Composite oil well cement and preparation method therefor

Abstract

The present application provides a composite oil well cement and a preparation method therefor. In the present application, carbon nanotubes and nano-magnesia are introduced, and are compounded with oil well cement, water and an oil well cement dispersant, it is defined that the nano-magnesia has been subjected to a calcination treatment at 300-1000°C, and the contents of the above components are also defined; therefore, the compressive strength and tensile strength of set cement formed by the composite oil well cement are effectively improved, the elastic modulus of the set cement is significantly reduced, and the occurrence of self-shrinkage is also suppressed. The set cement formed by the composite oil well cement meets the requirements of well-cementing construction, exhibits good construction performance, can be used for the design, research, development and application of high-performance well-cementing cement systems for high-temperature and high-pressure gas wells, unconventional oil and gas wells, gas storage wells, etc., satisfies the long-term sealing requirements of a cement sheath under complex stress environments, and has promising application prospects.

Description

A composite oil well cement and its preparation method

[0001] This application claims priority to Chinese Patent Application No. 202411883760.1, filed on December 19, 2024, entitled “A Composite Oil Well Cement and a Method for Preparing the Same”, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to oil well cement material technology, specifically to a composite oil well cement and its preparation method. Background Technology

[0003] Oil well cement is specifically used in cementing engineering for oil and gas wells. Its main function is to bond and seal the casing to the surrounding rock formation, isolating oil, gas, and water layers within the formation to prevent crosstalk and create a well-isolated oil flow channel from the oil layer to the surface. Therefore, the long-term effective sealing of the cement stone obtained after oil well cement hardening is an important guarantee for the safe production of oil and gas resources. In recent years, with the continuous exploration and development towards deeper, low-permeability, and unconventional resources, the proportion of complex oil and gas wells has been increasing year by year, and the service environment faced by cement stone has become increasingly harsh. This places higher demands on the mechanical properties and volumetric stability of oil well cement in cementing engineering.

[0004] However, cement stone, as a brittle material, has characteristics such as low tensile strength, poor impact resistance, weak deformation resistance, and susceptibility to brittle cracking and shrinkage. Downhole, cement stone is subject to its own shrinkage stress, temperature, and load effects, which can easily lead to microcracks and microannuations, forming fluid flow channels, causing interlayer seal failure, and consequently affecting the long-term sealing of the wellbore.

[0005] Therefore, the oil and gas sector is developing toughened cement systems and micro-expansion cement systems to improve the sealing integrity of cement ring stones in complex environments. Generally, this is achieved by adding elastic or expansive materials to cement to reduce the elastic modulus and suppress shrinkage, respectively. However, while conventional elastic and expansive materials reduce the elastic modulus of cement and improve shrinkage, they also have a significant adverse effect on the strength of cement stone. For example, Chinese patent CN115872680A discloses a lightweight toughened cementing material and its preparation method. This cementing material consists of the following components in parts by weight: 30-40 parts of Grade G oil well cement, 20-35 parts of vacuum glass microspheres, 8-15 parts of silica fume, 4-8 parts of plant fiber, 2-5 parts of rubber powder, 2.5-5 parts of an early-strength agent, 3-5 parts of a fluid loss reducer, 0.1-0.4 parts of a retarder, and 0.1-0.3 parts of a stabilizer. The cement stone prepared from this cementing material has an elastic modulus of only 4.5-5.5 GPa, but its early strength is 7.0-8.6 MPa (45℃ / 48h). How to effectively reduce the elastic modulus of cement stone and inhibit shrinkage without affecting its strength, thereby improving its crack resistance, impact resistance, and deformation resistance, is a current research challenge and hot topic. Summary of the Invention

[0006] This application provides a composite oil well cement. By introducing carbon nanotubes and nano-magnesium oxide and compounding them with oil well cement, water, and oil well cement dispersant, and by limiting the calcination treatment of nano-magnesium oxide at 300℃-1000℃ and limiting the content of the above components, the compressive strength and tensile strength of the cement stone formed by the composite oil well cement are effectively improved, the elastic modulus of the cement stone is significantly reduced, and the occurrence of autogenous shrinkage is suppressed.

[0007] This application also provides a method for preparing the above-mentioned composite oil well cement, which can effectively solve the problem of nanomaterial agglomeration, help promote the synergistic effect of nano-magnesium oxide and carbon nanotubes, and ultimately achieve the effect that the cement stone can maintain high strength while having low elastic modulus and low self-shrinkage value, thus meeting the long-term sealing requirements of cement stone under harsh mechanical conditions.

[0008] This application provides a composite oil well cement comprising the following raw materials in parts by weight:

[0009] Oil well cement 90-120 parts, water 40-60 parts, carbon nanotubes 0.02-0.1 parts, nano magnesium oxide 1-3 parts, oil well cement dispersant 0.1-0.5 parts;

[0010] Nano-magnesium oxide is obtained by calcination at 300℃-1000℃.

[0011] The composite oil well cement described above is obtained by calcining nano-magnesium oxide in the composite oil well cement at 450℃-550℃ when the curing temperature is 20℃-40℃.

[0012] The composite oil well cement described above is obtained by calcining nano-magnesium oxide in the composite oil well cement at 550℃-650℃ when the curing temperature is 50℃-70℃.

[0013] The composite oil well cement described above is obtained by calcining nano-magnesium oxide in the composite oil well cement at 750℃-850℃ when the curing temperature is 80℃-100℃.

[0014] The composite oil well cement described above contains nano-magnesium oxide with an average particle size of 45-55 nm and a specific surface area of ​​30-50 m² before calcination at 300-1000 °C. 2 / g.

[0015] The composite oil well cement described above contains carbon nanotubes with a length of 5-15 μm, an inner diameter of 3-5 nm, an outer diameter of 8-15 nm, and a specific surface area ≥250 m². 2 / g.

[0016] The composite oil well cement as described above further includes the following raw materials in parts by weight: 0.02-0.05 parts of carbon nanotube dispersant and 0.2-0.8 parts of nano-magnesium oxide dispersant.

[0017] This application also provides a method for preparing the above-mentioned composite oil well cement, comprising the following steps:

[0018] Nano-magnesium oxide is dispersed in water to obtain a nano-magnesium oxide suspension;

[0019] Carbon nanotubes are dispersed in water to obtain a carbon nanotube suspension;

[0020] The nano-magnesium oxide suspension and the carbon nanotube suspension were stirred, and then oil well cement and oil well cement dispersant were added to obtain composite oil well cement.

[0021] The preparation method described above further includes a nano-magnesium oxide dispersant in the nano-magnesium oxide suspension.

[0022] The composite oil well cement described above also includes a carbon nanotube dispersant in the carbon nanotube suspension.

[0023] The preparation method described above, wherein stirring the nano-magnesium oxide suspension and the carbon nanotube suspension further includes:

[0024] The nano magnesium oxide suspension and the carbon nanotube suspension were first stirred at a speed of 3000-5000 r / min for 10-20 s, and then stirred again at a speed of 11000-13000 r / min for 30-40 s.

[0025] The preparation method described above, wherein dispersing nano-magnesium oxide in water further includes: adding nano-magnesium oxide to water, stirring at 3000-5000 r / min for 15 s, stirring at 11000-13000 r / min for 30-40 s, and then sonicating for 2-8 min to obtain a nano-magnesium oxide suspension.

