A graphene oxide-carbon nanotube composite oil displacement agent and its application

Abstract

The present application provides a graphene oxide-carbon nanotube composite oil-displacing agent and its application, the oil-displacing agent includes a graphene oxide-carbon nanotube composite; the graphene oxide-carbon nanotube composite is a sheet-like-fibrous composite structure; the sheet diameter of the graphene oxide in the sheet-like-fibrous composite structure is selected from 100nm to 1000nm; the tube diameter of the carbon nanotube in the sheet-like-fibrous composite structure is selected from 5nm to 10000nm, and the tube length of the carbon nanotube in the sheet-like-fibrous composite structure is selected from 50nm to 50000nm. After the oil-displacing agent provided by the present application is injected into the stratum, on the one hand, the co-adsorption of graphene oxide and carbon nanotubes at the liquid-liquid interface can greatly reduce the interfacial tension, thereby promoting the expansion of the affected range and the recovery of clustered residual oil; on the other hand, due to the curved shape of the carbon nanotube, the residual oil in the blind-end hole or the blind-end hole in the affected area is stripped out.

Description

[0001] This application is a divisional application of the application with the application date of November 3, 2022, application number CN 202211370949.1, and invention title "A Graphene Oxide-Carbon Nanotube Composite Oil Displacement Agent and Its Application". Technical Field

[0002] This article relates to, but is not limited to, the field of new energy and energy efficiency, and particularly relates to, but is not limited to, nano-material oil displacement agents and methods for improving oil recovery using the same. Background Art

[0003] With the development of social economy, the problem of high external dependence on crude oil in China has become increasingly prominent, and the development of oil and gas resources is an important guarantee for national energy security. Therefore, it is of great significance to develop new technologies and methods for improving oil recovery. Among them, chemical flooding can significantly improve oil recovery by adding chemical reagents to the displacement fluid to change the fluid properties. However, traditional chemical flooding methods often have problems such as high cost, large dosage, and formation damage.

[0004] In recent years, the development of nanotechnology has made nanoparticle suspensions increasingly attract attention as a new displacement agent. The small size and high specific surface area of nanomaterials endow them with special surface and interface properties, such as regulating surface wettability and reducing interfacial tension. However, the applications of existing nano oil displacement agents are often only applicable to specific conditions, and the action mechanisms are relatively single, with limited action effects, making it difficult to fully play their roles in practical applications. Summary of the Invention

[0005] The following is an overview of the subject matter described in detail in this article. This overview is not intended to limit the scope of protection of the claims.

[0006] This application provides a graphene oxide-carbon nanotube composite oil displacement agent and its application method. By pretreatment, a graphene-carbon nanotube composite system that can be well dispersed and suitable for injection is prepared, and a displacement fluid suitable for different reservoir environments is formulated; during the displacement process, the multiple effects of nanomaterials can be fully exerted to improve oil recovery without modifying or decorating the nanomaterials themselves; the use of a stepwise increasing injection concentration can maximize the effect of the composite oil displacement agent while reducing costs and formation damage.

[0007] This application provides an oil displacement agent, and the oil displacement agent includes a graphene oxide-carbon nanotube composite;

[0008] The graphene oxide-carbon nanotube composite has a sheet-fibrous composite structure;

[0009] In the flake-fiber composite structure, on the one hand, carbon nanotubes can randomly adhere to graphene oxide sheets; on the other hand, the presence of graphene oxide sheets can prevent contact and aggregation between carbon nanotubes.

[0010] The sheet diameter of the graphene oxide in the flake-fiber composite structure is selected from 100 nm to 1000 nm; the tube diameter of the carbon nanotubes in the flake-fiber composite structure is selected from 5 nm to 10000 nm, and the tube length of the carbon nanotubes in the flake-fiber composite structure is selected from 50 nm to 50000 nm.

[0011] In an embodiment provided by the present application, the salinity range of the oil displacement agent is 100 ppm to 2500 ppm.

[0012] In an embodiment provided by the present application, the salt used to adjust the salinity range is selected from any one or more of NaCl, KCl, CaCl 2 , NaHCO 3 and MgSO 4 among others.

