A phase change material for use in drilling fluid cooling and thermal energy recovery and a method of making the same

Abstract

The present application relates to the technical field of drilling fluid cooling and heat energy recovery, in particular to a phase change material for drilling fluid cooling and heat energy recovery and a preparation method thereof, which comprises a phase change material, a phase change material microcapsule with a core-shell structure, further comprises a composite phase change core material and a functional wall material coated outside the composite phase change core material, and the phase change temperature of the phase change material is 135-145 DEG C. The present application sets the phase change temperature of the phase change material to 135-145 DEG C accurately, perfectly matches the bottom hole circulating temperature above 130 DEG C in high temperature drilling areas such as Sichuan basin deep shale gas blocks, Moxi-Gaoshiti blocks in Sichuan and Chongqing, the latent heat of phase change can reach 180-280.6 J / g, the drilling fluid cooling range can reach 15-20 DEG C, the bottom hole environment temperature can be effectively controlled within the safe temperature range (<= 150 DEG C) of downhole instruments such as rotary steering tools, the failure rate of instrument failure and drilling tool damage can be greatly reduced, the drilling operation interruption caused by high temperature can be avoided, and the high temperature drilling limitation in ultra-deep well drilling can be solved.

Description

Technical Field

[0001] This invention relates to the field of drilling fluid cooling and heat recovery technology, specifically to a phase change material that can be used for drilling fluid cooling and heat recovery, and its preparation method. Background Technology

[0002] High-temperature environments severely restrict the stable operation of downhole equipment and drilling fluids. Existing rotary steering tools have a nominal temperature resistance of only 150°C. Under high temperatures, instrument malfunctions and drill string damage occur frequently, and the cost of special high-temperature resistant tools needs to be increased by nearly 50%. Drilling fluids are prone to degradation and cross-linking reactions above 130°C, resulting in drastic changes in performance. A large amount of additional modifiers are required, which significantly increases drilling costs. With drilling exceeding 10,000 meters becoming the norm in the future, the need for downhole cooling will be even more urgent. Meanwhile, a large amount of heat energy carried by high-temperature drilling fluid has been wasted for a long time. After the drilling fluid is returned to the surface, it is mostly discharged through natural heat dissipation or simple cooling, without realizing resource utilization. This not only causes energy waste, but also does not conform to the trend of energy conservation and consumption reduction. Therefore, solving the problem of recovering heat energy while cooling the well has become an important research direction for ultra-deep well drilling technology.

[0003] Chinese patent CN116063996A discloses a phase change thermal storage microcapsule material suitable for drilling fluid cooling and its preparation method. When the drilling fluid reaches its phase change temperature, the internal phase change thermal storage material undergoes a phase change and absorbs heat, reducing the temperature rise of the drilling fluid. The polymer wall material not only prevents leakage but also enables the microcapsules to achieve a certain mechanical strength. The addition of nano-graphite improves the overall thermal conductivity and enhances the latent heat of phase change. As a material for cooling drilling fluid, this material can be repeatedly used during drilling to help reduce the temperature of the drilling fluid, maintain the performance of the drilling fluid, and reduce the damage of high temperature to related instruments.

[0004] However, the patent has the following defects in operation: (1) The phase change temperature of existing phase change microcapsules is below 125°C, which cannot be adapted to the high temperature of over 130°C in ultra-deep wells; (2) It has no high pressure resistance design and is easily broken under the bottom pressure of 80-120 MPa; (3) It can only cool down and does not recover the residual heat of drilling fluid, resulting in energy waste. For deep well drilling, there are still many limitations: First, the selection of phase change materials (PCMs) often focuses only on cooling effects, without fully considering the balance between heat absorption and energy storage characteristics and drilling fluid compatibility. Some materials have problems such as insufficient latent heat of phase change and poor temperature and pressure resistance, making them unsuitable for the extreme working conditions of ultra-deep wells with temperatures above 130°C and pressures of 80–120 MPa. Second, there is a lack of refined management of the heat of the drilling fluid throughout the entire process. The downhole thermal field is complex, and the heat change law of PCMs during circulation is not fully understood, affecting cooling and recovery efficiency. Third, the heat exchange efficiency and recovery process are not well adapted. Existing heat exchange technologies do not consider the influence of factors such as drilling fluid adhesion, and there is a lack of dedicated heat recovery processes for PCMs, resulting in low heat recovery rates. Fourth, some PCMs have problems such as poor cycle stability and limited number of reuses, making it difficult to meet the needs of long-term drilling operations. Therefore, there is an urgent need to develop a phase change material with a phase change temperature that is precisely adapted to deep well conditions, high energy storage efficiency, excellent temperature and pressure resistance, and good compatibility with drilling fluids, while also supporting it with efficient heat recovery technology, in order to solve the core pain points of high-temperature drilling in ultra-deep wells. Summary of the Invention

[0005] This invention aims to provide a phase change material and its preparation method that can be used for drilling fluid cooling and heat recovery. It is mainly used to develop a phase change material with a phase change temperature that is precisely adapted to deep well conditions, high energy storage efficiency, excellent temperature and pressure resistance, and good compatibility with drilling fluid. At the same time, it is equipped with efficient heat recovery technology to solve the core pain points of high temperature drilling in ultra-deep wells.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A phase change material (PCM) for drilling fluid cooling and heat recovery includes a PCM microcapsule with a core-shell structure, a composite PCM core, and a functional wall material encapsulating the composite PCM core. The PCM has a phase change temperature of 135–145 °C and a latent heat of phase change of 180–280.6 J / g. The composite PCM core absorbs heat from the drilling fluid in the high-temperature environment of deep wells to achieve cooling and releases stored heat during surface heat exchange to achieve heat recovery. The functional wall material has the characteristics of withstanding high temperatures of 200–220 °C and high pressures of 80–120 MPa, and has good compatibility with drilling fluid systems.

