Solid-free well killing fluid for shale reservoir and preparation method thereof

Abstract

The present application relates to the field of oil well killing fluid, and particularly relates to a solid-free well killing fluid for shale oil reservoir and a preparation method thereof. The solid-free well killing fluid for shale oil reservoir comprises the following raw materials: 50-70 parts by mass of water, 10-30 parts by mass of a density regulator, 3-6 parts by mass of a copolymer tackifier, 1-3 parts by mass of a dispersant, 3-7 parts by mass of a fluid loss additive, and 2-4 parts by mass of an inhibitor. The copolymer tackifier is obtained by radical copolymerization of acrylamide, a monomer containing a quaternary ammonium salt, an unsaturated silane coupling agent, and polyethylene glycol diacrylamide. The dispersant comprises fumed nano-silicon dioxide and a fluorine-containing surfactant. The present application plays a multi-functional role through reasonable compounding of the components of the formula, especially the copolymer tackifier, and simultaneously improves the high-temperature resistance, corrosion inhibition, and clay stability of the well killing fluid.

Description

Technical Field

[0001] This invention relates to the field of oil well control fluid production, and particularly to a solid-free well control fluid for shale oil reservoirs and its preparation method. Background Technology

[0002] In shale oil extraction, the performance of the kill fluid is crucial. Shale oil reservoirs are characterized by complex formation pressures, high temperatures, and susceptibility to reservoir damage, making it difficult for traditional kill fluids to meet extraction requirements. Conventional kill fluids are ill-suited to the needs of shale oil reservoirs. For example, their density adjustment range is limited, making them ineffective in high-pressure shale oil reservoirs; at high temperatures, their rheological properties deteriorate, leading to increased friction and pressure loss; significant filtration loss can severely damage shale reservoirs and reduce reservoir permeability; insufficient inhibition properties cannot effectively prevent shale formation expansion and wellbore instability; poor lubrication performance leads to severe drill string wear, increasing extraction costs and safety risks; and in high-salt formation water environments, their poor salt resistance affects the stability and effectiveness of the kill fluid. With the development of shale oil reservoirs and the advancement of complex formations, the development of high-performance kill fluids is urgently needed.

[0003] CN119331579A discloses a high-density, solid-free kill fluid with high temperature resistance and low damage. By weight, it comprises the following components: 50-80 parts water, 10-40 parts calcium chloride, 10-40 parts calcium bromide, 0.5-2 parts temperature-resistant viscosifier, 1-3 parts temperature-resistant filtration reduction agent, 0.5-2 parts high-temperature stabilizer, 0.5-3 parts high-temperature corrosion inhibitor, 0.5-2 parts salt crystallization inhibitor, and 0.1-1 parts water-locking agent. Using CaBr2-CaCl2 composite brine as the base fluid, the kill fluid density can be maintained at 1.4 g / cm³. 3 -1.6g / cm 3 Adjustments were made within a specified range. A heat-resistant treatment agent was selected during the preparation of the kill fluid, maintaining stable performance even at 150°C. The kill fluid constructed in this invention exhibits low-damage and weak-corrosion properties, effectively reducing damage to the reservoir and corrosion of the wellbore. Furthermore, this kill fluid demonstrates good stability in high-temperature formation environments and good compatibility with formation water.

[0004] CN117285914A discloses a kill fluid, wherein the total mass of the kill fluid is 100 wt%, the mass concentration of the weighting agent is 30 wt%-90 wt%, and the dispersant is selected from at least one of magnesium aluminum silicate and magnesium lithium silicate.

[0005] The unique physicochemical properties of shale reservoirs (ultra-low permeability, high water sensitivity, and well-developed natural fractures) and the large-scale hydraulic fracturing development method place higher demands on kill fluids in shale oil reservoir operations. Shale contains nanoscale pores and microfractures. Conventional kill fluid filtrate, once it enters, will clog these tiny channels, causing permanent damage and making it difficult to flow back. Therefore, kill fluids with lower HTHP filtrate loss are required. In addition, shale is rich in clay minerals, such as illite and montmorillonite, which easily swell, disperse, and migrate upon contact with water, leading to wellbore instability and pore blockage. Therefore, kill fluids used in shale oil reservoirs must also have good ability to inhibit clay hydration swelling and dispersion. Deep shale has a high temperature, requiring kill fluids with stable viscosity and density at high temperatures. Summary of the Invention

