An in situ method for determining the size of asphaltene precipitates formed by CO2

Abstract

The invention relates to the technical field of crude oil production, and discloses a method for in-situ determination of the size of asphaltene precipitates formed by CO2 driving. The method adopts in-situ determination of the size of asphaltene precipitates formed by CO2 driving for determination. The method can adjust the operating conditions such as pressure, temperature, crude oil type, CO2 purity, etc. according to actual conditions, simulate the in-situ generation of asphaltene precipitate particles under the dynamic driving conditions of CO2, accurately and reliably determine the size of asphaltene precipitates, and the determination result can be used to evaluate whether the cause of reservoir blockage is surface deposition, entrainment or pore throat blockage, and take targeted measures, which has practical application value.

Description

Technical Field

[0001] The invention relates to the technical field of crude oil production, and in particular to a method for in-situ determination of the size of asphaltene precipitation formed by CO2 driving. Background Art

[0002] During crude oil extraction, CO2 needs to be injected to improve oil recovery, but it is often accompanied by asphaltene precipitation. The precipitated asphaltene particles gather together, resulting in the generation of new solid phases in the oil flow, and deposition on the rock surface, causing surface deposition, entrainment or pore throat blockage, which in turn blocks the reservoir pore throats, reduces reservoir permeability, changes rock wettability, causes serious damage to the reservoir, and affects the recovery rate of CO2 flooding.

[0003] According to research experience, the ratio of asphaltene particles to rock porosity plays a leading role in the control mechanism of surface deposition or pore blockage. For example, the main cause of the reduction in permeability of limestone samples is surface deposition, and the effect of pore blockage can be ignored. This is because the ratio of pore throat to particle size is large. In sandstone samples, surface deposition and pore throat blockage have similar effects on permeability reduction. In dolomite samples, pore blockage is dominant, which causes the sample permeability to drop sharply almost instantly. It can be seen that the different sizes of precipitated asphaltene have different damage mechanisms to the formation. Determining the size of asphaltene precipitation under different actual conditions helps to determine the mechanism of formation damage.

[0004] At present, the research on the damage of asphaltene to reservoirs mainly uses nuclear magnetic resonance to measure the volume change of pore throats of different sizes after core displacement experiments, or uses electron microscope scanning to observe the deposition of asphaltene on the core section after displacement. However, the above methods can only reflect local surface phenomena, and cannot determine whether the cause of blockage is the surface deposition, entrainment or pore throat blockage of asphaltene particles. It cannot directly and accurately determine the mechanism of formation damage, and cannot provide targeted countermeasures and preventive measures. In addition, the operation is relatively complicated and the testing cost is relatively high.

[0005] Some researchers used a microscope to observe the asphaltene deposition process under high pressure, and then analyzed it through image processing software to determine the amount of asphaltene deposition and its particle size under different conditions (Zanganeh, Dashti and Ayatollahi, 2015). However, the size of the asphaltene precipitation measured in this experiment is inaccurate, because the light and the thickness of the asphaltene flocs are not uniform, which may cause the measured size to be too large. In addition, the microscopic observation method is carried out under static conditions, and the interaction between the flow field and the porous medium and the precipitation particles is not considered. Furthermore, the flocs may be deformed or dispersed when passing through the pores under pressure, and the particle size measured by the microscope is different from the actual size of the asphalt particles in the formation flow field. Summary of the invention

[0006] The purpose of the present invention is to provide an in-situ size determination device for asphaltene precipitates formed by CO2 drive, which can set operating conditions according to actual conditions, simulate the asphaltene precipitation during the dynamic flow of crude oil under CO2 injection, and accurately and reliably determine the size of the formed asphaltene precipitates in situ.

[0007] Another object of the present invention is to provide a method for in situ measuring the size of formed asphaltene precipitates using the above device.

[0008] The present invention provides the following technical solutions:

[0009] A device for in-situ determination of the size of asphaltene precipitation formed by CO2 drive comprises a first reactor and a second reactor for containing crude oil to be tested, wherein the first reactor and the second reactor are respectively provided with material flow valves, a filtering device is connected in series via a pipeline between the two material flow valves, a cumulative flow meter is provided on the pipeline, a filtering membrane is installed in the filtering device, and a pressurizing valve is also provided on the first reactor and the second reactor, respectively, and connected to the pressurizing device.

