Method for improving recovery ratio of low-permeability reservoir

Abstract

The invention discloses a method for improving the recovery ratio of a low-permeability oil reservoir. The method comprises the following steps: (1) screening a target oil reservoir; (2) the injection system and the injection amount of each slug are determined, specifically, (21) according to the oil reservoir temperature of a target oil reservoir, an oil displacement system matched with the oil reservoir temperature is determined, and the oil displacement system is composed of a spore profile control system and an oil washing system; the spore profile control system is composed of a spore microbial agent system and a spore activation system; (22) determining the injection amount of the injection system of each slug; and (3) performing a field injection process. The preparation method has the advantages of low production cost, simple production process, high targeting property, no stratum damage, no environmental pollution and the like; by means of a multi-round multi-slug on-site injection mode, profile control of a deep main flow line of the low-permeability reservoir and efficient oil washing of secondary flow line remaining oil are achieved, the maximum decreasing amplitude of the water content of the low-permeability reservoir reaches 20% or above, the input-output ratio is larger than 1: 10, and the recovery rate is increased by 20% or above.

Description

Technical Field

[0001] The present invention belongs to the technical field of oil development, and particularly relates to a method for improving the recovery rate of low-permeability oil reservoirs. Background Art

[0002] In the middle and late stages of oilfield development, long-term water injection exacerbates the heterogeneity of the reservoir, and the phenomenon of water channeling intensifies, significantly reducing the development effect.

[0003] Conventional oil reservoirs can adjust the water injection profile by injecting polymer systems and other systems to improve the swept volume of water flooding, thereby further improving the recovery rate. However, due to the poor physical properties of the reservoir, low-permeability oil reservoirs (permeability range of 0.1 - 50 mD) have contradictions such as high initial pressure of crude oil and high water injection pressure during the development process, and problems such as low water flooding efficiency, slow oil production rate, and low recovery degree. After the water flooding of such low-permeability oil reservoirs reaches a high water cut, due to the large molecular weight of conventional plugging and profile control systems such as polymer gels, they are prone to filtration blockage near the injection wells, with high injection pressure and unable to effectively enter the deep part of the oil reservoir for profile adjustment.

[0004] In response to the above problems, in recent years, major oilfields and research institutes have developed a series of enhanced oil recovery systems and technologies for low-permeability oil reservoirs by means of chemical synthesis, modification, and compounding. Among them, in the article "Research Progress on the Application Status of Profile Control and Displacement Agents in Low-Permeability Oil Reservoirs", the author reviewed the research progress of profile control and displacement systems for low-permeability oil reservoirs in recent years. The article elaborated in detail on the advantages and disadvantages of existing gel-based, gel-like, particle-based, and various composite profile control and displacement systems and corresponding processes, and pointed out that with the increasing development difficulty of low-permeability oil reservoirs, the existing systems and technologies cannot meet the requirements of significantly improving the recovery rate in the middle and late stages of low-permeability oil reservoir development, and new profile control and displacement systems and supporting process technologies need to be designed.

[0005] Chinese Patent CN115044356B discloses a profile control and displacement agent applicable to high-temperature and high-salt oil reservoirs and its preparation method, which includes, by weight percentage: 1% - 8% of a precipitation main agent, 10% - 30% of a first precipitation control agent, 0.1% - 2.0% of a second precipitation control agent, and the balance of liquid preparation water; wherein, the precipitation main agent is soluble silicate. The present invention also provides a preparation method of the profile control and displacement agent, including: adding the first precipitation control agent to the liquid preparation water and mixing evenly to obtain a first solution; adding the second precipitation control agent to the first solution and mixing evenly to obtain a second solution; adding the precipitation main agent to the second solution and mixing evenly to obtain the profile control and displacement agent. However, this patented technology synthesizes a low-viscosity profile control and displacement agent through chemical means, which only solves the problem of deep injection in medium and low-permeability oil reservoirs. However, the synthetic raw materials are chemical raw materials, which pollute the environment and have high synthesis costs, not meeting the current low-carbon and environmental protection concept of oilfield development.

[0006] Chinese Patent CN114539470B discloses an acrylamide-based functional polymer, its preparation method and application. The acrylamide-based functional polymer includes a structural unit A having the structure shown in formula (1), a structural unit B having the structure shown in formula (2), and a structural unit C having the structure shown in formula (3). However, the patent discloses a low-permeability reservoir profile control and flooding system centered on a modified polymer. This polymer has a high viscosity and is prone to near-wellbore plugging after injection, causing damage to the formation. Moreover, the industrial production cost is high after polymer modification, making it impossible to be widely applied in the field.

[0007] Chinese Patent CN109679623B discloses a displacement oil composition containing a sulfobetaine surfactant, its preparation method and application. The present invention adopts a displacement oil composition including a betaine-type amphoteric surfactant and an inorganic salt. Among them, the structure of the betaine-type amphoteric surfactant is shown in formula (I), R1 is selected from any one of C6-C29 alkyl and alkenyl, R3 and R4 are selected from any one of C1-C12 alkylene and hydroxy-substituted alkylene, m + n = 0-100, x + y = 0-100; the molar ratio of the betaine-type amphoteric surfactant to the inorganic salt is 1:0.01-1:100. This technical solution preferably solves this technical problem and can be used in the oil displacement production of oilfields. The medium-low permeability oil displacement system developed by this patent solves the injection problem, but it cannot solve the deep profile control problem in the later stage of medium-low permeability reservoir development. The injection system channels along the main flow path and cannot effectively contact the remaining oil.

[0008] Chinese Patent CN114181688B discloses a surfactant combination displacement oil system suitable for low-permeability reservoirs in multiple slugs and its preparation method. This system consists of a low-viscosity ultra-low interfacial tension surfactant displacement slug and a high-viscosity ultra-low interfacial tension surfactant profile control agent slug. However, the low-permeability reservoir profile control system developed by this patent consists of two systems. Among them, the high-viscosity profile control system also has the problem of high injection pressure in low-permeability reservoirs and is prone to near-wellbore plugging after long-term injection, making it impossible to effectively solve the problem of difficult injection and production in low-permeability reservoirs.

[0009] In addition, there are also a large number of low-permeability enhanced oil recovery systems and technologies using chemical synthesis and compounding as means. These systems and technologies also face problems such as complex processes, single functions and environmental protection, and are limited in on-site transformation.

[0010] Microbial enhanced oil recovery technology uses the growth and metabolism of microorganisms and the metabolites they produce to interact with crude oil and reservoirs to improve oil recovery. The preparation of the microbial enhanced oil recovery system mainly relies on waste materials from industries such as agriculture and food, with low technical costs. At the same time, microorganisms can grow and metabolize in-situ in the formation, and the metabolites produced will gradually degrade in the formation without causing environmental pollution and formation damage. Traditional microbial enhanced oil recovery technology has certain limitations when applied to medium and low permeability oil reservoirs. For example, Chinese patent application CN115895931A discloses a method for microbial enhanced oil recovery in medium and low permeability oil reservoirs, including the following steps: (1) screening of target oil reservoirs; (2) screening of high-protein activator systems for protease-producing microorganisms; (3) screening of activator systems for amine-producing microorganisms; (4) screening of oil-activated activator systems for surfactant-producing microorganisms; (5) determining the field injection process using physical simulation enhanced oil recovery experiments; (6) field tests and effect evaluation. This patent application uses the principle of endogenous microbial enhanced oil recovery technology to carry out microbial enhanced oil recovery in medium and low permeability oil reservoirs. Although the process is simple and the cost is low, it ignores the problem of strong heterogeneity in the middle and late stages of the development of medium and low permeability oil reservoirs. The microbial activation system mainly enters the preferential channels and cannot effectively mobilize the remaining oil that has not been produced by water flooding. In addition, the activator injected by the endogenous microbial enhanced oil recovery technology will preferentially enter the channeling channels with the injected water, and the activated microorganisms cannot contact the preferential remaining oil in the areas not swept by water flooding, resulting in limited improvement in oil recovery. During the implementation of exogenous microbial enhanced oil recovery technology, due to the certain molecular size of commonly used oil-producing strains (cell length 1-5 μm), they cannot migrate deep into low permeability oil reservoirs (throat radius < 1 μm). In addition, microbial enhanced oil recovery technology also has the defect of low oil washing efficiency. Summary of the Invention

[0011] Object of the Invention: Aiming at the deficiencies of the above-mentioned existing technologies, the present invention provides a method for improving the oil recovery of low permeability oil reservoirs, which can simultaneously solve the problems of deep profile control and efficient oil washing in low permeability oil reservoirs. The enhanced oil recovery system of the present invention includes a spore profile control system and a biosurfactant oil washing system; the injection process is multi-round multi-slug injection. By injecting the spore profile control system through the front slug, small-sized spores preferentially enter the preferential channels deep in the oil reservoir. Then, under the action of the activated activator, the spores are activated and grow into normal-sized bacilli to block the preferential channels. The subsequent slug injection of the biosurfactant oil washing slug without cells can enter the non-mainstream channels and contact the preferential remaining oil. Through multi-round injection, the oil recovery is greatly improved.