[0006] To overcome the lack of a comprehensive and high-performance kill fluid specifically designed for shale oil reservoirs in existing technologies, this invention proposes a solids-free kill fluid for shale oil reservoirs and its preparation method. Through the rational formulation of its components, particularly the copolymer thickener, and the copolymerization of multiple functional monomers, a multi-functional agent is achieved, simultaneously improving the kill fluid's high-temperature resistance, corrosion inhibition, and clay stability. This results in excellent properties such as low reservoir damage rate, effective suppression of expansion, and ultra-low filtration loss, meeting the demands of complex shale oil well production conditions, improving production efficiency and safety, and reducing reservoir damage. Specifically, this invention provides the following technical solutions to address the aforementioned technical problems:

[0007] A solids-free kill fluid for shale oil reservoirs comprises the following raw materials: 50-70 parts by weight of water, 10-30 parts by weight of density regulator, 3-6 parts by weight of copolymer thickener, 1-3 parts by weight of dispersant, 3-7 parts by weight of filtration reducer, and 2-4 parts by weight of corrosion inhibitor; the copolymer thickener is obtained by free radical initiation copolymerization of acrylamide, quaternary ammonium salt monomer, unsaturated silane coupling agent, and polyethylene glycol diacrylamide, and the number average molecular weight of the copolymer thickener is 1.7 million to 2.4 million; the dispersant comprises fumed silica and fluorinated surfactant, with the fumed silica accounting for 10-15% by weight.

[0008] The inventors discovered that the copolymer thickener prepared from the aforementioned monomers has multiple functions. Besides its inherent thickening effect, the introduction of functional monomers, quaternary ammonium salt cationic polymers, and unsaturated silane coupling agents endows the thickener with new functions. The quaternary ammonium salt groups provide positive charges, increasing the electrostatic repulsion between molecular chains. Even at high temperatures, this repulsion helps resist the entanglement and depolymerization of molecular chains, maintaining the viscosity of the solution and preventing coiling and breakage caused by thermal motion at high temperatures. Polyethylene glycol diallylamide, as a crosslinking agent, forms a lightly crosslinked polymer with a certain three-dimensional network structure, which also helps maintain the thickening effect at high temperatures. However, the amount of crosslinking agent used must be appropriate; insufficient or excessive crosslinking is not conducive to maintaining the excellent overall performance of the kill fluid. The introduction of quaternary ammonium salt groups also provides excellent clay swelling inhibition ability, especially after high temperatures, it still has sufficient ability to inhibit clay swelling. Since shale oil reservoirs are generally buried at great depths, they may experience high-temperature environments exceeding 130°C, and some even exceeding 160°C. The copolymer thickener of this invention can improve the shale rolling recovery rate of the kill fluid after high-temperature exposure. Conventional kill fluids typically add polyquaternary ammonium salts to prevent clay swelling. However, polyquaternary ammonium salts are prone to failure at high temperatures and can cause significant formation damage. The copolymer of this invention has a high molecular weight, and even with some degradation at high temperatures, it retains longer polymer chain segments, effectively maintaining its inhibitory effect on clay swelling. The copolymer thickener of this invention also provides a certain degree of corrosion inhibition. This is likely due to the electrostatic forces of the quaternary ammonium salt, where the polymer adsorbs onto the metal surface to form an isolation layer. The nitrogen atoms in the quaternary ammonium salt provide lone pairs of electrons, coordinating with empty d orbitals in the metal, promoting the formation of a passivation film. The introduction of silanoxy groups into unsaturated silane coupling agents can further improve the high-temperature resistance, clay stability, and corrosion inhibition properties of the kill fluid. The inventors believe this is likely because the silanols produced by the hydrolysis of silanoxy groups can undergo a condensation reaction with clay minerals at high temperatures, physically locking the clay sheets and preventing their hydration, expansion, and dispersion. Similarly, the formed silanol groups adsorb onto the metal surface, and the hydrophobic alkyl chains of the silane coupling agent form a dense hydrophobic barrier, blocking corrosive media. Finally, siloxanes and fumed silica form a high-temperature stable gel network, further enhancing the high-temperature resistance of the kill fluid. However, the amount and specific surface area of ​​fumed silica need to be controlled; otherwise, it is easy to form precipitates that are difficult to depolymerize, leading to abnormal system viscosity, dispersant failure, and increased risk of formation pore throat blockage. However, it is necessary to control the molecular weight of the copolymer thickener, with a number-average molecular weight in the range of 1.7 million to 2.4 million, to achieve the multi-functional purpose of the copolymer thickener. If the molecular weight is too low, it cannot achieve high-temperature resistance, nor can it impart corrosion inhibition and clay stabilization effects to the copolymer as a thickener; if the molecular weight is too high, it is prone to flocculation and failure. The process parameters for adjusting the molecular weight are well known in the art, such as the amount of initiator, chain transfer agent, polymerization temperature, and polymerization time.

