A critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive and its preparation method

Abstract

The invention discloses a critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss reducer and a preparation method thereof. The preparation steps of the fluid loss reducer are as follows: S1, N, N-dimethylacrylamide, vinyl pyrrolidone, sodium p-acrylamidobenzoate, acryloylmorphine, sodium methylallyl sulfonate, and sodium 3-allyloxy-2-hydroxy-1-1 propanesulfonate are added to water, stirred and dissolved to obtain a reaction solution; S2, the reaction solution is heated to 65°C ± 2°C, an initiator solution is added dropwise within 3 to 5 minutes, and then the reaction is carried out at a constant temperature of 80°C ± 2 for 4 hours; S3, after the reaction time reaches 4 hours, a terminator thiourea solution is added to terminate the reaction; and circulating cooling water is opened and the temperature is lowered to below 40°C under stirring. The obtained solution is the fluid loss reducer. The fluid loss reducer of the present invention has hydrophilic criticality, can form a temperature-sensitive synergistic effect at high temperature and a salting-out synergistic effect under high salt, and is particularly suitable for ultra-high temperature, ultra-high temperature and saturated brine cementing slurry systems.

Description

Technical Field

[0001] The present invention relates to the technical field of oil drilling, in particular to a critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive and a preparation method thereof. Background Art

[0002] Cementing is the process of injecting cement slurry into the annular space between the drilled wellbore and the lowered casing, allowing the cement slurry to solidify and isolate formation fluids. Cementing provides the foundation for subsequent oil and gas well stimulation, reservoir transformation, and workover projects. It is a critical step in the well construction process, impacting the well's lifespan, production, and overall economic benefits.

[0003] During the cementing process in oilfields, the liquid phase of cement slurry seeps into the permeable formation under the influence of pressure differentials as it flows through the formation. This phenomenon is called water loss in the cement slurry. Excessive water loss from the cement slurry not only contaminates the oil and gas reservoirs, reduces oil and gas well production, and affects the consistency of downhole cement slurry performance with designed properties, but can also lead to cementing accidents such as bridging, annular air crossflow, "flagpole insertion," and "sausage stuffing." Oilwell cement fluid loss additives are commonly added to cementing slurry systems to control water loss. Oilwell cement fluid loss additives are one of the three main admixtures required for cementing slurry systems.

[0004] With the decline of shallow and intermediate oil and gas resources, oil and gas exploration and development are shifting towards deep, ultra-deep, complex formations, and deepwater reservoirs, all facing demanding conditions. Cementing is increasingly subject to harsh conditions such as high temperatures (120°C to 150°C), ultra-high temperatures (150°C to 180°C), and ultra-high temperatures (180°C to 260°C), as well as saline and salt-gypsum formations. For example, the bottomhole temperature of oil formations in ultra-deep wells exceeding 10,000 meters can reach 230°C to 250°C, corresponding to circulating temperatures of 180°C to 220°C. In areas with significant geothermal gradients, such as the Shengke 1 well in the eastern region, at a depth of 7,026 meters, has a bottomhole temperature of 235°C, and the Gulong 1 well in the Daqing Oilfield, at a depth of 6,300 meters, has a maximum temperature of 232°C. Oil well cement fluid loss additives (FLAs) can experience chain scission, functional group degradation, high-temperature desorption, and high-temperature dehydration under ultra-high and ultra-high temperature and alkaline conditions. As a result, these additives fail to fully meet the requirements for ultra-deep and ultra-deep well cementing in terms of fluid loss control, thermal stability, and salt tolerance. Based on the FLAs' mechanism of action, failure causes, and group chemical properties, a molecular structure design approach is being developed to fundamentally address these temperature and salt tolerance issues. Summary of the Invention

[0005] In response to the problems of high-temperature chain scission, functional group degradation, high-temperature desorption, and high-temperature dehydration in current oil well cement fluid loss additives under ultra-high temperature, extra-high temperature and alkaline conditions, the present invention provides a critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive.

