An annulus protection fluid and a preparation method and application thereof

Abstract

The application discloses an organic annulus protection fluid for oil and gas wells and a preparation method thereof. The annulus protection fluid comprises 0.01-3wt% of active polymer, 0.1-5wt% of oxygen scavenger, 0.1-2.5wt% of bactericide, 0.1-0.5% of pH regulator, and the rest is water. The active polymer in the annulus protection fluid is copolymerized from N-acryloyl morpholine and acrylate compounds. The annulus protection fluid has good chemical stability, can effectively delay the corrosion rate of the casing and the oil and gas pipeline, solve the problems of fouling, bacterial reproduction, under-deposit corrosion and plugging caused thereby, and reduce the pressure difference between the oil and gas pipeline and the annulus. The system is suitable for annulus protection of oil and gas wells, and is especially suitable for high-acid and high-sulfur oil and gas wells.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas extraction technology, and specifically relates to a protective fluid for annular protection in oil and gas wells. Background Technology

[0002] Annular protective fluid is a liquid that fills the annular space between the tubing and casing. It is primarily used to protect the casing, tubing, and downhole tools within the annulus, reducing reservoir pressure on the casing head or packer, lowering the pressure difference between the tubing / gas line and the annulus, and inhibiting blockages caused by corrosion, scaling, and bacterial growth in the tubing / gas line and casing, thus extending the life of injection and production wells. During production, the annular medium typically has high salinity and a high concentration of scale-forming ions, leading to severe scaling of the tubing and casing, resulting in pipeline blockage and under-scale corrosion. Simultaneously, sealing issues in the annulus often result in the presence of Cl- in the annular medium. - Corrosive factors such as CO2 and H2S cause corrosion of oil and casing lines. In addition, the annular space also harbors certain microorganisms, such as sulfate-reducing bacteria and iron bacteria, leading to deterioration of the stagnant water quality in the annulus, microbial corrosion of the oil and casing, and blockage. Therefore, the annular protection fluid must possess the main functions of corrosion inhibition, scale inhibition, and sterilization. Under necessary conditions, additives such as density and pH adjusters are also added to meet production requirements.

[0003] Patent CN201610829130.5 discloses an annular protective fluid containing halides, organic acid salts and imidazoline corrosion inhibitors, dimethyl ketoxime and acetaldehyde oxime oxygen scavengers, glycine amphoteric bactericides, and polyepoxysuccinic acid (PESA) and polyaspartic acid (PASP) polymer scale inhibitors, using water as a solvent. The introduction of halide ions into this system poses a potential pitting corrosion risk to tubing and casing, and the application process differs significantly from actual operating conditions.

[0004] Patents CN201110078493.7 and 201210326961.2 disclose several formulations of water-based annular protective liquids using organic acid salts, imidazoline quaternary ammonium salts, quaternary ammonium salt-type cationic surfactants, and isoascorbic acid alkali metal salts or their optical isomers as oxygen scavengers. In these formulations, organic acid salts are used as density adjusters, imidazoline quaternary ammonium salts are used as corrosion inhibitors, quaternary ammonium salt-type cationic surfactants are used as bactericides, and isoascorbic acid alkali metal salts or their optical isomers are used as oxygen scavengers. This system has a density adjustment function, but it has a high corrosion rate on N80 steel sheets and does not consider the influence of acidic gases on the corrosion performance of the annular protective liquid.

[0005] CN102061155 discloses a multipurpose annular protective fluid formulated with amine compounds, composite phosphates, composite quaternary ammonium salt cationic surfactants and sulfites, which is used for corrosion protection of carbon steel tubing in water injection wells. The concentration is 1000-5000 mg / L and it has good corrosion inhibition and scale inhibition effects.

[0006] CN105220157 discloses an annular protective fluid for use in offshore oil fields. Its main components are sodium metaborate, sodium bicarbonate and bactericide. The annular protective fluid has good compatibility with seawater, but its anti-corrosion and scale inhibition performance needs further investigation and improvement.

