Method for improving recovery efficiency of low-permeability sandstone reservoir through P-Dots nano-composite fluid

Abstract

The invention belongs to the technical field of low-permeability reservoir oil recovery, and particularly relates to a method for improving the recovery efficiency of a low-permeability sandstone reservoir through P-Dots nano-composite fluid. The preparation method comprises the following steps: dissolving a conjugated polymer in an organic solvent to obtain a mixed solution, then adding an inorganic solvent into the mixed solution under an ultrasonic condition, and carrying out conjugated polymer self-assembly to obtain P-Dots nanoparticles; then dispersing the P-Dots nano-particles in water to prepare a P-Dots nano mother solution, and dissolving a zwitterionic surfactant in water to prepare a zwitterionic surfactant mother solution; dropwise adding the P-Dots nano mother liquor into the zwitterionic surfactant mother liquor under a stirring condition, and stirring after dropwise adding is finished, so as to prepare the nano fluid with the salt resistance. The nano-particles with the particle size of 20 nm prepared by the invention have smaller particle size, and after being prepared into nano-fluid with salt resistance, the nano-fluid is diluted, dissolved and dispersed before being used, the nano-fluid can better enter a reservoir pore space and fully act with a rock surface, so that the wettability of the reservoir is improved, the resistance of fluid flow can be reduced, and the stability of the fluid is improved. Therefore, the recovery efficiency of the low-permeability reservoir is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of oil recovery in low-permeability reservoirs, and more specifically relates to a method for improving the recovery rate of low-permeability sandstone reservoirs using P-Dots nanocomposite fluid. Background Art

[0002] With the improvement of exploration level and the continuous growth of demand for oil and gas resources, whether from the perspective of the potential of remaining oil and gas resources or future development trends, low permeability reservoirs will be the main target of future oil and gas exploration and development, and the mainstream and inevitable trend of future oil and gas industry development. Therefore, improving the recovery rate of low permeability reservoirs and stabilizing or even increasing crude oil production have great practical and strategic significance.

[0003] Low permeability reservoirs are in urgent need of enhanced oil recovery technology due to their low primary and secondary recovery rates. With the development of nanotechnology, nanofluids prepared from nanoparticles have gradually been applied to the field of oil development. From the perspective of matching the pore throat size of the reservoir with the particle size of the oil displacement agent, the development of low permeability reservoirs requires small-particle nanofluids. At the same time, the formation water in the reservoir has a certain degree of mineralization. The nanofluids used here need to have a certain temperature and salt resistance, and the rapid preparation of nanoparticles is conducive to promoting the industrial application of nanofluids. Therefore, how to quickly prepare small-particle nanofluids with certain temperature and salt resistance, and then use them for the development of low permeability sandstone reservoirs has become a difficult problem that technicians in this field urgently need to overcome. Summary of the invention

[0004] The purpose of the present invention is to provide a method for improving the recovery rate of low permeability sandstone reservoirs by using P-Dots nanocomposite fluid, so as to solve the problems existing in the above-mentioned prior art.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] One of the technical solutions of the present invention is to provide a method for preparing P-Dots nanoparticles, comprising the following steps:

[0007] dissolving the conjugated polymer in an organic solvent to obtain a mixed solution;

[0008] An inorganic solvent is added to the mixed solution under ultrasonic conditions, and the P-Dots nanoparticles are obtained through self-assembly of the conjugated polymer.

[0009] The preparation method of P-Dots nanoparticles provided by the present invention is based on nanoprecipitation synthesis, wherein a conjugated polymer is first dissolved in a "good" solvent (a "good" solvent is a solvent that easily dissolves the conjugated polymer, usually an organic solvent), and an excess of a "bad" solvent (a "bad" solvent is a solvent that does not easily dissolve the conjugated polymer, usually an inorganic solvent) is added under ultrasonic dispersion, and then the conjugated polymer is self-assembled to obtain P-Dots nanoparticles of ~20nm.

