CN114106247B

CN114106247B - Modified starch and preparation method thereof, micro-bubble drilling fluid and application

Info

Publication number: CN114106247B
Application number: CN202111328685.9A
Authority: CN
Inventors: 朱文茜; 郑秀华
Original assignee: China University of Geosciences Beijing
Current assignee: China University of Geosciences Beijing
Priority date: 2020-11-11
Filing date: 2021-11-10
Publication date: 2022-09-23
Anticipated expiration: 2041-11-10
Also published as: CN114106247A

Abstract

The invention relates to the field of oil exploitation, and discloses modified starch, a preparation method thereof, a micro-bubble drilling fluid and application. The preparation method comprises the following steps: (1) preparing starch and water into gelatinized starch solution, preparing a first monomer, a second monomer, a third monomer and water into monomer solution, and then mixing the gelatinized starch solution, the monomer solution and a hydrophilic emulsifier to prepare water phase solution; (2) mixing oleophilic emulsifier with liquid paraffin, and emulsifying to obtain oil phase solution; adding the water phase solution into the oil phase solution to prepare a water-in-oil type inverse emulsion system; (3) in the presence of an initiator, carrying out graft copolymerization on the water-in-oil type inverse emulsion system to obtain a reaction product system; (4) and performing demulsification, cleaning and subsequent treatment on the reaction product system to obtain the modified starch. The modified starch has excellent high-temperature foam stabilizing performance, can resist the temperature of 180 ℃, and has better salt and calcium resistance.

Description

Modified starch and preparation method thereof, micro-bubble drilling fluid and application

Technical Field

The invention relates to the field of oil exploitation, and in particular relates to modified starch, a preparation method thereof, a micro-bubble drilling fluid and application.

Background

The drilling fluid is a circulating fluid which plays a role in carrying rock debris, stabilizing a well wall, balancing formation pressure, cooling and lubricating a drilling tool and the like in drilling engineering of oil gas, coal beds, geothermal heat and the like, and well leakage is a major drilling problem which needs to be solved urgently when drilling strata with complex geological conditions such as low-pressure exhausted oil and gas reservoirs, pore/fracture development strata, broken or unconsolidated sandstone reservoirs and the like. In recent years, a near-equilibrium drilling technology using a recyclable microbubble drilling fluid can realize negative pressure drilling in a low-pressure stratum and effectively solve the problems of serious well leakage and differential pressure sticking. The advantages of this technique include: (1) compared with the conventional water-based/oil-based drilling fluid, the mechanical drilling speed is improved, the drilling period is shortened, and the cost is reduced; (2) the micro-bubble drilling fluid has extremely high low shear viscosity, is beneficial to carrying rock debris from the bottom of a well and cleaning the well; (3) the micro-bubble drilling fluid is ejected at a high speed through a drill nozzle to generate micro-bubbles, and compared with other gas type under-balanced drilling fluids, expensive gas injection equipment is not needed; (4) the micro-bubbles are high-stability foams with a multilayer film structure, can realize the plugging of formation pores/fractures, reduce the fluid invasion and are beneficial to reservoir protection.

For micro-bubble drilling fluids, the existing research and existing problems are mainly as follows: (1) the high temperature resistance research of the microbubble-free drilling fluid system is as follows: as conventional reservoir reserves decrease and drilling techniques advance, drilling is increasingly moving to deeper reservoirs with increasing bottom hole temperatures. At present, the temperature resistance of the micro-bubble drilling fluid is researched at 100 ℃ in foreign countries, the domestic research temperature is within 150 ℃, and the temperature does not reach the bottom temperature of a high-temperature well, so that the research of a micro-bubble drilling fluid system with the high temperature resistance of more than or equal to 150 ℃ is still a challenge in order to promote the application of the micro-bubble drilling fluid technology in the development of high-temperature reservoirs; (2) the existing foam stabilizer has poor high temperature resistance: the micro-bubble drilling fluid is a gas drilling fluid containing micro-bubbles with the bubble stabilizing diameter of 10-200 mu m, which is prepared by a foaming agent and a bubble stabilizing agent through high-speed stirring, wherein xanthan gum has been proved to be the best bubble stabilizing agent in the market at present, but the main defects of the natural biological polymer are poor temperature resistance and the using temperature is usually within 120 ℃. Has already been preparedResearches prove that the main reason for the sudden increase of the filtration loss of the bentonite-based microbubble drilling fluid system at the temperature of more than 120 ℃ is the fluid viscosity reduction and microbubble instability caused by the high-temperature failure of a foam stabilizer in the system; (3) when a drilling tool encounters a salt-gypsum layer containing a large amount of metal cations, the drilling fluid is subjected to NaCl and CaCl ₂ For the typical cationic pollution, which has adverse effects on the microbubble stability and the rock-carrying capacity of the drilling fluid, most of the existing natural bio-based polymers (such as xanthan gum, cellulose and the like) cannot reach a saturated salt solution (36%), and the high-temperature calcium ion resistant concentration is even lower than 10%. Therefore, the improvement of the high temperature resistance and the salt and calcium resistance of the foam stabilizer is an important direction for developing high temperature resistant micro-bubble drilling fluid.

Compared with xanthan gum, starch is a biopolymer which has rich source, huge yield, greenness, no toxicity and easy biodegradation, has lower price, and can effectively reduce the cost when being used in a large amount in a drilling site. However, starch also has the problem of poor temperature resistance, and the property of starch needs to be improved by modification. At present, water solution polymerization is mostly adopted for starch graft modification research, the problems of gelatinization, low product conversion rate, low molecular weight and the like in the synthesis process exist, the performance and the use effect of the product are reduced, the method is limited in the application field, and the development of a modified starch product of a high-temperature resistant foam stabilizer is not related.

The existing natural biopolymer foam stabilizer generally has the problem of poor high temperature resistance, the optimization and compounding of the conventional foam stabilizer cannot meet the requirement of the increasing drilling temperature, so that the research on the temperature resistance of the micro-bubble drilling fluid based on the foam stabilizer is still limited to be within 150 ℃, and the development of the micro-bubble drilling fluid system capable of resisting the temperature of more than 150 ℃ needs to be broken through as the drilling progresses to a deep reservoir stratum.

In conclusion, the development of the modified starch high-temperature resistant foam stabilizer for meeting the application of the micro-bubble drilling fluid in the development of a high-temperature reservoir stratum has important significance.

Disclosure of Invention

The invention aims to solve the problems of insufficient temperature resistance, unsatisfactory salt and calcium resistance and high use cost of a foam stabilizer for micro-bubble drilling fluid in the prior art, and provides modified starch, a preparation method thereof, micro-bubble drilling fluid and application thereof.

In order to achieve the above object, a first aspect of the present invention provides a method for preparing a modified starch, comprising:

(1) preparing starch and water into gelatinized starch solution, preparing a first monomer, a second monomer, a third monomer and water into monomer solution, and then mixing the gelatinized starch solution, the monomer solution and a hydrophilic emulsifier to prepare water phase solution;

(2) mixing the oleophilic emulsifier with liquid paraffin, and emulsifying to obtain an oil phase solution; adding the water phase solution into the oil phase solution to prepare a water-in-oil type inverse emulsion system;

(3) in the presence of an initiator, carrying out graft copolymerization on the water-in-oil type inverse emulsion system to obtain a reaction product system;

(4) and performing demulsification, cleaning and subsequent treatment on the reaction product system to obtain the modified starch.