[0010] The inventors found in their research that pressure, temperature, crude oil composition, CO2 purity, etc. will affect the interaction between CO2 and asphaltene, and thus affect the degree of flocculation and precipitation of asphaltene, so it is necessary to measure the asphaltene deposition in situ. At the same time, if the formed deposition can be deformed through the pores under shearing, it will not cause damage to the pores, so it is necessary to consider the pressure, the compatibility relationship between the deposited particles or floccules and the porous medium in the shear flow field, and link the deposition with the driving force. The device of the present application can simulate the dynamic penetration process of crude oil under different pressures, temperatures, crude oil types, and CO2 purities, and then examine the precipitation of asphaltene in the in-situ dynamic process, and the results obtained are more accurate, true, and reliable.

[0011] As a preferred embodiment of the present invention, the filter membrane is a hydrophilic microporous filter membrane, preferably a cellulose acetate microporous filter membrane.

[0012] As a preferred embodiment of the present invention, the first reactor and the second reactor are provided with pressure gauges.

[0013] A method for in-situ determination of the size of formed asphaltene precipitates using the above device comprises the following steps:

[0014] (1) Close the pressurizing valves and material flow valves of the first and second reactors, introduce a fixed amount of crude oil to be tested and CO2 gas into the feeding ports of the first and second reactors at a set temperature, and close the feeding ports;

[0015] (2) Open the pressure valve, pressurize the first and second reactors to P1 and P2, P1>P2, close the pressure valve, and then let it stand for ≥36 hours;

[0016] (3) Open the material flow valves of the two reactors to allow the mixture of crude oil and CO2 in the first reactor to flow into the second pressure reactor through the filter membrane, and record the filtration volume according to the time change;

[0017] (4) After the flow stops, close the material flow valve and remove the filter membrane;

[0018] (5) Replace the filter membrane with a different pore size and repeat steps (1) to (4) until no precipitation is produced;

[0019] (6) Based on the data recorded in step (3), a curve of the flow rate changing with time is obtained and a linear fit is made. If r ≥ 0.95, the corresponding minimum pore size of the filter membrane is the upper limit of the size of the asphaltene precipitate particles produced under the operating conditions.

[0020] The measurement method of the present application simulates the actual crude oil production conditions to set the pressure, temperature, and CO2, and uses the produced crude oil as the treatment object to demonstrate the precipitation of asphaltene in a dynamic process. It innovatively measures the size of asphaltene precipitation particles through the pore size of a microporous filter membrane, thus avoiding direct measurement under static conditions such as microscopic observation in the prior art.

[0021] When judging the size of asphaltene precipitation particles, it can be estimated by the pore size of the microporous filtration membrane when no precipitation is produced, that is, there is no need to measure the filtration volume. However, the accuracy of this judgment method is still insufficient, because slight precipitation may occur under certain unexpected circumstances, or fine impurities originally contained in the crude oil may cause the estimated particle size to be larger than the actual size.

[0022] The inventors found in the study that when the pore size of the filter membrane is large, no precipitation will be generated, and the relationship between the filtration volume and the filtration time of the CO2-crude oil system passing through the filter membrane pore is basically a linear relationship (the slope of the straight line is the filtration rate), while when the pore size of the filter membrane is small, precipitation blockage is easily formed, and the relationship between the filtration volume and the filtration time of the CO2-crude oil system passing through the filter membrane pore is basically a parabolic relationship with an opening downward. Therefore, the relationship between the change of the filtration volume and time is more regular and is not affected by the slight precipitation generated under unexpected circumstances (because no asphaltene precipitation particles are generated, no continuous accumulation will occur, and the overall linear relationship under other slight precipitation will not change). Therefore, by fitting the curve relationship between the two, it can be considered that no precipitation is generated under the linear relationship, and the pore size corresponding to the linear curve with the smallest slope is the upper limit of the size of the asphaltene precipitation, and the accuracy is significantly improved compared with directly based on whether precipitation is generated.

[0023] As a preferred embodiment of the method of the present invention, the crude oil to be tested in step (1) is filtered before use.

[0024] As a preferred embodiment of the method of the present invention, in step (2), P1-P2≥2MPa. An appropriate pressure difference is maintained to simulate the actual condition of the high pressure environment of the formation.

[0025] As a preferred embodiment of the method of the present invention, the method for determining the generation of precipitation in step (5) includes: washing out excess crude oil and other soluble impurities through a filter membrane with cyclohexane until the filtered cyclohexane is clear and transparent, taking out the filter membrane and drying it, weighing the filter membrane, and the increase in the mass of the filter membrane indicates the generation of precipitation. The drying temperature is 150±1°C and the drying time is 24h. It has been verified that this method does not affect the quality of the filter membrane itself.

[0026] As a preferred embodiment of the method of the present invention, r≥0.99.

