Preparation method of high-salt-resistant ultra-deep low-pressure oil and gas well workover fluid

Abstract

The invention discloses a preparation method of a high-salt-resistant ultra-deep low-pressure oil and gas well workover fluid, which is characterized by comprising the following steps: adding 10 parts by weight of nano SiO2 and 90-300 parts by weight of solvent into a three-mouth reaction bottle with a reflux condensing tube, stirring and dispersing the nano SiO2 at room temperature, then heating to 60-85 DEG C, adding 1.0-8 parts by weight of benzene ring-containing anion modifier, reacting for 6-24 hours under reflux, and cooling to room temperature to obtain the high-salt-resistant ultra-deep low-pressure oil and gas well workover fluid. Then centrifuging to remove the solvent and the unreacted modifier in the reaction system, and carrying out vacuum drying on the product under heating to obtain nano SiO2-BA containing heat-resistant benzene rings and anions; the preparation method comprises the following steps: pouring 1.0-40 parts of nano SiO2-BA into 1000 parts of a solvent at room temperature, stirring until the nano SiO2-BA is uniformly dispersed, then pouring 1.0-20 parts of a thickening agent into a nano SiO2-BA system, stirring until the thickening agent is completely dissolved, then sequentially adding 3.0-90 parts of a polymer stabilizer, 0.1-8 parts of a bactericide and 0.1-8 parts of a heat stabilizer into the thickening agent solution while stirring, and uniformly stirring to obtain a finished product. The workover fluid capable of resisting high salt and multivalent salt at the ultrahigh temperature of 180-240 DEG C is obtained.

Description

Technical Field

[0001] The invention relates to a method for preparing a high-salt resistant ultra-deep low-pressure oil and gas well repair fluid, and belongs to the fields of petroleum extraction, oilfield chemistry and oilfield polymers. Background Art

[0002] At present, the exploitation of ultra-deep oil and gas with a depth of more than 8,000 m has become an important oil and gas resource channel in my country. Therefore, in order to maintain the normal production of oil and gas wells, the number of ultra-deep oil and gas well workover operations is also increasing. Workover fluids that can resist high salt and ultra-high temperatures are the basis for the success of ultra-deep oil and gas well workover operations. During workover construction, the role of the workover fluid is to balance the formation pressure and carry cuttings back to clean the wellbore. This requires low leakage of the workover fluid and also requires that the workover fluid is harmless to the reservoir. The oil and gas wells in Shunbei Oil and Gas Field in my country are all ultra-deep oil and gas wells with a depth of 8,000 m and above; the mineralization of the formation water at the bottom of the well is high, at 22×10 4 mg / L and above, among which the total content of multivalent salts CaCl2 and MgCl2 is high, which is 4.0×10 4 mg / L or higher, also containing trace amounts of FeCl2 and / or FeCl3; bottomhole temperatures of 160°C or higher, with some wells exceeding 210°C; and the vast majority of wells are low-pressure, with an average formation pressure coefficient of 0.65. Commonly used workover fluids in such demanding workover environments present numerous challenges, including the following. The most commonly used water or salt water workover fluid on site has extremely low viscosity and extremely serious leakage. It cannot circulate in the oil pipe and annulus, making it difficult to carry the rock cuttings at the bottom of the well to the surface; the rock carrying capacity of the solid-phase mud workover fluid is improved, but due to the effect of positive pressure difference, the workover fluid also has serious leakage, which causes the solid phase particles in the system to block the seepage channel, causing damage to the reservoir, and the production capacity cannot be fully restored. The oil and gas production capacity may even drop by 30% to 50% after the well is repaired; the commonly used thickened polymer workover fluid has the advantages of low leakage and low damage. The commonly used polymers are xanthan gum or water-soluble modified cellulose ethers such as sodium carboxymethyl cellulose, sodium polyanionic cellulose, and hydroxyethyl cellulose. Among these polymers, xanthan gum has the strongest salt and temperature resistance, but its resistance to high salt and high temperature, especially its resistance to multivalent salts at high temperature, is limited. Xanthan gum undergoes severe thermal degradation at 120°C, resulting in a significant decrease in the viscosity of the system. If the temperature exceeds 130°C, even if the workover fluid contains only 10 2 The workover fluid is completely ineffective with polyvalent salts such as CaCl2 and MgCl2 at mg / L level and has almost no viscosity. The water used to prepare the workover fluid on site is usually shallow water in the well area. Moreover, most ultra-deep oil and gas wells are located in deserts, and the salinity of shallow water in the well area is (1.0~3.5)×10 4 mg / L, the contents of CaCl2 and MgCl2 were 10 3The total content of polyvalent salts, CaCl₂ and MgCl₂, in bottomhole formation water is, as previously mentioned, even higher. While adding high-concentration organic salts, potassium or sodium formate, exceeding 30% by weight, to xanthan gum workover fluid can improve its resistance to polyvalent salts and temperature tolerance to a certain extent, low-pressure wells require the fluid density to be as low as possible, and excessively high concentrations of organic salts cannot be added to enhance salt and temperature tolerance. Therefore, the current workover fluid cannot meet the workover operation requirements of ultra-deep low-pressure oil and gas wells. Research is being conducted to develop a high-salinity-resistant ultra-deep low-pressure oil and gas well polymer workover fluid that can maintain good viscosity-increasing and anti-aging properties when continuously circulated and heated in a high-salinity environment with a high temperature of 160°C to 240°C at the bottom of the well (temperatures exceeding 180°C are considered ultra-high temperatures) and a high-salinity environment where the mineralization of the formation water at the bottom of the well and the total content of polyvalent salts CaCl2 and MgCl2 are both high. This has practical application value in improving the workover operation efficiency of ultra-deep low-pressure oil and gas wells and ensuring the complete recovery of oil and gas well productivity after workover. Zhang Zhixuan et al., Contemporary Chemical Industry, 2022, 51(6), 1431−1434. The study found that the workover fluid with xanthan gum as a viscosifier has a strong sand-carrying and rock-carrying capacity and can be naturally degraded. In addition, it was used in 346 wells in the Sebei gas field from 2018 to 2021, where the formation pressure coefficient was between 0.72 and 1.00. However, the water used to prepare the workover fluid was fresh water, and the formation pressure coefficient was also higher than 0.70. The bottom hole temperature of the Sebei gas field was lower than 110 °C, and the mineralization of the bottom hole formation water was much lower than 22×10 4 mg / L. Zhao Yi, Research on Low Permeability Reservoir Protection Technology in Duanliubo Oilfield [D], 2016, Beijing: China University of Petroleum (Beijing), 0.4% xanthan gum was added to KCl brine base fluid to form a low-loss solid-free polymer well repair fluid. It was found that the filtration loss of the system was 14.4 mL at 90 °C and 3.5 MPa. In order to avoid the influence of multivalent salts on the viscosity of xanthan gum brine solution, only monovalent salt KCl was used in the system, and the experimental temperature was much lower than 160 °C. Xie Jun et al., Journal of Shandong University of Science and Technology, 2007, 26 (5), 16-20, reported that at 120 °C, a higher concentration of Ca 2+ This will cause precipitation in the xanthan gum solution, rendering the solution ineffective. Furthermore, the viscosity loss of the solution increases with the extension of aging time and the increase of temperature. Li Hongmei et al., Drilling and Completion Fluids, 2010, 27(6), 12−15, developed a low-damage, solid-free workover fluid using xanthan gum as a viscosity enhancer. Its API filtration loss is less than 8 mL. This workover fluid solves the problems of workover fluid leakage and severe liquid lock in the formation, but the formation temperature is only 100°C. Summary of the Invention

