阅读说明：本技术 一种利用纤维素纳米晶体模拟石油采收的方法 (Method for simulating petroleum recovery by using cellulose nanocrystals ) 是由 边航 陆程 罗万静 崔玉东 关子越 沙志斌 于 2020-12-24 设计创作，主要内容包括：本发明公开了一种利用纤维素纳米晶体模拟石油采收的方法,包括以下步骤：处理原油,利用真空过滤器和滤纸过滤原油；制备纤维素纳米晶体-氯化钠混合胶体溶液,一次筛选纤维素纳米晶体-氯化钠混合胶体溶液中的稳定浓度范围,再通过检测界面张力测量方式和接触角测量方式二次筛选出影响在砂岩中渗透率的纤维素纳米晶体-氯化钠胶体系统浓度；将二次筛选出的纤维素纳米晶体-氯化钠混合胶体溶液按照两次驱替模式注入砂岩岩心内；对比两次驱替模式的产出液,计算并对比两次驱替模式的采收率的数据；本发明利用纤维素纳米晶体颗粒调堵高渗透条带,扩大波及体积；利用纤维素纳米晶体-氯化钠混合胶体溶液改善油水界面性质提高原油采收率。(The invention discloses a method for simulating petroleum recovery by using cellulose nanocrystals, which comprises the following steps: processing the crude oil, and filtering the crude oil by using a vacuum filter and filter paper; preparing a cellulose nanocrystal-sodium chloride mixed colloidal solution, screening a stable concentration range in the cellulose nanocrystal-sodium chloride mixed colloidal solution for the first time, and screening the concentration of a cellulose nanocrystal-sodium chloride colloidal system influencing the permeability in sandstone for the second time by a detection interfacial tension measurement mode and a contact angle measurement mode; injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution screened twice into the sandstone core according to a twice displacement mode; comparing the produced fluids in the two displacement modes, and calculating and comparing the data of the recovery ratio in the two displacement modes; the invention utilizes cellulose nanocrystal particles to adjust and block a high permeability strip, and enlarges swept volume; the oil-water interface property is improved by using the cellulose nanocrystal-sodium chloride mixed colloidal solution to improve the crude oil recovery ratio.)

1. A method for simulating oil recovery using cellulose nanocrystals, comprising the steps of:

step 100, processing crude oil, and filtering the crude oil by using a vacuum filter and filter paper;

200, preparing a cellulose nanocrystal-sodium chloride mixed colloidal solution, screening a stable concentration range in the cellulose nanocrystal-sodium chloride mixed colloidal solution by using a stability measurement mode for the first time, and screening the concentration of a cellulose nanocrystal-sodium chloride colloidal system influencing the permeability in sandstone for the second time in the cellulose nanocrystal-sodium chloride mixed colloidal solution in the stable concentration range;

step 300, pretreating the sandstone core, and injecting the secondarily screened cellulose nanocrystal-sodium chloride mixed colloidal solution into the pretreated sandstone core according to a twice displacement mode;

and step 400, comparing the produced fluids in the two displacement modes, and calculating and comparing the data of the recovery ratio in the two displacement modes.

2. The method for simulating oil recovery by using cellulose nanocrystals according to claim 1, wherein the step of preparing the cellulose nanocrystal-sodium chloride mixed colloidal solution in step 200 comprises the following steps:

obtaining a concentrated solution of cellulose nanocrystals at a concentration of 12.18 wt%;

mixing and diluting deionized water and the original cellulose nanocrystals to form cellulose nanocrystal colloidal solutions with different concentrations;

mixing sodium chloride solutions of different concentrations with the original cellulose nanocrystal colloidal solution to dilute the cellulose nanocrystal colloidal solution to different concentrations and generate different classes of the cellulose nanocrystal-sodium chloride mixed colloidal solution;

the cellulose nanocrystal-sodium chloride mixed colloidal solution of the same category is divided into two groups after being uniformly stirred, and the two groups of the cellulose nanocrystal-sodium chloride mixed colloidal solution are respectively stored in a constant temperature box at 20 ℃ and a constant temperature box at 60 ℃.