A second aspect of the present invention provides a modified starch obtained by the method of the first aspect, wherein the modified starch contains a graft backbone, an amide group, a sulfonic acid group, a five-membered heterocyclic group and/or a phenyl group, and the graft backbone: amide group: sulfonic acid group: the molar ratio of (five-membered heterocyclic group and/or phenyl) is (1-10): (1-30): (1-20): (1-10).

In a third aspect, the present invention provides the use of a modified starch according to the second aspect as a foam stabilizer in a micro-bubble drilling fluid.

The fourth aspect of the invention provides a micro-bubble drilling fluid, which comprises the following components: 5000 parts of water, 5-15 parts of sodium carbonate, 50-250 parts of clay, 10-150 parts of foaming agent and 50-200 parts of foam stabilizer; wherein the foam stabilizer is the modified starch of the second aspect.

A fifth aspect of the invention provides the use of the micro-bubble drilling fluid of the fourth aspect in drilling for oil and gas.

Through the technical scheme, the invention has the following beneficial effects:

(1) the modified starch prepared by the method provided by the invention has excellent high-temperature foam stabilizing performance, can resist the temperature of 180 ℃, and solves the problems of high-temperature viscosity reduction and foam instability of the conventional microbubble drilling fluid taking xanthan gum as a foam stabilizing agent at the temperature of more than 120 ℃; the microbubble drilling fluid prepared by adopting the modified starch as a component can keep higher apparent viscosity and higher low-shear viscosity at 180 ℃, and has stronger shear dilutability and rock-carrying capacity;

(2) the modified starch prepared by the method provided by the invention has salt resistance and calcium resistance as a foam stabilizer, and has high-concentration NaCl aqueous solution (with the concentration of 36 weight percent) or CaCl at the temperature of 150 DEG C ₂ The aqueous solution (with the concentration of 20 weight percent) still has higher foam stabilizing performance;

(3) the modified starch prepared by the method solves the problem that the conventional foam drilling fluid taking xanthan gum as a foam stabilizer has sudden increase of the filtration loss at high temperature, can control the filtration loss of the microbubble drilling fluid within the allowable range of a drilling site within 180 ℃, and realizes that the medium-pressure filtration loss of the foam drilling fluid aged for 16 hours at 180 ℃ is lower than 13 mL;

(4) the modified starch prepared by the method provided by the invention has excellent water solubility, the slurry preparation time is shorter than 1/10 of the existing foam stabilizer xanthan gum, and the method is favorable for rapid slurry preparation on a drilling site.

Drawings

FIG. 1 is a micrograph (polarizing microscope) of a microbubble drilling fluid prepared according to example 1 of the present invention at normal temperature (25 ℃);

FIG. 2 is a micrograph (polarizing microscope) of the microbubble drilling fluid prepared in example 1 of the present invention after aging at 180 ℃ for 16 hours;

FIG. 3 is a graph showing shear thinning performance test of microbubble drilling fluids prepared in examples 1, 4 and 6 of the present invention after aging at 180 ℃ for 16 h.

Detailed Description

The endpoints of the ranges and any values disclosed herein are not limited to the precise range or value, and such ranges or values should be understood to encompass values close to those ranges or values. For ranges of values, between the endpoints of each of the ranges and the individual points, and between the individual points may be combined with each other to give one or more new ranges of values, and these ranges of values should be considered as specifically disclosed herein.

The following describes in detail specific embodiments of the present invention. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.

In a first aspect, the present invention provides a method for preparing a modified starch, the method comprising:

(2) mixing oleophilic emulsifier with liquid paraffin, and emulsifying to obtain oil phase solution; adding the water phase solution into the oil phase solution to prepare a water-in-oil type inverse emulsion system;

The modified starch is obtained by adopting an inverse emulsion polymerization method, taking a nonpolar solvent as an oil phase, taking an aqueous solution of starch and a monomer as a water phase, forming a water-in-oil (W/O) inverse emulsion system by the oil phase and the water phase by means of an emulsifier, realizing the graft copolymerization of the starch and a polymerization monomer under the action of an initiator, and introducing a strong adsorption group amide group, a strong hydration group sulfonic group and a rigid cyclic group on a starch graft main chain.

According to the present invention, in step (1), the starch may be a conventional soluble starch, which is commercially available, and the present invention is not particularly limited thereto. The preparation of the gelatinized starch solution from starch and water can be carried out by adopting a conventional method in the field, and preferably, the starch can be added into the water to form a mixed solution, and the mixed solution is heated at the temperature of 60-95 ℃ for 0.5-2h for gelatinization treatment until the mixed solution is a transparent colloidal fluid, thus obtaining the gelatinized starch solution. Preferably, the concentration of the gelatinized starch solution is 20 to 80% by weight, and more preferably 30 to 60% by weight.

According to the present invention, in the step (1), in the process of preparing the first monomer, the second monomer and the third monomer into the monomer solution with water, the addition manner and the order of the above monomers are not particularly limited, and the monomers may be added separately or simultaneously, as long as the first monomer, the second monomer and the third monomer are sufficiently dissolved in water to form the monomer solution.

In the present invention, the first monomer is capable of providing a rigid cyclic group, and preferably, the first monomer is at least one selected from the group consisting of 1-vinyl-2-pyrrolidone (NVP), dimethyldiallylammonium chloride (DMDAAC), and Sulfonated Styrene (SS), and more preferably, 1-vinyl-2-pyrrolidone (NVP).

In the present invention, the second monomer is capable of providing a sulfonic acid group, and preferably, the second monomer is selected from at least one of 2-acrylamide-2-methylpropanesulfonic Acid (AMPS), sodium Vinylsulfonate (VS), and sodium p-styrenesulfonate (SSS), and more preferably, 2-acrylamide-2-methylpropanesulfonic Acid (AMPS).

In the present invention, the third monomer is capable of providing an amide group, preferably, the third monomer is selected from Acrylamide (AM) and/or N, N Dimethylacrylamide (DMAM), more preferably Acrylamide (AM).

According to the present invention, in step (1), preferably, the first monomer: a second monomer: the molar ratio of the third monomer is (1-10): 1-20): 1-30, and more preferably (1-5): 1-10): 1-15, so that the modified starch prepared has better high-temperature foam stabilizing performance, salt resistance and calcium resistance, rheological adjustment performance and fluid loss reduction performance. Preferably, the concentration of the monomer solution is 10 to 70% by weight, and more preferably 15 to 60% by weight.

According to the present invention, in step (1), preferably, after the first monomer, the second monomer and the third monomer are sufficiently dissolved in water to form a monomer solution, the pH of the monomer solution is adjusted to 7 to 11, more preferably 7 to 8, using a solution of an alkali metal hydroxide. The solution of the alkali metal hydroxide is selected in a wide range, and is preferably an aqueous solution of sodium hydroxide and/or potassium hydroxide, and the concentration of the aqueous solution can be flexibly selected according to the requirement of adjusting the pH value.

According to the invention, in the step (1), preferably, the gelatinized starch solution is mixed with the monomer solution, and then the hydrophilic emulsifier is added to continue mixing until the mixed solution is uniform and stable, so as to prepare the aqueous phase solution. Preferably, the hydrophilic emulsifier is at least one selected from Tween 20, Tween 40, Tween 60, Triton X-100 and OP-10, and is further preferably Triton X-100.