[0006] The critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive provided by the present invention is prepared by copolymerizing six raw monomers: N,N-dimethylacrylamide, vinyl pyrrolidone, sodium p-acrylamidobenzoate, acryloylmorpholine, sodium methyl allyl sulfonate, and sodium 3-allyloxy-2-hydroxy-1-propanesulfonate in a mass ratio of (330-370):(40-60):(3-7):(35-45):(10-14):(25-35). The preparation method is as follows:

[0007] S1. Add six raw material monomers, namely, N,N-dimethylacrylamide, vinyl pyrrolidone, sodium p-acrylamidobenzoate, acryloylmorpholine, sodium methyl allyl sulfonate, and sodium 3-allyloxy-2-hydroxy-1-propanesulfonate, into water, stir and dissolve to obtain a reaction solution;

[0008] S2. Raise the temperature of the reaction solution to 65°C ± 2°C, add the initiator solution dropwise within 3 to 5 minutes, and then react at a constant temperature of 80°C ± 2 for 4 hours;

[0009] S3. After the reaction time reaches 4 hours, add the terminator thiourea solution to terminate the reaction; and start the circulating cooling water, and cool down to below 40°C while stirring. The obtained solution is the fluid loss additive.

[0010] Preferably, the initiator is sodium persulfate, and the amount of the initiator is 1.5% of the total mass of the six monomers, which is added into water and dissolved to obtain an initiator solution with a mass percentage concentration of 25%.

[0011] Preferably, in step S2, the reaction temperature can be controlled not to exceed 82° C. by circulating cooling water during the reaction process.

[0012] Preferably, the thiourea solution is an aqueous solution formed by dissolving thiourea in water with a mass percentage concentration of 8%, and the amount of thiourea used is 0.1% of the total mass of the six monomers.

[0013] Preferably, in the entire reaction system, the mass ratio of water to the total mass of the six monomers is 76:14. The water amount includes the total amount of water used in step S1 and the water in the initiator solution and the thiourea solution.

[0014] The preparation reaction formula of the fluid loss additive is as follows:

[0015]

[0016] The main technical improvements of the fluid loss reducer of the present invention are as follows: (1) hydrophilic criticality, temperature-sensitive dehydration effect and salting-out effect are generated by the benzene ring, pyrrole ring and morphine ring structures in the fluid loss reducer molecules, thereby promoting the adsorption of the fluid loss reducer molecules on the surface of cement particles, turning unfavorable factors into favorable factors, and improving the water loss control ability of the fluid loss reducer in the cement slurry system under ultra-high temperature and high salt conditions; (2) the high bond energy of the -CC-σ bond is utilized to solve the temperature stability of the main chain of the fluid loss reducer; (3) the two hydrogen atoms on the amidoamine are replaced by methyl groups with a power supply effect, and the power supply of the methyl groups is improved. The effect (weakening the tendency of the electron pair of the CN bond to lean toward the nitrogen atom) weakens the positive charge of the carbon atom, preventing the negatively charged hydroxyl group from attacking the positively charged carbon atom and causing hydrolysis, thereby fundamentally solving the problem of the high-temperature degradation of the fluid loss reducer on the performance of the cement slurry; (4) The weak polar bond is used to connect the temperature-resistant and salt-resistant sulfonic acid group, thereby avoiding the hydrolysis of the functional group in a high-temperature and high-alkaline environment, achieving the stability of the fluid loss reducer function and the engineering performance of the cement slurry system under ultra-high temperature and ultra-high temperature conditions, and solving the key technical problems of the consistency "thermal dilution", "bulging", and "stepping" during the thickening process of the cement slurry system.

[0017] Based on the above technical improvements, compared with the prior art, the fluid loss additive of the present invention has the following beneficial effects:

[0018] (1) The fluid loss additive prepared by the present invention has a hydrophilic critical point of 43.0% for a weakly polar solvent (anhydrous ethanol). It can form a temperature-sensitive synergistic effect at high temperature and a salting-out synergistic effect at high salinity. It is suitable for medium- and high-temperature cement slurry systems as well as ultra-high-temperature, extra-high-temperature and saturated brine cement slurry systems.

[0019] (2) The fluid loss additive prepared by the present invention has the ability to resist ultra-high temperature (250°C). 3 ) When the addition amount in cement slurry system is 8.0%, API water loss is 45.6mL (reaching the industry standard requirement of first-class product: ≤50mL).