3. The method for simulating oil recovery by using cellulose nanocrystals according to claim 2, wherein the method for measuring stability of the mixed colloidal solution of cellulose nanocrystals and sodium chloride comprises the following steps:

sampling the cellulose nanocrystal-sodium chloride mixed colloidal solution stored in a constant temperature box at 20 ℃ and 60 ℃ at regular time to carry out a stability measurement experiment;

and comparing the measurement results of each stability measurement experiment, and screening out the cellulose nanocrystal-sodium chloride colloidal solution which is still stable after being prepared for one month.

4. The method for simulating oil recovery by using cellulose nanocrystals, as claimed in claim 3, wherein the cellulose nanocrystal-sodium chloride mixed colloidal solution with a stable concentration range obtained by the primary screening is secondarily screened by the interfacial tension measurement method and the contact angle measurement method, and the concentration of the cellulose nanocrystal-sodium chloride mixed colloidal solution affecting permeability is filtered to increase the recovery ratio, and the method is implemented by:

and (3) carrying out an interfacial tension measurement experiment on the crude oil and the cellulose nanocrystal-sodium chloride colloidal solution screened out to keep stability, and a measurement experiment on a contact angle between the colloidal solution and the sandstone surface, and further screening out a colloidal solution with a proper concentration range from the stable colloidal solution so as to improve the injectability of the colloidal system in the sandstone.

5. The method for simulating oil recovery by using cellulose nanocrystals according to claim 4, wherein the contact angle is measured by dropping the selected stable concentration range of the cellulose nanocrystal-sodium chloride mixed colloidal solution on the polished sandstone surface.

6. The method for simulating oil recovery by using cellulose nanocrystals according to claim 5, wherein the polished sandstone pre-soaking is subjected to saturated aging in the crude oil environment of the step 100.

7. The method for simulating oil recovery by using cellulose nanocrystals according to claim 4, wherein the method comprises the following steps: in step 300, injecting the secondarily screened cellulose nanocrystal-sodium chloride mixed colloidal solution into the pretreated sandstone core according to a twice-displacement mode, wherein the twice-displacement mode comprises a mode of injecting 0.1 wt% NaCl solution as a first displacement mode until oil-free production and stable pressure difference are achieved; and then injecting the screened cellulose nanocrystal-sodium chloride mixed colloidal solution as a secondary displacement mode until oil-free production is achieved and the pressure difference is stable.

8. The method for simulating oil recovery by using cellulose nanocrystals according to claim 7, wherein the step of establishing the model body in step 300 is realized by:

cleaning the sandstone to prevent porosity blockage of the sandstone, and performing vacuum saturation on the sandstone by using a NaCl solution;

injecting the crude oil filtered in the step 100 into the sandstone, and performing crude oil saturation on a sandstone core to simulate the storage environment of the crude oil in the sandstone.

9. The method for simulating oil recovery using cellulose nanocrystals according to claim 8, wherein: in step 300, the specific implementation method for injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution into the model main body is as follows:

301, performing a primary oil recovery displacement experiment on a 0.1 wt% sodium chloride solution by adopting a sequence of low flow rate of 0.3mL/min and high flow rate of 3mL/min, and collecting a first output liquid by using a test tube with scales;

step 302, stopping injecting the sodium chloride solution when no obvious oil drop is visible in the produced liquid and the pressure difference is stable, performing a secondary oil recovery displacement experiment on the screened cellulose nanocrystal-sodium chloride mixed colloidal solution with the appropriate concentration range by adopting a sequence of firstly low flow rate of 0.3mL/min and then high flow rate of 3mL/min, and collecting a second produced liquid again;

and step 303, stopping injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution when no obvious oil drop is visible in the second output liquid and the pressure difference is stable.

10. The method for simulating oil recovery using cellulose nanocrystals according to claim 9, wherein: in step 303, after stopping the injection of the cellulose nanocrystal-sodium chloride mixed colloidal solution, the 0.1 wt% sodium chloride solution is re-injected to flush out the retained cellulose nanocrystals in the Berea sandstone core.

