Preparation method of amino-modified carbon nanotubes and application thereof in carbon nanotube cement slurry with self-healing property upon exposure to carbon dioxide

Abstract

The present invention discloses a preparation method of amino-modified carbon nanotubes, comprising the following steps: (1) adding hydrochloric acid to amino-carbon nanotubes, adding deionized water and stirring at 45-55 °C, filtering by suction, and drying to obtain solid A; (2) sequentially adding iodoisobutane and dichloromethane to the solid A, mixing, stirring at room temperature for 18-25 min, filtering by suction, and drying to obtain black solid B; (3) sequentially adding triethylamine, potassium hydroxide and dichloromethane to the black solid B to obtain a mixed solution, stirring at 35-40 °C for 1.5-2 h, filtering, rotary evaporating to remove dichloromethane, and then rinsing with deionized water and drying. Meanwhile, the present invention also discloses the application of the amino-modified carbon nanotubes in a carbon nanotube cement slurry with self-healing property upon encountering carbon dioxide. The amino-modified carbon nanotubes can undergo a chemical reaction with carbon dioxide, enabling the cement slurry system prepared therefrom to exhibit a self-healing phenomenon.

Description

Technical Field

[0001] The present invention belongs to the technical field of cement slurries for well cementing, and particularly relates to a preparation method of amine-modified carbon nanotubes and their application in carbon nanotube cement slurries with self-healing ability upon exposure to carbon dioxide. Background Art

[0002] Carbon capture, utilization and storage (CCUS) is a technology that captures and purifies carbon dioxide emitted during the production process and then inputs it into a new production process for recycling or storage. The global CCUS projects have shown an upward trend for four consecutive years, with the total capture capacity increasing by 32% and the annual capture capacity reaching 40 million tons.

[0003] Currently, carbon dioxide flooding and geological storage are important aspects of CCUS. This technology injects carbon dioxide into the formation to improve the oil and gas recovery rate, which can not only meet the needs of oilfield development but also solve the problem of carbon dioxide storage. However, with the large-scale application of carbon dioxide utilization and geological storage technologies, the sealing safety problem of injection and production wellbores has become prominent, attracting the high attention of experts, scholars and engineering managers. Previous studies have shown that adding additives with certain anti-corrosion functions to oil well cement is an effective way to improve the corrosion resistance of well cementing slurries, such as fly ash, resin, latex, microsilica, etc. However, carbon dioxide flooding and geological storage wells are different from traditional high-carbon dioxide wells. Taking injection wells as an example, during the production process of some wells, it may be necessary to release and set packers multiple times, as well as run and pull tubing strings, alternate gas injection, etc., which will inevitably impact the casing, resulting in microcracks inside the external cement sheath and between the cement sheath and the casing. Coupled with the existing hidden dangers of well cementing quality such as micro-clearances, it is extremely easy for carbon dioxide to leak and flow uncontrollably along the cement sheath, threatening the shallow surface ecology and the safety of the well site. Therefore, to ensure the safety of the well site and the storage effect, the cement slurry is required to have strong mechanical properties and the ability to self-heal microcracks upon exposure to carbon dioxide, and it is necessary to develop a carbon dioxide-responsive corrosion-resistant cement slurry system.

[0004] As a tubular carbon material with graphite crystallization, carbon nanotubes are lightweight, hard, high-strength, and have excellent mechanical properties. In recent years, they have been studied as reinforcements for cement-based composites. Patent CN109020316A discloses a carbon nanotube early-strength and high-tensile well cementing slurry and its preparation method, which improves the mechanical properties such as compressive, tensile and impact resistance of oil well cement by hydroxyl-modified carbon nanotubes, but does not further explore and improve its self-healing ability upon exposure to carbon dioxide. The inventor of the present invention found through experiments that the self-healing ability of hydroxyl-modified or pure carbon nanotube cement slurries upon exposure to carbon dioxide is limited, mainly because the existing carbon nanotube cement slurries have not been specifically developed in a targeted manner. Therefore, it is necessary to adopt more effective carbon nanotube modification technologies to construct a new carbon nanotube cement slurry system with better mechanical properties and self-healing ability upon exposure to carbon dioxide. Summary of the Invention

[0005] In view of the deficiencies of the prior art, the present invention provides a method for preparing amino-modified carbon nanotubes, and also provides an application of using the amino-modified carbon nanotubes in a carbon nanotube cement slurry that self-heals upon exposure to carbon dioxide. The amino-modified carbon nanotubes can react with carbon dioxide to form a self-healing phenomenon.