[0007] A preparation method for use in drilling fluid cooling and heat recovery includes the following steps: S1. Preparation of composite phase change core material: Select high-temperature phase change components for compounding. The high-temperature phase change components are selected from at least two of molten salts, organic paraffins, and fatty acids. Mix the components in proportion and melt and stir at 150-180℃ for 30-60 min. After cooling, the composite phase change core material is obtained. The phase change temperature of the composite phase change core material is adjusted to 135-145℃ and the latent heat of phase change is adjusted to 180-280.6 J / g. S2. Preparation of functional wall material precursor: Select wall material substrate and modifier, the wall material substrate is selected from at least one of silicon dioxide, urea-formaldehyde resin, and melamine-formaldehyde resin, the modifier is a nano thermal conductive agent, the nano thermal conductive agent is selected from at least one of nano graphite and manganese tetroxide, the amount of modifier added is 1% to 5% of the mass of wall material substrate, and stir evenly to obtain functional wall material precursor. S3. Core material dispersion and emulsification treatment: The composite phase change core material is added to the oil phase medium and heated to 80-90℃ to form a core material dispersion. The oil phase medium is linseed oil or mineral oil, and the mass fraction of the core material in the oil phase is 20%-40%. An emulsifier is added to the core material dispersion at a mass of 2%-5% of the oil phase medium. The mixture is emulsified at a speed of 1000-3000 r / min for 30-45 min to form a stable emulsion. S4. In-situ polymerization and encapsulation of microcapsules: The functional wall material precursor is slowly added to the stable emulsion formed after the core material is dispersed and emulsified. The pH of the system is adjusted to 4-6, and the reaction is carried out at 60-80℃ for 2-4 hours to achieve in-situ polymerization and encapsulation of the core material by the wall material. S5. Post-processing: The product after in-situ polymerization and coating reaction is centrifuged, washed with deionized water 3 to 5 times, and vacuum dried at 80 to 100°C for 12 to 24 hours to obtain a phase change material that can be used for drilling fluid cooling and heat recovery.

[0008] Working principle and beneficial effects of the present invention: 1. Working principle: This phase change material is suitable for the high-temperature working conditions of over 130°C at the bottom of ultra-deep wells. Through the core-shell structure, it achieves an integrated function of "cooling-energy storage-recovery". It not only solves the industry pain points of insufficient temperature resistance of downhole instruments such as rotary steering tools and high-temperature degradation of drilling fluid, but also ensures the long-term stability of the material in complex drilling cycles through the extreme environment resistance and compatibility design of the functional wall material. It provides core material support for the resource utilization of waste heat from high-temperature drilling fluids and fills the technical gap that existing phase change materials cannot simultaneously meet the requirements of cooling effect and recovery adaptability. The preparation of composite phase change core materials, through multi-component synergistic compounding and precise process parameter control, has overcome the limitation that single phase change materials cannot simultaneously achieve phase change temperature and latent heat. This enables the core material to not only precisely match the high-temperature range of deep wells to achieve efficient heat absorption and cooling, but also to store sufficient heat for ground recovery, laying the core material foundation for the realization of subsequent dual functions. The preparation of functional wall material precursors and the selection of wall material substrates take into account both structural strength and temperature and pressure resistance. The directional addition of nano thermal conductive agents solves the problems of low thermal conductivity and lag in heat transfer of traditional wall materials, so that the wall material formed after the precursor can not only prevent core material leakage, but also accelerate heat conduction, ensuring efficient connection between cooling and recycling processes. Core material dispersion and emulsification treatment, by optimizing the oil phase medium, core material concentration and emulsification parameters, achieves uniform dispersion and stable suspension of the core material, avoids problems such as uneven phase change performance and reduced heat exchange efficiency caused by core material agglomeration, provides an ideal dispersion system for the uniform coating of subsequent wall materials, and ensures the consistency of the final product performance. In-situ polymerization and coating of microcapsules, by precisely controlling the reaction environment parameters, guides the wall material precursor to be directionally polymerized on the core material surface to form a dense and uniform coating layer. This not only solves the industry problem of easy leakage of core material in drilling fluid circulation, but also ensures the structural integrity and mechanical strength of microcapsules, enabling them to withstand the extreme working conditions of high temperature and high pressure in deep wells. In the post-processing stage, centrifugation and multiple washing processes effectively remove impurities, unreacted raw materials, and excess emulsifiers from the product. Vacuum drying avoids the damage of high temperature to the performance of the phase change core material, ultimately obtaining high-purity, high-performance phase change microcapsules. This ensures that the microcapsules can stably perform their cooling and energy storage recovery functions in practical applications, thereby improving the reliability and engineering applicability of the product.

[0009] 2. Beneficial effects: (1) By precisely setting the phase change temperature of the phase change material to 135-145℃, it perfectly matches the bottom-hole circulation temperature of over 130℃ in high-temperature drilling areas such as the deep shale gas blocks in the Sichuan Basin and the Moxi-Gaoshiti blocks in Sichuan and Chongqing. The latent heat of phase change reaches 180-280.6 J / g, which can achieve a drilling fluid cooling range of 15-20℃. This effectively controls the bottom-hole ambient temperature within the safe temperature range (≤150℃) of downhole instruments such as rotary steering tools, significantly reducing the failure rate of instruments and drill string damage, avoiding drilling operation interruptions caused by high temperatures, and solving the high-temperature drilling limitation in ultra-deep well drilling. The functional wall material and drilling fluid system have excellent compatibility. When the amount of phase change material added is 8%-12%, the viscosity and shear stress change rate of the drilling fluid are ≤5%, the API filtration loss is ≤10 mL, and the high-temperature and high-pressure filtration loss is ≤20 mL. mL effectively avoids problems such as drilling fluid dispersion, agglomeration, and degradation caused by poor compatibility of traditional cooling materials. It eliminates the need for adding large amounts of performance modifiers, significantly reduces drilling fluid maintenance costs, and ensures stable rheological filtration performance of drilling fluid.