[0003] The purpose of the present invention is to enable a polymer solution free of solid phase to maintain good viscosity increasing and anti-aging properties when continuously circulated in a harsh environment of high temperature and ultra-high temperature at the bottom of the well of 160 ℃ to 240 ℃, and in a harsh environment where the mineralization of the formation water at the bottom of the well and the total content of polyvalent salts CaCl2 and MgCl2 are both high. The present invention provides a method for preparing a polymer well repair fluid for ultra-deep low-pressure oil and gas wells that can be used in ultra-deep low-pressure wells with a formation pressure coefficient of less than 1.0, and that still has good high-salt resistance at ultra-high temperature and ultra-high temperature bottom of the well of 160 ℃ and above, especially can resist high content of polyvalent salts CaCl2 and MgCl2 at ultra-high temperature, has low leakage, can be circulated for a long time, has strong sand washing and rock carrying ability, and does not contain solid materials that block the reservoir such as bentonite and sulfonated asphalt. The method is characterized by using sodium (x+1)-[4-(chloromethyl)phenyl]alkylate Cl(CH2)C6H4(CH2) x COONa, sodium (x+1)-[4-bromomethyl)phenyl]alkylate Br(CH2)C6H4(CH2) x COONa, sodium (x+1)-[4-iodomethyl)phenyl]alkylate I(CH2)C6H4(CH2) x At least one of COONa is a modifier, and x is an integer from 1 to 11. Nano-SiO2 is modified, and the particle size of nano-SiO2 is less than 100 nm. Modified nano-SiO2, namely nano-SiO2-BA (see attached Figure 1 ), then the nano-SiO2-BA is compounded with at least one of the heat-resistant polymers welan gum, diutan gum, gellan gum and sclerotin in at least one of clean water and oilfield well water, and at least one of trisodium citrate, sodium diethylenetriamine pentaacetate, tetrasodium ethylenediaminetetraacetate, trisodium nitrilotriacetate, sodium hexametaphosphate, sodium formate, potassium formate, sodium acetate, potassium acetate, sodium propionate, potassium propionate and sodium 2-hydroxypropionate is added as a polymer stabilizer, and benzene is added. A high-salt resistant workover fluid for ultra-deep low-pressure oil and gas well workover is prepared by adding at least one of phenol, catechol, hydroquinone, resorcinol, malondialdehyde, succinyldialdehyde, glutaraldehyde, adipaldehyde, sodium trichlorophenate, sodium disulfide carbamate and potassium disulfide carbamate as a bactericide, and adding at least one of sodium sulfite, potassium sulfite, sodium sulfide, potassium sulfide, sodium disulfite, potassium dithionite, sodium bisulfite, potassium bisulfite, thiourea and o-xylene thiourea as a heat stabilizer.

[0004] The inventors have found that the concentration of the thickener, the mass ratio of the modified nanomaterial to the polymer thickener, and the amount of each additive in the high-salinity resistant ultra-deep low-pressure oil and gas well workover fluid can significantly affect the viscosity, high-salinity resistance and anti-aging properties of the workover fluid.

[0005] The purpose of the present invention is achieved by the following technical measures, wherein the parts of the raw materials are all parts by weight unless otherwise specified.