[0006] A method for preparing amino-modified carbon nanotubes includes the following steps:

[0007] (1) Hydrochloric acid is added to amino-carbon nanotubes, deionized water is added at 45 - 55 °C and stirred for 25 - 30 min, then filtered by suction and dried to obtain solid A;

[0008] (2) Iodo-isobutane and dichloromethane are successively added to the solid A, mixed, and stirred at room temperature for 18 - 25 min, then filtered by suction and dried to obtain black solid B;

[0009] (3) Triethylamine, potassium hydroxide, and dichloromethane are successively added to the black solid B to obtain a mixed solution, which is stirred at 35 - 40 °C for 1.5 - 2 h, filtered, the dichloromethane is removed by rotary evaporation, then rinsed with deionized water and dried to obtain amino-modified carbon nanotubes.

[0010] Preferably, the weight ratios of the raw materials are as follows: the mass ratio of amino-carbon nanotubes, hydrochloric acid, iodo-isobutane, the dichloromethane in step (2), triethylamine, potassium hydroxide, and the dichloromethane in step (3) is 1:(1.2 - 1.8):(1 - 1.5):(46 - 53):(18 - 24):(1.2 - 1.8):(46 - 53).

[0011] More preferably, the ratios of the raw materials are as follows: the mass ratio of amino-carbon nanotubes, hydrochloric acid, iodo-isobutane, the dichloromethane in step (2), triethylamine, potassium hydroxide, and the dichloromethane in step (3) is 1:1.5:1.2:50:20:1.5:50.

[0012] Preferably, the drying condition is drying to constant weight at 105 - 110 °C.

[0013] Preferably, the concentration of the hydrochloric acid is 1 - 2 mol / L.

[0014] A carbon nanotube cement slurry that self-heals upon exposure to carbon dioxide is composed of the following raw materials in parts by weight: 100 parts of oil well cement, 0.5 - 1.5 parts of fluid loss reducer, 0.03 - 0.08 parts of amino-modified carbon nanotubes, 0.3 - 0.5 parts of defoamer, and 44 - 60 parts of water, wherein the amino-modified carbon nanotubes are prepared by the method for preparing amino-modified carbon nanotubes of the present invention.

[0015] Preferably, the carbon nanotube cement slurry that self-heals upon encountering carbon dioxide is composed of the following raw materials in parts by weight: 100 parts of oil well cement, 1.5 parts of fluid loss reducer, 0.05 part of amine-modified carbon nanotubes, 0.5 part of defoamer, and 44 parts of water.

[0016] Preferably, the oil well cement is G-class oil well cement.

[0017] Preferably, the defoamer is a polyether defoamer or a silicone defoamer.

[0018] Preferably, the fluid loss reducer is a solid water-soluble polymer fluid loss reducer.

[0019] Preferably, the fluid loss reducer is a liquid water-soluble polymer fluid loss reducer.

[0020] When the fluid loss reducer is a solid water-soluble polymer fluid loss reducer, the preparation method of the carbon nanotube cement slurry that self-heals upon encountering carbon dioxide includes the following steps:

[0021] (a) Take the oil well cement and the fluid loss reducer and mix them to obtain mixture A;

[0022] (b) Take water, amine-modified carbon nanotubes and the defoamer and mix them, and perform ultrasonic treatment at a power of 350 W for 25 - 30 min to obtain a mixed solution;

[0023] (c) Under the stirring condition of a rotation speed of 4500 ± 200 r / min, add mixture A thereto within 10 - 15 s, and then continue to stir at a rotation speed of 10000 ± 200 r / min for 50 - 60 s to obtain the carbon nanotube cement slurry that self-heals upon encountering carbon dioxide.