[0010] Furthermore, the conjugated polymer includes at least two of polybutylene terephthalate (PFBT), styrene-methyl acrylate (PSMA), polybutylene terephthalate (PBT), polyoxyethylene acrylate (PCT), polyethylene terephthalate (PET) and polyethylene naphthalate (PEN).

[0011] Optionally, the conjugated polymer is polybutylene terephthalate (PFBT) and styrene-methyl acrylate (PSMA) in a mass ratio of 0.1-5:0.1-5.

[0012] The conjugated polymer used in the present invention contains an ester group, and the ester group is easy to hydrolyze, and a carboxylic acid group is generated after the hydrolysis.

[0013] Furthermore, the organic solvent includes at least one of tetrahydrofuran (THF), ethanol, acetone and chloroform.

[0014] Optionally, the organic solvent is tetrahydrofuran (THF).

[0015] Furthermore, the inorganic solvent is water.

[0016] The step of adding the inorganic solvent to the mixed solution is not specifically limited, and the inorganic solvent may be sucked into the pipette and then directly added to the mixed solution, or the step may be completed within 1-5 seconds.

[0017] Furthermore, the usage ratio of the conjugated polymer, the organic solvent and the inorganic solvent is 0.2-10 mg: 2-10 mL: 5-30 mL.

[0018] Furthermore, the power of the ultrasound is 350-1000W.

[0019] Furthermore, the self-assembly further includes the steps of removing the organic solvent and concentrating.

[0020] Optionally, the temperature for removing the organic solvent is 40-50°C.

[0021] Optionally, the concentration temperature is 60-80°C.

[0022] Optionally, the concentration is to concentrate until the concentration of the P-Dots nanoparticles is 1-5 wt.%.

[0023] The second technical solution of the present invention is to provide a P-Dots nanoparticle, wherein the P-Dots nanoparticle is prepared by the above-mentioned preparation method.

[0024] The third technical solution of the present invention is to provide a method for preparing a nanofluid with salt resistance, the steps comprising:

[0025] Dispersing the above-mentioned P-Dots nanoparticles in water to prepare a P-Dots nano mother solution;

[0026] dissolving a zwitterionic surfactant in water to prepare a zwitterionic surfactant mother solution;

[0027] The P-Dots nano mother solution is added dropwise into the zwitterionic surfactant mother solution under stirring conditions, and stirred after the dropwise addition is completed to obtain the nanofluid with salt resistance.

[0028] The nanofluid with salt resistance prepared by the present invention is diluted, dissolved and dispersed to obtain a P-Dots nanocomposite fluid, which can be used to exploit reservoir crude oil.

[0029] Furthermore, the concentration of the P-Dots nano mother solution is 0.5-3wt.%.

[0030] Furthermore, the zwitterionic surfactant includes at least one of a betaine-type surfactant, an amino acid-type surfactant, a phosphate-type surfactant, and a sulfonate-type surfactant.

[0031] Furthermore, the concentration of the zwitterionic surfactant mother solution is 0.5-3% v / v.

[0032] Furthermore, the volume ratio of the P-Dots nano mother solution to the zwitterionic surfactant mother solution is 1:1-10.

[0033] Furthermore, the stirring time after the dropwise addition is 20-60 minutes.

[0034] Nanofluids are prepared based on the electrostatic force between positive and negative charges and the steric hindrance between surfactant molecules adsorbed on the surface of nanoparticles. Since P-Dots nanoparticles are negatively charged, positively charged surfactants are selected for preparation. At the same time, considering that sandstone reservoirs are negatively charged, the adsorption loss of cationic surfactants in the formation is large, so zwitterionic surfactants are selected for the preparation of nanofluids.

[0035] The fourth technical solution of the present invention is to provide a nanofluid with salt resistance, wherein the nanofluid with salt resistance is prepared by the above-mentioned preparation method.

