A self-healing cement slurry for well cementing and its preparation method

Abstract

The present invention relates to the technical field of oil and gas field development, and specifically relates to a self-healing cement slurry for well cementing and a preparation method thereof. The self-healing cement slurry comprises the following components: 4-6 parts by weight of a repair agent, 41-53 parts by weight of cement, 18-22 parts by weight of silica fume, 0.1-0.5 parts by weight of basalt fiber, 1-5 parts by weight of an expansion agent, 0.3-0.9 parts by weight of a fluid loss reducer, 0.2-0.4 parts by weight of bentonite, 1.7-2.3 parts by weight of a naphthalene sulfonate formaldehyde condensate, 0.1-0.2 parts by weight of dimethyl silicone oil, and 1-3 parts by weight of a retarder; wherein, the repair agent is modified by ethyl cyanoacrylate, candelilla wax and 3-isocyanatopropyltrimethoxysilane in a mass ratio of 1:1-2:0.1-0.3. The present invention is designed to use ethyl cyanoacrylate to repair the microcracks of the cement, use candelilla wax and nano-silica to perform microencapsulation to protect ethyl cyanoacrylate, and use the methoxy groups of 3-isocyanatopropyltrimethoxysilane to enhance the interfacial bonding force between the microcapsules and the cement slurry system, thereby effectively extending the service life of the cement structure.

Description

Technical Field

[0001] The present invention relates to the technical field of oil and gas field development, and more specifically, to a self-healing cement slurry for well cementing and a preparation method thereof. Background Art

[0002] Well cementing refers to fixing the casing in the wellbore through specific technological means and using materials such as cement slurry to seal formations such as oil, gas, and water. Its basic steps include bottom hole preparation, cement slurry calculation, cement slurry mixing, well cementing injection, mud backflow, and curing, etc. The purpose is to ensure the smooth progress of drilling and the safe and efficient subsequent oil and gas production, and it has wide applications and significance in the drilling and completion operations of oil and gas wells.

[0003] However, after the cement slurry in the oil and gas well solidifies, due to the long-term stress of the cement, microcracks are formed in the cement. These microcracks are small and their specific positions cannot be determined. Therefore, using conventional repair methods is time-consuming and laborious, and the success rate is low. In view of this, we propose a self-healing cement slurry for well cementing and a preparation method thereof. Summary of the Invention

[0004] The purpose of the present invention is to provide a self-healing cement slurry for well cementing and a preparation method thereof to solve the problems raised in the above background art.

[0005] To achieve the above purpose, the present invention provides a self-healing cement slurry for well cementing, which comprises the following components: 4-6 parts by weight of a repair agent, 41-53 parts by weight of cement, 18-22 parts by weight of silica fume, 0.1-0.5 parts by weight of basalt fiber, 1-5 parts by weight of an expansion agent, 0.3-0.9 parts by weight of a fluid loss reducer, 0.2-0.4 parts by weight of bentonite, 1.7-2.3 parts by weight of a naphthalene sulfonate formaldehyde condensate, 0.1-0.2 parts by weight of dimethyl silicone oil, and 1-3 parts by weight of a retarder;

[0006] Among them, the repair agent is modified from ethyl cyanoacrylate, candelilla wax, and 3-isocyanatopropyltrimethoxysilane according to a mass ratio of 1:1-2:0.1-0.3.

[0007] Preferably, the expansion agent is a CaO-MgO composite system with an expansion rate of 0.05%-0.2%.

[0008] Preferably, the fluid loss reducer is one of hydroxyethyl cellulose and polyacrylamide, and the retarder is one of calcium lignosulfonate and tartaric acid.

[0009] Preferably, the preparation method of the repair agent is as follows:

[0010] Under nitrogen protection, candelilla wax is heated and melted at 60 - 70 °C. Nano-silica powder is added under continuous stirring to ensure complete and uniform dispersion. Then, 3-isocyanatopropyltrimethoxysilane is dropped in to obtain the wall material solution for standby.

