation damage and a drilling fluid used in coal seam gas development drilling. The drilling fluid for reducing formation damage comprises 100 unit masses of fresh water, 1.5-4.5 unit masses of fluid loss 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 alkali. The invented drilling fluid can effectively reduce the surface tension of reservoir liquid, thereby producing wetting or reverse wetting, and reducing the capillary resistance of the reservoir, so as to greatly reduce the water lock effect generated in the drilling process. The invented drilling fluid is especially suitable for use in coal seam gas development drilling.

[0005] Patent CN106467726A (A Clean Polyfilm Drilling Fluid for Coalbed Methane and Its Preparation Method) provides a clean polyfilm drilling fluid for coalbed methane and its preparation method. By weight, it comprises the following components: 80-100 parts water, 3-5 parts film-forming agent, 1-3 parts solidifying agent, 1-3 parts biodegradable film-strengthening agent, 0.5-1.5 parts coal seam cleaner, and 1-3 parts biodegradable emulsion filtration reduction agent. This invention provides a clean polyfilm drilling fluid for coalbed methane. The drilling fluid possesses unique rheological and filtration-building properties, effectively controlling filtrate entry into the coal seam, strongly inhibiting coal seam collapse and rockfall, fully leveraging the advantages of a combination of multiple treatment agents for anti-collapse technology, exhibiting excellent performance, ease of maintenance and treatment, rapid gel breaking without residue, and minimal 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 drilling coal-bearing formations and its preparation method. This anti-collapse drilling fluid comprises 100 parts water, 1.0–6 parts bentonite, 0.1–0.3 parts sodium carbonate, 0.25–7.5 parts filtration loss reducer, 0.5–3 parts polyamine inhibitor, 2–8 parts sulfonated material, 1–5 parts asphalt material, 1–4 parts plugging agent, 1–6 parts lubricant, and 0.5–5 parts temporary plugging agent. The pH value of the anti-collapse drilling fluid is 9–10, and its density is 1.10–2.00 g / cm³. This drilling fluid exhibits good rheological properties, lubricity, strong anti-blocking ability, and a wide applicable density range, meeting the wellbore anti-collapse requirements in formations with fractures up to 180℃ and low strength, and is particularly suitable for drilling in coal-slurry interbedded formations.

[0007] The drilling fluids described in the prior art were developed to address the challenges of preventing collapse in certain coal seam sections encountered during drilling into other target formations, or in the development of shallow coalbed methane (depth less than 1000 meters). However, in deep coalbed methane (depth > 1500 meters) drilling, the horizontal section almost entirely needs to traverse the coal seam (the coal seam penetration rate needs to be maintained above 90%), with wellbore lengths reaching 1000 meters or even 2000 meters. This places extremely high demands on the wellbore stability of the drilling fluid, making the drilling fluid technologies described in the above literature insufficient to meet the drilling requirements of horizontal wells in deep coalbed methane. Summary of the Invention

[0008] The purpose of this invention is to provide a multi-stage wellbore sealing and strengthening water-based drilling fluid to overcome the aforementioned technical problems in the prior art.

[0009] Another objective of this invention is to provide a method for preparing a multi-stage wellbore sealing and strengthening water-based drilling fluid, which guides the preparation and application of drilling fluid at the drilling site and can maximize the performance advantages of the drilling fluid.

[0010] Therefore, the technical solution provided by the present invention is as follows:

[0011] A multi-stage wellbore sealing and strengthening water-based drilling fluid comprises the following components in parts by weight: 100 parts water, 2-8 parts bentonite, 0.1-0.3 parts cutting agent, 0.5-2 parts strong adsorption coal and rock nano-plugging agent, 0.5-3 parts coal and rock wellbore strengthening agent, 2-5 parts asphalt-based plugging and filtration reduction agent, 1-3 parts lubricant, 6-15 parts organic salt, 0.1-0.5 parts polymer filtration reduction agent, and 1-6 parts multi-size plugging material.

[0012] The bentonite is a mixture of ordinary sodium-based bentonite and salt-resistant bentonite in a weight ratio of 2:1.

[0013] The cutting agent is gelatin, guar gum, or xanthan gum, and the asphalt-based plugging and filtration reduction agent is white asphalt, emulsified asphalt, or sulfonated asphalt.