[0024] When the fluid loss reducer is a liquid water-soluble polymer fluid loss reducer, the preparation method of the carbon nanotube cement slurry that self-heals upon encountering carbon dioxide includes the following steps:

[0025] (a) Take amine-modified carbon nanotubes, water and the defoamer and mix them, perform ultrasonic treatment at a power of 350 W for 25 - 30 min, add the fluid loss reducer thereto, and stir and mix to obtain a mixed solution;

[0026] (b) Under the stirring condition of a rotation speed of 4500 ± 200 r / min, add the oil well cement thereto within 10 - 15 s, and then continue to stir at a rotation speed of 10000 ± 200 r / min for 50 - 60 s to obtain the carbon nanotube cement slurry that self-heals upon encountering carbon dioxide.

[0027] Advantages of the present invention:

[0028] (1) The amino-modified carbon nanotubes provided by the present invention do not damage the original material properties of the carbon nanotubes. Therefore, the cement slurry system prepared by using the amino-modified carbon nanotubes has strong compressive strength and tensile strength, improving the resistance of the cement stone to impact damage under different working conditions in the later stage of carbon dioxide injection and production wells;

[0029] (2) As a nano-microscale material, the amino-modified carbon nanotubes can more effectively fill the nano-micro gaps in the cement stone, improving the compactness of the cement stone and enabling the cement stone matrix to have strong carbon dioxide corrosion resistance;

[0030] (3) When carbon dioxide invades the cement stone matrix or the bonding surface between the cement stone and the casing or formation along the microcracks, the amino-modified carbon nanotubes can chemically react with carbon dioxide to form insoluble quaternary ammonium salts, block the microcrack channels, form a self-healing phenomenon, and delay the continuous invasion of carbon dioxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 TEM image of amino-modified carbon nanotubes. DETAILED DESCRIPTION OF THE INVENTION

[0032] The parts mentioned in the present invention refer to parts by weight.

[0033] Example 1

[0034] A preparation method of amino-modified carbon nanotubes, comprising the following steps:

[0035] (1) Take 1 part of amino-carbon nanotubes, add 1.5 parts of hydrochloric acid with a concentration of 1 mol / L thereto, add deionized water and stir at 50 °C for 30 min, filter, and dry to constant weight at 105 °C to obtain solid A;

[0036] (2) Add 1.2 parts of iodoisobutane and 50 parts of dichloromethane to the solid A in sequence, mix, stir at room temperature for 20 min, filter, and dry to constant weight at 105 °C to obtain black solid B;

[0037] (3) Add 20 parts of triethylamine, 1.5 parts of potassium hydroxide, and 50 parts of dichloromethane to the black solid B in sequence to obtain a mixed solution, stir at 35 °C for 2 h, filter, rotary evaporate to remove dichloromethane, then rinse with deionized water, and dry to constant weight at 105 °C to obtain amino-modified carbon nanotubes.

[0038] Perform TEM testing on the amino-modified carbon nanotubes, and the test results are shown in Figure 1 From Figure 1 it can be seen that the amino-modified carbon nanotubes obtained by the present invention do not damage the self-structure of the carbon nanotubes and still have the material properties of high aspect ratio, nano-scale material filling, and network formation.

[0039] Example 2

[0040] A preparation method of amino-modified carbon nanotubes comprises the following steps:

[0041] (1) Take 1 part of amino carbon nanotubes, add 1.2 parts of hydrochloric acid with a concentration of 2 mol / L thereto, add deionized water and stir for 30 min at 45 °C, carry out suction filtration, and dry to constant weight at 110 °C to obtain solid A;

[0042] (2) Sequentially add 1 part of iodo-isobutane and 46 parts of dichloromethane to the solid A, mix, stir at room temperature for 18 min, carry out suction filtration, and dry to constant weight at 110 °C to obtain black solid B;

[0043] (3) Sequentially add 18 parts of triethylamine, 1.2 parts of potassium hydroxide, and 46 parts of dichloromethane to the black solid B to obtain a mixed solution, stir at 35 °C for 1.5 h, filter, remove dichloromethane by rotary evaporation, then rinse with deionized water, and dry to constant weight at 110 °C to obtain amino-modified carbon nanotubes.

