Method for improving oil reservoir recovery efficiency by microbe-CO2 combination flooding based on reduction of minimum miscible pressure of crude oil

Abstract

The invention provides a method for improving the yield of an oil recovery reservoir based on microbial-CO2 combination flooding for reducing the minimum miscible pressure of crude oil, and the method comprises the following steps: carrying out a water-displacing oil experiment on a rock core, then alternately injecting a microbial solution slug and a CO2 slug into the rock core, injecting the microbial solution slug for the first time, and injecting the CO2 slug into the rock core for the second time; when microorganisms appear in the produced liquid, carrying out standing culture treatment, and finally carrying out a water-oil displacement experiment on the rock core; the microbial solution slug comprises first microbial functional bacteria with a long-chain alkane decomposition effect or second microbial functional bacteria with a biological surfactant generation effect. According to the method provided by the invention, the microbial functional bacteria strains (or metabolites) for reducing the minimum miscible pressure of the crude oil are combined with the CO2 flooding, so that the synergistic effect of various technologies is fully exerted, the effect of improving the crude oil recovery rate is further realized, and a reference is provided for indoor experimental research and field development.

Description

Technical Field

[0001] The present invention relates to the technical field of petroleum development, and in particular to a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil, and in particular to a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. Background Art

[0002] As easily exploitable oil and gas resources are becoming increasingly depleted, crude oil production has gradually decreased in recent years. Low-grade oil reservoir resources have become a strategic replacement resource with great potential for oil and are the main body of crude oil production. However, the traditional method of improving oil recovery is not ideal for the development of low-grade oil reservoirs, and a new method is urgently needed to further improve oil recovery.

[0003] Compared with other tertiary oil recovery technologies, CO2 flooding has many advantages in the development of low-grade oil reservoirs: (1) CO2 is easy to flow, and it is easy to establish an effective displacement pressure system; (2) CO2 injection methods are flexible and can be applied to various formation conditions; (3) After injection, CO2 can be miscible with crude oil, and has significant effects such as expansion, viscosity reduction, extraction, and reduction of interfacial tension, which can effectively expand the swept volume and thus improve the recovery rate. CO2 flooding can also store CO2 underground for a long time, thereby reducing greenhouse gas emissions.

[0004] CO2 flooding has good advantages and development prospects in improving oil recovery, and has become a hot topic of research in countries around the world. However, there are obvious differences in the oil recovery efficiency of CO2 immiscible flooding, near-miscible flooding and miscible flooding, and the difference in oil recovery efficiency between immiscible flooding and miscible flooding is about 20%. The suitable reserves for CO2 flooding selected in my country are mainly concentrated in immiscible and near-miscible flooding, and there are fewer miscible flooding. The recovery rate of low-grade oil reservoirs suitable for CO2 flooding needs to be further improved. Therefore, in order to further improve the displacement effect of CO2 flooding, it is necessary to start with the above problems, reduce and improve the properties of reservoir crude oil, reduce the minimum miscible pressure of crude oil, and improve the recovery rate of CO2 flooding.

[0005] At present, chemical agents are often used to reduce the minimum miscible pressure of oil reservoirs at home and abroad, but the cost of using chemical agents is high and the environmental pollution is serious. Microbial enhanced oil recovery (MEOR) can make up for this shortcoming. It can reduce the minimum miscible pressure of crude oil by degrading long-chain alkanes in crude oil through biological metabolism or producing biosurfactants to increase crude oil production. Compared with the use of chemical agents to act on reservoirs and crude oil, microorganisms can effectively reduce the environmental damage caused by the oil recovery process and have good development prospects. Therefore, before development, the use of microbial functional bacteria that can reduce the minimum miscible pressure of crude oil to act on reservoir crude oil can alleviate the impact of excessive miscible pressure of crude oil on CO2 flooding recovery. However, both domestic and foreign studies have been carried out on the two technologies of single CO2 flooding or microbial flooding to improve oil recovery, and the mechanism of improving oil recovery is relatively clear, but there is a lack of research on the use of microorganisms to reduce the miscible pressure of crude oil and assist CO2 flooding.