[0026] The preparation method described above, wherein dispersing carbon nanotubes in water further includes: adding carbon nanotubes to water, stirring at 300-500 r / min for 15-25 min, and then subjecting to ultrasonic treatment for 30-50 min.

[0027] This application provides a composite oil well cement. By introducing carbon nanotubes and nano-magnesium oxide and compounding them with oil well cement, water, and an oil well cement dispersant, and by limiting the calcination treatment of nano-magnesium oxide at 300℃-1000℃ and the content of the above components, this application effectively improves the compressive and tensile strength of the cement stone formed by the composite oil well cement, significantly reduces the elastic modulus of the cement stone, and inhibits the occurrence of self-shrinkage. This results in the cement stone formed by the composite oil well cement exhibiting good mechanical and shrinkage properties. Its rheological properties meet the requirements of cementing construction, and it has good construction performance. It can be used for the design, research and development, and application of high-performance cementing systems for high-temperature and high-pressure gas wells, unconventional oil and gas wells, and gas storage wells. It meets the long-term sealing requirements of cement sheaths under complex stress environments and has good application prospects. Attached Figure Description

[0028] Figure 1 shows the autogenous shrinkage curves of the cement stone formed by the oil well cement obtained in Comparative Examples 1-5 and Comparative Example 8 at a curing temperature of 30℃.

[0029] Figure 2 shows the autogenous shrinkage curves of the cement stone formed by the oil well cement obtained in Comparative Example 1, Examples 1-4 and Example 7 at a curing temperature of 30°C.

[0030] Figure 3 shows the autogenous shrinkage curves of the cement stone formed by the oil well cement obtained in Comparative Examples 1 and 4-8 at a curing temperature of 60℃.

[0031] Figure 4 shows the autogenous shrinkage curves of the cement stone formed by the oil well cement obtained in Comparative Example 1 and Example 3-7 at a curing temperature of 60℃.

[0032] Figure 5 shows the autogenous shrinkage curves of the cement stone formed by the oil well cement obtained in Comparative Examples 1 and 4-8 at a curing temperature of 90℃.

[0033] Figure 6 shows the autogenous shrinkage curves of the cement stone formed by the oil well cement obtained in Comparative Example 1 and Examples 3-7 at a curing temperature of 90℃.

[0034] Figure 7 is an electron microscope image of the oil well cement C2M3 obtained in Example 1 after curing at 30°C for 28 days;

[0035] Figure 8 is an electron microscope image of the oil well cement C2M3 obtained in Example 1 after curing at 30°C for 28 days. Detailed Implementation

[0036] To enable those skilled in the art to better understand the solutions of this application, a further detailed description of this application is provided below. The specific embodiments listed below are merely descriptions of the principles and features of this application; the examples are only for explaining this application and are not intended to limit its scope. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without inventive effort are within the scope of protection of this application.

[0037] Oil well cement is a type of cement specifically used in cementing engineering for oil and gas wells. Its main function is to bond and seal the casing to the surrounding rock formation, isolating oil, gas, and water layers within the formation to prevent crosstalk and create a well-isolated oil flow channel from the oil layer to the surface. The long-term effective sealing of the cement stone obtained after the oil well cement has hardened is an important guarantee for the safe production of oil and gas resources. However, as a brittle material, cement stone is inherently prone to low tensile strength, poor impact resistance, weak deformation resistance, and is susceptible to brittle cracking and shrinkage.

[0038] Therefore, in order to improve compressive strength and tensile strength and reduce elastic modulus and self-shrinkage rate, this application provides a composite oil well cement comprising the following raw materials in parts by weight: 90-120 parts oil well cement, 40-60 parts water, 0.02-0.1 parts carbon nanotubes, 1-3 parts nano-magnesium oxide, and 0.1-0.5 parts oil well cement dispersant, wherein the nano-magnesium oxide is obtained by calcination treatment at 300℃-1000℃.

[0039] In detail, the oil well cement in this application is the matrix component of composite oil well cement. In some embodiments, the oil well cement may be Grade G oil well cement.

[0040] Oil well cement dispersant is a dispersant that adjusts the surface charge of cement particles to achieve optimal rheological properties of cement slurry. It can reduce the consistency of cement slurry, improve its fluidity, and enhance its flowability (variation), thereby helping to improve cementing quality, reduce pump pressure, and accelerate cementing speed. To better disperse oil well cement and improve the quality of composite oil well cement, the oil well cement dispersant of this application can be sulfonated acetone-formaldehyde condensate (SAF). SAF is an aliphatic hydroxysulfonate oligomer obtained by condensation and sulfonation of acetone, formaldehyde, sulfite, etc. It has advantages such as readily available raw materials and simple production processes, and is widely used in various engineering projects. In some embodiments, the oil well cement dispersant used is a reddish-brown solid powder of sulfonated acetone-formaldehyde condensate.

[0041] To better disperse and blend the components of the composite oil well cement and achieve synergistic effects, this application also adds water to the composite oil well cement. In some embodiments, the water can be deionized water.

[0042] The composite oil well cement of this application includes carbon nanotubes, which are one-dimensional quantum materials with special structures. Their radial dimensions are on the order of nanometers and their axial dimensions are on the order of micrometers. They consist of several to dozens of layers of coaxial circular tubes composed of carbon atoms arranged in a hexagonal pattern, with a diameter generally of 2-20 nm.

[0043] Carbon nanotubes possess excellent mechanical properties. When uniformly dispersed in a cement matrix, they can significantly improve the compressive and tensile strength of composite oil well cement. The presence of carbon nanotubes can significantly reduce porosity and promote uniform pore distribution, thereby increasing the bulk density of composite oil well cement and improving its mechanical properties. Carbon nanotubes can also bridge microcracks in the cement matrix, preventing the propagation of internal microcracks, thus improving the toughness and crack resistance of composite oil well cement, and consequently increasing its compressive and tensile strength. Carbon nanotubes can also provide a nucleation effect, initiating the formation of a large number of cement hydration products on their surface, resulting in good adhesion between carbon nanotubes and the cement matrix. This improves the microstructure of the hydration process and hydration products, increases the crystallinity of the binder material, and ultimately enhances the compressive and tensile strength of composite oil well cement.

[0044] The composite oil well cement of this application also includes nano-magnesium oxide, which consists of magnesium oxide particles with a diameter on the nanometer scale. These particles have a small size, a large specific surface area, and possess optical, electrical, magnetic, and chemical properties that differ from those of the bulk material.

[0045] Nano-magnesium oxide also possesses excellent mechanical properties, improving the compressive and tensile strength of composite oil well cement. Furthermore, due to the high specific surface area of ​​nanoparticles, they can be more uniformly distributed within the cement matrix, thus improving the microstructure of the composite oil well cement. The presence of nano-magnesium oxide can fill the pores in the cement paste, reducing its porosity and increasing its bulk density, thereby improving the mechanical properties of the composite oil well cement. Nano-magnesium oxide can also adsorb some free water in the cement paste, thereby reducing the water-cement ratio to some extent and inhibiting the precipitation of free liquid.

[0046] Furthermore, nano-magnesium oxide also undergoes a hydration reaction during cement hydration to produce magnesium hydroxide. This process is accompanied by volume expansion, which can offset some of the shrinkage of the cement matrix caused by hydration and drying during hardening, thereby reducing overall autogenous shrinkage. Experiments have shown that after calcination at 300℃-1000℃, the nano-magnesium oxide particles aggregate, the particle size increases, and the reactivity is affected, leading to a longer time for the hydration reaction to begin. Therefore, by controlling the calcination temperature of nano-magnesium oxide, the expansion effect of nano-magnesium oxide can be matched with the progress of cement hydration, thereby maximizing the control of the expansion performance of composite oil well cement and improving the autogenous shrinkage phenomenon of composite oil well cement.

