Self-healing-shape memory phase transition microcapsules for drilling fluid cooling and preparation method and application thereof

Abstract

The application provides a self-healing-shape memory phase change microcapsule for drilling fluid cooling and a preparation method and application thereof, and belongs to the technical field of drilling fluid cooling and downhole thermal management. The preparation method of the phase change microcapsule comprises the following steps: dispersing polyurethane prepolymer, polyimide solution and hydrophobic nanocellulose in an organic solvent to obtain an oil phase; dissolving a phase change material in deionized water to obtain an aqueous phase; under shearing conditions, the oil phase is added dropwise into the aqueous phase, and a stable oil-in-water Pickering emulsion is formed by emulsification; a chain extender aqueous solution is added, and an interface polymerization reaction is carried out, and then centrifugation, washing and drying are carried out to obtain the phase change microcapsule. The phase change microcapsule exhibits excellent cyclic service performance under downhole high temperature and high shear conditions, and even after the shell is damaged, the structure repair and morphology recovery can be realized by relying on the self-healing and shape memory effect, so that the service life of the phase change microcapsule is prolonged, and the cooling persistence of the drilling fluid is improved.

Description

Technical Field

[0001] This invention belongs to the field of drilling fluid cooling and downhole thermal management technology, specifically relating to a self-healing-shape memory phase change microcapsule for drilling fluid cooling, its preparation method and application. Background Technology

[0002] With the development of ultra-deep well and high-temperature, high-pressure well drilling projects, the bottom hole circulation temperature generally reaches 200–300 ℃. The deterioration of the wellbore thermal environment leads to a decline in drilling fluid performance, wellbore instability, and frequent occurrence of complex downhole conditions. How to achieve wellbore temperature control under high-temperature conditions is a key issue in ensuring drilling safety and improving efficiency while reducing costs.

[0003] Current drilling fluid cooling technologies mainly rely on surface heat exchange and external cooling devices. However, this approach is limited by well depth and heat conduction path, resulting in limited cooling effects and difficulty in effectively controlling localized high temperatures at the bottom of the well. Some studies have attempted to add phase change materials directly to the drilling fluid to regulate well temperature using their heat absorption and release effects. However, this approach suffers from problems such as easy dissolution or leakage of phase change materials during circulation, low utilization rate, and difficulty in maintaining stability at high temperatures.

[0004] To improve the dispersibility and circulation stability of phase change materials (PCMs) in drilling fluids, researchers have proposed a microencapsulation strategy. By encapsulating the PCM core material, the dissolution and loss of the core material in drilling fluids are effectively reduced. For example, patent CN117701255A discloses a PCM capsule for drilling fluid cooling, using NaNO3 / KNO3 as the core material and a silicate shell formed by the hydrolysis of tetraethyl orthosilicate, a prepolymer of the shell material. However, existing PCM microcapsules for drilling fluids mostly adopt a single-material shell approach. Once damaged in the high-temperature and high-shear downhole circulation, they are difficult to recover, easily leading to core material leakage and degradation of cooling performance, making it difficult to achieve long-term stable service.

[0005] Therefore, there is an urgent need to develop a phase change microcapsule that combines high temperature resistance and self-healing properties, and to apply it to high-temperature drilling fluid systems to improve the stability and recycling efficiency of the microcapsule under extreme conditions. Summary of the Invention

[0006] The purpose of this invention is to provide a self-healing, shape memory phase change microcapsule for drilling fluid cooling, its preparation method, and its application. This addresses the technical problems of existing drilling fluid phase change microcapsules, which are difficult to repair once damaged in high-temperature and high-shear downhole circulation conditions, easily leading to core material leakage and decreased cooling performance, and hindering long-term stable service. The phase change microcapsules of this invention exhibit excellent cyclic service performance under high-temperature and high-shear downhole conditions. Even after shell damage, they can still achieve structural repair and morphological restoration through self-healing and shape memory effects, thereby extending the service life of the phase change microcapsules and improving the cooling persistence of the drilling fluid.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] A self-healing shape memory phase change microcapsule for cooling drilling fluid, wherein the core material of the phase change microcapsule is a phase change material and the wall material is a polyurethane / polyimide / hydrophobic nanocellulose composite shell.