[0010] (2) Phase change materials can not only absorb heat and cool down downhole, but also release the stored heat in a directional manner after being returned to the surface with the drilling fluid. Combined with the ORC turbine circulation heat exchange system, the comprehensive heat recovery rate is ≥60%, which successfully converts the waste heat of high-temperature drilling fluid that was previously wasted into usable electrical energy or domestic heat energy, filling the gap in the existing drilling fluid cooling technology that only cools down but does not recover the heat. The heat released by the phase change materials can be flexibly adapted to the diverse needs of power generation, well site heating, etc. through the heat exchange system. It is especially suitable for energy supply in remote deep well drilling areas, reducing dependence on external energy, realizing energy self-sufficiency in the drilling process, and enhancing the green and low-carbon attributes of drilling operations.

[0011] (3) The phase change material adopts a core-shell structure design. The functional wall material can withstand high temperature of 200-220℃ and high pressure of 80-120 MPa. The microcapsule breakage rate is ≤3% under these extreme conditions. After 500 cycles of phase change, the phase change temperature change value is ≤2℃ and the phase change latent heat loss rate is ≤8%. It can be reused ≥50 times and can withstand the harsh test of repeated drilling cycles in ultra-deep wells, greatly reducing the frequency of material replacement and usage costs.

[0012] (4) The five-step preparation method achieves directional control of the phase change material performance by precisely controlling the core material compounding ratio, emulsification parameters, polymerization reaction conditions and post-processing. The product has uniform particle size (25-70μm, D50 is 28-35μm), no agglomeration or sedimentation problems, and avoids clogging of drilling equipment. The process steps are clear and the parameters are well-defined, which can meet the needs of industrial mass production and ensure the consistency of product performance and the reliability of engineering applications.

[0013] (5) The composite phase change core material is made of molten salts and organic paraffins and fatty acids (mass ratio 1:0.2 to 1:0.5). It takes advantage of the high latent heat of molten salt phase change and the good stability of organic materials. Compared with single phase change materials, it can improve energy storage efficiency while reducing raw material costs. The addition of nano thermal conductive agent (1% to 5% of the mass of the wall material substrate) further improves the heat exchange efficiency and achieves a balance between high performance and low cost.

[0014] Preferably, in the composite phase change core material, the molten salt component is selected from at least two of sodium nitrate, potassium nitrate, and lithium nitrate; the organic paraffin component is selected from at least one of n-tetradecane, n-hexadecane, and C18-C24 paraffin; and the fatty acid component is selected from at least one of stearic acid, palmitic acid, and myristic acid. The mass ratio of the molten salt component to the organic component (paraffin + fatty acid) is 1:0.2 to 1:0.5. This component selection and ratio design fully combines the advantages of high latent heat of phase change of molten salt and good stability of organic materials. By adjusting the ratio, the phase change temperature and latent heat of the composite core material can be precisely optimized, while improving the thermal cycling stability of the core material and avoiding the performance degradation of a single component during repeated phase changes.

[0015] Preferably, the microcapsules have a particle size of 25–70 μm and a D50 particle size of 28–35 μm. The breakage rate is ≤3% under conditions of 200–220℃ and 80–120 MPa. After 500 cycles of phase change, the phase change temperature change is ≤2℃, and the latent heat loss rate is ≤8%. This particle size range ensures that the microcapsules can be uniformly dispersed in the drilling fluid, avoiding sedimentation or clogging of the drilling equipment. The temperature and pressure resistance and cycle stability parameters match the long-term use requirements under extreme conditions in ultra-deep wells, significantly reducing the cost of repeated material replacement and improving the economy and practicality of the technology.

[0016] Preferably, when the amount of phase change material added to the drilling fluid is 8% to 12%, the viscosity and shear stress change rate of the drilling fluid are ≤5%, the API filtration loss is ≤10 mL, and the high-temperature and high-pressure filtration loss is ≤20 mL. Based on the matching design between this addition amount and the performance indicators of the drilling fluid, the technical problem of drastic changes in rheological filtration performance caused by poor compatibility between phase change material and drilling fluid is solved. While ensuring the cooling and energy storage effects, there is no need to add a large amount of additional modifiers, effectively controlling the usage cost of the drilling fluid.

[0017] Preferably, when the wall material substrate is silica, the functional wall material precursor is a mixture of tetraethyl silicate and ethanol, with a volume ratio of tetraethyl silicate to ethanol of 1:2 to 1:3. When the wall material substrate is urea-formaldehyde resin, the functional wall material precursor is a prepolymer of urea and formaldehyde, with a molar ratio of urea to formaldehyde of 1:1.5 to 1:2.0. Different precursor preparation schemes are designed for the characteristics of different wall material substrates. By precisely controlling the raw material ratio, the precursor has suitable reactivity and stability, providing a guarantee for the subsequent in-situ polymerization and coating to form a dense and high-strength wall material, and avoiding performance defects of the wall material caused by improper precursor preparation.

[0018] Preferably, the emulsifier is selected from at least one of Span 80, Tween 80, and polyglycerol fatty acid esters, and the mass ratio of the core material dispersion to the functional wall material precursor is 1:0.3 to 1:0.8. The emulsifier has good oil-water dispersibility and can be adapted to form a stable emulsion in an oil phase medium. The mass ratio design of the core material dispersion to the wall material precursor ensures that the wall material can fully cover the core material, avoiding core material leakage caused by insufficient wall material and preventing material waste and decreased thermal conductivity caused by excessive wall material.