[0047] In this application, the introduction of carbon nanotubes and nano-magnesium oxide, along with the controlled ratio of carbon nanotubes and nano-magnesium oxide, leverages their synergistic effect. This not only significantly improves the compressive and tensile strength of the composite oil well cement but also effectively reduces its elastic modulus and autogenous shrinkage. If only nano-magnesium oxide is used in oil well cement without carbon nanotubes, while the autogenous shrinkage is somewhat mitigated, the expansion caused by the hydration reaction of nano-magnesium oxide leads to cracking of the cement stone, resulting in reduced compressive and tensile strength, making it unsuitable for practical engineering applications. Therefore, the simultaneous use of nano-magnesium oxide and carbon nanotubes in oil well cement not only effectively reduces its elastic modulus and autogenous shrinkage but also alleviates cement stone cracking, effectively improving the compressive and tensile strength of the oil well cement. Furthermore, since both carbon nanotubes and nano-magnesium oxide are nanoparticles, and nanoparticles possess excellent mechanical properties and high specific surface area, they can act as fillers in the cement matrix, thereby improving the pore structure and reducing the porosity of the composite oil well cement. They can also provide nucleation sites for cement hydration, accelerating it. After adjusting the ratio of carbon nanotubes to nano-magnesium oxide, their adsorption of free water in the cement paste can be increased, reducing the water-cement ratio to some extent and inhibiting the precipitation of free liquid. Moreover, because the rheology of oil well cement is temperature-dependent, the use of nanoparticles will not significantly affect the rheology of the composite oil well cement. In addition, nanomaterials can reduce the size of calcium hydroxide crystals formed during cement hydration, thus improving the microstructure of the composite oil well cement.

[0048] By introducing carbon nanotubes and nano-magnesium oxide and compounding them with oil well cement, water, and oil well cement dispersant, while limiting the calcination treatment of nano-magnesium oxide at 300℃-1000℃ and limiting the content of the above components, this application effectively improves the compressive and tensile strength of the cement stone formed by composite oil well cement, significantly reduces the elastic modulus of the cement stone, and inhibits the occurrence of self-shrinkage. This results in the cement stone formed by composite oil well cement exhibiting good mechanical and shrinkage properties. Its rheological properties meet the requirements of cementing construction, and it has good construction performance. It can be used for the design, research and development, and application of high-performance cementing systems for high-temperature and high-pressure gas wells, unconventional oil and gas wells, and gas storage wells. It meets the long-term sealing requirements of cement sheaths under complex stress environments and has good application prospects.

[0049] Furthermore, since other components in the composite oil well cement are prone to foaming during mixing, defoamers can be introduced into the composite oil well cement. Defoamers themselves have low surface tension, thus effectively controlling foam generation within the cement system and resulting in a denser and brighter cement stone. To better eliminate foam and improve the quality of the composite oil well cement, the defoamer in this application can be an organophosphate defoamer. Organophosphate defoamers are a class of defoamers based on phosphate ester compounds, which can effectively eliminate foam by reducing the surface tension of the liquid and thus disrupting the stability of the foam.

[0050] Specifically, the composite oil well cement with added defoamer includes the following raw materials in parts by weight: 90-120 parts oil well cement, 40-60 parts water, 0.02-0.1 parts carbon nanotubes, 1-3 parts nano magnesium oxide, 0.1-0.5 parts oil well cement dispersant, and 0.2-1 parts defoamer.

[0051] The temperature during the cementing process has a significant impact on the hydration reaction rate. To achieve the best results, the expansion time of nano-magnesium oxide must be matched with the overall hydration rate of the cement slurry system.

[0052] Specifically, the curing temperature of oil well cement affects the hydration reactions of both nano-magnesium oxide and the oil well cement itself. At high curing temperatures, the hydration reaction of nano-magnesium oxide is rapid and tends to complete before the hydration reaction of the oil well cement. Conversely, at low curing temperatures, the hydration reaction of nano-magnesium oxide is slower and tends to occur after the oil well cement has hardened. The expansion effect of nano-magnesium oxide is achieved through the formation of magnesium hydroxide via its hydration reaction. The process from the initial setting to complete hardening of oil well cement is achieved through the formation of hydration products, such as calcium silicate hydrate, in the hydration reaction of silicate cement. The above research results indicate that the curing temperature of oil well cement causes a discrepancy between the start times of the hydration reactions of nano-magnesium oxide and the oil well cement, thus preventing the expansion effect of nano-magnesium oxide from occurring within the period from the initial setting to complete hardening of the oil well cement. Ultimately, this affects the improvement of the autogenous shrinkage of oil well cement by nano-magnesium oxide. Therefore, the applicant proposed that the reactivity of nano-magnesium oxide can be controlled, that is, using nano-magnesium oxide with low reactivity at high curing temperatures and nano-magnesium oxide with high reactivity at low curing temperatures, in order to avoid the inconsistency between the hydration reaction of nano-magnesium oxide and the hydration reaction of oil well cement caused by curing temperature, thereby reducing the problem of auto-shrinkage rate of composite oil well cement.

[0053] Experiments have verified that controlling the calcination temperature of nano-magnesium oxide can control its reactivity, and the higher the calcination temperature, the lower the reactivity of the nano-magnesium oxide. If a high curing temperature is required, low-reactivity nano-magnesium oxide is needed, thus requiring nano-magnesium oxide with a high calcination temperature; conversely, if a low curing temperature is required, high-reactivity nano-magnesium oxide is needed, thus requiring nano-magnesium oxide materials with a low calcination temperature. Specifically, nano-magnesium oxide in composite oil well cement is obtained by calcination at 450℃-550℃ when the curing temperature is 20℃-40℃; by calcination at 550℃-650℃ when the curing temperature is 50℃-70℃; and by calcination at 750℃-850℃ when the curing temperature is 80℃-100℃.

[0054] In one embodiment of this application, the average particle size of the nano-magnesium oxide before calcination at 300℃-1000℃ is 45-55nm, and the specific surface area is 30-50m². 2 / g. This application specifies that the average particle size and specific surface area of ​​nano-magnesium oxide before calcination at 300℃-1000℃ meet the above range, which helps to further promote the improvement of the microstructure and mechanical properties of composite oil well cement by nano-magnesium oxide, increase the compressive strength and tensile strength of composite oil well cement, reduce the overall autogenous shrinkage, and to a certain extent reduce the water-cement ratio and inhibit the precipitation of free liquid.

[0055] In another embodiment of this application, the carbon nanotubes have a length of 5-15 μm, an inner diameter of 3-5 nm, an outer diameter of 8-15 nm, and a specific surface area ≥250 m². 2 / g.

[0056] This application specifies that the length, inner diameter, outer diameter, and specific surface area of ​​carbon nanotubes meet the above-mentioned ranges, which helps to further promote the improvement of the microstructure and mechanical properties of composite oil well cement by carbon nanotubes, and improve the compressive strength and tensile strength of composite oil well cement.