[0007] The above-mentioned annular protection fluids are all water-based systems. In practical applications, they cannot meet the corrosion and scale inhibition requirements of oil well annulus with high salinity and strong acidity. Although oil-based systems have the advantage of better corrosion inhibition compared to water-based systems, their system composition is more complex, which can easily cause environmental pollution. They are also prone to deposition and blockage in highly salinized water environments, and are more expensive, resulting in limited field applications. Therefore, water-based annular protection fluids remain the preferred choice for post-well completion annular protection due to their simple construction and lower cost. Thus, developing fully organic water-based annular protection fluids suitable for oil well conditions with coexisting CO2, O2, and H2S and complex bacteria is a new demand in annular protection fluid development. This aims to address the safety hazards caused by corrosion and scaling, ensure safe oil well production, and meet increasingly stringent environmental protection requirements. Summary of the Invention

[0008] The purpose of this invention is to develop an organic system annular protective fluid that can effectively inhibit corrosion of oil and gas pipelines and casings, solve the problems of under-deposit corrosion and annular blockage caused by scaling and bacterial growth, extend the life of injection and production wells, reduce the frequency of well workover operations, and reduce the cost of oil and gas extraction.

[0009] In a first aspect, this invention proposes an organic system annular protective liquid containing an active hydrophilic polymer, a bactericide, an oxygen scavenger, and a pH adjuster. Based on the total mass of the system, the active hydrophilic polymer comprises 0.01%–3.0%, preferably 0.5%–1.0%; the bactericide comprises 0.1%–2.5%, preferably 1.0%–2.0%; the oxygen scavenger comprises 0.1%–5%, preferably 1.0%–3.0%; the pH adjuster comprises 0.1%–0.5%, preferably 0.2%–0.3%; and the remainder is primarily water. The above proportions are calculated by mass fraction.

[0010] The oxygen scavenger is one or a mixture of two or more of isoascorbic acid alkali metal salts and dimethyl ketoxime.

[0011] The bactericide is one or more of a quaternary ammonium salt cationic surfactant or isothiazolinone and its derivatives;

[0012] The pH adjuster is an alkyl alcohol amine with a carbon number of C2 to C10, preferably one or more of methyl diethanolamine (MEDA), triethanolamine, and isopropanolamine.

[0013] The active polymer has the structural formula shown in formula (1) or formula (2):

[0014]

[0015]

[0016] In formula (1), a and b are integers greater than or equal to 1, and a / (a+b) is 70% to 96%, preferably 80% to 90%;

[0017] In formula (2), a and b are integers greater than or equal to 1, and a / (a+b) is 70% to 96%, preferably 80% to 90%;

[0018] In formula (1) and / or formula (2), R1 is a C1-C4 straight-chain alkyl or H, and R2 is a C1-C16 straight-chain alkyl, preferably a C2-C14 straight-chain alkane.

[0019] The active polymer is copolymerized from N-acryloylmorpholine and acrylate compounds. The specific method includes: adding the reactant acrylate compounds and N-acryloylmorpholine to a solvent, heating to 30-70°C and then adding an initiator, stirring the reaction to obtain a mixture, separating the precipitate and drying it to obtain the active polymer.

[0020] Preferably, the solvent is selected from dipolar aprotic solvents, such as dimethyl sulfoxide and dimethylformamide, with dimethyl sulfoxide being preferred. Such solvents are aprotic polar solvents and have the characteristics of high polarity, high boiling point, good thermal stability, and miscibility with water.

[0021] Preferably, the acrylate compound can be an alkyl-substituted or unsubstituted acrylate, with the structure shown in the figure:

[0022]

[0023] Wherein, R1 is a C1-C4 alkyl or H, such as methyl, ethyl, propyl, isopropyl, etc., and R2 is a C1-C16 alkyl, preferably a C2-C14 alkyl, and more preferably a C10-C14 straight alkyl.

[0024] Preferably, the total mass of the two reactive monomers accounts for 5% to 30% of the total mass fraction of the solvent, more preferably 10% to 25%.

[0025] Preferably, the mass ratio of the reactive monomer N-acryloylmorpholine to the acrylate compound is 70%–96%:4%–30%, more preferably 80%–90%:10%–20%.

[0026] Preferably, the initiator is one or a combination of at least two of azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, azobisisobutylamidine hydrochloride, benzoyl peroxide, and lauroyl peroxide, and more preferably azobisisobutylamidine hydrochloride.

[0027] Preferably, the initiator concentration is 0.05 to 1.0 wt% of the total monomer concentration, more preferably 0.1 to 0.5 wt%.

[0028] Preferably, the reaction temperature is 30–70°C, more preferably 50–60°C; the reaction time is 2–10 hours, more preferably 4–8 hours.

[0029] Preferably, the preparation method further includes: washing, drying, and pulverizing the reaction product with a precipitant to obtain a solid product; optionally, the precipitant is selected from methanol, ethanol, and diethyl ether; optionally, the drying temperature is 60–100°C; optionally, the drying time is 4–12 h.