[0012] Technical Solution: A method for improving the oil recovery of low permeability oil reservoirs includes the following steps:

[0013] (1) Screening of target oil reservoirs

[0014] (2) Determination of the injection system and injection volume for each slug

[0015] (21) Determine a displacement system adapted to the reservoir temperature of the target reservoir, where:

[0016] The displacement system consists of a spore profile control system and an oil washing system;

[0017] The spore profile control system consists of a spore bacterium agent system and a spore activation and activation system;

[0018] (22) Determine the injection volume of each slug injection system;

[0019] (3) Field injection process.

[0020] Furthermore, the screening criteria for the target reservoir in step (1) are: underground crude oil viscosity 50 - 250 mPa·s, permeability 0.1 - 50 mD, oil layer thickness 2 - 10 m, reservoir temperature 30 - 75 °C, formation water salinity 5000 - 20000 mg / L, well pattern is an area well pattern, and the maximum injection-production well spacing ≤ 200 m.

[0021] Furthermore, in step (21): The spore profile control system consists of a spore bacterium agent system and a spore activation and activation system, and their volume ratio is 1:2 - 1:5, where:

[0022] The spore bacterium agent system consists of a spore powder of Bacillus bacteria and the formation water of the target reservoir, where:

[0023] Based on the total mass of the spore bacterium agent system, the spore powder of Bacillus bacteria accounts for 30 - 50% by mass;

[0024] The spore activation and activation system consists of a spore activation activator and the formation water of the target reservoir, where: Based on the total mass of the spore activation and activation system, the spore activation activator accounts for 0.15 - 0.45% by mass.

[0025] Even further, the spore powder of Bacillus bacteria is prepared by freeze-drying the spore-producing fermentation broth of Bacillus bacteria, where:

[0026] The Bacillus bacteria are one of Bacillus subtilis, Bacillus licheniformis, Bacillus cereus, Bacillus coagulans, and Bacillus thermophilus.

[0027] Even more further, in step (21), select the type of spore powder of Bacillus bacteria and the spore-producing fermentation medium of Bacillus spores adapted to the reservoir temperature of the target reservoir according to the following specific selection criteria:

[0028] For a target reservoir with a reservoir temperature of 70 - 75 °C, select Bacillus thermophilus;

[0029] For a target reservoir with a reservoir temperature of 60 - 69 °C, select Bacillus licheniformis;

[0030] For a target reservoir with a reservoir temperature of 50 - 59°C, select Bacillus coagulans;

[0031] For a target reservoir with a reservoir temperature of 40 - 49°C, select Bacillus cereus;

[0032] For a target reservoir with a reservoir temperature of 30 - 39°C, select Bacillus subtilis.

[0033] Even further, the sporulation fermentation medium for Bacillus subtilis is starch 2 - 3%, yeast powder 1.0 - 2.0%, dipotassium hydrogen phosphate 0.2 - 0.3%, MnSO 4 0.03 - 0.05%, CuCl 2 0.01 - 0.02%, ZnSO 4 0.01 - 0.03%, and the pH value is 7.0;

[0034] The sporulation fermentation medium for Bacillus cereus is dextrin 3 - 4%, beef extract 1.5 - 2.0%, potassium dihydrogen phosphate 0.3 - 0.4%, MnSO 4 0.02 - 0.05%, CuCl 2 0.01 - 0.02%, ZnCl 2 0.02 - 0.03%, and the pH value is 8.0;

[0035] The sporulation fermentation medium for Bacillus coagulans is molasses 3 - 4%, corn dry powder 1.2 - 1.5%, dipotassium hydrogen phosphate 0.4 - 0.5%, MnSO 4 0.01 - 0.03%, CuCl 2 0.02 - 0.04%, ZnSO 4 0.01 - 0.05%, and the pH value is 8.0;

[0036] The sporulation fermentation medium for Bacillus licheniformis is cellulose 3.2 - 4.1%, ammonium sulfate 2.5 - 3.0%, dipotassium hydrogen phosphate 0.25 - 0.35%, MnSO 4 0.03 - 0.05%, CuCl 2 0.02 - 0.04%, ZnCl 2 0.01 - 0.04%, and the pH value is 8.0;

[0037] The sporulation fermentation medium for Bacillus stearothermophilus is xylose 2.5 - 3.5%, ammonium nitrate 1.8 - 2.1%, potassium dihydrogen phosphate 0.35 - 0.43%, MnSO 4 0.03 - 0.05%, MgSO 4 0.01 - 0.03%, ZnCl 2 0.01 - 0.04%, and the pH value is 8.5.

[0038] Even further, the spore-forming fermentation broth of the Bacillus bacteria is obtained by fermenting Bacillus bacteria at 30 - 75°C for 48 - 96 hours in the spore-forming fermentation medium of the corresponding Bacillus bacteria, where:

[0039] The fermentation temperature of Bacillus stearothermophilus is controlled at 70 - 75°C;

[0040] The fermentation temperature of Bacillus licheniformis is controlled at 60 - 69°C;

[0041] The fermentation temperature of Bacillus coagulans is controlled at 50 - 59°C;

[0042] The fermentation temperature of Bacillus cereus is controlled at 40 - 49°C;

[0043] The fermentation temperature of Bacillus subtilis is controlled at 30 - 39°C.

[0044] Even even further, the plate colony counting method is used to evaluate the spore formation rate in the spore-forming fermentation broth of the above-mentioned Bacillus bacteria. If the spore formation rate reaches more than 80%, it is a qualified fermentation broth and can be used for freezing to prepare Bacillus spore powder.

[0045] Even further, the spore activation activator is composed of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and their mass ratio is (0.1 - 0.3) : (0.02 - 0.05) : (0.02 - 0.05) : (0.01 - 0.05).

[0046] Further, the oil washing system in step (21) is composed of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and the formation water of the target reservoir, where:

[0047] Based on the total mass of the oil washing system, the biosurfactant fermentation broth accounts for 5 - 10% by mass.

[0048] Even further, the preparation steps of the oil washing system in step (21) are as follows:

[0049] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37°C for 72 - 96 hours to obtain the fermented bacterial liquid, and then heat it to not less than 100°C and keep it for 20 - 30 minutes to lyse the bacterial cells, and then a cell-free biosurfactant fermentation broth is obtained, where:

[0050] The inoculation amount of the Pseudomonas aeruginosa is 2 - 5%.

[0051] Furthermore, the Pseudomonas aeruginosa fermentation medium consists of the following components in mass percentage: 3 - 5% glycerol, 0.8 - 1.2% sodium nitrate, 0.4 - 0.6% dipotassium hydrogen phosphate, 0.4 - 0.6% potassium dihydrogen phosphate, 0.1 - 0.2% NaCl, 0.02 - 0.05% MgSO 4 , 0.01 - 0.03% CaCl 2 , with the balance being water and the pH value being 8.0.

[0052] Furthermore, in step (22), the injection volume of each slug profile control system and oil washing system is determined by the following formula:

[0053] V = β × k / λ × H

[0054] Where: V—the correlation coefficient value of the injection volume of each slug injection system;

[0055] k—the reservoir permeability, with the unit of mD;

[0056] H—the effective thickness of the reservoir, with the unit of m;

[0057] λ—the viscosity of underground crude oil, mPa·s;

[0058] β—the dosage coefficient, with a value range of 2 - 5, where:

[0059] When the calculated V value ≤ 1, each slug of the spore profile control system and the oil washing system is injected with 100 m 3 ;

[0060] When 1 < calculated V value ≤ 2, each slug of the spore profile control system and the oil washing system is injected with 200 m 3 ;

[0061] When 2 < calculated V value ≤ 3, each slug of the spore profile control system and the oil washing system is injected with 300 m 3 ;

[0062] When 3 < calculated V value ≤ 4, each slug of the spore profile control system and the oil washing system is injected with 400 m 3 ;

[0063] When the calculated V value > 4, each slug of the spore profile control system and the oil washing system is injected with 500 m 3 .