[0006] 1. Preparation of Nano-SiO2-BA 10 parts of unmodified nano-SiO2 with an average particle size of less than 100 nm and 90 to 300 parts of solvent are added to a three-necked reaction flask equipped with a reflux condenser, stirred at room temperature for 0.5 hour to disperse the nano-SiO2, then heated to 60°C to 85°C, and 1.0 to 8 parts of an anion modifier containing a benzene ring are slowly added. The reaction is carried out under reflux for 6 to 24 hours, and then the reaction system is centrifuged to remove the solvent and the anion modifier that does not participate in the reaction. Finally, the product is vacuum dried at 55°C to 80°C to obtain modified nano-SiO2 containing heat-resistant benzene rings and anions, namely nano-SiO2-BA. The modification degree of the synthesized nano-SiO2-BA is 35% to 70%.

[0007] The solvent is at least one of N,N-dimethylformamide, triethylamine, xylene, benzene, toluene, pyridine, tetrahydrofuran, cyclohexane and ethyl acetate; the anion modifier is sodium (x+1)-[4-(chloromethyl)phenyl]alkylate Cl(CH2)C6H4(CH2) x COONa, sodium (x+1)-[4-bromomethyl)phenyl]alkylate Br(CH2)C6H4(CH2) x COONa, sodium (x+1)-[4-iodomethyl)phenyl]alkylate I(CH2)C6H4(CH2) x At least one of COONa, x is an integer from 1 to 11.

[0008] 2. The formula components of high-salinity resistant ultra-deep low-pressure oil and gas well workover fluid are as follows:

[0009] The solvent is at least one of clean water and water from a well in an oilfield; the thickener is at least one of temperature-resistant polymers including welan gum, diutan gum, gellan gum and sclerotin; the polymer stabilizer is at least one of trisodium nitrotriacetate, trisodium citrate, sodium diethylenetriamine pentaacetate, tetrasodium ethylenediaminetetraacetate, sodium hexametaphosphate, sodium formate, potassium formate, sodium acetate, potassium acetate, sodium propionate, potassium propionate and sodium 2-hydroxypropionate; the bactericide is at least one of catechol, hydroquinone, resorcinol, phenol, malondialdehyde, succinic dialdehyde, glutaraldehyde, adipaldehyde, sodium trichlorophenate, sodium disulfide carbamate and potassium disulfide carbamate; and the heat stabilizer is at least one of sodium sulfite, potassium sulfite, sodium sulfide, potassium sulfide, sodium disulfite, potassium dithionite, sodium bisulfite, potassium bisulfite, thiourea and o-xylene thiourea.

[0010] 3. Preparation of high-salinity resistant ultra-deep low-pressure oil and gas well workover fluid At room temperature, 1.0 to 40 parts of nano-SiO2-BA are slowly poured into 1000 parts of a stirring solvent, and stirred continuously until the dispersion is uniform. Then, 1.0 to 20 parts of a thickener are slowly poured into the stirring nano-SiO2-BA dispersion system, and stirred until the thickener is completely dissolved to obtain a polymer solution with thickening properties. Subsequently, 3.0 to 90 parts of a polymer stabilizer are added to the prepared polymer solution under stirring, and then 0.1 to 8 parts of a fungicide are added and continued to be stirred evenly. Finally, 0.1 to 8 parts of a heat stabilizer are added and stirred to obtain a density of 1.005 to 1.100 g / cm 3 It is an ultra-deep, low-pressure oil and gas well workover fluid that can withstand high salt and high content of polyvalent salts CaCl2 and MgCl2 in formation water at ultra-high bottomhole temperatures of 160°C to 240°C, does not contain solid materials such as bentonite and sulfonated asphalt, but has low leakage.

[0011] The solvent is at least one of clean water and water from a well in an oilfield; the thickener is at least one of temperature-resistant polymers including welan gum, diutan gum, gellan gum and sclerotin; the polymer stabilizer is at least one of trisodium nitrotriacetate, trisodium citrate, sodium diethylenetriamine pentaacetate, tetrasodium ethylenediaminetetraacetate, sodium hexametaphosphate, sodium formate, potassium formate, sodium acetate, potassium acetate, sodium propionate, potassium propionate and sodium 2-hydroxypropionate; the bactericide is at least one of catechol, hydroquinone, resorcinol, phenol, malondialdehyde, succinic dialdehyde, glutaraldehyde, adipaldehyde, sodium trichlorophenate, sodium disulfide carbamate and potassium disulfide carbamate; and the heat stabilizer is at least one of sodium sulfite, potassium sulfite, sodium sulfide, potassium sulfide, sodium disulfite, potassium dithionite, sodium bisulfite, potassium bisulfite, thiourea and o-xylene thiourea.

[0012] 4. Performance of high-salinity-resistant ultra-deep low-pressure oil and gas well workover fluid The following method for preparing a high-salinity-resistant, ultra-deep, low-pressure oil and gas well workover fluid sample is described in Example 1. It is important to note that the following description of the properties of the workover fluid sample prepared according to Example 1 is intended solely to further illustrate the present invention and is not to be construed as limiting the scope of protection of the present invention. Researchers in this field may make non-essential improvements and adjustments to the present invention based on the disclosure herein.