[0044] Example 3

[0045] A preparation method of amino-modified carbon nanotubes comprises the following steps:

[0046] (1) Take 1 part of amino carbon nanotubes, add 1.8 parts of hydrochloric acid with a concentration of 1 mol / L thereto, add deionized water and stir for 25 min at 55 °C, carry out suction filtration, and dry to constant weight at 110 °C to obtain solid A;

[0047] (2) Sequentially add 1.5 parts of iodo-isobutane and 53 parts of dichloromethane to the solid A, mix, stir at room temperature for 25 min, carry out suction filtration, and dry to constant weight at 110 °C to obtain black solid B;

[0048] (3) Sequentially add 24 parts of triethylamine, 1.8 parts of potassium hydroxide, and 53 parts of dichloromethane to the black solid B to obtain a mixed solution, stir at 35 °C for 1.5 h, filter, remove dichloromethane by rotary evaporation, then rinse with deionized water, and dry to constant weight at 110 °C to obtain amino-modified carbon nanotubes.

[0049] Example 4

[0050] A carbon nanotube cement slurry that self-heals upon exposure to carbon dioxide is composed of the following raw materials in parts by weight: 100 parts of G-class oil well cement, 0.8 part of fluid loss reducer, 0.04 part of amino-modified carbon nanotubes, 0.3 part of defoamer, and 44 parts of water;

[0051] Among them, the fluid loss reducer is a liquid type with the model BH-F201L and is a water-soluble polymer type fluid loss reducer;

[0052] The antifoaming agent has the model number MX-906 and is a polyether antifoaming agent;

[0053] The amino-modified carbon nanotubes are prepared by the preparation method described in Example 1.

[0054] The preparation method of the carbon nanotube cement slurry with self-healing property upon encountering carbon dioxide includes the following steps:

[0055] (a) Take the amino-modified carbon nanotubes, water and the antifoaming agent and mix them, ultrasonically treat them at a power of 350 W for 25 min, add a fluid loss reducer thereto, and stir and mix to obtain a mixed solution;

[0056] (b) Under the stirring condition of a rotation speed of 4500 r / min, add the oil well cement thereto within 12 s, and then continue to stir at a rotation speed of 10000 r / min for 58 s to obtain the carbon nanotube cement slurry with self-healing property upon encountering carbon dioxide.

[0057] Example 5

[0058] A carbon nanotube cement slurry with self-healing property upon encountering carbon dioxide is composed of the following raw materials in parts by weight: 100 parts of G-class oil well cement, 1.0 part of fluid loss reducer, 0.06 part of amino-modified carbon nanotubes, 0.4 part of antifoaming agent, and 45 parts of water;

[0059] Among them, the fluid loss reducer is a solid type with the model number G33S and is a water-soluble polymer fluid loss reducer;

[0060] The antifoaming agent has the model number XP-3 and is a silicone antifoaming agent;

[0061] The amino-modified carbon nanotubes are prepared by the preparation method described in Example 1.

[0062] The preparation method of the carbon nanotube cement slurry with self-healing property upon encountering carbon dioxide includes the following steps:

[0063] (a) Take the oil well cement and the fluid loss reducer and mix them to obtain mixture A;

[0064] (b) Take water, amino-modified carbon nanotubes and the antifoaming agent and mix them, ultrasonically treat them at a power of 350 W for 25 min to obtain a mixed solution;

[0065] (c) Under the stirring condition of a rotation speed of 4600 r / min, add mixture A thereto within 15 s, and then continue to stir at a rotation speed of 10200 r / min for 55 s to obtain the carbon nanotube cement slurry with self-healing property upon encountering carbon dioxide.

[0066] Example 6

[0067] A carbon nanotube cement slurry that self-heals upon exposure to carbon dioxide, which is composed of the following raw materials in parts by weight: 100 parts of G-class oil well cement, 0.5 part of fluid loss reducer, 0.03 part of amino-modified carbon nanotubes, 0.5 part of defoamer, and 60 parts of water;

[0068] Among them, the fluid loss reducer is a solid type with the model G303, which is a water-soluble polymer fluid loss reducer;

[0069] The defoamer has the model XP-3 and is a silicone defoamer;

[0070] The amino-modified carbon nanotubes are prepared by the preparation method described in Example 2.

[0071] The preparation method of the carbon nanotube cement slurry that self-heals upon exposure to carbon dioxide includes the following steps:

[0072] (a) Take the oil well cement and the fluid loss reducer and mix them to obtain mixture A;

[0073] (b) Take water, amino-modified carbon nanotubes and the defoamer and mix them, and perform ultrasonic treatment for 30 min at a power of 350 W to obtain a mixed solution;

[0074] (c) Under the stirring condition of a rotation speed of 4300 r / min, add mixture A thereto within 10 s, and then continue to stir at a rotation speed of 8000 r / min for 50 s to obtain the carbon nanotube cement slurry that self-heals upon exposure to carbon dioxide.

