PH response release type oil well cement fluid loss agent as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of oilfield chemical materials, and relates to a pH response release type oil well cement fluid loss agent as well as a preparation method and application thereof. The pH response release type oil well cement fluid loss agent provided by the invention comprises an inner core, a middle modification layer and a shell, wherein the inner core is a multi-component copolymer; the middle modification layer is silanol attached to the surface of the inner core, and the shell is SiO2. According to the pH response release type oil well cement fluid loss agent disclosed by the invention, by introducing the pH response type SiO2 shell layer, an intelligent release mechanism of'seawater shielding-alkaline activation 'is realized, and the limitation of the prior art is broken through from double dimensions of physical isolation (avoiding ion contact) and chemical triggering (releasing as required).

Description

Technical Field

[0001] The present invention belongs to the technical field of oilfield chemical materials and relates to a pH-responsive release type oil well cement fluid loss additive and a preparation method and application thereof, and in particular to a pH-responsive intelligent fluid loss additive for metal ion interference under seawater slurry mixing conditions and a preparation method and application thereof. Background Art

[0002] Fluid loss reducers are mainly used to control the loss of the liquid phase in oil well cement slurry to the permeable formation in order to maintain an appropriate water-cement ratio of the cement slurry. Fresh water resources are scarce in offshore platform operations, and the cost of transporting fresh water over long distances is high. Using seawater to prepare the slurry locally is an inevitable economical and efficient choice. However, in the case of seawater preparation, existing oil well cement fluid loss reducers have some disadvantages, which bring risks to cementing construction. Specifically, these disadvantages include: (1) There are many metal ions in seawater, especially high-valent Ca 2+ Mg 2+ Plasma, these metal ions will react with the carboxyl functional groups of the fluid loss additive to form a complex, causing the polymer to undergo a cross-linking reaction, thereby making the cement slurry have a strong thixotropy, especially under low temperature conditions; (2) Mg in seawater 2+ High ion content, Mg 2+ Magnesium hydroxide precipitates will form in the alkaline environment of cement slurry, which will absorb a large amount of fluid loss additives, resulting in increased water loss in the cement slurry and increasing the risk of cementing operations. In recent years, researchers have developed a variety of polymer fluid loss additives through molecular structure optimization to meet the needs of seawater slurry cementing, but the following bottlenecks still exist:

[0003] (1) High dependence on dosage

[0004] The journal article "Preparation and Performance of Dispersed Fluid Loss Reducers for Seawater Cement Slurry" uses a tetrapolymer of 2-acrylamido-2-methylpropanesulfonic acid / maleic anhydride / N,N-dimethylacrylamide / allyl polyethylene glycol. Although it improves dispersibility through long comb-shaped side chains, it requires a 3% addition (BWOC) to control the water loss of cement slurry to below 50 mL at 60°C, which is not economical enough; the journal article "Development of Seawater-Type Polymer Fluid Loss Reducers" alleviates the thixotropy problem by reducing the molecular weight, but a 4% addition is required to meet the API water loss standard in a seawater environment, significantly increasing construction costs.

[0005] (2) Temperature adaptability limitations

[0006] The above studies all focused on low-temperature environments (50-60°C) and did not verify the performance degradation problem under medium- and high-temperature conditions (>80°C). However, the actual bottom-hole circulation temperature (BHCT) of cementing is relatively high. Existing fluid loss additives are prone to molecular chain breakage or group oxidation at high temperatures, resulting in a sharp increase in water loss.

[0007] (3) Metal ion interference has not been completely resolved

[0008] Existing technologies all adopt a "passive salt resistance" strategy and fail to actively avoid the contact between seawater ions and functional groups. For example, the journal article "Synthesis and Performance of Seawater Cement Slurry System Fluid Loss Reducer LTF-100L" introduces the cationic monomer dimethyldiallylammonium chloride, attempting to reduce the effect of Mg on the Mg by charge neutralization. 2+ Complexation, but experiments show that its effect is limited and does not solve the problem of Mg 2+ Cross-linking and thickening problems caused by this.

[0009] The root cause of these shortcomings lies in the fact that the linear molecular design of traditional fluid loss additives relies solely on chemical modification of functional groups, lacking the ability to dynamically respond to differences in the seawater-cement slurry environment. Therefore, developing an oil well cement fluid loss additive that can dynamically respond to differences in the seawater-cement slurry environment is crucial. Summary of the Invention

[0010] Regarding Ca in seawater slurry 2+ Mg 2+ The present invention provides a pH-responsive release type oil well cement fluid loss additive and its preparation method and application.