[0009] Further, the unsaturated silane coupling agent is selected from at least one of vinyltriethoxysilane, vinyltrimethoxysilane, vinyltriisopropoxysilane, and vinyltri(β-methoxyethoxy)silane; the quaternary ammonium salt monomer is selected from at least one of dimethyl diallyl ammonium chloride, acryloyloxyethyltrimethyl ammonium chloride, methacryloyloxyethyltrimethyl ammonium chloride, methacryloyloxyethyldimethylbenzyl ammonium chloride, and acrylamidepropyltrimethyl ammonium chloride; the number average molecular weight of the polyethylene glycol (PEG) segment in polyethylene glycol diacrylamide is 200-400.

[0010] Preferably, the quaternary ammonium salt-containing monomer is a blend of cyclic and chain quaternary ammonium salt monomers in a molar ratio of 1:6-10. The cyclic quaternary ammonium salt monomer is dimethyl diallyl ammonium chloride, and the chain quaternary ammonium salt monomer is selected from at least one of acryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyltrimethylammonium chloride, methacryloyloxyethyldimethylbenzylammonium chloride, and acrylamidopropyltrimethylammonium chloride. The inventors unexpectedly discovered that the blend of cyclic and chain quaternary ammonium salt monomers can simultaneously improve various properties of high-pressure well fluids.

[0011] Furthermore, the molar ratio of acrylamide, quaternary ammonium salt monomer, unsaturated silane coupling agent, and polyethylene glycol diacrylamide is 60-90:20-30:6-10:2-5.

[0012] Further, the copolymer tackifier is prepared by a method including the following steps: under an inert atmosphere, a mixture of monomers, namely acrylamide, quaternary ammonium salt-containing monomers, unsaturated silane coupling agent, and polyethylene glycol diacrylamide, along with a cosolvent and an alcohol solvent, is added to water and mixed evenly. Then, 30-50 wt% of a composite initiator system aqueous solution and 40-60 wt% of bisulfite are slowly added, and the temperature is controlled at 5-20°C for low-temperature free radical polymerization. Afterward, the temperature is raised to 50-60°C, and the remaining composite initiator system aqueous solution and bisulfite are added in 1-3 batches to continue the polymerization reaction. After the polymerization reaction is completed, the mixture is cooled, precipitated with alcohol, and dried to obtain the copolymer tackifier. The composite initiator system aqueous solution contains persulfate and azo hydrochloride as compound initiators.

[0013] Furthermore, the inert atmosphere is nitrogen and / or argon; the aqueous solution of the composite initiation system contains 2-5 wt% persulfate, 1-2 wt% azo hydrochloride, and 0.5-1 wt% complexing agent; the amount of the aqueous solution added to the composite initiation system is such that the persulfate is 1.8-2.6 wt% of the mass of the mixed monomers; the amount of water added is 5-10 times the mass of the mixed monomers; the persulfate is selected from at least one of sodium persulfate, potassium persulfate, and ammonium persulfate; the azo hydrochloride is selected from at least one of azobisisobutyramidine hydrochloride, 2,2-azo(2-amidinylpropane) dihydrochloride, and 2,2-azo[2-(2-imidazoline-2-yl)propane] dihydrochloride; the complexing agent is selected from at least one of disodium ethylenediaminetetraacetate and dipotassium ethylenediaminetetraacetate; the bisulfite is selected from at least one of sodium bisulfite and potassium bisulfite, and the amount of bisulfite added is 1-2 wt% of the mass of the mixed monomers. In the composite initiation system, the two initiators work synergistically and are added in batches to ensure high initiation activity in both the early and late stages of polymerization, resulting in copolymers with high molecular weights. The co-solvent is selected from at least one of urea, acetamide, and triethylamine, and the amount added is 10-15 wt% of the mixed monomers. The alcohol solvent is selected from at least one of ethanol, isopropanol, and ethylene glycol, and the amount added is 20-35 wt% of the mixed monomers. The low-temperature free radical polymerization reaction takes 2-3 hours, after which the remaining aqueous solution of the composite initiation system is added in 1-3 batches, with each batch differing by no more than 20 wt% and with an interval of 15-30 minutes between batches. After heating to 50-60°C, the reaction continues for a total time of 4-6 hours. The slow addition occurs dropwise over 0.5-1 hour; alcohol precipitation is performed using anhydrous methanol or anhydrous ethanol.