[0014] The polymer filtration loss reducer is a polyanionic cellulose PAC-LV, starch, carboxymethyl cellulose, or acrylamide copolymer NAT20.

[0015] The lubricant is graphite, extreme pressure lubricant, liquid polymeric alcohol, or solid polymeric alcohol.

[0016] The organic salt is a mixture of potassium formate, sodium formate, ammonium formate and cesium formate in a weight ratio of 3:2:2:4.

[0017] The multi-size sealing material is a mixture of ultrafine calcium carbonate, attapulgite, and graphite oxide sheets in a weight ratio of 4:2:0.5-1, and the ultrafine calcium carbonate has a mesh size of 800-1200.

[0018] The coal and rock well wall strengthening agent is composed of the following components in parts by weight: 100 parts water, 1-4 parts chitosan quaternary ammonium salt, 1-2.5 parts polyvinyl alcohol, and 0.2-1.2 parts polyethyleneimine.

[0019] A method for preparing a multi-stage wellbore sealing and strengthening water-based drilling fluid includes the following steps:

[0020] Step 1) Add water to the bentonite of the specified amount, then add sodium hydroxide to adjust the pH to 9-10, and stir thoroughly for more than 12 hours to ensure that the bentonite is hydrated and dispersed evenly to obtain bentonite slurry;

[0021] Step 2) Add the prescribed amount of cutting agent and polymer filtration reducer to water while stirring, and then stir for more than 3 hours to fully dissolve them and prepare a solution for later use;

[0022] Step 3) Under stirring conditions, add the prepared solution from Step 2) to the bentonite slurry, continue stirring and add the formulated amounts of strong adsorption coal and rock nano plugging agent, coal and rock well wall strengthening agent, asphalt-based plugging and filtration reduction agent, lubricant, multi-size plugging material and organic salt in sequence. After adding all the ingredients, continue stirring for more than 6 hours.

[0023] Step 4) Measure the pH and density. If the pH is below 9, add sodium hydroxide to adjust the pH to 9-10, then add barite to adjust the density to 1.30-1.50 g / cm³. 3 In between, it is obtained.

[0024] The preparation method of the strongly adsorbent coal and rock nano-plugging agent described in step 3) is as follows:

[0025] 1) Add 1-3 parts by weight of nano-silica to 100 parts by weight of ethanol under ultrasonic dispersion, and then stir with a stirrer at 60 r / min for 30-50 minutes; wherein the nano-silica particle size distribution is between 50-200 nm.

[0026] 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;

[0027] 3) Add the mixed liquid to 1), stir magnetically at 60°C for 12 hours, and then obtain the strongly adsorbed coal and rock nano-blocking agent by centrifugation.

[0028] The beneficial effects of this invention are:

[0029] This invention provides a multi-stage wellbore reinforcement water-based drilling fluid. Through bentonite sheets and cutting agent polymer segments forming a spatial network structure via hydrogen bonding and electrostatic interactions, it achieves efficient carrying of coal and rock cuttings, improving wellbore cleanliness. The multi-sized sealing materials include nano- and micro-sized materials in granular, fibrous, and film forms, with a wide size distribution and diverse morphologies. These materials can perform multi-stage sealing of nano- and micro-sized pores and fractures in the wellbore. Furthermore, asphalt-based sealing and filtration reduction agents, along with polymer-based filtration reduction agents, fill the micro-sized pores in the wellbore and the wellbore filter cake, forming a dense filter cake that protects the wellbore and reduces the amount of drilling fluid lost to the coal seam. The less drilling fluid seeps into the coal seam, the more stable the coal seam. Simultaneously, the coal and rock wellbore reinforcement agent and the highly adsorbent nano-sealing agent further enhance the adsorption and retention capacity of the sealing materials in coal and rock pores.

