Microbubble workover fluid system for low-pressure old well and preparation method of microbubble workover fluid system

Abstract

The invention discloses a microbubble workover fluid system for a low-pressure old well and a preparation method of the microbubble workover fluid system. The microbubble workover fluid system comprises the following components in percentage by weight: 1-2% of a composite foaming agent, 1-2% of a foam stabilizer, 0.1-0.5% of a deoxidant, 0.05-0.2% of a pH regulator and the balance of water. The invention not only has the characteristic of low density of the conventional foam system, but also forms a composite temporary plugging system through the synergistic effect of the micro-foam and the self-made resin foam stabilizing system, thereby greatly reducing the leakage of working fluid in the early stage of construction, and greatly improving the stability and loading capacity of the temporary plugging system. The microbubble workover fluid has the characteristics of high temperature resistance, high shear resistance, low viscosity, high shear force and the like, is helpful for a system to suspend and carry underground solid-phase impurities, and prevents system viscosity reduction, defoaming, leakage-proof performance reduction and the like caused by underground circulating shear of the microbubble workover fluid. A water-based material and an easily degradable material are adopted and can be naturally degraded in a downhole environment, so that no adverse effect is caused on recovery after well repair operation.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas exploitation, and in particular to a microbubble well repairing fluid system for low-pressure old wells and a preparation method thereof. Background Art

[0002] Because horizontal wells have the characteristics of large reservoir contact area and effective increase in single well production, they are widely used in the development of low-permeability, low-abundance unconventional oil and gas such as shale gas and coalbed methane. In the late stage of oil and gas field development, the formation energy is insufficient and the bottom hole pressure is lower than the hydrostatic column pressure. When performing well repair operations, if conventional well repair fluids are used, because their system density is often greater than or equal to 1.0g / ml, they are prone to leakage and cannot effectively establish a circulation to carry downhole spit sand, milling iron filings, corrosion iron filings, residual bridge plugs and other high-density, large-particle solids out of the wellbore. The microbubble well repair fluid system has a lower density, which can greatly reduce the leakage of the well repair fluid and the damage caused by the well repair fluid to the formation. It is a good well repair fluid for low-pressure oil and gas wells.

[0003] Chinese patent CN202011305537.0 discloses a microbubble well repairing fluid and a preparation method thereof, wherein a microbubble well repairing fluid having stronger microbubble stability and maintaining its low density performance under high bottom hole pressure is formed by compounding sodium sulfate of octadecyl polyoxyethylene ether (3), nonylphenol polyoxyethylene ether (3), xanthan gum, scleroglucan, carboxymethyl starch, hydroxypropyl starch, formaldehyde, KCl, and a synthetic compound of acrylamide, 2-acrylamido-2-methylpropane sulfonic acid and dimethylallyl dodecyl ammonium chloride to improve its leak-proof performance.

[0004] Chinese patent CN202211429055.5 discloses a low-density foam workover fluid for reducing leakage in low-pressure gas well workover and its application, which is prepared by adding clean water into a stirring cup, adding sodium dodecylbenzene sulfonate, ethylene glycol, hexadecyl dimethyl carboxy betaine, N, N-bishydroxyethyl alkylamide, carboxymethyl cellulose, xanthan gum, carboxymethyl modified starch, carboxyethyl modified starch, sodium salt of sulfonic fatty acid methyl ester, and fatty acid diethanolamide under stirring, and continuing to stir until the mixture is uniform to obtain a low-density foam workover fluid. This technology is based on a foam system, and by introducing surfactants and macromolecular components, a relatively dense multi-layer "membrane" structure is formed on the periphery of the bubble, so that different foam monomers are separated from each other, thereby improving the structural stability and temperature and salt resistance of the foam monomer to improve its leakproof performance.

[0005] The applicant has found that the prior art has at least the following technical problems:

[0006] 1. In the prior art, the leakage prevention performance of the micro-bubble workover fluid system is improved only by increasing the viscosity of the system and improving the stability of the foam to reduce the bottom hole pressure difference during construction. However, due to the dilution of the agent in the early stage of construction, the foam stability is reduced, the foam fails to be effectively temporarily blocked, and the system leakage is large, which will cause additional workover fluid loss.