[0030] Secondly, the present invention provides a method for preparing the annular protective liquid, comprising: adding an active hydrophilic polymer, an oxygen scavenger, a bactericide and a pH adjuster to water, and stirring until fully dissolved.

[0031] According to the method of the present invention, the preparation temperature is between 5°C and 85°C, and more preferably between 15°C and 30°C.

[0032] The annulus protection fluid provided by this invention can be used for annulus protection in oil and gas wells, and is especially suitable for annulus protection in oil and gas wells in acidic atmospheres.

[0033] Compared to existing annular protective fluids, the active hydrophilic polymer in this invention system simultaneously possesses corrosion and scale inhibition and pH adjustment functions. Through chemical coordination and the physical adsorption of long-chain alkyl groups on the metal surface, a hydrophobic adsorption film is formed, preventing H+ from entering the acidic medium. + By approaching the metal surface, reducing the number of active sites for electrode reactions, and altering the double-layer structure, corrosion inhibition is achieved; scale inhibition is achieved by relying on the N and O on the branched chains to form complexes with metal ions in the water, and the N on the branched chains to react with H in the solution. + The reaction produces NH + Reduce free H in the system + Concentration, inhibiting acid corrosion.

[0034] Compared with the prior art, the present invention has the following advantages: the annular protection system described in the present invention is an organic system with low concentration, high corrosion inhibition and scale inhibition efficiency, and the active polymers and alkanolamines therein can not only adjust the pH of the annular fluid, but also absorb acidic gases such as carbon dioxide and hydrogen sulfide that enter the annulus from the bottom of the well, making it suitable for high acid and high sulfur oil and gas wells.

[0035] Furthermore, the active hydrophilic polymers in this annular protective fluid form associated molecules with other components through intermolecular electrostatic and hydrogen bonding interactions, which can improve the suspension performance of the annular protective fluid. Since the annular protective fluid is placed in the annular space between the oil or gas pipe and the casing for a long period of time, improving the suspension performance of the annular protective fluid for detached and scaled substances can effectively enhance its performance. Attached Figure Description

[0036] Figure 1 The infrared spectrum of the active polymer prepared in Example 2 is shown. Detailed Implementation

[0037] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0038] Unless otherwise specified, the instruments, reagents, and materials used in the following examples are all conventional instruments, reagents, and materials that are already available in the prior art and can be purchased through legitimate commercial channels.

[0039] Unless otherwise specified, the experimental methods and detection methods involved in the embodiments are existing conventional experimental methods and detection methods.

[0040] Examples 1-4 illustrate the preparation method of the active polymer.

[0041] Example 1

[0042] 4.0 g (0.028 mol) of NAM (N-acryloylmorpholine) and 1.0 g (0.01 mol) of ethyl acrylate were weighed and dissolved in 50 g of dimethyl sulfoxide (DMSO). After stirring at room temperature and purging with nitrogen for 45 min to remove oxygen, 0.005 g of azobisisobutylamidine hydrochloride (AIBA) was added, and the polymerization reaction was initiated at 60 °C for 6 h. The reactants were precipitated with anhydrous ethanol, filtered, collected, redissolved in a small amount of water, and then precipitated again with ethanol. After three precipitation-dissolution-precipitation treatments, the resulting product was dried in a vacuum drying oven at 50 °C for 12 h to obtain the active polymer product A1.

[0043] Example 2

[0044] 4.0 g of NAM (N-acryloylmorpholine) and 1.0 g of dodecyl acrylate were weighed and dissolved in 50 g of DMSO. After stirring at room temperature and purging with nitrogen for 45 min to remove oxygen, 0.010 g of azobisisobutylamidine hydrochloride (AIBA) was added, and the polymerization reaction was initiated at 60 °C for 8 h. The reactants were precipitated with a large amount of anhydrous ethanol, filtered and collected, redissolved in a small amount of water, and then precipitated again with a large amount of ethanol. After three precipitation-dissolution-precipitation treatments, the resulting product was dried in a vacuum drying oven at 50 °C for 12 h to obtain the active polymer product A2.