[0036] The fifth technical solution of the present invention is to provide an application of the above-mentioned P-Dots nanoparticles or the above-mentioned nanofluid with salt resistance in the development of low permeability oil reservoirs.

[0037] The present invention discloses the following technical effects:

[0038] In low permeability reservoirs, the pore throats between rock particles are relatively small, and nanoparticles with larger particle sizes are prone to block the pore throats, hindering the seepage of oil displacement agents in the formation, and affecting the development effect. The nanoparticles of 20 nm prepared by the present invention have a smaller particle size and can better enter the pore space of the reservoir, fully interact with the rock surface, improve the wettability of the reservoir, and reduce the resistance to fluid flow, thereby improving the recovery rate of low permeability reservoirs.

[0039] The P-Dots nanoparticles prepared by the present invention are negatively charged, and one end of the zwitterionic surfactant is positively charged. Based on the electrostatic force between the positive and negative charges, a large number of zwitterionic surfactant molecules can be adsorbed on the surface of the P-Dots nanoparticles. Since one end of the zwitterionic surfactant molecule is bonded to the surface of the P-Dots nanoparticle and the other end is negatively charged, the zwitterionic surfactant molecules adsorbed on the surface of the P-Dots nanoparticles repel each other on the one hand, thereby preventing the molecules from approaching each other. On the other hand, due to the steric hindrance, the probability of precipitation due to collision between the P-Dots nanoparticles is suppressed, thereby enhancing the stability of the nano system.

[0040] The nanofluid prepared based on P-Dots nanoparticles and zwitterionic surfactants has better ability to improve reservoir wettability, higher recovery rate than water flooding and surfactant flooding, and has the ability to improve the development effect of low permeability oil reservoirs. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0042] Figure 1 is the particle size of the P-Dots nanoparticles prepared in Example 1.

[0043] Figure 2 This is the potential distribution of the P-Dots nanoparticles prepared in Example 1.

[0044] Figure 3 Particle size variation of nanofluids with salt tolerance.

[0045] Figure 4 The change of wetting angle with different P-Dots nanoparticle concentrations.

[0046] Figure 5 This is the recovery factor change curve during the core flooding experiment.

[0047] Figure 6 This is the pressure change curve during the core flooding experiment. DETAILED DESCRIPTION

[0048] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0049] It should be understood that the terms described in the present invention are only for describing a particular embodiment and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0050] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of a conflict with any incorporated document, the content of this specification shall prevail.

[0051] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to those skilled in the art. The present invention description and examples are exemplary only.

[0052] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0053] Low permeability sandstone core (permeability less than 50×10 -3 μm 2) After vacuum saturating the formation water and then saturating the crude oil, the pore volume (PV) of the core is determined, and the prepared nanocomposite fluid is used to displace the crude oil in the low permeability sandstone core at the reservoir temperature, which can greatly improve the crude oil recovery factor.

[0054] However, commercial solid nanoparticles sold on the market are difficult to disperse, and commercial liquid nanoparticles contain a large amount of stabilizers and dispersants with a low effective content. Therefore, the present invention dissolves a conjugated polymer in an organic solvent to obtain a mixed solution; adds an inorganic solvent to the mixed solution under ultrasonic conditions, and obtains P-Dots nanoparticles with higher nanoparticle purity through self-assembly of the conjugated polymer.

[0055] In some specific embodiments, the conjugated polymer includes at least two of polybutylene terephthalate (PFBT), styrene-methyl acrylate (PSMA), polybutylene terephthalate (PBT), polyoxyethylene acrylate (PCT), polyethylene terephthalate (PET) and polyethylene naphthalate (PEN); preferably polybutylene terephthalate (PFBT) and styrene-methyl acrylate (PSMA), with a mass ratio of 0.1-5:0.1-5.

[0056] In some specific embodiments, the organic solvent includes at least one of tetrahydrofuran (THF), ethanol, acetone and chloroform; preferably tetrahydrofuran (THF).