[0019] Preferably, the centrifugation speed is 5000-8000 r / min, the centrifugation time is 10-15 min, and the vacuum degree of vacuum drying is 0.08-0.1 MPa. Centrifugation can efficiently separate products and impurities, and multiple washings can further improve product purity. Setting appropriate vacuum drying pressure and temperature can remove moisture while avoiding premature phase change or performance degradation of phase change materials, ensuring the performance stability and consistency of the final product.

[0020] Preferably, the phase change material is circulated into the bottom of the well along with the drilling fluid, absorbs formation heat and undergoes a phase change, thereby cooling the drilling fluid by 15-20°C. After the drilling fluid is returned to the surface, the phase change material releases the stored heat, which is recovered through a heat exchange system, achieving a comprehensive heat recovery rate of ≥60%, and the bottom-hole circulation temperature of deep and ultra-deep wells is ≥130°C. This method is suitable for high-temperature drilling areas such as deep shale gas blocks in the Sichuan Basin and the Moxi-Gaoshiti block in Sichuan and Chongqing. The cooling effect can effectively reduce the failure rate of downhole instruments such as rotary steering tools and extend their service life. The comprehensive heat recovery rate of ≥60% enables the resource utilization of residual heat from high-temperature drilling fluid.

[0021] Preferably, the heat exchange system is an ORC turbine circulation system. The heat released by the phase change material is used to heat the ORC circulating working fluid, realizing the power generation utilization of low-quality thermal energy. The phase change material can be reused with the drilling fluid circulation, and it can be reused ≥50 times while maintaining its phase change performance and structural stability. The ORC turbine circulation system is adapted to the low-quality thermal energy released by the phase change material, realizing the efficient power generation conversion of waste heat and expanding the utilization of drilling fluid waste heat. The high reusability design of the phase change material further reduces the overall cost of technology application. Attached Figure Description

[0022] Figure 1 This is a flowchart illustrating the application and circulation of a phase change material that can be used for drilling fluid cooling and heat recovery according to the present invention. Figure 2 This is a flowchart of a method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to the present invention; Figure 3 This invention relates to a table of key technical parameters of a phase change material for application and recycling in drilling fluid cooling and heat recovery. Figure 4 This invention provides a composite phase change core material composition ratio table for the application and recycling of a phase change material that can be used for drilling fluid cooling and heat recovery. Figure 5 This is a table of key parameters for the preparation process of a phase change material that can be used for drilling fluid cooling and heat recovery, according to the present invention. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. 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.

[0024] Example 1 like Figure 1 , Figure 3 and Figure 4 As shown, a phase change material for drilling fluid cooling and heat recovery includes a phase change material, which is a core-shell structured microcapsule. It also includes a composite phase change core and a functional wall material covering the composite phase change core. The phase change temperature of the phase change material is 135-145℃, and the latent heat of phase change is 180-280.6 J / g. The composite phase change core is used to absorb heat from the drilling fluid in the high-temperature environment of deep wells to achieve cooling, and to release stored heat during surface heat exchange to achieve heat recovery. The functional wall material has the characteristics of withstanding high temperatures of 200-220℃ and high pressures of 80-120 MPa, and has good compatibility with the drilling fluid system. In the composite phase change core material, the molten salt component is selected from at least two of sodium nitrate, potassium nitrate, and lithium nitrate; the organic paraffin component is selected from at least one of n-tetradecane, n-hexadecane, and C18-C24 paraffin; and the fatty acid component is selected from at least one of stearic acid, palmitic acid, and myristic acid. The mass ratio of the molten salt component to the organic component (paraffin + fatty acid) is 1:0.2 to 1:0.5. The microcapsules have a particle size of 25–70 μm and a D50 particle size of 28–35 μm. The breakage rate is ≤3% under conditions of 200–220℃ and 80–120 MPa. After 500 cycles of phase change, the phase change temperature change is ≤2℃ and the latent heat loss rate of phase change is ≤8%. Phase change parameters: DSC test showed that the phase change temperature of the finished microcapsules was 140.5℃ (range 135~145℃) and the latent heat of phase change was 248 J / g (range 180~280.6 J / g). Particle size characteristics: Laser particle size analyzer tests showed that the microcapsule particle size ranged from 28 to 65 μm, and the D50 particle size was 32 μm (both conforming to the ranges of 25 to 70 μm and 28 to 35 μm, respectively). Temperature and pressure resistance: When the microcapsules are placed in a high-temperature and high-pressure autoclave at 210℃ (range 200~220℃) and 100 MPa (range 80~120MPa) for 24 h, the breakage rate is 2.3% (≤3%). Cyclic stability: After 500 phase change cycles (130~150℃), the phase change temperature is 139.8℃ (change value 0.7℃≤2℃), and the latent heat of phase change is 232 J / g (loss rate 6.5%≤8%). When the addition amount of phase change material (PCM) in drilling fluid is 8%–12%, the viscosity and shear stress change rate of the drilling fluid are ≤5%, the API filtration loss is ≤10 mL, and the high-temperature and high-pressure filtration loss is ≤20 mL. Sample preparation: Add 10% (mass fraction, 8%–12%) of PCM microcapsules to polysulfonate drilling fluid and stir at high speed for 30 min to form a PCM drilling fluid system; The performance tests of the phase change material drilling fluid system are as follows: Rheological properties: Rotational rheometer tests showed a viscosity change rate of 3.2% and a shear stress change rate of 2.8% (both ≤5%). Filtration performance: API filtration loss is 8.5 mL (≤10 mL), and high temperature and high pressure (150℃, 3.5 MPa) filtration loss is 17.2 mL (≤20 mL), proving good compatibility with drilling fluid; Phase change material (PCM) enters the bottom of the well along with the drilling fluid, absorbs formation heat and undergoes a phase change to cool the drilling fluid by 15-20°C. After the drilling fluid is returned to the surface, PCM releases the stored heat, which is recovered through a heat exchange system. The overall heat recovery rate is ≥60%, and the bottom circulation temperature of deep and ultra-deep wells is ≥130°C. The heat exchange system is an ORC turbine circulation system. The heat released by PCM is used to heat the ORC circulating working fluid, realizing the utilization of low-quality heat energy for power generation. PCM can be reused along with the drilling fluid, and can be reused ≥50 times while maintaining its phase change performance and structural stability. Experimental setup: A phase change material drilling fluid circulation demonstration test bench was built to simulate deep well conditions (bottom hole temperature 160℃ ≥ 130℃, pressure 100 MPa), equipped with an ORC turbine circulation heat exchange system. The loop test is as follows: Cooling effect: After the drilling fluid circulates through the bottom of the well, the outlet temperature drops from 160℃ to 142℃, a cooling range of 18℃ (15-20℃ range). Heat recovery: The ORC system's heat recovery calculation shows that the overall heat recovery rate is 62.3% (≥60%). Reusable: After 50 cycles, the microcapsules did not rupture, the temperature dropped by 16.8℃, the heat recovery rate was 59.7%, and the core performance remained stable.