[0013] In an embodiment provided by the present application, the pH value of the oil displacement agent is from 3 to 11;

[0014] In an embodiment provided by the present application, the pH regulator used to adjust the pH value is selected from any one or more of HCl, NaOH, KOH, NH 3 ·H 2 O, Na 2 CO 3 and CaO.

[0015] In an embodiment provided by the present application, the carbon nanotubes are selected from any one or more of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes.

[0016] In an embodiment provided by the present application, the carbon nanotubes are selected from multi-walled carbon nanotubes. Preferably, the tube diameter of the multi-walled carbon nanotubes is selected from 30 nm to 50 nm, and the tube length of the multi-walled carbon nanotubes is selected from 100 nm to 1000 nm.

[0017] On the other hand, the present application provides a preparation method of the above-mentioned oil displacement agent, and the preparation method includes:

[0018] Mix graphene oxide powder and carbon nanotube powder, and fully stir and grind in the powder state to obtain a mixed powder;

[0019] Mix the mixed powder with water evenly to obtain the oil displacement agent.

[0020] In an embodiment provided by the present application, the mass ratio of the graphene oxide powder to the carbon nanotubes is from 20:1 to 1:1. In an embodiment provided by the present application, the mass ratio of the graphene oxide powder to the carbon nanotubes is from 5:1 to 2:1.

[0021] In an embodiment provided by the present application, the graphene oxide powder and the carbon nanotube powder are mixed and thoroughly stirred and ground in the powder state to obtain a mixed powder; the thorough stirring and grinding need to ensure uniformity both macroscopically and microscopically;

[0022] Macroscopically, ensure that the relative difference between the mass ratio of the two nanomaterials in the sampled specimen and the set mass ratio is less than 20%; the specific operation process is to randomly take a mixed powder with a mass of more than 0.1 g, disperse it in water. At this time, the graphene oxide can be well dispersed in water, while the carbon nanotubes are difficult to disperse; filter, and the mass of the carbon nanotubes can be obtained after drying the filter cake, so as to determine the mass ratio of the sample;

[0023] Microscopically, observe the dispersion of the powder through a high-magnification microscope. The powders of the two materials can be distinguished by the differences in morphology and color. It is necessary to ensure that the powder presents a uniform dispersion state, and the proportion of the aggregated area in the observed area is less than 10%.

[0024] In an embodiment provided by the present application, the weight ratio of the mixed powder to water is (0.005 to 5):100; in an embodiment provided by the present application, the weight ratio of the mixed powder to water is (0.1 to 1):100.

[0025] In an embodiment provided by the present application, the mixture of the mixed powder and water is stirred at 400 rpm to 2000 rpm for 1 h to 2 h.

[0026] In an embodiment provided by the present application, the mixture of the mixed powder and water is ultrasonically treated at 300 W to 1500 W for 1 to 2 h; in an embodiment provided by the present application, the ultrasonic power is 600 W to 800 W, and the ultrasonic treatment is carried out under the condition of an ice-water bath; in an embodiment provided by the present application, after every 25 min of ultrasonic treatment, there is an interval of 10 min.

[0027] In an embodiment provided by the present application, the preparation method of the graphene oxide powder includes the following steps: preparing graphene oxide stock solution from natural graphite flakes by the improved Hummers method;

[0028] Redisperse the graphene oxide stock solution with deionized water and centrifuge it at 5000 rpm to 20000 rpm to remove unreacted and unexfoliated graphite residues, obtaining the supernatant; in an embodiment provided by the present application, the centrifugation speed is 8000 - 10000 rpm to ensure no graphite residues and maximum retention of the product;

[0029] Take the supernatant and ultrasonicate it in a water bath at 50 W to 200 W for 0.5 h to 2.5 h to obtain a graphene oxide dispersion, and place it in a vacuum drying oven to dry to obtain graphene oxide powder.