Technical Field

The invention relates to the technical field of petroleum recovery simulation experiment methods, in particular to a method for simulating petroleum recovery by using cellulose nanocrystals.

Background

The polymer flooding method is a method for increasing the viscosity of water by using a water-soluble high molecular polymer and using the water-soluble high molecular polymer as an oil field development injection agent to improve the oil recovery rate. The basic principle of polymer flooding for improving oil recovery is that the fluidity of an oil displacement agent is reduced by increasing the viscosity of injected water, so that fingering and channeling are reduced, and the sweep efficiency is increased to improve the oil recovery.

Currently, the most commonly used polymers for polymer flooding methods are partially Hydrolyzed Polyacrylamide (HPAM) and xanthan gum, both of which reduce its flowability by increasing the viscosity of the water. However, both of these materials have substantial performance limitations in enhanced oil recovery: xanthan gum is easily degraded by bacteria, fragments easily cause pore blockage, and thermal stability is poor; HPAM is susceptible to a variety of chemical, thermal, and mechanical degradation effects, and thus its viscosifying effect gradually decreases as it passes through the pore media. Because many polymers used in current polymer flooding are toxic and harmful to the environment, a polymer flooding method needs a stable, environmentally friendly polymer to enhance oil recovery.

Disclosure of Invention

The invention aims to provide a method for simulating oil recovery by using cellulose nanocrystals, which aims to solve the technical problems of rock pore blockage, poor thermal stability, toxicity of various polymers and harm to the environment caused by a polymer flooding method in the prior art.

In order to solve the technical problems, the invention specifically provides the following technical scheme:

a method for simulating oil recovery using cellulose nanocrystals, comprising the steps of:

step 100, processing crude oil, and filtering the crude oil by using a vacuum filter and filter paper;

200, preparing a cellulose nanocrystal-sodium chloride mixed colloidal solution, screening a stable concentration range in the cellulose nanocrystal-sodium chloride mixed colloidal solution by using a stability measurement mode for the first time, and screening the concentration of a cellulose nanocrystal-sodium chloride colloidal system influencing the permeability in sandstone through the cellulose nanocrystal-sodium chloride mixed colloidal solution in the stable concentration range by detecting an interfacial tension measurement mode and a contact angle measurement mode for the second time;

step 300, pretreating the sandstone core, and injecting the secondarily screened cellulose nanocrystal-sodium chloride mixed colloidal solution into the pretreated sandstone core according to a twice displacement mode;

and step 400, comparing the produced fluids in the two displacement modes, and calculating and comparing the data of the recovery ratio in the two displacement modes.

As a preferable embodiment of the present embodiment, in step 200, the preparation of the cellulose nanocrystal-sodium chloride mixed colloidal solution is performed by:

obtaining an original concentrated solution of cellulose nanocrystals having a concentration of 12.18 wt%;

mixing and diluting deionized water and the original cellulose nanocrystals to form cellulose nanocrystal colloidal solutions with different concentrations;

mixing sodium chloride solutions of different concentrations with the original cellulose nanocrystal colloidal solution to dilute the cellulose nanocrystal colloidal solution to different concentrations and generate different classes of the cellulose nanocrystal-sodium chloride mixed colloidal solution;

the cellulose nanocrystal-sodium chloride mixed colloidal solution of the same category is divided into two groups after being uniformly stirred, and the two groups of the cellulose nanocrystal-sodium chloride mixed colloidal solution are respectively stored in a constant temperature box at 20 ℃ and a constant temperature box at 60 ℃.

As a preferred embodiment of the present invention, in step 200, a specific implementation method for screening a stable concentration range in the cellulose nanocrystal-sodium chloride mixed colloidal solution by using a stability measurement manner at a time includes:

sampling the cellulose nanocrystal-sodium chloride mixed colloidal solution stored in a constant temperature box at 20 ℃ and 60 ℃ at regular time to carry out a stability measurement experiment;

and comparing the measurement results of each stability measurement experiment, and screening out the cellulose nanocrystal-sodium chloride colloidal solution which is still stable after being prepared for one month.