[0064] Furthermore, the specific steps of step (3) are as follows:

[0065] Multi - round and multi - slug injection is adopted. For the first slug of each round, the spore agent system is injected into the formation through the injection well using a high - pressure pump. For the second slug of each round, the spore activation activator is injected using a high - pressure pump and air is accompanied throughout the process using a compression pump (displacement 50 m 3 / d). After the injection is completed, the injection well is shut in for 3 - 5 days. For the third slug, a washing oil system is injected using a high - pressure pump. After the injection is completed, it is shut in for 1 - 3 days, and then continuous water flooding is carried out. Among them:

[0066] When the water cut is higher than 95%, the second - round injection is carried out, and the cycle is 5 - 10 times.

[0067] The present invention discloses a method for improving the recovery rate of low - permeability oil reservoirs. The spores in the oil - displacement system are dormant bodies with low water content and strong resistance formed by some bacterial vegetative bodies when the nutrients are exhausted. They have various stress resistances such as heat resistance and salt tolerance, can tolerate a maximum temperature of 120 - 140 °C, can remain active in extreme environments, and can quickly return to the normal cell morphology under suitable conditions. The size of the spore of the bacterial cell is smaller than that of the normal bacterial cell, usually <0.3 μm, and can enter the main flow channels in the deep part of the low - permeability oil reservoir. After injecting the activation activator in the later stage, it is quickly activated and grows in situ at the pores of the main flow channels to achieve microscopic plugging. The sterile biosurfactant fermentation broth injected in the next slug can enter the secondary flow line area rich in remaining oil, and improve the oil - washing efficiency by reducing the interfacial tension and changing the wettability, etc. The process of this patent has the advantages of simple process, low cost, strong pertinence, and obvious effect of improving the recovery rate. At the same time, the oil - displacement system used in this technology can be gradually degraded through biological growth and metabolism, will not cause damage to the formation, and has no hidden danger of environmental pollution.

[0068] The advantages and beneficial effects of the present invention are as follows:

[0069] (1) The present invention has the advantages of low production cost, simple production process, strong targeting, no formation damage, and no environmental pollution;

[0070] (2) Through the multi - round and multi - slug on - site injection method, the present invention realizes the profile control of the main flow line in the deep part of the low - permeability oil reservoir and the efficient oil washing of the remaining oil in the secondary flow line. The maximum reduction of the water cut in the low - permeability oil reservoir reaches more than 20%, the input - output ratio is greater than 1:10, and the recovery rate is increased by more than 20%. Brief Description of the Drawings

[0071] Figure 1 It is a flow chart of a method for improving the recovery rate of low - permeability oil reservoirs disclosed by the present invention. Detailed Description of the Specific Embodiments

[0072] The following is a detailed description of the specific embodiments of the present invention.

[0073] In the ranges disclosed in this application, the endpoints and any values are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, and between the endpoint values of each range and individual point values, can be gradually combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed in this application.

[0074] The following is a detailed description of the specific embodiments of the present invention.

[0075] Example 1

[0076] A method for improving the recovery rate of low-permeability reservoirs includes the following steps:

[0077] (1) Screening of target reservoirs

[0078] Test reservoir A 1 Underground crude oil viscosity is 53 mPa·s, permeability is 15 mD, oil layer thickness is 1.3 m, reservoir temperature is 45 °C, formation water salinity is 7600 mg / L, well pattern is 1 injection and 4 production, well spacing is 130 m, test reservoir A 1 Meets the screening criteria.

[0079] (2) Determination of each slug injection system and injection volume

[0080] According to test reservoir A 1 For the reservoir temperature, the bacillus spore powder in the profile control system is prepared by freeze-drying the sporulation fermentation broth of Bacillus cereus.

[0081] Bacillus cereus is fermented in a Bacillus cereus sporulation fermentation medium for 84 h, and the fermentation temperature is controlled at 45 °C to obtain a Bacillus cereus fermentation broth.

[0082] The plate colony counting method is used to evaluate the sporulation rate in the above-mentioned Bacillus cereus sporulation fermentation broth. The sporulation rate is 85%, and it is judged as a qualified fermentation broth and can be used for freeze-drying to prepare bacillus spore powder.

[0083] The spore profile control system is composed of a spore agent system and a spore activation system, and their volume ratio is 1:2, where:

[0084] The spore agent system is composed of bacillus spore powder and the formation water of the target reservoir, where:

[0085] Calculated based on the total mass of the spore agent system, the bacillus spore powder accounts for 30% by mass;

[0086] The spore activation system is composed of a spore activation activator and the formation water of the target reservoir, where: calculated based on the total mass of the spore activation system, the spore activation activator accounts for 0.15% by mass.

[0087] The sporulation fermentation medium of Bacillus cereus is 3.5% dextrin, 1.5% beef extract, 0.3% potassium dihydrogen phosphate, MnSO 4 0.02%, CuCl 2 0.01%, ZnCl 2 0.02%, and the pH value is 8.0; in another embodiment, the sporulation fermentation medium of Bacillus cereus is 3% dextrin, 1.7% beef extract, 0.35% potassium dihydrogen phosphate, MnSO 4 0.03%, CuCl 2 0.015%, ZnCl 2 0.025%, and the pH value is 8.0; in another embodiment, the sporulation fermentation medium of Bacillus cereus is 4% dextrin, 2.0% beef extract, 0.4% potassium dihydrogen phosphate, MnSO 4 0.05%, CuCl 2 0.02%, ZnCl 2 0.03%, and the pH value is 8.0;

[0088] The spore activation activator is composed of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and the mass ratio is 0.1:0.02:0.02:0.01.

[0089] The oil washing system is composed of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and the formation water of the target reservoir, wherein:

[0090] Based on the total mass of the oil washing system, the biosurfactant fermentation broth accounts for 5% by mass.

[0091] Furthermore, the preparation steps of the oil washing system in step (21) are as follows:

[0092] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37°C for 72 h to obtain the fermented bacterial liquid, and then heat to 100°C and keep for 20 min. After lysing the bacterial cells, a cell-free biosurfactant fermentation broth is obtained, wherein:

[0093] The inoculation amount of Pseudomonas aeruginosa is 2%.

[0094] The Pseudomonas aeruginosa fermentation medium is composed of the following components in mass percentage:

[0095] 3% glycerol, 0.8% sodium nitrate, 0.4% dipotassium hydrogen phosphate, 0.4% potassium dihydrogen phosphate, 0.1% NaCl, 0.02% MgSO 4 , 0.01% CaCl 2, and the pH value is 8.0. In another embodiment, the Pseudomonas aeruginosa fermentation medium, by mass percentage, consists of the following components: 3% glycerol, 0.8% sodium nitrate, 0.4% dipotassium hydrogen phosphate, 0.4% potassium dihydrogen phosphate, 0.1% NaCl, 0.02% MgSO 4 , 0.01% CaCl 2 , with the balance being water and the pH value being 8.0. In another embodiment, the Pseudomonas aeruginosa fermentation medium, by mass percentage, consists of the following components: 5% glycerol, 1.2% sodium nitrate, 0.6% dipotassium hydrogen phosphate, 0.6% potassium dihydrogen phosphate, 0.2% NaCl, 0.05% MgSO 4 , 0.03% CaCl 2 , with the balance being water and the pH value being 8.0.

[0096] The injection volume of the profile control system and the oil displacement system for each slug is determined by the following formula:

[0097] V = β × k / λ × H

[0098] Where: V—the correlation coefficient value of the injection volume of the profile control agent for each slug;

[0099] k—the reservoir permeability, in mD;

[0100] H—the effective thickness of the reservoir, in m;

[0101] λ—the viscosity of the underground crude oil, in mPa·s;

[0102] β—the dosage coefficient, with a value of 2;

[0103] The V value = 0.74. Because when the V value ≤ 1, for each slug, the spore profile control system and the oil displacement system are each injected 100 m 3 ;

[0104] The volume ratio of the spore agent system to the spore activation and activation system in the profile control system is 1:2. The spore agent system is 33 m 3 , and the spore activation and activation system is 67 m 3 ;

[0105] (3) Field injection process

[0106] Multi-round and multi-slug injection is adopted. For the first slug of each round, 33 m of the spore agent system is injected into the formation through the injection well using a high-pressure pump. For the second slug, 67 m of the spore activation and activation system is injected using a high-pressure pump, and air is accompanied throughout the injection using a compression pump (displacement 50 m 3 / d). After the injection is completed, the injection well is shut in for 3 days. For the third slug, 100 m is injected using a high-pressure pump 3 3 , and after the injection is completed, the injection well is shut in for 3 days. For the third slug, 100 m is injected using a high-pressure pump / d), and after the injection is completed, the injection well is shut in for 3 days. For the third slug, 100 m is injected using a high-pressure pump 3The washing oil system is stopped for 1 day after injection is completed, and subsequent water flooding is carried out. After the water cut is higher than 95%, the second round of injection is carried out, and the cycle is repeated 5 times.