[0020] (3) The fluid loss additive prepared by the present invention has a weak hydrophilicity. According to the principle of like dissolves like, the high-temperature dehydration effect causes the fluid loss additive to tend to be distributed on the surface of cement particles, which can effectively block the pores of the filter cake. Therefore, at the same dosage, the water loss increases only slightly with increasing temperature. Considering that the higher the temperature, the more intense the thermal motion of water molecules (an inherent characteristic) can cause a significant increase in water loss, the fluid loss additive actually has a high-temperature synergistic effect (low water loss), especially at high temperatures (above 150°C), ultra-high temperatures (above 180°C), and ultra-high temperatures (250°C), it exhibits a good temperature-sensitive effect.

[0021] (4) The fluid loss additive prepared by the present invention has good compatibility with other commonly used cement additives and has no adverse effects on the rheological properties, stability, thickening time, strength and other properties required by cementing engineering. It can be used to prepare 250°C ultra-high temperature cement slurry systems and high-density and low-density cement slurry systems.

[0022] (5) The synthesis method of the fluid loss additive of the present invention is simple and easy to operate, and the raw materials are reasonably priced and readily available, which is convenient for industrial promotion.

[0023] Other advantages, objectives and features of the present invention will be reflected in part from the following description and will be understood by those skilled in the art through study and practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is the infrared spectrum of the fluid loss additive prepared in Example 1.

[0025] Figure 2 This is the HNMR spectrum of the fluid loss additive prepared in Example 1.

[0026] Figure 3 This is the thermal analysis diagram of the fluid loss additive prepared in Example 1.

[0027] Figure 4 This is a diagram of water loss trend of cement slurry system under different salt contents.

[0028] Figure 5 This is the thickening curve of 4-1# formulation (180℃).

[0029] Figure 6 This is the thickening curve of 4-1# formulation (200℃).

[0030] Figure 7 This is the thickening curve of 4-3# formulation (220℃).

[0031] Figure 8 This is the thickening curve of 4-4# formulation (250℃). DETAILED DESCRIPTION

[0032] The preferred embodiments of the present invention are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only used to illustrate and explain the present invention, and are not used to limit the present invention.

[0033] Example 1

[0034] A preparation method of a fluid loss additive comprises the following steps:

[0035] (1) Weigh: 50.31 kg of N,N-dimethylacrylamide (DMAM); 7.19 kg of vinyl pyrrolidone (NVP); 0.72 kg of sodium p-acrylamidobenzoate (AMBA); 5.75 kg of acryloylmorpholine (AMPL); 1.73 kg of sodium methyl allyl sulfonate (MAS); and 4.31 kg of sodium 3-allyloxy-2-hydroxy-1-propanesulfonate (AHPS).

[0036] (2) Weigh 1.05 kg of sodium persulfate (Na2S2O8) as an initiator and dissolve it in 2.95 kg of water for later use;

[0037] (3) Weigh 0.080 kg of thiourea [(NH2)2S], a terminator, and dissolve it in 0.920 kg of water for later use;

[0038] (4) Add 430.0 kg of water to the reactor, add all six monomers to the reactor, and stir for 30 minutes to completely dissolve them; then heat the contents of the reactor to 65°C ± 2°C, and drop the initiator solution into the contents of the reactor within 3 to 5 minutes; after the initiator is added, control the contents of the reactor to react at 80°C ± 2°C for 6 hours;

[0039] (5) After 6 hours of reaction, the terminator thiourea solution was added to the reactor material at one time to terminate the reaction; and the circulating cooling water was turned on and the temperature was lowered to below 40°C while stirring to obtain a fluid loss additive product, which was numbered BS100L-CT.

[0040] Example 2

[0041] A preparation method of a fluid loss additive comprises the following steps:

[0042] (1) Weigh: 1006.16 kg of N,N-dimethylacrylamide (DMAM); 143.74 kg of vinyl pyrrolidone (NVP); 14.37 kg of sodium p-acrylamidobenzoate (AMBA); 114.99 kg of acryloylmorpholine (AMPL); 35.40 kg of sodium methyl allyl sulfonate (MAS); and 86.24 kg of sodium 3-allyloxy-2-hydroxy-1-propanesulfonate (AHPS).