[0014] Furthermore, the dispersant is a compound of fumed silica and a nonionic fluorinated surfactant, wherein the fumed silica accounts for 10-15% by mass. Even further, the specific surface area of ​​the fumed silica is 130-180 m². 2 / g, wherein the nonionic fluorinated surfactant is selected from at least one of perfluorooctyl sulfonamide, perfluorodecyl sulfonamide, perfluorododecyl sulfonamide, and ammonium perfluorooctanoate.

[0015] Furthermore, the filtration loss reducing agent is selected from at least one of hydroxypropyl starch, carboxymethyl starch, carboxymethyl cellulose, and sulfonated phenolic resin. Note that the filtration loss reducing agent used in this invention cannot be a sulfonate copolymer, because it contains anion and will bind to copolymers containing quaternary ammonium salt cations through electrostatic interactions, thus losing its effect. Similarly, anionic surfactants should not be used.

[0016] Furthermore, the corrosion inhibitor is selected from at least one of imidazoline, benzotriazole, 1,3-dimethyl-2-imidazoline, 5-mercaptobenzimidazole, and 2-alkyl-N-hydroxyethyl-N-hydroxypropylsulfonylimidazoline.

[0017] Furthermore, the density regulator is selected from at least one of calcium bromide, calcium chloride, sodium bromide, potassium bromide, sodium formate, sodium acetate, potassium formate, potassium acetate, cesium formate, and cesium acetate. The dosage of the density regulator can be flexibly adjusted according to the actual density required by the well control fluid, and can be between 1.4 and 2.2 g / cm³. 3 Adjustments will be made within the specified range.

[0018] The present invention also provides a method for preparing the above-mentioned solids-free kill fluid for shale oil reservoirs, comprising the following steps: adding water, density regulator, copolymer thickener, dispersant, filtration loss reducer, and corrosion inhibitor into a mixing device and mixing them evenly.

[0019] Furthermore, the preparation method of the solid-free kill fluid for shale oil reservoirs includes the following steps: adding water and dispersant to a high-speed stirred tank, stirring evenly, then adding copolymer thickener, dispersant, filtration loss reducer, corrosion inhibitor, and density regulator, stirring evenly to obtain the solid-free kill fluid for shale oil reservoirs.

[0020] Furthermore, the stirring speed is 200-800 rpm, preferably 300-500 rpm; the stirring time is 0.5-5 h, preferably 1-3 h. When the mixture is homogeneous, ultrasonic dispersion at 20-100 kHz can also be applied.

[0021] Compared with the prior art, the present invention achieves the following beneficial effects:

[0022] This invention utilizes a self-made copolymer thickener, obtained by copolymerizing four monomers. This copolymer thickener provides multiple benefits, enhancing viscosity while simultaneously improving the high-temperature resistance, clay stability, and corrosion inhibition of the kill fluid. After being hot-rolled at 180°C for 24 hours, the kill fluid of this invention showed no significant adverse effects on apparent viscosity, filtration loss, or clay swelling inhibition rate. The kill fluid of this invention exhibits a reservoir damage rate of <6%, effectively protecting the reservoir. It possesses excellent rheological, thermal stability, filtration loss, inhibition, lubrication, corrosion resistance, and salt resistance properties, comprehensively improving extraction efficiency. It contains no harmful substances, has a low corrosion rate, and protects downhole equipment and the environment. The preparation process of the kill fluid of this invention is simple to operate and easy to industrialize and control in terms of quality. Detailed Implementation

[0023] The present invention will now be described in detail with reference to specific embodiments.

[0024] Hydroxypropyl starch was purchased from Henan Hengrui Starch Technology Co., Ltd.