As a preferable embodiment of the present invention, the cellulose nanocrystal-sodium chloride mixed colloidal solution in the stable concentration range obtained by the primary screening is secondarily screened by detecting an interfacial tension measurement method and a contact angle measurement method, and the concentration of the cellulose nanocrystal-sodium chloride mixed colloidal solution affecting permeability is filtered to improve the recovery ratio, and the specific implementation method is as follows:

and (3) carrying out an interfacial tension measurement experiment on the crude oil and the cellulose nanocrystal-sodium chloride colloidal solution screened out to keep stability, and a measurement experiment on a contact angle between the colloidal solution and the sandstone surface, and further screening out a colloidal solution with a proper concentration range from the stable colloidal solution so as to improve the injectability of the colloidal system in the sandstone.

In a preferred embodiment of the present invention, the contact angle measurement method is specifically a contact angle of the selected stable concentration range of the cellulose nanocrystal-sodium chloride mixed colloidal solution on the polished sandstone surface.

As a preferable embodiment of the present invention, the polished sandstone is presoaked in the crude oil environment of step 100 for saturated aging.

As a preferable scheme of the embodiment, in step 300, injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution screened twice into the pretreated sandstone core according to a twice-displacement mode, wherein the twice-displacement mode comprises injecting a 0.1 wt% NaCl solution as a once-displacement mode until oil-free production and stable pressure difference are achieved; and then injecting the screened cellulose nanocrystal-sodium chloride mixed colloidal solution as a secondary displacement mode until oil-free production is achieved and the pressure difference is stable.

As a preferable embodiment of the present invention, in step 300, the step of establishing the model body includes:

cleaning the sandstone to prevent porosity blockage of the sandstone, and soaking the sandstone by using a vacuum saturated NaCl solution;

injecting the crude oil filtered in the step 100 into the sandstone, and performing crude oil saturation on a sandstone core to simulate the storage environment of the crude oil in the sandstone.

As a preferable embodiment of the present invention, in step 300, a specific implementation method of injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution into the model main body is as follows:

301, performing a primary oil recovery displacement experiment on a 0.1 wt% sodium chloride solution by adopting a sequence of low flow rate of 0.3mL/min and high flow rate of 3mL/min, and collecting a first output liquid by using a test tube with scales;

step 302, stopping injecting the sodium chloride solution when no obvious oil drop is visible in the produced liquid and the pressure difference is stable, performing a secondary oil recovery displacement experiment on the screened cellulose nanocrystal-sodium chloride mixed colloidal solution with the appropriate concentration range by adopting a sequence of firstly low flow rate of 0.3mL/min and then high flow rate of 3mL/min, and collecting a second produced liquid again;

and step 303, stopping injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution when no obvious oil drop is visible in the second output liquid and the pressure difference is stable.

In a preferred embodiment of the present invention, in step 303, after the injection of the cellulose nanocrystal-sodium chloride mixed colloidal solution is stopped, a 0.1 wt% sodium chloride solution is re-injected to flush out the retained cellulose nanocrystals in the Berea sandstone core.

Compared with the prior art, the invention has the following beneficial effects:

according to the injectability of the nano-cellulose-sodium chloride solution colloid system in sandstone, the concentration range of colloid components is screened, and the risk of pore throat blockage caused by instability of the colloid system due to overhigh concentration is reduced; the high permeability strip is adjusted and blocked by utilizing the cellulose nanocrystal particles, and the swept volume is enlarged; the oil-water interface property is improved by using the cellulose nanocrystal-sodium chloride mixed colloidal solution to improve the crude oil recovery ratio.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. It should be apparent that the drawings in the following description are merely exemplary, and that other embodiments can be derived from the drawings provided by those of ordinary skill in the art without inventive effort.