[0006] Based on the above reasons, it is necessary to establish a method for improving oil recovery by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil, select microbial functional bacteria that can reduce the miscible pressure for composite action with CO2, combine the two methods of improving oil recovery, CO2 flooding and microbial flooding, give full play to the synergistic effect of various technologies, to a certain extent achieve high-efficiency and low-cost exploitation of crude oil, create a good ecological environment and higher economic benefits, and provide a reference for indoor experimental research and field development. Summary of the invention

[0007] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for improving the recovery rate of oil reservoirs by combining microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The present invention combines the functional microbial strains (or metabolites) that reduce the minimum miscible pressure of crude oil with CO2 flooding to give full play to the synergistic effect of various technologies, thereby achieving the effect of improving the recovery rate of crude oil, and providing a reference for indoor experimental research and field development.

[0008] To achieve this object, the present invention adopts the following technical solutions:

[0009] The present invention provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil, the method comprising the following steps:

[0010] (1) Conduct water flooding experiments on cores;

[0011] (2) injecting microbial solution plugs and CO2 plugs alternately into the core of step (1), and injecting the microbial solution plugs for the first time, and performing static culture treatment when microorganisms appear in the produced fluid;

[0012] (3) performing a water flooding experiment on the core after the static culture treatment in step (2);

[0013] The microbial solution slug in step (2) contains the first microbial functional bacteria or the second microbial functional bacteria;

[0014] The first microbial functional bacteria has the function of decomposing long-chain alkanes; and the second microbial functional bacteria has the function of generating biosurfactants.

[0015] The present invention combines the microbial functional bacteria strain (or metabolites) that reduces the minimum miscible pressure of crude oil with CO2 flooding, giving full play to the synergistic effect of various technologies, thereby achieving the effect of improving crude oil recovery, and providing a reference for indoor experimental research and field development.

[0016] It is worth noting that the mechanism of action of the first microbial functional bacteria with the function of decomposing long-chain alkanes in the present invention is: the oil displacement efficiency of CO2 immiscible flooding and miscible flooding has obvious differences, and the oil displacement efficiency of immiscible flooding and miscible flooding differs by about 20%. Since CO2 is more likely to have a miscible effect with the light components in crude oil, when there are more long-chain heavy alkanes in underground crude oil, the minimum miscible pressure between CO2 and crude oil will also increase accordingly, thereby causing insufficient miscibility between crude oil and CO2, reducing the oil displacement efficiency. The first microbial functional bacteria capable of decomposing long-chain alkanes can degrade high-carbon chains into low-carbon chains through biological metabolic degradation during its growth and reproduction, reduce heavy components and increase light components, thereby reducing crude oil viscosity and improving its fluidity, while improving the quality of the output crude oil, thereby achieving the purpose of reducing the miscible pressure of crude oil. Due to the reduction of the miscible pressure of crude oil, CO2 is more likely to have a miscible effect with crude oil under formation conditions, greatly improving the crude oil recovery rate.

[0017] The mechanism of action of the second microbial functional bacteria that has the function of generating biosurfactants in the present invention is that the biosurfactants can reduce interfacial tension, increase the solubility of CO2 in crude oil and enhance the ability of CO2 to extract light hydrocarbons, thereby achieving the effect of reducing the minimum miscible pressure. At the same time, since microorganisms can be artificially propagated and cultivated outside the oil reservoir, the biosurfactants produced by the microorganisms after metabolism outside the oil reservoir can be injected into the low-permeability and dense reservoirs where microorganisms are difficult to inject, thereby achieving the purpose of reducing the minimum miscible pressure. Compared with traditional chemical surfactants, it will not pollute the reservoir and is more environmentally friendly.

[0018] As a preferred technical solution of the present invention, the permeability of the core in step (1) is ≥20mD, for example, it can be 20mD, 22mD, 24mD, 26mD, 28mD or 30mD, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0019] It is worth noting that if the method provided by the present invention is used to sample a core with a permeability of less than 20 mD, a surfactant needs to be injected into the core in advance. The surfactant is a surfactant produced by the metabolism of microorganisms cultivated externally.