[0107] After adopting the technical solution of this embodiment, the maximum water cut reduction is 21%, and the ultimate oil recovery rate is increased to 25%.

[0108] Example 2

[0109] A method for improving the oil recovery rate of low-permeability reservoirs includes the following steps:

[0110] (1) Screening of target reservoirs

[0111] The viscosity of the underground crude oil in test reservoir B is 87 mPa·s, the permeability is 1.8 mD, the oil layer thickness is 4.2 m, the reservoir temperature is 65 °C, the formation water salinity is 8940 mg / L, and the well pattern is 1 injection and 6 production, with a well spacing of 150 m, meeting the screening criteria;

[0112] (2) Determination of each slug injection system and injection volume

[0113] According to the reservoir temperature, the spore bacillus spore powder in the spore profile control system is prepared by freeze-drying the sporulation fermentation broth of Bacillus licheniformis.

[0114] Bacillus licheniformis is fermented in the sporulation fermentation medium of Bacillus licheniformis for 60 h, and the fermentation temperature is controlled at 65 °C to obtain the fermentation broth of Bacillus licheniformis.

[0115] The spore formation rate in the above-mentioned sporulation fermentation broth of Bacillus licheniformis is evaluated by the plate colony counting method, and its spore formation rate is 89%, which is judged to be a qualified fermentation broth and can be used for freeze-drying to prepare spore bacillus spore powder.

[0116] The spore profile control system is composed of a spore bacterium agent system and a spore activation and activation system, and their volume ratio is 1:3, where:

[0117] The spore bacterium agent system is composed of spore bacillus spore powder and the formation water of the target reservoir, where:

[0118] Based on the total mass of the spore bacterium agent system, the spore bacillus spore powder accounts for 35% by mass;

[0119] The spore activation and activation system is composed of a spore activation activator and the formation water of the target reservoir, where: based on the total mass of the spore activation and activation system, the spore activation activator accounts for 0.28% by mass.

[0120] The sporulation fermentation medium of Bacillus licheniformis is 3.8% cellulose, 2.5% ammonium sulfate, 0.30% dipotassium hydrogen phosphate, MnSO 4 0.03%, CuCl 2 0.02%, ZnCl2 0.01%, with a pH value of 8.0. In another embodiment, the sporulation fermentation medium for Bacillus licheniformis is 3.2% cellulose, 2.5% ammonium sulfate, 0.25% dipotassium hydrogen phosphate, MnSO 4 0.03%, CuCl 2 0.02%, ZnCl 2 0.01%, with a pH value of 8.0. In another embodiment, the sporulation fermentation medium for Bacillus licheniformis is 4.1% cellulose, 3.0% ammonium sulfate, 0.35% dipotassium hydrogen phosphate, MnSO 4 0.05%, CuCl 2 0.04%, ZnCl 2 0.04%, with a pH value of 8.0;

[0121] The spore activation activator consists of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and has a mass ratio of 0.2:0.03:0.03:0.02.

[0122] The oil washing system consists of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and the formation water of the target reservoir, where:

[0123] Based on the total mass of the oil washing system, the biosurfactant fermentation broth accounts for 6% by mass.

[0124] The preparation steps of the oil washing system are as follows:

[0125] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, then ferment at 37°C for 80 h to obtain the fermented broth, and then heat to 100°C and hold for 25 min. After lysing the cells, a cell-free biosurfactant fermentation broth is obtained, where:

[0126] The inoculation amount of Pseudomonas aeruginosa is 5%.

[0127] The Pseudomonas aeruginosa fermentation medium, in terms of mass percentage, consists of the following components: 4% glycerol, 1.0% sodium nitrate, 0.5% dipotassium hydrogen phosphate, 0.5% potassium dihydrogen phosphate, 0.15% NaCl, 0.03% MgSO 4 , 0.02% CaCl 2 , and the pH value is 8.0.

[0128] The injection amount of each slug of the spore profile control system and the oil washing system is determined by the following formula:

[0129] V = β × k / λ × H

[0130] where: V—the correlation coefficient value of the injection amount of each slug of the profile control agent;

[0131] k—reservoir permeability, in mD;

[0132] H—effective thickness of reservoir, in m;

[0133] λ—underground crude oil viscosity mPa·s;

[0134] β—dosage coefficient, the value is 4;

[0135] V value = 0.34, when V value ≤ 1, each slug spore profile control system and oil washing system is injected with 100m 3 ;

[0136] The volume ratio of the spore agent system and the spore activation system in the spore profile control system is 1:3, and the spore agent system is 25m 3 , spore activation system 75m 3 .

[0137] (3) On-site injection process

[0138] The injection was carried out in multiple rounds and multiple segments. In each round, the first segment was injected with a high-pressure pump to inject 25m 3 The spore-forming agent is injected into the formation through the water injection well, and the second plug is injected into the formation by a high-pressure pump for 75m 3 The spore activation system uses a compression pump to inject air (displacement 50m 3 / d), the injection well was stopped for 4 days after the injection was completed, and the third section was injected with 100m 3 For the oil washing system, stop injection for 2 days after the injection is completed, and then water drive. When the water content is higher than 95%, the second round of injection is carried out, and the cycle is repeated for 9 rounds.

[0139] After adopting the technical solution of this embodiment, the maximum reduction of water content is 28%, and the ultimate recovery rate is increased to 35%.

[0140] Example 3

[0141] A method for improving the recovery rate of a low permeability oil reservoir comprises the following steps:

[0142] (1) Target reservoir screening

[0143] The test reservoir C has an underground crude oil viscosity of 130mPa·s, a permeability of 30.8mD, an oil layer thickness of 2.5m, a reservoir temperature of 35℃, a formation water salinity of 5120mg / L, a well pattern of 1 injection and 4 production, and a well spacing of 160m, which meets the screening criteria;

[0144] (2) Determination of injection system and injection volume for each plug

[0145] According to the reservoir temperature, the Bacillus spore powder in the spore profile control system is prepared by freeze-drying the spore fermentation liquid of Bacillus subtilis.

[0146] Bacillus subtilis is fermented in a sporulation fermentation medium of Bacillus subtilis for 96 h, and the fermentation temperature is controlled at 37 °C to obtain a Bacillus subtilis fermentation broth.

[0147] The plate colony counting method is used to evaluate the sporulation rate in the above-mentioned sporulation fermentation broth of Bacillus subtilis. The sporulation rate is 87%, and it is judged as a qualified fermentation broth, which can be used for freeze-drying to prepare Bacillus spore powder.

[0148] The spore profile control system is composed of a spore agent system and a spore activation and activation system, and their volume ratio is 1:4, where:

[0149] The spore agent system is composed of Bacillus spore powder and formation water of the target reservoir, where:

[0150] Based on the total mass of the spore agent system, the Bacillus spore powder accounts for 50% by mass;

[0151] The spore activation and activation system is composed of a spore activation and activator and formation water of the target reservoir, where: based on the total mass of the spore activation and activation system, the spore activation and activator accounts for 0.41% by mass.

[0152] The sporulation fermentation medium of Bacillus subtilis is 3.0% starch, 2.0% yeast powder, 0.2% dipotassium hydrogen phosphate, MnSO 4 0.03%, CuCl 2 0.01%, ZnSO 4 0.03%, and the pH value is 7.0. In another embodiment, the sporulation fermentation medium of Bacillus subtilis is 2% starch, 1.0% yeast powder, 0.2% dipotassium hydrogen phosphate, MnSO 4 0.03%, CuCl 2 0.01%, ZnSO 4 0.01%, and the pH value is 7.0. In another embodiment, the sporulation fermentation medium of Bacillus subtilis is 3% starch, 2.0% yeast powder, 0.3% dipotassium hydrogen phosphate, MnSO 4 0.05%, CuCl 2 0.02%, ZnSO 4 0.03%, and the pH value is 7.0.

[0153] The spore activation and activator is composed of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and their mass ratio is 0.3:0.04:0.04:0.03.

[0154] The oil washing system is composed of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and formation water of the target reservoir, where:

[0155] Based on the total mass of the wash oil system, the biosurfactant fermentation broth accounts for 7% by mass.