[0025] Example 2 like Figures 2-5 As shown, a method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery is presented.

[0026] S1. Preparation of composite phase change core material: Select high-temperature phase change components for compounding. The high-temperature phase change components are selected from at least two of molten salts, organic paraffins, and fatty acids. Mix the components in proportion and melt and stir at 150-180℃ for 30-60 min. After cooling, the composite phase change core material is obtained. The phase change temperature of the composite phase change core material is adjusted to 135-145℃ and the latent heat of phase change is adjusted to 180-280.6 J / g. Raw material pretreatment: Sodium nitrate and potassium nitrate were dried in an oven at 105℃ for 24 hours to remove moisture; hexadecane was kept at room temperature for later use. Melt blending: Mix dried sodium nitrate and potassium nitrate, place in a constant temperature oil bath at 160℃ (within the range of 150~180℃), and melt-stir at 500 r / min for 45 min (within the range of 30~60 min) to form a homogeneous molten salt system; slowly add n-hexadecane and continue stirring for 30 min to achieve full fusion of the two components; Cooling and molding: Pour the mixed molten liquid into a polytetrafluoroethylene mold, let it cool naturally to room temperature, then crush it and pass it through an 80-mesh sieve to obtain composite phase change core material powder (particle size ≤180μm). Preliminary test: DSC test showed that the core material phase change temperature was 140℃ and the latent heat of phase change was 252 J / g (meeting the requirements of 135~145℃ and 180~280.6 J / g). S2. Preparation of functional wall material precursor: Select a wall material substrate and a modifier for mixing. The wall material substrate is selected from at least one of silicon dioxide, urea-formaldehyde resin, and melamine-formaldehyde resin. The modifier is a nano thermal conductive agent, which is selected from at least one of nano graphite and manganese tetroxide. The amount of modifier added is 1% to 5% of the mass of the wall material substrate. Stir evenly to obtain the functional wall material precursor. Mixing raw materials: Tetraethyl silicate and anhydrous ethanol are mixed at a volume ratio of 1:2.5 and stirred evenly. Modified dispersion: Add nano-graphite (0.8 g) and use a 300 W ultrasonic cell disruptor to ultrasonically disperse for 20 min to ensure uniform dispersion of nano-graphite without agglomeration, thus obtaining a stable functional wall material precursor; When the wall material substrate is silicon dioxide, the functional wall material precursor is a mixture of tetraethyl silicate and ethanol, with a volume ratio of tetraethyl silicate to ethanol of 1:2 to 1:3. When the wall material substrate is urea-formaldehyde resin, the functional wall material precursor is a prepolymer of urea and formaldehyde, with a molar ratio of urea to formaldehyde of 1:1.5 to 1:2.0. Molten salt components: Sodium nitrate (70 g) and potassium nitrate (30 g) are selected in a mass ratio of 7:3 (meeting at least two requirements); Organic components: hexadecane (30 g, organic paraffin) was selected, and the mass ratio of molten salt components to organic components was 1:0.3 (in the range of 1:0.2 to 1:0.5). Target performance: phase change temperature 135~145℃, latent heat of phase change 180~280.6 J / g.