[0043] (2) Weigh 2.10 kg of sodium persulfate (Na2S2O8) as an initiator and dissolve it in 59.00 kg of water for later use;

[0044] (3) Weigh 1.60 kg of thiourea [(NH2)2S], a terminator, and dissolve it in 18.4 kg of water;

[0045] (4) Add 8600.0 kg of water to the reactor, add all six monomers to the reactor, and stir for 30 minutes to completely dissolve them; then heat the contents of the reactor to 65°C ± 2°C, and dropwise add the initiator solution to the contents of the reactor within 3 to 5 minutes; after the initiator is added, control the contents of the reactor to react at 80°C ± 2°C for 6 hours;

[0046] (5) After 6 hours of reaction, add the terminator thiourea solution to the reactor material at one time to terminate the reaction; and start circulating cooling water, and cool down to below 40°C while stirring to obtain the fluid loss additive product.

[0047] The critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive BS100L-CT prepared in Example 1 was repeatedly dissolved and purified by precipitation with acetone and water to remove unreacted monomers and by-products in the system; the product was chopped, dried, and ground into powder; a certain amount of the powder product was characterized by infrared spectroscopy and nuclear magnetic resonance hydrogen spectrum for molecular structure; the results are shown in FIG. Figure 1 、 Figure 2 A certain amount of powder product was used to perform thermal scanning in the range of 25℃~650℃ to characterize its thermal stability. The results are shown in Figure 3 .

[0048] Figure 1 The infrared spectrum of the product is analyzed as follows: wave number 3200 cm -1 ~3600cm -1 It is the vibration peak of water molecules and alcohol hydroxyl groups adsorbed by fluid loss additive; 2933cm -1 The CH stretching vibration peaks of the main chain -CH3, -CH2-, -CH-; 2130cm -1 1630cm is the benzene ring double bond stretching vibration peak; -1 is the carbonyl C=O vibration peak; 1546cm -1 Compared with the C=O vibration peak in alkanone; 1457cm -1 -COO- stretching vibration peak; 1405cm -1 is the CN vibration peak of the amide group; 1365cm -1 C-OH stretching vibration peak; 1188cm -1 The CN stretching vibration peak in morphine; 1112 cm -1 -O- stretching vibration peak; 1042cm -1 、622cm -1 It is the characteristic peak of sulfonic acid group; 569cm -1 Benzene ring CH out-of-plane bending vibration peak.

[0049] Figure 2The nuclear magnetic hydrogen spectrum analysis is as follows: δ=7.9 is the amide-CO-NH- hydrogen peak in sodium acrylamidobenzoate; δ=7.8 is the hydrogen peak of the benzene ring; δ=4.7 is the solvent peak; 3.6 singlet is the hydrogen peak on the hydroxyl carbon; 2.7-3.1 quartet is the hydrogen peak of the carbon atom connected to the N atom in vinyl pyrrolidone and acryloylmorpholine; 2.6 singlet hydroxyl hydrogen peak; 1.2~1.8 multiplets are the methylene (C-CH2-C) and methylene (C-CH-C) hydrogen atoms in the main chain and the alkyl hydrogen peaks in bis(methylacrylamide) and sodium methylallyl sulfonate.

[0050] Depend on Figure 3 Thermal analysis shows that: 8% of the weight loss at 25℃~320℃ is due to the loss of polymer adsorbed water and crystallization water; the decomposition temperature starts at 320℃, 320℃~330℃ is due to the thermal decomposition of amide groups, sulfonic acid groups and other groups on the fluid loss reducer polymer, and 330℃~340℃ is due to the thermal decomposition of the fluid loss reducer polymer main chain; the thermal stability of the fluid loss reducer is 320℃.

[0051] Figure 1 and Figure 2 The infrared spectrum and nuclear magnetic hydrogen spectrum showed that the characteristic peaks of the functional groups of the fluid loss additive appeared in both the infrared and nuclear magnetic spectra, indicating that all the selected monomers participated in the polymerization reaction and the synthesized fluid loss additive was the expected target molecule. Figure 3 Thermal analysis shows that the fluid loss agent has excellent temperature resistance, its thermal stability reaches 320℃, and it will not thermally decompose under ultra-high temperature (200-250℃) conditions, which can effectively ensure the temperature resistance of the fluid loss agent.

[0052] The performance test of the fluid loss additive prepared in Example 1 is as follows:

[0053] Cement slurry engineering performance was evaluated according to the industry standard SY / T5504.2-2013, "Evaluation Methods for Oil Well Cement Admixtures - Part 2: Fluid Loss Reducers," and the national standard GB / T 19139-2015, "Test Methods for Oil Well Cement." The experiments involved retarder BS200R-G, expansive agent BS500, flow pattern modifier CT-18, and defoamer BP-1, all of which are oil well cement admixtures produced and widely used by Sichuan Hongsheng Petroleum Engineering Technology Service Co., Ltd.