[0030] On the other hand, the present application provides the use of the above-mentioned oil displacement agent, and the use includes:

[0031] (1) Select a target well, and the target well is an oil production well with water in the produced fluid of the oil production well. Optionally, the proportion of the water in the produced fluid of the oil production well exceeds 10%;

[0032] (2) Inject the oil displacement agent through an injection well, and the initial injection concentration is 1 / 20 to 1 / 2 of the designed concentration; after no new oil is produced, increase the injection concentration to 1.5 to 5 times the original; and so on until the injection concentration reaches the designed concentration; optionally, the designed concentration is 0.0001 wt.% to 0.5 wt.%, and preferably, the designed concentration is 0.05 wt.% to 0.2 wt.%;

[0033] In an embodiment provided by the present application, when performing the operation of step (2), after the injection pressure rises to reach 0.5 MPa to 2 MPa, restart water injection, that is, repeat steps (1) and (2).

[0034] In an embodiment provided by the present application, when the average pore diameter of the reservoir is less than 1 μm, or the formation water salinity is higher than 80000 ppm, and the concentration of the oil displacement agent is higher than 0.1 wt.%, a surfactant is added to assist dispersion, and the mass fraction of the surfactant is lower than the total mass fraction of graphene oxide and carbon nanotubes.

[0035] In an embodiment provided by the present application, the surfactant is selected from any one or more of anionic surfactants, sulfobetaine 12, and polyethylene glycol;

[0036] In an embodiment provided by the present application, the anionic surfactant is selected from any one or more of sodium dodecyl sulfate and sodium dodecylbenzenesulfonate.

[0037] After the oil displacement agent provided by this application is injected into the formation, on the one hand, it can significantly reduce the interfacial tension through the co-adsorption of graphene oxide and carbon nanotubes at the liquid-liquid interface, induce interfacial instability by using the dynamic adsorption effect, and have a certain emulsifying effect on crude oil, thereby promoting the expansion of the swept area and the production of clustered residual oil; on the other hand, due to the curved shape of carbon nanotubes, the graphene oxide-carbon nanotube composite can also form a multi-level rough structure after adsorption at the solid-liquid interface, stripping the residual oil in blind-end pores or quasi-blind-end pores within the swept area. The composite system and synergistic effect of the two materials enable it to effectively play a role in different types of reservoir environments, and the effect can accumulate with the increase in concentration. At the same time, low-concentration injection can reduce formation damage. Therefore, using a stepwise increasing injection concentration can maximize the effect of the composite oil displacement agent and reduce costs.

[0038] Other features and advantages of this application will be described in the subsequent specification, and part of them will become obvious from the specification or be understood by implementing this application. Other advantages of this application can be achieved and obtained through the solutions described in the specification. Brief Description of the Drawings

[0039] The drawings are used to provide an understanding of the technical solutions of this application, and constitute a part of the specification. Together with the embodiments of this application, they are used to explain the technical solutions of this application and do not constitute a limitation to the technical solutions of this application.

[0040] Figure 1 In (a), it is the transmission electron microscope characterization result of the graphene oxide-carbon nanotube composite prepared in the embodiment;

[0041] Figure 1 In (b), it is the transmission electron microscope characterization result of graphene oxide sheets (the difference from the embodiment is that carbon nanotube powder is not added in step (1.4), and other conditions are the same);

[0042] Figure 1 In (c), it is the transmission electron microscope characterization result of carbon nanotubes (the difference from the embodiment is that graphene oxide powder is not added in step (1.4), and other conditions are the same);

[0043] Figure 1 In (d), it is the particle size distribution diagram of the graphene oxide-carbon nanotube composite and graphene oxide dispersion (concentration is 0.2 wt.%) prepared in the embodiment.

[0044] Figure 2 In (a), it is a schematic diagram of the graphene oxide-carbon nanotube composite oil displacement agent under an optical microscope after being prepared in the embodiment;

[0045] Figure 2Figure (b) is a schematic diagram of the graphene oxide-carbon nanotube composite oil displacement agent under an optical microscope 8 hours after the formulation in the example;

[0046] Figure 2 Figure (c) is a schematic diagram of the carbon nanotube dispersion (concentration: 0.2 wt.%) under an optical microscope at the time of formulation completion;

[0047] Figure 2 Figure (d) is a schematic diagram of the carbon nanotube dispersion (concentration: 0.2 wt.%) under an optical microscope 8 hours after the formulation completion;

[0048] Figure 2 In Figure (e), the left figure is the carbon nanotube dispersion, and the right figure is the graphene oxide-carbon nanotube composite oil displacement agent prepared in the example. After both have been placed for 8 hours, sedimentation occurs in the carbon nanotube dispersion, while no sedimentation occurs in the graphene oxide-carbon nanotube composite oil displacement agent prepared in the example.