According to the present invention, in step (1), preferably, the hydrophilic emulsifier: the weight ratio of the first monomer to the second monomer to the third monomer is (0.1-20): 100. Preferably, the starch: the weight ratio of the first monomer to the second monomer to the third monomer is (5-60): 100.

According to the present invention, in the step (2), the oleophilic emulsifier is dissolved in the liquid paraffin, mixed and emulsified under high-speed shearing and stirring conditions to form an oil phase solution; and (2) adding the water phase solution prepared in the step (1) into the oil phase solution under the condition of high-speed shearing and stirring to prepare a water-in-oil type inverse emulsion system. In the above process, the aqueous phase solution is preferably added dropwise to the oil phase solution, so that a better emulsification effect can be obtained, and the formation of a water-in-oil type inverse emulsion system is facilitated.

In the present invention, preferably, the oleophilic emulsifier is at least one selected from span 65, span 80 and span 85, and further preferably span 80.

In the present invention, preferably, the oleophilic type emulsifier: the weight ratio of the first monomer to the second monomer to the third monomer is (5-50): 100.

In the present invention, preferably, the liquid paraffin: the weight ratio of the first monomer to the second monomer to the third monomer is (50-600): 100.

In the invention, the Hydrophilic Lipophilic Balance (HLB) of an emulsification system is controlled by adopting the compounding regulation of a hydrophilic emulsifier and an oleophilic emulsifier. Preferably, the HLB value of the water-in-oil invert emulsion system is from 3 to 9, more preferably from 4 to 8.

According to the invention, in the step (3), preferably, the water-in-oil inverse emulsion system obtained in the step (2) is placed in a reaction vessel, stirring is started, and an inert gas is introduced to remove oxygen from the reaction system so as to keep the activity of the initiator. When the reaction vessel is filled with inert gas and reaches the temperature required by the reaction, adding an initiator to initiate graft copolymerization reaction to obtain a reaction product system.

In the present invention, preferably, the inert gas may be selected from at least one of nitrogen, helium, neon, and argon, and is further preferably nitrogen.

In the present invention, preferably, the initiator consists of an oxidizing agent and a reducing agent, wherein the oxidizing agent is at least one selected from the group consisting of ammonium persulfate, sodium persulfate, potassium persulfate, hydrogen peroxide, sodium hypochlorite and potassium permanganate, and more preferably at least one selected from the group consisting of ammonium persulfate, potassium persulfate and sodium persulfate; the reducing agent is selected from at least one of sodium bisulfite, potassium sulfite, sodium thiosulfate, potassium thiosulfate and sodium sulfide, and is further preferably sodium bisulfite and/or potassium sulfite; the oxidant is: the mass ratio of the reducing agent is (1-10) to (1-7).

In the present invention, preferably, the initiator: the weight ratio of the first monomer to the second monomer to the third monomer is (0.1-3): 100.

In the present invention, preferably, the conditions of the graft copolymerization reaction include: the temperature is 30-90 ℃ and the time is 0.5-12 h. Further preferably, the temperature is 55-65 ℃ and the time is 1-3h, so that the prepared modified starch has better high-temperature foam stabilizing performance, salt resistance and calcium resistance, rheological regulation performance and fluid loss reduction performance.

According to the present invention, in step (4), the demulsification may employ a method conventional in the art, such as ultrasonic dispersion, centrifugal separation, electro-sedimentation, chemical demulsification, and the like. The washing is preferably with an organic solvent such as acetone, isopropanol, absolute ethanol, etc.

According to the invention, in step (4), the subsequent treatment comprises drying and pulverization carried out in this order. The drying and pulverization are not particularly limited in the present invention, and a method conventional in the art may be employed. For the drying, for example, drying, freeze drying, spray drying, etc. may be employed, and preferably, the drying conditions include: the temperature is 60-80 ℃, and the time is 48-72 h. For the pulverization, for example, mechanical pulverization, air stream pulverization, or the like can be employed.

A second aspect of the present invention provides a modified starch obtained by the method of the first aspect, wherein the modified starch contains a graft backbone, an amide group, a sulfonic acid group, a five-membered heterocyclic group and/or a phenyl group, and the graft backbone: amide group: sulfonic acid group: (five-membered heterocyclic group and/or phenyl group) in a molar ratio of (1-10): (1-30): (1-20): (1-10).

According to the invention, the modified starch is used as a foam stabilizer for the micro-bubble drilling fluid, shows excellent high-temperature foam stabilization performance, can resist the temperature of 180 ℃, has salt resistance and calcium resistance, can keep higher apparent viscosity and higher low-shear viscosity of the micro-bubble drilling fluid, and has stronger shear dilution property and rock carrying capacity at high temperature; the modified starch has good filtration loss effect under high-temperature conditions, can control the filtration loss of the micro-bubble drilling fluid within an allowable range of a drilling site at 180 ℃, and improves the temperature resistance of a micro-bubble drilling fluid system to 180 ℃; in addition, the modified starch also has excellent water solubility, and is favorable for rapid slurry preparation in a drilling site.

In a third aspect, the present invention provides the use of a modified starch according to the second aspect as a foam stabiliser in a micro-bubble drilling fluid.

The invention provides a microbubble drilling fluid, which comprises the following components: 5000 parts of water, 5-15 parts of sodium carbonate, 50-250 parts of clay, 10-150 parts of foaming agent and 50-200 parts of foam stabilizer; wherein the foam stabilizer is the modified starch of the second aspect.

According to the invention, in the components of the micro-bubble drilling fluid, the clay is selected in a wide range, preferably at least one selected from bentonite, attapulgite, sepiolite and chlorite, and further preferably bentonite; the foaming agent is also selected from a wide range, and may be at least one selected from an anionic foaming agent, a cationic foaming agent, a zwitterionic foaming agent and a nonionic foaming agent, and more preferably at least one selected from sodium fatty alcohol polyoxyethylene ether sulfate, sodium lauryl sulfate, alpha-olefin sulfonate, sodium dodecyl benzene sulfonate, CTAB, sodium dodecyl iminodiacetate, lauryl dimethyl ammonium oxide and cocamidopropyl betaine.

The microbubble drilling fluid provided by the invention can be prepared by the following steps:

(1) weighing the raw materials according to the weight parts of the components in the micro-bubble drilling fluid, adding sodium carbonate into water, and stirring for 10-30min under the condition of 300-800 rpm until the sodium carbonate is completely dissolved to obtain a sodium carbonate aqueous solution; then adding clay and continuing to stir for 30-120min until the clay is fully dispersed, and standing for 16-24h to obtain drilling fluid base slurry;

(2) adding the modified starch provided by the invention into the drilling fluid base slurry under the high-speed stirring condition of 5000 plus 10000 r/min, and stirring for 15-30min until the modified starch is fully dispersed and dissolved;

(3) and (3) adding a foaming agent into the slurry obtained in the step (2) under the high-speed stirring condition of 8000-12000 rpm, and stirring for 1-5min to obtain the micro-bubble drilling fluid.

The present invention will be described in detail below by way of examples. In the following preparations, examples and comparative examples, the molar ratio of the main groups in the prepared modified starch was determined using nuclear magnetic resonance hydrogen spectroscopy (nuclear magnetic resonance hydrogen spectrometer, Bruker model 400, germany).

In the case where no specific description is made, the materials used are those which are generally commercially available.