[0156] The preparation steps of the wash oil system are as follows:

[0157] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37 °C for 72 h - 96 h to obtain the fermented bacterial liquid. Then heat it to 110 °C and maintain for 25 min. After lysing the bacterial cells, the cell-free biosurfactant fermentation broth is obtained, where:

[0158] The inoculation amount of Pseudomonas aeruginosa is 2%.

[0159] The Pseudomonas aeruginosa fermentation medium consists of the following components in mass percentage: 5% glycerol, 1.1% sodium nitrate, 0.55% dipotassium hydrogen phosphate, 0.55% potassium dihydrogen phosphate, 0.15% NaCl, 0.04% MgSO 4 , 0.04% CaCl 2 , and the pH value is 8.0.

[0160] The injection volume of each slug of the spore profile control system and the wash oil system is determined by the following formula:

[0161] V = β × k / λ × H

[0162] Where: V—the correlation coefficient value of the injection volume of each slug of the displacement agent;

[0163] k—the reservoir permeability, in mD;

[0164] H—the effective thickness of the reservoir, in m;

[0165] λ—the underground crude oil viscosity, in mPa·s;

[0166] β—the dosage coefficient, with a value of 3;

[0167] When the V value = 1.77 and 1 < calculated V value ≤ 2, each slug of the spore profile control system and the wash oil system are each injected with 200 m 3 ;

[0168] The volume ratio of the spore agent system to the spore activation and activation system in the spore profile control system is 1:4. The spore agent system is 40 m 3 and the spore activation and activation system is 160 m 3 .

[0169] (3) Field injection process

[0170] Adopt multi-round and multi-slug injection. In the first slug of each round, use a high-pressure pump to inject 40 m 3The spore agent is injected into the formation through the injection well, and the second slug is injected with a high-pressure pump for 160 m 3 The spore activation and activation system is used, and air is injected concomitantly with a compression pump throughout the process (displacement 50 m 3 / d). After the injection is completed, the injection well is shut in for 5 d. The third slug is injected with a high-pressure pump for 200 m 3 of the oil washing system. After the injection is completed, it is shut in for 3 d, and then water flooding is carried out. When the water cut is higher than 95%, the second round of injection is carried out, and it is circulated 8 times.

[0171] After adopting the technical solution of this embodiment, the maximum reduction of water cut is 29%, and the ultimate recovery factor is increased to 25%.

[0172] Example 4

[0173] A method for improving the recovery factor of a low-permeability oil reservoir includes the following steps:

[0174] (1) Screening of the target oil reservoir

[0175] The viscosity of the underground crude oil in the test oil reservoir D is 243 mPa·s, the permeability is 28 mD, the oil layer thickness is 4.8 m, the reservoir temperature is 52 °C, the formation water salinity is 12,000 mg / L, the well pattern is 1 injection and 6 production, and the well spacing is 180 m, which meets the screening criteria;

[0176] (2) Determination of the injection system and injection volume for each slug

[0177] According to the reservoir temperature, the spore bacillus spore powder in the spore profile control system is prepared by fermenting Bacillus coagulans.

[0178] Bacillus coagulans is fermented in the Bacillus coagulans sporulation fermentation medium for 72 h, and the fermentation temperature is controlled at 55 °C to obtain the Bacillus coagulans fermentation broth.

[0179] The sporulation rate in the above-mentioned Bacillus coagulans sporulation fermentation broth is evaluated by the plate colony counting method, and its sporulation rate is 90%. It is judged that it is a qualified fermentation broth and can be used for freeze-drying to prepare the spore powder of bacillus.

[0180] The spore profile control system is composed of a spore agent system and a spore activation and activation system, and their volume ratio is 1:5, where:

[0181] The spore agent system is composed of the spore powder of bacillus and the formation water of the target oil reservoir, where:

[0182] Based on the total mass of the spore agent system, the spore powder of bacillus accounts for 40% by mass;

[0183] The spore activation and activation system is composed of a spore activation activator and the formation water of the target oil reservoir, where: based on the total mass of the spore activation and activation system, the spore activation activator accounts for 0.39% by mass.

[0184] The sporulation fermentation medium of Bacillus coagulans is 4% molasses, 1.2% corn dry powder, 0.4% dipotassium hydrogen phosphate, MnSO 4 0.01%, CuCl 2 0.03%, ZnSO 4 0.03%, and the pH value is 8.0. In another embodiment, the sporulation fermentation medium of Bacillus coagulans is 3% molasses, 1.2% corn dry powder, 0.4% dipotassium hydrogen phosphate, MnSO 4 0.01%, CuCl 2 0.02%, ZnSO 4 0.01%, and the pH value is 8.0. In another example, the sporulation fermentation medium of Bacillus coagulans is 4% molasses, 1.5% corn dry powder, 0.5% dipotassium hydrogen phosphate, MnSO 4 0.03%, CuCl 2 0.04%, ZnSO 4 0.05%, and the pH value is 8.0.

[0185] The spore activation activator is composed of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and the mass ratio is 0.25:0.05:0.05:0.04.

[0186] The oil washing system is composed of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and the formation water of the target reservoir, where:

[0187] Based on the total mass of the oil washing system, the biosurfactant fermentation broth accounts for 8% by mass.

[0188] The preparation steps of the oil washing system are as follows:

[0189] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37°C for 72 h to obtain the fermented bacterial liquid, and then heat to 100°C and keep for 20 min. After lysing the bacterial cells, a cell-free biosurfactant fermentation broth is obtained, where:

[0190] The inoculation amount of Pseudomonas aeruginosa is 3%.

[0191] The Pseudomonas aeruginosa fermentation medium, in terms of mass percentage, is composed of the following components: 3.5% glycerol, 1.2% sodium nitrate, 0.6% dipotassium hydrogen phosphate, 0.6% potassium dihydrogen phosphate, 0.2% NaCl, 0.05% MgSO 4 , 0.05% CaCl 2 , and the pH value is 8.0.

[0192] The injection volume of each slug of the Bacillus plugging and profile control system and the oil washing system is determined by the following formula:

[0193] V = β × k / λ × H

[0194] Where: V—the correlation coefficient value of the injection volume of each slug of the displacement agent;

[0195] k—the reservoir permeability, in mD;

[0196] H—the effective thickness of the reservoir, in m;

[0197] λ—the viscosity of underground crude oil, in mPa·s;

[0198] β—the dosage coefficient, with a value of 5;

[0199] When the value of V = 2.76 and 2 < the calculated value of V ≤ 3, each slug of the Bacillus plugging and profile control system and the oil washing system is injected with 300 m 3 ;

[0200] The volume ratio of the Bacillus agent system to the Bacillus activation and activation system in the Bacillus plugging and profile control system is 1:5, with the Bacillus agent system being 50 m 3 and the Bacillus activation and activation system being 250 m 3 .

[0201] (3) On-site injection process

[0202] Multi-round and multi-slug injection is adopted. In the first slug of each round, 50 m 3 of the Bacillus agent is injected into the formation through the injection well using a high-pressure pump. In the second slug, 250 m 3 of the Bacillus activation and activation system is injected using a high-pressure pump, and air is accompanied throughout the injection using a compression pump (displacement 50 m 3 / d). After the injection is completed, the injection well is shut in for 4 d. In the third slug, 300 m 3 of the oil washing system is injected using a high-pressure pump. After the injection is completed, it is shut in for 2 d, and then subsequent water flooding is carried out. When the water cut is higher than 95%, the second round of injection is carried out, and the cycle is repeated 6 times.

[0203] After adopting the technical solution of this embodiment, the maximum reduction in water cut is 22%, and the final enhanced oil recovery reaches 27%.

[0204] Example 5

[0205] A method for enhancing oil recovery in low-permeability reservoirs, comprising the following steps:

[0206] (1) Target reservoir screening

[0207] The viscosity of the underground crude oil in reservoir E is 249 mPa·s, the permeability is 49 mD, the thickness of the oil layer is 3.2 m, the reservoir temperature is 73 °C, the salinity of the formation water is 15,000 mg / L, the well pattern is 2 injection wells and 11 production wells, and the well spacing is 197 m, meeting the screening criteria;

[0208] (2) Determination of each slug injection system and injection volume

[0209] According to the reservoir temperature, the spore powder of Bacillus in the spore profile control system is prepared by freeze-drying the spore fermentation broth of thermophilic Bacillus.

[0210] The thermophilic Bacillus is fermented in the spore fermentation medium of thermophilic Bacillus for 50 h, and the fermentation temperature is controlled at 73 °C to obtain the fermentation broth of thermophilic Bacillus.

