CN109054791B - Water-based carbon nanofluid and preparation method thereof - Google Patents

Water-based carbon nanofluid and preparation method thereof Download PDF

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CN109054791B
CN109054791B CN201810779954.5A CN201810779954A CN109054791B CN 109054791 B CN109054791 B CN 109054791B CN 201810779954 A CN201810779954 A CN 201810779954A CN 109054791 B CN109054791 B CN 109054791B
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CN109054791A (en
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赵明伟
戴彩丽
李玉阳
吴一宁
赵光
孙永鹏
李琳
吕文娇
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China University of Petroleum East China
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Abstract

The invention discloses a water-based carbon nanofluid, which consists of carbon nanoparticles and an alkaline aqueous solution, wherein the mass percent of the carbon nanoparticles is 0.001-5%. The invention also discloses a preparation method of the water-based carbon nanofluid, which is characterized in that the water-based carbon nanofluid can be obtained by dispersing the carbon nanoparticles in the proportion into an alkaline aqueous solution with the pH value of 8-12. The surface wettability of the carbon nano-particles can change along with the change of the pH value of the environment, carboxylic acid groups on the surfaces of the carbon nano-particles are converted into carboxylate ionic groups in an alkaline environment, the wettability of the carbon nano-particles is converted from hydrophobicity into hydrophilicity, and the carbon nano-particles can be stably dispersed in an aqueous medium; in a neutral or acidic environment, the carboxylate ion groups on the surface of the carbon nanoparticles are converted into carboxylic acid groups, and the wettability of the carbon nanoparticles is changed from hydrophilic to hydrophobic. The nano fluid has the advantages of simple preparation process, uniform dispersion, good stability, good effects of reducing pressure, increasing injection and improving recovery ratio, and easy field preparation and large-scale application.

Description

Water-based carbon nanofluid and preparation method thereof
Technical Field
The invention belongs to the technical field of nano functional materials and oil field chemicals, and particularly relates to a water-based carbon nanofluid and a preparation method thereof.
Background
Petroleum has an irreplaceable role in national economy as a high-efficiency energy source. With the rapid increase of national economy, the demand for petroleum is continuously increased. With the depth of oil and gas exploration and development, the proportion of low-permeability oil and gas resource exploration and development is getting larger and larger, and particularly, the reserves of ultra-low-permeability oil reservoirs and compact oil reservoirs are especially rich newly discovered in recent years. However, the micro-nano pore throats of the reservoir layer of the ultra-low-permeability and compact oil reservoir develop, the water injection pressure is high, the oil absorption and discharge are difficult, and the improvement of the effective utilization rate of the reserve volume faces challenges.
In recent years, with the development of nanotechnology and the increasing difficulty of oil and gas field development, nanotechnology is gradually applied to the field of oil and gas field development. "Enhanced Heavy Oil Recovery in Sandstone Cores Using TiO" volume 28, volume 1, 20142Nanofluides reports that a nano titanium dioxide dispersion is used for improving the recovery efficiency of heavy oil, and the result shows that the recovery efficiency can be improved from 49% to 80% by nano particles, the main action mechanism of the nano titanium dioxide is wettability conversion, the nano titanium dioxide converts an oil-wet surface into a water-wet surface, oil is stripped from a wall surface, the interaction and adsorption mechanism of nano materials play a key role in the wettability conversion, but the nano titanium dioxide dispersion has a large dispersed particle size (the average particle size is 63nm), and when the nano titanium dioxide dispersion is applied to a low-permeability reservoir, the pores of a matrix are easily blocked, so that the reservoir is damaged. Russian-produced polysilicone materials (bleachel) are prepared by using gamma-ray radioactive additives to react with SiO2The product subjected to chemical modification has the discrete particle size of 10-500 nm, and has good pressure reduction and injection enhancement effects when polysilicon materials are used for treating more than 200 wells in oil fields such as Sisiberia, Qiuming, Uldmouji and the like.