[0027] The beneficial effects of the present invention are as follows:

[0028] The device of the present invention can adjust the operating conditions such as pressure, temperature, crude oil type, CO2 purity, etc. according to actual conditions, simulate the in-situ generation of asphaltene precipitation particles under CO2 dynamic driving conditions, and accurately and reliably measure the asphaltene precipitation size. The measurement results can be used to evaluate whether the cause of reservoir blockage is surface deposition, entrainment or pore throat blockage, and take targeted measures, which has practical application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is a structural diagram of the device of the present invention.

[0030] Figure 1 Among them, 1. the first reactor, 11. the pressurizing valve, 2. the second reactor, 21. the material flow valve, 3. the filtering device, 31. the cumulative flow meter, 4. the ISCO pump.

[0031] Figures 2 to 4 In the figure, the nanometer scale is the pore size specification of the filter membrane. DETAILED DESCRIPTION

[0032] The specific implementation modes of the present invention are further described below.

[0033] Unless otherwise specified, the raw materials used in the present invention can be purchased from the market or are commonly used in the art. Unless otherwise specified, the methods in the following embodiments are all conventional methods in the art.

[0034] An in situ device for measuring the size of asphaltene precipitates formed by CO2 drive, such as Figure 1As shown, it includes a first reactor 1 and a second reactor 2 for containing the crude oil to be tested, and the reactors are resistant to pressure of 40MPa and temperature of 500℃. The first reactor and the second reactor are respectively provided with a sealable feeding port and a pressure gauge, and a material flow valve 21 is respectively provided on the top. A filter device 3 is connected in series through a pipeline between the two material flow valves, and a cumulative flow meter 31 is provided on the pipeline. A replaceable filter membrane is installed in the filter device, which is a hydrophilic acetate fiber microporous filter membrane, and different pore size specifications can be replaced according to actual conditions. A pressure valve 11 is respectively provided at the bottom of the first reactor and the second reactor, and is connected to the pressure device ISCO pump 4.

[0035] Application Example 1

[0036] The above device is used to measure the precipitation of the CO2-crude oil mixed system (CO2 accounts for 75% by volume at normal pressure) under different pressures to determine the matching of the formed sediments with pores of different sizes. The specific steps are as follows:

[0037] (1) The crude oil to be tested was pre-filtered through a microporous filter membrane with a diameter of 1.5 μm to remove the influence of impurities. A microporous filter membrane made of cellulose acetate with a mass of m1 was installed in the filtration device. The pressurization valve and material flow valve of the first reactor and the second reactor were closed. A fixed amount of 2.5 L of crude oil to be tested and 7.5 L of CO2 gas with a purity of 99.9% were introduced into the feeding ports of the first reactor and the second reactor at 40°C, and the feeding ports were closed.

[0038] (2) Open the pressure valve, pressurize the first and second reactors to P1 and P2, where P1-P2 is 11 MPa, close the pressure valve, and then let stand at 40°C for 48 hours;

[0039] (3) Open the material flow valves of the two reactors to allow the mixture of crude oil and CO2 in the first reactor to flow into the second pressure reactor through the filter membrane, and record the filtration volume at different times;

[0040] (4) After the inflow stops, close the material flow valve and remove the filter membrane from the filtration device;

[0041] (5) Replace the microporous filter membrane with a different pore size and repeat steps (1) to (4) until no precipitation is produced;

[0042] (6) Based on the data recorded in step (3), a curve of the flow rate changing with time is obtained and a linear fit is made, requiring r ≥ 0.95. The corresponding minimum pore size of the filter membrane is the upper limit of the size of the asphaltene precipitate particles produced under the operating conditions.

[0043] According to the above process, the conditions when P1-P2 are 16MPa and 20MPa respectively are tested.

[0044] In the above application example, when the crude oil-CO2 system passes through filter membranes with different pore sizes at 11MPa, 16MPa and 20MPa, the filtration volume changes with filtration time as shown in the following table: Figure 1 , 2 As shown in Figure 3, it can be seen that under different pressures, there are certain differences in the filtration rates of the crude oil-CO2 system when passing through filter membranes with different pore sizes. Based on this difference, the compatibility relationship between the size of the asphaltene deposit and the pore size of the filter membrane can be judged.