FIG. 1 is a schematic flow chart of a simulation experiment method according to an embodiment of the present invention;

FIG. 2 is a diagram of the solution sample ratios, i.e., labels, of the mixed colloidal solution of cellulose nanocrystals and sodium chloride provided in the embodiment of the present invention;

FIG. 3 is a graph showing the measurement results of the particle size of particles provided in the examples of the present invention;

FIG. 4 is a graph showing the results of measurement of zeta potential provided by an embodiment of the present invention;

FIG. 5 is a graph of interfacial tension and contact angle measurements provided by an embodiment of the present invention;

FIG. 6 is a representation of core data for simulated oil production provided by an embodiment of the present invention;

FIG. 7 is a schematic diagram of the amount of oil produced in simulating oil production according to an embodiment of the present invention;

fig. 8 is a schematic operation flow diagram of an experimental method according to an embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

As shown in fig. 1, the present invention provides a method for simulating oil recovery using cellulose nanocrystals, comprising the steps of:

step 100, processing crude oil, and filtering the crude oil by using a vacuum filter and filter paper;

200, preparing a cellulose nanocrystal-sodium chloride mixed colloidal solution, screening a stable concentration range in the cellulose nanocrystal-sodium chloride mixed colloidal solution by using a stability measurement mode for the first time, and screening the concentration of the cellulose nanocrystal-sodium chloride colloidal system influencing the permeability in the sandstone through the cellulose nanocrystal-sodium chloride mixed colloidal solution in the stable concentration range for the second time by detecting an interfacial tension measurement mode and a contact angle measurement mode.

In the step, the preparation of the cellulose nanocrystal-sodium chloride mixed colloidal solution is realized by the following steps:

(1) a concentrated colloidal solution of the original cellulose nanocrystals was obtained at a concentration of 12.18 wt%.

(2) Deionized water and original cellulose nanocrystals are mixed and diluted to form cellulose nanocrystal colloidal solutions with different concentrations.

(3) Mixing sodium chloride solutions with different concentrations with the original cellulose nanocrystal colloidal solution to dilute the cellulose nanocrystal colloidal solution to different concentrations and generate different classes of cellulose nanocrystal-sodium chloride mixed colloidal solutions.

Specifically, in the step (2), the original concentrated colloidal solution of cellulose nanocrystals with a concentration of 12.18 wt% is diluted to a colloidal solution of cellulose nanocrystals with a concentration of 0.05 wt%, 0.1 wt%, 0.5 wt%, 1.0 wt% using deionized water.

In step (3), 0.1 wt% and 0.5 wt% sodium chloride solutions are mixed with 12.18 wt% original cellulose nanocrystal concentrated colloidal solutions, respectively, and the 12.18 wt% original cellulose nanocrystal concentrated colloidal solutions are diluted to 0.05 wt%, 0.1 wt%, 0.5 wt%, and 1.0 wt% cellulose nanocrystal colloidal solutions, respectively.

(4) The cellulose nanocrystal-sodium chloride mixed colloidal solution of the same category is uniformly stirred and divided into two groups, the two groups of cellulose nanocrystal-sodium chloride mixed colloidal solutions are respectively placed in thermostats at 20 ℃ and 60 ℃ for storage, and the solution sample ratio, namely the label of the specifically prepared cellulose nanocrystal-sodium chloride mixed colloidal solution is shown in figure 2, so that 24 colloidal solutions are obtained.

It should be added that when sodium chloride solutions with different solubilities are mixed with the diluted cellulose nanocrystal colloidal solution, a dispersion machine is used to stir the colloidal solution to make the particles distributed more uniformly so as to avoid aggregating together to form aggregates and settling.

The specific implementation method for respectively measuring the particle size and the zeta potential of the 24 colloidal solutions according to the planned time points and screening the stable concentration range in the cellulose nanocrystal-sodium chloride mixed colloidal solution by using the stability measurement mode comprises the following steps:

firstly, sampling cellulose nanocrystal-sodium chloride mixed colloid solution stored in a constant temperature box at 20 ℃ and 60 ℃ at regular time to carry out a stability measurement experiment. Wherein, the time selection points for timing sampling measurement are the day, 24 hours, one week and one month of solution preparation.

