Temperature-sensitive nano material, preparation method thereof, oil-displacing agent and application of oil-displacing agent

Abstract

The invention provides a temperature-sensitive nano-material and a preparation method thereof as well as an oil-displacing agent and application thereof, the temperature-sensitive nano-fluid comprises nano-silica and a polymer grafted on the surface of the nano-silica, and the polymer is formed by polymerizing a temperature-sensitive monomer and a vinyl hydrophilic monomer, the temperature-sensitive type monomer comprises isopropyl acrylamide and / or dimethylamino ethyl methacrylate. The temperature-sensitive nano material has excellent dispersity, thermal stability and temperature response characteristics, has good adaptability at the environment temperature of the low-permeability oil reservoir, and can improve the recovery rate of the low-permeability oil reservoir.

Description

Technical Field

[0001] The invention relates to the technical field of oil displacement agents used for oil recovery in oil reservoirs, and in particular to a temperature-sensitive nano material and a preparation method thereof, as well as an oil displacement agent and applications thereof. Background Art

[0002] Low permeability reservoirs refer to reservoirs with low permeability and low oil production. Generally, reservoirs with a permeability of less than 50mD are collectively referred to as low permeability reservoirs. Among them, reservoirs with an average permeability of 10-50mD are general low permeability reservoirs, reservoirs with an average permeability of 1-10mD are extra-low permeability reservoirs, and reservoirs with an average permeability of less than 1mD are ultra-low permeability reservoirs. Low permeability reservoirs have complex microscopic pore structures, and the problem of "injection failure" is often encountered in tertiary oil recovery, resulting in unsatisfactory recovery results, low oil displacement efficiency, and an average recovery rate of only about 20%. Most of the crude oil has not been recovered and remains in the reservoir.

[0003] Compared with conventional chemical flooding, the size of nanomaterials is at the nanometer level, which matches the pore throat size of low permeability layers (low permeability reservoirs) and can adapt to the pore size of low permeability reservoirs. Therefore, nanofluid flooding using nanofluids formed by nanomaterials as oil displacement agents can be applied to oil production in low permeability reservoirs. In recent years, nanosilicon dioxide particles, as a nanomaterial, have the characteristics of high surface energy, strong rigidity, and high thermal stability. They have been widely used in drilling and production development, oil recovery and other related fields. Nanofluid flooding using nanofluids formed by nanosilicon dioxide particles has broad application prospects in the field of improving the recovery rate of low permeability reservoirs.

[0004] However, when nano-silica is used as an oil displacement agent, there are generally problems such as easy agglomeration of silica, poor dispersibility, and large particle size, which are not conducive to infiltration into low permeability reservoirs, and the recovery rate of low permeability reservoirs is low. In addition, as the underground depth of low permeability reservoirs increases, the formation temperature will gradually rise. The change in formation temperature plays a vital role in the formation, migration and aggregation of oil and gas resources. During the oil production process, the formation temperature will have a great impact on the stability of the injected fluid (the oil displacement agent injected into the low permeability reservoir) and its migration in the reservoir, and is also an important factor affecting the recovery rate.

[0005] Therefore, how to improve the dispersibility and thermal stability of nanomaterials, while improving the adaptability of nanomaterials and oil displacement agents formed by nanomaterials at the ambient temperature of low permeability reservoirs, and increasing the recovery rate of low permeability reservoirs is a technical problem that needs to be urgently solved in this field. Summary of the invention

[0006] The present invention provides a temperature-sensitive nanomaterial, which has excellent dispersibility, thermal stability and temperature response characteristics, has good adaptability at the ambient temperature of low-permeability oil reservoirs, and can significantly improve the recovery rate of low-permeability oil reservoirs.

[0007] The invention also provides a method for preparing the temperature-sensitive nano material. The preparation method has simple process and is convenient for industrial production.

[0008] The present invention also provides an oil displacing agent, which has good adaptability at the environmental temperature of the low permeability oil reservoir and further improves the recovery rate of the low permeability oil reservoir.

[0009] The present invention also provides an oil displacement method, which uses the above oil displacement agent to displace crude oil in the oil reservoir, thereby improving the recovery rate of the low permeability oil reservoir.

[0010] In a first aspect, the present invention provides a temperature-sensitive nanomaterial, comprising nano-silica and a polymer grafted onto the surface of the nano-silica, wherein the polymer is polymerized from a temperature-sensitive monomer and a vinyl hydrophilic monomer, and the temperature-sensitive monomer comprises isopropyl acrylamide and / or dimethylaminoethyl methacrylate.

[0011] The thermosensitive nanomaterial as described above, wherein the minimum critical solution temperature of the thermosensitive nanomaterial in water is 69.8° C.; and / or,

[0012] The particle size range of the temperature-sensitive nano material dispersed in water is 131-512 nm.

[0013] The thermosensitive nanomaterial as described above, wherein the vinyl hydrophilic monomer comprises one or more of methylene succinic acid, crotonic acid, and maleic acid.

[0014] The temperature-sensitive nanomaterial as described above, wherein the temperature-sensitive nanomaterial is formed by a raw material composition, and the raw material composition includes base oil, emulsifier, modified nano-silica, temperature-sensitive monomer, vinyl hydrophilic monomer, initiator and water; wherein the modified nano-silica is prepared by a grafting reaction of nano-silica and a silane coupling agent.

[0015] The temperature-sensitive nanomaterial as described above, wherein the silane coupling agent comprises γ-methacryloxypropyltrimethoxysilane; and / or,

[0016] The base oil comprises white oil; and / or,

[0017] The initiator comprises potassium persulfate; and / or,

[0018] The emulsifier includes sorbitan fatty acid ester and / or polyoxyethylene sorbitan monooleate;

[0019] Preferably, the mass ratio of the sorbitan fatty acid ester to the polyoxyethylene sorbitan monooleate is (7-9): (1-3);

[0020] Preferably, the sorbitan fatty acid ester includes one or more of Span80, Span60 and Span40;

[0021] Preferably, the polyoxyethylene sorbitan monooleate includes one or more of Tween80, Tween60 and Tween40.