[0013] (1) The temperature resistance data of the high-salinity resistant ultra-deep low-pressure oil and gas well workover fluid are shown in Table 1. The temperature resistance of the workover fluid is the effect of temperature on the apparent viscosity. When the temperature is within the range of 30°C to 130°C, the apparent viscosity of the high-salinity resistant workover fluid increases as the temperature rises. When the temperature continues to rise, the apparent viscosity begins to decrease. The results show that the high-salinity resistant workover fluid has good temperature resistance and has little effect on the apparent viscosity of the high-salinity resistant workover fluid over the entire temperature range.

[0014] Table 1 Effect of temperature on the apparent viscosity of nano-SiO2-BA anti-high-salt workover fluid before aging

[0015] Note: Test shear rate: 170.3 s -1 (2) The aging resistance data of high-salt-resistant ultra-deep, low-pressure oil and gas well workover fluids at different temperatures are shown in Table 2. The aging resistance of the workover fluid is the change in apparent viscosity after a period of hot rolling at a constant temperature. The aging time for all samples was 16 h. This simulated evaluation of the high-salt-resistant ultra-deep, low-pressure oil and gas well workover fluids after continuous cyclic heating at ultra-high temperatures in the presence of multivalent salts CaCl2 and MgCl2, and their aging resistance at ultra-high temperatures and in the presence of multivalent salts.

[0016] 220 g of NaCl, 36 g of CaCl, 6 g of MgCl, and 12 mg of FeCl were added to the workover fluid sample from Example 1 and the unmodified nano-SiO sample, stirred and mixed, and then aged for 16 h at 140°C, 160°C, 200°C, and 240°C. The aging data are shown in Table 3. This simulates the evaluation of the high-salinity resistance and aging resistance of the high-salinity-resistant, ultra-deep, low-pressure oil and gas well workover fluid after continuous cyclic heating at the bottom of the well at ultra-high temperature and high salinity.

[0017] Table 2 shows that the viscosity retention rate of the high-salt resistant workover fluid containing nano-SiO2-BA after aging at 140°C for 16 h is 101.2%. After aging at ultra-high temperatures of 200°C and 240°C for 16 h, it still retains a high viscosity retention rate. This is because nano-SiO2-BA is more dispersed than unmodified nano-SiO2, and compared with the unheated system, the molecular chains of the heat-resistant polymer are more extended and the intermolecular aggregation structure is better after aging at 140°C for 16 h, resulting in a viscosity retention rate higher than 100%.

[0018] The results in Tables 2 and 3 show that at ultra-high temperatures, the dispersed nano-SiO2-BA particles containing benzene rings undergo chemical and physical compounding with the thickening polymer molecular chains in the system, making the composite system contain benzene rings with stable structure and strong intermolecular force, which increases the rigidity and heat resistance of the composite system. In the presence of high temperature, ultra-high temperature and high salt, the polymer can still form a good supramolecular aggregation structure in the well repair fluid, thereby enhancing the system's resistance to high salt and aging at ultra-high temperature, so that the well repair fluid still has high apparent viscosity and good sand washing and rock carrying performance at the bottom of the well at ultra-high temperature and high salt, and can resist Ca in formation water. 2+ Mg 2+ 、Fe 2+ and Fe 3+ and other multivalent metal cations.

[0019] Table 2 Anti-aging data of nano-SiO2-BA anti-high-salt workover fluid at different aging temperatures

[0020] Note: Test temperature: 30°C; shear rate: 170.3 s -1 ; Aging time: 16 h; The nanomaterial content in the two workover fluids in the table is 25 g / L, and the polymer, other additives and their respective addition amounts are the same.

[0021] Table 3 Anti-aging data of nano-SiO2-BA anti-high-salt workover fluid under high temperature, ultra-high temperature and high salt conditions

[0022] Note: Test temperature: 30°C; shear rate: 170.3 s -1 ; Aging time: 16 h; The nanomaterial content in the two workover fluids in the table is 25 g / L, and the polymer, other additives and their respective addition amounts are the same.

[0023] (3) The fluid loss reduction performance of the high-salinity ultra-deep low-pressure oil and gas well workover fluid before and after aging at different temperatures is shown in Table 4. 220 g of NaCl, 36 g of CaCl2, 6 g of MgCl2, and 12 mg of FeCl2 were added to the workover fluid sample of Example 1 and the unmodified nano-SiO2, respectively, and stirred and mixed. Then, the samples were aged at 140°C, 160°C, 200°C, and 240°C for 16 h. The API fluid loss data after aging are shown in Table 5. This is a simulation evaluation of the high-salinity resistance and fluid loss reduction performance of the high-salinity ultra-deep low-pressure oil and gas well workover fluid under ultra-high temperature and high salt conditions after continuous circulation heating at the bottom of the well.

[0024] Tables 4 and 5 show that the nano-SiO2-BA workover fluid still exhibits good fluid loss reduction performance after aging for 16 h under high salinity and high temperature and ultra-high temperature conditions. Compared with the unmodified nano-SiO2 composite workover fluid, the workover fluid has significantly lower bottom hole loss in high salinity and high temperature and ultra-high temperature conditions. This is because compared with unmodified nano-SiO2, nano-SiO2-BA can be better dispersed in the well repair fluid, and undergoes chemical and physical compounding with the heat-resistant polymer at high temperature and ultra-high temperature to form a composite material of inorganic nanomaterials and polymers. In addition, benzene rings are introduced into the composite system, which increases the rigidity and heat resistance of the composite system. A good polymer supramolecular aggregation structure can be formed under high salinity and high temperature and ultra-high temperature, so that the well repair fluid still has good filtration loss reduction properties under high salinity, high temperature and ultra-high temperature and ultra-deep low pressure. It can also have a strong shielding and temporary plugging ability for the well bottom under such a high positive pressure difference and harsh well bottom environment, so that the well repair fluid has lower leakage at the well bottom, and can realize the circulation of the well repair fluid in the wellbore and annulus of ultra-deep low-pressure wells, thereby carrying the rock cuttings to the ground.