And (II) comparing the stability measurement results of each measurement experiment, and screening out the cellulose nanocrystal-sodium chloride colloidal solution which is still stable after being prepared for one month.

It should be added that the parameters for measuring the stability of the prepared cellulose nanocrystal-sodium chloride mixed colloidal solution are as follows: the particle size of the colloidal solution measured at the above sampling time point and the zeta potential of the colloidal solution measured at the sampling time point are shown in fig. 3 and 4.

In addition, a nanoscale Malvern Zetasizer Nano ZS potential analyzer was used in particle measurement experiments on cellulose nanocrystal-sodium chloride mixed colloidal solution samples to measure the nanoparticle size and Zeta potential of the cellulose nanocrystal-sodium chloride mixed colloidal solution.

According to the experimental results, the data of both particle size and zeta potential prove that the particle size of the colloidal solution of cellulose nanocrystals (abbreviated as CNC (USDA)) with any concentration mixed with the 0.5 wt% NaCl solution is larger than that of the colloidal solution with the same concentration CNC (USDA)) in the NaCl solution with the weight lower than 0.5 wt% and the zeta potential absolute value of the former is lower than that of the latter at the room temperature of 20 ℃ or the high temperature of 60 ℃.

Furthermore, the 0.05 wt% and 0.1 wt% cnc (usda) colloidal solutions remain substantially larger than 1000nm in size in 0.5 wt% NaCl solution, the absolute value of the zeta potential is less than 30, so that neither of the two stoichiometric colloidal solutions, i.e., 0.05 wt% and 0.1 wt% cnc (usda) colloidal solutions, remains stable, the nanoparticles in the colloidal system flocculate and settle in a short period of time, and the colloidal system consisting of other concentrations of cnc (usda) and 0.5 wt% NaCl solution exhibits greater instability than it does in low concentration salt solutions.

As shown in fig. 3 and 4, according to the contact angle experiment result, the colloid solution with low salt concentration and high CNC (usda) concentration should be selected in the stable concentration range for the experiment, and since the salinity of the seawater is not zero, 1.0 wt% CNC +0.1 wt% NaCl colloid solution is finally selected as the displacement fluid for the displacement experiment for improving the recovery ratio.

As a preferable scheme of the embodiment, the concentration of the cellulose nanocrystal-sodium chloride mixed colloidal solution which is beneficial to improving the recovery ratio is secondarily screened from the cellulose nanocrystal-sodium chloride mixed colloidal solution in the stable concentration range which is screened for the first time by detecting an interfacial tension measurement mode and a contact angle measurement mode, and the specific implementation method is as follows:

and (3) carrying out an interfacial tension measurement experiment on the crude oil and the cellulose nanocrystal-sodium chloride colloidal solution screened out to keep the stability, and a measurement experiment on a contact angle between the colloidal solution and the sandstone surface, and further filtering the solution concentration of the cellulose nanocrystal-sodium chloride colloidal solution influencing the permeability from the stable colloidal solution.

The interfacial tension measurement experiment between the crude oil and the screened colloidal solution and the measurement experiment of the contact angle between the colloidal solution and the sandstone surface are used for analyzing the permeability influence factors (interfacial tension and contact angle) to reduce the interfacial tension by adding CNC (USDA) colloidal solution into the displacement fluid and increase the contact angle between the crude oil and the sandstone, and the conclusion shows that the recovery ratio can be improved by adding the cellulose nanocrystals.

The contact angle measurement mode is specifically that the polished sandstone is soaked in advance and saturated and aged in the crude oil environment in the step 100, and the contact angle of the selected cellulose nanocrystal-sodium chloride mixed colloidal solution with the stable concentration range on the polished sandstone surface is dropped.