[0009] According to a preferred embodiment of the present invention, the phase change material is one or a combination of two of erythritol, galactitol or mannitol.

[0010] According to the present invention, the mass ratio of core material to wall material is preferably 20:20.6-32.6, more preferably 20:26-27, and even more preferably 20:26.8.

[0011] The preparation method of the above-mentioned self-healing-shape memory phase change microcapsules for drilling fluid cooling includes the following steps:

[0012] (1) The polyurethane prepolymer, polyimide solution and hydrophobic nanocellulose were fully dispersed in an organic solvent to obtain an oil phase;

[0013] (2) Dissolve the phase change material in deionized water to obtain an aqueous phase;

[0014] (3) Under shear conditions, the oil phase is added dropwise to the aqueous phase and emulsified to form a stable oil-in-water Pickering emulsion; an aqueous chain extender solution is added, and after interfacial polymerization, centrifugation, washing and drying are performed to obtain self-healing-shape memory phase change microcapsules.

[0015] According to a preferred embodiment of the present invention, in step (1), the polyurethane prepolymer is a polyether-type polyurethane prepolymer.

[0016] According to a preferred embodiment of the present invention, in step (1), the solid content of the polyimide solution is 10-20 wt%.

[0017] According to a preferred embodiment of the present invention, in step (1), the organic solvent is chlorobenzene.

[0018] According to a preferred embodiment of the present invention, in step (1), the mass ratio of the polyurethane prepolymer, the polyimide solution and the organic solvent is 1:(0.27-0.82):(0.76-1.52), preferably 1:(0.40-0.60):1, and more preferably 1:0.50:1.

[0019] According to a preferred embodiment of the present invention, in step (1), the mass ratio of the hydrophobic nanocellulose and the polyurethane prepolymer is 1: (3.64-18.2), preferably 1: (7.50-8.50), and more preferably 1: 7.91.

[0020] According to a preferred embodiment of the present invention, in step (2), the phase change material is one or a combination of two of erythritol, galactitol or mannitol.

[0021] According to a preferred embodiment of the present invention, in step (2), the mass ratio of the phase change material to deionized water is 1:(2.0-3.0), preferably 1:2.25.

[0022] According to a preferred embodiment of the present invention, in step (3), the shear rate of the shearing condition is 8000-10000 rpm.

[0023] According to a preferred embodiment of the present invention, in step (3), the mass ratio of the polyurethane prepolymer in the oil phase to the phase change material in the aqueous phase is 12-24:20, preferably 18-19:20, and more preferably 18.2:20.

[0024] According to a preferred embodiment of the present invention, in step (3), the emulsification is performed by shearing at a rotation speed of 8000-10000 rpm for 15-25 min.

[0025] According to a preferred embodiment of the present invention, in step (3), the chain extender aqueous solution is a 1,4-butanediol aqueous solution with a concentration of 2-10%, preferably 5%; the mass ratio of the chain extender to the polyurethane prepolymer in the oil phase is 1:2.02-9.10, preferably 1:3.50-4.50, and more preferably 1:4.04.

[0026] According to a preferred embodiment of the present invention, in step (3), the reaction temperature of the interfacial polymerization reaction is 60-70°C, the reaction time is 3-5 h, and the reaction is carried out under the protection of a protective gas and under stirring conditions. Preferably, the protective gas is nitrogen or argon.

[0027] According to a preferred embodiment of the present invention, in step (3), the drying temperature is 100-140°C and the drying time is 10-14 hours.

[0028] The above-mentioned self-healing-shape memory phase change microcapsules for drilling fluid cooling are used in drilling fluid cooling.

[0029] According to a preferred embodiment of the present invention, the mass of the self-healing-shape memory phase change microcapsule is 4%-8% of the mass of the drilling fluid.