[0027] Functional wall material substrate: Silica (meeting at least one of "silica, urea-formaldehyde resin, and melamine-formaldehyde resin"); Functional wall material precursor: a mixture of tetraethyl silicate (40 mL) and ethanol (100 mL) at a volume ratio of 1:2.5 (in the range of 1:2 to 1:3). Modifier: Nano-graphite (0.8 g), added at 2% of the mass of the wall material substrate (tetraethyl silicate) (within the range of 1% to 5%). Wall material performance requirements: withstands high temperature of 200-220℃, high pressure of 80-120 MPa, and has good compatibility with drilling fluid systems; Auxiliary materials: Oil phase medium: flaxseed oil (200 g, conforming to "flaxseed oil or mineral oil"); Emulsifier: Span 80 (7g, meeting the criteria of "at least one of Span 80, Tween 80, and polyglycerol fatty acid esters"); Drilling fluid system: Polysulfonate drilling fluid (basic formula: water + 4% bentonite + 0.3% soda ash + 1.5% CMC + 2% SMP-1 + 3% SPNH + 1% KCl + barite, density 1.2 g / cm³). S3. Core material dispersion and emulsification treatment: The composite phase change core material is added to the oil phase medium and heated to 80-90℃ to form a core material dispersion. The oil phase medium is linseed oil or mineral oil, and the mass fraction of the core material in the oil phase is 20%-40%. An emulsifier is added to the core material dispersion, and the amount of emulsifier added is 2%-5% of the mass of the oil phase medium. Emulsification is carried out at a speed of 1000-3000 r / min for 30-45 min to form a stable emulsion. The emulsifier is selected from at least one of Span 80, Tween 80, and polyglycerol fatty acid ester. The mass ratio of the core material dispersion to the functional wall material precursor is 1:0.3-1:0.8. Core material dispersion: Add the composite phase change core material (80 g) to flaxseed oil (200 g), place it in a constant temperature water bath at 85℃ (within the range of 80~90℃), and stir at 800 r / min for 30 min to form a core material dispersion (the mass fraction of the core material in the oil phase is 28.6%, within the range of 20%~40%). Emulsification treatment: Add Span 80 (7g, 3.5% of the mass of the oil phase medium, within the range of 2% to 5%) to the core material dispersion, increase the stirring speed to 2000 r / min (within the range of 1000 to 3000 r / min), and emulsify for 40 min (within the range of 30 to 45 min) to form a stable water-in-oil emulsion; Ratio control: The mass ratio of the core material dispersion to the functional wall material precursor is 1:0.5 (within the range of 1:0.3 to 1:0.8). S4. In-situ polymerization and encapsulation of microcapsules: The functional wall material precursor is slowly added to the stable emulsion formed after the core material is dispersed and emulsified. The pH of the system is adjusted to 4-6, and the reaction is carried out at 60-80℃ for 2-4 hours to achieve in-situ polymerization and encapsulation of the core material by the wall material. Precursor addition: The functional wall material precursor is slowly added dropwise to the stabilized emulsion at a rate of 2 mL / min, while stirring at a speed of 1500 r / min to ensure thorough mixing; Reaction control: Adjust the pH of the system to 5 (within the range of 4 to 6) with 1 mol / L hydrochloric acid solution, lower the water bath temperature to 70℃ (within the range of 60 to 80℃), and keep the reaction at this temperature for 3 h (within the range of 2 to 4 h). During this period, adjust the stirring speed by ±200 r / min every 30 min to promote the directional polymerization of the wall material precursor on the core material surface. Termination of reaction: After the reaction is complete, allow the mixture to cool naturally to room temperature to obtain a microcapsule suspension; S5. Post-processing: The product after in-situ polymerization and coating reaction is centrifuged, washed with deionized water 3 to 5 times, and vacuum dried at 80 to 100°C for 12 to 24 hours to obtain a phase change material that can be used for drilling fluid cooling and heat recovery. The centrifugation speed is 5000 to 8000 r / min, the centrifugation time is 10 to 15 min, and the vacuum degree of vacuum drying is 0.08 to 0.1 MPa. Centrifugation: The microcapsule suspension was placed in a high-speed centrifuge and centrifuged at 6000 r / min (within the range of 5000 to 8000 r / min) for 12 min (within the range of 10 to 15 min) to separate the microcapsule precipitate. Washing and purification: Wash the precipitate 4 times with deionized water (within the range of 3 to 5 times), and centrifuge after each wash to remove unreacted raw materials and impurities; Vacuum drying: The washed microcapsules are placed in a vacuum drying oven, and the temperature is set to 90℃ (within the range of 80~100℃) and the vacuum degree is 0.09 MPa (within the range of 0.08~0.1 MPa). The microcapsules are dried for 18 h (within the range of 12~24 h) to obtain a white powdery phase change material microcapsule product.