In this step, the crude oil from the North sea was filtered through a vacuum filter and 5 μm filter paper, and then the interfacial tension between the 24 colloidal solutions and the crude oil as shown in FIG. 2 was measured, and the contact angle of each colloidal solution drop in the crude oil with the polished quartz surface was measured.

The results of 24 experiments on the interfacial tension of the colloidal solution and the crude oil, and the contact angle of the crude oil droplet with the polished quartz surface in the colloidal solution environment are shown in fig. 5.

It can be known from the interfacial tension data that the interfacial tension of crude oil and colloidal solution is slightly lower than that of the crude oil and colloidal solution at the temperature of 60 ℃, and the temperature of the oil reservoir environment is generally higher than room temperature, so for the subsequent displacement experiment, the effect under the oil reservoir condition is deduced from the experiment result of the part of the experiment to be better than that obtained by the displacement simulation experiment at room temperature.

According to the contact angle data, the higher the concentration of the cellulose nanocrystals is, the larger the contact angle is under the same salt concentration; when the concentrations of the cellulose nanocrystals are the same, the higher the salt concentration is, the smaller the contact angle is; the contact angle of the colloidal solution with the sandstone surface is integrally larger under the condition of 60 ℃ than under the condition of 20 ℃. The smaller the contact angle, the lower the wettability of the crude oil to quartz, and the higher the wettability to water, the more easily the crude oil is removed from the rock surface and recovered.

Step 300, pretreating the sandstone core, and injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution with stable concentration into the pretreated sandstone core according to a twice-displacement mode, wherein a specific experimental flow chart is shown in fig. 8.

The cellulose nanocrystal-sodium chloride mixed colloidal solution screened twice is injected into the pretreated sandstone core according to a twice-displacement mode, wherein the twice-displacement mode comprises the step of injecting a 0.1 wt% NaCl solution as a once-displacement mode until oil-free production and stable pressure difference are achieved; and then injecting the screened cellulose nanocrystal-sodium chloride mixed colloidal solution as a secondary displacement mode until oil-free production is achieved and the pressure difference is stable.

It should be further specifically noted that, in the present embodiment, the cellulose nanocrystal-sodium chloride mixed colloidal solution may be used not only for simulating oil exploitation, but also for simulating hydrate exploitation, as for the stability screening and permeability screening of the cellulose nanocrystal-sodium chloride colloidal solution are not changed, the creation mode of the model body is changed to establish a storage environment (low temperature and high pressure) for simulating natural gas hydrate in the formation, and the development amount of the cellulose nanocrystal-sodium chloride colloidal solution for natural gas hydrate and the specific exploitation rate increase range for natural gas hydrate may be calculated by injecting the cellulose nanocrystal-sodium chloride colloidal solution and collecting the yield of natural gas.

The realization principle of improving the oil recovery ratio by using the cellulose nanocrystal-sodium chloride colloidal solution is as follows: after the cellulose nanocrystal-sodium chloride colloidal solution with certain concentration and stable determination is injected into a stratum, the cellulose nanocrystal-sodium chloride colloidal solution firstly flows into a communicating pore with a larger throat radius, along with the flow of fluid, the cellulose nanocrystal is retained at the throat, so that the throat radius is narrowed, the inflow pressure is increased, the fluid further flows into a passage with a narrower throat radius, the sweep efficiency is improved, and the recovery ratio is improved.

In the step, the method for preprocessing the sandstone core comprises the following steps:

(a) performing pretreatment operation on the Berea sandstone core, wherein the pretreatment operation comprises cleaning and drying the Berea sandstone core and saturated soaking by using saline water so as to prevent porosity of the sandstone from being blocked;

(b) injecting crude oil into the pretreated Berea sandstone core, and saturating the Berea sandstone core with the crude oil at a low flow rate of 1mL/min and a high flow rate of 10mL/min in the injection process so as to simulate the storage environment of the crude oil in the sandstone.