Preparation example 1

(1-A) dissolving soluble starch in deionized water, and heating and stirring for 0.75h at 90 ℃ by using a magnetic heating stirrer to carry out pre-gelatinization treatment to obtain a transparent colloidal gelatinized starch solution (the concentration is 30 weight percent);

(1-B) in a monomer molar ratio of 1: 2: 4, weighing grafting monomers NVP, AMPS and AM, dissolving the monomers NVP, AMPS and AM in deionized water to obtain a monomer solution (the total concentration of each monomer is 60 weight percent), and then adjusting the pH value of the monomer solution to 8 by using NaOH aqueous solution;

(1-C) mixing the gelatinized starch solution with a monomer solution, adding triton X-100, and continuously mixing to obtain an aqueous phase solution;

in the above steps, soluble starch: (NVP + AMPS + AM) in a weight ratio of 50: 100, respectively; triton X-100: (NVP + AMPS + AM) in a weight ratio of 3: 100, respectively;

(2) dissolving span 80 in liquid paraffin, stirring, mixing and emulsifying at 10000 r/min by using a high-speed shearing emulsifying machine to form an oil phase solution; dropwise adding the water-phase solution into the oil-phase solution while maintaining the stirring rate to obtain a water-in-oil type inverse emulsion system (HLB value of 5.2);

wherein, span 80: (NVP + AMPS + AM) in a weight ratio of 30: 100, respectively; liquid paraffin: (NVP + AMPS + AM) weight ratio 240: 100;

(3) placing the water-in-oil type inverse emulsion system prepared in the step (2) into a three-neck flask, stirring at the speed of 350 r/min, introducing nitrogen for 0.5h, and then adding an initiator (ammonium persulfate: sodium bisulfite with the weight of 2: 1) at the temperature of 60 ℃ to perform graft copolymerization for 2h to obtain an emulsion product system;

wherein, the initiator: (NVP + AMPS + AM) in a weight ratio of 0.12: 100, respectively;

(4) and pouring the emulsion product system into 100mL of isopropanol, stirring for demulsification, flocculating to obtain a milky solid, repeatedly cleaning with absolute ethyl alcohol for three times, drying in a drying oven at 65 ℃ for 48 hours, taking out, and crushing to obtain a milky powdery product, namely modified starch (recorded as S1).

S1, graft backbone: amide group: sulfonic acid group: the mol ratio of the five-membered heterocyclic group is 2: 4: 2: 1.

preparation example 2

(1-A) dissolving soluble starch in deionized water, and heating and stirring for 0.5h at 85 ℃ by using a magnetic heating stirrer to carry out pre-gelatinization treatment to obtain a transparent colloidal gelatinized starch solution (the concentration is 30 weight percent);

(1-B) in a monomer molar ratio of 1: 4: 6, weighing grafting monomers NVP, AMPS and AM, dissolving the monomers NVP, AMPS and AM in deionized water to obtain a monomer solution (the total concentration of all the monomers is 60 weight percent), and then adjusting the pH value of the monomer solution to 7.5 by using a NaOH aqueous solution;

(1-C) mixing the gelatinized starch solution with a monomer solution, adding triton X-100, and continuously mixing to prepare an aqueous phase solution;

in the above steps, soluble starch: (NVP + AMPS + AM) in a weight ratio of 50: 100; triton X-100: (NVP + AMPS + AM) in a weight ratio of 16: 100;

(2) dissolving span 80 in liquid paraffin, stirring, mixing and emulsifying at 10000 r/min by using a high-speed shearing emulsifying machine to form an oil phase solution; dropwise adding the water-phase solution into the oil-phase solution while maintaining the stirring rate to obtain a water-in-oil type inverse emulsion system (HLB value is 8);

wherein, span 80: (NVP + AMPS + AM) in a weight ratio of 25.5: 100, respectively; liquid paraffin: (NVP + AMPS + AM) in a weight ratio of 500: 100;

(3) placing the water-in-oil type inverse emulsion system prepared in the step (2) into a three-neck flask, stirring at the speed of 350 revolutions per minute, introducing nitrogen for 0.5h, and then adding an initiator (ammonium persulfate: sodium bisulfite with the weight of 1: 1) at the temperature of 65 ℃ to carry out graft copolymerization for 1.5h to obtain an emulsion product system;

wherein, the initiator: (NVP + AMPS + AM) in a weight ratio of 1.8: 100, respectively;

(4) and pouring the emulsion product system into 100mL of isopropanol, stirring for demulsification, flocculating to obtain a milky white solid, repeatedly washing with absolute ethyl alcohol for three times, drying in a drying oven at 70 ℃ for 48 hours, taking out, and crushing to obtain a milky white powdery product, namely modified starch (marked as S2).

S2, graft backbone: amide group: sulfonic acid group: the mole ratio of the five-membered heterocyclic group is 2: 6: 4: 1.

preparation example 3

(1-A) dissolving soluble starch in deionized water, and heating and stirring for 1h at 90 ℃ by using a magnetic heating stirrer to perform pre-gelatinization treatment to obtain a transparent colloidal gelatinized starch solution (the concentration is 30 weight percent);

(1-B) in a monomer molar ratio of 2: 3: 5, weighing grafting monomers NVP, AMPS and AM, dissolving the monomers NVP, AMPS and AM in deionized water to obtain a monomer solution (the total concentration of all the monomers is 60 weight percent), and then adjusting the pH value of the monomer solution to 7.5 by using NaOH aqueous solution;

in the above steps, soluble starch: (NVP + AMPS + AM) in a weight ratio of 50: 100; triton X-100: (NVP + AMPS + AM) in a weight ratio of 10: 100, respectively;

(2) dissolving span 80 in liquid paraffin, stirring, mixing and emulsifying at 10000 r/min by using a high-speed shearing emulsifying machine to form an oil phase solution; dropwise adding the water-phase solution into the oil-phase solution while keeping the stirring speed to obtain a water-in-oil type inverse emulsion system (the HLB value is 6);

wherein, span 80: (NVP + AMPS + AM) in a weight ratio of 46.5: 100, respectively; liquid paraffin: (NVP + AMPS + AM) in a weight ratio of 100: 100;

(3) placing the water-in-oil type inverse emulsion system prepared in the step (2) into a three-neck flask, stirring at the speed of 300 revolutions per minute and introducing nitrogen for 0.5h, and then adding an initiator (ammonium persulfate: sodium bisulfite with the weight of 7: 3) at the temperature of 55 ℃ to carry out graft copolymerization for 3h to obtain an emulsion product system;

wherein, the initiator: (NVP + AMPS + AM) in a weight ratio of 3: 100, respectively;

(4) and pouring the emulsion product system into 100mL of isopropanol, stirring for demulsification, flocculating to obtain a milky solid, repeatedly cleaning with absolute ethyl alcohol for three times, drying in a drying oven at 75 ℃ for 72 hours, taking out, and crushing to obtain a milky powdery product, namely modified starch (recorded as S3).