[0022] In a second aspect, the present invention provides a method for preparing the above-mentioned temperature-sensitive nanomaterial, comprising the following steps:

[0023] mixing the base oil and the emulsifier to obtain an oil phase;

[0024] The modified nano-silica, the temperature-sensitive monomer, the vinyl hydrophilic monomer and water are mixed to obtain a water phase;

[0025] Mixing the aqueous phase with the oil phase to obtain an inverse microemulsion;

[0026] After the reverse microemulsion is mixed with an initiator, a polymerization reaction is carried out at 65 to 75° C. for 16 to 32 hours under an inert atmosphere to obtain the temperature-sensitive nanomaterial; wherein the modified nano-silicon dioxide is obtained by a grafting reaction of nano-silicon dioxide and a silane coupling agent.

[0027] The preparation method as described above, wherein the preparation method of the modified nano-silica comprises the following steps: activating the nano-silica at 100-110°C for 12-36 hours, mixing it with a solvent to obtain a mixed solution; then adding a silane coupling agent to the mixed solution, reacting it at 65-75°C for 10-12 hours under an inert atmosphere to obtain the modified nano-silica.

[0028] In a third aspect, the present invention provides an oil-displacing agent comprising water and the temperature-sensitive nanomaterial described in the first aspect.

[0029] The oil-displacing agent as described above, wherein the concentration of the temperature-sensitive nanomaterial in the oil-displacing agent is 0.01 wt % to 0.2 wt %.

[0030] In a fourth aspect, the present invention provides an oil displacement method, comprising the following steps: using the oil displacement agent described in the third aspect to displace crude oil in an oil reservoir.

[0031] The present invention can improve the dispersibility of the temperature-sensitive nanomaterial, avoid its agglomeration, keep its particle size small, and facilitate the adaptation to the pore throat size of the low-permeability oil reservoir, so that the oil displacement agent (nanofluid) formed by the temperature-sensitive nanomaterial can migrate and distribute more effectively in the oil reservoir, thereby improving the recovery rate of the low-permeability oil reservoir; at the same time, the polymer formed by the temperature-sensitive monomer and the vinyl hydrophilic monomer is grafted on the surface of the silicon dioxide, which can also improve the thermal stability and temperature response characteristics of the temperature-sensitive nanomaterial, so that the temperature-sensitive nanomaterial and the oil displacement agent formed by the temperature-sensitive nanomaterial have good adaptability at the environmental temperature of the low-permeability oil reservoir, and the conformational change of the oil displacement agent can be triggered by the change of the reservoir temperature, so that it can adapt to a wider temperature range, thereby further improving the recovery rate of the low-permeability oil reservoir. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is the infrared spectrum of nano-silicon dioxide (nano-SiO2) and temperature-sensitive nano-material (NIPAm / IA / SiO2) in Example 1 of the present invention.

[0033] Figure 2 This is a thermal gravimetric curve of the temperature-sensitive nanomaterial of Example 1 of the present invention.

[0034] Figure 3 This is a transmission electron microscope image of the nano-silicon dioxide in Example 1 of the present invention.

[0035] Figure 4 This is a transmission electron microscope image (100 nm) of the temperature-sensitive nanomaterial (NIPAm / IA / SiO2) of Example 1 of the present invention.

[0036] Figure 5 This is a transmission electron microscope image (20 nm) of the temperature-sensitive nanomaterial (NIPAm / IA / SiO2) of Example 1 of the present invention.

[0037] Figure 6 This is a particle size distribution diagram of nano-silicon dioxide and temperature-sensitive nano-material (NIPAm / IA / SiO2) in Example 1 of the present invention.

[0038] Figure 7 This is a curve diagram showing the change in transmittance of the temperature-sensitive nanofluid according to Example 1 of the present invention as a function of temperature.

[0039] Figure 8 This is a curve diagram of the recovery rate change of temperature-sensitive nanofluids with different concentrations in Example 1 of the present invention.

[0040] Fig. 9 This is a bar chart of the recovery rate of temperature-sensitive nanofluids with different injection volumes according to Example 1 of the present invention. DETAILED DESCRIPTION

[0041] In order to enable those skilled in the art to better understand the scheme of the present invention, the present invention is further described in detail below. The specific implementation methods listed below are only for describing the principles and features of the present invention. The examples are only used to explain the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other implementation methods obtained by ordinary technicians in the field without creative work are within the scope of protection of the present invention.

[0042] The invention provides a temperature-sensitive nano material, comprising nano silicon dioxide and a polymer grafted on the surface of the nano silicon dioxide, wherein the polymer is polymerized from a temperature-sensitive monomer and a vinyl hydrophilic monomer, wherein the temperature-sensitive monomer comprises isopropyl acrylamide and / or dimethylaminoethyl methacrylate.

[0043] Compared with unmodified nano-silica, by grafting a polymer formed by polymerization of a thermosensitive monomer and a vinyl hydrophilic monomer on the surface of nano-silica, the surface energy of the thermosensitive nanomaterial can be reduced, thereby reducing the interaction force between the particles of the thermosensitive nanomaterial, thereby improving the dispersibility of the thermosensitive nanomaterial, reducing its agglomeration phenomenon, and keeping the thermosensitive nanomaterial at a smaller particle size, with uniform particle size distribution and regular shape, which is conducive to adapting to the pore throat size of low permeability reservoirs, thereby making the oil displacement agent (nanofluid) formed by the temperature-sensitive nanomaterial more effective in migration and distribution in the reservoir, thereby improving the recovery rate of low permeability reservoirs.