[0045] Example 3

[0046] 4.0 g of NAM (N-acryloylmorpholine) and 1.0 g of tetradecyl acrylate were weighed and dissolved in 50 g of DMSO. After stirring at room temperature and purging with nitrogen for 45 min to remove oxygen, 0.010 g of azobisisobutylamidine hydrochloride (AIBA) was added, and the polymerization reaction was initiated at 60 °C for 10 h. The reactants were precipitated with a large amount of anhydrous ethanol, filtered and collected, redissolved in a small amount of water, and then precipitated again with a large amount of ethanol. After three precipitation-dissolution-precipitation treatments, the resulting product was dried in a vacuum drying oven at 50 °C for 12 h to obtain the active polymer product A3.

[0047] Example 4

[0048] 4.0 g of NAM (N-acryloylmorpholine) and 1.0 g of methyl methacrylate were weighed and dissolved in 50 g of dimethylformamide. After stirring at room temperature and purging with nitrogen for 45 min to remove oxygen, 0.010 g of azobisisobutyronitrile (AIBN) was added, and the polymerization reaction was initiated at 60 °C for 8 h. The reactants were precipitated with a large amount of anhydrous ethanol, filtered and collected, redissolved in a small amount of water, and then precipitated again with a large amount of ethanol. After three precipitation-dissolution-precipitation treatments, the resulting product was dried in a vacuum drying oven at 50 °C for 12 h to obtain the active polymer product A4.

[0049] Examples 5-8 illustrate the preparation method of the annular protective liquid of the present invention, and the components are expressed as a percentage by mass.

[0050] Example 5

[0051] At room temperature, 0.5% of active polymer A1, 3.0% of isoascorbic acid alkali metal salt, 1.0% of 1227 and 0.3% of methyldiethanolamine were dissolved in distilled water and stirred evenly to obtain annular protective solution B1.

[0052] Example 6

[0053] At room temperature, 1.0% of active polymer A2, 3.0% of dimethyl ketoxime, 1.5% of dodecyl dimethyl ammonium chloride and 0.3% of methyl diethanolamine were dissolved in distilled water and stirred until homogeneous to obtain annular protective solution B2.

[0054] Example 7

[0055] At room temperature, 1.0% of active polymer A3, 1.0% of dimethyl ketoxime, 2.0% of isothiazolinone and 0.3% of isopropanolamine were dissolved in distilled water and stirred until homogeneous to obtain annular protective solution B3.

[0056] Example 8

[0057] At room temperature, 1.0% of active polymer A4, 2.5% of dimethyl ketoxime, 1.6% of isothiazolinone and 0.2% of isopropanolamine were dissolved in distilled water and stirred until homogeneous to obtain annular protective solution B4.

[0058] Comparative Example 1

[0059] At room temperature, 1.0% of commercially available active polymer polyacrylamide-2-acrylamide-2-methylpropanesulfonic acid (purchased from Aisen (China) Co., Ltd.), 3.0% of dimethyl ketoxime, 2.0% of isothiazolinone and 0.3% of isopropanolamine were dissolved in distilled water and stirred until homogeneous to obtain annular protective solution D1.

[0060] Comparative Example 2

[0061] A commercially available imidazoline-based ring osmotic protectant D2 (whose main components are mercaptoimidazoline, alkylimidazoline quaternized derivatives, 1227 surfactant, and carbohydrazide) was selected as a reference comparative example 2.

[0062] Example 9 Corrosion Inhibition Experiment

[0063] Referring to the SYT5273 standard for evaluating the performance of corrosion inhibitors for oilfield produced water reinjection, six 1L stoppered bottles were taken. Oilfield source well water, 1% of annular protection fluids B1-B4 and D1-D2 were added to each bottle, and the volume was adjusted to 1L. Prepared N80 hanging plates (40mm × 13mm × 2mm, with a round hole drilled in the middle at 5mm from the edge at one end, and a surface area of ​​12cm²) were then attached. 2 CO2 and H2S were introduced until saturation was achieved to simulate the working conditions of annular protective fluid in acidic oil and gas wells. The plates were then placed in a constant temperature oven at 90℃ for 14 days. After removing the plates, the corrosion products on the surface of the plates were cleaned off, and the plates were dried with cold air and weighed. The corrosion rate of the steel plates was calculated according to formula (1). The results are shown in Table 1.

[0064]

[0065] Where, γ c Corrosion rate, in mm / a;

[0066] ω0 is the initial mass of the hanging plate, in g; ω1 is the mass of the hanging plate at the end of the experiment, in g.

[0067] S represents the surface area of ​​the hanging patch, in cm². 2 ;

[0068] t is the reaction time, in hours (h).

[0069] ρ is g / cm 3

[0070] Comparative experiment: CO2 and H2S were introduced into the water source well of the oilfield until saturation, without adding annular protection fluid, and the remaining steps were the same as those described above.