S3, graft backbone: amide group: sulfonic acid group: the mole ratio of the five-membered heterocyclic group is 2: 5: 3: 2.

preparation example 4

(1-A) dissolving soluble starch in deionized water, and heating and stirring for 0.5h at 90 ℃ by using a magnetic heating stirrer to perform pre-gelatinization treatment to obtain a transparent colloidal gelatinized starch solution (the concentration is 20 weight percent);

(1-B) in a monomer molar ratio of 7: 8: 15 weighing graft monomers SS, VS and DMAM, dissolving the graft monomers SS, VS and DMAM in deionized water to obtain a monomer solution (the total concentration of each monomer is 60 weight percent), and then adjusting the pH value of the monomer solution to 9 by using a NaOH aqueous solution;

(1-C) mixing the gelatinized starch solution with a monomer solution, adding OP-10, and continuously mixing to prepare an aqueous phase solution;

in the above steps, soluble starch: (SS + VS + DMAM) in a weight ratio of 33: 100, respectively; OP-10: (SS + VS + DMAM) in a weight ratio of 20: 100;

(2) dissolving span 85 in liquid paraffin, stirring, mixing and emulsifying at 10000 r/min by using a high-speed shearing emulsifying machine to form an oil phase solution; dropwise adding the water-phase solution into the oil-phase solution while keeping the stirring speed to prepare a water-in-oil type inverse emulsion system (the HLB value is 8);

wherein, span 85: (SS + VS + DMAM) in a weight ratio of 21: 100, respectively; liquid paraffin: (SS + VS + DMAM) in a weight ratio of 300: 100, respectively;

(3) placing the water-in-oil type inverse emulsion system prepared in the step (2) into a three-neck flask, stirring at the speed of 300 revolutions per minute, introducing nitrogen for 0.5h, and then adding an initiator (sodium hypochlorite: potassium thiosulfate with the weight of 2: 1) at 50 ℃ to perform graft copolymerization for 6h to obtain an emulsion product system;

wherein, the initiator: (SS + VS + DMAM) in a weight ratio of 1: 100, respectively;

(4) and pouring the emulsion product system into 100mL of isopropanol, stirring for demulsification, flocculating to obtain a milky solid, repeatedly cleaning with absolute ethyl alcohol for three times, drying in a drying oven at 60 ℃ for 48 hours, taking out, and crushing to obtain a milky powdery product, namely modified starch (recorded as S4).

S4, graft backbone: amide group: sulfonic acid group: molar ratio of phenyl groups 1.33: 15: 8: 7.

preparation example 5

(1-B) in a monomer molar ratio of 1: 2: 3, weighing grafting monomers DMDAAC, SSS and DMAM, dissolving the monomers in deionized water to obtain a monomer solution (the total concentration of all the monomers is 60 weight percent), and then adjusting the pH value of the monomer solution to 8 by using an NaOH aqueous solution;

in the above steps, the soluble starch: (DMDAAC + SSS + DMAM) in a weight ratio of 33: 100; OP-10: (DMDAAC + SSS + DMAM) in a weight ratio of 20: 100, respectively;

(2) dissolving span 85 in liquid paraffin, stirring, mixing and emulsifying at 10000 r/min by using a high-speed shearing emulsifying machine to form an oil phase solution; dropwise adding the water-phase solution into the oil-phase solution while keeping the stirring speed to prepare a water-in-oil type inverse emulsion system (the HLB value is 9);

wherein, span 85: (DMDAAC + SSS + DMAM) in a weight ratio of 15: 100, respectively; liquid paraffin: (DMDAAC + SSS + DMAM) in a weight ratio of 500:100, respectively;

(3) placing the water-in-oil type inverse emulsion system prepared in the step (2) into a three-neck flask, stirring at the speed of 300 revolutions per minute, introducing nitrogen for 0.5h, and then adding an initiator (sodium hypochlorite: potassium thiosulfate with the weight of 1: 1) at the temperature of 70 ℃ to perform graft copolymerization for 5h to obtain an emulsion product system;

wherein, the initiator: (DMDAAC + SSS + DMAM) in a weight ratio of 2: 100, respectively;

(4) and pouring the emulsion product system into 100mL of isopropanol, stirring for demulsification, flocculating to obtain a milky white solid, repeatedly washing with absolute ethyl alcohol for three times, drying in a drying oven at 70 ℃ for 48 hours, taking out, and crushing to obtain a milky white powdery product, namely modified starch (marked as S5).

S5, graft backbone: amide group: sulfonic acid group: the mole ratio of the five-membered heterocyclic group is 1.33: 3: 2: 1.

preparation example 6

(1-A) dissolving soluble starch in deionized water, and heating and stirring for 0.5h at 90 ℃ by using a magnetic heating stirrer to perform pre-gelatinization treatment to obtain a transparent colloidal gelatinized starch solution (the concentration is 30 weight percent);

(1-B) in a monomer molar ratio of 1: 1: 3 weighing graft monomers SS, VS and AM, dissolving the graft monomers SS, VS and AM in deionized water to obtain a monomer solution (the total concentration of each monomer is 60 wt%), and then adjusting the pH value of the monomer solution to 7.8 by using NaOH aqueous solution;

in the above steps, soluble starch: (SS + VS + AM) in a weight ratio of 33: 100, respectively; OP-10: (SS + VS + AM) in a weight ratio of 20: 100, respectively;

(2) dissolving span 85 in liquid paraffin, stirring, mixing and emulsifying at 10000 r/min by using a high-speed shearing emulsifying machine to form an oil phase solution; dropwise adding the water-phase solution into the oil-phase solution while maintaining the stirring rate to obtain a water-in-oil type inverse emulsion system (HLB value of 8.5);

wherein, span 85: (SS + VS + AM) in a weight ratio of 18: 100, respectively; liquid paraffin: (SS + VS + AM) in a weight ratio of 350: 100, respectively;

(3) placing the water-in-oil type inverse emulsion system prepared in the step (2) into a three-neck flask, stirring at the speed of 350 revolutions per minute, introducing nitrogen for 0.5h, and then adding an initiator (the weight of sodium hypochlorite and potassium sulfite is 3: 2) at the temperature to perform graft copolymerization for 4h to obtain an emulsion product system;

wherein, the initiator: (SS + VS + AM) in a weight ratio of 0.8: 100, respectively;

(4) and pouring the emulsion product system into 100mL of isopropanol, stirring for demulsification, flocculating to obtain a milky solid, repeatedly cleaning with absolute ethyl alcohol for three times, drying in a drying oven at 60 ℃ for 72 hours, taking out, and crushing to obtain a milky powdery product, namely modified starch (recorded as S6).

S6, graft backbone: amide group: sulfonic acid group: the molar ratio of phenyl groups is 2: 3: 1: 1.

preparation example 7

(1) Dissolving soluble starch in deionized water, and heating and stirring for 0.75h at 90 ℃ by using a magnetic heating stirrer to perform pre-gelatinization treatment to obtain a transparent colloidal gelatinized starch solution (the concentration is 30 weight percent);

(2) according to the monomer molar ratio of 1: 2: 4, weighing grafting monomers NVP, AMPS and AM, dissolving the monomers NVP, AMPS and AM in deionized water to obtain a monomer solution (the total concentration of each monomer is 60 wt%), then adjusting the pH value of the monomer solution to 8 by using a NaOH aqueous solution, and mixing the gelatinized starch solution and the monomer solution to obtain an aqueous phase solution; wherein, the soluble starch: (NVP + AMPS + AM) in a weight ratio of 50: 100, respectively;

(3) placing the aqueous phase solution prepared in the step (2) into a three-neck flask, stirring at the speed of 350 r/min, introducing nitrogen for 0.5h, and then adding an initiator (ammonium persulfate: sodium bisulfite in a weight ratio of 2: 1) at the temperature of 60 ℃ to perform graft copolymerization for 2h to obtain an aqueous solution polymerization product system; wherein, the initiator: (NVP + AMPS + AM) in a weight ratio of 0.12: 100, respectively;

(4) and taking out the colloidal aqueous solution product, drying in a drying oven at 65 ℃ for 48h, and crushing to obtain the modified starch (recorded as S7).