[0044] At the same time, the polymer grafted on the surface of silica is polymerized by thermosensitive monomers and vinyl hydrophilic monomers, so that the polymer has thermosensitive monomer structural units and vinyl hydrophilic monomer structural units. Under such a polymer structural system, on the one hand, the vinyl hydrophilic monomer structural unit contains hydrophilic functional groups, and the hydrophilic functional groups can form hydrogen bonds with water molecules, thereby giving the thermosensitive nanomaterial good hydrophilicity and wettability, making it uniformly dispersed in water, and further improving the recovery rate of the reservoir. On the other hand, the polymer contains thermosensitive monomer structural units, which can give the thermosensitive nanomaterial excellent temperature response characteristics. Specifically, the thermosensitive nanomaterial has a minimum critical solution temperature (LCST). When in a temperature environment lower than the LCST, the nitrogen-containing and oxygen-containing functional groups in the polymer, that is, the nitrogen-containing functional groups (amide groups) in the thermosensitive monomer structural units and the oxygen-containing functional groups (carboxyl groups and hydroxyl groups) from the vinyl hydrophilic monomer structural units can form hydrogen bonds with water molecules, so that the hydrophilicity of the polymer molecular chain on the surface of the thermosensitive nanomaterial is enhanced, thereby making the thermosensitive nanomaterial have It has excellent hydrophilicity and can be evenly dispersed in water, thereby changing the wettability of the rock surface in low permeability reservoirs, increasing capillary force, promoting imbibition, allowing more oil to be displaced and improving the recovery rate of the reservoir; when in a temperature environment higher than LCST, the hydrogen bonds formed between at least part of the nitrogen-containing amide groups in the polymer and water are destroyed, and the hydrophobic effect of the hydrophobic group isopropyl in the polymer becomes more obvious, which reduces the hydrophilicity of the thermosensitive nanomaterial. The thermosensitive nanomaterial has both hydrophilicity and lipophilicity, improves the fluidity of the oil-water phase, and improves the oil recovery efficiency.

[0045] Therefore, when the oil displacing agent (or nanofluid, nano oil displacing agent) formed by the above-mentioned temperature-sensitive nanomaterial is applied to a low permeability oil reservoir, the temperature-sensitive nanomaterial and the oil displacing agent formed by it have temperature response characteristics, have good adaptability at the ambient temperature of the low permeability oil reservoir, and can trigger the conformational change of the oil displacing agent by changes in the reservoir temperature, thereby improving the recovery rate of the low permeability oil reservoir.

[0046] In addition, the above-mentioned temperature-sensitive nanomaterials use nano-silicon dioxide as a matrix, which can give the temperature-sensitive nanomaterials excellent thermal stability and further improve their adaptability in low-permeability oil reservoir environments.

[0047] In some embodiments, the minimum critical solution temperature LCST of the thermosensitive nanomaterial in water is 69.8° C., which is beneficial for the thermosensitive nanomaterial to adapt to the ambient temperature of the low permeability reservoir and improve the recovery rate of the low permeability reservoir.

[0048] According to further research by the inventors, after the above-mentioned thermosensitive nanomaterials are dispersed in water, the transmittance of the obtained oil-displacing agent is high and relatively stable in the temperature range below LCST within 25°C-95°C (the transmittance of the oil-displacing agent does not change much), and the dispersion is clear; when the temperature is higher than LCST, the transmittance of the oil-displacing agent drops rapidly and becomes turbid. Therefore, the thermosensitive nanomaterial has excellent temperature response characteristics and is suitable for low-permeability reservoir environments. When the temperature is lower than LCST, it can change the wettability of the rock surface, increase capillary force, promote the occurrence of imbibition, and allow more oil to be displaced. When the temperature is higher than LCST, the thermosensitive nanomaterial has both hydrophilicity and lipophilicity, improves the fluidity of the oil-water phase, and improves the oil displacement efficiency.

[0049] In specific implementation, a UV-visible spectrophotometer connected to a thermostatic bath may be used to test the transmittance of the dispersion and its change with temperature.

[0050] In addition, the temperature of the thermosensitive nanomaterial was increased from room temperature (25°C) to 800°C at a heating rate of 10±2°C / min to perform thermogravimetric analysis (TGA) on the thermosensitive nanomaterial. The weight loss rate of the thermosensitive nanomaterial at 25-310°C was 4% to 4.5%, and the weight loss at this stage was mainly due to the evaporation of free water and crystal water adsorbed on the surface of the thermosensitive nanomaterial. The weight loss rate of the thermosensitive nanomaterial at 310-800°C was 32% to 33%, and the weight loss at this stage was due to the thermal decomposition of the thermosensitive nanomaterial. The thermosensitive nanomaterial has excellent thermal stability and can maintain good thermal stability in the ambient temperature of the low permeability reservoir. It is not easy to thermally decompose, thereby further improving the recovery rate of the low permeability reservoir.

[0051] In some embodiments, the particle size range of the thermosensitive nanomaterial dispersed in water is 131 to 512 nm, and the median particle size is 264 nm. The thermosensitive nanomaterial has good dispersibility in water, which is beneficial to its effective migration and distribution in low permeability reservoirs, thereby improving the recovery rate of low permeability reservoirs.

[0052] In the embodiment of the present invention, the temperature-sensitive nanomaterial can be synthesized by a reverse microemulsion method.

[0053] Specifically, the temperature-sensitive nanomaterial can be formed by a raw material composition, which may include base oil, emulsifier, modified nano-silica, temperature-sensitive monomer, vinyl hydrophilic monomer, and initiator; wherein the modified nano-silica is prepared by modifying nano-silica with a silane coupling agent, which can improve the thermal stability, dispersibility and temperature response characteristics of the temperature-sensitive nanomaterial and increase the recovery rate of low permeability oil reservoirs.

[0054] In some embodiments, the base oil may include white oil, which has stable chemical properties and is not easy to react with reactants such as temperature-sensitive monomers and vinyl hydrophilic monomers, which is conducive to the formation of a stable reverse microemulsion. At the same time, white oil, as a continuous phase in the emulsification system (i.e., a raw material composition containing an emulsifier), can provide mild reaction conditions, make the polymerization reaction stable and controllable, reduce the occurrence of side reactions, further improve the properties of the temperature-sensitive nanomaterials, and increase the recovery rate.