[0049] Figure 3 Figure (a) is a schematic diagram of the contact angle measurement results of the graphene oxide-carbon nanotube composite oil displacement agent, graphene oxide dispersion, and silica nanoparticle dispersion prepared in the example at the same concentration (concentration: 0.2 wt.%);

[0050] Figure 3 Figure (b) is a schematic diagram of the interfacial tension measurement results of the graphene oxide-carbon nanotube composite oil displacement agent, graphene oxide dispersion, and silica nanoparticle dispersion prepared in the example at the same concentration (concentration: 0.2 wt.%).

[0051] Figure 4 For the emulsification test results, the volume ratio of the oil phase to the water phase in the test tube is 1:1. The oil phase is n-decane (the transparent part at the upper part of the test tube before oscillation), Figure 4 the water phase in Figure (a) is deionized water, Figure 4 the water phase in Figure (b) is the silica nanoparticle dispersion (concentration: 0.2 wt.%), Figure 4 the water phase in Figure (c) is the graphene oxide dispersion (concentration: 0.2 wt.%), Figure 4 the water phase in Figure (d) is the graphene oxide-carbon nanotube composite oil displacement agent prepared in the example (concentration: 0.2 wt.%).

[0052] Figure 5 For the wall surface adsorption roughness measurement results of the graphene oxide-carbon nanotube composite Figure 5 prepared in the example (Figure (a)), graphene oxide Figure 5 Figure (b)), and silica nanoparticles Figure 5 Figure (c)) at the same concentration (concentration: 0.2 wt.%).

[0053] Figure 6 Schematic diagram comparing the phase distribution and oil recovery curve at the final moment during the displacement process of the graphene oxide-carbon nanotube composite oil displacement agent prepared in the example with water flooding. Detailed implementation manners

[0054] To make the objectives, technical solutions, and advantages of this application clearer, the embodiments of this application are described in detail below. It should be noted that, without conflict, the embodiments in this application and the features in the embodiments can be combined arbitrarily with each other.

[0055] Embodiment

[0056] (1) Preparation and formulation of the oil displacement agent

[0057] In the preparation and formulation parameters of the oil displacement agent, the mass ratio of carbon nanotubes to graphene oxide, the salinity and pH of the composite dispersion liquid, etc. are all determined according to the conditions in the displacement experiment.

[0058] The specific values of the salinity and pH of the oil displacement agent will affect the stability of the composite dispersion liquid system and the strength of the liquid-liquid interface / solid-liquid interface interaction, so they should also be determined according to the reservoir conditions;

[0059] When the average pore diameter of the reservoir is smaller, the pH of the oil displacement agent is higher to promote dispersion; when the formation water salinity of the reservoir is higher, the pH of the oil displacement agent is higher to promote dispersion, and the salinity is higher to maintain the salinity matching relationship.

[0060] Generally, the higher the pH and the lower the salinity are more conducive to dispersion, but if the formation water salinity is relatively high, the salinity needs to be increased accordingly to avoid salinity mismatch.

[0061] For example, when the average pore diameter of the reservoir is less than 1 μm, or the formation water salinity is higher than 80000 ppm, the salinity can be 2500 ppm and the pH can be 11; when the average pore diameter of the reservoir is greater than 20 μm, or the formation water salinity is less than 20000 ppm, the salinity can be 100 ppm and the pH can be 3; when the average pore diameter of the reservoir is between 1 μm and 20 μm, or the formation water salinity is between 20000 ppm and 80000 ppm, the salinity and pH can be flexibly selected between the above values.