[0057] The carbon nanotubes and nano-magnesium oxide used in this application have high specific surface area and high surface energy, making them difficult to disperse. The simultaneous use of these two nanomaterials in this application makes their dispersion even more difficult. Therefore, in order to better disperse carbon nanotubes and nano-magnesium oxide, carbon nanotube dispersants and nano-magnesium oxide dispersants can be added, and their dispersing effect can be further enhanced by limiting their types and amounts. Specifically, the composite oil well cement may include the following raw materials in parts by weight: 0.02-0.05 parts of carbon nanotube dispersant and 0.2-0.8 parts of nano-magnesium oxide dispersant. That is, the composite oil well cement after adding carbon nanotube dispersant and nano-magnesium oxide dispersant includes the following raw materials in parts by weight: 90-120 parts of oil well cement, 40-60 parts of water, 0.02-0.1 parts of carbon nanotubes, 1-3 parts of nano-magnesium oxide, 0.1-0.5 parts of oil well cement dispersant, 0.2-1 parts of defoamer, 0.02-0.05 parts of carbon nanotube dispersant, and 0.2-0.8 parts of nano-magnesium oxide dispersant. Among them, the carbon nanotube dispersant can be TNWDIS. TNWDIS is a nonionic surfactant with a long carbon chain structure, one end of which is an aromatic group and the other end is a hydrophilic group. Since the aromatic group has good affinity with the wall of the carbon nanotube, the aromatic group at one end can be adsorbed on the tube wall, and the hydrophilic group at the other end can be uniformly dispersed in water, making it particularly suitable for preparing carbon nanotube dispersions. The nano magnesium oxide dispersant can be PEG400 (polyethylene glycol 400). PEG400 has long-chain ethylene glycol repeating units. These units have polar hydroxyl groups (-OH), which can form hydrogen bonds with the surface of nano magnesium oxide particles, helping to form a stable adsorption layer on the particle surface and preventing particle aggregation.

[0058] To ensure the dispersion effect of the dispersant, this application limits the amount of carbon nanotube dispersant in the composite oil well cement to 0.02-0.05 parts and the amount of nano-magnesium oxide dispersant to 0.2-0.8 parts. At this time, the amount of carbon nanotubes is 0.02-0.1 parts and the amount of nano-magnesium oxide is 1-3 parts, that is, the mass ratio of carbon nanotube dispersant to carbon nanotubes is (1-2.5):(1-5.0), and the mass ratio of nano-magnesium oxide dispersant to nano-magnesium oxide is (1-4):(5-15).

[0059] A second aspect of this application provides a method for preparing the above-mentioned composite oil well cement, comprising the following steps:

[0060] Nano-magnesium oxide is dispersed in water to obtain a nano-magnesium oxide suspension;

[0061] Carbon nanotubes are dispersed in water to obtain a carbon nanotube suspension;

[0062] The nano-magnesium oxide suspension and the carbon nanotube suspension were stirred, and then oil well cement and oil well cement dispersant were added to obtain composite oil well cement.

[0063] Since the nano-magnesium oxide and carbon nanotubes used in the preparation method of this application are both nanomaterials, their high specific surface area and high surface energy easily lead to agglomeration. Agglomerated nanomaterials result in uneven distribution within the composite oil well cement, affecting both the stability of the composite oil well cement and preventing them from exerting their intended effects, thus hindering their contribution to improving the mechanical and shrinkage properties of the composite oil well cement. Therefore, this application proposes a method for preparing composite oil well cement that is compatible with nano-magnesium oxide and carbon nanotubes. Based on the composite oil well cement composition ratio and corresponding preparation method proposed in this application, the agglomeration problem of nanomaterials can be effectively solved, promoting a synergistic effect between nano-magnesium oxide and carbon nanotubes. Ultimately, this achieves the effect of maintaining high strength while possessing low elastic modulus and low autogenous shrinkage value in the cement stone, meeting the long-term sealing requirements of the cement stone under harsh mechanical environments.

[0064] Because this application uses both carbon nanotubes and nano-magnesium oxide, the dispersion of nanomaterials becomes more difficult. Therefore, either nano-magnesium oxide dispersant or carbon nanotube dispersant can be introduced into the scheme of this application. In addition, experimental verification has shown that the nano-magnesium oxide dispersant can be PEG400 and the carbon nanotube dispersant can be TNWDIS. Using the above types of dispersants can provide better dispersion effects.

[0065] To further improve the dispersion of nanomaterials, the applicant discovered through long-term research that a two-stage stirring process can be performed. This involves first performing a low-speed stirring to premix the nano-magnesium oxide suspension and the carbon nanotube suspension, followed by a high-speed stirring to further and thoroughly mix the premixed nanomaterials. By limiting the speed and time of the first and second stirring processes, the uniform dispersion of the nanomaterials can be effectively promoted. Specifically, the nano-magnesium oxide suspension, carbon nanotube suspension, and water can be first stirred at a speed of 3000-5000 r / min for 10-20 s, followed by a second stirring at a speed of 11000-13000 r / min for 30-40 s.

[0066] Since the components in composite oil well cement are prone to foaming during mixing, defoamers can be introduced into the composite oil well cement. Specifically, organophosphate defoamers can be added dropwise during the last 3-8 seconds of the second mixing.

[0067] Building upon this, to further enhance the dispersion effect of nano-magnesium oxide, nano-magnesium oxide can be added to water, stirred at 3000-5000 r / min for 10-20 s, stirred at 11000-13000 r / min for 30-40 s, and then ultrasonically treated for 2-8 min to obtain a nano-magnesium oxide suspension. By limiting the stirring speed, stirring time, and ultrasonic time during the preparation of the nano-magnesium oxide suspension, it is helpful to disperse the nano-magnesium oxide more uniformly.

[0068] Understandably, to further promote uniform dispersion, a nano-magnesium oxide dispersant can be added. In practice, the nano-magnesium oxide dispersant and water can be stirred to dissolve the nano-magnesium oxide dispersant. Then, when stirring at 3000-5000 r / min for 10-20 s, the nano-magnesium oxide can be added. Subsequently, the mixture can be stirred at 11000-13000 r / min for 30-40 s and ultrasonically treated for 2-8 min.

[0069] To further enhance the dispersion of carbon nanotubes, they can be added to water, stirred at 300-500 rpm for 15-25 minutes, and then sonicated for 30-50 minutes. By limiting the stirring speed, stirring time, and sonication time during the preparation of the carbon nanotube suspension, it is helpful to disperse the carbon nanotubes more uniformly.

[0070] Understandably, to further promote uniform dispersion, carbon nanotube dispersant can be added. In practice, the carbon nanotube dispersant and water can be stirred to dissolve the carbon nanotube dispersant, then carbon nanotubes can be added and stirred at 300-500 r / min for 15-25 min, followed by ultrasonic treatment for 30-50 min.

[0071] Since both nano-magnesium oxide and carbon nanotubes require ultrasonic treatment, in order to improve the dispersion effect, reduce costs, and simplify experimental operations, the ultrasonic treatment for preparing nano-magnesium oxide suspension and the ultrasonic treatment for preparing carbon nanotube suspension can be combined. That is, the nano-magnesium oxide suspension to be ultrasonicated is added at the 35-minute mark of the ultrasonic treatment of carbon nanotubes.

[0072] The technical solution of this application will be further explained below with reference to specific embodiments. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions or as recommended by the manufacturer. Unless otherwise specified, all reagents used are commercially available or obtained through public channels.