The 'application research of water-based nano-silicone emulsion system in the 2 nd stage of 2012 of oilfield chemistry' reports that hydrophobic nano-silicone with water dispersibility is prepared, water is used as a carrying agent to replace a conventional organic dispersant, the nano-polysilicon emulsion is clear and transparent, the formula of the water-based nano-additive solution is 1.75 per mill nano-polysilicon and 2 per mill auxiliary dispersant, the water-based nano-polysilicon emulsion can change the hydrophilicity of the rock surface into strong hydrophobicity, the core flow experiment result shows that the water-phase permeability is averagely improved by 40 percent after the water-based nano-polysilicon is treated, but the product introduces a large amount of emulsifier to form microemulsion in order to improve water solubility, and emulsion breaking needs to be induced when the product is used on site, thereby increasing the operation complexity and the product price, and the surfactant is used for stably dispersing the nano particles by virtue of physical action, and the physical action is damaged in the washing process, so that the nano material is seriously aggregated and settled.
Because the rock of the ultra-low-permeability and compact oil reservoir is buried compactly, and has the characteristics of fine pore throat, small porosity, poor permeability and the like, the application of the nano oil displacement technology is greatly limited due to factors such as toxicity, dispersity, stability and the like.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provide the water-based carbon nanofluid and the preparation method thereof.
In order to achieve the purpose, the invention adopts the following technical scheme:
a water-based carbon nanofluid is composed of carbon nanoparticles and an alkaline aqueous solution, wherein the mass percent of the carbon nanoparticles is 0.001% -5%.
Preferably, the pH value of the alkaline aqueous solution is 8-12.
Preferably, the solute in the alkaline aqueous solution is one or a combination of several of sodium hydroxide, potassium hydroxide, sodium carbonate, ammonia monohydrate, sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, potassium monohydrogen phosphate and potassium dihydrogen phosphate.
Preferably, the dispersed particle size of the carbon nanoparticles is 5-20 nm.
Preferably, the method for preparing the carbon nanoparticles comprises the following steps:
(1) uniformly mixing organic acid, exothermic agent and water for reaction until the mixture is cooled to room temperature to obtain a first reaction system;
(2) and washing and separating the solid in the first reaction system, and then drying to finally obtain the carbon nano-particles.
Preferably, the weight ratio of the organic acid, the exothermic agent and the water in the step (1) is as follows: 10-100: 25-250: 1-10.
Preferably, the organic acid in the step (1) is one or more of small molecular acid and unsaturated fatty acidCombining; the chemical formula of the micromolecular acid is CH3(CH2)nCOOH, wherein n is more than or equal to 0 and less than or equal to 3 and is an integer; the unsaturated fatty acid has a chemical formula of CH3(CH2)aHC＝CH(CH2)bCOOH or CH3(CH2)cHC＝CH(CH2)dHC＝CH(CH2)eCOOH or CH3(CH2)fHC＝CH(CH2)gHC＝CH(CH2)hHC＝CH(CH2)iCOOH, wherein a is more than or equal to 1 and less than or equal to 10, b is more than or equal to 1 and less than or equal to 10, c is more than or equal to 1 and less than or equal to 5, d is more than or equal to 1 and less than or equal to 5, e is more than or equal to 1 and less than or equal to 4, g is more than or equal to 1 and less than or equal to 4, h is more than or equal to 1 and less than or equal to 4, and a, b, c, d, e, f, g.
Preferably, the exothermic agent in step (1) is one or a combination of sodium oxide, potassium oxide, calcium oxide and phosphorus pentoxide.
Preferably, in the step (2), the process of washing and separating the solid in the first reaction system is repeated 4 times, the separation mode is centrifugation or filtration, and the drying condition is vacuum drying at 50 ℃ for 24 h.
The invention also discloses a preparation method of the water-based carbon nanofluid, which is characterized in that the water-based carbon nanofluid can be obtained by dispersing the carbon nanoparticles in the proportion into an alkaline aqueous solution with the pH value of 8-12.
The beneficial effect of the invention is that,
(1) the surface wettability of the carbon nano-particles prepared by the invention can be changed along with the change of the pH value of the environment due to the carboxylic acid groups on the surface of the carbon nano-particles, and when the carbon nano-particles are in an alkaline environment, the carboxylic acid groups on the surface of the carbon nano-particles are converted into carboxylate ionic groups, so that the wettability of the carbon nano-particles is changed from hydrophobicity to hydrophilicity, and the carbon nano-particles can be stably dispersed in an aqueous medium; when in a neutral or acidic environment, the carboxylate ionic groups on the surface of the carbon nanoparticles are converted into carboxylic acid groups, so that the wettability of the carbon nanoparticles is changed from hydrophilic to hydrophobic.