[0025] Preparation Example 1

[0026] (T1) Prepare an aqueous solution containing 3 wt% sodium persulfate, 1.5 wt% azobisisobutyramidine hydrochloride, and 0.7 wt% disodium ethylenediaminetetraacetate as the aqueous solution of the composite initiation system;

[0027] (T2) Accurately weigh acrylamide, quaternary ammonium salt monomer (a mixture of dimethyl diallyl ammonium chloride and acryloyloxyethyl trimethyl ammonium chloride in a molar ratio of 1:6), vinyltriethoxysilane, and polyethylene glycol diacrylamide (PEG segment number average molecular weight 200) in a molar ratio of 60:20:10:3 and mix them as mixed monomers;

[0028] (T3) Add the mixed monomers to 6 times their mass of water, purge with nitrogen to remove air, add 10 wt% urea as a co-solvent, add 30 wt% ethanol as a co-solvent, mix well, slowly add the aqueous solution of the composite initiation system, the amount of the aqueous solution of the composite initiation system added is such that sodium persulfate is 0.9 wt% of the mass of the mixed monomers, add 0.6 wt% sodium sulfite and sodium hydrogen persulfite as a co-solvent, control the temperature at 10±2℃, and carry out the low-temperature free radical polymerization reaction for 2 hours. The temperature was then raised to 60°C, and the aqueous solution of the composite initiation system and sodium bisulfite were added in two batches. The amount of the aqueous solution added in each batch was such that sodium persulfate was 0.45 wt% of the mixed monomers and sodium bisulfite was 0.3 wt% of the mixed monomers. That is, the total amount of the aqueous solution added in the composite initiation system met the requirement that sodium persulfate was 1.8 wt% of the mixed monomers and sodium bisulfite was 1.2 wt% of the mixed monomers. Each batch was added over a period of 20 min, with a 30 min interval between batches. After raising the temperature to 60°C, the reaction was continued for 5 h. After cooling, the mixture was precipitated with ethanol to obtain a viscous liquid. This liquid was dissolved in water and precipitated with ethanol again. This process was repeated twice. The final precipitate yielded a viscous liquid, which was then vacuum dried to obtain the copolymer thickener. Its number average molecular weight was tested to be 2.45 million.

[0029] Preparation Example 2

[0030] Step (T1) is the same as in Preparation Example 1;

[0031] (T2) Accurately weigh acrylamide, quaternary ammonium salt monomer (a mixture of dimethyl diallyl ammonium chloride and methacryloyloxyethyl dimethyl benzyl ammonium chloride in a molar ratio of 1:10), vinyltrimethoxysilane, and polyethylene glycol diacrylamide (PEG segment number average molecular weight 400) in a molar ratio of 90:30:6:3 and mix them as mixed monomers;

[0032] (T3) Add the mixed monomers to 6 times their mass of water, purge with nitrogen to remove air, add 10 wt% urea as a co-solvent, add 30 wt% ethanol as a co-solvent, mix well, slowly add the aqueous solution of the composite initiation system, the amount of the aqueous solution of the composite initiation system added is such that sodium persulfate is 1.3 wt% of the mass of the mixed monomers, add 1 wt% sodium sulfite and sodium hydrogen persulfite as a co-solvent, control the temperature at 10±2℃, and carry out the low-temperature free radical polymerization reaction for 2 hours. The temperature was then raised to 60°C, and the aqueous solution of the composite initiator system and sodium bisulfite were added in two batches. The amount of the aqueous solution added in each batch was such that sodium persulfate was 0.65 wt% of the mixed monomers and sodium bisulfite was 0.5 wt% of the mixed monomers. That is, the total amount of the aqueous solution added in the composite initiator system met the requirement that sodium persulfate was 2.6 wt% of the mixed monomers and sodium bisulfite was 2 wt% of the mixed monomers. Each batch was added over a period of 20 min, with a 30 min interval between batches. After raising the temperature to 60°C, the reaction was continued for 5 h. After cooling, the mixture was precipitated with ethanol to obtain a viscous liquid. This liquid was dissolved in water and precipitated with ethanol again. This process was repeated twice. The final precipitate yielded a viscous liquid, which was then vacuum dried to obtain the copolymer thickener. Its number average molecular weight was measured to be 1.72 million.