[0007] 2. Well repair fluid is often used in conjunction with well repair tools. The well repair process is often accompanied by high temperatures downhole, bacterial contamination downhole, and tool circulation shear, which can lead to a significant reduction in the performance of viscosity-increasing and foam-stabilizing additives, resulting in a decrease in the defoaming and leak-proof performance of the micro-foam well repair fluid system. Summary of the invention

[0008] The purpose of the present invention is to provide a microbubble well repairing fluid system for low-pressure old wells and a preparation method thereof, so as to solve the technical problems in the prior art of microbubble well repairing fluid, such as rapid decrease in viscosity of the system and decrease in foam stabilization performance during underground high temperature and cyclic shearing process, and decrease in defoaming and temporary plugging performance, which may lead to operation failure.

[0009] To achieve the above object, the present invention provides the following technical solutions:

[0010] The invention provides a micro-bubble well repairing fluid system for low-pressure old wells, comprising the following components in weight percentage: 1-2% composite foaming agent, 1-2% foam stabilizer, 0.1-0.5% deoxidizer, 0.05-0.2% pH regulator, and the balance is water.

[0011] Furthermore, the composite foaming agent is a mixture of betaine surfactant, alkyl glyceryl ether carboxylate, alkyl glyceryl ether sulfate, and fatty alcohol polyether surfactant in a mass ratio of (2-3): (0.2-0.5): (0.2-0.5): (0.5-1).

[0012] The foam stabilizer of the present invention introduces strong hydration groups, electrostatic adsorption groups, rigid structural groups, and organic silicon polymerization monomers. Due to the large Si-O-Si bond angle, large Si-O bond length, relatively easy internal rotation, good film-forming property, hydrogen bond formation, strong adhesion between molecules, and the formation of a multi-polymer from the perspective of multi-group synergistic enhancement, the foam stabilizer has the characteristics of high temperature resistance, high shear resistance, low viscosity, high shear force, self-repair, etc., adjusts the viscosity of the system, forms rigid foam, reduces the probability of foam merging, prolongs the time for foam to float to the surface, increases the stability of the foam, helps the system to suspend and carry downhole solid phase impurities, and prevents the viscosity of the system from decreasing due to downhole circulation shear of the microbubble workover fluid, resulting in defoaming and temporary plugging performance decrease. A low-pressure microbubble workover fluid system for old wells is formed, which has a large adjustable density range, a stable system before and after hot rolling, little viscosity and shear change of the fluid, good rheological properties, and high shear force, indicating that it has strong sand carrying capacity and circulation, and can effectively reduce circulation resistance and improve pumping efficiency.

[0013] Furthermore, the foam stabilizer is a multi-polymer, which is prepared by a free radical polymerization reaction of multiple monomers selected from acrylamide monomers (N,N-dimethylacrylamide, N,N-diethylacrylamide, acrylamide, etc.), anionic monomers (2-acrylamide-2-methylpropanesulfonic acid (AMPS), itaconic acid (IA), acrylic acid (AA), sodium allyl sulfonate), cationic monomers (diallyldimethylammonium chloride (DMDAAC), methacryloyloxyethyltrimethylammonium chloride (DMC), sodium p-styrene sulfonate (SSS)), cyclic monomers (1-vinyl-2-pyrrolidone, N-vinylcaprolactam), and silicone monomers (3-(methacryloyloxy)propyltrimethoxysilane, vinyltrimethoxysilane, 5-hexenyltrimethoxysilane) through reverse suspension polymerization.