[0045] It can be seen from the figure that when the pore size of the filter membrane is large, the relationship between the filtration volume and the filtration time of the CO2-crude oil system passing through the filter membrane is basically a linear relationship (the slope of the straight line is the filtration rate), and the larger the pore size of the filter membrane, the greater the filtration rate of the CO2-crude oil system through the filter membrane, and the smaller the flow resistance. Figure 2 As shown in the figure, the filtration rate of the CO2-crude oil system through the 100nm filter membrane is significantly lower than that through the 200nm and 450nm filter membranes. When the pore size of the filter membrane is small, the relationship between the filtration volume and filtration time of the CO2-crude oil system through the filter membrane is not a linear relationship, but a parabola with a downward opening. The smaller the pore size of the filter membrane, the longer it takes for the same volume of CO2-crude oil system to pass through the filter membrane. Moreover, the filtration rate through the filter membrane gradually decreases with the increase of the filtration volume, indicating that asphaltene is retained on the surface of the filter membrane at this time. The smaller the pore size of the filter membrane, the more asphaltene is retained on the surface of the nuclear pore membrane. As a result, the retention of asphaltene on the surface of the filter membrane causes the filtration rate of the CO2-crude oil system through the nuclear pore membrane to be significantly slowed down. Therefore, it is determined that the size of the asphaltene precipitate formed at 11MPa is about 50nm, the size of the asphaltene precipitate formed at 16MPa is about 100nm, and the size of the asphaltene precipitate formed at 20MPa is about 220nm, which is consistent with the result of curve fitting, r is not less than 0.95, and under the filter membrane with a larger pore size, r can reach 0.99 or even higher. And the above judgment result is basically consistent with the result of judging whether precipitation is generated by the following method: use cyclohexane to wash out the excess crude oil and other soluble impurities through the filter membrane until the filtered cyclohexane is clear and transparent, take out the filter membrane and dry it at 150℃ for 24h, weigh the filter membrane m2, and the filter membrane mass m2 becomes larger than m1, indicating that precipitation has occurred.

[0046] Application Example 2

[0047] Repeat the test process at 11MPa in Application Example 1, with the following differences:

[0048] The asphaltene precipitation particles are directly judged based on whether precipitation occurs and the pore size of the corresponding microporous filtration membrane without fitting the curve.

[0049] The application example was repeated three times. For the 50nm microporous filtration membrane, the results of three repetitions at 11MPa in Application Example 1 were consistent, while a slight precipitation occurred once in Application Example 2 and the test passed smoothly twice. It is speculated that the slight precipitation may be the impurities originally contained in the crude oil. Compared with the fitting curve method in Application Example 1, the accuracy is lower.

[0050] Comparative Example 1

[0051] Repeat the test process at 11MPa in Application Example 1, with the following differences:

[0052] The standing time in step (2) is 24 hours. Since CO2 and crude oil are not fully mixed, the results of the three repetitions of comparative example 1 show greater fluctuations compared with the repeated application example 1.

Claims

1. A method for in-situ determination of the size of asphaltene precipitates formed by CO2 drive, characterized in that: The measurement is performed using an in-situ measuring device for the size of asphaltene precipitation formed by CO2 drive, the measuring device comprising a first reactor and a second reactor for containing the crude oil to be measured, the first reactor and the second reactor are respectively provided with material flow valves, a filter device is connected in series via a pipeline between the two material flow valves, a cumulative flow meter is provided on the pipeline, and a filter membrane is installed in the filter device; The assay consists of the following steps: (1) Close the pressurizing valves and material flow valves of the first and second reactors, introduce a fixed amount of crude oil to be tested and CO2 gas into the feeding ports of the first and second reactors at a set temperature, and close the feeding ports; (2) Open the pressure valve, pressurize the first and second reactors to P1 and P2, P1>P2, close the pressure valve, and then let it stand for ≥36 hours; (3) Open the material flow valves of the two reactors to allow the mixture of crude oil and CO2 in the first reactor to flow into the second pressure reactor through the filter membrane, and record the filtration volume according to the time change; (4) After the flow stops, close the material flow valve and remove the filter membrane; (5) Replace the filter membrane with one with a larger pore size and repeat steps (1) to (4) until no precipitation is produced; (6) Based on the data recorded in step (3), a curve of the flow rate changing with time is obtained and a linear fit is made. If r ≥ 0.95, the corresponding minimum pore size of the filter membrane is the upper limit of the size of asphaltene precipitation under the operating conditions.

2. The method according to claim 1, characterized in that The filter membrane is a hydrophilic microporous filter membrane.

3. The method according to claim 1, characterized in that The first reactor and the second reactor are provided with pressure gauges.

4. The method according to claim 1, characterized in that In step (1), the crude oil to be tested is filtered before use.

5. The method according to claim 1, characterized in that In step (2), P1-P2≥2MPa.

6. The method according to claim 1, characterized in that The method for determining the generation of precipitation in step (5) includes: washing out excess crude oil and other soluble impurities through a filter membrane with cyclohexane until the filtered cyclohexane is clear and transparent, removing the filter membrane and drying it, and weighing the filter membrane. An increase in the mass of the filter membrane indicates the generation of precipitation.

7. The method according to claim 1, characterized in that r≥0.99。

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