[0057] In some specific embodiments, the inorganic solvent is generally water, which can be distilled water or deionized water.

[0058] In some specific embodiments, the step of adding the inorganic solvent to the mixed solution is not specifically limited, and the inorganic solvent can be directly added to the mixed solution after being sucked into the pipette, or it can be completed within 1-5 seconds.

[0059] In some specific embodiments, the ratio of the conjugated polymer, the organic solvent and the inorganic solvent is 0.2-10 mg: 2-10 mL: 5-30 mL.

[0060] In some specific embodiments, the power of the ultrasound is 350-1000W.

[0061] In some specific embodiments, the self-assembly further comprises the steps of removing the organic solvent and concentrating, wherein the temperature for removing the organic solvent is 40-50°C, the temperature for concentrating is 60-80°C, and the concentration is to concentrate to a concentration of 1-5 wt.% of the P-Dots nanoparticles.

[0062] The synthesis of nanoparticles includes physical method, chemical method, biological method and physical and chemical method. Most of these methods have complicated preparation processes and require a specific synthesis environment. The synthesis method of P-Dots nanoparticles in this application is very simple, which can be simply summarized as three steps of dissolution, reaction and evaporation. The reaction equipment involved in the whole preparation stage is very few and very conventional, only an ultrasonic instrument and a heating stirrer are needed, no special reaction conditions are required, and heating is only required in the final purification stage. It takes a short time and the preparation of nanoparticles can be completed in about 2 hours.

[0063] If the nanoparticles are directly dissolved in simulated formation water, the charges on the surface of the nanoparticles are neutralized by the ions in the simulated formation water, resulting in a reduction in the repulsive force between the nanoparticles and causing agglomeration. Therefore, compared with the direct contact of the nanoparticles with simulated formation water, the method of preparing the nanocomposite mother solution is adopted to ensure that the zwitterionic surfactant is adsorbed on the surface of the P-Dots nanoparticles through electrostatic force. The other end of the surfactant molecules adsorbed on the surface of the nanoparticles has a negative charge. On the one hand, these surfactant molecules repel each other due to the same charge, and on the other hand, due to the steric hindrance effect, the two together enhance the stability of the nanocomposite system. Then, the simulated formation water is added to make the prepared P-Dots nanocomposite fluid more stable. Specifically:

[0064] The P-Dots nanoparticles are dispersed in distilled water or deionized water to obtain a P-Dots nano mother liquor; a zwitterionic surfactant is dissolved in distilled water or deionized water to obtain a zwitterionic surfactant mother liquor; the P-Dots nano mother liquor is added dropwise to the zwitterionic surfactant mother liquor under stirring conditions, and the mixture is stirred after the addition is completed to obtain the nanofluid with salt resistance.

[0065] After the nanofluid with salt resistance is diluted, dissolved and dispersed, the P-Dots nanocomposite fluid is obtained, which can be used to exploit reservoir crude oil.

[0066] In some specific embodiments, the concentration of the P-Dots nanoparticle solution is 0.5-3% v / v.

[0067] In some specific embodiments, the zwitterionic surfactant includes at least one of an amino acid-type surfactant, a phosphate-type surfactant, and a sulfonate-type surfactant.

[0068] In some specific embodiments, the concentration of the zwitterionic surfactant mother solution is 0.5-3 wt.%.

[0069] In some specific embodiments, the volume ratio of the P-Dots nano mother solution to the zwitterionic surfactant mother solution is 1:1-10.

[0070] In some specific embodiments, the stirring time after the dropwise addition is 20-60 minutes, so that the zwitterionic surfactant molecules are fully adsorbed onto the surface of the P-Dots nanoparticles.

[0071] From the perspective of matching the reservoir pore throat size with the oil displacement agent particle size, the development of low permeability reservoirs requires nanofluids with small particle sizes. Therefore, the present invention synthesizes the above-mentioned nanoparticles of 20 nm for preparing nanofluids.