[0020] Further preferably, the concentration of the surfactant is 2 to 10 g / L, for example, 2 g / L, 4 g / L, 6 g / L, 8 g / L or 10 g / L, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0021] As a preferred technical solution of the present invention, the displacement rate of the water-to-oil experiment in step (1) is 0.01 to 0.5 mL / min, for example, it can be 0.05 mL / min, 0.1 mL / min, 0.15 mL / min, 0.2 mL / min, 0.25 mL / min, 0.3 mL / min, 0.35 mL / min, 0.4 mL / min or 0.45 mL / min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0022] As a preferred technical solution of the present invention, the endpoint of the water drive oil experiment in step (1) is: the water content of the produced fluid is ≥98%, for example, it can be 98.2%, 98.4%, 98.6%, 98.8%, 99%, 99.2%, 99.4% or 99.6%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0023] As a preferred technical solution of the present invention, the first functional microorganism includes Pseudomonas aeruginosa, preferably Pseudomonas aeruginosa.

[0024] Preferably, the second microbial functional bacteria include Bacillus subtilis or Pseudomonas stutzeri, preferably Bacillus subtilis.

[0025] It is worth noting that the functional microorganisms described in the present invention are preferably derived from the target oil reservoir for further improving the recovery rate, so as to ensure good compatibility with the reservoir / fluid conditions of the target oil reservoir. It is also possible to select strains that grow widely in the natural environment, or to construct engineered bacteria through bioengineering, genetic engineering, and genetic engineering according to the reservoir and fluid characteristics of the oil reservoir, but it is best to give priority to natural strains to avoid harm to the human body and the environment, and at the same time, functional bacteria strains other than the target oil reservoir must be tested and screened for compatibility.

[0026] As a preferred technical solution of the present invention, the concentration of the microbial solution in step (2) is 10 5 -10 7 / mL, for example, 3×10 5 / mL, 5×10 5 / mL, 7×10 5 / mL, 1×10 6 / mL, 3×10 6 / mL, 5×10 6 / mL or 7×10 6 Pieces / mL, but not limited to the listed values, other values ​​not listed within the numerical range are also applicable.

[0027] As a preferred technical solution of the present invention, the volume of the CO2 plug in step (2) is 0.04-0.1HCPV, for example, it can be 0.05HCPV, 0.06HCPV, 0.07HCPV, 0.08HCPV, 0.09HCPV or 0.1HCPV, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0028] Preferably, the volume ratio of the CO2 plug to the microbial plug in step (2) is 1:(0.3-3), for example, it can be 1:1, 1:2, 1:3, 2:1 or 3:1, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0029] As a preferred technical solution of the present invention, the speed of the alternating injection in step (2) is 0.01-0.1 mL / min, for example, 0.02 mL / min, 0.04 mL / min, 0.06 mL / min or 0.08 mL / min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably, it is 0.03-0.08 mL / min.

[0030] Preferably, the time of the static culture treatment in step (2) is ≥3 days, for example, it can be 3.5 days, 4 days, 4.5 days, 5 days, 6 days, 7 days, 8 days or 10 days, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably, it is 5 to 10 days.

[0031] Preferably, the temperature of the static culture treatment in step (2) is 20-65°C, for example, 20°C, 30°C, 40°C, 50°C or 60°C, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable; preferably, it is 25-45°C.

[0032] As a preferred technical solution of the present invention, the displacement rate of the water-to-oil experiment in step (3) is 0.01-0.5 mL / min, for example, it can be 0.05 mL / min, 0.1 mL / min, 0.15 mL / min, 0.2 mL / min, 0.25 mL / min, 0.3 mL / min, 0.35 mL / min, 0.4 mL / min or 0.45 mL / min, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0033] Preferably, the oil recovery efficiency of the method is ≥ 65%, for example, it can be 65%, 67%, 69%, 71%, 73% or 75%, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable.