[0071] Table 1 Results of corrosion inhibition experiments

[0072]

[0073] Example 10: Experiment on bactericidal performance evaluation

[0074] According to SY / T5890 "Evaluation Method for Performance of Bactericides", the bactericidal effect of the annular protective solution on sulfate-reducing bacteria, iron bacteria and saprophytic bacteria was determined by the trace dilution method. The concentration of the annular protective solution was 1%. (The bacterial test bottle SRB-7 is the reading on day 7, SRB-14 is the reading on day 14, and TGB is the reading on day 7). The experimental results are shown in Table 2.

[0075] Table 2. Evaluation results of bactericidal performance

[0076]

[0077]

[0078] Example 11 Scale Inhibition Experiment

[0079] Referring to the standard SY / T5673-93 "Performance Evaluation Method of Oilfield Scale Inhibitors", prepare a Ca solution of the appropriate concentration. 2+ CO3 2- SO4 2- The calcium ion concentration changes before and after the addition of the annular protective solution were compared, and the scale inhibition rate was calculated using the following formula. The experimental results are shown in Table 3.

[0080]

[0081] Among them, c 环 This indicates the Ca content in the supernatant after adding the circulatory protective solution. 2+ concentration;

[0082] C0 represents the initial Ca content in the solution. 2+ concentration;

[0083] C 未 This indicates the Ca content in the supernatant without added annulus protective fluid. 2+ concentration

[0084] Calcium carbonate scale inhibition rate:

[0085] 1) Prepare CaCl2 solutions with a concentration of 10 g / L and Na2CO3 solutions with a concentration of 10 g / L respectively;

[0086] 2) Take 200 mL of distilled water into a 250 mL volumetric flask, accurately add 10.00 mL of the CaCl2 preparative solution prepared in step 1), add a certain amount of annular protection solution to make the concentration 1%, then add 10 mL of Na2CO3 preparative solution, dilute with distilled water to the mark, shake well, and let stand in a 90℃ oven for 14 days. After that, take it out, extract the supernatant, and test the Ca in the supernatant according to the specifications in SY / T5523. 2+ Concentration C 环 .

[0087] Comparative experiment: Without adding annular protective solution, all other steps were the same. After standing in a 90℃ oven for 14 days, the supernatant was measured to obtain Ca. 2+ Concentration C 未 .

[0088] Calcium sulfate scale inhibition rate:

[0089] (1) Prepare CaCl2 solution with a concentration of 20 g / L and Na2SO4 solution with a concentration of 20 g / L respectively.

[0090] (2) Take 200 mL of distilled water into a 250 mL volumetric flask, accurately add 25.00 mL of the CaCl2 preparative solution prepared in step 1), add annular protection solution to make its concentration 1%, then add 25 mL of Na2SO4 preparative solution, dilute with distilled water to the mark, shake well, and let it stand in a 90℃ oven for 14 days. After that, take it out, extract the supernatant, and test the Ca in the supernatant according to the provisions of SY / T5523. 2+ Concentration C 环 .

[0091] Comparative experiment: Without adding annular protective solution, all other steps were the same. After standing in a 90℃ oven for 14 days, the supernatant was measured to obtain Ca. 2+ Concentration C 未 .

[0092] Table 3 Scale inhibition test results

[0093]

[0094] In the above embodiments, during the CaCO3 scale inhibition experiment, CaCO3 without the addition of annular protective fluid... 2+ Concentration C 未 The concentration was 0.00133 g / L; in the above examples, during the CaSO4 scale inhibition experiment, the CaSO4 concentration without annular protection solution was 0.00133 g / L. 2+ Concentration C未 It is 0.3765 g / L.

[0095] Corrosion, scale inhibition, and bactericidal tests show that the annular protective fluid in this invention has a good corrosion inhibition effect under simulated working conditions, with a corrosion inhibition rate of less than 0.05 mm / a, which is far below the corresponding industry standard. The scale inhibition rate and bactericidal rate meet the industry standard. It has a good protective function for the inner wall of the casing and the outer wall of the oil and gas pipeline in the annular space under acidic environment, extends the service life, and ensures the safe production of the oilfield.

[0096] The applicant declares that the detailed process equipment and process flow of this invention are illustrated through the above embodiments, but this invention is not limited to the above detailed process equipment and process flow, that is, it does not mean that this invention must rely on the above detailed process equipment and process flow to be implemented. Those skilled in the art should understand that any improvements to this invention, equivalent substitutions of raw materials for the products of this invention, additions of auxiliary components, and selection of specific methods, all fall within the protection scope and disclosure scope of this invention.