[0062] The selection of the mass ratio of the carbon nanotubes and graphene oxide can be determined according to the reservoir conditions. The smaller the average pore diameter of the reservoir and the higher the formation water salinity, the lower the proportion of the carbon nanotubes should be;

[0063] For example, when the average pore size of the reservoir is less than 1 μm, or the salinity of the formation water is higher than 80,000 ppm, the mass ratio can be 20:1; when the average pore size of the reservoir is greater than 20 μm, or the salinity of the formation water is less than 20,000 ppm, the mass ratio can be 1:1; when the average pore size of the reservoir is between 1 μm and 20 μm, or the salinity of the formation water is between 20,000 ppm and 80,000 ppm, the mass ratio can be flexibly selected between 20:1 and 1:1.

[0064] In this embodiment, the average pore size of the reservoir chip used in the displacement experiment is about 7.5μm, which is between 1μm and 20μm. At the same time, the displacement experiment does not involve a high-salinity environment of more than 20,000ppm, so the mass ratio is selected to be 3:1, the pH is 8 to 10, and the salinity is 1500ppm to 2000ppm.

[0065] (1.1) Prepare graphene oxide slurry from natural graphite flakes using the improved Hummers method;

[0066] (1.2), re-dispersing the graphene oxide slurry obtained in step (1.1) with deionized water, and centrifuging at 9000 rpm to remove unreacted / unexfoliated graphite residues;

[0067] (1.3), take the supernatant obtained in step (1.2), and ultrasonicate it in a water bath at 100W for 1h to obtain a graphene oxide dispersion, and dry it in a vacuum drying oven to obtain a single-layer graphene oxide powder (the microscopic morphology of graphene oxide is flaky, and the flake diameter is about 1μm to 10μm);

[0068] (1.4), premix the graphene oxide powder obtained in step (1.3) with the carbon nanotube powder, stir and grind them thoroughly to ensure the contact area;

[0069] The carbon nanotubes used are multi-walled carbon nanotubes, the specifications of which are 30nm to 50nm in diameter, 100nm to 1000nm in length, and the mass ratio of graphene oxide to carbon nanotubes is 3:1;

[0070] The mixing degree of carbon nanotubes and graphene oxide was tested: 0.1g of mixed powder was taken, ultrasonicated in a water bath at 100W for 1h, filtered, and the filter cake was dried and weighed. The mass ratios of the three samples (the mass ratio of graphene oxide to carbon nanotubes) were 3.2:1, 3.3:1, and 2.9:1, respectively, and the relative errors were all less than 20%; the dispersion of the powder was observed under a high-power microscope, and the area in a clustered state accounted for about 6% of the observed area.

[0071] (1.5) Add 0.2 parts of the mixed powder described in step (1.4) to 100 parts of deionized water. After stirring at 1200 rpm for 1 h, perform strong ultrasonic treatment at 750 W for 2 h (it can be probe-type ultrasonic) using an ultrasonic crusher. After every 25 min of ultrasonic treatment under ice-water bath conditions, there is an interval of 10 min. The total concentration of the graphene oxide-carbon nanotube composite in the obtained dispersion is 0.2 wt.%. The sheet diameter of the graphene oxide in the prepared graphene oxide-carbon nanotube composite is about 100 nm to 1000 nm.

[0072] (1.6) Add 1500 ppm of NaCl to the dispersion obtained in (1.5) (i.e., the salinity of the dispersion is 1500 ppm), and adjust the pH value of the composite dispersion to 8.5 using HCl and NaOH.

[0073] Figure 1 Figure (a) shows the transmission electron microscopy characterization results of the prepared graphene oxide-carbon nanotube composite, and is compared with Figure 1 Figure (b) of the graphene oxide dispersion (i.e., in step (1.4), carbon nanotube powder is not added and other conditions are the same) and Figure 1 Figure (c) of the carbon nanotube dispersion (i.e., in step (1.4), graphene oxide powder is not added and other conditions are the same). From the transmission electron microscopy characterization results, the layered structure of graphene oxide and the fibrous structure of carbon nanotubes can be clearly observed. Moreover, the presence of graphene oxide can effectively achieve the stable dispersion of carbon nanotubes, while serious agglomeration occurs under the condition of only carbon nanotubes.