[0011] Ethyl cyanoacrylate is mixed evenly with hydroquinone and dioctyl phthalate to obtain the core material solution. The core material solution is dropped into the wall material solution drop by drop, and stirring is continued for 30 - 50 min to obtain the repair agent.

[0012] Ethyl cyanoacrylate will initiate an anionic polymerization reaction when encountering water. Once the polymerization reaction is initiated, monomer molecules will quickly link together to form long-chain polymers. As the polymerization reaction proceeds, ethyl cyanoacrylate gradually changes from liquid to solid, forming a tough polymer network that tightly binds the surface of the adherend. Due to the characteristics of rapid reaction and strong adhesion of ethyl cyanoacrylate, it becomes an effective crack repair agent. However, its sensitivity to humidity also affects its application in self-healing cement slurries for well cementing. Therefore, microencapsulation is carried out using a mixture of candelilla wax and nano-silica. Since candelilla wax, as a natural material, has hydrophobic and barrier properties, wrapping the repair agent inside it can isolate moisture and effectively prevent premature curing of ethyl cyanoacrylate, thus ensuring the normal release and function of the ethyl cyanoacrylate repair agent. To further improve the protective effect of candelilla wax and its stability in the cement slurry system, 3-isocyanatopropyltrimethoxysilane is additionally added. The hydroxyl groups of candelilla wax and nano-silica react with the isocyanate groups of 3-isocyanatopropyltrimethoxysilane to crosslink, thereby improving the chemical stability, thermal stability, mechanical strength, and barrier effect of candelilla wax. And through the methoxy groups of 3-isocyanatopropyltrimethoxysilane, in the highly alkaline environment of the cement slurry, the methoxy groups directly undergo a condensation reaction with the hydroxyl groups on the surface of cement particles to generate stable Si - O - Si bonds, thereby enhancing the interfacial binding force between the microcapsules and the cement slurry system, without relying on the hydrolysis to generate silanol groups and improving compatibility.

[0013] Since the ethyl cyanoacrylate monomer is prone to polymerization, hydroquinone is added to prevent anionic polymerization and free radical polymerization. To improve the brittleness after curing, dioctyl phthalate is added to increase the impact strength of the cured layer.

[0014] Preferably, the nano-silica is 15 - 28% of the mass of candelilla wax.

[0015] Preferably, the hydroquinone accounts for 0.5 - 0.8% of the mass of ethyl cyanoacrylate.

[0016] Preferably, the dioctyl phthalate accounts for 11.2 - 13.5% of the mass of ethyl cyanoacrylate.

[0017] Preferably, the particle size distribution of the repair agent is 25 - 50 μm.

[0018] On the other hand, the present invention provides a method for preparing a self - healing cement slurry for well cementing, which is used to prepare the above - mentioned self - healing cement slurry for well cementing, and includes the following steps:

[0019] Add cement, silica fume, basalt fiber, expansive agent, fluid loss reducer, bentonite, naphthalene sulfonate formaldehyde condensate and repair agent into a stirrer, premix at room temperature of 100 - 200 rpm for 5 - 10 min, then add dimethyl silicone oil and retarder, and then calculate the required water addition amount according to the water - cement ratio, add water and mix and stir at 500 - 1000 rpm for 10 - 15 min to obtain the self - healing cement slurry for well cementing.

[0020] Preferably, the water - cement ratio is 0.38 - 0.45.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] In the self - healing cement slurry for well cementing and its preparation method, a repair agent is prepared by using ethyl cyanoacrylate, candelilla wax and 3 - isocyanatopropyltrimethoxysilane as main raw materials. The anionic polymerization reaction of ethyl cyanoacrylate is used to bond and repair the micro - cracks of the cement. Micro - encapsulation is carried out with candelilla wax and nano - silica to protect ethyl cyanoacrylate and prevent its premature curing and failure. The methoxy group of 3 - isocyanatopropyltrimethoxysilane is used to enhance the interfacial bonding force between the micro - capsule and the cement slurry system, improve the compatibility, and further improve the chemical stability, thermal stability, mechanical strength and barrier effect of the micro - capsule wall material, thereby effectively extending the service life of the cement structure. Specific Embodiments