[0075] Example 7

[0076] A carbon nanotube cement slurry that self-heals upon exposure to carbon dioxide, which is composed of the following raw materials in parts by weight: 100 parts of G-class oil well cement, 1.5 parts of fluid loss reducer, 0.08 part of amino-modified carbon nanotubes, 0.4 part of defoamer, and 50 parts of water;

[0077] Among them, the fluid loss reducer is a liquid type with the model BH-F201L, which is a water-soluble polymer fluid loss reducer;

[0078] The defoamer has the model MX-906 and is a polyether defoamer;

[0079] The amino-modified carbon nanotubes are prepared by the preparation method described in Example 3.

[0080] The preparation method of the carbon nanotube cement slurry that self-heals upon exposure to carbon dioxide includes the following steps:

[0081] (a) Take amino-modified carbon nanotubes, water and the defoamer and mix them, perform ultrasonic treatment for 30 min at a power of 350 W, and add the fluid loss reducer thereto and stir and mix to obtain a mixed solution;

[0082] (b) Under the stirring condition of 4700 r / min, the well cement is added into the mixed solution within 15 s, and then the mixture is continuously stirred at 10000 r / min for 50 s to obtain the carbon nanotube cement slurry with self-healing property upon exposure to carbon dioxide.

[0083] Comparative Example 1

[0084] Without adding amino-modified carbon nanotubes, specifically as follows:

[0085] A cement slurry is composed of the following raw materials in parts by weight: 100 parts of G-class well cement, 0.8 part of fluid loss reducer, 0.3 part of defoamer, and 44 parts of water; the others are the same as in Example 4.

[0086] The preparation method of the cement slurry includes the following steps:

[0087] (a) Mix water and defoamer, and perform ultrasonic treatment for 25 min at a power of 350 W. Add the fluid loss reducer thereto and stir to obtain a mixed solution.

[0088] (b) Under the stirring condition of 4500 r / min, the well cement is added into the mixed solution within 12 s, and then the mixture is continuously stirred at 10000 r / min for 58 s to obtain the cement slurry.

[0089] Comparative Example 2

[0090] Use amino carbon nanotubes to replace amino-modified carbon nanotubes, and the others are the same as in Example 4.

[0091] Comparative Example 3

[0092] Use carbon nanotubes to replace amino-modified carbon nanotubes, and the others are the same as in Example 4.

[0093] I. Mechanical property detection

[0094] Use a BSRD-299 type multi-functional pressure testing machine to conduct mechanical property tests on the cement stones formed by the cement slurries of Example 4, Example 5, Comparative Example 1, Comparative Example 2, and Comparative Example 3. Before the test, the curing conditions of each sample are 70 °C, 20.8 MPa, and 72 h (water bath), and the curing device is a DFC-0720 type pressurized curing autoclave. The test methods for different mechanical properties are all carried out according to the standards. Among them, the compressive strength is based on GB / T 19139-2012 "Test Methods for Well Cement", and the test method for the tensile strength is based on the method in NB / T 14004.2-2016 "Shale Gas Cementing Engineering - Part 2: Technical Requirements and Evaluation Methods for Cement Slurries". The test results are shown in Table 1.

[0095] Table 1 Mechanical properties of cement stones

[0096]

[0097] As can be seen from the data in Table 1, the compressive strength and tensile strength of the cement stone without carbon nanotubes (Comparative Example 1) are both relatively low. After adding carbon nanotubes and amino carbon nanotubes, the strength of the cement stone increases slightly. After adding the modified carbon nanotubes provided by the present invention, the strength of the cement stone further increases, and the increase amplitude of the tensile strength is relatively large. Compared with Comparative Example 1, the tensile strength of the cement stone in Example 4 can be increased by 64%. The test results show that the amino carbon nanotubes of the present invention have a better dispersion effect, can better exert the characteristics of the carbon nanotubes themselves with higher strength and larger aspect ratio, can form a network structure inside the cement stone, and when subjected to an external load, increase the resistance to being damaged, and the macroscopic manifestation is the increase of the compressive strength and tensile strength.