[0014] Preferably, the monomers are N,N-dimethylacrylamide, itaconic acid (IA), sodium allyl sulfonate, N-vinyl caprolactam, 5-hexenyl trimethoxysilane, and preferably, the molar ratio of the monomer compounds is 2.5-4: 0.5-2: 1-3: 0.2-1: 0.1-1, preferably: 2.8-3.2: 1: 1-1.3: 0.2-0.5: 0.1-0.3. In order to be suitable for the free radical polymerization reaction and reasonably control the molecular weight of the obtained copolymer, preferably, the total molar amount of the compound is 0.8-3 mol, preferably 1.9-2.3 mol, relative to 100 g of water.

[0015] Further, a free radical polymerization reaction of reverse suspension polymerization is adopted, and a conventional redox free radical initiator ammonium persulfate and sodium bisulfite system is introduced to carry out the polymerization reaction. The amount of the free radical initiator system can vary within a wide range. In order to properly initiate the polymerization to obtain a more preferred copolymer of the present invention, preferably, based on 1 mol of the total molar amount of the polymer compound, the amount of the free radical initiator system is 0.1-8 mmol, preferably 2-4 mmol.

[0016] Furthermore, the dispersion medium used in the polymerization reaction is one of n-pentane, n-hexane, gasoline, cyclohexane, toluene, xylene, benzene, light oil, carbon tetrachloride, solvent naphtha, solvent oil, and dearomatized solvent oil.

[0017] The present invention preferably uses D80 dearomatized solvent oil as the organic dispersed phase. The dearomatized solvent oil is a solvent oil product obtained by ultra-deep refining to remove impurities such as aromatics, sulfur and nitrogen. It has the characteristics of being colorless, odorless, non-toxic, chemically inert, and having excellent light and thermal stability. The amount of the dispersion medium is 1-5 times the total amount of the functional monomer water-soluble, and the preferred amount is 1.2-1.8 times.

[0018] Furthermore, a dispersant is used in the polymerization reaction, and the dispersant is one or more of a Span surfactant and a Tween surfactant. Preferably, Span60 and Span80 are compounded as dispersants in a ratio of 2-8:1, and the amount of the dispersant is 2%-6% of the total amount of the reaction. Under more optimal conditions, the ratio of Span60 and Span80 is 3-4:1, and the amount of the dispersant is 3.5%-4.5% of the total amount of the reaction.

[0019] Further, the polymerization process is carried out under acidic conditions, preferably, the pH of the acidic conditions is 4-6. The pH can be adjusted by various alkaline compounds conventionally used in the art, such as one or more of lithium hydroxide, sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, sodium bicarbonate and their respective aqueous solutions (e.g., concentration of 5-20 wt%).

[0020] Furthermore, the conditions of the free radical polymerization reaction include: a temperature of 20-80° C. (preferably 50-70° C.); a time of 3-10 h (preferably 4-6 h). The reaction process needs to be carried out at a stirring speed of 400-800 rpm / min.

[0021] In order to maintain the activity of the initiator, etc., the method may also include: adding a metal ion complexing agent to the solution prepared during the reaction liquid process, the metal ion complexing agent may be added to one or more of ethylenediaminetetraacetic acid (EDTA), aminotriacetic acid, sodium gluconate, etc., and the present invention preferably uses ethylenediaminetetraacetic acid (EDTA) in an amount of 0.1‰ of the mass ratio of the aqueous solution; before introducing the free radical initiation system, first keep the system in an inert atmosphere, for example, one or more of nitrogen, helium, neon, argon, etc. may be introduced into the system and replace the atmosphere therein (this process may be carried out for 5-300 minutes, for example).

[0022] In order to make the reaction proceed more smoothly, the method includes: first adding EDTA metal chelating agent to water and stirring evenly, respectively mixing the compounds of the polymerization reaction with water (the mixing conditions may include, for example, temperature of 10-40°C and time of 10-30min), adjusting the pH value if the pH value is less than 4, (the mixing conditions may include, for example, temperature of 10-50°C and time of 10-60min) (optionally performing atmosphere replacement to maintain an inert atmosphere), adding a dispersant to the dispersion medium and performing atmosphere replacement to maintain an inert atmosphere, and then adding all the functional monomer aqueous solutions while maintaining a stirring rate of 400-800rpm / min, and then introducing a free radical initiation system to carry out the free radical polymerization reaction.