[0030] The technical features and beneficial effects of this invention are as follows:

[0031] 1. This invention employs a polyurethane / polyimide / hydrophobic nanocellulose composite shell structure, overcoming the shortcomings of existing drilling fluid phase change microcapsules, which are mostly single-shell structures and difficult to recover from damage during high-temperature and high-shear downhole circulation. Polyurethane imparts self-healing properties to the shell, while polyimide provides excellent temperature resistance and shape memory function, thus maintaining shell integrity and long-term stability even under ultra-deep well high-temperature conditions. This invention introduces modified hydrophobic nanocellulose as a Pickering stabilizer into the shell, which not only improves the interfacial stability of the emulsion polymerization process but also significantly enhances the mechanical properties and density of the shell, effectively reducing the breakage rate and core material leakage of the microcapsules during circulation. The self-healing-shape memory phase change microcapsules of this invention exhibit excellent cyclic service performance under high-temperature and high-shear downhole conditions. Even after shell damage, they can still achieve structural repair and morphological restoration through self-healing and shape memory effects, thereby extending the service life of the phase change microcapsules and improving the cooling retention of the drilling fluid.

[0032] 2. The method of this invention is simple, the raw materials are readily available, and it is suitable for industrial production. All steps and conditions of the method work together as a whole to achieve the excellent effects of this invention; if the type or ratio of raw materials is unsuitable, or the reaction conditions are unsuitable, the performance of the resulting microcapsules will be reduced. Attached Figure Description

[0033] Figure 1 The image shows a SEM image of the microcapsules prepared in Example 1.

[0034] Figure 2 The graphs show the tensile strength recovery ability of the microcapsules prepared for the examples and comparative examples after damage.

[0035] Figure 3 The images show the deformation recovery ability of the microcapsules prepared for the examples and comparative examples after damage. Detailed Implementation

[0036] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0037] Example 1

[0038] A method for preparing self-healing-shape memory phase change microcapsules with erythritol as the core material and polyurethane / polyimide / hydrophobic nanocellulose as the composite shell, the specific steps of which are as follows:

[0039] Step 1: Dissolve 20.0 g of erythritol in 45.0 g of deionized water and stir until completely dissolved to obtain an aqueous phase for later use.

[0040] Step 2: Weigh 18.2 g of polyether-type polyurethane prepolymer and dissolve it in 18.2 g of chlorobenzene. Then add 9.0 g of a commercially available polyimide solution with a solid content of 20 wt% and 2.3 g of commercially available hydrophobic nanocellulose (paste, solid content 2 wt%, fiber diameter approximately 3–5 nm, fiber length approximately 500–1000 nm). Sonicate the mixture for 20 minutes to ensure uniform dispersion of the hydrophobic nanocellulose in the oil phase, forming a stable oil phase.

[0041] Step 3: Under shear conditions of 9000 rpm, the oil phase prepared in step 2 is slowly added dropwise to the aqueous phase prepared in step 1, and shear emulsification is continued for 20 minutes to form a stable oil-in-water Pickering emulsion.

[0042] Step 4: Transfer the obtained emulsion to a three-necked flask equipped with a mechanical stirrer. Under a nitrogen atmosphere, at 30 °C and a stirring speed of 2500 rpm, slowly add 4.5 g of a diluted solution of 1,4-butanediol in 85.5 g of deionized water. Subsequently, heat the system to 65 °C and maintain the temperature at this temperature under a nitrogen atmosphere with constant stirring for 4 hours to allow the polyurethane prepolymer and chain extender to undergo interfacial polymerization, forming a polyurethane / polyimide composite shell.

[0043] Step 5: After the reaction is complete, allow the system to cool naturally to room temperature. Centrifuge to obtain microcapsule solids, and wash three times with 200 mL of anhydrous ethanol to remove residual monomers and solvents. Finally, vacuum dry the obtained microcapsules in a vacuum oven at 120 °C for 12 hours to obtain self-healing shape memory phase change microcapsule powder.

[0044] The SEM image of the microcapsules obtained in this embodiment is shown below. Figure 1 As shown, it is a spherical structure.

[0045] Example 2

[0046] A method for preparing self-healing-shape memory phase change microcapsules with mannitol as the core material and polyurethane / polyimide / hydrophobic nanocellulose as the composite shell is as described in Example 1, except that in step 1, erythritol is replaced with mannitol; other steps and conditions are the same as in Example 1.