[0073] Example 1

[0074] This embodiment provides a method for preparing nano-magnesium oxide, including the following steps:

[0075] With an average particle size of 50 nm and a specific surface area of ​​40 m², 2 / g of nano-magnesium oxide was calcined at 300℃, 400℃, 500℃, 600℃, 700℃, 800℃, 900℃, and 1000℃ for 1 hour to obtain nano-magnesium oxide calcined at 300℃ for 1 hour, 400℃ for 1 hour, 500℃ for 1 hour, 600℃ for 1 hour, 700℃ for 1 hour, 800℃ for 1 hour, 900℃ for 1 hour, and 1000℃ for 1 hour. Take 1.7g of the nano-magnesium oxide calcined for 1 hour and add it to 200mL of citric acid standard solution (with 4 drops of phenolphthalein indicator solution added beforehand). At the same time as adding the nano-magnesium oxide calcined for 1 hour, turn on the stopwatch and start the magnetic stirrer to carry out the neutralization reaction at a speed of 700r / min. Stop the stopwatch as soon as the solution turns red. The time period displayed by the stopwatch is the time required for the nano-magnesium oxide calcined for 1 hour to reach the neutralization reaction (hereinafter referred to as the neutralization time), as shown in Table 1.

[0076] Table 1

[0077] Table 1 shows that adjusting the calcination temperature of nano-magnesium oxide can control its neutralization time, and the neutralization time increases with increasing calcination temperature, indicating that the reactivity of nano-magnesium oxide decreases with increasing calcination temperature. Therefore, if a high curing temperature is required, low-reactivity nano-magnesium oxide is needed, which necessitates nano-magnesium oxide with a high calcination temperature; conversely, if a low curing temperature is required, low-reactivity nano-magnesium oxide is needed, which necessitates nano-magnesium oxide materials with a low calcination temperature. However, a more precise relationship between calcination and curing temperatures still needs to be clarified through further mechanical property testing and self-shrinkage testing.

[0078] This embodiment also provides an oil well cement and its preparation method, including the following steps:

[0079] (1) Preparation of nano magnesium oxide suspension: Weigh 3g of PEG400 and add it to the cement slurry cup, then weigh 132g of deionized water and add it to the cement slurry cup and stir until the PEG400 is uniformly dissolved. Then place the cement slurry cup on a corrugated mixer and add 12g of the above-mentioned nano magnesium oxide calcined at 300℃ for 1 hour. Stir at 4000r / min for 15s and then at 12000r / min for 35s to obtain nano magnesium oxide suspension.

[0080] (2) Preparation of composite nanomaterial suspension: Carbon nanotube material with product number CAS:308068-56-6 produced by Shenzhen Suiheng Technology Co., Ltd. was selected. It is a carboxylated multi-walled carbon nanotube with a length of 5-15 μm, an inner diameter of 3-5 nm, an outer diameter of 8-15 nm, and a specific surface area of ​​250-270 m². 2 / g. To avoid changes in the water-cement ratio due to water loss during liquid transfer, the beaker was pre-wetted with deionized water. 0.24g of TNWDIS was added to the wetted beaker, followed by 132g of deionized water. The beaker was then placed on a magnetic stirrer (HJ-2 type) and stirred until the TNWDIS was completely dissolved. At this point, a timer was turned on, and 0.24g of the weighed carbon nanotube material was added to the beaker. The mixture was stirred at 400 rpm for 20 minutes. Immediately after stirring, the beaker was placed in an ultrasonic cleaner (KH3200DB type) for 40 minutes of ultrasonic dispersion. During the last 5 minutes of ultrasonication, the nano-magnesium oxide suspension was immediately added to the beaker, and ultrasonication continued for another 5 minutes to obtain a composite nanomaterial suspension. Because ultrasonic dispersion generates a large amount of energy, causing the water bath temperature in the ultrasonic cleaner to rise, the water was changed 20 minutes after the start of ultrasonication, and then every 5 minutes thereafter. To prevent water evaporation during stirring and ultrasonication, the beaker was sealed with plastic wrap.

[0081] (3) Preparation of oil well cement: Weigh 600g of Grade G oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above composite nanomaterial suspension to the cement slurry cup, then place the cement slurry cup on a corrugated mixer, and add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C2M3.

[0082] Example 2

[0083] This embodiment provides an oil well cement and its preparation method. The specific steps can be referred to in Embodiment 1. The only difference is that in this embodiment, the nano-magnesium oxide calcined at 400°C for 1 hour is selected instead of the nano-magnesium oxide calcined at 300°C for 1 hour.

[0084] The oil well cement prepared by calcining nano-magnesium oxide at 400℃ for 1 hour was named C2M4.

[0085] Example 3

[0086] This embodiment provides an oil well cement and its preparation method. The specific steps can be referred to in Embodiment 1. The only difference is that in this embodiment, the nano-magnesium oxide calcined at 500°C for 1 hour is selected instead of the nano-magnesium oxide calcined at 300°C for 1 hour.

[0087] The oil well cement prepared by calcining nano-magnesium oxide at 500℃ for 1 hour was named C2M5.

[0088] Example 4

[0089] This embodiment provides an oil well cement and its preparation method. The specific steps can be referred to in Embodiment 1. The only difference is that in this embodiment, the nano-magnesium oxide calcined at 600°C for 1 hour is used instead of the nano-magnesium oxide calcined at 300°C for 1 hour.

[0090] The oil well cement prepared by calcining nano-magnesium oxide at 600℃ for 1 hour was named C2M6.

[0091] Example 5

[0092] This embodiment provides an oil well cement and its preparation method. The specific steps can be referred to in Embodiment 1. The only difference is that in this embodiment, the nano-magnesium oxide calcined at 700°C for 1 hour is selected instead of the nano-magnesium oxide calcined at 300°C for 1 hour.

[0093] The oil well cement prepared by calcining nano-magnesium oxide at 700℃ for 1 hour was named C2M7.

[0094] Example 6

[0095] This embodiment provides an oil well cement and its preparation method. The specific steps can be referred to in Embodiment 1. The only difference is that in this embodiment, the nano-magnesium oxide calcined at 800°C for 1 hour is used instead of the nano-magnesium oxide calcined at 300°C for 1 hour.

[0096] The oil well cement prepared by calcining nano-magnesium oxide at 800℃ for 1 hour was named C2M8.

[0097] Example 7

[0098] This embodiment provides an oil well cement and its preparation method. The specific steps can be referred to in Embodiment 1. The only difference is that in this embodiment, the nano-magnesium oxide calcined at 900°C for 1 hour is selected instead of the nano-magnesium oxide calcined at 300°C for 1 hour.

[0099] The oil well cement prepared by calcining nano-magnesium oxide at 900℃ for 1 hour was named C2M9.

[0100] Example 8

[0101] This embodiment provides an oil well cement and its preparation method. The specific steps can be referred to in Embodiment 1. The only difference is that in this embodiment, the nano-magnesium oxide calcined at 1000°C for 1 hour is selected instead of the nano-magnesium oxide calcined at 300°C for 1 hour.

[0102] The oil well cement prepared using nano-magnesium oxide calcined at 1000℃ for 1 hour was named C2M10.

[0103] Comparative Example 1

[0104] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0105] Weigh 600g of Grade G oil well cement powder and 1.2g of powdered sulfonated acetone-formaldehyde condensate and mix them evenly to obtain a powder mixture. Add 264g of deionized water to the cement slurry cup, then place the cement slurry cup on a corrugated mixer and add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue mixing for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0MO.