Claims

1. An annular protective liquid comprising an active hydrophilic polymer, a bactericide, an oxygen scavenger, and a pH adjuster, wherein the active hydrophilic polymer has the structural formula shown in formula (1) or formula (2): Equation (1) Equation (2) In formulas (1) and (2), a and b are integers greater than or equal to 1, and a / (a+b) is 70%~96%; R1 is a C1~C4 straight-chain alkyl group or H, and R2 is a C1~C16 straight-chain alkyl group, wherein, The active hydrophilic polymer comprises 0.01% to 3.0%, the bactericide comprises 0.1% to 2.5%, the oxygen scavenger comprises 0.1% to 5%, the pH adjuster comprises 0.1% to 0.5%, and the balance is water. The active hydrophilic polymer is copolymerized from N-acryloylmorpholine and acrylate compounds, and the mass ratio of the reactive monomer N-acryloylmorpholine to the acrylate compounds is 70% to 96%: 4% to 30%.

2. The annular protective fluid according to claim 1, wherein, In formula (1) and / or formula (2), a / (a+b) is 80%~90%; R2 is a C2-C14 straight-chain alkyl group.

3. The annular protective fluid according to claim 1, wherein, The oxygen scavenger is one or a mixture of two or more of isoascorbic acid alkali metal salts and dimethyl ketoxime.

4. The annular protective fluid according to claim 1, wherein, The bactericide is one or more of a quaternary ammonium salt cationic surfactant or isothiazolinone and its derivatives.

5. The annular protective fluid according to claim 1, wherein, The pH adjuster is an alkyl alcohol amine with a carbon number of C2 to C10.

6. The annular protective fluid according to claim 1 or 5, wherein, The pH adjuster is one or more of methyldiethanolamine, triethanolamine, and isopropanolamine.

7. The annular protective fluid according to claim 1, based on the total mass of the system, wherein, The active hydrophilic polymer accounts for 0.5%~1.0%, the bactericide accounts for 1.0%~2.0%, the oxygen scavenger accounts for 1.0%~3.0%, the pH adjuster accounts for 0.2%~0.3%, and the balance is water.

8. The annular protective fluid according to claim 1, wherein, The preparation method of the active hydrophilic polymer includes: adding the reactive monomers acrylate compounds and N-acryloylmorpholine to a solvent, heating to 30~70℃ and then adding an initiator, stirring the reaction to obtain a mixture, separating the precipitate and drying it to obtain the active hydrophilic polymer.

9. The annular protective fluid according to claim 1 or 8, wherein, The acrylate compounds are alkyl-substituted or unsubstituted acrylates, with the following structures: Wherein, R1 is a C1~C4 alkyl or H, and R2 is a C1~C16 alkyl.

10. The annular protective fluid according to claim 9, wherein, R2 is a C2~C14 straight-chain alkyl group.

11. The annular protective fluid according to claim 9, wherein, R2 is a C10~C14 straight alkyl group.

12. The annular protective fluid according to claim 8, wherein, The total mass of the two reactive monomers accounts for 5% to 30% of the total mass fraction of the solvent.

13. The annular protective fluid according to claim 8, wherein, The total mass of the two reactive monomers accounts for 10% to 25% of the total mass of the solvent.

14. The annular protective fluid according to claim 1 or 8, wherein, The mass ratio of the reactive monomer N-acryloylmorpholine to the acrylate compound is 80%~90%:10%~20%.

15. The annular protective fluid according to claim 8, wherein, The initiator is one or a combination of at least two of the following: azobisisobutyronitrile, azobisisoheptanenitrile, dimethyl azobisisobutyrate, azobisisobutylamidine hydrochloride, benzoyl peroxide, and lauroyl peroxide.

16. The annular protective fluid according to claim 8, wherein, The initiator concentration is 0.05~1.0 wt% of the total monomer concentration.

17. The annular protective fluid according to claim 8, wherein, The initiator concentration is 0.1 to 0.5 wt% of the total monomer concentration.

18. A method for preparing the annular protective fluid according to any one of claims 1-17, comprising: Add the active hydrophilic polymer, oxygen scavenger, bactericide and pH adjuster to the water and stir until fully dissolved.

19. The use of the annulus protection fluid according to any one of claims 1-17 for annulus protection in oil and gas wells.

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