[0047] Example 3

[0048] A method for preparing phase change microcapsules, as described in Example 1, except that in step 2, the amount of polyimide solution added is adjusted from 9.0 g to 5.0 g; other steps and conditions are the same as in Example 1.

[0049] Example 4

[0050] A method for preparing phase change microcapsules, as described in Example 1, except that in step 2, the amount of polyimide solution added is adjusted from 9.0 g to 15.0 g; other steps and conditions are the same as in Example 1.

[0051] Example 5

[0052] A method for preparing phase change microcapsules, as described in Example 1, except that in step 2, the amount of hydrophobic nanocellulose added is adjusted from 2.3 g to 1.0 g; other steps and conditions are the same as in Example 1.

[0053] Example 6

[0054] A method for preparing phase change microcapsules, as described in Example 1, except that in step 2, the amount of hydrophobic nanocellulose added is adjusted from 2.3 g to 5.0 g; other steps and conditions are the same as in Example 1.

[0055] Example 7

[0056] A method for preparing phase change microcapsules, as described in Example 1, except that in step 2, the amount of polyurethane prepolymer added is adjusted from 18.2 g to 12.0 g; other steps and conditions are the same as in Example 1.

[0057] Example 8

[0058] A method for preparing phase change microcapsules, as described in Example 1, except that in step 2, the amount of polyurethane prepolymer added is adjusted from 18.2 g to 24.0 g; other steps and conditions are the same as in Example 1.

[0059] Example 9

[0060] A method for preparing phase change microcapsules, as described in Example 1, except that in step 4, the amount of chain extender 1,4-butanediol added is adjusted from 4.5 g in Example 1 to 2.0 g; other steps and conditions are the same as in Example 1.

[0061] Example 10

[0062] A method for preparing phase change microcapsules, as described in Example 1, except that in step 4, the amount of chain extender 1,4-butanediol added is adjusted from 4.5 g in Example 1 to 9.0 g; other steps and conditions are the same as in Example 1.

[0063] Comparative Example 1

[0064] A method for preparing phase change microcapsules, as described in Example 1, except that: in step 2, polyamic acid solution is not added; other steps and conditions are the same as in Example 1.

[0065] Comparative Example 2

[0066] A method for preparing phase change microcapsules, as described in Example 1, except that hydrophobic nanocellulose is not added in step 2; other steps and conditions are the same as in Example 1.

[0067] Comparative Example 3

[0068] A method for preparing phase change microcapsules, as described in Example 1, except that in step 4, the chain extender 1,4-butanediol and the diluent solution of deionized water are not added; specifically as follows:

[0069] Step 4: Transfer the resulting emulsion to a three-necked flask equipped with a mechanical stirrer, heat the system to 65 °C, and stir the reaction at this temperature under a nitrogen atmosphere for 4 hours.

[0070] The other steps and conditions are the same as in Example 1.

[0071] Experimental Example 1

[0072] Damage-self-healing strength recovery test

[0073] This experiment used microcapsule / epoxy resin composite sheets as the mechanical testing carrier. 0.50 g of each group of microcapsule powder was mixed evenly with 4.50 g of epoxy resin matrix, ultrasonically defoamed, and then poured into a strip mold. After curing at room temperature for 24 hours, tensile specimens with a thickness of approximately 1.0 mm and a gauge length of 20 mm were obtained. Subsequently, a linear scratch tester was used to inscribe mechanical damage with a length of 10 mm and a depth of 50–70 μm in the center of the specimen to form a damaged sample. The damaged sample was then heated for 30 min at a temperature approximately 15 ℃ higher than the microcapsule phase transition temperature and allowed to cool naturally to room temperature to obtain a self-healing sample. The original, damaged, and self-healing specimens were tested on a universal testing machine at a tensile rate of 5 mm / min, and their maximum tensile strength σ0 (original strength) and σ... d (damaged state strength) and σ h (Self-healing strength), and the self-healing strength recovery rate Rσ=σ h / σ0 is used to characterize the mechanical recovery ability of the microcapsule shell after damage.