[0098] II. Detection of carbon dioxide corrosion resistance

[0099] The method of testing the carbon dioxide corrosion depth was used to investigate the carbon dioxide corrosion resistance of different cement slurry systems. The specific scheme was as follows: First, Examples 4 and 5, Comparative Example 1, Comparative Example 2, and Comparative Example 3 were cured in a water bath at 70°C and 20.8 MPa for 72 h. Then, the cured cement stones were subjected to a 28-day immersion experiment with carbon dioxide in an XWL-18 type high-temperature and high-pressure carbon dioxide reaction kettle, and the temperature and pressure conditions were 50°C × 10 MPa and 70°C × 15 MPa respectively. After the reaction was completed, the cement stones were cut in half from the middle, and phenolphthalein reagent was evenly applied to the cross-section of the cement stones. Since the hydration products of cement are alkaline, after being corroded by carbon dioxide, the corroded area will not change color, while the non-corroded area will turn red. The thickness of the corroded area was measured and averaged to obtain the corrosion depth. The test results of different systems are shown in Table 2.

[0100] Table 2 Test results of corrosion depth

[0101]

[0102] As can be seen from Table 2, even at a relatively low temperature of 50°C, the corrosion penetration depth of the cement stone without additives (Comparative Example 1) still reached 3.48 mm. After adding amino-carbon nanotubes or carbon nanotubes, due to the filling effect of the nanomaterials, the corrosion penetration depth of the cement stone decreased, but still remained in a relatively high range. The corrosion depth of the cement stone in the examples of the present invention is generally low. The corrosion penetration depth of Example 4 of the present invention is only 1.03 mm. After increasing the temperature and pressure, the corrosion depth of the cement stone in Comparative Example 1 increased significantly, and there were also significant increases in Comparative Examples 2 and 3. However, the overall change in the corrosion penetration depth of the cement stone in the examples of the present invention was small, only slightly increasing. It is analyzed that this is mainly because the modified carbon nanotube cement stone of the present invention has a better dispersion effect in cement (which can also be confirmed by the strength test results in Table 1), forming a three-dimensional network structure to reduce the intrusion of carbon dioxide. On the other hand, the modified carbon nanotube of the present invention can react with carbon dioxide to generate insoluble quaternary ammonium salts, which can fill the shrinkage caused by cement corrosion and carbonization, reducing the intrusion path of carbon dioxide.

[0103] III. Evaluation of Self-Healing Ability against Carbon Dioxide Corrosion

[0104] The method of evaluating the change in the size of prefabricated cracks before and after carbon dioxide corrosion is used to evaluate the self-healing ability of the cement stone against carbon dioxide. The self-healing abilities of the cracks inside the cement stone, between the cement stone and the steel column, and between the cement stone and the core are respectively investigated to simulate the crack self-healing abilities of the materials under three different working conditions of the cement sheath matrix, the cement sheath and the casing, and the cement sheath and the formation.

[0105] The specific method is as follows: (1) For the self-healing evaluation of the cracks inside the cement stone, first, a cylindrical cement stone with a diameter of 25 mm and a length of 50 mm that has been cured in a water bath under certain temperature and pressure conditions for 72 h is wrapped with a heat shrinkable tube. The Brazilian splitting method is used with a pressure testing machine to generate tiny longitudinal cracks inside the cement stone, and its permeability is measured. Then, the cement stone column is placed in a carbon dioxide curing kettle under certain temperature and pressure conditions for 28 days, and the permeability is measured again. The permeability before curing is compared to evaluate its self-healing rate;

[0106] (2) For the self-healing evaluation of the cracks between the cement stone and the steel column, a semi-cylindrical carbon steel component with a diameter of 25 mm and a length of 50 mm is processed and placed in a cylindrical curing mold with a diameter of 25 mm and a length of 50 mm. Then, the cement slurries of different examples and comparative examples are poured in, and it is cured in a water bath under certain temperature and pressure conditions for 72 h to prepare a combined cylinder with a diameter of 25 mm and a length of 50 mm. The combined body is gently disassembled, then reassembled and sealed with a heat shrinkable tube, and its permeability is measured. Then, the combined cylinder is placed in a carbon dioxide curing kettle under certain temperature and pressure conditions for 28 days, and the permeability is measured again. The permeability before curing is compared to evaluate its self-healing rate;