S7, graft backbone: amide group: sulfonic acid group: the mol ratio of the five-membered heterocyclic group is 15: 3: 2: 1.

preparation example 8

The process of preparation 1 is followed, with the difference that in step (1-B) the monomer NVP: AMPS: the molar ratio of AM is 12: 8: 35. the other conditions were the same as in preparation example 1. Modified starch (denoted as S8) was obtained.

S8, graft backbone: amide group: sulfonic acid group: the mol ratio of the five-membered heterocyclic group is 2: 12: 8: 35.

example 1

(1) Adding 12 parts by weight of sodium carbonate into 5000 parts by weight of water, stirring for 10min under the condition of 800 revolutions per minute until the sodium carbonate is completely dissolved, then adding 150 parts by weight of bentonite (purchased from Weifang Boda bentonite Co., Ltd.), continuing stirring for 60min, and standing for 16h to obtain a drilling fluid base slurry;

(2) under the high-speed stirring condition of 8000 revolutions per minute, 100 parts by weight of modified starch S1 (foam stabilizer) is added into the drilling fluid base slurry, and stirring is carried out for 20min so that S1 is fully dispersed and dissolved;

(3) and (3) under the high-speed stirring condition of 10000 r/min, adding 100 parts by weight of fatty alcohol-polyoxyethylene ether sodium sulfate (foaming agent) into the slurry obtained in the step (2), and continuing stirring for 3min to complete foaming to obtain the micro-bubble drilling fluid (marked as P1).

Fig. 1 and fig. 2 are micrographs (polarizing microscope) of the microbubble drilling fluid P1 prepared in example 1 of the present invention after aging at room temperature (25 ℃) and 180 ℃ for 16 hours, respectively. As can be seen from FIGS. 1 and 2, after aging at room temperature and 180 ℃, a great deal of microfoam with a particle size of 50-150 μm can be seen in the micrographs, and the exterior of the microfoam is provided with a thicker black film and a semitransparent hydrated film, which indicates the successful preparation of the microfoam drilling fluid.

Examples 2 to 6

The procedure of example 1 was followed except that modified starches S2 to S6 obtained in preparation examples 2 to 6 were used as foam stabilizers, respectively. The other conditions were the same as in example 1. And preparing the micro-bubble drilling fluid (respectively marked as P2-P6).

Comparative examples 1 to 3

The procedure of example 1 was followed except that modified starches S7 and S8 obtained in preparation examples 7 and 8 and commercially available xanthan gum (available from Tianjin Kogyo Co., Ltd.) were used as foam stabilizers, respectively. Other conditions were the same as in example 1 to produce a micro-bubble drilling fluid (designated D1-D3, respectively).

Test example

The modified starch S1-S8 prepared in preparation examples 1-8 and the commercially available xanthan gum (purchased from Guangfu Fine chemical research institute of Tianjin) serving as a foam stabilizer are subjected to foam stabilizing performance test and salt and calcium resistance test; rheological and fluid loss tests were performed on the foamed drilling fluids P1, P4, P6, and D1-D3 prepared in examples 1, 4, 6, and 1-3. In the following test examples, the following test examples were carried out,

the apparent viscosity (AV, mPas) is measured with a six-speed viscometer according to the method specified in GB/T29170-2012;

low shear viscosity (LSRV, cP) was measured using a Brookfield viscometer at 0.3rpm (corresponding to a shear rate of 0.11S) ^-1 ) The lower viscosity value is taken as the low shear viscosity value;

medium pressure fluid loss (API, mL) was measured using a fluid loss meter and according to the method specified in GB/T29170-2012;

the fluidity index is calculated by the method specified in GB/T29170-2012;

the six-speed viscometer is manufactured by Qingdao Haitoda special instrument, model ZNN-D6S;

brookfield viscometer is available from Brookfield corporation, USA, model number Brookfield DV-II;

the manufacturer of the fluid loss instrument is Beijing research institute of mineral exploration, model SD-6.

1. Foam stability test

Foam stability at room temperature (25 ℃): taking 100mL of clear water, respectively adding 2g of modified starch S1-S8 and commercially available xanthan gum, stirring and dispersing at 8000 rpm for 20min, then adding 2g of foaming agent fatty alcohol-polyoxyethylene ether sodium sulfate, stirring at 10000 rpm for 3min, pouring into a measuring cylinder, and recording the volume V (mL) of foam liquid and the time (namely half-life period T) for precipitating 50mL of clear water _1/2 ) The results are shown in Table 1.

And (3) aging at 180 ℃ for 16h and testing foam stabilizing performance: taking 100mL of clean water, respectively adding 2g of modified starch S1-S8 and commercially available xanthan gum, stirring and dispersing for 20min at 8000 rpm, then adding 2g of foaming agent fatty alcohol-polyoxyethylene ether sodium sulfate, stirring for 3min at 10000 rpm, pouring into an aging tank, placing the sealed aging tank into a roller furnace, rolling and aging for 16h at 180 ℃, taking out and standing, cooling to room temperature, taking out liquid, stirring for 3min at 10000 rpm, pouring into a measuring cylinder, and recording the volume V (mL) of the foam liquid and the time (namely half-life period T) for separating out 50mL of clean water _1/2 ) The results are shown in Table 1.

TABLE 1

As can be seen from Table 1, the foam stabilizing effect of S1-S6 is good, the half-life period of the foam system at room temperature is more than 72h, and the foam stabilizing effect of S1-S6 can still be achieved after the foam system is aged at the high temperature of 180 ℃ for 16h, wherein the half-life period is more than 30 min. In addition, S1-S6 as a foam stabilizer does not inhibit the foaming capability of the foaming agent, and the foaming system has higher foaming multiplying power (4-5 times) all the time after aging for 16 hours at room temperature or 180 ℃.

Compared with S1-S6, the stability of the foam system is greatly reduced after the S7, the S8 and the commercially available xanthan gum are aged at the high temperature of 180 ℃ for 16, particularly the commercially available xanthan gum has low foaming capability after being aged at the high temperature, the foam half-life period is less than 1min, and the use requirement of assisting the generation of a stable foam system at the high temperature cannot be met.

2. Salt and calcium resistance test

Salt and calcium resistance at room temperature (25 ℃): the same procedure was followed as described above for the foam stabilizing performance test at room temperature (25 ℃ C.) except that 100mL of an aqueous 36 wt% sodium chloride solution and 100mL of an aqueous 20 wt% calcium chloride solution were used instead of 100mL of clean water, and 5g of sodium dodecyliminodiacetate was used as a foaming agent. The foam concentrate volume, V (mL), and the time it takes to precipitate 50mL of clear water (i.e., half-life, T) were recorded _1/2 ) The results are shown in Table 2.