[0072] The raw materials and reagents used in the specific embodiments of the present invention are all commercially available products.

[0073] Unless otherwise specified, the "room temperature" and "normal temperature" referred to in the specific embodiments of the present invention refer to 20-30°C.

[0074] It should be pointed out that the matters not described in detail in the present invention are conventional operating means in the art and are not the focus of the present invention.

[0075] Example 1

[0076] Preparation of P-Dots nanoparticles:

[0077] 1 mg PFBT and 0.2 mg PSMA were dissolved in 10.0 mL THF to obtain a mixed solution, and then 20.0 mL deionized water was added to the mixed solution under ultrasonic conditions (500 W). After the addition was completed, ultrasonic treatment was continued for 2 minutes, and then the organic solvent THF was evaporated at 50° C. to obtain P-Dots nanoparticles.

[0078] The above P-Dots nanoparticles were evenly dispersed in distilled water, and their particle size and potential distribution were detected. The results are as follows: Figure 1 shown.

[0079] Figure 1 is the particle size of the P-Dots nanoparticles prepared in Example 1. As can be seen from the figure, the average hydrodynamic diameter of the P-Dots nanoparticles in distilled water measured by DLS is 20.1±2.6 nm, which can ensure that the nanoparticles can freely enter and exit the pore throats of low permeability reservoirs.

[0080] Figure 2 The potential distribution of the P-Dots nanoparticles prepared in Example 1. As shown in the figure, the zeta potential of the distilled water P-Dots nanoparticles is -33.2±3.1 mV, which confirms that the surface of the P-Dots nanoparticles is negatively charged.

[0081] Example 2

[0082] Preparation of P-Dots nanoparticle concentrate:

[0083] 1 mg PFBT and 0.2 mg PSMA were dissolved in 10.0 mL THF to obtain a mixed solution, and then 20.0 mL deionized water was added to the mixed solution under ultrasonic conditions (500 W). After the addition was completed, ultrasonic treatment was continued for 2 minutes, and the organic solvent THF was evaporated at 50°C, and then concentrated at 70°C to obtain a 1 wt.% P-Dots nanoparticle concentrate.

[0084] Example 3

[0085] The steps for preparing the nanofluid with salt tolerance include:

[0086] S1, adding distilled water to the P-Dots nanoparticle concentrate prepared in Example 2 or the P-Dots nanoparticles prepared in Example 1 to prepare a 0.5 wt. % P-Dots nano mother solution;

[0087] S2, dissolving a zwitterionic surfactant (betaine-type surfactant) in distilled water to prepare a 0.5% v / v zwitterionic surfactant mother solution;

[0088] S3. Under stirring conditions, the P-Dots nano mother liquor is added dropwise to the zwitterionic surfactant mother liquor. After the addition is completed, stirring is continued for 40 minutes to prepare a nanofluid with salt resistance, wherein the volume ratio of the P-Dots nano mother liquor to the zwitterionic surfactant mother liquor is 1:4.

[0089] Test Example 1

[0090] The salt-resistant nanofluid prepared in Example 3 was mixed with simulated formation water in an equal volume ratio to prepare P-Dots nanocomposite fluid, which was divided into two groups and placed in a constant temperature oven at 60°C and 80°C, respectively. The particle size of the samples was measured every five days to monitor whether the nanofluid aggregated under high temperature and formation water mineralization conditions. The composition of the simulated formation water is shown in Table 1. The particle size change of the nanofluid within 30 days is shown in Table 1. Figure 3 shown.

[0091] Table 1 Simulated formation water composition

[0092]

[0093] Figure 3The particle size change of the nanofluid with salt tolerance is shown in the figure. It can be seen that after the first five days, the particle size of the nanofluid increased from 22.1±2.2nm to 24.0±1.7nm at 60℃ and from 24.1±1.5nm to 27.4±1.6nm at 80℃, which increased by about 9.1% and 14.2% respectively compared with the original size. However, no obvious aggregation or sedimentation was observed. After that, the particle size remained stable at both temperatures, which were 26.4±2.4nm (60℃) and 29.6±2.6nm (80℃). Therefore, the nanofluid prepared by the present invention is sufficiently stable under higher temperature and formation water salinity conditions and can be used for further EOR applications.