(2) The carbon nano-particles can be self-dispersed in an alkali solution without adding a dispersing agent and by means of auxiliary equipment, and the corresponding nano-fluid can be prepared by means of the dispersing agent and dispersing equipment (stirring or ultrasonic and the like) in the prior art.
(3) After the water-based carbon nanofluid is injected into a stratum, the nanofluid spreads on the surface of a reservoir fracture or a matrix, oil is isolated from the surface of reservoir rock, an oil phase is converted into a free phase, and the free oil phase is discharged along with the flow of the fluid in the reservoir, so that the oil absorption and discharge efficiency of an ultralow-permeability and dense oil reservoir is improved. Meanwhile, the carbon nano particles are tightly adsorbed on the surface of the reservoir rock to form a nano adsorption layer. In the subsequent water injection process, the pH value in the stratum water environment is changed from alkalinity to neutrality, carboxylate groups on the surfaces of the carbon nano particles are changed into carboxylic acid groups, and the wettability of the surfaces of the nano particles is changed from hydrophilicity to hydrophobicity, so that the adhesion work of the nano adsorption layer to a water phase is reduced, the purposes of reducing water flow resistance and injection pressure are further achieved, and the water injection effect of the ultra-low-permeability and compact oil reservoir is improved. In the prior art in the field, a nano particle and surfactant compound system and a microemulsion system are mostly adopted, the separation of nano particles and a surfactant and the emulsion breaking of microemulsion are required to be induced when the nano particle and surfactant compound system is used on site, the operation complexity and the product price are increased, and the surfactant stably disperses nano particles by virtue of physical action, so that the physical action is destroyed in the scouring process, and the nano material is seriously aggregated and settled.
(4) The nano fluid has the advantages of simple preparation process, uniform dispersion, good stability, good effects of reducing pressure, increasing injection, absorbing and discharging oil and improving the recovery ratio, and easy field preparation and large-scale application.
Drawings
FIG. 1 is a graph of the particle size distribution of carbon nanoparticles in a carbon nanofluid in example 1;
fig. 2 is a schematic diagram of the effect of ambient pH on the surface group properties of carbon nanoparticles prepared according to the present invention.
Detailed Description
The invention is further illustrated with reference to the following figures and examples.
A water-based carbon nanofluid is composed of carbon nanoparticles and an alkaline aqueous solution, wherein the mass percent of the carbon nanoparticles is 0.001% -5%, the pH value of the alkaline aqueous solution is 8-12, solutes in the alkaline aqueous solution are one or a combination of more of sodium hydroxide, potassium hydroxide, sodium carbonate, ammonium monohydrate, sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, potassium monohydrogen phosphate and potassium dihydrogen phosphate, and the dispersed particle size of the carbon nanoparticles is 5-20 nm.
The preparation method of the carbon nano-particles comprises the following steps:
(1) uniformly mixing organic acid, exothermic agent and water for reaction until the mixture is cooled to room temperature to obtain a first reaction system;
(2) and washing and separating the solid in the first reaction system, wherein the washing and separating process is repeated for four times, and the separation process adopts a centrifugal or filtering mode and vacuum drying at 50 ℃ for 24 hours to finally obtain the carbon nano-particles.