[0023] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0024] A self-healing cement slurry for well cementing of the present invention comprises the following components: 4-6 parts by weight of a repair agent, 41-53 parts by weight of cement, 18-22 parts by weight of silica fume, 0.1-0.5 parts by weight of basalt fiber, 1-5 parts by weight of an expansive agent, 0.3-0.9 parts by weight of a fluid loss control agent, 0.2-0.4 parts by weight of bentonite, 1.7-2.3 parts by weight of a naphthalene sulfonate formaldehyde condensate, 0.1-0.2 parts by weight of dimethyl silicone oil, and 1-3 parts by weight of a retarder;

[0025] Among them, the repair agent is modified from ethyl cyanoacrylate, candelilla wax, and 3-isocyanatopropyltrimethoxysilane in a mass ratio of 1:1-2:0.1-0.3;

[0026] Preferably, the fluid loss control agent is hydroxyethyl cellulose and the retarder is calcium lignosulfonate.

[0027] Example 1: A preparation method of a self-healing cement slurry for well cementing comprises the following steps:

[0028] Prepare the components: 6 parts by weight of a repair agent, 53 parts by weight of cement, 22 parts by weight of silica fume, 0.5 parts by weight of basalt fiber, 5 parts by weight of a CaO-MgO composite system, 0.9 parts by weight of hydroxyethyl cellulose, 0.4 parts by weight of bentonite, 2.3 parts by weight of a naphthalene sulfonate formaldehyde condensate, 0.2 parts by weight of dimethyl silicone oil, and 3 parts by weight of calcium lignosulfonate;

[0029] Among them, the repair agent is modified from ethyl cyanoacrylate, candelilla wax, and 3-isocyanatopropyltrimethoxysilane in a mass ratio of 1:1:0.1; nano-silica is 20% of the mass of candelilla wax; hydroquinone accounts for 0.8% of the mass of ethyl cyanoacrylate; dioctyl phthalate accounts for 13.5% of the mass of ethyl cyanoacrylate;

[0030] Under nitrogen protection, melt candelilla wax at 60 °C, add nano-silica powder under continuous stirring to ensure complete and uniform dispersion, and then dropwise add 3-isocyanatopropyltrimethoxysilane to obtain a wall material solution for standby; mix ethyl cyanoacrylate with hydroquinone and dioctyl phthalate evenly to obtain a core material solution, and dropwise add the core material solution to the wall material solution, and continuously stir for 30 min to obtain a repair agent with a particle size distribution of 30 μm;

[0031] Add the cement, silica fume, basalt fiber, expansive agent, fluid loss control agent, bentonite, naphthalene sulfonate formaldehyde condensate, and repair agent to a stirrer, premix at room temperature at 100 rpm for 5 min, then add dimethyl silicone oil and retarder, and then calculate the required water addition amount according to a water-cement ratio of 0.41, and add water and mix and stir at 1000 rpm for 10 min to obtain a self-healing cement slurry for well cementing.

[0032] Example 2: A preparation method of a self-healing cement slurry for well cementing, comprising the following steps:

[0033] Prepare components: 6 parts by weight of a repair agent, 53 parts by weight of cement, 22 parts by weight of silica fume, 0.5 part by weight of basalt fiber, 5 parts by weight of a CaO-MgO composite system, 0.9 part by weight of hydroxyethyl cellulose, 0.4 part by weight of bentonite, 2.3 parts by weight of naphthalene sulfonate formaldehyde condensate, 0.2 part by weight of dimethyl silicone oil, and 3 parts by weight of calcium lignosulfonate;

[0034] Among them, the repair agent is modified from ethyl cyanoacrylate, candelilla wax, and 3-isocyanatopropyltrimethoxysilane according to a mass ratio of 1:1.5:0.2; nano-silica is 20% of the mass of candelilla wax; hydroquinone accounts for 0.8% of the mass of ethyl cyanoacrylate; dioctyl phthalate accounts for 13.5% of the mass of ethyl cyanoacrylate;