[0034] As a preferred technical solution of the present invention, the method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil provided by the present invention comprises the following steps:

[0035] (1) conducting a water-to-oil drive experiment on the core at a displacement rate of 0.01 to 0.5 mL / min until the water content of the produced fluid is ≥ 98%; and the permeability of the core is ≥ 20 mD;

[0036] (2) injecting microbial solution plugs and CO2 plugs alternately into the core of step (1) at a volume ratio of (0.3-3):1 at a rate of 0.01-0.1 mL / min, and injecting the microbial solution plugs for the first time, and when microorganisms appear in the produced fluid, performing a static culture treatment at 20-65°C for ≥3 days;

[0037] The microbial solution slug contains a first microbial functional bacterium or a second microbial functional bacterium; the first microbial functional bacterium includes Pseudomonas aeruginosa having the function of decomposing long-chain alkanes; the second microbial functional bacterium includes Bacillus subtilis having the function of generating a biosurfactant;

[0038] The concentration of the microbial solution is 10 5 -10 7 Pieces / mL;

[0039] The volume of the CO2 slug is 0.04-0.1 HCPV;

[0040] (3) performing a water flooding experiment on the core after the static culture treatment in step (2) at a displacement rate of 0.01-0.5 mL / min.

[0041] The present invention also provides a practical mine application method for improving oil reservoir recovery rate by using microorganisms, specifically comprising: utilizing the optimal functional bacteria for reducing miscible pressure and the optimal displacement method obtained in the above method, and scaling up the application to actual oil field mine development according to similar principles (such as determining the injection amount (segment size, microbial concentration), injection timing, injection rounds and injection segment ratio of the oil reservoir based on the pore volume of the experimental rock, etc.).

[0042] The numerical range described in the present invention not only includes the point values ​​listed above, but also includes any point values ​​between the above numerical ranges that are not listed. Due to space limitations and for the sake of simplicity, the present invention no longer exhaustively lists the specific point values ​​included in the range.

[0043] Compared with the prior art, the present invention has the following beneficial effects:

[0044] (1) The present invention combines microbial flooding with CO2 flooding. While utilizing the advantages of CO2 flooding to improve oil reservoir recovery, microorganisms are used to improve the properties of crude oil before CO2 flooding, thereby reducing the miscible pressure of crude oil and further improving the recovery rate of CO2 flooding in oil reservoirs.

[0045] (2) The method provided by the present invention provides a complete set of process methods for the study of the mechanism of improving oil recovery by combining microbial functional bacteria that reduce the miscible pressure of crude oil with CO2, as well as the evaluation of the optimal functional bacteria that reduce the miscible pressure of crude oil and the microbial-CO2 composite flooding method that reduces the minimum miscible pressure of crude oil.

[0046] (3) The present invention can, through similar principles, apply the optimal functional bacteria for reducing crude oil miscibility pressure and the optimal microbial-CO2 composite flooding method for reducing the minimum miscibility pressure of crude oil obtained from indoor evaluation experiments to real oil reservoirs, forming a mine development method that is convenient for direct application on the oil field site;

[0047] (4) The method provided by the present invention can flexibly adjust the displacement mode of the microbial functional bacteria-CO2 composite drive that reduces the miscible pressure of crude oil according to the development effect of the same oil reservoir and the characteristics of different oil reservoirs, and has universal applicability. DETAILED DESCRIPTION

[0048] The technical solution of the present invention is further described below by specific implementation methods. It should be understood by those skilled in the art that the embodiments are only to help understand the present invention and should not be regarded as specific limitations of the present invention.

[0049] It is worth noting that the method provided by the present invention is only applicable to oil reservoirs that have not achieved miscible displacement under reservoir pressure conditions; for oil reservoirs that have achieved miscible displacement, the effect of the method provided by the present invention may not be obvious.

[0050] Example 1

[0051] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil, the method comprising the following steps:

[0052] (1) a water flooding experiment was conducted on the core at a displacement rate of 0.3 mL / min until the water content of the produced fluid was 98.6%; the permeability of the core was 29.5684 mD;

[0053] (2) injecting microbial solution plugs and CO2 plugs alternately into the core of step (1) at a volume ratio of 1:1 at a rate of 0.06 mL / min, and injecting the microbial solution plugs for the first time, and when microorganisms appear in the produced fluid, performing a static culture treatment at 30° C. for 7 days;

[0054] The microbial solution slug contains Pseudomonas aeruginosa that has the function of decomposing long-chain alkanes;

[0055] The concentration of the microbial solution is 10 6 Pieces / mL;

[0056] The volume of the CO2 slug is 0.06HCPV;

[0057] (3) A water flooding experiment was performed on the core after the static culture treatment in step (2) at a displacement rate of 0.3 mL / min.