The weight ratio of the organic acid, the exothermic agent and the water in the step (1) is as follows: 10-100: 25-250: 1-10; the organic acid in the step (1) is one or a combination of a plurality of micromolecular acids and unsaturated fatty acids; the chemical formula of the micromolecular acid is CH3(CH2)nCOOH, wherein n is more than or equal to 0 and less than or equal to 3 and is an integer; the unsaturated fatty acid has a chemical formula of CH3(CH2)aHC＝CH(CH2)bCOOH or CH3(CH2)cHC＝CH(CH2)dHC＝CH(CH2)eCOOH or CH3(CH2)fHC＝CH(CH2)gHC＝CH(CH2)hHC＝CH(CH2)iCOOH, wherein a is more than or equal to 1 and less than or equal to 10, b is more than or equal to 1 and less than or equal to 10, c is more than or equal to 1 and less than or equal to 5, d is more than or equal to 1 and less than or equal to 5, e is more than or equal to 1 and less than or equal to 4, g is more than or equal to 1 and less than or equal to 4, h is more than or equal to 1 and less than or equal to 4, and a, b, c, d, e, f, g; the exothermic agent in the step (1) is one or a combination of more of sodium oxide, potassium oxide, calcium oxide and phosphorus pentoxide.
As shown in fig. 2, the carbon nanoparticles prepared by the invention have carboxylic acid groups on the surface, so that the surface wettability of the carbon nanoparticles can be changed along with the change of the environmental pH, and the hydrophilicity and hydrophobicity of the nanoparticles can be changed according to the needs of different conditions, on one hand, in an alkaline environment, the self-dispersion stability of the nanoparticles in water can be enhanced, the nanoparticles can be uniformly adsorbed on the surface of rock of a reservoir stratum after being injected into a stratum, and the nanoparticles cannot be aggregated to generate self-loss, so that the adsorption efficiency is reduced, on the other hand, in the subsequent injection, the environmental pH value is reduced, the hydrophilicity is changed into hydrophobicity, the adhesion work of the nanoparticle adsorption layer to a water phase is reduced, so that the adhesion work of the nanoparticle adsorption layer to the water phase is reduced, the purposes of reducing the water flow resistance and the injection pressure are achieved, and the.
Example 1
Adding 10g of CH3Uniformly mixing COOH, 25g of sodium oxide and 1g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, centrifuging, repeating for 4 times, and vacuum-drying at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
0.01g of the prepared carbon nanoparticles was weighed and dispersed in 99.99g of an aqueous sodium hydroxide solution having a pH of 8, to obtain the water-based carbon nanofluid.
The particle size distribution of the carbon nanoparticles in the nanofluid is measured by using a laser particle size analyzer, and as shown in fig. 1, the particle size distribution range of the carbon nanoparticles is 5-20 nm.
Example 2
20g of CH3CH2CH2Uniformly mixing COOH, 50g of potassium oxide and 2g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, filtering, repeating for 4 times, and drying in vacuum at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
The water-based carbon nanofluid was obtained by weighing 0.1g of the prepared carbon nanoparticles and dispersing them in 99.9g of an aqueous potassium hydroxide solution having a pH of 10.
And (3) determining the particle size distribution of the carbon nanoparticles in the nanofluid by using a laser particle size analyzer, wherein the particle size distribution range of the carbon nanoparticles is 6-18 nm.
Example 3
50g of CH3CH2CH2COOH, 150g of pentoxideUniformly mixing phosphorus oxide and 6g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, centrifuging, repeating for 4 times, and vacuum-drying at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
And weighing 1g of the prepared carbon nanoparticles, and dispersing the carbon nanoparticles in 99g of a mixed aqueous solution of potassium hydroxide and sodium dihydrogen phosphate with the pH value being 9 to obtain the water-based carbon nanofluid.
And (3) determining the particle size distribution of the carbon nanoparticles in the nanofluid by using a laser particle size analyzer, wherein the particle size distribution range of the carbon nanoparticles is 4-21 nm.
Example 4
50g of CH3(CH2)6HC＝CH(CH2)6Uniformly mixing COOH, 200g of phosphorus pentoxide and 9g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, filtering, repeating for 3 times, and drying in vacuum at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
0.1g of the prepared carbon nanoparticles was weighed and dispersed in 99.9g of a mixed aqueous solution of potassium hydroxide and sodium hydroxide with a pH of 10, to obtain the water-based carbon nanofluid.
And (3) determining the particle size distribution of the carbon nanoparticles in the nanofluid by using a laser particle size analyzer, wherein the particle size distribution range of the carbon nanoparticles is 7-17 nm.