[0055] In some embodiments, the initiator includes potassium persulfate, which decomposes under the above-mentioned polymerization reaction conditions to generate free radicals, which initiate the polymerization reaction of the thermosensitive monomer and the vinyl hydrophilic monomer on the surface of the modified silica, and promote the grafting reaction of the polymer formed by the polymerization of the thermosensitive monomer and the vinyl hydrophilic monomer on the surface of the modified silica.

[0056] In some embodiments, the emulsifier includes sorbitan fatty acid ester and / or polyoxyethylene sorbitan monooleate, that is, the emulsifier can be sorbitan fatty acid ester, can also be polyoxyethylene sorbitan monooleate, and can also be a combination of sorbitan fatty acid ester and polyoxyethylene sorbitan monooleate.

[0057] In some specific embodiments, the emulsifier includes sorbitan fatty acid ester and polyoxyethylene sorbitan monooleate. The hydrophilic-lipophilic balance (HLB) value of sorbitan fatty acid ester is relatively low, tending to be lipophilic, while the HLB value of polyoxyethylene sorbitan monooleate is relatively high, tending to be hydrophilic. The use of such a mixed emulsifier can not only effectively disperse the oil phase, but also stabilize the water phase, thereby achieving a better emulsification effect, further improving the preparation efficiency of the temperature-sensitive nanomaterial, and further improving the thermal temperature, dispersibility and temperature response characteristics of the temperature-sensitive nanomaterial, thereby improving the recovery rate of low permeability oil reservoirs.

[0058] In some embodiments, the mass ratio of sorbitan fatty acid ester and polyoxyethylene sorbitan monooleate can be (7-9): (1-3), for example, it can be 7:1, 7:2, 7:3, 8:1, 8:2, 8:3, 9:1, 9:2, 9:3, which is beneficial to improve the stability of the reaction system, further improve the preparation efficiency of the temperature-sensitive nanomaterials, and make the temperature-sensitive nanomaterials have better thermal temperature properties, dispersibility and temperature response characteristics.

[0059] Furthermore, the sorbitan fatty acid ester includes one or more of Span80, Span60, and Span40, and the polyoxyethylene sorbitan monooleate may include one or more of Tween80, Tween60, and Tween40, which helps to improve the compatibility of sorbitan fatty acid ester, polyoxyethylene sorbitan monooleate with components such as temperature-sensitive monomers and vinyl hydrophilic monomers.

[0060] In some embodiments, the silane coupling agent includes γ-methacryloxypropyltrimethoxysilane (KH570). The methoxy (or ethoxy) in KH570 is hydrolyzed to generate silanol (Si-OH), and the silanol group reacts with the hydroxyl (Si-OH) group on the surface of the nano-silicon dioxide to form a covalent bond. At the same time, the acryloxy group of KH570 can further react with the unsaturated bond or functional group in the polymer formed by the polymerization of the thermosensitive monomer and the vinyl hydrophilic monomer, so that the nano-silicon dioxide and the polymer matrix formed by the polymerization of the thermosensitive monomer and the vinyl hydrophilic monomer are more firmly combined, thereby improving the stability and temperature responsiveness of the thermosensitive nanomaterial.

[0061] In some embodiments, the vinyl hydrophilic monomer includes one or more of methylene succinic acid, crotonic acid, and maleic acid, which is beneficial to improving the hydrophilicity of the thermosensitive nanomaterial and its dispersibility in water. At the same time, it cooperates with the thermosensitive monomer to improve the temperature response characteristics of the thermosensitive nanomaterial, thereby making the thermosensitive nanomaterial suitable for low permeability reservoirs and improving the recovery rate of low permeability reservoirs.

[0062] The present invention also provides a method for preparing the above-mentioned temperature-sensitive nanomaterial, comprising the following steps: mixing a base oil and an emulsifier to obtain an oil phase; mixing modified nano-silicon dioxide, a temperature-sensitive monomer, a vinyl hydrophilic monomer, and water to obtain an aqueous phase; mixing the aqueous phase with the oil phase to obtain an inverse microemulsion (i.e., a W / O (water-in-oil) inverse microemulsion); mixing the inverse microemulsion with an initiator, and reacting at 65 to 75° C. for 16 to 32 hours under an inert atmosphere to obtain the temperature-sensitive nanomaterial. Among them, the modified nano-silicon dioxide is obtained by grafting nano-silicon dioxide and a silane coupling agent.

[0063] In some embodiments, the preparation method of modified nano-silica includes the following steps: activating the nano-silica at 100-110°C for 12h-36h, mixing it with a solvent to obtain a mixed solution; then adding a silane coupling agent thereto, reacting at 65-75°C for 10h-12h under an inert atmosphere to obtain the modified nano-silica.

[0064] Specifically, the activation temperature of nano-silica can be 100°C, 103°C, 105°C, 108°C, 110°C or a range consisting of any two thereof, and the activation time of nano-silica can be 12h, 15h, 20h, 25h, 30h, 36h or a range consisting of any two thereof.

[0065] Specifically, the solvent mixed with the activated nano-silica may include an organic solvent, specifically toluene (or toluene solvent).

[0066] In the specific implementation, after activating the nano-silicon dioxide (unmodified nano-silicon dioxide particles), a solvent is added, and the nano-silicon dioxide is uniformly dispersed in the solvent by ultrasound or other means to obtain a mixed solution; then the mixed solution is transferred to a reaction device such as a three-necked flask, a magnetic stirrer is added thereto for stirring, a silane coupling agent is added to the mixed solution, and nitrogen is continuously introduced to exhaust the air in the reaction device (the nitrogen introduction time can be 10min±2min); then, a thermometer is inserted into the reaction device, and the temperature begins to rise. After the temperature rises to 70°C, the reaction is carried out for 6h. After the reaction is completed, the reaction product is filtered by a vacuum filtration pump, and anhydrous ethanol is added to the obtained solid product for washing. The product is stirred during the washing process. After the solid product particles are uniformly dispersed, the filtration-washing process is completed once, and the filtration-washing process is repeated three times in total; then, the washed solid product is dried and ground in turn to obtain modified nano-silicon dioxide.

[0067] In the present invention, conventional ultrasonic machines in the art may be used for ultrasonic treatment, and there is no particular limitation thereto.