[0030] Among them, the hydroxyl, tertiary amine, and quaternary ammonium groups in the coal wellbore strengthening agent form multiple adsorption sites through hydrogen bonds, electrostatic forces, and van der Waals forces, improving the adsorption and adhesion of the coal pores and the nano- and micro-sized plugging materials therein. On the other hand, the polar groups such as carboxyl and tertiary amine groups on the strongly adsorbing nano-plugging agent improve the adsorption and retention capacity of the nano-plugging agent in the coal pores through hydrogen bonds and van der Waals forces between the nanoparticles and the coal pores, thereby further synergistically improving the plugging effect and playing a role in stabilizing and strengthening the coal wellbore. As the length of the horizontal section increases, the lubricant can form a lubricating film on the wellbore and drill string surface, preventing drag and pressure and drill string sticking, and improving the drilling efficiency of the horizontal section. At the same time, the potassium and ammonium ions in the organic salt can inhibit the hydration and dispersion of clay minerals in the coal and coal drill cuttings, which is conducive to maintaining the stability of the coal structure. Moreover, the organic salt in the system can give the drilling fluid good rheological properties, improve the degassing of the drilling fluid, and ensure the stability of the drilling fluid performance.

[0031] The drilling fluid of this invention has low filtration loss (high temperature and high pressure filtration loss <7mL, low temperature and low pressure filtration loss <2mL), excellent rheological properties, strong rock-carrying capacity, good degassing performance, and high cleanliness of the horizontal section of the wellbore. Detailed Implementation

[0032] The following specific embodiments illustrate the implementation 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.

[0033] Exemplary embodiments of the present invention are now described; however, the present invention may be embodied in many different forms and is not limited to the embodiments described herein. These embodiments are provided to fully and completely disclose the invention and to fully convey its scope to those skilled in the art. The terminology used in the exemplary embodiments is not intended to limit the invention.

[0034] Unless otherwise stated, the terms used herein (including technical terms) have their common meaning as understood by one of ordinary skill in the art. Furthermore, it is understood that terms defined in commonly used dictionaries should be understood to have a meaning consistent with the context of their relevant field, and not to be interpreted as having an idealized or overly formal meaning.

[0035] Example 1

[0036] This invention provides a multi-stage wellbore-strengthening water-based drilling fluid, comprising the following components in parts by weight: 100 parts water, 2-8 parts bentonite, 0.1-0.3 parts cutting agent, 0.5-2 parts strong adsorption coal and rock nano-plugging agent, 0.5-3 parts coal and rock wellbore strengthening agent, 2-5 parts asphalt-based plugging and filtration reduction agent, 1-3 parts lubricant, 6-15 parts organic salt, 0.1-0.5 parts polymer filtration reduction agent, and 1-6 parts multi-size plugging material.

[0037] The bentonite is a mixture of ordinary sodium-based bentonite and salt-resistant bentonite in a weight ratio of 2:1.

[0038] The cutting agent is gelatin, guar gum, or xanthan gum, and the asphalt-based plugging and filtration reduction agent is white asphalt, emulsified asphalt, or sulfonated asphalt.

[0039] The polymer filtration loss reducer is a polyanionic cellulose PAC-LV, starch, carboxymethyl cellulose, or acrylamide copolymer NAT20.

[0040] The lubricant is graphite, extreme pressure lubricant, liquid polymeric alcohol, or solid polymeric alcohol.

[0041] The organic salt is a mixture of potassium formate, sodium formate, ammonium formate and cesium formate in a weight ratio of 3:2:2:4.

[0042] The multi-size sealing material is a mixture of ultrafine calcium carbonate, attapulgite, and graphite oxide sheets in a weight ratio of 4:2:0.5-1, and the ultrafine calcium carbonate has a mesh size of 800-1200.

[0043] The coal and rock well wall strengthening agent is composed of the following components in parts by weight: 100 parts water, 1-4 parts chitosan quaternary ammonium salt, 1-2.5 parts polyvinyl alcohol, and 0.2-1.2 parts polyethyleneimine.

[0044] Example 2

[0045] Based on Example 1, this example provides a multi-stage wellbore reinforcement water-based drilling fluid, comprising the following components in parts by weight: 100 parts water, 8 parts bentonite, 0.1 parts cutting agent, 0.5 parts strong adsorption coal and rock nano plugging agent, 3 parts coal and rock wellbore reinforcement agent, 3 parts asphalt-based plugging and filtration reduction agent, 3 parts lubricant, 15 parts organic salt, 0.1 parts polymer filtration reduction agent, and 1 part multi-size plugging material.