[0074] Figure 1 Figure (d) is the particle size distribution diagram of the graphene oxide-carbon nanotube composite measured based on the principle of dynamic light scattering, and is compared with the graphene oxide dispersion under the same conditions. From the measurement results, it can be seen that the graphene oxide-carbon nanotube composite has good uniform dispersion; the average value of the hydrodynamic diameter of graphene oxide is 303 nm, that is, small-sized thin sheets are formed; the graphene oxide-carbon nanotube composite has a relatively wider particle size distribution due to the random attachment of carbon nanotubes.

[0075] Figure 2 are the observation results under an optical microscope. From Figure 2 Figures (a) and (b), it can be seen that no flocculent distribution can be observed for the graphene oxide-carbon nanotube composite at high resolution, and it has long-term stability, thus ensuring that it can play a role under reservoir conditions; while as shown in Figure 2 Figures (c) and (d), under the condition of no graphene oxide, carbon nanotubes are extremely easy to agglomerate; Figure 2Similar results were also obtained in the static observation experiment shown in (e), that is, the carbon nanotube dispersion settled while the graphene oxide-carbon nanotube composite oil displacement agent remained stable.

[0076] Figure 3 The contact angle and interfacial tension measurement results of the graphene oxide-carbon nanotube composite oil displacement agent, graphene oxide dispersion, and silica nanoparticle dispersion at the same concentration are shown (the oil phase is n-decane); among them, the particle size of the silica nanoparticles is 20 nm, and the preparation method can refer to the preparation method of the reservoir chip in Chinese Patent CN113881415B. It can be seen that the interfacial action of the graphene oxide-carbon nanotube composite is the most significant, and it can most effectively reduce the contact angle and interfacial tension.

[0077] For the carbon nanotube dispersion, due to its extremely unstable dispersion state and very low effective content in the dispersion, its influence on the contact angle and interfacial tension is very small.

[0078] Figure 4 The results of the emulsification test are shown. The ratio of the oil phase (n-decane) to the water phase in the test tube is 1:1. After sufficient shaking, the formation of the water-in-oil emulsion is compared. It can be seen from the figure that neither deionized water nor the silica nanoparticle dispersion has an emulsifying effect, and the upper and lower layer interfaces are clear, while both the graphene oxide dispersion and the graphene oxide-carbon nanotube composite oil displacement agent show certain emulsifying abilities, and the upper transparent oil phase is replaced by the emulsion. Among them, the emulsifying effect of the graphene oxide-carbon nanotube composite oil displacement agent is the most intense. Figure 4 The internal microstructure of the formed emulsion is shown in.

[0079] For the carbon nanotube dispersion, although it will also form an emulsion due to its hydrophobic properties, the carbon nanotubes flocculate severely after encountering the oil phase and have no practical application value.

[0080] Figure 5 The wall surface adsorption roughness measurement results of the graphene oxide-carbon nanotube composite, graphene oxide, and silica nanoparticles at the same concentration are shown; it can be seen that the graphene oxide-carbon nanotube composite not only has a significant liquid-liquid interfacial action but also can form a multi-level rough structure through solid-liquid adsorption to displace more oil.

[0081] (2) Displacement experiment

[0082] The porous medium model used in this example is a reservoir chip designed according to the core structure of the Changqing Oilfield. The preparation method of the reservoir chip can refer to the preparation method of the reservoir chip in Chinese Patent CN110302853B. The surface properties of the chip are hydrophilic and neutral. The experimental steps are as follows:

[0083] (2.1) Inject water into the injection port of the reservoir chip at a flow rate of 1 μL / min for 15 min (the total volume of the chip reservoir is 0.84 μL (excluding the external fixing structure)). At this time, water appears at the outlet pipe.

[0084] (2.2) Inject the oil displacement agent with a concentration of 0.05 wt.% (obtained by diluting 0.2 wt.% of the oil displacement agent with deionized water) at a constant flow rate until no new oil is produced in the observation area.

[0085] When the volume change of the oil phase in the produced liquid is less than 0.1% after injecting 1 PV of the oil displacement agent, it is considered that no new oil is produced, and the concentration is increased.