[0211] The spore production rate in the above-mentioned spore fermentation broth of thermophilic Bacillus is evaluated by the plate colony counting method. The spore production rate is 92%, and it is judged as a qualified fermentation broth and can be used for freeze-drying to prepare the spore powder of Bacillus.

[0212] The spore profile control system is composed of a spore agent system and a spore activation system, and their volume ratio is 1:3, where:

[0213] The spore agent system is composed of the spore powder of Bacillus and the formation water of the target reservoir, where:

[0214] Based on the total mass of the spore agent system, the spore powder of Bacillus accounts for 45% by mass;

[0215] The spore activation system is composed of a spore activation activator and the formation water of the target reservoir, where: based on the total mass of the spore activation system, the spore activation activator accounts for 0.4% by mass.

[0216] The spore fermentation medium of thermophilic Bacillus is 2.5% xylose, 1.8% ammonium nitrate, 0.35% potassium dihydrogen phosphate, MnSO 4 0.03%, MgSO 4 0.01%, ZnCl 2 0.01%, and the pH value is 8.5. In another embodiment, the spore fermentation medium of thermophilic Bacillus is 3% xylose, 2% ammonium nitrate, 0.4% potassium dihydrogen phosphate, MnSO 4 0.04%, MgSO 4 0.02%, ZnCl 2 0.02%, and the pH value is 8.5. In another embodiment, the spore fermentation medium of thermophilic Bacillus is 3.5% xylose, 2.1% ammonium nitrate, 0.43% potassium dihydrogen phosphate, MnSO 4 0.05%, MgSO 4 0.03%, ZnCl2 0.04%, and the pH value is 8.5.

[0217] The spore activation activator consists of glucose, alanine, dipotassium hydrogen phosphate, and CaCl 2 and their mass ratio is 0.3:0.025:0.025:0.05.

[0218] The oil-washing system consists of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and the formation water of the target reservoir, where:

[0219] Based on the total mass of the oil-washing system, the biosurfactant fermentation broth accounts for 9% by mass.

[0220] The preparation steps of the oil-washing system are as follows:

[0221] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37°C for 96 h to obtain the fermented broth. Then heat it to 100°C and keep it for 30 min to lyse the cells, and a cell-free biosurfactant fermentation broth is obtained, where:

[0222] The inoculation amount of Pseudomonas aeruginosa is 5%.

[0223] The Pseudomonas aeruginosa fermentation medium consists of the following components in mass percentage: 4.5% glycerol, 0.9% sodium nitrate, 0.4% dipotassium hydrogen phosphate, 0.4% potassium dihydrogen phosphate, 0.2% NaCl, 0.03% MgSO 4 , 0.03% CaCl 2 , and the pH value is 8.0.

[0224] The injection amount of each slug of the spore profile control system and the oil-washing system is determined by the following formula:

[0225] V = β × k / λ × H

[0226] Where: V—the correlation coefficient value of the injection amount of each slug of the profile control agent;

[0227] k—the reservoir permeability, in mD;

[0228] H—the effective thickness of the reservoir, in m;

[0229] λ—the viscosity of underground crude oil, in mPa·s;

[0230] β—the dosage coefficient, with a value of 5;

[0231] When the V value = 3.15 and 3 < the calculated V value ≤ 4, each slug of the spore profile control system and the oil-washing system is injected with 400 m 3 ;

[0232] In the spore profile control system, the volume ratio of the spore bacterium agent system to the spore activation and activation system is 1:3. The spore bacterium agent system is 100 m 3 , and the spore activation and activation system is 300 m 3 .

[0233] (3) Field injection process

[0234] Multi-round and multi-slug injection is adopted. In the first slug of each round, 100 m 3 of the spore bacterium agent is injected into the formation through the injection well by using a high-pressure pump. In the second slug, 300 m 3 of the spore activation and activation system is injected by using a high-pressure pump, and air is accompanied by injection throughout the process by using a compression pump (displacement 50 m 3 / d). After the injection is completed, the injection well is shut off for 5 d. In the third slug, 400 m 3 of the oil washing system is injected. After the injection is completed, it is shut off for 3 d, and then water flooding is carried out. When the water cut is higher than 95%, the second round of injection is carried out, and it is circulated 8 times.

[0235] After adopting the technical solution of this embodiment, the maximum reduction of water cut is 33%, and the ultimate recovery rate is increased to 38%.

[0236] Example 6

[0237] A method for improving the recovery rate of low-permeability oil reservoirs includes the following steps:

[0238] (1) Target oil reservoir screening

[0239] The underground crude oil viscosity of the test oil reservoir F is 210 mPa·s, the permeability is 35 mD, the oil layer thickness is 6.3 m, the oil reservoir temperature is 62 °C, the formation water salinity is 17,000 mg / L, the well pattern is 3 injection wells and 16 production wells, and the well spacing is 150 m, which meets the screening criteria;

[0240] (2) Determination of each slug injection system and injection volume

[0241] According to the temperature of this oil reservoir, the spore bacillus powder in the spore profile control system is prepared by fermentation with Bacillus licheniformis liquid.

[0242] Bacillus licheniformis is fermented in the Bacillus licheniformis sporulation fermentation medium for 70 h, and the fermentation temperature is controlled at 62 °C to obtain the Bacillus licheniformis fermentation broth.

[0243] The plate colony counting method is used to evaluate the sporulation rate in the above-mentioned Bacillus licheniformis sporulation fermentation broth. The sporulation rate is 93%, and it is judged that it is a qualified fermentation broth and can be used for freeze-drying to prepare the spore bacillus powder.

[0244] The spore profile control system is composed of a spore bacterium agent system and a spore activation and activation system, and their volume ratio is 1:2, where:

[0245] The spore bacteria agent system consists of a spore powder of Bacillus and formation water of the target reservoir, where:

[0246] Based on the total mass of the spore bacteria agent system, the spore powder of Bacillus accounts for 45% by mass;

[0247] The spore activation and activation system consists of a spore activation activator and formation water of the target reservoir, where: based on the total mass of the spore activation and activation system, the spore activation activator accounts for 0.23% by mass.

[0248] The sporulation fermentation medium of Bacillus licheniformis is 4.04% cellulose, 3.0% ammonium sulfate, 0.35% dipotassium hydrogen phosphate, MnSO 4 0.05%, CuCl 2 0.04%, ZnCl 2 0.04%, and the pH value is 8.0.

[0249] The spore activation activator consists of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and their mass ratio is 0.15:0.035:0.035:0.01.

[0250] The oil washing system consists of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and formation water of the target reservoir, where:

[0251] Based on the total mass of the oil washing system, the biosurfactant fermentation broth accounts for 10% by mass.

[0252] The preparation steps of the oil washing system are as follows:

[0253] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37°C for 96 h to obtain the fermented bacterial liquid, and then heat to 120°C and keep for 20 min to lyse the bacteria to obtain the cell-free biosurfactant fermentation broth, where:

[0254] The inoculation amount of Pseudomonas aeruginosa is 2%.

[0255] The Pseudomonas aeruginosa fermentation medium consists of the following components in mass percentage: 3.5% glycerol, 1.0% sodium nitrate, 0.5% dipotassium hydrogen phosphate, 0.5% potassium dihydrogen phosphate, 0.15% NaCl, 0.04% MgSO 4 0.01% CaCl 2 and the pH value is 8.0.

[0256] The injection amount of each slug spore profile control system and oil washing system is determined by the following formula:

[0257] V = β × k / λ × H

[0258] Wherein: V—the correlation coefficient value of the injection volume of each slug of the profile control agent;

[0259] k—the reservoir permeability, with the unit of mD;

[0260] H—the effective thickness of the reservoir, with the unit of m;

[0261] λ—the viscosity of underground crude oil, mPa·s;

[0262] β—the dosage coefficient, with a value of 5;

[0263] The value of V = 5.25. When calculating the value of V, when 4 < the calculated result of the value of V, for each slug, 500 m of the spore profile control system and the oil washing system are injected respectively; 3 ;

[0264] In the spore profile control system, the volume ratio of the spore agent system to the spore activation and activation system is 1:2. The spore agent system is 167 m 3 and the spore activation and activation system is 333 m 3 .