[0046] Preparation process:

[0047] Step 1) Add water to the bentonite of the specified amount, then add sodium hydroxide to adjust the pH to 9-10, and stir thoroughly for more than 12 hours to ensure that the bentonite is hydrated and dispersed evenly to obtain bentonite slurry;

[0048] Step 2) Add the prescribed amount of cutting agent and polymer filtration reducer to water while stirring, and then stir for more than 3 hours to fully dissolve them and prepare a solution for later use;

[0049] Step 3) Under stirring conditions, add the prepared solution from Step 2) to the bentonite slurry, continue stirring and add the formulated amounts of strong adsorption coal and rock nano plugging agent, coal and rock well wall strengthening agent, asphalt-based plugging and filtration reduction agent, lubricant, multi-size plugging material and organic salt in sequence. After adding all the ingredients, continue stirring for more than 6 hours.

[0050] Step 4) Measure the pH and density. If the pH is below 9, add sodium hydroxide to adjust the pH to 10, then add barite to adjust the density to 1.40 g / cm³. 3 In between, it is obtained.

[0051] In this embodiment, the cutting agent is gelatin, the asphalt-based sealing and filtration reduction agent is white asphalt, the lubricant is graphite, and the polymer filtration reduction agent is polyanionic cellulose PAC-LV.

[0052] The preparation process of the strongly adsorbent coal and rock nano-plugging agent is as follows:

[0053] (1) 1.5 parts by weight of nano silica (particle size distribution between 50-100 nm) were added to 100 parts by weight of ethanol under ultrasonic dispersion, and then stirred with a stirring paddle at a speed of 60 r / min for 30 minutes.

[0054] (2) Take 9.5 parts by weight of N,N-dimethylformamide liquid as a solvent, and add 2 parts by weight of aminopropyltriethoxysilane, 2 parts by weight of succinic anhydride and 0.01 parts by weight of catalyst 4-dimethylaminopyridine.

[0055] (3) Add the neutralized mixed liquid from (2) to (1) and stir magnetically at 60°C for 12 hours. Then, obtain the strongly adsorbed coal and rock nano-blocking agent QA-1 by centrifugation.

[0056] The polar groups such as carboxyl 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, carbonyl, and carboxyl groups on the coal and rock surface. At the same time, the van der Waals forces between the nanoparticles and the coal and rock surface can improve the adsorption and retention capacity of the strong adsorption nano-plugging agent in the pores of coal and rock, thereby further improving the plugging effect.

[0057] The preparation process of coal and rock well wall strengthening agent is as follows:

[0058] Add 2.5 parts by weight of chitosan quaternary ammonium salt (molecular weight 45,000-70,000, degree of quaternization 60%) to 100 parts by weight of water under stirring conditions, then add 1.5 parts by weight of polyvinyl alcohol (molecular weight 15,000-30,000, degree of alcoholysis 82%), and finally add 0.4 parts by weight of polyethyleneimine (molecular weight 500-65,000). Continue stirring for 0.5 hours to obtain coal and rock well wall strengthening agent MQ-1.

[0059] Chitosan quaternary ammonium salt, polyvinyl alcohol, and polyethyleneimine all possess functional groups with strong adsorption properties. Polyvinyl alcohol has a large number of hydroxyl groups on its molecular chain, chitosan quaternary ammonium salt has hydroxyl groups, tertiary amine groups, and cationic quaternary ammonium groups, and polyethyleneimine has a large number of tertiary amine groups on its molecular chain. Through interactions such as hydrogen bonding, electrostatic forces, and van der Waals forces, these substances can synergistically enhance the adsorption and adhesion of coal and rock well wall strengthening agents to coal and rock, thereby improving the well wall stability of coal and rock sections.

[0060] Multi-size sealing materials can be obtained through physical mixing. MF-1 is obtained by compounding 1200-mesh ultrafine calcium carbonate, attapulgite clay, and graphite oxide sheets in a weight ratio of 4:2:1.

[0061] Multi-size sealing materials include micron-sized ultrafine calcium carbonate particles, rod-shaped attapulgite, and nanoscale film-like graphite oxide sheets. The material size ranges from micron to nanometer, and the material morphology ranges from granular, fibrous, to film-like. The wide size distribution and diverse morphology enable multi-level sealing of nano- and micron-sized pores and fractures in coal and rock.