[0094] Test Example 2

[0095] In the related field of petroleum engineering, the sessile drop method is used to determine the contact angle of the oil phase between crude oil in a test solution and a rock sample slice.

[0096] The core slices had a diameter of 0.3 cm and a length of 0.1 cm. Before testing, the core slices were aged in crude oil at 80 °C for 10 days to ensure that the rock surface was wetted by the oil.

[0097] The salt-resistant nanofluid was mixed with different amounts of simulated formation water in an equal volume ratio to prepare P-Dots nanocomposite fluids containing different concentrations (wt.%) of P-Dots nanoparticles, wherein the concentration of the P-Dots nanoparticles was 0.00wt.% (adding an equal amount of betaine-type surfactant), 0.01wt.%, 0.02wt.%, 0.03wt.%, 0.04wt.%, 0.05wt.%, 0.06wt.%, 0.07wt.%, 0.08wt.%, 0.09wt.%, and 0.1wt.%.

[0098] The P-Dots nanocomposite fluid was heated to 60°C, and the three-phase wetting angle between the core / P-Dots nanocomposite fluid / crude oil was tested using a wetting angle tester. Figure 4 shown.

[0099] Figure 4The change of wetting angle for different P-Dots nanoparticle concentrations. As can be seen from the figure, when there is only surfactant solution and no nanoparticles, the contact angle is 100.5°, indicating that the betaine surfactant alone can change the oil-wet formation to moderate wetting. When the concentration of P-Dots nanoparticles is lower than 0.06%, the contact angle of the nanocomposite fluid continues to increase rapidly with the increase of the concentration of P-Dots nanoparticles. In the range of 0.06% to 0.10% of the concentration of P-Dots nanoparticles, the contact angle increases slowly. Finally, after using 0.08% concentration of nanoparticles, the oil phase contact angle slowly reaches 115.8°, indicating that water phase wetting is achieved. It can be seen that P-Dots nanoparticles play an important role in this wettability transition.

[0100] Test Example 3

[0101] The nanofluid with salt resistance prepared in Example 3 was mixed with simulated formation water in an equal volume ratio to prepare a P-Dots nanocomposite fluid.

[0102] The gas permeability is 20×10 -3 μm 2 The sandstone core (size and related physical properties are shown in Table 2) was vacuum-saturated with simulated formation water (the ion composition in the simulated formation water is shown in Table 1) and then saturated with crude oil to determine the pore volume (PV) of the core. The core was then aged at 60°C reservoir conditions for 24 hours before a core flooding experiment was conducted. The flooding scheme was as follows:

[0103] 1PV simulated formation water drive + 0.5PV chemical drive (zwitterionic surfactant drive or P-Dots nanocomposite fluid drive) + 1PV subsequent water drive, where the C1 core was driven by zwitterionic surfactant and the C2 core was driven by P-Dots nanocomposite fluid. The recovery results of the two are shown in Table 3. The recovery curve is shown in Figure 5 The pressure change curve is shown in Figure 6.

[0104] Table 2 Core properties

[0105]

[0106]

[0107] Table 3 Recovery rate at different stages

[0108]

[0109] Figure 5 This is the recovery factor change curve during the core flooding experiment; Figure 6 This is the pressure change curve during the core flooding experiment.