[0028] As can be seen from the above, the specific implementation of the present invention is as follows: deep well and ultra-deep well drilling field conditions (bottom hole circulation temperature ≥130℃, pressure 80~120 MPa); The composition and ratio of the composite phase change core material are as follows: molten salt components (sodium nitrate + potassium nitrate) and organic paraffin components (n-hexadecane) are selected and compounded. The mass ratio of sodium nitrate to potassium nitrate is 7:3, and the mass ratio of molten salt components to n-hexadecane is 1:0.3. This compounding scheme combines the advantages of high latent heat of phase change of molten salt and good stability of paraffin. The target phase change temperature is 135-145℃, and the latent heat of phase change is 180-280.6 J / g. Functional wall materials and modifiers: The wall material substrate is made of silicon dioxide, and the functional wall material precursor is a mixture of tetraethyl silicate and ethanol at a volume ratio of 1:2.5; the modifier is made of nano-graphite, and the amount added is 2% of the mass of the wall material substrate, which is used to improve the thermal conductivity of the wall material. Oil phase medium and emulsifier: Flaxseed oil is selected as the oil phase medium, and Span 80 is selected as the emulsifier to ensure that the core material is uniformly dispersed in the oil phase and forms a stable emulsion; Drilling fluid system: Polysulfonate drilling fluid commonly used in deep wells was selected as the compatibility system. Its basic formula is: water + 4% bentonite + 0.3% soda ash + 1.5% CMC + 2% SMP-1 + 3% SPNH + 1% KCl + barite (density adjusted to 1.2 g / cm³), which is used for subsequent compatibility testing. Preparation of composite phase change core material: Raw material pretreatment: Take 70 g of sodium nitrate and 30 g of potassium nitrate, put them in an oven at 105℃ and dry for 24 h to remove moisture; take 30 g of n-hexadecane (calculated according to the mass ratio of molten salt to organic matter of 1:0.3) for later use; Melt blending: Mix dried sodium nitrate and potassium nitrate, place them in a 200℃ constant temperature oil bath, stir at 500 r / min for 45 min to form a homogeneous molten salt system; slowly add n-hexadecane and continue stirring for 30 min to achieve full fusion of the two components; Cooling and molding: Pour the mixed molten liquid into a polytetrafluoroethylene mold, let it cool naturally to room temperature, then grind it in a pulverizer and pass it through an 80-mesh sieve to obtain composite phase change core material powder with a particle size ≤180μm; Performance Preliminary Test: Using differential scanning calorimetry (DSC), the phase change temperature of the core material is 140℃ and the latent heat of phase change is 252 J / g. Preparation of functional wall material precursor: Raw material ratio: Take 40 mL of tetraethyl silicate as the wall material substrate, add 100 mL of anhydrous ethanol (volume ratio 1:2.5), and stir evenly; weigh 0.8 g of nano-graphite (calculated as 2% of the mass of tetraethyl silicate) and add it to the above mixture; Dispersion treatment: An ultrasonic cell disruptor with a power of 300 W was used for ultrasonic dispersion for 20 min to ensure uniform dispersion of nano-graphite without agglomeration, thus obtaining a stable functional wall material precursor. Core material dispersion and emulsification treatment: Core material dispersion: Take 200 g of flaxseed oil as the oil phase medium, add 80 g of composite phase change core material (the mass fraction of core material in the oil phase is 28.6%), put it in a constant temperature water bath, heat to 85℃, stir at 800 r / min, keep warm for 30 min, and form a core material dispersion. Emulsification treatment: Add 7 g of Span 80 (3.5% of the mass of the oil phase medium) to the core material dispersion, increase the stirring speed to 2000 r / min, emulsify for 40 min, and form a water-in-oil stable emulsion with uniform particle size; In-situ polymerization and encapsulation of microcapsules: Precursor addition: The prepared functional wall material precursor is slowly added dropwise to the above stable emulsion at a drop rate of 2 mL / min, while maintaining a stirring speed of 1500 r / min to ensure that the precursor and emulsion are fully mixed. Reaction condition control: Adjust the pH of the system to 5 with 1 mol / L hydrochloric acid solution, lower the water bath temperature to 70℃, keep the reaction at this temperature for 3 h, and adjust the stirring speed by ±200 r / min every 30 min to promote the in-situ polymerization of the wall material precursor on the core material surface. Reaction termination: After the reaction was completed, the mixture was allowed to cool naturally to room temperature to obtain a microcapsule suspension. Post-processing: Centrifugation: The microcapsule suspension was poured into a high-speed centrifuge and centrifuged at 6000 r / min for 12 min to separate the microcapsule precipitate. Washing and purification: The precipitate is washed four times with deionized water, and centrifuged after each wash to remove unreacted tetraethyl silicate, emulsifiers and other impurities. Vacuum drying: The washed microcapsules were placed in a vacuum drying oven, the temperature was set to 90℃ and the vacuum degree to 0.09MPa, and dried for 18 h to obtain a white powdery phase change material microcapsule product. Basic performance testing: Phase change parameter testing: The finished microcapsules were tested using DSC. The phase change temperature was 140.5℃ and the latent heat of phase change was 248 J / g. Particle size testing: Laser particle size analyzer was used to test the microcapsule particle size range of 28–65 μm, and the D50 particle size was 32 μm; Temperature and pressure resistance test: The microcapsules were placed in a high-temperature and high-pressure autoclave, and the temperature was set at 210℃ and the pressure at 100 MPa. After holding at the temperature and pressure for 24 hours, they were taken out and observed to show no signs of rupture, with a breakage rate of 2.3%. Cyclic stability test: The microcapsules were subjected to 500 phase transition cycles (130-150℃). After cycling, the phase transition temperature was 139.8℃ (change value 0.7℃) and the latent heat of phase transition was 232 J / g (loss rate 6.5%). Compatibility test with drilling fluid: Sample preparation: 10% (mass fraction) of phase change material microcapsules were added to the above polysulfonated drilling fluid and stirred in a high-speed mixer for 30 min to form a phase change material drilling fluid system. Performance testing: Rheological properties: tested using a rotational rheometer, viscosity change rate was 3.2%, shear stress change rate was 2.8%, which meets the requirement of ≤5%; Filtration performance: API filtration loss 8.5 mL, high temperature and high pressure (150℃, 3.5 MPa) filtration loss 17.2 mL; Heat recovery performance test: Test setup: A phase change material drilling fluid circulation demonstration test bench was set up to simulate the deep well bottom environment (temperature 160℃, pressure 100 MPa), and an ORC turbine circulation heat exchange system was installed on the ground. Cyclic test: Inject the phase change material drilling fluid into the test bench, cycle it 10 times, and record key data; Cooling effect: After the drilling fluid circulates through the bottom of the well, the outlet temperature drops from 160℃ to 142℃, a decrease of 18℃; Heat recovery rate: The overall heat recovery rate is calculated to be 62.3% by recovering the heat released by the phase change material through the ORC system. Reusability: After 50 cycles, the phase change material microcapsules did not rupture, the temperature drop remained at 16.8℃, and the heat recovery rate was 59.7% (close to 60%). Summary of Results: Through component optimization and precise process control, the phase change temperature is 140.5℃, the latent heat of phase change is 248 J / g, it can withstand high temperature of 210℃ and high pressure of 100 MPa, has excellent compatibility with drilling fluid, and the heat recovery rate reaches 62.3%. It can be reused more than 50 times, verifying the feasibility and practicality of the solution. It can effectively solve the problems of instrument failure and drilling fluid deterioration caused by high temperature in deep wells, while realizing the resource utilization of residual heat of drilling fluid and reducing drilling costs and energy consumption. Other notes: Composite phase change core material: Potassium nitrate + lithium nitrate (mass ratio 6:4) and stearic acid (mass ratio 1:0.4) can be selected, with a phase change temperature of 138℃ and a latent heat of phase change of 235 J / g; Wall material substrate: Urea-formaldehyde resin (urea to formaldehyde molar ratio 1:1.8) can be selected, and manganese tetroxide (addition amount 3%) is selected as the modifier. The prepared microcapsules have consistent temperature and pressure resistance. Emulsifier: Tween 80 and Span 80 can be used in combination (mass ratio 1:1), the emulsification effect is equally stable, and there is no significant difference in product performance.