[0033] Preparation Example 3

[0034] The other conditions were the same as in Preparation Example 1, except that in step (T2), all quaternary ammonium salt monomers were dimethyl diallyl ammonium chloride. The final copolymer tackifier had a number-average molecular weight of 1.88 million.

[0035] Preparation Example 4

[0036] The other conditions were the same as in Preparation Example 2, except that in step (T2), all quaternary ammonium salt monomers were methacryloyloxyethyl dimethylbenzylammonium chloride. The final copolymer tackifier had a number-average molecular weight of 2.36 million.

[0037] Comparative Preparation Example 1

[0038] The other conditions are the same as in Preparation Example 1, except that no quaternary ammonium salt monomer is added in step (T1).

[0039] Comparative Preparation Example 2

[0040] The other conditions are the same as in Preparation Example 1, except that vinyltriethoxysilane is not added in step (T1).

[0041] Comparative preparation example 3

[0042] The other conditions are the same as in Preparation Example 1, except that polyethylene glycol diacrylamide is not added in step (T1). Example 1

[0043] Mix 60 parts by weight of water and 2.5 parts by weight of dispersant (BET 160 m). 2 A mixture of fumed silica and perfluorooctyl sulfonamide (10 wt%) was added to a high-speed stirred tank and stirred until homogeneous. Then, 3.7 parts by weight of the copolymer thickener prepared in Example 1, 4.9 parts by weight of the filtration loss reducer hydroxypropyl starch, 2.7 parts by weight of the corrosion inhibitor benzotriazole, 12 parts by weight of sodium formate, and 5 parts by weight of calcium bromide were added. After all materials were added, the mixture was stirred until homogeneous to obtain a solids-free well-killing fluid for shale oil reservoirs. Example 2

[0044] The other conditions were the same as in Example 1, except that the copolymer tackifier prepared in Preparation Example 1 was replaced with an equal mass of the copolymer tackifier prepared in Preparation Example 2. Example 3

[0045] The other conditions were the same as in Example 1, except that the copolymer tackifier prepared in Preparation Example 1 was replaced with an equal mass of the copolymer tackifier prepared in Preparation Example 3. Example 4

[0046] The other conditions were the same as in Example 1, except that the copolymer tackifier prepared in Preparation Example 1 was replaced with an equal mass of the copolymer tackifier prepared in Preparation Example 4. Example 5

[0047] Mix 70 parts by weight of water and 2.5 parts by weight of dispersant (BET 180 m). 2 A mixture of fumed silica and perfluorodecyl sulfonamide (15 wt%) was added to a high-speed stirred tank and stirred until homogeneous. Then, 4.5 parts by weight of the copolymer thickener prepared in Example 1, 6.1 parts by weight of the filtration loss reducer hydroxypropyl starch, 3.2 parts by weight of the corrosion inhibitor benzotriazole, 13.7 parts by weight of sodium formate, and 6.2 parts by weight of calcium bromide were added. After all materials were added, the mixture was stirred until homogeneous to obtain a solids-free well-killing fluid for shale oil reservoirs. Comparative Example 1

[0048] The other conditions were the same as in Example 1, except that the copolymer tackifier prepared in Example 1 was replaced with an equal mass of the copolymer tackifier prepared in Comparative Example 1. Comparative Example 2

[0049] The other conditions were the same as in Example 1, except that the copolymer tackifier prepared in Example 1 was replaced with an equal mass of the copolymer tackifier prepared in Comparative Example 2. Comparative Example 3

[0050] The other conditions were the same as in Example 1, except that the copolymer tackifier prepared in Example 1 was replaced with an equal mass of the copolymer tackifier prepared in Comparative Example 3. Comparative Example 4

[0051] The other conditions are the same as in Example 1, except that fumed silica accounts for 8.5 wt% in the dispersant. Comparative Example 5

[0052] The other conditions are the same as in Example 1, except that fumed silica accounts for 17.3 wt% in the dispersant.

[0053] Application examples

[0054] The performance of the kill fluids from the above examples and comparative examples was tested. The results are shown in Tables 1 and 2 below.

[0055] Core samples were selected for permeability recovery rate analysis, with pre-damage permeability ranging from 16.0 to 21.0. -3 μm 2 Within the range.

[0056] Hot rolling is the process of hot rolling the kill fluid in a high-pressure sealed tank at a high temperature of 3.5 MPa and 180°C for 24 hours.