In addition, the concrete implementation steps of the pretreatment operation on the Berea sandstone core are as follows:

wherein the content of the first and second substances,

(I) cleaning the Berea sandstone core by using a 3% NaCl solution and methanol, drying, and measuring the dry weight, porosity and air permeability of the dried Berea sandstone core;

(II) soaking the Berea sandstone core by using a vacuum and saturated 0.1 wt% NaCl solution, and weighing the saturated wet weight of the Berea sandstone core;

(III) injecting the filtered crude oil obtained in the step 100 into the Berea sandstone core, displacing NaCl solution in the Berea sandstone core to leach until the inflow and outflow of the crude oil are consistent, and calculating the irreducible water saturation of the Berea sandstone core at the moment, wherein the irreducible water saturation of the Berea sandstone core calculated by the embodiment is 28.45%.

When the filtered crude oil obtained in the step 100 is injected into the Berea sandstone core, the crude oil is injected at the low speed of 1mL/min, and after the crude oil is stabilized, the crude oil is injected at the high speed of 10 mL/min.

It should be added that, after the filtered crude oil obtained in the step 100 is injected into the Berea sandstone core and the irreducible water saturation of the Berea sandstone core at this time is calculated, the Berea sandstone core is soaked in the crude oil obtained in the step 100 and stands still, so that the pores of the Berea sandstone core are fully saturated with the crude oil.

As one of the innovative points of the embodiment, the amount of oil injected into sandstone can be calculated in real time by injecting a proper amount of oil until saturation according to the porosity and air permeability of sandstone, and the influence of the displacement experiment of the cellulose nanocrystal-sodium chloride mixed colloid solution on oil recovery and the data change of oil recovery rate can be calculated according to the amount of oil injected into sandstone.

In addition, the concrete implementation method for injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution into the model main body comprises the following steps:

301, performing a primary oil recovery displacement experiment on a 0.1 wt% sodium chloride solution by adopting a sequence of low flow rate of 0.3mL/min and high flow rate of 3mL/min, and collecting a first output liquid by using a test tube with scales;

step 302, stopping injecting the sodium chloride solution when no obvious oil drop is visible in the produced liquid and the pressure difference is stable, performing a secondary oil recovery displacement experiment on the screened cellulose nanocrystal-sodium chloride mixed colloidal solution with the appropriate concentration range by adopting a sequence of firstly low flow rate of 0.3mL/min and then high flow rate of 3mL/min, and collecting a second produced liquid again;

step 303, stopping injecting the cellulose nanocrystal-sodium chloride mixed colloidal solution when no obvious oil drop is visible in the second output liquid and the pressure difference is stable.

In step 303, after the injection of the cellulose nanocrystal-sodium chloride mixed colloidal solution is stopped, a 0.1 wt% sodium chloride solution is injected to flush out the retained cellulose nanocrystals in the Berea sandstone core.

And step 400, comparing the produced fluids in the two displacement modes, and calculating the recovery ratio of the two displacement modes.

The specific experimental results are shown in fig. 6 and 7, and the experimental results show that on the basis of the original 0.1 wt% sodium chloride solution secondary displacement, 1.0 wt% cnc (usda) +0.1 wt% NaCl colloidal solution tertiary displacement improves the recovery efficiency by about 2.76%, because cnc (usda) is added, the interfacial tension of the water phase and the oil phase is reduced, and the crude oil is light crude oil, and the emulsion output is seen in the experimental process.

Therefore, according to the injectability of the nano-cellulose-sodium chloride solution colloid system in sandstone, the concentration range of the colloid composition is screened, and the risk of pore throat blockage caused by instability of the colloid system due to overhigh concentration is reduced; the high permeability strip is adjusted and blocked by utilizing the cellulose nanocrystal particles, and the swept volume is enlarged; the oil-water interface property is improved by using the cellulose nanocrystal-sodium chloride mixed colloidal solution to improve the crude oil recovery ratio.

The above embodiments are only exemplary embodiments of the present application, and are not intended to limit the present application, and the protection scope of the present application is defined by the claims. Various modifications and equivalents may be made by those skilled in the art within the spirit and scope of the present application and such modifications and equivalents should also be considered to be within the scope of the present application.