[0062] Example 3

[0063] Based on Example 1, this example provides a multi-stage wellbore reinforcement water-based drilling fluid, comprising the following components in parts by weight: 100 parts water, 4.5 parts bentonite, 0.15 parts shearing agent, 2 parts strong adsorption coal and rock nano plugging agent, 2 parts coal and rock wellbore reinforcement agent, 5 parts asphalt-based plugging and filtration reduction agent, 2 parts lubricant, 10 parts organic salt, 0.3 parts polymer filtration reduction agent, and 3 parts multi-size plugging material.

[0064] The preparation method is the same as in Example 2. The difference is that barite was added to adjust the density to 1.50 g / cm³. 3 .

[0065] In this embodiment, the cutting agent is guar gum, the asphalt-based plugging and filtration reduction agent is emulsified asphalt, the lubricant is liquid polymeric alcohol, the polymer filtration reduction agent is carboxymethyl cellulose, and the multi-size plugging material is MF-1 from Example 2.

[0066] The preparation process of the strongly adsorbent coal and rock nano-blocking agent is as follows:

[0067] (1) 2.5 parts by weight of nano silica (particle size distribution between 150-200 nm) were added to 100 parts by weight of ethanol under ultrasonic dispersion, and then stirred with a stirring paddle at a speed of 60 r / min for 50 minutes.

[0068] (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 parts by weight of catalyst 4-dimethylaminopyridine.

[0069] (3) Add the neutralized mixed liquid from (2) to (1) and stir magnetically at 60°C for 12 hours. Then, obtain the strongly adsorbed coal and rock nano-blocking agent QA-2 by centrifugation.

[0070] The preparation process of coal and rock well wall strengthening agent is as follows:

[0071] In 100 parts by weight of water, 3.0 parts by weight of chitosan quaternary ammonium salt (molecular weight 45,000-70,000, degree of quaternization 70%) is slowly added under gentle stirring conditions. Then, 2.5 parts by weight of polyvinyl alcohol (molecular weight 15,000-30,000, degree of alcoholysis 87%) is added. Finally, 0.6 parts by weight of polyethyleneimine (molecular weight 500-65,000) is added, and stirring is continued for 1 hour to obtain coal and rock well wall strengthening agent MQ-2.

[0072] Example 4

[0073] Based on Example 1, this example provides a multi-stage wellbore reinforcement water-based drilling fluid, comprising the following components in parts by weight: 100 parts water, 2 parts bentonite, 0.3 parts shearing agent, 1.5 parts strong adsorption coal and rock nano plugging agent, 0.5 parts coal and rock wellbore reinforcement agent, 2 parts asphalt-based plugging and filtration reduction agent, 1 part lubricant, 6 parts organic salt, 0.5 parts polymer filtration reduction agent, and 6 parts multi-size plugging material.

[0074] The preparation method is the same as in Example 2. The difference is that barite was added to adjust the density to 1.30 g / cm³. 3 .

[0075] In this embodiment, the cutting agent is xanthan gum, the asphalt-based plugging and filtration reduction agent is sulfonated asphalt, the lubricant is solid polymeric alcohol, and the polymeric filtration reduction agent is acrylamide-based copolymer NAT20. The strongly adsorbing coal and rock nano-plugging agent is the same as in Example 2, and the coal and rock well wall strengthening agent is the same as in Example 3. Multi-size plugging materials can be obtained through physical mixing. MF-2 is obtained by compounding 800-mesh ultrafine calcium carbonate, attapulgite, and graphite oxide sheets in a weight ratio of 4:2:0.5.

[0076] Comparative Example 1

[0077] Drilling fluid was prepared according to the method in Example 2, except that DQA-1 was used instead of the strong adsorption coal and rock nano plugging agent QA-1 and the multi-size plugging material MF-1. That is, the amount of DQA-1 added was 5.5 parts by weight, and QA-1 and MF-1 were not used, resulting in drilling fluid DM-1.

[0078] DQA-1 was prepared according to the preparation method of the strong adsorption coal and rock nano-blocking agent in Example 2, except that the amount of N,N-dimethylformamide liquid was 4.5 parts by weight and the amount of succinic anhydride was 0.3 parts by weight. It was obtained after magnetic stirring at 30°C for 12 hours.