[0106] Comparative Example 2

[0107] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0108] (1) Preparation of nano magnesium oxide suspension: Weigh 12g of nano magnesium oxide calcined at 300℃ for 1 hour in Example 1 and disperse it into 264g of deionized water. Sonicate for 5 minutes to obtain nano magnesium oxide suspension.

[0109] (2) Preparation of oil well cement: Weigh 600g of G-grade oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above-mentioned nano magnesium oxide suspension to the cement slurry cup, and then place the cement slurry cup on a corrugated mixer. Add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0M3.

[0110] Comparative Example 3

[0111] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0112] (1) Preparation of nano magnesium oxide suspension: Weigh 12g of nano magnesium oxide calcined at 400℃ for 1 hour in Example 1 and disperse it into 264g of deionized water. Sonicate for 5 minutes to obtain nano magnesium oxide suspension.

[0113] (2) Preparation of oil well cement: Weigh 600g of Grade G oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above-mentioned nano magnesium oxide suspension to the cement slurry cup, and then place the cement slurry cup on a corrugated mixer. Add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0M4.

[0114] Comparative Example 4

[0115] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0116] (1) Preparation of nano magnesium oxide suspension: Weigh 12g of nano magnesium oxide calcined at 500℃ for 1 hour in Example 1 and disperse it into 264g of deionized water. Sonicate for 5 minutes to obtain nano magnesium oxide suspension.

[0117] (2) Preparation of oil well cement: Weigh 600g of G-grade oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above-mentioned nano magnesium oxide suspension to the cement slurry cup, and then place the cement slurry cup on a corrugated mixer. Add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0M5.

[0118] Comparative Example 5

[0119] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0120] (1) Preparation of nano magnesium oxide suspension: Weigh 12g of nano magnesium oxide calcined at 600℃ for 1 hour in Example 1 and disperse it into 264g of deionized water. Sonicate for 5 minutes to obtain nano magnesium oxide suspension.

[0121] (2) Preparation of oil well cement: Weigh 600g of G-grade oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above-mentioned nano magnesium oxide suspension to the cement slurry cup, and then place the cement slurry cup on a corrugated mixer. Add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0M6.

[0122] Comparative Example 6

[0123] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0124] (1) Preparation of nano magnesium oxide suspension: Weigh 12g of nano magnesium oxide calcined at 700℃ for 1 hour in Example 1 and disperse it into 264g of deionized water. Sonicate for 5 minutes to obtain nano magnesium oxide suspension.

[0125] (2) Preparation of oil well cement: Weigh 600g of Grade G oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above-mentioned nano magnesium oxide suspension to the cement slurry cup, and then place the cement slurry cup on a corrugated mixer. Add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0M7.

[0126] Comparative Example 7

[0127] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0128] (1) Preparation of nano magnesium oxide suspension: Weigh 12g of nano magnesium oxide calcined at 800℃ for 1 hour in Example 1 and disperse it into 264g of deionized water. Sonicate for 5 minutes to obtain nano magnesium oxide suspension.

[0129] (2) Preparation of oil well cement: Weigh 600g of G-grade oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above-mentioned nano magnesium oxide suspension to the cement slurry cup, and then place the cement slurry cup on a corrugated mixer. Add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0M8.

[0130] Comparative Example 8

[0131] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0132] (1) Preparation of nano magnesium oxide suspension: Weigh 12g of nano magnesium oxide calcined at 900℃ for 1 hour in Example 1 and disperse it into 264g of deionized water. Sonicate for 5 minutes to obtain nano magnesium oxide suspension.

[0133] (2) Preparation of oil well cement: Weigh 600g of Grade G oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above-mentioned nano magnesium oxide suspension to the cement slurry cup, and then place the cement slurry cup on a corrugated mixer. Add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0M9.

[0134] Comparative Example 9

[0135] This comparative example provides an oil well cement and its preparation method, including the following steps:

[0136] (1) Preparation of nano magnesium oxide suspension: Weigh 12g of nano magnesium oxide calcined at 1000℃ for 1 hour in Example 1, disperse it into 264g of deionized water, and sonicate for 5min to obtain nano magnesium oxide suspension.

[0137] (2) Preparation of oil well cement: Weigh 600g of G-grade oil well cement powder and 1.2g of powdered sulfonated acetone formaldehyde condensate and mix them evenly to obtain a powder mixture. Add the above-mentioned nano magnesium oxide suspension to the cement slurry cup, and then place the cement slurry cup on a corrugated mixer. Add the powder mixture evenly to the cement slurry cup at a speed of 4000r / min within 15s. Then switch to a speed of 12000r / min and continue stirring for 35s. In the last 3-8 seconds, add 1.2g of organophosphate defoamer to obtain oil well cement, named C0M10.

[0138] Test case

[0139] (1) Evaluation of mechanical properties: The oil well cement obtained in Examples 1-8 and Comparative Examples 1-9 was poured into steel compression molds, tensile molds, and elastic modulus molds in three separate batches. The compression mold was a cubic mold with a side length of 50.8 mm, the tensile mold was a circular mold with a diameter of 50 mm and a height of 25 mm, and the elastic modulus mold was a cylindrical mold with a diameter of 50 mm and a height of 100 mm. After each batch was poured to 1 / 3 of its volume, a plastic tamping rod was used to vibrate the edges and corners to eliminate air bubbles and promote uniformity of the oil well cement slurry. After pouring, the excess slurry on the top was scraped off with a scraper, and the mixture was covered with a metal cover and placed in water baths with pre-set curing temperatures (30℃, 60℃, 90℃) for water bath curing until the specified age (3d, 28d). At curing temperatures of 30℃ and 60℃, the molded specimens were placed in 27℃ water 45 minutes before the specified curing age. At a curing temperature of 90℃, the specimens were removed 1 hour and 45 minutes before the specified curing age and placed in 90℃ water. After natural cooling at room temperature for 1 hour, the specimens were demolded and then placed back in cooling water to prevent dehydration. Mechanical data were collected at the specified curing age. Compressive strength test data were obtained using a uniform force loading method, with a loading rate of 1.2 kN / s selected according to GB10238-2015. Tensile strength data were obtained using an indirect method, namely the Brazilian disc splitting test. Before obtaining the elastic modulus data, both sides of the cement should be ground smooth with a grinder, and then the specimen should be placed in the measuring device in the center. A uniform displacement loading method should be used, with the speed set at 0.1 mm / min. The pressure sensor at the bottom of the measuring device and the four surrounding axial LVDTs can output the force and axial deformation of the specimen in real time through the device software, thereby obtaining the stress-strain curve. The slope obtained by linear fitting within the stress range of 20% to 50% is the elastic modulus. Specific results are shown in Tables 2-4.

[0140] (2) Engineering performance evaluation: The water bath temperature in the atmospheric pressure thickener was preheated to 27℃ and maintained at a constant temperature for at least 1 hour. The oil well cement obtained in Examples 1-8 and Comparative Examples 1-9 was poured into the slurry cup of the atmospheric pressure thickener to the graduation mark within 1 minute. It was then placed in the atmospheric pressure thickener and thickened at 300 r / min for 20 minutes. After thickening, it was poured into a 500 ml conical flask within 1 minute, ensuring a transfer mass of 760 ± 5 g. The density and mass of the oil well cement at this point were recorded. Subsequently, it was sealed and allowed to stand on a vibration-free horizontal table for 2 hours. At the end, the supernatant in the conical flask was transferred to a graduated cylinder using a pipette, and the volume was measured and recorded. The proportion of free liquid was calculated using the following formula:

[0141] Among them, V f The volume of the supernatant is expressed in mL, ρ represents the density of the oil well cement, and M represents the density of the supernatant. s The value represents the mass (g) of cement in the oil well. The specific results can be found in Table 5.