[0086] (2.3) Inject the oil displacement agent with a concentration of 0.1 wt.% (obtained by diluting 0.2 wt.% with deionized water) at a constant flow rate until no new oil is produced in the observation area.

[0087] When the volume change of the oil phase in the produced liquid is less than 0.1% after injecting 1 PV of the oil displacement agent, it is considered that no new oil is produced, and the concentration is increased.

[0088] (2.4) Finally, inject the oil displacement agent with a concentration of 0.2 wt.% (designed concentration) at a constant flow rate until the experiment ends after no new oil is produced.

[0089] When the volume change of the oil phase in the produced liquid is less than 0.1% after injecting 1 PV of the oil displacement agent, it is considered that no new oil is produced, and the experiment ends.

[0090] To illustrate the effect of the graphene oxide-carbon nanotube composite oil displacement agent provided by this application and the advantages of the above application method, the above experiment was compared with the experimental effects of directly injecting 0.2 wt.% of the graphene oxide-carbon nanotube composite oil displacement agent, directly injecting water, directly injecting 0.2 wt.% of the graphene oxide dispersion, and directly injecting 0.2 wt.% of the silica nanoparticle dispersion. The injection flow rate and total time of all experiments are the same. Since the chip was severely blocked quickly when injecting the carbon nanotube dispersion, the recovery effect could not be statistically analyzed.

[0091] Tables 1 and 2 show the final recovery rates under different conditions. It can be seen from the results that, on the one hand, using a stepwise increasing injection concentration can achieve a similar effect of increasing the recovery rate as directly injecting at a high concentration, while the consumption of the oil displacement agent is greatly reduced; on the other hand, the graphene oxide-carbon nanotube composite oil displacement agent is significantly better than any one of water, graphene oxide, and silica nanoparticles.

[0092] Table 1: Final recovery rates under different conditions (hydrophilic surface)

[0093]

[0094]

[0095] Table 2: Ultimate Recovery Factor under Different Conditions (Neutral Surface)

[0096]

[0097] Figure 6 shows the comparison of the recovery curve during the displacement process of the graphene oxide-carbon nanotube composite flooding agent injected by the injection method in steps (2.1) to (2.4) above and the distribution of the displacement phase at the final moment with water flooding; Figure 6 It can be seen that the graphene oxide-carbon nanotube composite flooding agent can not only expand the swept volume but also promote the improvement of the displacement efficiency within the swept area. Therefore, it can play a role in porous media with any surface property and can continuously and stably recover residual oil even after breakthrough.

[0098] Exemplarily, when performing steps (2.1) to (2.4), after the injection pressure rises to 0.5 MPa to 2 MPa, water injection is restarted, that is, steps (2.1) to (2.4) are repeated.

[0099] Exemplarily, when the average pore diameter of the reservoir is less than 1 μm, or the formation water salinity is higher than 80,000 ppm, and the concentration of the flooding agent is higher than 0.1 wt.%, a surfactant is added for auxiliary dispersion, and the mass fraction of the surfactant is lower than the total mass fraction of graphene oxide and carbon nanotubes.

[0100] Exemplarily, the surfactant is selected from any one or more of anionic surfactants, sulfobetaine 12, and polyethylene glycol.

[0101] Exemplarily, the anionic surfactant is selected from any one or more of sodium dodecyl sulfate and sodium dodecylbenzenesulfonate.

Claims

1. An oil displacing agent, characterized in that: The oil displacing agent includes a graphene oxide-carbon nanotube composite; The graphene oxide-carbon nanotube composite is a sheet-like-fiber composite structure; The sheet diameter of the graphene oxide in the sheet-like-fibrous composite structure is selected from 100 nm to 1000 nm; the tube diameter of the carbon nanotube in the sheet-like-fibrous composite structure is selected from 5 nm to 10000 nm, and the tube length of the carbon nanotube in the sheet-like-fibrous composite structure is selected from 50 nm to 50000 nm; The salinity range of the oil displacement agent is 100 ppm to 2500 ppm; The pH value of the oil displacing agent is 3 to 11; The preparation method of the oil displacing agent comprises: Mixing graphene oxide powder and carbon nanotube powder, stirring and grinding them in a powder state to obtain a mixed powder; the mass ratio of the graphene oxide powder to the carbon nanotube is 20:1 to 1:1; The mixed powder is mixed evenly with water to obtain the oil-displacing agent.