[0079] Comparative Example 2

[0080] Drilling fluids were prepared according to the method of Example 2, except that equal parts by weight of polymer mixtures DMQ-1 and DMQ-2 were used instead of MQ-1 to obtain drilling fluids DM-2 and DM-3, respectively.

[0081] DMQ-1 was prepared according to the preparation method of the coal and rock well wall strengthening agent in Example 2, except that chitosan quaternary ammonium salt was not added.

[0082] DMQ-2 is prepared according to the coal and rock well wall strengthening agent preparation method in Example 3, except that polyvinyl alcohol is not added.

[0083] Comparative Example 3

[0084] Drilling fluid was prepared according to the method in Example 3, except that the coal and rock wellbore strengthening agent MQ-2 was used instead of the strong adsorption coal and rock plugging agent QA-2 and the multi-size plugging material MF-1. That is, the amount of coal and rock wellbore strengthening agent MQ-2 added was 9.5 parts by weight, and QA-2 and MF-1 were not used, resulting in drilling fluid DM-4.

[0085] Comparative Example 4

[0086] Drilling fluid was prepared according to the method in Example 3, except that an equal part by weight of strong adsorption coal and rock plugging agent QA-2 was used instead of coal and rock well wall strengthening agent MQ-2, that is, MQ-2 was not used, and drilling fluid DM-5 was obtained.

[0087] The drilling fluids prepared in Examples 2-4 and Comparative Examples 1-4 were subjected to performance tests, and the experimental results are shown in Table 1.

[0088] I. Drilling fluid 6-turn reading and filtrate loss

[0089] The drilling fluid's 6-turn reading, high-temperature high-pressure filtration loss (100℃), and low-temperature low-pressure filtration loss were measured according to the measurement methods described in GB / T 16783.1-2014 "Field Testing of Drilling Fluids for Petroleum and Natural Gas Industry - Part 1: Water-based Drilling Fluids".

[0090] II. Recovery Rate

[0091] The measurement process for the coal and rock rolling recovery rate of drilling fluid is as follows: Take 20g of coal and rock cuttings (passed through a 6-10 mesh sieve), and put them into 350mL of the above drilling fluid. Then, place them in an aging tank at 100℃ for 16h for hot rolling. Pass the rock cuttings in the hot-rolled drilling fluid through a 40-mesh sieve. Dry the rock cuttings remaining on the sieve and weigh them (m1). The rolling recovery rate (R) can be calculated according to the following formula:

[0092]

[0093] III. Coal and Rock Expansion

[0094] The process for measuring the coal and rock expansion of drilling fluid is as follows:

[0095] First, 5g of coal and rock powder was pressed at 10MPa for 5min to obtain coal and rock flakes. Then, the expansion amount of the coal and rock flakes was measured using a Qingdao Tongchun CPZ-2 dual-channel linear dilatometer. The pressed coal and rock flakes were placed in the sample tank of the instrument and 20mL of the above-mentioned drilling fluid was added to completely immerse the coal and rock flakes. The expansion height of the coal and rock flakes was then continuously recorded for 24h to obtain the coal and rock expansion amount.

[0096] Table 1 Drilling fluid performance test results

[0097]

[0098] As shown in Table 1, the multi-stage plugging and wellbore strengthening water-based drilling fluid for deep coal and rock gas provided by this invention has a low filtration loss: <7 mL at 100℃ and a pressure differential of 3.5 MPa, and <2 mL at a pressure differential of 0.69 MPa. It exhibits excellent rheological properties, strong rock-carrying capacity, good degassing performance, and high cleanliness in the horizontal wellbore section.

[0099] During the drilling of the horizontal section of the Mi172H well, a deep coal-rock gas well in the Ordos Basin, the drilling fluid of this invention ensured the stability of the coal and rock mass within the wellbore. The rock cuttings returned by the vibrating screen were sharp, uniform in size, and no complex downhole situations such as coal seam collapse, block falling, or stuck drill occurred, guaranteeing the safe and efficient completion of the well. The well was drilled to a depth of 4006 meters, with a drilling cycle of 40.79 days and a horizontal section length of 1314 meters. This marked the first time that a horizontal section of coal-rock gas had been drilled in a single run, setting a new record for the shortest drilling cycle for a three-stage coal-rock gas horizontal well.