[0035] Under nitrogen protection, melt candelilla wax at 60 °C, add nano-silica powder under continuous stirring to ensure complete and uniform dispersion, and then dropwise add 3-isocyanatopropyltrimethoxysilane to obtain a wall material solution for standby; mix ethyl cyanoacrylate with hydroquinone and dioctyl phthalate evenly to obtain a core material solution, and dropwise add the core material solution into the wall material solution, and continuously stir for 30 min to obtain a repair agent with a particle size distribution of 30 μm;

[0036] Add cement, silica fume, basalt fiber, an expansion agent, a fluid loss reducer, bentonite, naphthalene sulfonate formaldehyde condensate, and a repair agent to a stirrer, premix at room temperature at 100 rpm for 5 min, then add dimethyl silicone oil and a retarder, and then calculate the required water addition amount according to a water-cement ratio of 0.41, and add water and mix and stir at 1000 rpm for 10 min to obtain a self-healing cement slurry for well cementing.

[0037] Example 3: A preparation method of a self-healing cement slurry for well cementing, comprising the following steps:

[0038] Prepare components: 6 parts by weight of a repair agent, 53 parts by weight of cement, 22 parts by weight of silica fume, 0.5 part by weight of basalt fiber, 5 parts by weight of a CaO-MgO composite system, 0.9 part by weight of hydroxyethyl cellulose, 0.4 part by weight of bentonite, 2.3 parts by weight of naphthalene sulfonate formaldehyde condensate, 0.2 part by weight of dimethyl silicone oil, and 3 parts by weight of calcium lignosulfonate;

[0039] Among them, the repair agent is modified from ethyl cyanoacrylate, candelilla wax, and 3-isocyanatopropyltrimethoxysilane according to a mass ratio of 1:2:0.3; nano-silica is 20% of the mass of candelilla wax; hydroquinone accounts for 0.8% of the mass of ethyl cyanoacrylate; dioctyl phthalate accounts for 13.5% of the mass of ethyl cyanoacrylate;

[0040] Under nitrogen protection, candelilla wax is heated and melted at 60 °C. Nano-silica powder is added under continuous stirring to ensure complete and uniform dispersion. Then, 3-isocyanatopropyltrimethoxysilane is dropped in to obtain a wall material solution for standby. Ethyl cyanoacrylate is mixed evenly with hydroquinone and dioctyl phthalate to obtain a core material solution, and the core material solution is dropped into the wall material solution drop by drop while continuously stirring for 30 min to obtain a repair agent with a particle size distribution of 30 μm.

[0041] Cement, silica fume, basalt fiber, expansive agent, fluid loss reducer, bentonite, naphthalene sulfonate formaldehyde condensate, and the repair agent are added to a stirrer and premixed at room temperature at 100 rpm for 5 min. Then, dimethyl silicone oil and retarder are added. Then, the required water addition amount is calculated according to a water-cement ratio of 0.41, and water is added and mixed and stirred at 1000 rpm for 10 min to obtain a well cementing self-healing slurry.

[0042] Example 4: A preparation method of a well cementing self-healing slurry, comprising the following steps:

[0043] Prepare components: 6 parts by weight of repair agent, 41 parts by weight of cement, 18 parts by weight of silica fume, 0.1 part by weight of basalt fiber, 1 part by weight of CaO-MgO composite system, 0.3 part by weight of hydroxyethyl cellulose, 0.2 part by weight of bentonite, 1.7 parts by weight of naphthalene sulfonate formaldehyde condensate, 0.1 part by weight of dimethyl silicone oil, and 1 part by weight of calcium lignosulfonate;

[0044] Among them, the repair agent is modified from ethyl cyanoacrylate, candelilla wax, and 3-isocyanatopropyltrimethoxysilane according to a mass ratio of 1:2:0.3; the nano-silica is 15% of the mass of candelilla wax; hydroquinone accounts for 0.5% of the mass of ethyl cyanoacrylate; dioctyl phthalate accounts for 11.2% of the mass of ethyl cyanoacrylate;

[0045] Under nitrogen protection, candelilla wax is heated and melted at 60 °C. Nano-silica powder is added under continuous stirring to ensure complete and uniform dispersion. Then, 3-isocyanatopropyltrimethoxysilane is dropped in to obtain a wall material solution for standby. Ethyl cyanoacrylate is mixed evenly with hydroquinone and dioctyl phthalate to obtain a core material solution, and the core material solution is dropped into the wall material solution drop by drop while continuously stirring for 30 min to obtain a repair agent with a particle size distribution of 25 μm.