[0025] Table 4 API filtration loss of samples after aging for 16 h at different aging temperatures

[0026] Note: Test temperature: 30°C; Test time: 0.5 h; The nanomaterial content in the two workover fluids in the table is 25 g / L, and the polymer, other additives and their respective addition amounts are the same.

[0027] Table 5 API filtration loss of samples after aging for 16 h at different aging temperatures and high salinity

[0028] Note: Test temperature: 30°C; Test time: 0.5 h; The nanomaterial content in the two workover fluids in the table is 25 g / L, and the polymer, other additives and their respective addition amounts are the same.

[0029] (4) After aging at 240℃ for 16 hours, the apparent viscosity of the high-salinity ultra-deep low-pressure oil and gas well workover fluid at different shear rates is shown in Table 6. 220g NaCl, 36g CaCl2, 6g MgCl2 and 12mg FeCl2 were added to the workover fluid sample in Example 1 and the sample containing unmodified nano-SiO2, respectively, and stirred and mixed. Then, the samples were aged at 240℃ for 16 hours. The apparent viscosity and rheological parameter data after aging are shown in Table 7. The flow index in Table 6 and Table 7 n The values ​​are all far below 1, and the consistency coefficient KThe values ​​are also relatively high, which shows that the nano-SiO2-BA and the heat-resistant polymer in the workover fluid undergo chemical and physical compounding at ultra-high temperature, so that the molecular structure of the heat-resistant polymer in the workover fluid remains relatively stable under high salt and ultra-high temperature conditions containing multivalent salts. The workover fluid also has good viscosity increasing and pseudoplasticity, which reduces its apparent viscosity and low friction at high shear rate in the wellbore. However, when the annular shear rate decreases, the apparent viscosity increases significantly due to recovery, which is beneficial to sand flushing and rock carrying.

[0030] Table 6 Apparent viscosity and rheological parameters of samples after aging at 240 ℃ for 16 h

[0031] Note: The test temperature is 30℃, the fluidity index n is a dimensionless quantity, the consistency coefficient K The unit is Pa·s n .

[0032] Table 7 Apparent viscosity and rheological parameters of samples after aging for 16 h at high salt and 240 °C

[0033] Note: The test temperature is 30℃, the fluidity index n is a dimensionless quantity, the consistency coefficient K The unit is Pa·s n .

[0034] 5. Advantages of high-salt resistant ultra-deep low-pressure oil and gas well workover fluid (1) Nano-SiO2-BA and heat-resistant polymers undergo chemical and physical compounding at high and ultra-high temperatures, making the workover fluid have good resistance to high salt at high and ultra-high temperatures. It can still show good viscosity increase, sand washing and rock carrying properties and filtration loss reduction properties when the salt content is high and the bottom hole temperature is 140℃~240℃. Moreover, it can resist high content of Ca in formation water at high and ultra-high temperatures. 2+ Mg 2+ 、Fe 3+ and Fe 2+ The surface of nano-SiO2-BA particles has a modified structure similar to that of surfactants, and also contains benzene ring groups with rigid structure and stable molecular structure, which makes its surface energy significantly lower than that of unmodified nano-SiO2, and its surface also carries anions - COO —The negative charge on the surface of the nanoparticles has an electrostatic repulsive force, which makes the nanoparticles more dispersed and less likely to agglomerate. Under high temperature and ultra-high temperature, the dispersed nano-SiO2-BA particles undergo chemical and physical compounding with the hydroxyl groups on the surface of the polymer molecular chain in the salt solution, and the modified benzene ring groups and -COO — Ions are also introduced into the composite system, which enhances the salt resistance, ultra-high temperature resistance, ultra-high temperature aging resistance and hydrophilicity of the composite system, so that the high-salt polymer system containing multivalent salts can still form a good supramolecular aggregation structure at high and ultra-high temperatures. This enables the composite well repair fluid to still have good viscosity enhancement, sand washing and rock carrying properties and temporary shielding and plugging effects at the bottom of the well at high salt and ultra-high temperature, thereby reducing the loss of the well repair fluid at the bottom of the ultra-deep low-pressure oil and gas reservoir, and allowing the well repair fluid to circulate in the wellbore and annulus to carry the cuttings to the ground.

[0035] (2) The heat-resistant polymers in the workover fluid, under the composite action of nano-SiO2-BA, are not only resistant to high salt content containing polyvalent salts, but also resistant to ultra-high temperatures and ultra-high temperature aging, and will not clog the reservoir. The polymers in the anti-high-salt workover fluid are all biopolymers, and their ultra-high temperature resistance and high temperature aging resistance are significantly stronger than the currently commonly used xanthan gum. However, without the composite action of nano-SiO2-BA, even if the content of polyvalent salts such as CaCl2 and MgCl2 in the workover fluid is low, the anti-aging temperature of these biopolymers is still below 140°C. Under the combined action of other additives, the biopolymers in the workover fluid can undergo chemical and physical composites with nano-SiO2-BA under high salt and ultra-high temperature conditions at the bottom of the well, which significantly improves the anti-aging temperature in the presence of high salt and polyvalent salts. Unlike synthetic polymers, these biopolymers will completely degrade naturally under high and ultra-high temperatures after completing their functions in the workover fluid system during well repair. The workover fluid circulates in the wellbore and annulus. Therefore, a small amount of biopolymers leaked into the formation will not cause damage to the reservoir, will not block the pores and throats of the reservoir, and of course it is impossible to block the micro-cracks in the formation.