[0107] (3) Self-healing evaluation of the cracks between the cement stone and the core: Cut the core into semi-cylindrical shapes with a diameter of 25 mm and a length of 50 mm, place them in cylindrical curing molds with a diameter of 25 mm and a length of 50 mm, and pour the cement slurries of different examples and comparative examples. Cure them in a water bath under certain temperature and pressure conditions for 72 h to prepare composite cylinders with a diameter of 25 mm and a length of 50 mm. Gently disassemble the composite, then reassemble and seal it with heat shrinkable tubing, and test its permeability. Then place the composite cylinder in a carbon dioxide curing kettle under certain temperature and pressure conditions for 28 days, test the permeability again, and compare it with the permeability before curing to evaluate its self-healing rate.

[0108] The permeability when the cement stone body is not split and the cement stone and the steel column or core are not disassembled is the original permeability. The permeability after the cement stone is split, the cement stone and the steel column or core are disassembled and then combined with heat shrinkable tubing is the combined permeability. The permeability after being soaked in carbon dioxide for 28 days is the permeability after self-healing. The calculation method of the self-healing rate is the difference between the permeability after self-healing and the original permeability divided by the difference between the combined permeability and the original permeability. The above carbon dioxide soaking experiment is still carried out using an XWL-18 type high-temperature and high-pressure carbon dioxide reaction kettle. The above temperature and pressure conditions are 50 °C × 10 MPa. The permeability is tested using an HK-type automatic permeability measuring instrument. The test results are shown in Table 3.

[0109] Table 3 Permeability test results of different cement slurry systems under different working conditions

[0110]

[0111] As can be seen from Table 3, there is no correlation and pattern between the combined permeability after prefabricating cracks and different cement slurry systems. This is mainly due to the differences in sample preparation during prefabricating cracks. However, generally, the combined permeabilities of different examples and comparative examples within the same working condition do not vary significantly, which lays a foundation for the effective comparative evaluation of the later self-healing rate. The cement slurry system without adding carbon nanotubes (Comparative Example 1) has a certain self-healing ability under different working conditions, but the self-healing rates are all relatively low. Especially in the channels of the first interface (cement stone and steel column) and the second interface (cement stone and core) where carbon dioxide is likely to channel underground in actual field operations, its self-healing ability is even lower. After adding amino carbon nanotubes (Comparative Example 2) or carbon nanotubes (Comparative Example 3), compared with Comparative Example 1, the original permeability of the cement stone decreases, and the self-healing rate increases to a certain extent. This is mainly because the nanomaterials are filled in the cement stone and can act as crystal nuclei in microcracks, promoting the reaction between carbon dioxide and cement hydration products to generate new substances to block the cracks. However, the overall promoting effect is weak, and the increase in the self-healing rate is small. Especially in the first interface (cement stone and steel column) with relatively weak bonding force, the effect is average. While for the cement slurry system of the present invention, the improvement effect on the self-healing ability of the cement-based material is significant. Inside the cement stone, the crack self-healing rate of the cement slurry system in Example 4 reached 75.65% after 28 days. Compared with the cement slurry system without special additives (Comparative Example 1), it increased by 61.12 percentage points. Even at the interface between the cement stone and the steel column where effective cementation is difficult, the self-healing rate of the system of the present invention exceeded 40%, while the self-healing rate of the system in Comparative Example 1 was lower than 10%. The main reasons and mechanisms for the above results are the technical solutions provided by the present invention. When carbon dioxide invades the cracks, it can react with amino carbon nanotubes to generate insoluble quaternary ammonium salts, block the microcrack channels, reduce the permeability of the cement stone or the composite body, and form self-healing cement stone.

[0112] Currently, the cement slurry system provided by the present invention has been applied in 7 wells in the 100,000-ton / year carbon dioxide flooding and storage demonstration area of Yanchang Oilfield. The continuous safety monitoring results show that during the stages of packer setting, gas injection, and water alternating gas injection, no carbon dioxide leakage occurred in the applied wells, indicating that the carbon dioxide self-healing system of the present invention has excellent characteristics in preventing carbon dioxide leakage and improving wellbore integrity.