Example 5
20g of CH3CH2HC＝CH(CH2)2HC＝CH(CH2)2Uniformly mixing COOH, 50g of potassium oxide and 9g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, centrifuging, repeating for 4 times, and vacuum-drying at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
The water-based carbon nanofluid was obtained by weighing 0.01g of the prepared carbon nanoparticles and dispersing them in 99.99g of an aqueous sodium phosphate solution having pH of 8.
And (3) determining the particle size distribution of the carbon nanoparticles in the nanofluid by using a laser particle size analyzer, wherein the particle size distribution range of the carbon nanoparticles is 5-20 nm.
Example 6
Adding 10g of CH3CH2HC＝CHCH2HC＝CHCH2HC＝CHCH2Uniformly mixing COOH, 50g of phosphorus pentoxide and 5g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, filtering, repeating for 4 times, and drying in vacuum at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
0.005g of the prepared carbon nanoparticles was weighed and dispersed in 99.995g of a mixed aqueous solution of sodium hydroxide and potassium dihydrogen phosphate having a pH of 11, to obtain the water-based carbon nanofluid.
And (3) determining the particle size distribution of the carbon nanoparticles in the nanofluid by using a laser particle size analyzer, wherein the particle size distribution range of the carbon nanoparticles is 6-21 nm.
Example 7
Adding 5g of CH3COOH、5g CH3(CH2)6HC＝CH(CH2)6Uniformly mixing COOH, 50g of phosphorus pentoxide and 5g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, filtering, repeating for 4 times, and drying in vacuum at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
0.005g of the prepared carbon nanoparticles was weighed and dispersed in 99.995g of a mixed aqueous solution of sodium hydroxide and potassium dihydrogen phosphate having a pH of 11, to obtain the water-based carbon nanofluid.
And (3) determining the particle size distribution of the carbon nanoparticles in the nanofluid by using a laser particle size analyzer, wherein the particle size distribution range of the carbon nanoparticles is 7-23 nm.
Example 8
Adding 10g of CH3Uniformly mixing COOH, 40g of phosphorus pentoxide, 10g of calcium oxide and 5g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, filtering, repeating for 4 times, and drying in vacuum at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
The water-based carbon nanofluid was obtained by weighing 0.005g of the prepared carbon nanoparticles and dispersing them in 99.995g of an aqueous sodium hydroxide solution having a pH of 10.
And (3) determining the particle size distribution of the carbon nanoparticles in the nanofluid by using a laser particle size analyzer, wherein the particle size distribution range of the carbon nanoparticles is 4-18 nm.
Example 9
Adding 5g of CH3COOH、5g CH3CH2Uniformly mixing COOH, 50g of phosphorus pentoxide and 5g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, filtering, repeating for 4 times, and drying in vacuum at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
The water-based carbon nanofluid was obtained by weighing 0.005g of the prepared carbon nanoparticles and dispersing them in 99.995g of an aqueous sodium hydroxide solution having a pH of 10.
And (3) determining the particle size distribution of the carbon nanoparticles in the nanofluid by using a laser particle size analyzer, wherein the particle size distribution range of the carbon nanoparticles is 5-21 nm.
Example 10
Adding 5g of CH3(CH2)6HC＝CH(CH2)6COOH、5g CH3CH2HC＝CHCH2HC＝CHCH2HC＝CHCH2Uniformly mixing COOH, 50g of phosphorus pentoxide and 5g of water for reaction, cooling to room temperature to obtain a first reaction system, washing the solid in the first reaction system with water, filtering, repeating for 4 times, and drying in vacuum at 50 ℃ for 24 hours to obtain the carbon nanoparticles.
The water-based carbon nanofluid was obtained by weighing 0.005g of the prepared carbon nanoparticles and dispersing them in 99.995g of an aqueous sodium hydroxide solution having a pH of 11.