[0011] Specifically, the present invention is achieved through the following technical solutions:

[0012] A pH-responsive release type oil well cement fluid loss additive comprises a core, an intermediate modification layer and an outer shell; wherein the core is a multi-polymer; the intermediate modification layer is a silanol attached to the surface of the core; and the outer shell is SiO2.

[0013] In the above-mentioned pH-responsive release type oil well cement fluid loss additive, the multi-component copolymer is a copolymer of 2-acrylamido-2-methylpropanesulfonic acid, N,N-dimethylacrylamide, and a carboxyl group-containing monomer.

[0014] In the pH-responsive release type oil well cement fluid loss additive, the molar ratio of the 2-acrylamido-2-methylpropanesulfonic acid, the N,N-dimethylacrylamide, and the carboxyl group-containing monomer is 1:(0.4-0.7):(0.01-0.04).

[0015] In the pH-responsive release type oil well cement fluid loss additive, the carboxyl group-containing monomer is one of acrylic acid, maleic acid, maleic anhydride, itaconic acid, fumaric acid, citraconic acid, and mesaconic acid.

[0016] In the above-mentioned pH-responsive release type oil well cement fluid loss additive, the molecular weight of the multi-polymer is in the range of 100,000-200,000 Da.

[0017] In the above-mentioned pH-responsive release type oil well cement fluid loss additive, the thickness of the shell is 10-50 nm.

[0018] A method for preparing a pH-responsive release type oil well cement fluid loss additive, comprising:

[0019] (1) dissolving 2-acrylamido-2-methylpropanesulfonic acid, N,N-dimethylacrylamide, and a carboxyl group-containing monomer in water, adjusting the pH, and then venting the oxygen in the container;

[0020] (2) adding a chain transfer agent and an initiator, heating, and spray drying to obtain a core polymer powder after the reaction is completed;

[0021] (3) dispersing the core polymer powder in a modification solution, adjusting the pH, heating, stirring, centrifuging, washing, and drying after the reaction is completed to obtain amino-modified core particles;

[0022] (4) dispersing the amino-containing core particles in ethanol, adding a coating liquid and a catalyst, heating and stirring, and after the reaction is completed, centrifuging and drying to obtain a pH-responsive release type oil well cement fluid loss additive with a core-shell structure.

[0023] In the preparation method of the pH-responsive release type oil well cement fluid loss additive, the molar ratio of the 2-acrylamido-2-methylpropanesulfonic acid, the N,N-dimethylacrylamide, and the carboxyl group-containing monomer is 1:(0.4-0.7):(0.01-0.04).

[0024] The preparation method of the pH-responsive release type oil well cement fluid loss additive comprises the following steps: step (1) dissolving 2-acrylamido-2-methylpropanesulfonic acid, N,N-dimethylacrylamide, and a carboxyl group-containing monomer in water, adjusting the solid content of the system to 20%-25%, and adjusting the pH to 6.5-7.5 using calcium hydroxide.

[0025] In the preparation method of the pH-responsive release type oil well cement fluid loss additive, the chain transfer agent is isopropyl alcohol, and the addition amount is 0.5%-1.5% of the total monomer mass; the initiator is ammonium persulfate, and the addition amount is 0.3%-2.0% of the total monomer mass.

[0026] In the preparation method of the pH-responsive release type oil well cement fluid loss additive, the heating temperature in step (2) is 40-60° C., and the reaction time is 4-7 hours.

[0027] In the preparation method of the pH-responsive release type oil well cement fluid loss additive, the modifying liquid is a mixture of KH-550, ethanol, and water in a volume ratio of 5:80:15.

[0028] In the preparation method of the pH-responsive release type oil well cement fluid loss additive, in step (3), the pH is 4-5, the heating temperature is 55-65° C., and the stirring time is 2-4 hours.

[0029] In the preparation method of the pH-responsive release type oil well cement fluid loss additive, the coating liquid is a mixture of tetraethoxysilane, ethanol and water in a molar ratio of 1:4:4; and the catalyst is 0.06-0.15 mol / L ammonia water.

[0030] In the preparation method of the pH-responsive release type oil well cement fluid loss additive, in step (4), the heating temperature is 40-50° C. and the stirring reaction time is 6-24 hours.

[0031] A pH-responsive release type oil well cement fluid loss additive is prepared by adopting the above-mentioned preparation method of the pH-responsive release type oil well cement fluid loss additive.

[0032] The application of the pH-responsive release type oil well cement fluid loss additive in the preparation of an oil well cement slurry system prepared with seawater.

[0033] In the above application, the dry mixing amount of the pH-responsive release type oil well cement fluid loss additive is 0.5%-2.0% of the cement mass.