[0023] According to the present invention, in order to obtain the copolymer of the present invention in solid form, the method further comprises: filtering the product of the free radical polymerization reaction through an organic dispersion medium and drying. The drying conditions may include, for example, a temperature of 70-100° C. and a time of 20-60 hours. There is no particular limitation on the drying method, and various conventional drying methods in the art may be used, such as a drying method, a reduced pressure drying method, and the like.

[0024] Furthermore, the deoxidizer is sodium sulfite, carbohydrazide, or isoascorbic acid.

[0025] Furthermore, the pH adjuster is sodium hydroxide, calcium hydroxide, sodium carbonate, calcium carbonate, or potassium hydroxide.

[0026] The present invention also provides a method for preparing a microbubble well repairing fluid system for low-pressure old wells, which is characterized by comprising the following steps:

[0027] (1) Add clean water into the reactor;

[0028] (2) adding a pH adjuster to a pH of 9-10;

[0029] (3) adding a deoxidizer and stirring until completely dissolved to perform liquid pretreatment;

[0030] (4) Add foam stabilizer and stir until completely dissolved to adjust viscosity;

[0031] (5) adding a composite foaming agent and stirring to form a uniform mixture;

[0032] (6) Adjust the rotation speed to 1200-2000 r / min to generate microfoam to reduce the system density and obtain microfoam workover fluid.

[0033] Based on the above technical solution, the embodiments of the present invention can at least produce the following technical effects:

[0034] (1) The low-pressure microbubble well repair fluid system for old wells provided by the present invention not only has the low-density characteristics of conventional foam systems, but also forms a composite temporary plugging system through the synergistic effect of microfoam and a self-made resin-type foam stabilization system, which greatly reduces the loss of working fluid in the early stage of construction and greatly improves the stability and pressure-bearing capacity of the temporary plugging system.

[0035] (2) The microbubble well repair fluid system for low-pressure old wells provided by the present invention has the characteristics of resistance to high temperature, resistance to high shear, low viscosity and high shear force, which helps the system to suspend and carry solid impurities underground, and prevents the viscosity of the system from decreasing due to the shearing of the microbubble well repair fluid underground circulation, resulting in a decrease in defoaming and leak-proof performance.

[0036] (3) The low-pressure microbubble well repair fluid system for old wells provided by the present invention uses water-based materials that are easily degradable and will naturally degrade in the bottom well environment, without adversely affecting the resumption of production after the well repair operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying creative work.

[0038] Figure 1 This is a graph showing the particle size detection results of Example 1 of the present invention. DETAILED DESCRIPTION

[0039] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0040] It should be understood that the terms described in the present invention are only for describing special embodiments and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. Each smaller range between the intermediate value in any stated value or stated range and any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0041] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0042] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to the skilled artisan. The present invention description and examples are exemplary only.

[0043] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0044] The oxygen scavenger is isoascorbic acid;

[0045] The pH adjuster is potassium hydroxide;

[0046] Example 1

[0047] Foam stabilizer ① Synthesis: N,N-dimethylacrylamide, itaconic acid (IA), sodium allyl sulfonate, N-vinyl caprolactam, 5-hexenyl trimethoxysilane, according to the monomer molar ratio of 2.8-3.2:1:1-1.3:0.2-0.5:0.1-0.3; Prepare into an aqueous solution, add ethylenediaminetetraacetic acid (EDTA) to the water in advance, the amount is 0.1‰ of the mass ratio of water, and stir evenly. Relative to 100g of water, the total molar amount of the compound is 1.9-2.3mol. Dearomatized solvent oil (D80) is used as an organic dispersed phase, and the amount is 1.2-1.8 times the total amount of the aqueous solution. The polymerization reaction is carried out using an ammonium persulfate and sodium bisulfite system. Based on 1 mol of the total molar amount of the polymer compound, the amount of the free radical initiation system is 2-4 mmol; Span60 and Span80 are compounded as dispersants in a ratio of 3-4:1, and the amount of the dispersant is 3.5%-4.5% of the total reaction amount.