And (3) aging at 150 ℃ for 16h, and testing the salt and calcium resistance: the same procedure was followed as described for the foam stabilizing performance test by aging at 180 ℃ for 16h, except that 100mL of 36 wt% aqueous sodium chloride solution and 100mL of 20 wt% aqueous calcium chloride solution were used instead of 100mL of clean water, and 5g of sodium dodecyliminodiacetate was used as the foaming agent. The foam concentrate volume, V (mL), and the time it takes to precipitate 50mL of clear water (i.e., half-life, T) were recorded _1/2 ) The results are shown in Table 2.

TABLE 2

As can be seen from Table 2, the salt (NaCl) resistance of S1-S6 was saturated (36%), and calcium (CaCl) resistance was achieved ₂ ) The capacity can reach at least 20%. In high concentration of NaCl or CaCl ₂ In the aqueous solution, the S1-S6 is used as a foam stabilizer to obtain high-stability foam, the half-life period of the foam system at room temperature is more than 72 hours, the half-life period after aging at 150 ℃ for 16 hours can be more than 30min, and the foam system has good foaming capacity and the foaming multiplying power is 4-5 times.

The performance of foam systems prepared from S7, S8 and commercially available xanthan gums was severely affected by cationic contamination, especially CaCl containing divalent cations ₂ The foaming volume and the foam stability are greatly reduced, because S7, S8 and the commercially available xanthan gum lose the effect of stabilizing the foam in a high-concentration salt solution, and cations directly act on the foaming agent, so that the foaming capacity of the foaming agent and the stability of a foam system are obviously reduced.

3. Rheological property test and fluid loss property test of foam drilling fluid

The Apparent Viscosity (AV), the low shear viscosity (LSRV), the fluidity index and the medium-pressure Filtration Loss (FL) of the foam drilling fluids P1, P4, P6 and D1-D3 after aging for 16h at normal temperature and 180 ℃ and cooling to room temperature after aging are respectively tested _API ) The results are shown in Table 3.

The viscosity values (cP) of the foam drilling fluids P1, P4, P6 after aging for 16h at 180 ℃ were measured at different rotation speeds (corresponding to different shear rates) using a brookfield viscometer to evaluate the shear dilution performance, and the results are shown in fig. 3.

TABLE 3

Note: the hot rolling condition is 180 ℃ hot rolling for 16h

As can be seen from Table 3, the apparent viscosity values of P1, P4 and P6 after aging for 16h at room temperature and 180 ℃ are all higher than those of D1-D3 under the same conditions, and although D3 also has a higher apparent viscosity value at room temperature, the phenomenon that the apparent viscosity value suddenly drops after aging at high temperature occurs.

The low shear viscosity values of P1, P4 and P6 after aging for 16h at room temperature and 180 ℃ are respectively 108000-125000cP and 53197-85512cP, which are much higher than those of D1, D2 and D3 under the same conditions; after high-temperature aging, the viscosity values of P1, P4 and P6 are still kept high, while the viscosity values of D1, D2 and D3 are suddenly reduced, and the high-temperature performance of the drilling fluid is deteriorated.

From the above results, the high apparent viscosity exhibited by P1, P4, P6, as well as the high viscosity values at low shear rates, are more favorable for carrying debris, cleaning the wellbore, and enhancing the stability of the foam system.

Free water in the drilling fluid can invade into stratum under the action of bottom hole differential pressure, the drilling fluid system with high filtration loss can cause the complex problems under the well such as differential pressure drill sticking, borehole wall instability and the like while causing damage to a reservoir layer, and the Filtration Loss (FL) of the high-temperature aged drilling fluid is obtained according to API standard and drilling site construction experience _API ) Within 15mL is acceptable. In Table 3, D1, D2, and D3 exhibited very poor fluid loss properties after aging at 180 ℃, all of which exceeded 30mL for medium pressure fluid loss, and even as high as 67.3mL for D3, which was far outside of the reasonable range. And the filtration loss can be controlled within 13mL within 180 ℃ for P1, P4 and P6, and the standard of API specification is met. For effectively carrying rock debris, the fluidity indexes of P1, P4 and P6 are kept between 0.4 and 0.5 at normal temperature, and can still be controlled within a reasonable range of 0.5 to 0.6 after being aged for 16 hours at 180 ℃, so that a good rock carrying effect is shown.

Furthermore, as can be seen from fig. 3, as the rotational speed increases (and the shear rate also increases), the viscosity values of the micro-bubble drilling fluids P1, P4 and P6 rapidly decrease, exhibiting strong shear thinning, with low viscosity at high shear rates facilitating drilling fluid pumping.

The preferred embodiments of the present invention have been described above in detail, but the present invention is not limited thereto. Within the scope of the technical idea of the invention, many simple modifications can be made to the technical solution of the invention, including combinations of various technical features in any other suitable way, and these simple modifications and combinations should also be regarded as the disclosure of the invention, and all fall within the scope of the invention.

Claims

1. The application of the modified starch as a foam stabilizer in micro-bubble drilling fluid is characterized in that the modified starch contains a grafting main chain, an amido group, a sulfonic group, a five-membered heterocyclic group and/or a phenyl group, wherein the grafting main chain: amide group: sulfonic acid group: the molar ratio of (five-membered heterocyclic group and/or phenyl) is (1-10): 1-30): 1-20): 1-10;

the preparation method of the modified starch comprises the following steps:

2. Use according to claim 1, wherein, in step (1),

the first monomer is selected from at least one of 1-vinyl-2-pyrrolidone (NVP), dimethyldiallylammonium chloride (DMDAAC) and Sulfonated Styrene (SS);

the second monomer is selected from at least one of 2-acrylamide-2-methyl propane sulfonic Acid (AMPS), sodium Vinyl Sulfonate (VS) and sodium p-styrene sulfonate (SSS);

the third monomer is selected from Acrylamide (AM) and/or N, N Dimethylacrylamide (DMAM);

the hydrophilic emulsifier is at least one selected from Tween 20, Tween 40, Tween 60, Triton X-100 and OP-10.

3. Use according to claim 2, wherein, in step (1),

the first monomer is 1-vinyl-2-pyrrolidone (NVP);

the second monomer is 2-acrylamide-2-methylpropanesulfonic Acid (AMPS);

the third monomer is Acrylamide (AM);

the hydrophilic emulsifier is triton X-100.

4. Use according to any one of claims 1 to 3, wherein, in step (1), the first monomer: a second monomer: the molar ratio of the third monomer is (1-10) to (1-20) to (1-30);

and/or, the starch: the weight ratio of the first monomer to the second monomer to the third monomer is (5-60): 100;

and/or, the hydrophilic emulsifier: the weight ratio of the first monomer to the second monomer to the third monomer is (0.1-20): 100;

and/or, adjusting the pH value of the monomer solution to 7-11 by adopting an alkali metal hydroxide solution;

and/or the concentration of the gelatinized starch solution is 20 to 80 weight percent;

and/or the concentration of the monomer solution is 10-70 wt%.

5. Use according to claim 4, wherein, in step (1), the first monomer: a second monomer: the molar ratio of the third monomer is (1-5) to (1-10) to (1-15);

and/or, adjusting the pH value of the monomer solution to 7-8 by adopting an alkali metal hydroxide solution;

and/or the concentration of the gelatinized starch solution is 30-60 wt%;

and/or the concentration of the monomer solution is 15-60 wt%.

6. The use according to any one of claims 1, 2, 3, 5, wherein, in step (2),

the oleophilic emulsifier is at least one selected from span 65, span 80 and span 85;

and/or, the oleophilic emulsifier: the weight ratio of the first monomer to the second monomer to the third monomer is (5-50): 100;

and/or, the liquid paraffin: the weight ratio of the first monomer to the second monomer to the third monomer is (50-600): 100;

and/or the HLB value of the water-in-oil inverse emulsion system is 3-9.