[0046] Cement, silica fume, basalt fiber, expansive agent, fluid loss reducer, bentonite, naphthalene sulfonate formaldehyde condensate, and the repair agent are added to a stirrer and premixed at room temperature at 100 rpm for 5 min. Then, dimethyl silicone oil and retarder are added. Then, the required water addition amount is calculated according to a water-cement ratio of 0.41, and water is added and mixed and stirred at 1000 rpm for 10 min to obtain a well cementing self-healing slurry.

[0047] Example 5: A preparation method of a self-healing cement slurry for well cementing, comprising the following steps:

[0048] Prepare the components: 4 parts by weight of a repair agent, 53 parts by weight of cement, 22 parts by weight of silica fume, 0.5 parts by weight of basalt fiber, 5 parts by weight of a CaO-MgO composite system, 0.9 parts by weight of hydroxyethyl cellulose, 0.4 parts by weight of bentonite, 2.3 parts by weight of naphthalene sulfonate formaldehyde condensate, 0.2 parts by weight of dimethyl silicone oil, and 3 parts by weight of calcium lignosulfonate;

[0049] Among them, the repair agent is modified from ethyl cyanoacrylate, candelilla wax, and 3-isocyanatopropyltrimethoxysilane in a mass ratio of 1:2:0.3; nano-silica is 20% of the mass of candelilla wax; hydroquinone accounts for 0.8% of the mass of ethyl cyanoacrylate; dioctyl phthalate accounts for 13.5% of the mass of ethyl cyanoacrylate;

[0050] Under nitrogen protection, melt candelilla wax at 60 °C, add nano-silica powder under continuous stirring to ensure complete and uniform dispersion, and then dropwise add 3-isocyanatopropyltrimethoxysilane to obtain a wall material solution for standby; mix ethyl cyanoacrylate with hydroquinone and dioctyl phthalate evenly to obtain a core material solution, and dropwise add the core material solution to the wall material solution, and continuously stir for 30 min to obtain a repair agent with a particle size distribution of 30 μm;

[0051] Add cement, silica fume, basalt fiber, expansion agent, fluid loss reducer, bentonite, naphthalene sulfonate formaldehyde condensate, and repair agent to a stirrer, premix at room temperature at 100 rpm for 5 min, then add dimethyl silicone oil and retarder, and then calculate the required water addition amount according to a water-cement ratio of 0.41, and add water and mix and stir at 1000 rpm for 10 min to obtain a self-healing cement slurry for well cementing.

[0052] Example 6: A preparation method of a self-healing cement slurry for well cementing, comprising the following steps:

[0053] Prepare the components: 5 parts by weight of a repair agent, 53 parts by weight of cement, 22 parts by weight of silica fume, 0.5 parts by weight of basalt fiber, 5 parts by weight of a CaO-MgO composite system, 0.9 parts by weight of hydroxyethyl cellulose, 0.4 parts by weight of bentonite, 2.3 parts by weight of naphthalene sulfonate formaldehyde condensate, 0.2 parts by weight of dimethyl silicone oil, and 3 parts by weight of calcium lignosulfonate;

[0054] Among them, the repair agent is modified from ethyl cyanoacrylate, candelilla wax, and 3-isocyanatopropyltrimethoxysilane in a mass ratio of 1:2:0.3; nano-silica is 20% of the mass of candelilla wax; hydroquinone accounts for 0.8% of the mass of ethyl cyanoacrylate; dioctyl phthalate accounts for 13.5% of the mass of ethyl cyanoacrylate;

[0055] Under nitrogen protection, candelilla wax was heated and melted at 60 °C. Nano-silica powder was added under continuous stirring to ensure complete and uniform dispersion. Then, 3-isocyanatopropyltrimethoxysilane was added dropwise to obtain the wall material solution for standby. Ethyl cyanoacrylate was mixed evenly with hydroquinone and dioctyl phthalate to obtain the core material solution. The core material solution was added dropwise to the wall material solution, and stirring was continued for 30 min to obtain a repair agent with a particle size distribution of 30 μm.