[0057] The corrosion rate was tested using the static plate weight loss method. The corrosion conditions were 180℃ for 7 days, and the corrosion test sample was a 50mm×15mm×2mm steel sheet.

[0058] The HTHP high-temperature and high-pressure filtration loss was tested at 180℃ and 3.5MPa.

[0059] Table 1. Well Killing Fluid Performance Test I

[0060] Kill fluid Permeability recovery rate (%) Apparent viscosity before hot rolling (mPa·s) Apparent viscosity after hot rolling (mPa·s) Filtration loss before hot rolling (mL, HTHP) Filtration loss after hot rolling (mL, HTHP) Example 1 96.5 43.8 38.6 4.4 5.8 Example 2 97.3 38.4 34.2 5.3 6.8 Example 3 95.4 42.1 33.6 6.5 7.8 Example 4 96.2 44.0 31.2 5.1 9.4 Example 5 95.8 44.2 32.4 5.3 6.5 Comparative Example 1 97.5 42.1 24.3 5.2 14.6 Comparative Example 2 96.4 43.5 20.1 6.5 19.2 Comparative Example 3 97.0 38.5 19.0 6.0 12.7 Comparative Example 4 97.5 42.6 18.7 4.8 14.2 Comparative Example 5 94.2 44.5 35.1 4.3 6.5

[0061] Table 2 Well Killing Fluid Performance Test II

[0062] Kill fluid Corrosion rate before hot rolling (mm / a) Corrosion rate after hot rolling (mm / a) Shale recovery rate before hot rolling (%) Shale recovery rate after hot rolling (%) Example 1 0.35 0.41 99.6 94.2 Example 2 0.37 0.44 99.2 92.7 Example 3 0.41 0.50 99.5 93.1 Example 4 0.35 0.57 99.5 90.6 Example 5 0.37 0.48 99.4 93.5 Comparative Example 1 0.63 0.81 95.3 84.7 Comparative Example 2 0.35 0.75 98.2 82.9 Comparative Example 3 0.36 0.63 97.4 83.2 Comparative Example 4 0.40 0.77 99.3 81.5 Comparative Example 5 0.35 0.59 96.1 86.0

Claims

1. A solids-free kill fluid for shale oil reservoirs, characterized in that, The product comprises the following raw materials: 50-70 parts by weight of water, 10-30 parts by weight of density regulator, 3-6 parts by weight of copolymer tackifier, 1-3 parts by weight of dispersant, 3-7 parts by weight of filtration loss reducer, and 2-4 parts by weight of corrosion inhibitor; the copolymer tackifier is obtained by free radical initiation copolymerization of acrylamide, quaternary ammonium salt-containing monomers, unsaturated silane coupling agents, and polyethylene glycol diacrylamide, and the number average molecular weight of the copolymer tackifier is 1.7 million to 2.4 million; the dispersant comprises fumed silica nanoparticles and fluorinated surfactants, with the fumed silica nanoparticles accounting for 10-15% by weight; the unsaturated silane coupling agent is selected from at least one of vinyltriethoxysilane, vinyltrimethoxysilane, vinyltriisopropoxysilane, and vinyltris(β-methoxyethoxy)silane; the quaternary ammonium salt-containing monomers are used in the copolymer. The monomer is selected from at least one of dimethyl diallyl ammonium chloride, acryloyloxyethyl trimethyl ammonium chloride, methacryloyloxyethyl trimethyl ammonium chloride, methacryloyloxyethyl dimethyl benzyl ammonium chloride, and acrylamide propyl trimethyl ammonium chloride; the number average molecular weight of the polyethylene glycol segment in polyethylene glycol diacrylamide is 200-400; the molar ratio of acrylamide, quaternary ammonium salt monomer, unsaturated silane coupling agent, and polyethylene glycol diacrylamide is 60-90:20-30:6-10:2-5; the dispersant is a compound of fumed silica and nonionic fluorinated surfactant, wherein the mass percentage of fumed silica is 10-15%; the nonionic fluorinated surfactant is selected from at least one of perfluorooctyl sulfonamide, perfluorodecyl sulfonamide, perfluorododecyl sulfonamide, and perfluorooctanoic acid ammonium.