[0036] (3) Nano-SiO2-BA can make the workover fluid resistant to high salt, high temperature and ultra-high temperature, but it will not damage the reservoir, and can fully restore the oil and gas production capacity of the reservoir after the workover. The particle size of nano-SiO2 particles with a particle size below 100 nm is modified by low molecular weight containing benzene rings. The particle size of nano-SiO2-BA is still below 100 nm. It also has a stable rigid structure at ultra-high temperature, and because of the introduction of benzene ring structure and anion -COO on its surface, it is easy to be oxidized. —, its rigidity is stronger. After chemical and physical compounding with the polymer in the well repair fluid at ultra-high temperature downhole, the rigidity of the polymer molecular chain is significantly enhanced, making its molecular structure and molecular chain conformation stable under the high salt and ultra-high temperature at the bottom of the well, and it can adapt to the harsh environment at the bottom of the well. However, the particle size of nano-SiO2-BA is much smaller than the pore throat size of the matrix, and it will not clog the formation like the mud that leaks into the reservoir. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 Schematic diagram of the molecular structure of nano-SiO2-BA. DETAILED DESCRIPTION

[0038] The present invention is described in detail below through examples. It is necessary to point out that the present examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Researchers in this field can make some non-essential improvements and adjustments to the present invention based on the above content of the present invention. Example

[0039] 10 grams of nano-SiO2 with an average particle size of 23 nm, 100 grams of toluene, and 55 grams of cyclohexane were added to a three-necked reaction flask equipped with a reflux condenser and stirred at room temperature for 0.5 hours to disperse the nano-SiO2. The temperature was then raised to 80°C, and 3.068 grams of anionic modifier 2-[4-bromomethyl)phenyl] sodium acetate was slowly added. The reaction was allowed to reflux for 16 hours, and the reaction system was subsequently centrifuged to remove toluene, cyclohexane, and unreacted anionic modifier. Finally, the product was vacuum-dried at 70°C to obtain modified nano-SiO2 containing heat-resistant benzene rings and anions, i.e., nano-SiO2-BA. The degree of modification of the synthesized nano-SiO2-BA was 61.3%.

[0040] At room temperature, 25 g of nano-SiO2-BA was slowly poured into 1000 g of stirring water from a water source well in an oil field well area, wherein the water from the water source well contained 19 g / L of sodium chloride, 7.0 g / L of calcium chloride, 2.0 g / L of magnesium chloride, and 8 mg / L of ferrous chloride, and stirred continuously until the dispersion was uniform. Then, 9 g of dimethicone and 4 g of welan gum were slowly poured into the stirring nano-SiO2-BA dispersion system and stirred until the thickener was completely dissolved to obtain a polymer solution with thickening properties. Subsequently, 68 g of potassium formate, 10 g of sodium propionate, 5 g of tetrasodium ethylenediaminetetraacetate, and 5 g of sodium diethylenetriaminepentaacetate were added to the prepared polymer solution under stirring, followed by 3 g of phenol, 0.5 g of adipaldehyde, and 1 g of potassium disulfide carbamate, and continued stirring. Finally, 1 g of sodium bisulfite, 0.5 g of potassium sulfide, and 1 g of thiourea were added and stirred to obtain a density of 1.086 g / cm 3It is an ultra-deep, low-pressure oil and gas well workover fluid that can withstand high salt and high content of multivalent salts CaCl2 and MgCl2 in formation water at ultra-high temperatures of 160℃~240℃ at the bottom of the well and has low leakage. Example

[0041] 10 grams of nano-SiO2 with an average particle size of 38 nm, 30 grams of tetrahydrofuran, 60 grams of toluene and 30 grams of N, N-dimethylformamide were added to a three-necked reaction flask equipped with a reflux condenser and stirred at room temperature for 0.5 hours to disperse the nano-SiO2. The temperature was then raised to 76°C and the anion modifiers 2.930 grams of 4-[4-bromomethyl)phenyl]sodium butyrate and 1.352 grams of 3-[4-chloromethyl)phenyl]sodium propionate were slowly added. The reaction was allowed to reflux for 20 hours. The reaction system was then centrifuged to remove tetrahydrofuran, toluene, N, N-dimethylformamide and the anion modifier that did not participate in the reaction. The product was finally dried in vacuum at 75°C to obtain modified nano-SiO2 containing heat-resistant benzene rings and anions, i.e., nano-SiO2-BA. The degree of modification of the synthesized nano-SiO2-BA was 65.3%.