[0113] In summary, the cement slurry system provided by the present invention has a strong microcrack self-healing ability when encountering carbon dioxide, can effectively prevent carbon dioxide escape during the process of carbon dioxide flooding and geological storage, and can provide important technical support for achieving the safety goal of carbon dioxide geological storage.

Claims

1. A preparation method of amino-modified carbon nanotubes, characterized in that: It includes the following steps: (1) Add hydrochloric acid to amino carbon nanotubes, add deionized water and stir for 25 - 30 min at 45 - 55 °C, perform suction filtration, and dry to obtain solid A; (2) Add iodo - isobutane and dichloromethane to the solid A in sequence, mix, stir at room temperature for 18 - 25 min, perform suction filtration, and dry to obtain black solid B; (3) Add triethylamine, potassium hydroxide and dichloromethane to the black solid B in sequence to obtain a mixed solution, stir at 35 - 40 °C for 1.5 - 2 h, filter, rotary evaporate to remove dichloromethane, then rinse with deionized water and dry to obtain amino - modified carbon nanotubes.

2. The preparation method of the amino-modified carbon nanotubes according to claim 1, wherein: The weight ratio of each raw material is as follows: the mass ratio of amino carbon nanotubes, hydrochloric acid, iodo - isobutane, dichloromethane in step (2), triethylamine, potassium hydroxide, dichloromethane in step (3) is 1:(1.2 - 1.8):(1 - 1.5):(46 - 53):(18 - 24):(1.2 - 1.8):(46 - 53).

3. The preparation method of the amino-modified carbon nanotubes according to claim 1, wherein: The drying condition is drying to constant weight at 105 - 110 °C, and the concentration of the hydrochloric acid is 1 - 2 mol / L.

4. A carbon nanotube cement slurry that self-heals upon encountering carbon dioxide, characterized in that: It is composed of the following raw materials in parts by weight: 100 parts of oil well cement, 0.5 - 1.5 parts of fluid loss reducer, 0.03 - 0.08 parts of amino - modified carbon nanotubes, 0.3 - 0.5 parts of defoamer, 44 - 60 parts of water, wherein the amino - modified carbon nanotubes are prepared by the preparation method described in any one of claims 1 - 3.

5. The self-healing carbon nanotube cement slurry upon exposure to carbon dioxide according to claim 4, wherein: The oil well cement is G - grade oil well cement.

6. The self-healing carbon nanotube cement slurry upon encountering carbon dioxide according to claim 5, wherein: The defoamer is a polyether - type defoamer or a silicone - type defoamer.

7. The self-healing carbon nanotube cement slurry upon encountering carbon dioxide according to claim 6, characterized in that: The fluid loss reducer is a solid - type water - soluble polymer fluid loss reducer.

8. The self-healing carbon nanotube cement slurry upon encountering carbon dioxide according to claim 6, wherein: The fluid loss reducer is a liquid - type water - soluble polymer fluid loss reducer.

9. The preparation method of a carbon nanotube cement slurry that self-heals upon encountering carbon dioxide according to claim 7, characterized in that: It includes the following steps: (a) Take oil well cement and fluid loss reducer and mix them to obtain mixture A; (b) Take water, amino - modified carbon nanotubes and defoamer and mix them, perform ultrasonic treatment at a power of 350 W for 25 - 30 min to obtain a mixed solution; (c) Under the stirring condition of a rotation speed of 4500 ± 200 r / min, add the mixture A thereto within 10 - 15 s, and then continue to stir at a rotation speed of 10000 ± 200 r / min for 50 - 60 s to obtain a carbon nanotube cement slurry with self - healing upon exposure to carbon dioxide.

10. The preparation method of a carbon nanotube cement slurry that self-heals upon encountering carbon dioxide according to claim 8, characterized in that: It includes the following steps: (a) Take amino - modified carbon nanotubes, water and defoamer and mix them, perform ultrasonic treatment at a power of 350 W for 25 - 30 min, add the fluid loss reducer thereto, and stir and mix to obtain a mixed solution; (b) Under the stirring condition of a rotation speed of 4500 ± 200 r / min, add the oil well cement thereto within 10 - 15 s, and then continue to stir at a rotation speed of 10000 ± 200 r / min for 50 - 60 s to obtain a carbon nanotube cement slurry with self - healing upon exposure to carbon dioxide.

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