[0058] Example 2

[0059] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil, the method comprising the following steps:

[0060] (1) conducting a water-to-oil experiment on the core at a displacement rate of 0.01 mL / min until the water content of the produced fluid is 98%; the permeability of the core is 21.1379 mD;

[0061] (2) injecting microbial solution plugs and CO2 plugs alternately into the core of step (1) at a volume ratio of 2:1 at a rate of 0.01 mL / min, and injecting the microbial solution plugs for the first time, and when microorganisms appear in the produced fluid, performing a static culture treatment at 25° C. for 10 days;

[0062] The microbial solution slug contains Bacillus subtilis which has the function of generating biosurfactant;

[0063] The concentration of the microbial solution is 10 5 Pieces / mL;

[0064] The volume of the CO2 slug is 0.04HCPV;

[0065] (3) A water flooding experiment was performed on the core after the static culture treatment in step (2) at a displacement rate of 0.01 mL / min.

[0066] Example 3

[0067] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil, the method comprising the following steps:

[0068] (1) conducting a water flooding experiment on the core at a displacement rate of 0.5 mL / min until the water content of the produced fluid is 99%; the permeability of the core is 38.6715 mD;

[0069] (2) injecting microbial solution plugs and CO2 plugs alternately into the core of step (1) at a volume ratio of 1:3 at a rate of 0.1 mL / min, and injecting the microbial solution plugs for the first time, and when microorganisms appear in the produced fluid, performing a static culture treatment at 45° C. for 5 days;

[0070] The microbial solution slug contains Pseudomonas aeruginosa that has the function of decomposing long-chain alkanes;

[0071] The concentration of the microbial solution is 10 7 Pieces / mL;

[0072] The volume of the CO2 slug is 0.1HCPV;

[0073] (3) A water flooding experiment was performed on the core after the static culture treatment in step (2) at a displacement rate of 0.5 mL / min.

[0074] Example 4

[0075] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from that of embodiment 1 only in that:

[0076] In this example, the concentration of the microbial solution in step (2) was changed to 10 4 Pieces / mL.

[0077] Example 5

[0078] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from that of embodiment 1 only in that:

[0079] In this example, the concentration of the microbial solution in step (2) was changed to 10 8 Pieces / mL.

[0080] Example 6

[0081] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from that of embodiment 1 only in that:

[0082] In this embodiment, the volume ratio of the microbial solution plug and the CO2 plug in step (2) is changed to 4:1.

[0083] Example 7

[0084] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from that of embodiment 1 only in that:

[0085] In this embodiment, the volume ratio of the microbial solution plug and the CO2 plug in step (2) is changed to 1:4.

[0086] Example 8

[0087] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from that of embodiment 1 only in that:

[0088] In this embodiment, the speed of the alternating injection in step (2) is changed to 0.2 mL / min.

[0089] Example 9

[0090] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from that of embodiment 1 only in that:

[0091] In this embodiment, the functional microorganisms in the microbial solution slug in step (2) are replaced with Ochrobactrum.

[0092] Example 10

[0093] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from that of embodiment 1 only in that:

[0094] In this example, the temperature of the static culture in step (2) was changed to 18°C.

[0095] Embodiment 11

[0096] This embodiment provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from that of embodiment 1 only in that:

[0097] In this example, the temperature of the static culture in step (2) is changed to 70°C.

[0098] Comparative Example 1

[0099] This comparative example provides a method for improving oil reservoir recovery, which differs from Example 1 only in that:

[0100] In this comparative example, step (2) is revised as follows: CO2 gas is injected into the core of step (1) at a rate of 0.06 mL / min until the gas-oil ratio is greater than 1300.

[0101] Comparative Example 2

[0102] This comparative example provides a method for improving oil reservoir recovery, which differs from Example 1 only in that:

[0103] In this comparative example, step (2) is revised as follows: injecting the microbial solution into the core of step (1) at a rate of 0.06 mL / min, and performing a static culture treatment for 5 days when microorganisms appear in the produced fluid.