2. The solids-free kill fluid for shale oil reservoirs according to claim 1, characterized in that, The quaternary ammonium salt monomer is a compound of cyclic quaternary ammonium salt monomer and chain quaternary ammonium salt monomer in a molar ratio of 1:6-10. The cyclic quaternary ammonium salt monomer is dimethyl diallyl ammonium chloride, and the chain quaternary ammonium salt monomer is selected from at least one of acryloyloxyethyltrimethyl ammonium chloride, methacryloyloxyethyltrimethyl ammonium chloride, methacryloyloxyethyldimethylbenzyl ammonium chloride, and acrylamidopropyltrimethyl ammonium chloride.

3. The solids-free kill fluid for shale oil reservoirs according to claim 1, characterized in that, The copolymer tackifier is prepared by a method comprising the following steps: under an inert atmosphere, a mixture of monomers, namely acrylamide, quaternary ammonium salt-containing monomers, unsaturated silane coupling agent, and polyethylene glycol diacrylamide, along with a cosolvent and an alcohol solvent, is added to water and mixed thoroughly. Then, 30-50 wt% of a composite initiator aqueous solution and 40-60 wt% of bisulfite are slowly added, and the temperature is controlled at 5-20°C for free radical polymerization. Afterward, the temperature is raised to 50-60°C, and the remaining composite initiator aqueous solution and bisulfite are added in 1-3 batches to continue the polymerization reaction. After the polymerization reaction is completed, the mixture is cooled, precipitated with alcohol, and dried to obtain the copolymer tackifier. The composite initiator aqueous solution contains persulfate and azo hydrochloride as compound initiators.

4. The solids-free kill fluid for shale oil reservoirs according to claim 3, characterized in that, The aqueous solution of the composite initiation system contains 2-5 wt% persulfate, 1-2 wt% azo hydrochloride, and 0.5-1 wt% complexing agent. The amount of persulfate added to the aqueous solution of the composite initiation system is such that the persulfate is 1.8-2.6 wt% of the mass of the mixed monomers. The persulfate is selected from at least one of sodium persulfate, potassium persulfate, and ammonium persulfate. The azo hydrochloride is selected from at least one of azobisisobutyramidine hydrochloride, 2,2-azo(2-amidinylpropane) dihydrochloride, and 2,2-azo[2-(2-imidazolin-2-yl)propane] dihydrochloride. The complexing agent is selected from at least one of disodium ethylenediaminetetraacetate and dipotassium ethylenediaminetetraacetate. The bisulfite is selected from at least one of sodium bisulfite and potassium bisulfite, and the amount of bisulfite added is 1-2 wt% of the mass of the mixed monomers.

5. The solids-free kill fluid for shale oil reservoirs according to claim 3, characterized in that, The co-solvent is selected from at least one of urea, acetamide, and triethylamine, and the amount of co-solvent added is 10-15 wt% of the mass of the mixed monomers; and / or, The alcohol solvent is selected from at least one of ethanol, isopropanol, and ethylene glycol, and the amount of alcohol solvent added is 20-35 wt% of the mass of the mixed monomers; and / or, The free radical polymerization reaction takes 2-3 hours. Then, the remaining aqueous solution of the composite initiator system is added in 1-3 batches, with the amount added in each batch not exceeding 20 wt% and the interval between each batch being 15-30 minutes. After heating to 50-60℃, the reaction continues for a total time of 4-6 hours.

6. The solids-free kill fluid for shale oil reservoirs according to claim 1, characterized in that, The filtration loss reducing agent is selected from at least one of hydroxypropyl starch, carboxymethyl starch, carboxymethyl cellulose, and sulfonated phenolic resin; and / or, The corrosion inhibitor is selected from at least one of imidazoline, benzotriazole, 1,3-dimethyl-2-imidazolineone, 5-mercaptobenzimidazole, and 2-alkyl-N-hydroxyethyl-N-hydroxypropylsulfonylimidazoline; and / or, The density regulator is selected from at least one of calcium bromide, calcium chloride, sodium bromide, potassium bromide, sodium formate, sodium acetate, potassium formate, potassium acetate, cesium formate, and cesium acetate.

7. The solids-free kill fluid for shale oil reservoirs according to claim 6, characterized in that, The specific surface area of ​​fumed silica is 130-180 m². 2 / g.

8. The method for preparing solids-free kill fluid for shale oil reservoirs according to any one of claims 1-7, characterized in that, Includes the following steps: Add water, density regulator, copolymer thickener, dispersant, filtration loss reducer, and corrosion inhibitor to the mixing equipment and mix thoroughly.