[0034] The technical solution of the present invention has the following beneficial effects:

[0035] (1) In the pH-responsive release type oil well cement fluid loss additive of the present invention, the SiO2 shell prevents Mg from being released during the mixing process of the fluid loss additive and the slurry seawater by physical barrier and chemical inertness. 2+ Contact with the core carboxyl group, avoiding complex cross-linking; after the seawater and cement are mixed to form cement slurry, the OH - With Mg 2+ Mg(OH)2 precipitation is formed, and then the strong alkaline environment triggers the dissolution of the SiO2 shell, releasing the core polymer to play the role of reducing water loss, avoiding the core polymer and Mg 2+ Cross-linking reaction leads to room temperature thixotropy;

[0036] (2) The pH-responsive release type oil well cement fluid loss additive of the present invention realizes the intelligent release mechanism of "seawater shielding-alkaline activation" by introducing a pH-responsive SiO2 shell, breaking through the limitations of existing technologies from the dual dimensions of physical isolation (avoiding ion contact) and chemical triggering (release on demand);

[0037] (3) The pH-responsive release type oil well cement fluid loss additive of the present invention has an addition dosage of only 0.5%-2.0% (BWOC), and the API water loss in seawater slurry is ≤50mL / 30min. This solves the technical bottleneck of traditional fluid loss additives with high addition dosage (3%-4%) and narrow temperature range, and significantly reduces marine cementing costs and operational risks. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Various other advantages and benefits will become apparent to those skilled in the art by reading the following detailed description of the preferred embodiment.The accompanying drawings are only for the purpose of illustrating the preferred embodiment and are not to be considered as limiting the present invention.

[0039] Figure 1 The figure shows the effect of the fluid loss additives prepared in Example 1 and Comparative Example 1 on the water loss of seawater-mixed cement slurry at the same effective content (80°C);

[0040] Figure 2 The figure shows the effect of the fluid loss additives prepared in Example 2 and Comparative Example 2 on the water loss of seawater-mixed cement slurry at the same effective content (80°C);

[0041] Figure 3 The figure shows the effect of the fluid loss additives prepared in Examples 1 to 4 on the water loss of seawater-mixed cement slurry at different temperatures (1% effective content). DETAILED DESCRIPTION

[0042] In order to fully understand the purpose, features and effects of the present invention, the present invention is described in detail through the following specific embodiments. Except for the following contents, the process of the present invention adopts conventional methods or devices in the art. Unless otherwise specified, the following terms have the meanings commonly understood by those skilled in the art.

[0043] When a numerical range is disclosed herein, the range is considered continuous and includes the minimum and maximum values of the range, as well as every value between such minimum and maximum values. Further, when a range refers to an integer, every integer between the minimum and maximum values of the range is included. In addition, when multiple ranges are provided to describe a feature or characteristic, the ranges can be combined. In other words, unless otherwise indicated, all ranges disclosed herein are understood to include any and all subranges subsumed therein.

[0044] Specifically, in the first aspect, the present invention provides a pH-responsive release-type oil well cement fluid loss additive, comprising a core, an intermediate modification layer and an outer shell; wherein the core is a multi-polymer; the intermediate modification layer is a silanol attached to the surface of the core, and the outer shell is SiO2.

[0045] The pH-responsive release type oil well cement fluid loss additive of the present invention realizes intelligent controlled release through a core-shell structure design: the core is copolymerized by AMPS, N,N-dimethylacrylamide and a carboxyl group-containing monomer, providing carboxyl / sulfonic acid group fluid loss reduction functions; the outer shell is a SiO2 layer, which blocks ion penetration when the seawater pH is approximately 8, and quickly dissolves and releases the core when the cement slurry pH is greater than 12, thus solving the problems of metal ion interference, high-temperature performance degradation and high dosage dependence in seawater slurry preparation.

[0046] Wherein, the multi-component copolymer is a copolymer of 2-acrylamido-2-methylpropanesulfonic acid, N,N-dimethylacrylamide, and a carboxyl group-containing monomer.

[0047] In some preferred embodiments, the molar ratio of the 2-acrylamido-2-methylpropanesulfonic acid, the N,N-dimethylacrylamide, and the carboxyl group-containing monomer is 1:(0.4-0.7):(0.01-0.04).

[0048] When the ratio of the monomers is not within the above protection range of the present invention, the performance of the fluid loss additive will deteriorate, which is reflected in the increase of water loss in the cement slurry or even the inability to control the water loss.

[0049] More preferably, the molar ratio of the 2-acrylamido-2-methylpropanesulfonic acid, the N,N-dimethylacrylamide, and the carboxyl group-containing monomer is 1:(0.5-0.7):(0.02-0.03).