[0265] (3) On-site injection process

[0266] Multi-round and multi-slug injection is adopted. In the first slug of each round, 167 m of the spore agent is injected into the formation through the injection well by using a high-pressure pump. In the second slug, 333 m of the spore activation and activation system is injected by using a high-pressure pump, and air is accompanied by injection throughout the process by using a compression pump (displacement 50 m 3 / d). After the injection is completed, the injection well is shut off for 3 d. In the third slug, 500 m 3 of the oil washing system is injected by using a high-pressure pump. After the injection is completed, it is shut off for 2 d, and then subsequent water flooding is carried out. When the water cut is higher than 95%, the second round of injection is carried out, and the cycle is repeated 7 times. 3 / d), and after the injection is completed, the injection well is shut off for 3 d. In the third slug, 500 m 3 of the oil washing system is injected by using a high-pressure pump. After the injection is completed, it is shut off for 2 d, and then subsequent water flooding is carried out. When the water cut is higher than 95%, the second round of injection is carried out, and the cycle is repeated 7 times.

[0267] After adopting the technical solution of this embodiment, the maximum reduction of water cut is 22%, and the ultimate oil recovery rate is increased to 31%.

[0268] Example 7

[0269] A method for improving the oil recovery rate of low-permeability reservoirs, comprising the following steps:

[0270] (1) Screening of target reservoirs

[0271] For the test reservoir G, the viscosity of underground crude oil is 30 mPa·s, the permeability is 0.3 mD, the oil layer thickness is 7.9 m, the reservoir temperature is 70 °C, the formation water salinity is 19,000 mg / L, the well pattern is 4 injection wells and 22 production wells, and the well spacing is 130 m, which meets the screening criteria;

[0272] (2) Determination of the injection system and injection volume for each slug

[0273] According to the reservoir temperature, the spore powder of Bacillus in the spore profile control system is prepared by freeze-drying the spore fermentation broth of heat-resistant Bacillus.

[0274] The heat-resistant Bacillus is fermented in a spore-producing fermentation medium of heat-resistant Bacillus for 50 h, and the fermentation temperature is controlled at 70 °C to obtain a heat-resistant Bacillus fermentation broth.

[0275] The spore production rate in the above-mentioned spore-producing fermentation broth of heat-resistant Bacillus is evaluated by the plate colony counting method. The spore production rate is 92%, and it is judged as a qualified fermentation broth, which can be used for freeze-drying to prepare spore powder of Bacillus.

[0276] The spore profile control system consists of a spore agent system and a spore activation system, and their volume ratio is 1:4, where:[[]]END]]

[0277] The spore agent system consists of spore powder of Bacillus and formation water of the target reservoir, where:[[]]END]]

[0278] Based on the total mass of the spore agent system, the spore powder of Bacillus accounts for 30% by mass;[[]]END]]

[0279] The spore activation system consists of a spore activation activator and formation water of the target reservoir, where: based on the total mass of the spore activation system, the spore activation activator accounts for 0.33% by mass.

[0280] The spore-producing fermentation medium of heat-resistant Bacillus is 3.5% xylose, 2.1% ammonium nitrate, 0.43% potassium dihydrogen phosphate, MnSO 4 0.05%, MgSO 4 0.03%, ZnCl 2 0.04%, and the pH value is 8.5.

[0281] The spore activation activator consists of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and their mass ratio is 0.2:0.04:0.04:0.05.

[0282] The oil washing system consists of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and formation water of the target reservoir, where:[[]]END]]

[0283] Based on the total mass of the oil washing system, the biosurfactant fermentation broth accounts for 5% by mass.

[0284] The preparation steps of the oil washing system are as follows:[[]]END]]

[0285] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37°C for 86 hours to obtain the fermented bacterial liquid. Then heat it to 120°C and maintain for 20 minutes. After lysing the bacteria, a cell-free biosurfactant fermentation broth is obtained, where:

[0286] The inoculation amount of the Pseudomonas aeruginosa is 5%.

[0287] The Pseudomonas aeruginosa fermentation medium consists of the following components in mass percentage: 4% glycerol, 0.8% sodium nitrate, 0.4% dipotassium hydrogen phosphate, 0.6% potassium dihydrogen phosphate, 0.1% NaCl, 0.03% MgSO 4 、0.03% CaCl 2 , and the pH value is 8.0.

[0288] The injection amount of each slug of the spore profile control system and the oil washing system is determined by the following formula:

[0289] V = β × k / λ × H

[0290] Where: V - the correlation coefficient value of the injection amount of each slug of the profile control agent;

[0291] k - the reservoir permeability, with the unit of mD;

[0292] H - the effective thickness of the reservoir, with the unit of m;

[0293] λ - the viscosity of underground crude oil, mPa·s;

[0294] β - the dosage coefficient, with a value of 2;

[0295] The value of V = 0.158. When the calculated value of V ≤ 1, each slug of the spore profile control system and the oil washing system are each injected with 100 m 3 ;

[0296] The volume ratio of the spore agent system to the spore activation and activation system in the spore profile control system is 1:4. The spore agent system is 20 m 3 The spore activation and activation system is 80 m 3 .

[0297] (3) On-site injection process

[0298] Adopt multi-round and multi-slug injection. In the first slug of each round, use a high-pressure pump to inject 20 m 3 of the spore agent into the formation through the injection well. In the second slug, use a high-pressure pump to inject 80 m 3 of the spore activation and activation system and accompany the injection of air throughout the process using a compression pump (displacement 50 m 3 / d). After the injection is completed, the injection well is shut in for 4 days. For the third slug, a 100 washing oil system is injected using a high-pressure pump. After the injection is completed, it is shut in for 3 days, and then followed by water flooding. When the water cut is higher than 95%, the second round of injection of 100 m 3 is carried out for 6 cycles.

[0299] After adopting the technical solution of this embodiment, the maximum reduction of water cut is 25%, and the final enhanced oil recovery reaches 28%.

[0300] Example 8

[0301] A method for enhancing oil recovery in low-permeability reservoirs, comprising the following steps:

[0302] (1) Screening of target reservoirs

[0303] For the test reservoir H, the underground crude oil viscosity is 55 mPa·s, the permeability is 25 mD, the oil layer thickness is 9.3 m, the reservoir temperature is 43 °C, the formation water salinity is 6419 mg / L, the well pattern is 2 injection wells and 15 production wells, and the well spacing is 160 m, which meets the screening criteria;

[0304] (2) Determination of the injection system and injection volume for each slug

[0305] According to the reservoir temperature, the bacillus spores in the bacillus profile control system are prepared by freeze-drying the sporulation fermentation broth of Bacillus cereus.

[0306] Bacillus cereus is fermented in the Bacillus cereus sporulation fermentation medium for 84 h, and the fermentation temperature is controlled at 45 °C to obtain the Bacillus cereus fermentation broth.

[0307] The spore formation rate in the above-mentioned Bacillus cereus sporulation fermentation broth is evaluated by the plate colony counting method, and its spore formation rate is 92%, which is judged to be a qualified fermentation broth and can be used for freeze-drying to prepare bacillus spores powder.

[0308] The bacillus profile control system is composed of a bacillus agent system and a bacillus activation and activation system, and their volume ratio is 1:5, where:

[0309] The bacillus agent system is composed of bacillus spores powder and the formation water of the target reservoir, where:

[0310] Based on the total mass of the bacillus agent system, the bacillus spores powder accounts for 40% by mass;

[0311] The bacillus activation and activation system is composed of a bacillus activation and activator and the formation water of the target reservoir, where: based on the total mass of the bacillus activation and activation system, the bacillus activation and activator accounts for 0.37% by mass.

[0312] The Bacillus cereus sporulation fermentation medium is 4% dextrin, 2.0% beef extract, 0.4% potassium dihydrogen phosphate, MnSO4 0.05%, CuCl 2 0.02%, ZnCl 2 0.03%, with a pH value of 8.0.

[0313] The spore activation activator consists of glucose, alanine, dipotassium hydrogen phosphate, and CaCl 2 and is composed in a mass ratio of 0.3:0.03:0.02:0.02.

[0314] The oil-washing system consists of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and the formation water of the target reservoir, where:

[0315] Based on the total mass of the oil-washing system, the biosurfactant fermentation broth accounts for 8% by mass.

[0316] The preparation steps of the oil-washing system are as follows:

[0317] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, then ferment at 37 °C for 96 h to obtain the fermented broth, and then heat it to 100 °C and hold for 30 min. After lysing the cells, a cell-free biosurfactant fermentation broth is obtained, where:

[0318] The inoculation amount of Pseudomonas aeruginosa is 4%.

[0319] The Pseudomonas aeruginosa fermentation medium consists of the following components in mass percentages: 3% glycerol, 0.9% sodium nitrate, 0.5% dipotassium hydrogen phosphate, 0.5% potassium dihydrogen phosphate, 0.15% NaCl, 0.04% MgSO 4 , 0.03% CaCl 2 , and the pH value is 8.0.