[0056] Cement, silica fume, basalt fiber, expansive agent, fluid loss reducer, bentonite, naphthalene sulfonate formaldehyde condensate, and the repair agent were added to a stirrer and premixed at room temperature at 100 rpm for 5 min. Then, dimethyl silicone oil and retarder were added. The required water addition was calculated according to a water-cement ratio of 0.41, and water was added and mixed and stirred at 1000 rpm for 10 min to obtain a well cementing self-healing slurry.

[0057] Comparative Example 1: The method of Example 3 was used without adding the repair agent.

[0058] Comparative Example 2: The method of Example 3 was used, and ethyl cyanoacrylate and candelilla wax were directly used without modifying ethyl cyanoacrylate and candelilla wax with 3-isocyanatopropyltrimethoxysilane.

[0059] A well cementing self-healing slurry prepared by using a repair agent according to the present invention, wherein the performance index test items and test standards of the slurry are as follows:

[0060] The compressive strength of the cement stone under standard curing conditions was tested to observe whether the self-healing components affect the initial strength and long-term strength development. Standard-sized specimens of 50 mm × 50 mm × 50 mm cubes were prepared and cured under standard curing conditions (20 °C ± 2 °C, relative humidity 95%) for 7 days. A universal material testing machine was used for compression testing, and the loading rate was kept constant at (0.5 ± 0.05) MPa / s. The maximum load at failure was recorded, and the compressive strength F c = F max / A, where F c is the compressive strength (MPa), F max is the maximum bearing capacity (N), and A is the bearing area of the specimen (m²). Then, a spherical indenter with a diameter of 2 mm was used to apply an initial load of 200 N to simulate the compressive strength recovery rate after 7 days of self-healing at 50 °C under the condition that the specimen was subjected to local stress concentration.

[0061] The permeability test is used to evaluate the permeability characteristics of the cement stone and determine whether the self-healing component can effectively reduce the permeability and improve the sealing performance. A cylindrical specimen with a diameter of 50 mm and a height of 100 mm cured under standard conditions for 7 days is installed on the permeameter. Nitrogen is used as the permeating gas, and a pressure of 20 MPa is applied and maintained for 1 h. Record the fluid volume passing through the specimen and the corresponding pressure difference. According to Darcy's law, calculate the permeability coefficient k = Q⋅L / A⋅ΔP⋅t, where Q is the fluid volume passing through the specimen (m³), L is the length of the specimen (m), A is the cross-sectional area of the specimen (m²), ΔP is the pressure difference (Pa), and t is the time (s). And calculate the self-healing rate = (permeability coefficient before self-healing - permeability coefficient after self-healing) / permeability coefficient before self-healing × 100%. Compare the self-healing rates of the specimens before and after adding the self-healing component to evaluate its effect on reducing permeability.

[0062] Through the above standards, the self-healing cement slurries for well cementing prepared in Examples 1-6 and Comparative Examples 1-2 were tested, and the obtained data are shown in Table 1:

[0063] Table 1 Performance data of Examples 1-6 and Comparative Examples 1-2

[0064]

[0065] The above data fully show that compared with Comparative Examples 1-2, in Examples 1-6, it can be clearly seen the role of the repair agent in the self-healing performance of the self-healing cement slurry for well cementing.