[0050] Wherein, the carboxyl group-containing monomer is one of acrylic acid, maleic acid, maleic anhydride, itaconic acid, fumaric acid, citraconic acid and mesaconic acid.

[0051] More preferably, the carboxyl group-containing monomer is one of acrylic acid, maleic acid, maleic anhydride and itaconic acid.

[0052] In some preferred embodiments, when preparing the multipolymer, the present invention uses a chain transfer agent to control the molecular weight of the multipolymer to be in the range of 100,000-200,000 Da, thereby effectively controlling the water loss of the cement slurry and improving the slurry fluidity.

[0053] If the molecular weight of the fluid loss additive is too small, it will not be able to control the water loss of the cement slurry; if the molecular weight of the fluid loss additive is too large, it will cause the cement slurry to thicken and affect the pumpability of the cement slurry.

[0054] The intermediate modified layer is chemically bonded to the core's multi-polymer and the outer shell's SiO2, enhancing the overall structural stability. This interfacial connection prevents premature shell peeling and ensures the fluid loss additive remains inert in non-target pH environments.

[0055] In some preferred embodiments, the shell has a thickness of 10-50 nm.

[0056] After testing, it was found that when the thickness of the shell is too small, the overall structure is unstable and easily damaged under mechanical force, resulting in loss of pH response release characteristics; when the thickness is too large, the fluid loss additive will not be released in time, causing the cement slurry to lose too much water in the target layer.

[0057] In a second aspect, the present invention also provides a method for preparing a pH-responsive release type oil well cement fluid loss additive, comprising: aqueous solution polymerization, surface amination modification and SiO2 shell coating process.

[0058] The preparation method of the pH-responsive release type oil well cement fluid loss additive of the present invention is introduced in detail below:

[0059] (1) Aqueous solution polymerization

[0060] The purpose of redox-initiated polymerization is to prepare a multi-polymer as the core.

[0061] Specifically, the process comprises: dissolving 2-acrylamido-2-methylpropanesulfonic acid, N,N-dimethylacrylamide, and a carboxyl group-containing monomer in water, adjusting the pH to 6.5-7.5 with calcium hydroxide, and exhausting oxygen from the container; adding a chain transfer agent and an initiator, heating to 40-60° C. (preferably 40° C.), reacting for 4-7 hours (preferably 4 hours), and then spray drying to obtain a core polymer powder.

[0062] In some preferred embodiments, the molar ratio of the 2-acrylamido-2-methylpropanesulfonic acid, the N,N-dimethylacrylamide, and the carboxyl group-containing monomer is 1:(0.4-0.7):(0.01-0.04); and the carboxyl group-containing monomer is one of acrylic acid, maleic acid, maleic anhydride, itaconic acid, fumaric acid, citraconic acid, and mesaconic acid.

[0063] The ratio of each monomer for preparing the multi-component copolymer, the effect of the type of carboxylic acid-containing monomer and its preferred embodiment are the same as those in the pH-responsive release type oil well cement fluid loss additive introduced in the first aspect of the present invention, and the present invention will not repeat them here.

[0064] In some preferred embodiments, after 2-acrylamido-2-methylpropanesulfonic acid, N,N-dimethylacrylamide, and carboxyl group-containing monomers are dissolved in water, the solid content of the system is adjusted to 20%-25% (preferably 20%).

[0065] In practice, when the solid content of the adjustment system is too high, the viscosity of the fluid loss additive will be too high, which is not conducive to construction; when the solid content is too low, the transportation cost of the fluid loss additive will increase.

[0066] In some preferred embodiments, the chain transfer agent is isopropanol, and the added amount is 0.5%-1.5% of the total weight of the monomers, thereby controlling the molecular weight range of the multi-polymer to be 100,000-200,000 Da.

[0067] In some preferred embodiments, the initiator is ammonium persulfate, and the amount added is 0.3%-2.0% of the total weight of the monomers.

[0068] Prior to spray drying, an appropriate polymer solution viscosity ensures good solution fluidity, facilitating efficient formation of fine, uniform droplets in the atomizer. This allows for rapid water removal during the drying process, preventing localized overheating and thermal degradation of the polymer. The particles formed at low viscosity exhibit a smooth surface and narrow size distribution, facilitating uniform coating of the subsequent amino-modified layer with the SiO2 shell, while also improving the powder's fluidity and storage stability.

[0069] In some preferred embodiments, the viscosity of the polymer solution before spray drying is lower than 200 mPa·s (25° C.), thereby significantly optimizing the drying process and product properties.

[0070] (2) Surface amino modification

[0071] Disperse the core polymer powder in the modified solution, adjust the pH to 4-5, heat to 55-65°C (preferably 60°C), and stir for 2-4 hours (preferably 2 hours). ) After centrifugation, washing and drying, amino-core particles were obtained.