[0104] Comparative Example 3

[0105] This comparative example provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from Example 1 only in that:

[0106] In this comparative example, the plug injection method described in step (2) is modified as follows: the microbial solution is injected into the core described in step (1) at a rate of 0.06 mL / min, and when microorganisms appear in the produced fluid, a static culture treatment is performed for 5 days, and then CO2 gas is injected into the core at a rate of 0.06 mL / min until the gas-oil ratio is >1300.

[0107] Comparative Example 4

[0108] This comparative example provides a method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil. The method is different from Example 1 only in that:

[0109] In this comparative example, the microbial solution plug described in step (2) is modified into a common formation water solution plug.

[0110] The oil recovery experiments were carried out on the cores using the methods provided in the above embodiments and comparative examples. The oil recovery efficiency results are shown in Table 1. The magnitude of the oil recovery efficiency reflects the effect of improving the recovery rate.

[0111] Table 1

[0112]

[0113]

[0114] From Table 1, we can draw the following conclusions:

[0115] (1) The method provided in Examples 1-3 of the present invention has an oil displacement efficiency of ≥65%, which significantly improves the recovery rate and realizes high-efficiency and low-cost exploitation of oil reservoirs;

[0116] (2) Analysis of Example 1, Examples 4-5, and Comparative Example 4 (equivalent to a microbial concentration of 0) shows that when the concentration of the injected microbial solution is low, the microorganisms are too few to decompose the long-chain alkanes in the crude oil in a large amount in a short period of time, thereby reducing the minimum miscibility pressure of the crude oil, and the CO2 displacement cannot reach the conditions of miscible displacement, and the effect of CO2 displacement cannot be fully exerted, resulting in an insufficient increase in crude oil recovery; when the concentration of the injected microbial solution is high, the microorganisms are too many, blocking some pores to form filter cakes, making it difficult for the microorganisms to be injected into the core or further penetrate into the fine pores to decompose the long-chain alkanes inside and thereby reduce the minimum miscibility pressure of the crude oil. At the same time, the blockage of the microorganisms also makes it difficult for CO2 to enter the fine pores, resulting in an insufficient increase in crude oil recovery;

[0117] (3) Analysis of Example 1 and Examples 6-9 shows that when the volume ratio of the CO2 slug to the microbial solution slug is too high, the non-miscible displacement effect of CO2 contributes most to the improvement of oil recovery efficiency. The microorganisms have an insignificant effect on decomposing long-chain alkanes due to the small injection amount, which leads to a weak reduction in the miscible pressure of crude oil, making CO2 in an non-miscible displacement state, resulting in an insufficient improvement in crude oil recovery. When the volume ratio of the CO2 slug to the microbial solution slug is too low, the microbial effect contributes most to the improvement of oil recovery efficiency. However, due to the small injection amount of CO2, even if the microorganisms degrade the long-chain alkanes, the viscosity of the crude oil is reduced, making the crude oil relatively easier to recover and making the CO2 displacement a miscible drive, the advantages of the CO2 miscible drive are not fully utilized, resulting in an insufficient improvement in crude oil recovery.

[0118] (4) Analysis of Example 1 and Comparative Examples 1-2 shows that the use of microorganism-CO2 composite flooding is more conducive to achieving high-efficiency exploitation of oil reservoirs. On the one hand, composite flooding slows down the gas channeling phenomenon of CO2 flooding. On the other hand, when the reservoir pressure cannot reach the miscible flooding, the role of microorganisms makes up for the shortcomings of CO2 flooding. By degrading long-chain alkanes, crude oil becomes relatively easy to exploit while reducing the minimum miscible pressure of crude oil, making CO2 miscible flooding possible, giving full play to the synergistic effect of the two.

[0119] (5) Analysis of Example 1 and Comparative Example 3 shows that, compared with sequential microbial flooding and CO2 flooding, the method provided by the present invention does not use a plugging method to displace the core. Therefore, although the microorganisms reduce the minimum miscible pressure of crude oil, CO2, as a gas, is prone to gas channeling. Although it is in a miscible displacement state, once a dominant channel is formed, it is difficult to displace crude oil in other locations in the core, resulting in a lack of obvious improvement in the oil recovery rate of crude oil.