[0100] In summary, the drilling fluid of this invention solves the problem of wellbore stability when traversing wells hundreds or even thousands of meters deep in coal seams, preventing coal seam collapse and rockfall, and enabling safe and rapid drilling of deep coal and gas horizontal wells.

[0101] The above examples are merely illustrative of the present invention and do not constitute a limitation on the scope of protection of the present invention. All designs that are the same as or similar to the present invention are within the scope of protection of the present invention.

Claims

1. A multi-stage wellbore sealing and strengthening water-based drilling fluid, characterized in that: The product comprises the following components in parts by weight: 100 parts water, 2-8 parts bentonite, 0.1-0.3 parts shearing agent, 0.5-2 parts strong adsorption coal and rock nano-plugging agent, 0.5-3 parts coal and rock well wall strengthening agent, 2-5 parts asphalt-based plugging and filtration reduction agent, 1-3 parts lubricant, 6-15 parts organic salt, 0.1-0.5 parts polymer filtration reduction agent, and 1-6 parts multi-size plugging material; The multi-size sealing material is a mixture of ultrafine calcium carbonate, attapulgite clay, and graphite oxide sheets in a weight ratio of 4:2:0.5-1, and the ultrafine calcium carbonate has a mesh size of 800-1200 mesh. The coal and rock well wall strengthening agent is composed of the following components in parts by weight: 100 parts water, 1-4 parts chitosan quaternary ammonium salt, 1-2.5 parts polyvinyl alcohol, and 0.2-1.2 parts polyethyleneimine. The preparation method of the strongly adsorbent coal and rock nano-plugging agent is as follows: 1) Add 1-3 parts by weight of nano-silica to 100 parts by weight of ethanol under ultrasonic dispersion, and then stir with a stirrer at 60 r / min for 30-50 minutes; wherein the nano-silica particle size distribution 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 hours, and then obtain the strongly adsorbed coal and rock nano-blocking agent by centrifugation.

2. The multi-stage wellbore sealing and 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 in a weight ratio of 2:

1.

3. The multi-stage wellbore sealing and strengthening water-based drilling fluid according to claim 1, characterized in that: The cutting agent is gelatin, guar gum, or xanthan gum, and the asphalt-based plugging and filtration reduction agent is white asphalt, emulsified asphalt, or sulfonated asphalt.

4. The multi-stage wellbore sealing and strengthening water-based drilling fluid according to claim 1, characterized in that: The polymer filtration loss reducer is a polyanionic cellulose PAC-LV, starch, carboxymethyl cellulose, or acrylamide copolymer NAT20.

5. The multi-stage wellbore sealing and strengthening water-based drilling fluid according to claim 1, characterized in that: The lubricant is graphite, extreme pressure lubricant, liquid polymeric alcohol, or solid polymeric alcohol.

6. The multi-stage wellbore sealing and strengthening water-based drilling fluid according to claim 1, characterized in that: The organic salt is a mixture of potassium formate, sodium formate, ammonium formate and cesium formate in a weight ratio of 3:2:2:

4.

7. A method for preparing a multi-stage wellbore sealing and strengthening water-based drilling fluid according to any one of claims 1-6, characterized in that: Includes the following steps: Step 1) Add water to the bentonite of the specified amount, then add sodium hydroxide to adjust the pH to 9-10, and stir thoroughly for more than 12 hours to ensure that the bentonite is hydrated and dispersed evenly to obtain bentonite slurry; Step 2) Add the prescribed amount of cutting agent and polymer filtration reducer to water while stirring, and then stir for more than 3 hours to fully dissolve them and prepare a solution for later use; Step 3) Under stirring conditions, add the prepared solution from Step 2) to the bentonite slurry, continue stirring and add the formulated amounts of strong adsorption coal and rock nano plugging agent, coal and rock well wall strengthening agent, asphalt-based plugging and filtration reduction agent, lubricant, multi-size plugging material and organic salt in sequence. After adding all the ingredients, continue stirring for more than 6 hours. Step 4) Measure the pH and density. If the pH is below 9, add sodium hydroxide to adjust the pH to 9-10, then add barite to adjust the density to 1.30-1.50 g / cm³. 3 In between, it is obtained.

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