[0072] In some preferred embodiments, the modifying liquid is a mixture of KH-550, ethanol, and water in a volume ratio of 5:80:15.

[0073] In this step, the ethoxy groups of KH-550 react with water to form silanols, simultaneously releasing ethanol, which acts as a cosolvent, promoting the dispersion of KH-550 and accelerating its hydrolysis. The resulting silanols (-Si-OH) undergo condensation reactions with the core polymer or inorganic substrate (such as pretreated SiO2 or hydroxyl-containing surfaces), forming stable Si-O-Si or Si-OC covalent bonds, anchoring the amino groups (-NH2) to the surface and thus constructing an amino layer.

[0074] (3) SiO2 shell coating

[0075] The amino core particles are dispersed in ethanol, a coating liquid and a catalyst are added, heated to 40-50° C. (preferably 40° C.), and stirred for 6-24 hours. After the reaction is completed, the mixture is centrifuged and dried to obtain a pH-responsive release oil well cement fluid loss additive with a core-shell structure.

[0076] In some preferred embodiments, the coating liquid is a mixture of tetraethoxysilane, ethanol, and water in a molar ratio of 1:4:4; and the catalyst is 0.06-0.15 mol / L (preferably 0.1 mol / L) ammonia water.

[0077] In this step, TEOS is first hydrolyzed to form silicic acid (Si(OH)4), which then undergoes dehydration and condensation to form a Si-O-Si network structure. This network is gradually deposited on the surface of the amino-containing core, where it forms a dense SiO2 shell through chemical bonding (e.g., Si-ON) and physical adsorption. Finally, heating at 40°C and prolonged stirring ensure uniform SiO2 coverage, forming a core-shell structure.

[0078] In some preferred embodiments, the reaction time can be adjusted to control the thickness of the SiO2 shell. The relationship between the reaction time and the shell thickness is: 6h→10-15nm, 12h→20-30nm, 24h→40-50nm.

[0079] The SiO2 shell in the present invention prevents Mg from being absorbed by the fluid loss additive and the slurry seawater through physical barrier and chemical inertness. 2+ Contact with the core carboxyl group, avoiding complex cross-linking; after the seawater and cement are mixed to form cement slurry, the OH - With Mg 2+ Mg(OH)2 precipitation is formed, and then the strong alkaline environment triggers the dissolution of the SiO2 shell, releasing the core polymer to play the role of reducing water loss, avoiding the core polymer and Mg 2+ Cross-linking reaction leads to room temperature thixotropy.

[0080] In a third aspect, the present invention further provides the use of the pH-responsive release type oil well cement fluid loss additive in the preparation of an oil well cement slurry system prepared with seawater.

[0081] The pH-responsive release type oil well cement fluid loss additive of the present invention has a dry mixing dosage of 0.5% to 2.0% of the cement mass. In a cement slurry environment with a pH greater than 12, the SiO2 shell dissolves and releases the core polymer to achieve fluid loss control.

[0082] Example

[0083] The present invention is further illustrated by way of examples below, but the invention is not limited to the scope of the examples. The experimental methods in the following examples where specific conditions are not specified were based on conventional methods and conditions. The raw materials used in the following examples were all commercially available.

[0084] Example 1

[0085] (1) Aqueous Solution Polymerization: 80 g of 2-acrylamido-2-methylpropanesulfonic acid, 20 g of N,N-dimethylacrylamide, and 0.8 g of acrylic acid were dissolved in water in appropriate proportions to adjust the solid content of the solution to 20%. The pH of the solution was then adjusted to 6.5-7.5 using calcium hydroxide. Oxygen was removed from the reaction vessel, and 0.5 g of isopropyl alcohol (a chain transfer agent) and 1.5 g of ammonium persulfate (an initiator) were added. The reaction was continued in a 40°C water bath for 4 hours to obtain a core polymer. After the reaction was completed, the core polymer powder was obtained by spray drying.

[0086] (2) Surface Amino Modification: The resulting core polymer powder was dispersed in 1000 L of a modification solution (comprising KH-550, ethanol, and water in a volume ratio of 5:80:15), the pH was adjusted to 4-5, and the mixture was stirred at 60°C for 2 hours. After the reaction, the mixture was washed by centrifugation and dried to obtain surface-amino-modified core particles.

[0087] (3) SiO2 Shell Coating: The resulting amino-modified core was dispersed in 100 g of ethanol, and 50 g of a coating solution (comprising tetraethoxysilane, ethanol, and water in a molar ratio of 1:4:4) was added. 0.1 mol / L ammonia water was added as a catalyst during the reaction. The system was stirred at 40°C for 6 hours to obtain a core-shell fluid loss additive with a SiO2 shell. After the reaction was completed, the mixture was centrifuged and dried to obtain the final product.