[0042] At room temperature, 500 g of clean water was added to 500 g of water from a source well in an oil field and stirred to prepare brine. The water from the source well contained 22 g / L of sodium chloride, 9.0 g / L of calcium chloride, 3.0 g / L of magnesium chloride and 12 mg / L ferrous chloride, 28 g of nano-SiO2-BA was slowly poured into 1000 g of stirring brine, and stirred continuously until it was evenly dispersed. Then, 6 g of dimethicone, 3 g of gellan gum and 5 g of welan gum were slowly poured into the stirring nano-SiO2-BA dispersion system, and stirred until the thickener was completely dissolved to obtain a polymer solution with thickening properties. Subsequently, 50 g of sodium formate, 10 g of potassium acetate, 8 g of trisodium nitrilotriacetate and 5 g of sodium diethylenetriamine pentaacetate were added to the prepared polymer solution under stirring, and then 2 g of m-phenol, 1.0 g of glutaraldehyde and 0.5 g of sodium trichlorophenate were added, and the stirring was continued. Finally, 1 g of sodium disulfite, 0.5 sodium sulfide and 1 g of o-xylene thiourea were added and stirred to obtain a density of 1.078 g / cm 3 It is an ultra-deep, low-pressure oil and gas well workover fluid that can withstand high salt and high content of multivalent salts CaCl2 and MgCl2 in formation water at ultra-high temperatures of 160℃~240℃ at the bottom of the well and has low leakage. Example

[0043] 10 grams of nano-SiO2, 20 grams of ethyl acetate, 30 grams of toluene and 40 grams of xylene with an average particle size of 15 nm were added to a three-necked reaction flask equipped with a reflux condenser. The mixture was stirred at room temperature for 0.5 hours to disperse the nano-SiO2. The mixture was then heated to 80°C and slowly added with anionic modifiers 1.682 grams of 8-[4-iodomethyl)phenyl]sodium octanoate and 0.705 grams of 5-[4-bromomethyl)phenyl]sodium valerate. The mixture was reacted under reflux for 20 hours. The reaction system was then centrifuged to remove ethyl acetate, toluene, xylene and any anionic modifier that did not react. The product was finally dried under vacuum at 75°C to obtain a modified nano-SiO2 containing a heat-resistant benzene ring and anions, i.e., nano-SiO2-BA. The degree of modification of the synthesized nano-SiO2-BA was 56.8%.

[0044] At room temperature, 15 g of nano-SiO2-BA was slowly poured into 1000 g of stirring water while continuously stirring until uniformly dispersed. Then, 7 g of diurnal gum, 5 g of welan gum and 4 g of sclerotin were slowly poured into the stirring nano-SiO2-BA dispersion system and stirred until the thickener was completely dissolved to obtain a polymer solution with thickening properties. Subsequently, 50 g of potassium formate, 20 g of sodium acetate, 5 g of sodium hexametaphosphate, 7 g of tetrasodium ethylenediaminetetraacetic acid and 3 g of sodium diethylenetriaminepentaacetic acid were added to the prepared polymer solution under stirring, and then 1 g of m-phenol, 1 g of p-phenol and 1 g of glutaraldehyde were added and continued to stir evenly. Finally, 1 g of sodium sulfite, 0.5 g of sodium sulfide and 1 g of potassium dithionite were added and stirred to obtain a density of 1.067 g / cm 3 It is an ultra-deep, low-pressure oil and gas well workover fluid that can withstand high salt and high content of multivalent salts CaCl2 and MgCl2 in formation water at ultra-high temperatures of 160℃~240℃ at the bottom of the well and has low leakage. Example

[0045] 10 grams of nano-SiO2 with an average particle size of 63 nm, 30 grams of triethylamine, 120 grams of toluene and 60 grams of pyridine were added to a three-necked reaction flask equipped with a reflux condenser and stirred at room temperature for 0.5 hours to disperse the nano-SiO2. The temperature was then raised to 85°C and 5.172 grams of anionic modifier 12-[4-iodomethyl)phenyl]sodium laurylate were slowly added. The reaction was allowed to reflux for 24 hours. The reaction system was then centrifuged to remove triethylamine, toluene, pyridine and any anionic modifier that did not participate in the reaction. The product was finally dried in vacuo at 75°C to obtain modified nano-SiO2 containing heat-resistant benzene rings and anions, i.e., nano-SiO2-BA. The degree of modification of the synthesized nano-SiO2-BA was 60.5%.

[0046] At room temperature, 36 g of nano-SiO2-BA was slowly poured into 1000 g of stirring water while stirring continuously until uniformly dispersed. Then, 10 g of welan gum, 5 g of gellan gum and 3 g of sclerotin were slowly poured into the stirring nano-SiO2-BA dispersion system and stirred until the thickener was completely dissolved to obtain a polymer solution with thickening properties. Subsequently, 40 g of sodium formate, 10 g of sodium 2-hydroxypropionate, 3 g of sodium diethylenetriamine pentaacetate and 5 g of trisodium citrate were added to the prepared polymer solution under stirring, and then 3 g of p-phenol, 1 g of succinaldehyde and 2 g of potassium disulfide carbamate were added and continued to stir evenly. Finally, 2 g of potassium bisulfite, 1 g of potassium sulfide and 2 g of o-xylene thiourea were added and stirred to obtain a density of 1.071 g / cm 3 It is an ultra-deep, low-pressure oil and gas well workover fluid that can withstand high salt and high content of multivalent salts CaCl2 and MgCl2 in formation water at ultra-high temperatures of 160℃~240℃ at the bottom of the well and has low leakage. Example

[0047] 10 grams of nano-SiO2, 50 grams of xylene, 130 grams of toluene, 30 grams of ethyl acetate and 60 grams of cyclohexane with an average particle size of 86 nm were added to a three-necked reaction flask equipped with a reflux condenser. The mixture was stirred at room temperature for 0.5 hours to disperse the nano-SiO2. The mixture was then heated to 85°C and slowly added with anionic modifiers 3.426 grams of 6-[4-chloromethyl)phenyl]sodium hexanoate and 4.130 grams of 10-[4-bromomethyl)phenyl]sodium decanoate. The mixture was reacted under reflux for 24 hours. The reaction system was then centrifuged to remove xylene, toluene, ethyl acetate, cyclohexane and any anionic modifier that did not react. The product was finally dried in vacuo at 75°C to obtain a modified nano-SiO2 containing a heat-resistant benzene ring and anions, i.e., nano-SiO2-BA. The degree of modification of the synthesized nano-SiO2-BA was 48.3%.