[0068] Specifically, the mass percentage of the emulsifier in the oil phase may be 5%-10%.

[0069] Specifically, the mass ratio of modified nano-silica, temperature-sensitive monomer, and vinyl hydrophilic monomer in the aqueous phase can be (0.2-0.5):1:1, for example, 0.2:1:1, 0.3:1:1, 0.4:1:1, and 0.5:1:1.

[0070] Specifically, the inert atmosphere may be nitrogen.

[0071] In specific implementation, the emulsifier can be added to the base oil and stirred evenly to form an oil phase; the modified nano-silica and monomers (thermosensitive monomers and vinyl hydrophilic monomers, etc.) used to form a polymer grafted on the surface of the nano-silica are added to water and stirred evenly to form a water phase.

[0072] Subsequently, the aqueous phase is slowly dripped into the oil phase under stirring to obtain a reverse microemulsion; the reverse microemulsion can be placed in a device such as a three-necked flask, nitrogen is filled into the reverse microemulsion and the temperature is increased, and an initiator is added to the reverse microemulsion to carry out a polymerization reaction, and a temperature-sensitive nanomaterial is obtained through the reverse microemulsion synthesis method.

[0073] The present invention further provides an oil-displacing agent, comprising water and a temperature-sensitive nanomaterial. The oil-displacing agent has advantages corresponding to those of the temperature-sensitive nanomaterial, which will not be described in detail.

[0074] In some embodiments, the concentration of the temperature-sensitive nanomaterial in the oil-displacing agent (i.e., the mass percentage of the temperature-sensitive nanomaterial in the oil-displacing agent) can be 0.01wt% to 0.2wt%, for example, 0.01wt%, 0.03wt%, 0.05wt%, 0.07wt%, 0.1wt%, 0.15wt%, or 0.2wt%.

[0075] The embodiment of the present invention further provides an oil displacement method, comprising the following steps: displacing crude oil in an oil reservoir with an oil displacement agent.

[0076] Specifically, the oil reservoir may be a low permeability oil reservoir, that is, the oil displacing agent according to the embodiment of the present invention is suitable for displacing oil in the low permeability oil reservoir, and can improve the recovery rate of the low permeability oil reservoir.

[0077] In some embodiments, the above-mentioned oil recovery method may include sequentially performing pre-water drive, nanofluid drive and post-water drive on the oil reservoir, wherein the pre-water drive stage uses water to displace the crude oil in the oil reservoir, the nanofluid drive stage uses the above-mentioned oil-displacing agent to displace the crude oil in the oil reservoir, and the post-water drive stage uses water to displace the crude oil in the oil reservoir. Through this oil recovery process, the recovery rate can be significantly improved in the nanofluid drive stage, and the remaining crude oil in the oil reservoir can be displaced through the post-water drive stage, and the recovery rate of the post-water drive stage can be improved.

[0078] In specific implementation, water may be first injected into the reservoir for pre-water flooding; after the pre-water flooding is completed, the above oil displacement agent may be injected into the reservoir for nanofluid flooding; after the nanofluid flooding is completed, water may be injected into the reservoir for post-water flooding.

[0079] According to the inventor's research, relatively speaking, when the injection volume of the oil-displacing agent is too small, the degree of contact between the oil-displacing agent and the rock surface in the low-permeability reservoir is poor, so that the nanofluid flooding stage cannot fully peel off the oil droplets attached to the rock, and then relatively more crude oil remains in the rock pores, affecting the recovery rate; and as the injection volume of the oil-displacing agent increases, the oil-displacing agent contacts the rock surface in the low-permeability reservoir more fully, which can increase the efficiency of the oil-displacing agent in stripping the oil attached to the rock surface, so that the nanofluid flooding stage can displace more crude oil and improve the recovery rate, and if the injection volume of the oil-displacing agent is too large, there will also be a risk of causing the pores and cracks of the reservoir to be blocked, which not only affects the recovery rate of the nanofluid flooding stage, but also hinders the further advancement of oil droplets in the post-water flooding stage, reducing the recovery rate of the post-water flooding stage (water flooding oil stage). Therefore, taking the above factors into consideration, it is preferred that in the nanofluid flooding stage, the injection volume of the above-mentioned oil-displacing agent in the reservoir can be 0.3PV~0.7PV.

[0080] The present invention is further described below through specific embodiments.

[0081] Embodiment 1:

[0082] 1. Preparation of modified nano-silica

[0083] 5 g of nano-silicon dioxide was placed in an oven at 110°C for activation for 24 hours, and then 100 mL of dehydrated toluene was added, and the nano-silicon dioxide was ultrasonically dispersed to obtain a mixed solution;

[0084] Then the mixed solution was transferred to a container, and 0.2 wt% of KH-570 was added to the mixed solution; nitrogen was continuously introduced into the container for 10 minutes to exhaust the air in the container; the reaction was carried out at 100° C. for 24 hours;

[0085] After the reaction is completed, the solid product is filtered and stirred and washed with anhydrous ethanol. After the solid product particles are evenly dispersed, the filtration-washing process is completed in sequence, and the filtration-washing process is repeated three times. The washed solid product is placed in a 60°C oven for drying, and modified nano-silica is obtained by grinding, which is recorded as KH570@SiO2.

[0086] 2. Preparation of temperature-sensitive nanomaterials

[0087] 66.7wt% white oil, 6.4wt% Span80 and 1.6wt% Tween80 (the mass percentages of white oil, Span80 and Tween80 are all based on the oil phase) are added to a container and mechanically stirred to form an oil phase; KH570@SiO2 is weighed and added to water for ultrasonic dispersion, and isopropyl acrylamide and itaconic acid are added to the water to form an aqueous phase; the aqueous phase is added dropwise to the oil phase and then stirred to obtain a W / O inverse microemulsion;

[0088] Nitrogen was introduced into a container containing W / O reverse microemulsion and heated to 74°C, and then 0.5 wt% potassium persulfate solution was added dropwise thereto, and the product was reacted at 74°C for 24 hours to obtain the product; after cooling to room temperature, 100 ml of anhydrous ethanol was added to the container for washing, and then filtered, and the washing was repeated three times; after washing, the product was placed in a constant temperature oven at 60°C for drying and then ground to obtain a temperature-sensitive nanomaterial, which was recorded as NIPAm / IA / SiO2.