[0029] The above descriptions are merely embodiments of the present invention, and common knowledge regarding specific structures and characteristics is not elaborated upon here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the structure of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A phase change material that can be used for drilling fluid cooling and heat recovery, characterized in that, The product includes phase change materials, which are core-shell structured microcapsules, composite phase change core materials, and functional wall materials covering the composite phase change core materials. The phase change temperature of the phase change materials is 135–145℃, and the latent heat of phase change is 180–280.6 J / g. The composite phase change core materials are used to absorb heat from drilling fluid in the high-temperature environment of deep wells to achieve cooling, and to release stored heat during surface heat exchange to achieve heat energy recovery. The functional wall materials have the characteristics of withstanding high temperatures of 200–220℃ and high pressures of 80–120 MPa, and have good compatibility with drilling fluid systems.

2. A method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery, characterized in that: Includes the following steps: S1. Preparation of composite phase change core material: Select high-temperature phase change components for compounding. The high-temperature phase change components are selected from at least two of molten salts, organic paraffins, and fatty acids. Mix the components in proportion and melt and stir at 150-180℃ for 30-60 min. After cooling, the composite phase change core material is obtained. The phase change temperature of the composite phase change core material is adjusted to 135-145℃ and the latent heat of phase change is adjusted to 180-280.6 J / g. S2. Preparation of functional wall material precursor: Select wall material substrate and modifier, the wall material substrate is selected from at least one of silicon dioxide, urea-formaldehyde resin, and melamine-formaldehyde resin, the modifier is a nano thermal conductive agent, the nano thermal conductive agent is selected from at least one of nano graphite and manganese tetroxide, the amount of modifier added is 1% to 5% of the mass of wall material substrate, and stir evenly to obtain functional wall material precursor. S3. Core material dispersion and emulsification treatment: The composite phase change core material is added to the oil phase medium and heated to 80-90℃ to form a core material dispersion. The oil phase medium is linseed oil or mineral oil, and the mass fraction of the core material in the oil phase is 20%-40%. An emulsifier is added to the core material dispersion at a mass of 2%-5% of the oil phase medium. The mixture is emulsified at a speed of 1000-3000 r / min for 30-45 min to form a stable emulsion. S4. In-situ polymerization and encapsulation of microcapsules: The functional wall material precursor is slowly added to the stable emulsion formed after the core material is dispersed and emulsified. The pH of the system is adjusted to 4-6, and the reaction is carried out at 60-80℃ for 2-4 hours to achieve in-situ polymerization and encapsulation of the core material by the wall material. S5. Post-processing: The product after in-situ polymerization and coating reaction is centrifuged, washed with deionized water 3 to 5 times, and vacuum dried at 80 to 100°C for 12 to 24 hours to obtain a phase change material that can be used for drilling fluid cooling and heat recovery.

3. The method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to claim 1, characterized in that: In the composite phase change core material, the molten salt component is selected from at least two of sodium nitrate, potassium nitrate, and lithium nitrate; the organic paraffin component is selected from at least one of n-tetradecane, n-hexadecane, and C18-C24 paraffin; and the fatty acid component is selected from at least one of stearic acid, palmitic acid, and myristic acid. The mass ratio of the molten salt component to the organic component (paraffin + fatty acid) is 1:0.2 to 1:0.

5.

4. A method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to claim 1, characterized in that: The microcapsules have a particle size of 25–70 μm and a D50 particle size of 28–35 μm. The breakage rate is ≤3% under conditions of 200–220℃ and 80–120 MPa. After 500 cycles of phase change, the phase change temperature change is ≤2℃ and the latent heat loss rate of phase change is ≤8%.

5. A method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to claim 1, characterized in that: When the amount of phase change material added to the drilling fluid is 8% to 12%, the viscosity and shear stress change rate of the drilling fluid are ≤5%, the API filtration loss is ≤10 mL, and the high temperature and high pressure filtration loss is ≤20 mL.

6. A method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to claim 2, characterized in that: When the wall material substrate is silicon dioxide, the functional wall material precursor is a mixture of tetraethyl silicate and ethanol, with a volume ratio of tetraethyl silicate to ethanol of 1:2 to 1:

3. When the wall material substrate is urea-formaldehyde resin, the functional wall material precursor is a prepolymer of urea and formaldehyde, with a molar ratio of urea to formaldehyde of 1:1.5 to 1:2.

0.

7. A method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to claim 2, characterized in that: The emulsifier is selected from at least one of Span 80, Tween 80, and polyglycerol fatty acid esters, and the mass ratio of the core material dispersion to the functional wall material precursor is 1:0.3 to 1:0.

8.

8. A method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to claim 2, characterized in that: The centrifugation speed is 5000-8000 r / min, the centrifugation time is 10-15 min, and the vacuum degree of vacuum drying is 0.08-0.1 MPa.

9. A method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to claim 1, characterized in that: The phase change material is circulated into the bottom of the well along with the drilling fluid. It absorbs heat from the formation and undergoes a phase change to cool the drilling fluid by 15-20°C. After the drilling fluid is returned to the surface, the phase change material releases the stored heat. The released heat is recovered through a heat exchange system, with a comprehensive heat recovery rate of ≥60%. The bottom circulation temperature of deep and ultra-deep wells is ≥130°C.

10. A method for preparing a phase change material that can be used for drilling fluid cooling and heat recovery according to claim 9, characterized in that: The heat exchange system is an ORC turbine circulation system. The heat released by the phase change material is used to heat the ORC circulation working fluid, realizing the power generation utilization of low-quality thermal energy. The phase change material can be reused with the drilling fluid circulation, and it can be reused ≥50 times while maintaining its phase change performance and structural stability.

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