[0054] (1) Hydrophilicity criticality test

[0055] According to the working principle of fluid loss additives, their distribution on the surface of cement particles is a key factor in controlling water loss. Adjusting the hydrophilicity of the fluid loss additive polymer to approach a critical state of water dissolution and, under high temperature and high salinity conditions, promoting its distribution on the cement particle surface can enhance its ability to control water loss. A simple, intuitive, and practical method for determining the hydrophilicity of a fluid loss additive is to observe the precipitation of water and a relatively weakly hydrophilic solvent (anhydrous ethanol) in a mixture of solvents with different affinities. A certain amount of anhydrous ethanol was added to a 5.0% aqueous solution of the fluid loss additive with stirring using a burette until turbidity or precipitation occurred. The hydrophilicity and critical water point were then examined. In this experiment, a terpolymer prepared from three commonly used raw monomers for fluid loss additive preparation in the industry—acrylamide (AM), acrylic acid (AA), and sodium 2-acrylamido-2-dimethylpropanesulfonate (AMPS)—was selected as a comparative sample. The resulting terpolymer was prepared using the same preparation method as in Example 1 and was designated Polymer A. The mass ratio of AM, AA, and AMPS was 3:1:5. The initiator was 2.0% of the total mass of the three monomers. The thiourea was 0.1% of the total mass of the three monomers. The mass ratio of water to the total mass of the three monomers in the reaction system was 74:16. The experimental results are shown in Table 1.

[0056] Table 1. Experimental data of evaluation of hydrophilicity criticality of fluid loss additive

[0057]

[0058] The experimental results in Table 1, combined with an analysis of the molecular structure characteristics of the fluid loss additive, indicate that while the conventional fluid loss additive copolymer A has a non-hydrophilic backbone, it contains a large number of strongly hydrophilic sulfonic acid and carboxyl groups, resulting in almost no critical hydrophilicity in weakly polar ethanol solvents. The fluid loss additive BS100L-CT of the present invention, in addition to a non-hydrophilic backbone, also incorporates a certain amount of lipophilic or weakly hydrophilic groups such as benzene, pyrrole, and morphine rings. Consequently, it exhibits strong non-hydrophilicity in weakly polar ethanol solutions, reaching a critical hydrophilic point at a volume ratio of 43.0% in weakly polar ethanol. This demonstrates that the fluid loss additive synthesized in the present invention possesses a certain critical hydrophilicity.

[0059] (2) Thermosensitivity test evaluation

[0060] According to the mechanism of action of fluid loss additives, their distribution on the surface of cement particles is a key factor in controlling fluid loss. Adjusting the hydrophilicity of the fluid loss additive polymer to a critical state of dissolution in water, and promoting its distribution on the surface of cement particles under high temperature and high salinity conditions, can enhance its ability to control fluid loss. The weak hydrophilicity of fluid loss additives, on the one hand, makes the dehydration effect more pronounced at high temperatures; on the other hand, due to their weak hydrophilicity, according to the principle of like dissolves like, the fluid loss additive tends to distribute on the surface of cement particles with the same affinity, thereby enhancing its fluid loss control ability. By adding the same amount of fluid loss additive to a conventional density cement slurry system, the temperature-dependent change in water loss in the slurry system was examined to compare and investigate whether different types of fluid loss additives exhibit temperature-sensitive effects. The formula for conventional density cement slurry at 60°C to 105°C is as follows: 800.0g of Grade G high-sulfur-resistant oil well cement + 5.0% fluid loss additive + 1.0% retarder BS200R-G + 2.0% expansion agent BS500 + 1.5% flow pattern modifier TC-18 + 0.5% defoamer BP-1 + water. The formula for conventional density cement slurry at 120°C to 220°C is as follows: 500.0g of Grade G high-sulfur-resistant oil well cement + 250.0g of high-temperature stabilizer (quartz sand) + fluid loss additive + 5.0% to 7.0% retarder BS200R-G + 2.0% expansion agent + 1.5% flow pattern modifier TC-18 + 0.5% defoamer BP-1 + water. The experimental results are shown in Table 2.