[0089] Among them, the mass ratio of Span80 to Tween80 is 4:1, and the mass fraction of Span80 and Tween80 in the reverse microemulsion is 8%; the mass ratio of KH570@SiO2, thermosensitive monomer and vinyl hydrophilic monomer is 0.3:1:1.

[0090] Performance Testing

[0091] 1. Infrared spectroscopy analysis

[0092] Fourier transform infrared spectrometer (Beijing Rayleigh Analytical Instrument Co., Ltd., WQF-520-FTIR) was used to perform infrared spectroscopy analysis on nano-silicon dioxide particles (nano-SiO2) and the temperature-sensitive nanomaterial (NIPAm / IA / SiO2) in Example 1. To ensure the accuracy of the test, all samples were vacuum dried under the same temperature and time conditions. The samples were prepared by potassium bromide tableting. The wave number range of the spectral scan was set to 4000cm -1 ~400cm -1 , the test results are shown in Figure 1 .

[0093] like Figure 1 As shown, at 1716cm -1 The peak at 1403 cm corresponds to the stretching vibration peak of C=O on the carboxyl group of IA (itaconic acid). -1 、910cm -1 The peak at 1360 cm corresponds to the bending vibration peak of OH on the carboxyl group in the IA molecular structural unit; -1 、2930cm -1 、2970cm -1 The peaks at 1554cm are the CH stretching vibration peaks of isopropyl, methylene and methyl in the molecular structure unit of NIPAm (N-isopropylacrylamide); -1 、1637cm -1 The peaks at the right and left sides represent the NH and C=O stretching vibration peaks on the amide group in the NIPAm molecular structure unit. The existence of these spectral characteristic peaks proves that both NIPAm and IA are successfully grafted on the surface of nano-silica.

[0094] 2. Thermogravimetric analysis (TGA)

[0095] The thermal stability of NIPAm / IA / SiO2 in Example 1 was tested by thermogravimetric analyzer (Mettler-Toledo International, Switzerland, TGA / DSC1). The test results are shown in Figure 2 The test was carried out in a nitrogen environment with a temperature range of 25°C to 800°C and a heating rate of 10°C / min.

[0096] like Figure 2As shown in the figure, when the temperature is below 310℃, the weight loss of NIPAm / IA / SiO2 is 4.23%. The weight loss at this stage is caused by the evaporation of free water and bound water adsorbed on the surface of NIPAm / IA / SiO2. When the temperature exceeds 310℃, NIPAm / IA / SiO2 decomposes rapidly with a weight loss rate of 32.75%. The weight loss at this stage is caused by the thermal decomposition of the amide groups on the surface of NIPAm / IA / SiO2. The quality of NIPAm / IA / SiO2 tends to be stable at around 600℃. The test results show that NIPAm / IA / SiO2 has excellent thermal stability.

[0097] 3. Transmission electron microscope (TEM) test

[0098] The field emission transmission electron microscope will shoot an electron beam at the sample to be tested. When the electron beam interacts with the atoms in the sample, scattering occurs. The microstructure of the sample can be revealed by subsequent processing of the data acquisition and imaging system. The S-TWIN-F20 field emission transmission electron microscope was used to test the microscopic morphology of nano-silicon dioxide particles and NIPAm / IA / SiO2 in Example 1, and the differences in the aggregation states of the two were compared and analyzed. The test results are shown in Figure 3 , Figure 4 and Figure 5 To prevent the electron beam from reacting with air molecules, all samples were vacuum dried at 60°C and -50 kPa for 24 h.

[0099] like Figure 3 As shown, unmodified silica particles tend to form aggregates, e.g. Figure 4 and Figure 5 As shown, the NIPAm / IA / SiO2 particles showed excellent dispersibility and uniform distribution, and no significant aggregation was observed, indicating that the excellent dispersibility of NIPAm / IA / SiO2 can increase the contact area between the oil displacement agent and crude oil, reduce the oil-water interfacial tension, promote the emulsification and dispersion of crude oil, and thus improve the oil displacement efficiency.

[0100] 4. Dynamic Light Scattering (DLS) Test

[0101] Nanosilica and NIPAm / IA / SiO2 in Example 1 were vacuum dried at 60°C, -50 kPa for 24 h, and respectively added into water for ultrasonic dispersion until the samples were evenly distributed in the water, thereby obtaining nanosilica fluid and NIPAm / IA / SiO2 fluid. The mass percentage of nanosilica in the nanosilica fluid was 0.01%, and the mass percentage of NIPAm / IA / SiO2 in the NIPAm / IA / SiO2 fluid was 0.01%.

[0102] The particle size distribution of nano-silica in nano-silica fluid and NIPAm / IA / SiO2 in NIPAm / IA / SiO2 fluid was measured using a BI-200SM laser scattering instrument. Figure 6 .

[0103] like Figure 6 As shown, the particle size distribution range of nano-silicon dioxide in nano-silicon dioxide fluid is relatively wide, even far beyond the nanometer size range. The distribution range of NIPAm / IA / SiO2 in NIPAm / IA / SiO2 fluid is 131-512nm, and the median particle size is 264nm, which shows that NIPAm / IA / SiO2 has a smaller particle size, and the particle size distribution of NIPAm / IA / SiO2 in NIPAm / IA / SiO2 fluid is more concentrated, which shows that the particle size distribution of NIPAm / IA / SiO2 prepared by the present invention is more uniform, which is conducive to the more effective migration and distribution of the oil displacement agent prepared by NIPAm / IA / SiO2 in low permeability reservoirs, thereby improving the recovery rate of low permeability reservoirs.