[0066] Since the present invention uses a repair agent to prepare the self-healing cement slurry for well cementing, the performance of the self-healing cement slurry for well cementing is effectively improved by the repair agent, specifically as follows:

[0067] It can be seen from Examples 1-3 that as the proportion of the repair agent component increases continuously, the self-healing performance of the self-healing cement slurry for well cementing is significantly improved. Since ethyl cyanoacrylate is a fast-curing adhesive, it can polymerize and cure rapidly when contacting moisture to form a strong polymer film, which helps to seal the cracks. Candelilla wax helps to protect the repair agent from premature curing. 3-isocyanatopropyltrimethoxysilane can enhance the adhesion between the repair agent and the cement matrix and promote better interfacial bonding. When the repair agent containing these components is introduced into the cement slurry, the repair agent will react after contacting water on the crack surface to generate a filler with high strength and durability, thereby effectively filling the cracks and restoring part of the mechanical properties.

[0068] It can be seen from Example 3 and Example 4 that as the contents of other components change continuously, the self-healing performance of the self-healing cement slurry for well cementing has no obvious change, indicating that small changes within a certain range of other components are not sufficient to significantly affect the self-healing effect of the self-healing cement slurry for well cementing.

[0069] It can be seen from Example 3, Example 5 and Example 6 that as the content of the healing agent changes continuously, the self-healing performance of the well cementing self-healing slurry changes continuously. With the increase of the healing agent content, the number of healing agents distributed in the cement matrix increases, which means that more healing materials can participate in the repair process when cracks or damages occur, thus improving the overall self-healing efficiency and effect. In addition, introducing microcapsules into the slurry will additionally increase the porosity inside the cement matrix, thereby weakening its overall density and resulting in a decrease in compressive strength. Therefore, as the healing agent content increases, the compressive strength of the slurry decreases instead.

[0070] According to the above test experiments, a well cementing self-healing slurry prepared according to Example 3 has the optimal performance, so Example 3 is taken as the optimal example.

[0071] It can be seen from the comparison between Example 3 and Comparative Examples 1-2:

[0072] No healing agent was added in Comparative Example 1, and the self-healing effect of the well cementing self-healing slurry was poor. Not adding the healing agent means that the cement matrix itself lacks components that can actively respond to cracks and repair them. Once cracks appear, the cement matrix can only rely on its own hydration products and fine particles to try to fill the cracks, but this is usually not enough to completely seal the cracks or restore the original strength, and the repair process is inefficient, thus significantly affecting the overall mechanical properties of the material.

[0073] In Comparative Example 2, ethyl cyanoacrylate and candelilla wax were directly used without modifying ethyl cyanoacrylate and candelilla wax with 3-isocyanatopropyltrimethoxysilane, and the self-healing effect of the well cementing self-healing slurry was even worse. Due to the lack of effective chemical bonding of 3-isocyanatopropyltrimethoxysilane, the compatibility and adhesion between the microcapsules and the cement matrix are weak, the interfacial adhesion is insufficient, and stress concentration points are easily formed. The areas with stress concentration are more likely to be damaged, thus reducing the load-bearing capacity and service life of the entire structure. Therefore, the self-healing effect of the well cementing self-healing slurry is poor.

[0074] In summary, by wrapping ethyl cyanoacrylate in candelilla wax and nano-silica and modifying it with 3-isocyanatopropyltrimethoxysilane, the performance and stability of the healing agent are significantly improved;

[0075] First, ethyl cyanoacrylate can rapidly polymerize upon contact with moisture to form a tough polymer network, possessing strong adhesiveness and good repair effects. However, it is extremely sensitive to moisture in the environment and is prone to premature curing during storage or construction, which affects its stability and release effect in the cement slurry. Therefore, microencapsulation is carried out using a mixture of candelilla wax and nano-silica, with the repair agent encapsulated inside, which can isolate moisture and effectively prevent premature curing of ethyl cyanoacrylate. When microcracks occur in the cement matrix due to stress, the mechanical stress at the crack causes the microcapsule wall material to rupture, releasing the ethyl cyanoacrylate in the core material. At the same time, the moisture infiltrating into the crack contacts the ethyl cyanoacrylate, triggering its anionic polymerization reaction to rapidly form a polymer network and repair the crack. To avoid premature polymerization of ethyl cyanoacrylate, the addition of nano-silica not only enhances the mechanical strength of the microcapsules but also provides more active hydroxyl sites, promoting subsequent chemical modification;