[0110] From Table 2 and Table 3 and Figure 5-Figure 6 It can be seen that the P-Dots nanocomposite fluid has a significant improvement over formation water flooding and surfactant flooding, which shows that the P-Dots nanocomposite nanofluid can further improve the recovery rate of low permeability sandstone reservoirs. Figure 5 and Figure 6 In the early stage of simulated brine flooding, the oil recovery and injection pressure of C1 and C2 cores are very similar, which also shows that the various properties of the two cores are very similar, providing a basis for comparison for the subsequent chemical flooding. However, when implementing the second stage 0.5PV chemical flooding, the recovery rate and injection pressure of nanofluid flooding are higher than those of surfactant flooding, indicating that the ability of nanofluid to recover crude oil is much higher than that of surfactant alone. In the subsequent water flooding stage of the third stage, although the oil recovery rate and pressure difference trends of both floodings are slowing down, the pressure difference of C2 cores that implement nanocomposite fluid flooding is still slightly higher than that of C1 core samples that use surfactant flooding in the same step because some nanocomposites are retained in the formation.

[0111] It can be seen that P-Dots nanocomposite fluid has broad potential in improving the recovery rate of low permeability sandstone reservoirs. It is also of great significance to how to improve the temperature and salt resistance of nanofluids and the principle of nanofluids to improve oil recovery.

[0112] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0113] The above description of the disclosed embodiments enables one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing P-Dots nanoparticles, characterized in that the steps include: dissolving the conjugated polymer in an organic solvent to obtain a mixed solution; An inorganic solvent is added to the mixed solution under ultrasonic conditions, and the P-Dots nanoparticles are obtained through self-assembly of the conjugated polymer.

2. The preparation method according to claim 1, characterized in that The conjugated polymer includes at least two of PFBT, PSMA, PBT, PCT, PET and PEN; and / or the organic solvent includes at least one of tetrahydrofuran, ethanol, acetone and chloroform; and / or the inorganic solvent is water.

3. The preparation method according to claim 2, characterized in that: The conjugated polymers are PFBT and PSMA in a mass ratio of 0.1-5:0.1-5; and / or the organic solvent is tetrahydrofuran.

4. The preparation method according to claim 1, characterized in that: The usage ratio of the conjugated polymer, the organic solvent and the inorganic solvent is 0.2-10 mg: 2-10 mL: 5-30 mL; and / or, the power of the ultrasound is 350-1000 W; and / or, after the self-assembly, the steps of removing the organic solvent and concentrating are also included.

5. The preparation method according to claim 4, characterized in that: The temperature for removing the organic solvent is 40-50° C.; and / or, the temperature for concentrating is 60-80° C.; and / or, the concentration is performed until the concentration of the P-Dots nanoparticles is 1-5 wt.%.

6. A P-Dots nanoparticle, characterized in that: The P-Dots nanoparticles are prepared by the preparation method according to any one of claims 1 to 5.

7. A method for preparing a nanofluid with salt resistance, characterized in that the steps include: Dispersing the P-Dots nanoparticles according to claim 6 in water to prepare a P-Dots nano mother solution; dissolving a zwitterionic surfactant in water to prepare a zwitterionic surfactant mother solution; The P-Dots nano mother solution is added dropwise into the zwitterionic surfactant mother solution under stirring conditions, and stirred after the dropwise addition is completed to obtain the nanofluid with salt resistance.

8. The preparation method according to claim 7, characterized in that: The concentration of the P-Dots nano mother solution is 0.5-3wt.%; and / or, the zwitterionic surfactant includes at least one of a betaine-type surfactant, an amino acid-type surfactant, a phosphate-type surfactant and a sulfonate-type surfactant; and / or, the concentration of the zwitterionic surfactant mother solution is 0.5-3% v / v; and / or, the volume ratio of the P-Dots nano mother solution to the zwitterionic surfactant mother solution is 1:1-10; and / or, the stirring time after the dropwise addition is 20-60min.

9. A nanofluid with salt resistance, characterized in that: The nanofluid with salt resistance is prepared by the preparation method described in claim 7 or 8.

10. Use of the P-Dots nanoparticles according to claim 6 or the nanofluid with salt tolerance according to claim 9 in the development of low permeability oil reservoirs.

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