[0074] This experiment was used to verify the effect of different shell structures on the mechanical strength recovery ability of microcapsules after damage. The test results are as follows: Figure 2 As shown in the figure. The results show that the embodiment using a polyurethane / polyimide / hydrophobic nanocellulose composite shell exhibits the best overall recovery performance. Among them, Example 1 shows the most significant self-healing effect, with the strength after self-healing essentially returning to the pre-damage level. This indicates that the reversible segments and shape memory network in the shell can effectively rearrange under heating conditions, rapidly repairing the damaged interface. The recovery ability of Example 2 is slightly lower than that of Example 1, but still at a high level. This indicates that when the shell structure remains consistent, changes in the core material do not significantly affect the self-healing mechanism, indirectly proving that the shell structure is the key factor determining the self-healing performance. Comparative Example 1, due to the complete lack of polyimide hard segments, is difficult to achieve effective reconstruction through thermal triggering after damage, resulting in significantly insufficient recovery ability, making it the weakest among all samples. The comparison effect is extremely intuitive. Example 3 has a low polyimide content, and its recovery effect is significantly weaker than that of Example 1. This indicates that when hard segments are insufficient, the deformation memory ability of the network is weakened and insufficient to support structural reconstruction. Although Example 4 has a high polyimide content, its recovery ability is similar to that of the best embodiment, verifying that polyimide does not continuously improve the self-healing effect within a certain range and has a specific performance plateau region. In Examples 5 and 6, where the hydrophobic nanocellulose content was too low or too high, the recovery levels were in the moderate range: when too low, the shell was not dense enough; when too high, the shell became hard and its dispersibility decreased. Both affected the interfacial rearrangement process, resulting in similar recovery abilities, but both were weaker than in Example 1. In Examples 7 and 8, where the polyurethane content was too low or too high, the shell thickness was either too thin or too thick, respectively. Both affected the closing speed of the damaged area, keeping its recovery performance at a moderate level, a clear contrast to Example 1, where the shell thickness was in the optimal range. In Comparative Examples 3, 9, and 10, where the chain extender was insufficient or excessive, the network crosslinking density deviated from the optimal range, resulting in limited chain segment flowability under heating. The recovery abilities exhibited were all in the low to moderate range, further illustrating that the degree of crosslinking needs to be kept within a suitable window to ensure good self-healing behavior.

[0075] Experimental Example 2

[0076] Damage-Self-Healing Deformation Recovery Test

[0077] This experiment used the same microcapsule / epoxy resin composite specimens and damage and self-healing treatment conditions as the self-healing tensile strength recovery performance test. Based on this, the elongation at break of each group of samples was further evaluated. Specifically, 0.50 g of microcapsule powder and 4.50 g of epoxy resin matrix were mixed evenly, degassed, and poured into a strip mold. The mixture was cured at room temperature for 24 h to obtain tensile specimens with a thickness of approximately 1.0 mm and a gauge length of 20 mm. Damaged specimens were formed by making mechanical scratches of 10 mm length and 50–70 μm depth in the center of the specimens using a linear scratcher. These damaged specimens were then heated for 30 min at a temperature approximately 15 ℃ higher than the microcapsule phase transition temperature and allowed to cool naturally to room temperature to obtain self-healing specimens. The original, damaged, and self-healing specimens were tested on a universal testing machine at a tensile rate of 5 mm / min. The strain values ​​at fracture were recorded to obtain the original state elongation at break A0 (original state elongation at break) and the damaged state elongation at break A... d (Elongation at break in damaged state) and elongation at break in self-healing state A h (Self-healing fracture elongation), and the recovery rate R of self-healing fracture elongation. A = A h / A0 characterizes the toughness recovery ability of the microcapsule shell after damage.