[0142] (3) Shrinkage Performance Evaluation: The bellows shrinkage test equipment was customized according to ASTM C 1698-09 requirements, and a circulating water bath was added to obtain data at different curing temperatures (30℃, 60℃, 90℃). First, one end of the bellows was sealed with a high-temperature resistant plastic plug and PTFE tape to ensure that water would not seep in. Then, it was placed in a transparent cylinder with a base to keep it vertical. The oil well cement obtained in Examples 1-8 and Comparative Examples 1-9 was then filled into the bellows in four layers. At the end of each layer, a vortex mixer was used to vibrate for 15 seconds, and a plastic tamping rod was used to tamp the slurry in the pipe to eliminate air bubbles and make the slurry uniform. After filling, the other end of the bellows was sealed with a high-temperature resistant plastic plug and PTFE tape to ensure that there was no water leakage. Then, the water bath was heated to ensure that the circulating water in the water bath where the bellows was placed was at the test temperature to ensure that the oil well cement sample was not twisted during subsequent operations. Place the corrugated pipe smoothly into the channel of the constant temperature water bath. Adjust the LVDT (range: 5mm, accuracy: 0.005mm) at one end of the corrugated pipe to the middle scale (2500μm) and then zero it. Start the experiment and set the instrument to automatically acquire a set of experimental data every 10 minutes. After 84 hours, export the data on the change in length of the oil well cement sample over time. The corresponding linear micro-strain calculation formula is shown below:

[0143] In the formula: με(t) represents the linear shrinkage deformation at time t, i.e., the self-shrinkage value (%); L(t) represents the sample length measured at time t; L(i) represents the sample length at the initial time. The specific results can be seen in Figures 1, 2, 3, 4, 5 and 6, as well as Tables 2, 3 and 4. In the self-shrinkage (%) in Tables 2, 3 and 4, “ / ” indicates that no self-shrinkage test was performed.

[0144] (4) Microstructure evaluation: The microstructure of the oil well cement C2M3 obtained in Example 1 was photographed using a Tescan Mira4 scanning electron microscope after curing at 30°C for 28 days. The specific images are shown in Figures 7 and 8. Figure 7 shows the mechanism of action of nano-magnesium oxide; Figure 8 shows the mechanism of action of carbon nanotubes.

[0145] Table 2

[0146] Table 3

[0147] Table 4

[0148] Table 5

[0149] The following conclusions can be drawn from Tables 2 to 5:

[0150] As shown in Table 2, under the conditions of curing temperature of 30℃ and curing time of 3 days, the compressive strength and tensile strength of cement stone prepared from oil well cement C2M5 were increased by 16.8% and 19.6%, respectively, compared with the cement stone prepared from oil well cement C0M0. Under the conditions of curing temperature of 30℃ and curing time of 28 days, the compressive strength and tensile strength of cement stone prepared from oil well cement C2M5 were increased by 7.1% and 14.9%, respectively, compared with the cement stone prepared from oil well cement C0M0 without the addition of carbon nanotubes. Compared with oil well cements C0M3, C0M4, C0M5, C0M6, C0M7, C0M8, C0M9 and C0M10 without the addition of carbon nanotubes, the cement stone prepared from oil well cement C2M5 showed a significant improvement in compressive strength and tensile strength. Compared to oil well cements C2M3, C2M4, C2M6, C2M7, C2M9, and C2M10 prepared using nano-magnesium oxide calcined at other calcination temperatures for 1 hour as experimental materials, the cement stone prepared from oil well cement C2M5 exhibits improved compressive and tensile strength. Furthermore, the elastic modulus and autogenous shrinkage ratio of the cement stone prepared from oil well cement C2M5 are significantly lower than those prepared from oil well cement C0M0. Therefore, at a curing temperature of 30℃, the oil well cement C2M5 of this application is the most suitable oil well cement for this curing temperature.

[0151] As shown in Table 3, under the conditions of curing temperature of 60℃ and curing time of 3 days, the compressive strength and tensile strength of cement stone prepared from oil well cement C2M6 were increased by 14.8% and 18.4% respectively compared with cement stone prepared from oil well cement C0M0; under the conditions of curing temperature of 60℃ and curing time of 28 days, the compressive strength and tensile strength of cement stone prepared from oil well cement C2M6 were increased by 7.2% and 8.6% respectively compared with cement stone prepared from oil well cement C0M4, C0M5, C0M6, C0M7, C0M8, and C0M9 without the addition of carbon nanotubes, the cement stone prepared from oil well cement C2M6 showed a significant improvement in compressive and tensile strength. Compared to oil well cements C2M4, C2M5, C2M7, C2M8, and C2M9 prepared using nano-magnesium oxide calcined at other calcination temperatures for 1 hour as experimental materials, the cement stone prepared from oil well cement C2M6 also exhibits improved compressive and tensile strength. Furthermore, the elastic modulus and autogenous shrinkage ratio of the cement stone prepared from oil well cement C2M6 are significantly lower than those prepared from oil well cement C0M0. Therefore, at a curing temperature of 60℃, the oil well cement C2M6 of this application is the most suitable oil well cement for this curing temperature.

[0152] As shown in Table 4, under the conditions of curing temperature of 90℃ and curing time of 3 days, the compressive strength and tensile strength of the cement stone prepared by oil well cement C2M8 were increased by 8.4% and 18.3% respectively compared with the cement stone prepared by oil well cement C0M0; under the conditions of curing temperature of 90℃ and curing time of 28 days, the compressive strength and tensile strength of the cement stone prepared by oil well cement C2M8 were increased by 10.9% and 29.6% respectively compared with the cement stone prepared by oil well cement C0M5, C0M6, C0M7, C0M8, C0M9 and C0M10 without the addition of carbon nanotubes, the cement stone prepared by oil well cement C2M8 showed a significant improvement in compressive strength and tensile strength. Compared to oil well cements C2M5, C2M6, C2M7, C2M9, and C2M10 prepared using nano-magnesium oxide calcined at other temperatures for 1 hour as experimental materials, the cement stone prepared from oil well cement C2M8 also exhibits improved compressive and tensile strength. Furthermore, the elastic modulus and autogenous shrinkage ratio of the cement stone prepared from oil well cement C2M8 are significantly lower than those prepared from oil well cement C0M0. Therefore, at a curing temperature of 90℃, the oil well cement C2M8 of this application is the most suitable oil well cement for this curing temperature.

[0153] As shown in Table 5, the free fluid ratios of the composite oil well cements C2M5, C2M6, and C2M8 in this application are all lower than that of C0M0, which meets the pumping requirements for on-site construction.

[0154] The following conclusions can be drawn from Figures 1 to 8:

[0155] Figure 1 shows the autogenous shrinkage curves of cement stone formed by oil well cement obtained in Comparative Examples 1-5 and Comparative Example 8 at a curing temperature of 30°C. Figure 2 shows the autogenous shrinkage curves of cement stone formed by oil well cement obtained in Comparative Examples 1, 1-4, and Example 7 at a curing temperature of 30°C. As shown in Figures 1 and 2, under 30°C conditions, the shrinkage of cement stone prepared from oil well cement C2M5 is significantly less than that from C0M0, and it exhibits a significant expansion trend. Compared with oil well cements C0M3, C0M4, C0M5, C0M6, and C0M9 without added carbon nanotubes, its shrinkage is also reduced, and its expansion trend is more pronounced. Compared with oil well cement C2M9 prepared using nano-magnesium oxide calcined at other calcination temperatures for 1 hour as the experimental material, its shrinkage is significantly reduced, and its expansion trend is more pronounced, while the difference compared with C2M3, C2M4, and C2M6 is not significant.