[0061] Table 2. Experimental data of temperature sensitivity evaluation of fluid loss additive

[0062]

[0063] According to the experimental results in Table 2, combined with the molecular structure characteristics of the fluid loss additive and the analysis of the action principle of the fluid loss additive, it can be seen that for the ordinary high-temperature resistant fluid loss additive copolymer A, on the one hand, due to its strong hydrophilicity, the fluid loss additive produced by thermal desorption and dehydration is distributed in the water phase, and the viscosity of the water phase containing the fluid loss additive decreases sharply with increasing temperature; on the other hand, the higher the temperature, the more intense the thermal motion of water molecules; therefore, at the same dosage, water loss increases with increasing temperature, especially at high temperatures (above 150°C) and ultra-high temperatures (above 180°C), water loss increases significantly. To better control water loss, it is necessary to increase the dosage of the fluid loss additive. For the critical temperature-sensitive high-temperature resistant fluid loss additive BS100L-CT, due to its weak hydrophilicity, according to the principle of like dissolves like, the high-temperature dehydration effect causes the fluid loss additive to tend to distribute on the surface of cement particles, which can effectively block the pores of the filter cake. Therefore, at the same dosage, water loss increases only slightly with increasing temperature. Considering that high temperature causes more intense thermal motion of water molecules (an inherent characteristic), which can lead to a significant increase in water loss, the fluid loss additive actually has a high-temperature synergistic effect; especially at high temperatures (above 150°C) and ultra-high temperatures (above 180°C), it exhibits a good temperature-sensitive effect (low water loss).

[0064] (3) Salt synergistic effect evaluation

[0065] In general, the water loss of a cement slurry system to which a conventional fluid loss reducer is added will increase significantly with the increase of salt content. Salt enhancement means that the ability of the fluid loss reducer to control water loss will not weaken or will be further enhanced with the increase of salt content. The currently commonly used high-temperature fluid loss reducer copolymer A was used to conduct an experimental comparison with the fluid loss reducer BS100L-CT of the present invention. When the water loss does not increase by more than 10mL or further decreases under the same conditions, it is judged to be salt enhanced. Cement slurry system formula: 500.0g of G-grade high sulfur resistant oil well cement + 250.0g of high temperature stabilizer (quartz sand) + 8.0% fluid loss reducer + 7.0% retarder BS200R-G + 2.0% expansion agent BS500 + 1.5% flow pattern regulator TC-18 + 0.5% defoaming agent BP-1 + water; experimental temperature 200°C. The experimental results are shown in Table 3 and Figure 4 shown.

[0066] Table 3. Formula of conventional density brine cement slurry system

[0067]

[0068] According to Table 3 and Figure 4The experimental results, combined with the molecular structure characteristics and action principle of the fluid loss additive, show that for the conventional structure fluid loss additive copolymer A, water loss increases sharply with increasing salt (NaCl) concentration. The first stage of rapid water loss increase occurs when the salt concentration is below 7.5%. This stage is mainly due to the ionic atmosphere of the salt cations shielding the fluid loss additive anions, causing the fluid loss additive to curl. The second stage of rapid water loss increase occurs when the salt concentration reaches approximately 10.0% to 15.0%. During this stage, water loss in the cement slurry system is mainly caused by the salting-out effect, which causes the fluid loss additive molecules to separate and precipitate from the aqueous phase. The third stage of increased water loss occurs when the salt concentration is 15.0% to 26.5% (saturated brine). The increase in water loss in this stage is mainly due to the salting-out effect, which causes the fluid loss additive to curl more tightly and precipitate more completely. However, this effect is much smaller than that from the dissolved state to the precipitated state. Therefore, the impact on water loss is smaller at lower concentrations, and the water loss increase is slightly slower than in the first two stages. For the fluid loss additive BS100L-CT of the present invention, the first stage of increased water loss occurs when the salt concentration is 0-7.5%. This is mainly due to the ionic atmosphere effect generated by the ionization of salt in water, which causes the fluid loss additive molecules in the aqueous phase to curl and reduces the viscosity of the aqueous phase. At a salt concentration of 7.5-26.5% (saturated brine), the water loss of the cement slurry system decreases slightly. This is mainly due to the presence of benzene rings, pyrrole rings, and morphine rings in its structure, which have weak hydrophilicity and a significant salting-out effect. The curled and precipitated fluid loss additive polymers produced by the salting-out effect are distributed more on the surface of cement particles according to the principle of like dissolves like, effectively blocking the pores of the filter cake and reducing water loss. However, since most of the fluid loss additive has already precipitated at a certain initial salting-out precipitation concentration (approximately 7.5%), further increasing the salt concentration only causes further precipitation of a small amount of polymer and a more compact curling and precipitation of the polymer. However, this effect is far less than that from the dissolved state to the precipitated state, and has little effect on the water loss control ability, resulting in a slow decrease in water loss. Experimental results show that the fluid loss additive synthesized by the present invention has a salt synergistic effect, and the critical concentration of salt synergism is about 7.5%.