[0088] Example 2

[0089] (1) Aqueous Solution Polymerization: 80 g of 2-acrylamido-2-methylpropanesulfonic acid, 27 g of N,N-dimethylacrylamide, and 1.8 g of maleic acid were dissolved in water in appropriate proportions to adjust the solid content of the solution to 20%. The pH of the solution was then adjusted to 6.5-7.5 using calcium hydroxide. Oxygen was removed from the reaction vessel, and 1 g of isopropyl alcohol (a chain transfer agent) and 1.2 g of ammonium persulfate (an initiator) were added. The reaction was continued in a 40°C water bath for 4 hours to obtain a core polymer. After the reaction was completed, the core polymer powder was obtained by spray drying.

[0090] (2) Surface Amino Modification: The obtained core polymer powder was dispersed in 1000 L of the modification solution (same as in Example 1), the pH was adjusted to 4-5, and the mixture was stirred and reacted at 60°C for 2 hours. After the reaction, the mixture was washed by centrifugation and dried to obtain surface amino-modified core particles.

[0091] (3) SiO2 Shell Coating: The amino-treated core was dispersed in 100 g of ethanol, 50 g of the coating solution (same as in Example 1) was added, and 0.1 mol / L ammonia water was added as a catalyst during the reaction. The system was stirred at 40°C for 12 hours to obtain a core-shell fluid loss additive with a SiO2 shell. After the reaction was completed, the mixture was centrifuged and dried to obtain the final product.

[0092] Example 3

[0093] (1) Aqueous Solution Polymerization: 80 g of 2-acrylamido-2-methylpropanesulfonic acid, 16 g of N,N-dimethylacrylamide, and 0.5 g of itaconic acid were dissolved in water in appropriate proportions to adjust the solid content of the solution to 20%. The pH of the solution was then adjusted to 6.5-7.5 using calcium hydroxide. Oxygen was removed from the reaction vessel, and 0.48 g of isopropyl alcohol (a chain transfer agent) and 0.3 g of ammonium persulfate (an initiator) were added. The reaction was continued in a 40°C water bath for 4 hours to obtain a core polymer. After the reaction was completed, the core polymer powder was obtained by spray drying.

[0094] (2) Surface Amino Modification: The obtained core polymer powder was dispersed in 1000 L of the modification solution (same as in Example 1), the pH was adjusted to 4-5, and the mixture was stirred and reacted at 60°C for 2 hours. After the reaction, the mixture was washed by centrifugation and dried to obtain surface amino-modified core particles.

[0095] (3) SiO2 Shell Coating: The amino-treated core was dispersed in 100 g of ethanol, 50 g of the coating solution (same as in Example 1) was added, and 0.1 mol / L ammonia water was added as a catalyst during the reaction. The system was stirred at 40°C for 24 hours to obtain a core-shell fluid loss additive with a SiO2 shell. After the reaction was completed, the mixture was centrifuged and dried to obtain the final product.

[0096] Example 4

[0097] (1) Aqueous Solution Polymerization: 80 g of 2-acrylamido-2-methylpropanesulfonic acid, 20 g of N,N-dimethylacrylamide, and 1 g of mesaconic acid were dissolved in water in appropriate proportions to adjust the solid content of the solution to 20%. The pH of the solution was then adjusted to 6.5-7.5 using calcium hydroxide. Oxygen was removed from the reaction vessel, and 1.5 g of isopropyl alcohol (a chain transfer agent) and 2 g of ammonium persulfate (an initiator) were added. The reaction was continued in a 40°C water bath for 4 hours to obtain a core polymer. After the reaction was completed, the core polymer powder was obtained by spray drying.

[0098] (2) Surface Amino Modification: The obtained core polymer powder was dispersed in 1000 L of the modification solution (same as in Example 1), the pH was adjusted to 4-5, and the mixture was stirred and reacted at 60°C for 2 hours. After the reaction, the mixture was washed by centrifugation and dried to obtain surface amino-modified core particles.

[0099] (3) SiO2 Shell Coating: The amino-treated core was dispersed in 100 g of ethanol, 50 g of the coating solution (same as in Example 1) was added, and 0.1 mol / L ammonia water was added as a catalyst during the reaction. The system was stirred at 40°C for 12 hours to obtain a core-shell fluid loss additive with a SiO2 shell. After the reaction was completed, the mixture was centrifuged and dried to obtain the final product.