[0156] Figure 3 shows the autogenous shrinkage curves of the cement stone formed by the oil well cement obtained in Comparative Examples 1 and 4-8 at a curing temperature of 60℃. Figure 4 shows the autogenous shrinkage curves of the cement stone formed by the oil well cement obtained in Comparative Examples 1 and 3-7 at a curing temperature of 60℃. As shown in Figures 3 and 4, under 60℃ conditions, the shrinkage of the cement stone prepared from oil well cement C2M6 is significantly less than that of C0M0, and it has a significant expansion trend. Compared with oil well cements C0M3, C0M4, C0M5, C0M6, and C0M9 without added carbon nanotubes, although the shrinkage is increased, the expansion trend is more obvious. Compared with oil well cement C2M5 prepared using nano-magnesium oxide calcined for 1 hour at other calcination temperatures as experimental materials, the shrinkage is increased, but compared with C2M7, C2M8, and C2M9, the shrinkage is reduced. This indicates that under 60℃ conditions, nano-magnesium oxide can exert its expansion performance according to the preparation method provided in this application, but carbon nanotubes will slightly reduce the expansion performance.

[0157] Figure 5 shows the autogenous shrinkage curves of cement stone formed by oil well cement obtained in Comparative Examples 1 and 4-8 at a curing temperature of 90℃; Figure 6 shows the autogenous shrinkage curves of cement stone formed by oil well cement obtained in Comparative Examples 1 and 3-7 at a curing temperature of 90℃. As shown in Figures 5 and 6, under 90℃ conditions, the shrinkage of cement stone prepared from oil well cement C2M8 is significantly less than that from C0M0, and it exhibits a significant expansion trend. Compared with oil well cements C0M5, C0M6, C0M7, C0M8, and C0M9 without added carbon nanotubes, the shrinkage is not significantly different, but the expansion trend is more obvious. Compared with oil well cement C2M9 prepared using nano-magnesium oxide calcined for 1 hour at other calcination temperatures as experimental material, the shrinkage is increased, but compared with C2M5, C2M6, and C2M7, the shrinkage is reduced. This indicates that under 90℃ conditions, nano-magnesium oxide can exert its expansion performance according to the preparation method provided in this application, but carbon nanotubes will slightly reduce the expansion performance.

[0158] Figure 7 shows the microstructure of the oil well cement C2M3 obtained in Example 1 after curing at 30°C for 28 days under an electron microscope. CSH represents hydrated calcium silicate, which is formed by the reaction of tricalcium silicate and dicalcium silicate with water and plays a crucial role in the strength and durability of the cement. The "I" in CSHI stands for "intermediate" or "incomplete," indicating that the structure may be some intermediate state or an incomplete calcium-silica hydrate. CH represents calcium hydroxide, and MH represents magnesium hydroxide. Figure 7 demonstrates that after the nano-magnesium oxide material undergoes a hydration reaction in the oil well cement, it generates magnesium hydroxide. Through interaction with other hydration products (such as calcium hydroxide), it can fill large capillary pores and reduce the formation of microcracks.

[0159] Figure 8 shows the microstructure of the oil well cement C2M3 obtained in Example 1 after curing at 30℃ for 28 days under an electron microscope. CSH represents calcium silicate hydrate, and spectral point 2 represents carbon nanotubes. Figure 8 illustrates the pull-out and bridging phenomena of carbon nanotubes. Under external force, carbon nanotubes may be pulled out of the cement matrix, thus observing the pull-out phenomenon. The pull-out of carbon nanotubes can improve the fracture toughness of the oil well cement by dissipating energy. The bridging phenomenon refers to the formation of a network structure by carbon nanotubes in the cement matrix, connecting or "bridging" microcracks or pores in the cement matrix. This bridging effect can improve the crack resistance and toughness of the cement, effectively preventing crack propagation.

[0160] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them; although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or make equivalent substitutions for some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A composite oil well cement, wherein, Including the following parts by weight of raw materials: Oil well cement 90-120 parts, water 40-60 parts, carbon nanotubes 0.02-0.1 parts, nano magnesium oxide 1-3 parts, oil well cement dispersant 0.1-0.5 parts; The nano-magnesium oxide was obtained by calcination at 300℃-1000℃.

2. The composite oil well cement according to claim 1, wherein, The nano-magnesium oxide in the composite oil well cement is obtained by calcination at 450℃-550℃ when the curing temperature is 20℃-40℃.

3. The composite oil well cement according to claim 1, wherein, The nano-magnesium oxide in the composite oil well cement is obtained by calcination at 550℃-650℃ when the curing temperature is 50℃-70℃.

4. The composite oil well cement according to claim 1, wherein, The nano-magnesium oxide in the composite oil well cement is obtained by calcination at 750℃-850℃ when the curing temperature is 80℃-100℃.

5. The composite oil well cement according to claim 1, wherein, The nano-magnesium oxide, before calcination at 300℃-1000℃, has an average particle size of 45-55nm and a specific surface area of ​​30-50m². 2 / g.

6. The composite oil well cement according to claim 1, wherein, The carbon nanotubes have a length of 5-15 μm, an inner diameter of 3-5 nm, an outer diameter of 8-15 nm, and a specific surface area ≥250 m². 2 / g.

7. The composite oil well cement according to any one of claims 1-6, wherein, The composite oil well cement also includes the following raw materials in parts by weight: 0.02-0.05 parts of carbon nanotube dispersant and 0.2-0.8 parts of nano magnesium oxide dispersant.

8. The method for preparing composite oil well cement according to any one of claims 1-7, wherein, Includes the following steps: Nano-magnesium oxide is dispersed in water to obtain a nano-magnesium oxide suspension; Carbon nanotubes are dispersed in water to obtain a carbon nanotube suspension; The nano-magnesium oxide suspension and the carbon nanotube suspension are stirred, and oil well cement and oil well cement dispersant are added to obtain the composite oil well cement.

9. The preparation method according to claim 8, wherein, The nano magnesium oxide suspension also includes a nano magnesium oxide dispersant.

10. The preparation method according to claim 8 or 9, wherein, The carbon nanotube suspension also includes a carbon nanotube dispersant.

11. The preparation method according to claim 8, wherein, The stirring of the nano-magnesium oxide suspension and the carbon nanotube suspension also includes: The nano-magnesium oxide suspension and the carbon nanotube suspension are first stirred at a speed of 3000-5000 r / min for 10-20 s, and then stirred at a speed of 11000-13000 r / min for 30-40 s.

12. The preparation method according to claim 8, wherein, Dispersing nano-magnesium oxide in water also includes: adding nano-magnesium oxide to water, stirring at 3000-5000 r / min for 10-20 s, stirring at 11000-13000 r / min for 30-40 s, and then sonicating for 2-8 min to obtain a nano-magnesium oxide suspension.

13. The preparation method according to claim 8, wherein, Dispersing carbon nanotubes in water also includes: adding carbon nanotubes to water, stirring at 300-500 r / min for 15-25 min, and then subjecting it to ultrasonic treatment for 30-50 min.

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