[0069] (4) Comprehensive performance evaluation of fluid loss additives

[0070] In actual cementing projects, each well faces different engineering conditions and requirements, requiring cement slurry systems with different properties, such as different densities and temperatures. Synthetic fluid loss additive BS100L-CT was used to prepare different cement slurry systems. The comprehensive engineering performance of the fluid loss additive at conventional density was investigated within the temperature range of 180°C to 250°C. The experimental formula is shown in Table 4, and the types of raw materials are the same as above. The experimental results are shown in Tables 5 and Figure 5-8 .

[0071] Table 4. Formulas for ultra-high temperature, extra-high temperature, and conventional density cement slurry systems

[0072]

[0073] Note: The expander in Table 4 is BS500. The types of other additives are the same as those used in the previous experiments.

[0074] Table 5. Comprehensive engineering performance experimental data of ultra-high temperature, extra-high temperature and conventional density cement slurry systems

[0075]

[0076] Table 5 and Figure 5-8 Experimental results show that the fluid loss additive BS100L-CT synthesized in the present invention can be used to prepare a conventional density ultra-high temperature and extra-high temperature cementing cement slurry system by properly adjusting the dosage of fluid loss additive and retarder and adding an expander, flow pattern regulator, and defoamer. The engineering performance indicators of the cement slurry system, such as water loss, rheology, free liquid, thickening time, compressive strength, and gravity stability, meet the requirements of industry standards.

[0077] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment as above, it is not intended to limit the present invention. Any technician familiar with this profession can make some changes or modifications to equivalent embodiments of the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive, characterized in that: It is prepared by copolymerizing six raw material monomers, namely, N,N-dimethylacrylamide, vinyl pyrrolidone, sodium p-acrylamidobenzoate, acryloylmorpholine, sodium methyl allyl sulfonate, and sodium 3-allyloxy-2-hydroxy-1-propanesulfonate, in a mass ratio of (330-370): (40-60): (3-7): (35-45): (10-14): (25-35). The molecular structure of the fluid loss additive is as follows:

2. A method for preparing the critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive according to claim 1, characterized in that: The following steps are involved: S1. Add six raw material monomers, namely, N,N-dimethylacrylamide, vinyl pyrrolidone, sodium p-acrylamidobenzoate, acryloylmorpholine, sodium methyl allyl sulfonate, and sodium 3-allyloxy-2-hydroxy-1-propanesulfonate, into water, stir and dissolve to obtain a reaction solution; S2. Raise the temperature of the reaction solution to 65°C ± 2°C, add the initiator solution dropwise within 3 to 5 minutes, and then keep the reaction at 80°C ± 2 for 4 hours; S3. After the reaction time reaches 4 hours, add the terminator thiourea solution to terminate the reaction; and start the circulating cooling water, and cool down to below 40°C while stirring. The obtained solution is the fluid loss additive.

3. The method for preparing the critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive according to claim 2, wherein: The initiator is sodium persulfate, and the amount of the initiator is 1.5% of the total mass of the six monomers, and is dissolved in water to form an initiator solution.

4. The method for preparing the critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive according to claim 2, wherein: In step S2, the reaction temperature is controlled not to exceed 82° C. by circulating cooling water during the reaction.

5. The method for preparing the critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive according to claim 2, characterized in that: The thiourea solution is a solution formed by dissolving thiourea in water, and the amount of thiourea used is 0.1% of the total mass of the six monomers.

6. The method for preparing the critical temperature-sensitive salt-enhanced ultra-high temperature oil well cement fluid loss additive according to claim 2, characterized in that: In the entire reaction system, the mass ratio of water to the total mass of the six monomers is 76:14.

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