[0048] Adjust the system pH to 4-6. The temperature is 50-70°C and the reaction time is 4-6 hours. The reaction process needs to be carried out at a stirring speed of 400-800 rpm / min.

[0049] The preparation of a micro-bubble well repairing fluid system for low-pressure old wells comprises the following components: 1-2% composite foaming agent, 0.5-1% resin foam stabilizer①, 0.1-0.5% deoxidizer, 0.05-0.2% pH regulator, and the balance is water.

[0050] The composite foaming agent is a mixture of betaine surfactant (cocamidopropyl betaine), alkyl glyceryl ether carboxylate, alkyl glyceryl ether sulfate and fatty alcohol polyether surfactant in a mass ratio of (2-3): (0.2-0.5): (0.2-0.5): (0.5-1).

[0051] (1) Add clean water into the reactor;

[0052] (2) adding a pH adjuster to a pH of 9-10;

[0053] (3) adding a deoxidizer and stirring until completely dissolved to perform liquid pretreatment;

[0054] (4) Add homemade resin-type foam stabilizer and stir until completely dissolved to adjust the viscosity;

[0055] (5) Add the composite foaming agent and stir until it is evenly mixed.

[0056] (6) Adjust the rotation speed to 1200-2000 r / min to generate microfoam to reduce the system density and obtain microfoam workover fluid.

[0057] Example 2

[0058] Foam stabilizer ② Synthesis: N,N-dimethylacrylamide, itaconic acid (IA), sodium allyl sulfonate, N-vinyl caprolactam; monomer molar ratio is 2.8-3.2:1:1-1.3:0.2-0.5; prepare into aqueous solution, add ethylenediaminetetraacetic acid (EDTA) to the water in advance, the amount is 0.1‰ of the mass ratio of water, and stir evenly. Relative to 100g of water, the total molar amount of the compound is 1.9-2.3mol. Dearomatized solvent oil (D80) is used as an organic dispersed phase, and the amount is 1.2-1.8 times the total amount of the aqueous solution. Ammonium persulfate and sodium bisulfite system are used for polymerization reaction. Based on the total molar amount of 1mol of the polymer compound, the amount of the free radical initiation system is 2-4mmol; Span60 and Span80 are compounded as dispersants, the ratio is 3-4:1, and the amount of dispersant is 3.5%-4.5% of the total reaction amount.

[0059] Adjust the system pH to 4-6. The temperature is 50-70°C and the reaction time is 4-6 hours. The reaction process needs to be carried out at a stirring speed of 400-800 rpm / min.

[0060] The preparation of a micro-bubble well repair fluid system for low-pressure old wells includes the following components: 1-2% composite foaming agent, 0.5-1% foam stabilizer ②, 0.1-0.5% deoxidizer, 0.05-0.2% pH regulator, and the balance is water.

[0061] The composite foaming agent is a mixture of betaine surfactant (cocamidopropyl betaine), alkyl glyceryl ether carboxylate, alkyl glyceryl ether sulfate and fatty alcohol polyether surfactant in a mass ratio of (2-3): (0.2-0.5): (0.2-0.5): (0.5-1).

[0062] (1) Add clean water into the reactor;

[0063] (2) adding a pH adjuster to a pH of 9-10;

[0064] (3) adding a deoxidizer and stirring until completely dissolved to perform liquid pretreatment;

[0065] (4) Add homemade resin-type foam stabilizer and stir until completely dissolved to adjust the viscosity;

[0066] (5) Add the composite foaming agent and stir until it is evenly mixed.

[0067] (6) Adjust the rotation speed to 1200-2000 r / min to generate microfoam to reduce the system density and obtain microfoam workover fluid.