7. Use according to claim 6, wherein, in step (2),

the oleophilic emulsifier is span 80;

and/or the HLB value of the water-in-oil inverse emulsion system is 4-8.

8. The use according to claim 4, wherein, in step (2),

and/or the HLB value of the water-in-oil inverse emulsion system is 3-9.

9. Use according to claim 8, wherein, in step (2),

the oleophilic emulsifier is span 80;

and/or the HLB value of the water-in-oil inverse emulsion system is 4-8.

10. The use according to any one of claims 1, 2, 3, 5, 7, 8, 9, wherein in step (3) the conditions of the graft copolymerization reaction comprise: the temperature is 30-90 ℃; the time is 0.5 to 12 hours;

and/or the initiator consists of an oxidizing agent and a reducing agent, wherein the oxidizing agent is selected from at least one of ammonium persulfate, sodium persulfate, potassium persulfate, hydrogen peroxide, sodium hypochlorite and potassium permanganate; the reducing agent is selected from at least one of sodium bisulfite, potassium sulfite, sodium thiosulfate, potassium thiosulfate and sodium sulfide; the oxidizing agent: the weight ratio of the reducing agent is (1-10) to (1-7);

and/or, the initiator: the weight ratio of the first monomer to the second monomer to the third monomer is (0.1-3): 100.

11. The use according to claim 10, wherein, in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 55-65 ℃; the time is 1-3 h;

and/or the oxidant is at least one of ammonium persulfate, potassium persulfate and sodium persulfate; the reducing agent is sodium bisulfite and/or potassium sulfite.

12. The use according to claim 4, wherein, in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 30-90 ℃; the time is 0.5 to 12 hours;

and/or the initiator consists of an oxidizing agent and a reducing agent, wherein the oxidizing agent is selected from at least one of ammonium persulfate, sodium persulfate, potassium persulfate, hydrogen peroxide, sodium hypochlorite and potassium permanganate; the reducing agent is selected from at least one of sodium bisulfite, potassium sulfite, sodium thiosulfate, potassium thiosulfate and sodium sulfide; the oxidant is: the weight ratio of the reducing agent is (1-10) to (1-7);

13. The use according to claim 12, wherein, in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 55-65 ℃; the time is 1-3 h;

14. The use according to claim 6, wherein, in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 30-90 ℃; the time is 0.5 to 12 hours;

15. The use according to claim 14, wherein, in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 55-65 ℃; the time is 1-3 h;

16. The use according to any one of claims 1 to 3, 5, 7 to 9, 11 to 15, wherein, in step (4), the subsequent treatment comprises drying and pulverization, which are carried out sequentially;

and/or, the drying conditions include: the temperature is 60-80 ℃ and the time is 48-72 h.

17. Use according to claim 4, wherein, in step (4), the subsequent treatment comprises drying and pulverization carried out in sequence;

18. Use according to claim 6, wherein, in step (4), the subsequent treatment comprises drying and pulverization carried out in sequence;

and/or, the drying conditions include: the temperature is 60-80 ℃, and the time is 48-72 h.

19. Use according to claim 10, wherein, in step (4), the subsequent treatment comprises drying and pulverization carried out in sequence;

20. The microbubble drilling fluid is characterized by comprising the following components: 5000 parts of water, 5-15 parts of sodium carbonate, 50-250 parts of clay, 10-150 parts of foaming agent and 50-200 parts of foam stabilizer; the foam stabilizer is modified starch, wherein the modified starch contains a grafting main chain, an amido group, a sulfonic group, a five-membered heterocyclic group and/or a phenyl group, and the grafting main chain is as follows: amide group: sulfonic acid group: the molar ratio of (five-membered heterocyclic group and/or phenyl) is (1-10): 1-30): 1-20): 1-10;

the preparation method of the modified starch comprises the following steps:

21. The micro-bubble drilling fluid according to claim 20, wherein, in step (1),

22. The micro-bubble drilling fluid of claim 21, wherein, in step (1),

the first monomer is 1-vinyl-2-pyrrolidone (NVP);

the second monomer is 2-acrylamide-2-methylpropanesulfonic Acid (AMPS);

the third monomer is Acrylamide (AM);

the hydrophilic emulsifier is triton X-100.

23. A micro bubble drilling fluid according to any one of claims 20 to 22 wherein in step (1) the first monomer: a second monomer: the molar ratio of the third monomer is (1-10) to (1-20) to (1-30);

and/or the concentration of the monomer solution is 10-70 wt%.

24. The micro-bubble drilling fluid of claim 23, wherein in step (1), the first monomer: a second monomer: the molar ratio of the third monomer is (1-5) to (1-10) to (1-15);

and/or the concentration of the gelatinized starch solution is 30-60 wt%;

and/or the concentration of the monomer solution is 15-60 wt%.

25. A micro bubble drilling fluid according to any one of claims 20, 21, 22 and 24, wherein in step (2),

and/or the HLB value of the water-in-oil inverse emulsion system is 3-9.

26. The micro-bubble drilling fluid of claim 25, wherein, in step (2),

the oleophilic emulsifier is span 80;

and/or the HLB value of the water-in-oil inverse emulsion system is 4-8.

27. The micro-bubble drilling fluid of claim 23, wherein, in step (2),

and/or the HLB value of the water-in-oil inverse emulsion system is 3-9.

28. The micro-bubble drilling fluid of claim 27, wherein, in step (2),

the oleophilic emulsifier is span 80;

and/or the HLB value of the water-in-oil inverse emulsion system is 4-8.

29. A micro bubble drilling fluid according to any one of claims 20, 21, 22, 24, 26, 27, 28, wherein in step (3) the conditions of the graft copolymerization reaction comprise: the temperature is 30-90 ℃; the time is 0.5 to 12 hours;

30. A micro bubble drilling fluid according to claim 29, wherein in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 55-65 ℃; the time is 1-3 h;

31. A micro bubble drilling fluid according to claim 23, wherein in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 30-90 ℃; the time is 0.5 to 12 hours;

32. A micro bubble drilling fluid according to claim 31, wherein in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 55-65 ℃; the time is 1-3 h;

33. A micro bubble drilling fluid according to claim 25, wherein in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 30-90 ℃; the time is 0.5 to 12 hours;

34. A micro bubble drilling fluid according to claim 33, wherein in step (3), the conditions of the graft copolymerization reaction comprise: the temperature is 55-65 ℃; the time is 1-3 h;

35. A micro bubble drilling fluid according to any one of claims 20 to 22, 24, 26 to 28 and 30 to 34 wherein in step (4) the subsequent treatment comprises drying and comminution in sequence;

36. The micro-bubble drilling fluid according to claim 23, wherein in step (4), the subsequent treatment comprises drying and pulverization which are carried out sequentially;

37. The micro-bubble drilling fluid according to claim 25, wherein in step (4), the subsequent treatment comprises drying and pulverization which are carried out sequentially;

38. The micro-bubble drilling fluid according to claim 29, wherein in step (4), the subsequent treatment comprises drying and pulverization, which are carried out sequentially;

39. Use of the micro-bubble drilling fluid according to any one of claims 20 to 38 in oil and gas drilling.