[0076] Secondly, to further improve the protective effect of candelilla wax and its stability in the cement slurry system, 3-isocyanatopropyltrimethoxysilane is introduced, which enhances the chemical stability, thermal stability, and mechanical strength of the microcapsules, and significantly enhances the interfacial bonding force between the microcapsules and the cement slurry system, improving the compatibility;

[0077] Finally, through this microencapsulation strategy, not only is the problem of the moisture sensitivity of ethyl cyanoacrylate solved, but the storage stability of the repair agent is also significantly enhanced, effectively extending the service life of the cement structure.

[0078] The above shows and describes the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited by the above embodiments. The above embodiments and descriptions in the specification are only preferred examples of the present invention and are not used to limit the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and all these changes and improvements fall within the scope of the present invention claimed.

Claims

1. A self-healing cement slurry for cementing, characterized in that: The invention comprises the following components: 4-6 parts by weight of a repairing agent, 41-53 parts by weight of cement, 18-22 parts by weight of silica fume, 0.1-0.5 parts by weight of basalt fiber, 1-5 parts by weight of an expansion agent, 0.3-0.9 parts by weight of a fluid loss reducer, 0.2-0.4 parts by weight of bentonite, 1.7-2.3 parts by weight of a naphthalenesulfonate formaldehyde condensate, 0.1-0.2 parts by weight of dimethyl silicone oil and 1-3 parts by weight of a retarder; The repair agent is modified from ethyl cyanoacrylate, candelilla wax and 3-isocyanate propyl trimethoxy silane in a mass ratio of 1:1-2:0.1-0.3; The repair agent is prepared by the following preparation method: Under nitrogen protection, heat the candelilla wax at 60-70°C to melt, add the nano-silica powder under continuous stirring to ensure complete and uniform dispersion, and then drop 3-isocyanate propyltrimethoxysilane to obtain a wall material solution for use; Ethyl cyanoacrylate was mixed evenly with hydroquinone and dioctyl phthalate to obtain a core material solution, and the core material solution was added dropwise into the wall material solution, and stirring was continued for 30-50 minutes to obtain a repair agent.

2. The self-healing cement slurry for cementing according to claim 1, characterized in that: The expansion agent is a CaO-MgO composite system, and the expansion rate is 0.05%-0.2%.

3. The self-healing cement slurry for cementing according to claim 1, characterized in that: The fluid loss reducer is one of hydroxyethyl cellulose and polyacrylamide, and the retarder is one of calcium lignin sulfonate and tartaric acid.

4. The self-healing cement slurry for cementing according to claim 1, characterized in that: The nano silicon dioxide is 15-28% of the mass of the candelilla wax.

5. The self-healing cement slurry for cementing according to claim 1, characterized in that: The hydroquinone accounts for 0.5-0.8% of the mass of ethyl cyanoacrylate.

6. The self-healing cement slurry for cementing according to claim 1, characterized in that: The dioctyl phthalate accounts for 11.2-13.5% of the mass of ethyl cyanoacrylate.

7. The self-healing cement slurry for cementing according to claim 1, characterized in that: The particle size distribution of the repair agent is 25-50 μm.

8. A method for preparing a self-healing cement slurry for cementing, used for preparing the self-healing cement slurry for cementing as claimed in any one of claims 1 to 7, characterized in that: The steps include: Add cement, silica fume, basalt fiber, expansion agent, fluid loss reducer, bentonite, naphthalenesulfonate formaldehyde condensate and repair agent into a stirrer, premix for 5-10 minutes at room temperature at 100-200rpm, then add dimethyl silicone oil and retarder, calculate the required amount of water according to the water-cement ratio, add water and mix at 500-1000rpm for 10-15 minutes to obtain cementing self-healing cement slurry.

9. The method for preparing the self-healing cement slurry for cementing according to claim 8, characterized in that: The water-cement ratio is 0.38-0.45.

Images