[0068] Example 3

[0069] Foam stabilizer ③ Synthesis: N,N-dimethylacrylamide, acrylic acid (AA), 2-acrylamide-2-methylpropanesulfonic acid (AMPS), N-vinyl caprolactam, 3-(methacryloyloxy)propyltrimethoxysilane, according to the monomer molar ratio of 2.5-3:1:1-1.3:0.2-0.5:0.2-0.5; prepared into an aqueous solution, ethylenediaminetetraacetic acid (EDTA) is added to the water in advance, the amount is 0.1‰ of the mass ratio of water, and stirred evenly. Relative to 100g of water, the total molar amount of the compound is 1.9-2.3mol. Dearomatized solvent oil (D80) is used as an organic dispersed phase, and the amount is 1.2-1.8 times the total amount of the aqueous solution. The polymerization reaction is carried out using an ammonium persulfate and sodium bisulfite system. Based on the total molar amount of 1 mol of the polymer compound, the amount of the free radical initiation system is 2-4 mmol; Span60 and Span80 are compounded as dispersants in a ratio of 3-4:1, and the amount of the dispersant is 3.5%-4.5% of the total reaction amount. The system pH is adjusted to 4-6. The temperature is 50-70°C; the time is 4-6h. The reaction process needs to be carried out at a stirring speed of 400-800rpm / min.

[0070] The preparation of a micro-bubble well repairing fluid system for low-pressure old wells comprises the following components: 1-2% composite foaming agent, 0.5-1% foam stabilizer ③, 0.1-0.5% deoxidizer, 0.05-0.2% pH regulator, and the balance is water.

[0071] The foaming agent is a mixture of betaine surfactant (cocamidopropyl betaine), alkyl glyceryl ether carboxylate, alkyl glyceryl ether sulfate, and fatty alcohol polyether surfactant in a mass ratio of (2-3): (0.2-0.5): (0.2-0.5): (0.5-1);

[0072] (1) Add clean water into the reactor;

[0073] (2) adding a pH adjuster to a pH of 9-10;

[0074] (3) adding a deoxidizer and stirring until completely dissolved to perform liquid pretreatment;

[0075] (4) Add homemade resin-type foam stabilizer and stir until completely dissolved to adjust the viscosity;

[0076] (5) Add the composite foaming agent and stir until it is evenly mixed.

[0077] (6) Adjust the rotation speed to 1200-2000 r / min to generate microfoam to reduce the system density and obtain microfoam workover fluid.

[0078] Example 4

[0079] Foam stabilizer ④ Synthesis: acrylamide, N,N-dimethylacrylamide, diallyldimethylammonium chloride, 1-vinyl-2-pyrrolidone, the monomer molar ratio is 1.5-2:1:0.5-1:0.2-0.5; prepare into aqueous solution, add ethylenediaminetetraacetic acid (EDTA) to the water in advance, the amount is 0.1‰ of the mass ratio of water, and stir evenly. Relative to 100g of water, the total molar amount of the compound is 1.9-2.3mol. Dearomatized solvent oil (D80) is used as the organic dispersed phase, and the amount is 1.2-1.8 times the total amount of the aqueous solution. Use ammonium persulfate and sodium bisulfite system to carry out polymerization reaction, based on the total molar amount of 1mol of the polymer compound, the amount of the free radical initiation system is 2-4mmol; Span60 and Span80 are compounded as dispersants, the ratio is 3-4:1, and the amount of dispersant is 3.5%-4.5% of the total reaction amount. Adjust the system pH to 4-6. The temperature is 50-70°C and the time is 4-6 hours. The reaction process needs to be carried out at a stirring speed of 400-800 rpm / min.

[0080] The preparation of a micro-bubble well repairing fluid system for low-pressure old wells comprises the following components: 1-2% composite foaming agent, 0.5-1% foam stabilizer④, 0.1-0.5% deoxidizer, 0.05-0.2% pH regulator, and the balance is water.

[0081] The foaming agent is a mixture of betaine surfactant (cocamidopropyl betaine), alkyl glyceryl ether carboxylate, alkyl glyceryl ether sulfate, and fatty alcohol polyether surfactant in a mass ratio of (2-3): (0.2-0.5): (0.2-0.5): (0.5-1);

[0082] (1) Add clean water into the reactor;

[0083] (2) adding a pH adjuster to a pH of 9-10;

[0084] (3) adding a deoxidizer and stirring until completely dissolved to perform liquid pretreatment;

[0085] (4) Add homemade resin-type foam stabilizer and stir until completely dissolved to adjust the viscosity;

[0086] (5) Add the composite foaming agent and stir until it is evenly mixed.