[0100] Comparative Example 1

[0101] A fluid loss additive was prepared according to the aqueous solution polymerization method described in Example 1. 80 g of 2-acrylamido-2-methylpropanesulfonic acid, 20 g of N,N-dimethylacrylamide, and 0.8 g of acrylic acid were dissolved in water in appropriate proportions to adjust the solids content of the solution to 20%. The pH of the solution was then adjusted to 6.5-7.5 using calcium hydroxide. Oxygen was removed from the reaction vessel, and 0.5 g of isopropyl alcohol (a chain transfer agent) and 1.5 g of ammonium persulfate (an initiator) were added. The mixture was reacted in a 40°C water bath for 4 hours to produce an aqueous fluid loss additive.

[0102] Comparative Example 2

[0103] A fluid loss additive was prepared according to the aqueous solution polymerization method described in Example 2. 80 g of 2-acrylamido-2-methylpropanesulfonic acid, 27 g of N,N-dimethylacrylamide, and 1.8 g of maleic acid were dissolved in water in appropriate proportions to adjust the solids content of the solution to 20%. The pH of the solution was then adjusted to 6.5-7.5 using calcium hydroxide. Oxygen was removed from the reaction vessel, and 1 g of isopropyl alcohol (a chain transfer agent) and 1.2 g of ammonium persulfate (an initiator) were added. The mixture was reacted in a 40°C water bath for 4 hours to produce an aqueous fluid loss additive.

[0104] Test Case

[0105] To evaluate the fluid loss control capabilities of the fluid loss additives prepared in each example and comparative example in seawater-mixed cement slurry, the water loss of each of the fluid loss additives prepared in each example and comparative example was measured, and the thixotropy of the slurries at 25°C was also measured. The water loss test for seawater-mixed cement slurry was conducted according to the "Static Fluid Loss Test for Cement Slurry" in the national standard "GB / T 19139-2012 Test Methods for Oil Well Cement" at a test temperature of 50°C. The thixotropy test for seawater-mixed cement slurry was conducted according to the "Determination of Gel Strength" in the national standard "GB / T 19139-2012 Test Methods for Oil Well Cement" at a test temperature of 25°C. Seawater preparation was performed according to the American Society for Testing and Materials standard "ASTM D1141-98(2021) Standard Practice for Preparation of Substitute Ocean Water." The seawater-mixed cement slurry formulation was: 800g oil well cement + 352g artificial seawater + 0.5% retarder. The retarder used is an organic phosphonate retarder.

[0106] from Figure 1 and Figure 2 The test results show that, at the same effective fluid loss additive content, the water loss of the seawater-mixed cement slurries added with the fluid loss additives of Examples 1 and 2 was significantly lower than that of the seawater-mixed cement slurries added with the fluid loss additives of Comparative Examples 1 and 2. At an effective fluid loss additive content of 1%, the fluid loss additives of Examples 1 and 2 both controlled the water loss of the seawater-mixed cement slurries to below 50 mL. However, at an effective fluid loss additive content of 2%, the fluid loss additives of Comparative Examples 1 and 2 were still unable to control the water loss of the seawater-mixed cement slurries to below 50 mL. This demonstrates that the fluid loss additives prepared in Examples 1 and 2 have excellent seawater resistance.

[0107] from Figure 3 The test results show that, within the temperature range of 20-90°C, the fluid loss additives prepared in Examples 1 to 4 can control the water loss of cement slurry to less than 50 mL at an effective content of 1%, showing good temperature adaptability.

[0108] The thixotropy test results of seawater-mixed cement slurry are shown in Table 1. Comparative Examples 1 and 2 used aqueous fluid loss additives with a solid content of 20%, while Examples 1 and 2 used solid fluid loss additives. Based on the principle of equal effective content, the aqueous fluid loss additive dosage was 5% in the seawater-mixed cement slurries containing Comparative Examples 1 and 2, while the solid fluid loss additive dosage was 1% in the seawater-mixed cement slurries containing Examples 1 and 2. The test results in Table 1 indicate that the 10s initial shear value and 10min final shear value of the seawater-mixed cement slurries containing Comparative Examples 1 and 2 were both high, with the 10s initial shear value exceeding 120 Pa and the 10min final shear value exceeding the measuring range, demonstrating very strong thickening and thixotropy. The 10s initial shear value and 10min final shear value of the cement slurry mixed with seawater added in Example 1 and Example 2 are both low, the 10s initial shear value is lower than 6Pa, the 10min final shear value is lower than 13Pa, and the initial and final shear interpolation value Δτ is less than 8Pa. The cement slurry does not show thixotropy and has good rheological properties.