[0120] In summary, the method provided by the present invention combines the functional microbial strains (or metabolites) that reduce the minimum miscible pressure of crude oil with CO2 flooding, giving full play to the synergistic effect of various technologies, thereby achieving the effect of improving crude oil recovery, providing a reference for indoor experimental research and field development.

[0121] The specific embodiments described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for improving oil reservoir recovery by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil, characterized in that: The method comprises the following steps: (1) Conduct water flooding experiments on cores; (2) injecting microbial solution plugs and CO2 plugs alternately into the core of step (1), and injecting the microbial solution plugs for the first time, and performing static culture treatment when microorganisms appear in the produced fluid; (3) performing a water flooding experiment on the core after the static culture treatment in step (2); The microbial solution slug in step (2) contains the first microbial functional bacteria or the second microbial functional bacteria; The first microbial functional bacteria has the function of decomposing long-chain alkanes; and the second microbial functional bacteria has the function of generating biosurfactants.

2. The method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to claim 1, characterized in that: The permeability of the core in step (1) is ≥20mD.

3. The method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to claim 1 or 2, characterized in that: The displacement rate of the water-to-oil experiment in step (1) is 0.01 to 0.5 mL / min; Preferably, the endpoint of the water flooding experiment in step (1) is: the water content of the produced fluid is ≥ 98%.

4. The method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to any one of claims 1 to 3, characterized in that: The first microbial functional bacteria include Pseudomonas aeruginosa; Preferably, the second microbial functional bacteria include Bacillus subtilis or Pseudomonas stutzeri, preferably Bacillus subtilis.

5. The method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to any one of claims 1 to 4, characterized in that: The concentration of the microbial solution in step (2) is 10 5 -10 7 Pieces / mL.

6. The method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to any one of claims 1 to 5, characterized in that: The volume of the CO2 slug in step (2) is 0.04-0.1 HCPV; Preferably, the volume ratio of the CO2 plug to the microbial plug in step (2) is 1:(0.3-3).

7. The method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to any one of claims 1 to 6, characterized in that: The speed of the alternating injection in step (2) is 0.01 to 0.1 mL / min, preferably 0.03 to 0.08 mL / min; Preferably, the static culture treatment time in step (2) is ≥ 3 days, preferably 5 to 10 days; Preferably, the temperature of the static culture treatment in step (2) is 20 to 65°C, preferably 25 to 45°C.

8. The method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to any one of claims 1 to 7, characterized in that: The displacement rate of the water-to-oil experiment in step (3) is 0.01-0.5 mL / min.

9. The method for improving oil reservoir recovery rate by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to any one of claims 1 to 8, characterized in that: The oil recovery efficiency of the method is ≥65%.

10. The method for improving oil reservoir recovery by microbial-CO2 composite flooding based on reducing the minimum miscible pressure of crude oil according to any one of claims 1 to 9, characterized in that: The method comprises the following steps: (1) conducting a water-to-oil drive experiment on the core at a displacement rate of 0.01 to 0.5 mL / min until the water content of the produced fluid is ≥ 98%; and the permeability of the core is ≥ 20 mD; (2) injecting microbial solution plugs and CO2 plugs alternately into the core of step (1) at a volume ratio of (0.3-3):1 at a rate of 0.01-0.1 mL / min, and injecting the microbial solution plugs for the first time, and when microorganisms appear in the produced fluid, performing a static culture treatment at 20-65°C for ≥3 days; The microbial solution slug contains a first microbial functional bacterium or a second microbial functional bacterium; the first microbial functional bacterium includes Pseudomonas aeruginosa having the function of decomposing long-chain alkanes; the second microbial functional bacterium includes Bacillus subtilis having the function of generating a biosurfactant; The concentration of the microbial solution is 10 5 -10 7 Pieces / mL; The volume of the CO2 slug is 0.04-0.1 HCPV; (3) performing a water flooding experiment on the core after the static culture treatment in step (2) at a displacement rate of 0.01-0.5 mL / min.

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