[0104] This result is different from that observed by transmission electron microscopy. The possible reason may be that the interaction between particles or the hydration in the aqueous solution causes the aggregation of NIPAm / IA / SiO2, resulting in a larger particle diameter than that measured by transmission electron microscopy.

[0105] 5. Evaluation of the temperature sensitivity of temperature-sensitive nanofluids

[0106] The temperature-sensitive nanomaterial in Example 1 was added into water to prepare a temperature-sensitive nanofluid with a mass fraction of the temperature-sensitive nanomaterial of 0.1% (ie, the mass percentage of the temperature-sensitive nanomaterial in the temperature-sensitive nanofluid was 0.1%).

[0107] The above-mentioned temperature-sensitive nanofluid was placed in a cuvette, and a UV-visible spectrophotometer connected to a thermostatic bath was used to measure the change in the transmittance of the nanofluid with temperature. The temperature range of the experiment was set at 25-95°C. The effect of temperature change on the dispersion state of NIPAm / IA / SiO2 in aqueous solution was analyzed, and the temperature responsiveness of NIPAm / IA / SiO2 was explored. The specific test results are shown in Figure 7 .

[0108] like Figure 7As shown in the figure, when the temperature is less than 69.8℃, the thermosensitive nanofluid has a high transmittance. At this time, the thermosensitive nanofluid is clear and NIPAm / IA / SiO2 can be well dispersed in water. When the temperature exceeds 69.8℃, the transmittance of the thermosensitive nanofluid drops rapidly and becomes turbid. The reason for this phenomenon is that the degree of hydration between the amide groups in NIPAm / IA / SiO2 and water molecules decreases. When the temperature is lower than 69.8℃, the amide groups, carboxyl groups and hydroxyl groups in NIPAm / IA / SiO2 form a large number of hydrogen bonds with water molecules, which enhances the hydrophilicity of the polymer molecular chains on the surface of NIPAm / IA / SiO2 and evenly disperses in the aqueous solution. In summary, the temperature response mechanism of NIPAm / IA / SiO2 is mainly determined by the polymer on its surface, and the lower critical solution temperature (LCST) of NIPAm / IA / SiO2 is 69.8℃.

[0109] 6. Recovery efficiency of temperature-sensitive nanofluids

[0110] NIPAm / IA / SiO2 was added to deionized water and ultrasonically dispersed to obtain NIPAm / IA / SiO2 nanofluid, and 0.01wt%, 0.05wt%, 0.1wt%, and 0.2wt% of NIPAm / IA / SiO2 nanofluid were configured respectively.

[0111] 6-1. Effect of Temperature-Sensitive Nanofluid Concentration on Oil Recovery

[0112] The above-mentioned temperature-sensitive nanofluid was used to carry out core displacement experiments at the same ambient temperature, with the same injection rate and injection volume.

[0113] The core flooding experiment steps are as follows:

[0114] a. Core permeability determination:

[0115] Soak the core in chloroform for 8 hours to remove organic pollutants; wash it three times with anhydrous ethanol to remove residual chloroform, and wash it three times with pure water. Dry the washed core in a constant temperature drying oven at 105°C for 24 hours to fully remove the moisture in the core. Then use a vernier caliper to measure the length and diameter of the dried core and calculate the volume V. Weigh the dry weight M of the core before saturation with water 干重 Then, the core is placed in a core holder and saturated with formation water simulation (water is injected into one end of the core holder, and the saturated formation water simulation is completed after the other end is discharged and the pressure is stable). Salt water with a salinity of 1000 mg / L is used as the simulated formation water. After the completion, the wet weight of the rock is weighed M 湿重 , calculate the core pore volume V0 and porosity Φ.

[0116]

[0117] The saturated core was left to stand for 24 hours to allow the water in the core to be evenly distributed. Confining pressure was applied to the core, and formation simulated water was injected. When the produced fluid flowed out and the pressure was stable, the pressure value was read. The core permeability was determined according to the following formula:

[0118]

[0119] Q-flow rate of simulated water injected into the formation, cm 3 / s;

[0120] K-absolute permeability of the core, D;

[0121] A-core cross-sectional area, cm 2 ;

[0122] Δp-pressure difference before the formation simulated water flows through the core, atm;

[0123] μ-Viscosity of formation simulated water, mPa·s.

[0124] b. Saturated crude oil: The core saturated oil experiment was conducted using the progressive flow rate method. At the beginning of the experiment, the initial saturation was achieved with an oil injection rate of 0.25 mL / min for 6 hours. The flow rate was then increased to 0.50 mL / min and saturation continued for 3 hours; finally, it was further accelerated to 0.75 mL / min and continued for 2 hours to complete the oil saturation process. The saturated oil core was placed in a 70°C oven for 48 hours of aging, and the volume of the water phase in the liquid receiving tube was read to determine the amount of saturated oil in the core;

[0125] c. Water flooding: Inject simulated water into the formation at a flow rate of 0.5 mL / min. Stop water flooding when the displacement pressure is stable and the water content of the produced fluid is stable at 98%, and analyze the volume of oil and water in the produced fluid at this stage;

[0126] d. Nanofluid flooding: Inject temperature-sensitive nanofluid at a flow rate of 0.5 mL / min. Stop the temperature-sensitive nanofluid flooding until the displacement pressure is stable and the water content of the produced fluid is stable at 98%, and analyze the oil and water volumes in the produced fluid at this stage;

[0127] e. Post-water flooding: Continue to inject simulated water into the formation at a flow rate of 0.5 mL / min. Stop water flooding until the displacement pressure is stable and the water content of the produced fluid is stable at 98%, and analyze the volume of oil and water in the produced fluid at this stage. Sum up the volume of oil and water in each flooding stage, and calculate the recovery rate during the flooding process. The recovery rate is the ratio of the produced oil volume to the saturated oil volume.

[0128] The results of the four displacement experiments are as follows Figure 8As shown in the figure, when the concentration of the temperature-sensitive nanofluid is 0.05wt%, the recovery rate of crude oil is the highest, which is 38.3%. The recovery rates of the four groups of experiments are similar in the pre-water flooding stage, but the recovery rates differ after entering the nanofluid flooding stage. In the subsequent water flooding process, the recovery rate gradually stabilizes.