Utilizing a laser particle size analyzer to measure the particle size distribution of carbon nanoparticles in the nanofluid, wherein the particle size distribution range of the carbon nanoparticles
7～27nm。
Test example 1
Measuring ultra-low permeability natural core with gas permeability of 2.1mD, saturated simulated oil (viscosity of 2mPa & s, 25 deg.C), displacing 30 Pore Volume (PV) with simulated water (3 wt% potassium chloride saline), establishing residual oil, and measuring stable water injection pressure P1The nanophase solution of example 3 of 1PV was then injected into the core and aged at room temperatureAfter 24h, continuing to inject simulated water, and recording the water injection pressure P after stabilization2. After being treated by the nano-liquid for reducing pressure and increasing injection, the nano-liquid is prepared by the following formula: the blood pressure lowering rate is (P)1-P2)/P1× 100, the calculation result shows that the water injection pressure is reduced by 31 percent, which indicates that the nanometer liquid for reducing the pressure and increasing the injection has good effects of reducing the pressure and increasing the injection.
Test example 2
Taking a compact natural rock core with gas permeability of 0.1mD, saturating simulated oil (viscosity of 1mPa & s, 25 ℃), displacing 30 Pore Volumes (PV) with simulated water (3 wt% potassium chloride saline), establishing residual oil, and measuring the stable water injection pressure P1Then, the nano-solution of example 4 of 1PV was injected into the core, aged at room temperature for 24h, and then simulated water was continuously injected, and the injection pressure P after stabilization was recorded2. After being treated by the nano-liquid for reducing pressure and increasing injection, the nano-liquid is prepared by the following formula: the blood pressure lowering rate is (P)1-P2)/P1× 100 percent, the water injection pressure is reduced by 26 percent, which shows that the nanometer liquid for reducing the pressure and increasing the injection has good effects of reducing the pressure and increasing the injection.
Test example 3
Taking a compact natural rock core with gas permeability of 0.5mD, saturating simulated oil (viscosity of 1mPa & s, 25 ℃), displacing 30 Pore Volumes (PV) with simulated water (3 wt% potassium chloride saline), establishing residual oil, and measuring the stable water injection pressure P1Then, the nano-solution of example 6 of 1PV was injected into the core, aged at room temperature for 24h, and then simulated water was continuously injected, and the injection pressure P after stabilization was recorded2. After being treated by the nano-liquid for reducing pressure and increasing injection, the nano-liquid is prepared by the following formula: the blood pressure lowering rate is (P)1-P2)/P1× 100 percent, the water injection pressure is reduced by 26 percent, which shows that the nanometer liquid for reducing the pressure and increasing the injection has good effects of reducing the pressure and increasing the injection.
Test example 4
Taking 12 ultra-low permeability natural cores with the length of 4cm and the diameter of 2.5cm, cleaning the cores, after vacuum drying at 90 ℃ for 24 hours, dividing gas permeability into 2.3mD, 2.2mD, 2.3mD, 2.1mD, 2.4mD, 1.8mD, 2.3mD, 2.2mD, 2.0mD and 2.1mD, saturating simulated oil (viscosity is 2mPa · s, 25 ℃), and recording saturated oil volume after aging for 24 hours; placing the 5 cores into an imbibition bottle, respectively soaking the cores in 3 wt% potassium chloride saline, 0.1 wt% sodium dodecyl benzene sulfonate surfactant solution and the nanofluids in examples 1-6, recording oil discharge of 8 cores in different imbibitions with time until the oil discharge does not change any more, recording final oil discharge, respectively calculating recovery ratios of the 8 imbibitions to be 13.2%, 18.6%, 30.1%, 29.2%, 33.2%, 28.7%, 30.0%, 29.1%, 27.1%, 24.2%, 28.7% and 33.8%, respectively, and increasing the nanofluids by more than 10% compared with the surfactant solution.
Test example 5
Taking 12 ultra-low permeability natural cores with the length of 2.5cm and the diameter of 2.5cm, cleaning the cores, after vacuum drying for 24 hours at 90 ℃, the gas permeability is divided into 0.2mD, 0.1mD, 0.2mD, 0.1mD, 0.2mD and 0.1mD, saturated simulated oil (viscosity is 1mPa & s, after aging for 24 hours, recording the saturated oil quantity; placing the 5 cores into an imbibition bottle, respectively soaking the cores in 3 wt% potassium chloride saline, 0.1 wt% sodium dodecyl benzene sulfonate surfactant solution and the nanofluids in examples 1-10, recording oil discharge of 8 cores in different imbibitions with time until the oil discharge does not change any more, recording final oil discharge, respectively calculating recovery ratios of 8 imbibitions to be 6.2%, 10.6%, 23.1%, 26.2%, 30.2%, 24.7%, 31.2%, 27.8%, 25.2%, 28.1%, 24.2% and 21.8%, respectively, and increasing the nanofluids by more than 10% compared with the surfactant solution.