2. The oil-displacing agent according to claim 1, characterized in that The salt used to adjust the salinity range is selected from any one or more of NaCl, KCl, CaCl2, NaHCO3 and MgSO4.

3. The oil-displacing agent according to claim 1, characterized in that The pH adjuster used to adjust the pH value is selected from any one or more of HCl, NaOH, KOH, NH3·H2O, Na2CO3 and CaO.

4. The oil-displacing agent according to claim 1, characterized in that The carbon nanotubes are selected from any one or more of single-walled carbon nanotubes, double-walled carbon nanotubes and multi-walled carbon nanotubes.

5. The oil-displacing agent according to claim 4, characterized in that The carbon nanotubes are selected from multi-walled carbon nanotubes.

6. The oil-displacing agent according to claim 5, characterized in that The tube diameter of the multi-walled carbon nanotube is selected from 30nm to 50nm, and the tube length of the multi-walled carbon nanotube is selected from 100nm to 1000nm.

7. The oil-displacing agent according to any one of claims 1 to 6, characterized in that The mass ratio of the graphene oxide powder to the carbon nanotubes is 5:1 to 2:

1.

8. The oil-displacing agent according to any one of claims 1 to 6, characterized in that The weight ratio of the mixed powder to water is (0.005 to 5):

100.

9. The oil-displacing agent according to claim 8, characterized in that The weight ratio of the mixed powder to water is (0.1 to 1):

100.

10. The oil displacing agent according to any one of claims 1 to 6, characterized in that The mixture of the mixed powder and the water is stirred at 400 rpm to 2000 rpm for 1 h to 2 h.

11. The oil displacing agent according to any one of claims 1 to 6, characterized in that The mixture of the mixed powder and the water is sonicated at 300 W to 1500 W for 1 to 2 h.

12. The oil displacing agent according to claim 11, characterized in that The ultrasonic power was 600 W to 800 W, and the ultrasonication was performed in an ice water bath.

13. The use of an oil displacing agent according to any one of claims 1 to 12, characterized in that: (1) selecting a target well, wherein the target well is an oil production well in which water is found in the produced fluid; (2) injecting the oil-displacing agent through an injection well, with an initial injection concentration of 1 / 20 to 1 / 2 of the designed concentration; after no new oil is produced, increasing the injection concentration to 1.5 to 5 times the original concentration; And so on, until the injection concentration reaches the designed concentration.

14. The use of the oil displacing agent according to claim 13, characterized in that: The proportion of water in step (1) is more than 10% of the produced fluid from the oil well.

15. The use of the oil displacing agent according to claim 13, characterized in that: The designed concentration in step (2) is 0.0001wt.% to 0.5wt.%.

16. The use of the oil displacing agent according to claim 15, characterized in that: The designed concentration is 0.05wt.% to 0.2wt.%.

17. The use of the oil displacing agent according to claim 13, characterized in that: When the step (2) is performed, after the injection pressure increases to 0.5 MPa to 2 MPa, water injection is restarted, i.e., steps (1) and (2) are repeated.

18. The use of an oil displacing agent according to any one of claims 13 to 17, characterized in that: When the average pore size of the reservoir is less than 1 μm, or the salinity of the formation water is higher than 80,000 ppm, and the concentration of the oil-displacing agent is higher than 0.1 wt.%, a surfactant is added to assist dispersion, and the mass fraction of the surfactant is lower than the total mass fraction of the graphene oxide and the carbon nanotubes.

19. The use of the oil displacing agent according to claim 18, characterized in that: The surfactant is selected from any one or more of anionic surfactants, sulfobetaine 12 and polyethylene glycol.

20. The use of the oil displacing agent according to claim 19, characterized in that: The anionic surfactant is selected from any one or more of sodium dodecyl sulfate and sodium dodecylbenzene sulfonate.

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