[0129] 6-2. Effect of injection volume of temperature-sensitive nanofluid on oil recovery Figure 8 Based on the results, a temperature-sensitive nanofluid with a concentration of 0.05wt% was selected for detection, and temperature-sensitive nanofluids with volumes of 0.3PV, 0.5PV, and 0.7PV were injected respectively, and core displacement experiments were carried out at 70℃ and 0.5mL / min.

[0130] The results of the three groups of experiments are as follows Fig. 9 As shown in the figure, with the increase of the injection volume of the temperature-sensitive nanofluid, the recovery rate of the nanofluid flooding stage was increased by 14.52%, 17.89%, and 18.73%, respectively. When the injection volume increased from 0.3PV to 0.5PV, the recovery rate increased the most; when the injection volume increased from 0.5PV to 0.7PV, the recovery rate continued to increase, but the growth rate decreased. According to the experimental results of the post-water flooding stage, the recovery rate of post-water flooding gradually decreased with the increase of the injection volume. When the injection volume of the nanofluid was 0.3PV, the recovery rate of post-water flooding was the largest, which was 3.68%.

[0131] When the injection volume of the thermosensitive nanofluid is small, the thermosensitive nanofluid is not in sufficient contact with the core surface, so that all attached oil droplets cannot be completely peeled off in the thermosensitive nanofluid displacement stage, and a certain amount of crude oil remains in the rock pores, leaving potential oil that has not been exploited. Therefore, in the post-water drive stage, these remaining oils may be further produced by conventional water drive methods, resulting in the displacement of part of the crude oil in the post-water drive stage. With the increase of the injection volume of the thermosensitive nanofluid, the contact between NIPAm / IA / SiO2 and the rock surface is more sufficient, which increases the efficiency of the thermosensitive nanofluid in stripping the attached oil on the rock surface, so that more crude oil can be displaced in the thermosensitive nanofluid drive stage. Since most of the easily exploitable oil has been displaced in this stage, the remaining residual oil is not easy to be displaced in the subsequent water drive (post-water drive) stage, so the oil recovery of the post-water drive is reduced. In addition, excessive injection of the thermosensitive nanofluid may also cause pores, cracks, etc. to be blocked, hindering the water drive from further advancing the oil droplets and reducing the effectiveness of the water drive stage. Therefore, in the oil displacement process, the oil displacement agent with a concentration of 0.05wt% and a volume of 0.5PV has a higher recovery rate and better effect.

[0132] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A temperature-sensitive nanomaterial, characterized in that: It comprises nano silicon dioxide and a polymer grafted on the surface of the nano silicon dioxide; The polymer is polymerized from a thermosensitive monomer and a vinyl hydrophilic monomer; The temperature-sensitive monomer includes isopropyl acrylamide and / or dimethylaminoethyl methacrylate.

2. The temperature-sensitive nanomaterial according to claim 1, characterized in that: The minimum critical dissolution temperature of the thermosensitive nanomaterial in water is 69.8° C.; and / or, The particle size of the temperature-sensitive nano material dispersed in water is 131-512 nm.

3. The temperature-sensitive nanomaterial according to claim 1 or 2, characterized in that: The vinyl hydrophilic monomer includes one or more of methylene succinic acid, crotonic acid, and maleic acid.

4. The temperature-sensitive nanomaterial according to any one of claims 1 to 3, characterized in that: The temperature-sensitive nanomaterial is formed from a raw material composition, which includes base oil, emulsifier, modified nano-silicon dioxide, temperature-sensitive monomer, vinyl hydrophilic monomer, initiator and water; wherein the modified nano-silicon dioxide is prepared by grafting nano-silicon dioxide and a silane coupling agent.

5. The temperature-sensitive nanomaterial according to claim 4, characterized in that: The silane coupling agent includes γ-methacryloxypropyltrimethoxysilane; and / or, The base oil comprises white oil; and / or, The initiator comprises potassium persulfate; and / or, The emulsifier includes sorbitan fatty acid ester and / or polyoxyethylene sorbitan monooleate; Preferably, the mass ratio of the sorbitan fatty acid ester to the polyoxyethylene sorbitan monooleate is (7-9): (1-3); Preferably, the sorbitan fatty acid ester includes one or more of Span80, Span60 and Span40; Preferably, the polyoxyethylene sorbitan monooleate includes one or more of Tween80, Tween60 and Tween40.

6. A method for preparing the temperature-sensitive nanomaterial according to claim 4 or 5, characterized in that: The following steps are involved: mixing the base oil and the emulsifier to obtain an oil phase; Mixing modified nano-silicon dioxide, a temperature-sensitive monomer, a vinyl hydrophilic monomer and water to obtain an aqueous phase; wherein the modified nano-silicon dioxide is obtained by grafting nano-silicon dioxide and a silane coupling agent; mixing the aqueous phase with the oil phase to obtain an inverse microemulsion; After the reverse microemulsion is mixed with the initiator, a polymerization reaction is carried out at 65 to 75° C. for 16 to 32 hours under an inert atmosphere to obtain the temperature-sensitive nanomaterial.

7. The method for preparing the temperature-sensitive nanomaterial according to claim 6, characterized in that: The preparation method of the modified nano-silica comprises the following steps: activating the nano-silica at 100-110° C. for 12-36 hours, mixing with a solvent to obtain a mixed solution; then adding a silane coupling agent to the mixed solution, reacting at 65-75° C. for 10-12 hours under an inert atmosphere to obtain the modified nano-silica.

8. An oil displacing agent, characterized in that: The invention comprises water and the temperature-sensitive nano material according to any one of claims 1 to 5.

9. The oil-displacing agent according to claim 8, characterized in that The concentration of the temperature-sensitive nanomaterial in the oil-displacing agent is 0.01 wt % to 0.2 wt %.

10. An oil displacement method, characterized in that: The method comprises the following steps: using the oil displacing agent according to claim 8 or 9 to displace the crude oil in the oil reservoir.

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