The test examples show that the water-based carbon nanofluid has good effects of reducing pressure, increasing injection and improving recovery ratio of ultra-low permeability and compact oil reservoirs, can reduce water injection pressure of compact and ultra-low permeability natural cores by more than 26%, and can improve the recovery ratio of crude oil by more than 10% compared with a surfactant solution. The preparation process of the water-based carbon nanofluid is extremely simple, and compared with the prior art in the field, the corresponding nanofluid can be prepared by means of a dispersing agent and dispersing equipment (stirring or ultrasound and the like). The water-based carbon nanofluid can be directly prepared on site without any equipment, has uniform dispersion and good stability, and greatly saves transportation, manpower and equipment cost, so that the water-based carbon nanofluid is easy to prepare on site and apply on a large scale.
The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and the modifications or the substitutions do not make the essence of the corresponding technical solutions depart from the scope of the technical solutions of the embodiments of the present invention.

Claims (6)

1. A water-based carbon nano-fluid is composed of carbon nano-particles and an alkaline aqueous solution, wherein the mass percent of the carbon nano-particles is 0.001% -5%, and the preparation method of the carbon nano-particles is characterized by comprising the following steps:
(1) uniformly mixing organic acid, exothermic agent and water for reaction until the mixture is cooled to room temperature to obtain a first reaction system;
(2) washing and separating the solid in the first reaction system, and then drying to finally obtain the carbon nano-particles;
the weight ratio of the organic acid, the exothermic agent and the water in the step (1) is as follows: 10-100: 25-250: 1-10;
the organic acid in the step (1) is one or a combination of a plurality of micromolecular acids and unsaturated fatty acids; the chemical formula of the micromolecular acid is CH3(CH2)nCOOH, wherein n is more than or equal to 0 and less than or equal to 3 and is an integer; the unsaturated fatty acid has a chemical formula of CH3(CH2)aHC＝CH(CH2)bCOOH or CH3(CH2)cHC＝CH(CH2)dHC＝CH(CH2)eCOOH or CH3(CH2)fHC＝CH(CH2)gHC＝CH(CH2)hHC＝CH(CH2)iCOOH, wherein a is more than or equal to 1 and less than or equal to 10, b is more than or equal to 1 and less than or equal to 10, c is more than or equal to 1 and less than or equal to 5, d is more than or equal to 1 and less than or equal to 5, e is more than or equal to 1 and less than or equal to 5, f is more than or equal to 1 and less than or equal to 4, g is more than4. I is more than or equal to 1 and less than or equal to 4, and a, b, c, d, e, f, g, h and i are integers;
the exothermic agent in the step (1) is one or a combination of more of sodium oxide, potassium oxide, calcium oxide and phosphorus pentoxide.
2. The water-based carbon nanofluid as claimed in claim 1, wherein the pH of the alkaline aqueous solution is 8 to 12.
3. The water-based carbon nanofluid of claim 2, wherein the solute in the alkaline aqueous solution is one or a combination of sodium hydroxide, potassium hydroxide, sodium carbonate, ammonium monohydrate, sodium phosphate, sodium monohydrogen phosphate, sodium dihydrogen phosphate, potassium monohydrogen phosphate, and potassium dihydrogen phosphate.
4. The water-based carbon nanofluid according to claim 1, wherein the dispersed particle size of the carbon nanoparticles is 5 to 20 nm.
5. The water-based carbon nanofluid according to claim 1, wherein in the step (2), the washing and separation of the solid in the first reaction system are repeated 4 times, the separation is performed by centrifugation or filtration, and the drying is performed under vacuum at 50 ℃ for 24 hours.
6. The method of any one of claims 1 to 5, wherein the aqueous-based carbon nanofluid is obtained by dispersing the carbon nanoparticles in the above ratio in an alkaline aqueous solution having a pH of 8 to 12.
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