[0048] At room temperature, 12 grams of nano-SiO2-BA was slowly poured into 1000 grams of stirring water from a water source well in an oil field well area, wherein the water from the water source well contained 13 g / L of sodium chloride, 5.0 g / L of calcium chloride, 4.0 g / L of magnesium chloride, and 12 mg / L of ferrous chloride. The mixture was stirred continuously until uniformly dispersed. Then, 10 grams of dimethicone and 3 grams of welan gum were slowly poured into the stirring nano-SiO2-BA dispersion system and stirred until the thickener was completely dissolved to obtain a polymer solution with thickening properties. Subsequently, 50 grams of sodium formate, 20 grams of potassium acetate, and 10 grams of tetrasodium ethylenediaminetetraacetic acid were added to the prepared polymer solution under stirring, followed by 1 gram of resorcinol and 1.5 grams of glutaraldehyde, and the mixture was continued to be stirred evenly. Finally, 2 grams of sodium disulfite and 1 gram of o-xylene thiourea were added and stirred to obtain a polymer solution with a density of 1.077 g / cm 3It is an ultra-deep, low-pressure oil and gas well workover fluid that can withstand high salt and high content of multivalent salts CaCl2 and MgCl2 in formation water at ultra-high temperatures of 160℃~240℃ at the bottom of the well and has low leakage.

Claims

1. A method for preparing a high-salinity resistant ultra-deep low-pressure oil and gas well workover fluid, comprising: (1) Preparation of nano-SiO2-BA 10 parts of unmodified nano-SiO2 with an average particle size of less than 100 nm and 90 to 300 parts of solvent are added to a three-necked reaction flask equipped with a reflux condenser, stirred at room temperature for 0.5 hours to disperse the nano-SiO2, then heated to 60°C to 85°C, slowly added with 1.0 to 8 parts of an anion modifier containing a benzene ring, and reacted under reflux for 6 to 24 hours. The reaction system is then centrifuged to remove the solvent and the anion modifier that does not participate in the reaction, and finally the product is vacuum dried at 55°C to 80°C to obtain modified nano-SiO2 containing heat-resistant benzene rings and anions, namely nano-SiO2-BA. The modification degree of the synthesized nano-SiO2-BA is 35% to 70%. The solvent is at least one of N,N-dimethylformamide, triethylamine, xylene, benzene, toluene, pyridine, tetrahydrofuran, cyclohexane and ethyl acetate; the anion modifier is sodium (x+1)-[4-(chloromethyl)phenyl]alkylate Cl(CH2)C6H4(CH2) x COONa, sodium (x+1)-[4-bromomethyl)phenyl]alkylate Br(CH2)C6H4(CH2) x COONa, sodium (x+1)-[4-iodomethyl)phenyl]alkylate I(CH2)C6H4(CH2) x At least one of COONa, x is an integer from 1 to 11; (2) Preparation of high-salinity resistant ultra-deep low-pressure oil and gas well workover fluid At room temperature, 1.0 to 40 parts of nano-SiO2-BA are slowly poured into 1000 parts of a stirring solvent, and stirred continuously until the dispersion is uniform. Then, 1.0 to 20 parts of a thickener are slowly poured into the stirring nano-SiO2-BA dispersion system, and stirred until the thickener is completely dissolved to obtain a polymer solution with thickening properties. Subsequently, 3.0 to 90 parts of a polymer stabilizer are added to the prepared polymer solution under stirring, and then 0.1 to 8 parts of a fungicide are added and continued to be stirred evenly. Finally, 0.1 to 8 parts of a heat stabilizer are added and stirred to obtain a density of 1.005 to 1.100 g / cm 3 It is an ultra-deep, low-pressure workover fluid for oil and gas wells that can withstand high salt content and high content of polyvalent salts such as CaCl2 and MgCl2 in formation water at extremely high bottomhole temperatures of 160°C to 240°C. It does not contain solid materials such as bentonite and sulfonated asphalt, but has low leakage. The solvent is at least one of clean water and water from a well in an oilfield; the thickener is at least one of temperature-resistant polymers including welan gum, diutan gum, gellan gum and sclerotin; the polymer stabilizer is at least one of trisodium nitrotriacetate, trisodium citrate, sodium diethylenetriamine pentaacetate, tetrasodium ethylenediaminetetraacetate, sodium hexametaphosphate, sodium formate, potassium formate, sodium acetate, potassium acetate, sodium propionate, potassium propionate and sodium 2-hydroxypropionate; the bactericide is at least one of catechol, hydroquinone, resorcinol, phenol, malondialdehyde, succinic dialdehyde, glutaraldehyde, adipaldehyde, sodium trichlorophenate, sodium disulfide carbamate and potassium disulfide carbamate; and the heat stabilizer is at least one of sodium sulfite, potassium sulfite, sodium sulfide, potassium sulfide, sodium disulfite, potassium dithionite, sodium bisulfite, potassium bisulfite, thiourea and o-xylene thiourea.

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