[0078] This experiment compares the elongation of microcapsule materials in their original, damaged, and post-heat-recovery states. This directly reflects the shell's deformation recovery ability after stress damage, and is an important indicator for verifying self-healing properties and shape memory effect. The test results are as follows: Figure 3As shown, from the overall trend, the recovery performance of the Example Group is significantly better than that of the comparative examples, indicating that the shell structure proposed in this invention can reclose after damage and restore its initial deformation capability. Examples 1 and 2 both use composite shells with good synergy between soft and hard segments. After damage, heating can achieve sufficient segment rearrangement, exhibiting the most obvious deformation recovery capability, indicating that the overall network structure of the shell is in a superior state. Comparative Example 1, due to the lack of necessary hard segment support, is difficult to form an effective recovery network after damage, has the weakest self-healing ability, and almost no obvious shape memory behavior. When the proportion of hard segments in the shell is insufficient (Example 3), the network stability is insufficient, and it is difficult to maintain the complete structure after damage, and its recovery capability is significantly weakened; while when the proportion of hard segments is too high, its self-healing efficiency is not improved (Example 4). Comparative Examples 2, 5, and 6 show that a low amount of stabilizing agent will lead to a non-dense shell and difficulty in closing cracks, while an excessive amount will make the shell too hard and difficult to reattach after damage. The recovery performance of both is significantly weaker than that of Example 1. Examples 7 and 8 show that when the shell layer is too thin, it is prone to penetrating cracks, and lacks sufficient recovery support after damage; when the shell layer is too thick, the chain segment flow is restricted, which also weakens the recovery ability after heating. Comparative Examples 3, 9, and 10 show that when the cross-linking structure is weak or too dense, the reversible deformation ability of the shell is affected, and the recovery level after heating is significantly lower than that of Example 1. In addition, since the shell layer of the present invention is formed in one step in the original state, the chain segment position may not be optimal. At the same time, there is internal stress in the shell layer during microcapsule solidification. Some samples showed a slightly higher elongation rate after self-healing than in the original state, which is a common behavior in self-healing systems. Since the heating self-healing process promotes the rearrangement of shell layer chains and further continuity of soft segment domains, the local microphase structure becomes more uniform, and the chain segment activity space is released, so that the material exhibits a slightly higher elongation at tensile fracture. This phenomenon is consistent with the self-healing research results based on the polyurethane-polyimide system, indicating that the shell layer of the present invention can enter a more stable and compliant configuration during the self-healing process.

[0079] Experimental Example 3

[0080] Cyclic stability test

[0081] A phase change microcapsule sample was taken and added to white oil at a ratio of 6 wt%, and stirred until a dispersion was obtained. The dispersion was placed in a heating-cooling cycle device and repeatedly circulated between room temperature and phase change temperature. Each cycle was recorded as room temperature - phase change temperature - room temperature. At the 1st, 10th, 50th, and 100th cycles, 25 mL of the circulating liquid was taken, an appropriate amount of anhydrous ethanol was added, and the mixture was centrifuged. The supernatant was discarded, and the microcapsule solids were washed with ethanol 1–2 times. The collected microcapsule solids were dried in an oven. The dried microcapsule samples were then placed in a differential scanning calorimeter to determine their phase change temperature and latent heat of phase change. By comparing the test results at different cycle numbers, the structural stability and phase change performance retention ability of the microcapsules under long-term thermal cycling conditions were evaluated. The results are shown in Table 1 below.

[0082] Table 1. Test data on latent heat of phase change

[0083]