[0087] (6) Adjust the rotation speed to 1200-2000 r / min to generate microfoam to reduce the system density and obtain microfoam workover fluid.

[0088] Comparative Example

[0089] The invention comprises the following components: 2% of a composite foaming agent, 1% of a foam stabilizer, and the balance of the foaming agent is water.

[0090] The composite foaming agent is a mixture of betaine surfactant (cocamidopropyl betaine), alkyl glyceryl ether carboxylate, alkyl glyceryl ether sulfate and fatty alcohol polyether surfactant in a mass ratio of (2-3): (0.2-0.5): (0.2-0.5): (0.5-1).

[0091] The foam stabilizer is a 1:1 mixture of carboxymethyl cellulose and xanthan gum.

[0092] (1) Add clean water into the reactor;

[0093] (2) adding a pH adjuster to a pH of 9-10;

[0094] (3) adding a deoxidizer and stirring until completely dissolved to perform liquid pretreatment;

[0095] (4) Add homemade resin-type foam stabilizer and stir until completely dissolved to adjust the viscosity;

[0096] (5) Add the composite foaming agent and stir until it is evenly mixed.

[0097] (6) Adjust the rotation speed to 1200-2000 r / min to generate microfoam to reduce the system density and obtain microfoam workover fluid.

[0098] Test Method

[0099] (1) Apparent viscosity

[0100] Test method: According to the provisions of GB / T 16783.1-2014 "Field Test of Drilling Fluids in Petroleum and Natural Gas Industry Part 1: Water-based Drilling Fluids", a direct-reading rotational viscometer is used for measurement.

[0101] Determination procedure: Inject the sample into the scale line of the measuring cup of the direct-reading rotational viscometer, and immerse the rotating cylinder just to the scale line; rotate the rotating cylinder at 600r / min, and after the dial reading stabilizes, read and record the dial reading at 600r / min, which is recorded as R600. Apparent viscosity, AV = R600 / 2.

[0102] (2) API filtration loss

[0103] Test method: According to the provisions of GB / T 16783.1-2014 "Field Test of Drilling Fluids in Petroleum and Natural Gas Industry Part 1: Water-based Drilling Fluids", the API filter loss tester is used for measurement. Measuring cup specifications: inner diameter: 76.2mm, height: 64mm, filter paper area: 45.8cm 2 ±0.6cm 2 .

[0104] Determination procedure: Pour the sample into the measuring cup so that the liquid level is at least 1cm-1.5cm from the top of the cup; place a dry measuring cylinder under the discharge pipe to receive the filtrate; close the pressure reducing valve and adjust the pressure regulator to 0.7Mpa within 30S, and start timing; read the volume of the filtrate in the measuring cylinder at 7min30S, recorded as V. API filtration loss = 2*V.

[0105] (3) Sealing pressure bearing performance

[0106] Testing method: According to the provisions of SY / T 5840-2007 "Indoor test method for bridging plugging materials for drilling fluids", use the GGS71 high temperature and high pressure filter loss tester for measurement.

[0107] Determination procedure: Add 300g of quartz sand (quartz sand mesh number: 70-140 mesh) to the GGS71 high-temperature and high-pressure filter loss tester mud cup, flatten and compact to form a sand bed. At room temperature, inject the sample to the upper scale of the test cup, and slowly pressurize the test container to the design value, and stabilize the pressure for 10 minutes; if there is no gas puncture during the pressurization and stabilization process, continue to pressurize according to the above procedure until gas puncture occurs.