[0109] Table 1

[0110]

[0111] The present invention has been disclosed above with reference to preferred embodiments. However, those skilled in the art will appreciate that these embodiments are intended only to illustrate the present invention and are not to be construed as limiting the scope of the present invention. It should be noted that any equivalent variations and substitutions to these embodiments are to be considered encompassed within the scope of the claims of the present invention. Therefore, the scope of protection of the present invention shall be subject to the scope defined in the claims.

Claims

1. A pH-responsive release type oil well cement fluid loss additive, characterized in that: It includes a core, an intermediate modified layer and an outer shell; The core is a multi-component copolymer; the intermediate modification layer is silanol attached to the surface of the core; and the outer shell is SiO2.

2. The pH-responsive release type oil well cement fluid loss additive according to claim 1, characterized in that The multi-component copolymer is a copolymer of 2-acrylamide-2-methylpropanesulfonic acid, N,N-dimethylacrylamide and a carboxyl group-containing monomer.

3. The pH-responsive release type oil well cement fluid loss additive according to claim 2, characterized in that: The molar ratio of the 2-acrylamido-2-methylpropanesulfonic acid, the N,N-dimethylacrylamide, and the carboxyl group-containing monomer is 1:(0.4-0.7):(0.01-0.04).

4. The pH-responsive release type oil well cement fluid loss additive according to claim 2, characterized in that: The carboxyl group-containing monomer is one of acrylic acid, maleic acid, maleic anhydride, itaconic acid, fumaric acid, citraconic acid, and mesaconic acid.

5. The pH-responsive release type oil well cement fluid loss additive according to claim 1, characterized in that: The molecular weight of the multi-polymer is in the range of 100,000-200,000 Da.

6. The pH-responsive release type oil well cement fluid loss additive according to claim 1, characterized in that: The thickness of the shell is 10-50 nm.

7. A method for preparing a pH-responsive release type oil well cement fluid loss additive, characterized in that: include: (1) dissolving 2-acrylamido-2-methylpropanesulfonic acid, N,N-dimethylacrylamide, and a carboxyl group-containing monomer in water, adjusting the pH, and then venting the oxygen in the container; (2) adding a chain transfer agent and an initiator, heating, and spray drying to obtain a core polymer powder after the reaction is completed; (3) dispersing the core polymer powder in a modification solution, adjusting the pH, heating, stirring, centrifuging, washing, and drying after the reaction is completed to obtain amino-modified core particles; (4) dispersing the amino-containing core particles in ethanol, adding a coating liquid and a catalyst, heating and stirring, and after the reaction is completed, centrifuging and drying to obtain a pH-responsive release type oil well cement fluid loss additive with a core-shell structure.

8. The preparation method according to claim 7, characterized in that The molar ratio of the 2-acrylamido-2-methylpropanesulfonic acid, the N,N-dimethylacrylamide, and the carboxyl group-containing monomer is 1:(0.4-0.7):(0.01-0.04).

9. The preparation method according to claim 7, characterized in that Step (1) comprises: dissolving 2-acrylamido-2-methylpropanesulfonic acid, N,N-dimethylacrylamide and a carboxyl group-containing monomer in water, adjusting the solid content of the system to 20%-25%, and adjusting the pH to 6.5-7.5 using calcium hydroxide.

10. The preparation method according to claim 7, characterized in that The chain transfer agent is isopropyl alcohol, and the added amount is 0.5%-1.5% of the total weight of the monomer; the initiator is ammonium persulfate, and the added amount is 0.3%-2.0% of the total weight of the monomer.

11. The preparation method according to claim 7, characterized in that The heating temperature of step (2) is 40-60° C., and the reaction time is 4-7 hours.

12. The preparation method according to claim 7, characterized in that The modified liquid is a mixture of KH-550, ethanol and water in a volume ratio of 5:80:

15.

13. The preparation method according to claim 7, characterized in that In step (3), the pH is 4-5, the heating temperature is 55-65° C., and the stirring time is 2-4 hours.

14. The preparation method according to claim 7, characterized in that The coating liquid is a mixture of tetraethoxysilane, ethanol and water in a molar ratio of 1:4:4; and the catalyst is 0.06-0.15 mol / L ammonia water.

15. The preparation method according to claim 7, characterized in that In step (4), the heating temperature is 40-50° C. and the stirring reaction time is 6-24 hours.

16. A pH-responsive release type oil well cement fluid loss additive, characterized in that: The preparation method is described in any one of claims 7 to 15.

17. Use of the pH-responsive release type oil well cement fluid loss additive according to any one of claims 1 to 6 and 16 in preparing an oil well cement slurry system prepared with seawater.

18. The use according to claim 17, characterized in that The dry mixing amount of the pH-responsive release type oil well cement fluid loss additive is 0.5%-2.0% of the cement mass.

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