[0084] Examples 1 and 2 exhibited the most stable latent heat retention capacity throughout the cycling process, showing only slight attenuation. After 100 cycles, the latent heat remained within a high proportion of the initial value. This is mainly due to the high integrity and heat resistance of the self-healing-shape memory composite functional shell, which effectively resists shell fatigue cracking during repeated melting and solidification, preventing core material leakage. The difference between the two mainly stems from the different core materials, resulting in a slightly lower absolute latent heat in Example 2, but the cycling trends are basically the same. Comparative Example 1 showed a significant decrease in latent heat in the early cycles, with the largest decrease after the 100th cycle. Due to the lack of thermally stable hard segments supported by polyimide, its shell is more prone to cracking and local rupture under long-term thermal stress, leading to core material loss and latent heat attenuation. Comparative Examples 2 and 3 also showed a relatively obvious attenuation trend during cycling. The former has insufficient interface stability and poor shell density; the latter has insufficient cross-linking resulting in lower overall mechanical strength. Both are more prone to structural loosening or damage in multiple cycles, thus causing a continuous decrease in latent heat. Although Example 3 showed better cycle stability than Comparative Examples 1–3 (which lacked the key component), its latent heat reduction was still significantly higher than that of Example 1. This indicates that insufficient PI content makes it difficult to form a complete composite network, limiting the shell durability during cycling. Example 4 exhibited cycle stability closer to that of Example 1, indicating that even after the PI content exceeded the optimal range, although the self-healing and shape memory networks were not further enhanced, the shell as a whole still maintained high heat resistance, thus partially maintaining cycle performance. Examples 5 and 6 showed slightly higher attenuation during cycling than Example 1, exhibiting a moderate stability range. Low hydrophobic nanocellulose content led to insufficient interfacial stability, while high content increased local rigidity of the shell and made it prone to fatigue microcracks, both of which are detrimental to long-term cycle retention. Examples 7 and 8 showed that deviations from the optimal shell thickness weakened cycle durability. Thinner shells are more prone to breakage, while thicker shells result in more significant stress accumulation, both leading to a gradual decrease in latent heat of phase transition. Examples 9 and 10 showed that their cycle retention capabilities were at a moderately low level. Too low a value leads to insufficient cross-linking and loose shell; too high a value restricts chain segment movement and easily forms brittle cracks, both of which affect cycle tolerance.

[0085] Test Example 4

[0086] Drilling fluid compatibility test

[0087] Taking a phase change microcapsule addition amount of 6 wt% of the oil-based drilling fluid slurry as an example, the performance of the microcapsules prepared in Examples 1 and 2 was evaluated as follows. Preparation of oil-based drilling fluid slurry: 320 mL of No. 3 diesel oil, 4.5 g of organic clay, 4 g of primary emulsifier (polyamide LH-OPE), 8 g of secondary emulsifier (amide-amine LH-OSE), 6 g of filtration reducer (oxidized asphalt LH-OFL), 12 g of calcium oxide, 6 g of wetting agent (alcohol ether LH-OWA), and 80 mL of 30 wt% calcium chloride aqueous solution were mixed and stirred at high speed for 20 min. Then, 350 g of barite was added and stirred at high speed for 30 min.

[0088] Drilling fluid preparation: Add phase change microcapsules to oil-based drilling fluid slurry and mix evenly; the amount of phase change microcapsules added is 6 wt% of the oil-based drilling fluid slurry.

[0089] Aging treatment of drilling fluid and base slurry: The drilling fluid samples and base slurry were aged using a roller heating furnace at a temperature of 200 ℃ for 16 h.

[0090] Rheological property testing: The readings Φ600, Φ300, Φ200, Φ100, Φ6, and Φ3 of the aged drilling fluid and base slurry were measured using a Fann 35 rotational viscometer at 600 r / min, 300 r / min, 200 r / min, 100 r / min, 6 r / min, and 3 r / min. The apparent viscosity AV, plastic viscosity PV, and dynamic shear force YP were calculated according to the standard method to evaluate the rheological properties of the drilling fluid system. The test results are shown in Table 2.

[0091] The high-temperature and high-pressure filtration loss was determined using a high-temperature and high-pressure filtration loss meter. Aging-treated oil-based drilling fluid samples and base slurry were loaded into the high-temperature and high-pressure filtration cup, filter paper was laid, and a sealing assembly was assembled. After stabilizing at the set temperature (200 ℃), the pressure in the upper and lower chambers was adjusted to 3.5 MPa, the lower valve was opened, and timing was started. The pressure was maintained for 30 min, and the volume of filtrate was recorded as the high-temperature and high-pressure filtration loss of the system. The test results are shown in Table 2.

[0092] Table 2 Drilling fluid compatibility test data

[0093]

[0094] In terms of rheological properties, the drilling fluid system with added microcapsules exhibited relatively complete structural characteristics before and after aging. The system viscosity and yield behavior remained stable, and no obvious structural damage or thinning occurred. This indicates that the microcapsules of the present invention are compatible with the drilling fluid system under high temperature and high shear conditions and will not damage the existing emulsion system. Compared with the base slurry, the structural support capacity of Examples 1 and 2 is enhanced, indicating that the composite shell microcapsules can provide additional spatial network effects under circulation conditions, improving the carrying capacity and flow stability of the drilling fluid.