[0108] (4) Thermal shear stability

[0109] Determination procedure: The microbubble well repairing fluid system was prepared according to the raw material ratio and preparation method in the embodiment, and the RS6000 rheometer was used to measure the microbubble well repairing fluid system at 93°C and 170s. -1 The thermal shear stability of the microbubble workover fluid system was evaluated at a shear rate of 1 hour, and the changes in the viscosity of the system before and after shearing were recorded.

[0110] (5) Thermal aging stability

[0111] Determination procedure: Prepare the microbubble workover fluid system according to the raw material ratio and preparation method in the embodiment, use a roller heating furnace, heat at 150°C for 12 hours, and evaluate the thermal aging stability of the microbubble workover fluid system. Record the change in the viscosity of the system before and after shearing.

[0112] Test results:

[0113]

[0114] Example 1 Particle size test results are as follows Figure 1 As shown:

[0115] The microbubble well repairing fluid of the present invention is detected by a laser particle size analyzer, and the D50 is 30.63um and the D90 is 94.68um. It has good plugging ability for micro pores and can effectively reduce underground leakage.

[0116] Finally, it should be noted that:

[0117] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions described in the above embodiments may still be modified, or some or all of the technical features may be replaced by equivalents. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A microbubble workover fluid system for low-pressure old wells, characterized in that: The invention comprises the following components in weight percentage: 1-2% of a composite foaming agent, 1-2% of a foam stabilizer, 0.1-0.5% of an oxygen scavenger, 0.05-0.2% of a pH regulator, and the balance is water.

2. The low-pressure microbubble workover fluid system for old wells according to claim 1, characterized in that: The composite foaming agent is a mixture of betaine surfactant, alkyl glyceryl ether carboxylate, alkyl glyceryl ether sulfate and fatty alcohol polyether surfactant in a mass ratio of (2-3): (0.2-0.5): (0.2-0.5): (0.5-1).

3. The low-pressure microbubble workover fluid system for old wells according to claim 1, characterized in that: The foam stabilizer is a multi-component copolymer, and is prepared by free radical polymerization of multiple monomers of acrylamide monomers, anionic monomers, cationic monomers, cyclic monomers, and silicone monomers through reverse suspension polymerization.

4. The low-pressure microbubble workover fluid system for old wells according to claim 3, characterized in that: The dispersion medium is used in the polymerization reaction, and the dispersion medium is one of n-pentane, n-hexane, gasoline, cyclohexane, toluene, xylene, benzene, light oil, carbon tetrachloride, solvent naphtha, solvent oil, and dearomatized solvent oil.

5. The low-pressure microbubble workover fluid system for old wells according to claim 3, characterized in that: A dispersant is used in the polymerization reaction, and the dispersant is one or more of a span surfactant and a tween surfactant.

6. The low-pressure microbubble workover fluid system for old wells according to claim 3, characterized in that: The polymerization reaction is initiated by using an ammonium persulfate and sodium bisulfite system, and the amount of the free radical initiation system is 0.1-8 mmol based on the total molar amount of 1 mol of the polymer compound; The polymerization process is carried out under acidic conditions with a pH of 4-6; the polymerization temperature is 20-80° C. and the reaction time is 3-10 hours.

7. The low-pressure microbubble workover fluid system for old wells according to claim 1, characterized in that: The deoxidizer is sodium sulfite, carbohydrazide and isoascorbic acid.

8. The low-pressure microbubble workover fluid system for old wells according to claim 1, characterized in that: The pH regulator is sodium hydroxide, calcium hydroxide, sodium carbonate, calcium carbonate, and potassium hydroxide.

9. The method for preparing a low-pressure old well microbubble workover fluid system according to any one of claims 1 to 8, characterized in that: The following steps are involved: (1) Add clean water to the reactor; (2) Add pH adjuster to pH 9-10; (3) Add deoxidizer and stir until completely dissolved to pre-treat the prepared liquid; (4) Add foam stabilizer and stir until completely dissolved, and adjust the viscosity; (5) Add the composite foaming agent and stir until it becomes a uniform mixture; (6) Adjust the rotation speed to 1200-2000 r / min to generate microfoam to reduce the system density and obtain microfoam workover fluid.

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