[0095] In terms of high-temperature and high-pressure filtration performance, drilling fluids with added microcapsules exhibited superior liquid intrusion inhibition compared to the base mud. Both embodiments were able to form a dense and stable filter cake structure at high temperatures, effectively reducing filtration loss. This indicates that the microcapsule shell maintains good integrity in the high-temperature downhole environment and will not damage the filter cake structure due to breakage or overflow. Examples 1 and 2 both demonstrated strong high-temperature tolerance, with better filter cake continuity and stability, further verifying the effect of the composite shell structure of this invention on improving the high-temperature filtration performance of drilling fluids.

[0096] Based on the combined rheological properties and filtration behavior, the self-healing-shape memory phase change microcapsules of this invention exhibit good compatibility with oil-based drilling fluids. They maintain system stability after high-temperature aging and reduce liquid intrusion, thereby improving the service performance of drilling fluids in high-temperature and complex downhole environments. These results verify the feasibility and effectiveness of the microcapsules of this invention in drilling fluid cooling applications.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to 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 the present invention.

Claims

1. An application of self-healing-shape memory phase change microcapsules for drilling fluid cooling, characterized in that, The core material of the phase change microcapsule is a phase change material, and the wall material is a polyurethane / polyimide / hydrophobic nanocellulose composite shell. The phase change material is one or a combination of two of erythritol, galactitol or mannitol; the mass ratio of the core material to the wall material is 20:26-27. The method for preparing the self-healing-shape memory phase change microcapsules for drilling fluid cooling is characterized by comprising the following steps: (1) A polyurethane prepolymer, a polyimide solution, and a hydrophobic nanocellulose paste are fully dispersed in an organic solvent to obtain an oil phase; the polyurethane prepolymer is a polyether-type polyurethane prepolymer; the solid content of the polyimide solution is 10-20 wt%; the mass ratio of the polyurethane prepolymer, the polyimide solution, and the organic solvent is 1:(0.5-0.82):(0.76-1.52); the mass ratio of the hydrophobic nanocellulose paste and the polyurethane prepolymer is 1:(7.50-8.50); ​​the solid content of the hydrophobic nanocellulose paste is 2 wt%, wherein the fiber diameter of the hydrophobic nanocellulose is 3–5 nm and the fiber length is 500–1000 nm; (2) The phase change material is dissolved in deionized water to obtain an aqueous phase; the phase change material is one or a combination of two of erythritol or mannitol; (3) Under shear conditions, the oil phase is added dropwise to the aqueous phase and emulsified to form a stable oil-in-water Pickering emulsion; an aqueous chain extender solution is added, and after interfacial polymerization, centrifugation, washing, and drying are performed to obtain self-healing-shape memory phase change microcapsules; the mass ratio of polyurethane prepolymer in the oil phase to phase change material in the aqueous phase is 18.2:20; the aqueous chain extender solution is a 1,4-butanediol aqueous solution with a concentration of 2-10%; the mass ratio of chain extender to polyurethane prepolymer in the oil phase is 1:3.50-4.

50.

2. The application according to claim 1, characterized in that, In step (1), the organic solvent is chlorobenzene.

3. The application according to claim 1, characterized in that, In step (2), the mass ratio of the phase change material to deionized water is 1: (2.0-3.0).

4. The method for preparing self-healing-shape memory phase change microcapsules for drilling fluid cooling according to claim 1, characterized in that, Step (3) includes one or more of the following conditions: i. The shear rate under shear conditions is 8000-10000 rpm; ii. The emulsification is performed by shearing at a speed of 8000-10000 rpm for 15-25 min; iii. The reaction temperature of the interfacial polymerization reaction is 60-70℃, the reaction time is 3-5 h, and the reaction is carried out under the protection of a protective gas and stirring conditions; the protective gas is nitrogen or argon.

5. The application according to claim 1, characterized in that, The mass of the self-healing shape memory phase change microcapsules is 4%-8% of the drilling fluid mass.

Images