[0320] The injection amount of each slug of the spore profile control system and the oil-washing system is determined by the following formula:

[0321] V = β × k / λ × H

[0322] where: V—the correlation coefficient value of the injection amount of each slug of the displacement agent;

[0323] k—the reservoir permeability, in mD;

[0324] H—the effective thickness of the reservoir, in m;

[0325] λ—the viscosity of underground crude oil, in mPa·s;

[0326] β—the dosage coefficient, with a value of 3;

[0327] When V value = 12.68, 4 < calculated result V, each slug spore profile control system and oil washing system injects 500m 3 ;

[0328] The volume ratio of the spore-forming agent system and the spore activation system in the spore profile control system is 1:5, and the spore-forming agent system is 83m 3 , spore activation system 417m 3 .

[0329] (3) On-site injection process

[0330] The injection was carried out in multiple rounds and multiple segments. In each round, the first segment was injected with a high-pressure pump to inject 83m 3 The spore-forming agent was injected into the formation through the water injection well, and the second plug was injected into the formation using a high-pressure pump at 417 m 3 The spore activation system uses a compression pump to inject air (displacement 50m 3 / d), the injection well was stopped for 5 days after the injection was completed, and the third section was injected with a high-pressure pump for 500m 3 For the oil washing system, stop injection for 3 days after the injection is completed, and then water drive is carried out. When the water content is higher than 95%, the second round of injection is carried out, and the cycle is repeated for 7 rounds.

[0331] After adopting the technical solution of this embodiment, the maximum reduction of water content is 30%, and the ultimate recovery rate is increased to 35%.

[0332] Example 9

[0333] A method for improving the recovery rate of a low permeability oil reservoir comprises the following steps:

[0334] (1) Target reservoir screening

[0335] The underground crude oil viscosity of test reservoir I is 110mPa·s, permeability is 10.8mD, oil layer thickness is 8.2m, reservoir temperature is 30℃, formation water salinity is 8400mg / L, well pattern is 4 injections and 17 productions, well spacing is 120m, which meets the screening criteria;

[0336] (2) Determination of injection system and injection volume for each plug

[0337] According to the reservoir temperature, the Bacillus spore powder in the spore profile control system is prepared by freeze-drying the spore fermentation liquid of Bacillus subtilis.

[0338] Bacillus subtilis was fermented in a Bacillus subtilis spore-forming fermentation medium for 96 hours, and the fermentation temperature was controlled at 37° C. to obtain a Bacillus subtilis fermentation liquid.

[0339] The spore production rate of the Bacillus subtilis spore fermentation broth was evaluated by plate colony counting method, and the spore production rate was 89%. The broth was judged to be qualified and could be used for freeze-drying to prepare Bacillus spore powder.

[0340] The spore profile control system consists of a spore bacterium agent system and a spore activation system, and their volume ratio is 1:3. Among them:

[0341] The spore bacterium agent system consists of a spore powder of Bacillus and formation water of the target oil reservoir. Among them:

[0342] Based on the total mass of the spore bacterium agent system, the spore powder of Bacillus accounts for 45% by mass;

[0343] The spore activation system consists of a spore activation agent and formation water of the target oil reservoir. Among them: based on the total mass of the spore activation system, the spore activation agent accounts for 0.35% by mass.

[0344] The sporulation fermentation medium of Bacillus subtilis is 3% starch, 2.0% yeast powder, 0.3% dipotassium hydrogen phosphate, MnSO 4 0.05%, CuCl 2 0.02%, ZnSO 4 0.03%, and the pH value is 7.0.

[0345] The spore activation agent consists of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and their mass ratio is 0.2:0.05:0.05:0.05.

[0346] The oil washing system consists of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and formation water of the target oil reservoir. Among them:

[0347] Based on the total mass of the oil washing system, the biosurfactant fermentation broth accounts for 9% by mass.

[0348] The preparation steps of the oil washing system are as follows:

[0349] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37°C for 84 hours to obtain the fermented bacterial liquid, and then heat it to 100°C and keep it for 30 minutes. After lysing the bacterial cells, a cell-free biosurfactant fermentation broth is obtained. Among them:

[0350] The inoculation amount of Pseudomonas aeruginosa is 2%.

[0351] The Pseudomonas aeruginosa fermentation medium consists of the following components in mass percentage: 4% glycerol, 1.2% sodium nitrate, 0.6% dipotassium hydrogen phosphate, 0.6% potassium dihydrogen phosphate, 0.2% NaCl, 0.05% MgSO 4 0.02% CaCl 2 , and the pH value is 8.0.

[0352] The injection volume of each slug of the Bacillus plugging and profile control system and the oil washing system is determined by the following formula:

[0353] V = β×k / λ×H

[0354] Where: V—the correlation coefficient value of the injection volume of each slug of the displacement control agent;

[0355] k—the reservoir permeability, unit: mD;

[0356] H—the effective thickness of the reservoir, unit: m;

[0357] λ—the viscosity of underground crude oil, mPa·s;

[0358] β—the dosage coefficient, with a value of 2;

[0359] When the value of V = 1.61 and 1 < the calculated value of V ≤ 2, 200 m of each slug of the Bacillus plugging and profile control system and the oil washing system are injected respectively. 3 ;

[0360] The volume ratio of the Bacillus agent system to the Bacillus activation and activation system in the Bacillus plugging and profile control system is 1:3, 50 m of the Bacillus agent system 3 and 150 m of the Bacillus activation and activation system. 3 .

[0361] (3) On-site injection process

[0362] Multi-round and multi-slug injection is adopted. In the first slug of each round, 50 m of the Bacillus agent is injected into the formation through the injection well by using a high-pressure pump. In the second slug, 150 m of the Bacillus activation and activation system is injected by using a high-pressure pump, and air is accompanied by injection throughout the process by using a compression pump (displacement 50 m 3 / d). After the injection is completed, the injection well is shut off for 4 days. In the third slug, 200 m of the oil washing system is injected by using a high-pressure pump. After the injection is completed, it is shut off for 2 days, and then water flooding is carried out. When the water cut is higher than 95%, the second round of injection is carried out, and the cycle is repeated 10 times. 3 3 3 / d), and after the injection is completed, the injection well is shut off for 4 days. In the third slug, 200 m of the oil washing system is injected by using a high-pressure pump. After the injection is completed, it is shut off for 2 days, and then water flooding is carried out. When the water cut is higher than 95%, the second round of injection is carried out, and the cycle is repeated 10 times. 3 After adopting the technical solution of this embodiment, the maximum reduction of water cut is 32%, and the ultimate recovery factor is increased to 37%.

[0363] After adopting the technical solution of this embodiment, the maximum reduction of water cut is 32%, and the ultimate recovery factor is increased to 37%.

[0364] Example 10

[0365] A method for improving the recovery factor of low-permeability reservoirs includes the following steps:

[0366] (1) Target reservoir screening

[0367] The underground crude oil viscosity of test reservoir J is 69 mPa·s, the permeability is 23 mD, the oil layer thickness is 2.7 m, the reservoir temperature is 58 °C, the formation water salinity is 19,000 mg / L, the well pattern is 7 injection wells and 19 production wells, and the well spacing is 110 m, meeting the screening criteria;

[0368] (2) Determination of each slug injection system and injection volume

[0369] According to the reservoir temperature, the Bacillus spores in the profile control system of Bacillus spores are prepared by freeze-drying the spore fermentation broth of Bacillus coagulans.

[0370] Bacillus coagulans is fermented in the spore-producing fermentation medium of Bacillus coagulans for 70 h, and the fermentation temperature is controlled at 58 °C to obtain the fermentation broth of Bacillus coagulans.

[0371] The plate colony counting method is used to evaluate the spore production rate in the above-mentioned spore fermentation broth of Bacillus coagulans. The spore production rate is 95%, and it is judged as a qualified fermentation broth, which can be used for freeze-drying to prepare Bacillus spores powder.

[0372] The profile control system of Bacillus spores consists of a Bacillus spore agent system and a Bacillus spore activation system, and their volume ratio is 1:5, where:

[0373] The Bacillus spore agent system consists of Bacillus spores powder and the formation water of the target reservoir, where:

[0374] Based on the total mass of the Bacillus spore agent system, the Bacillus spores powder accounts for 40% by mass;

[0375] The Bacillus spore activation system consists of a Bacillus spore activator and the formation water of the target reservoir, where: based on the total mass of the Bacillus spore activation system, the Bacillus spore activator accounts for 0.45% by mass.

[0376] The spore-producing fermentation medium of Bacillus coagulans is 4% molasses, 1.5% corn dry powder, 0.5% dipotassium hydrogen phosphate, MnSO 4 0.03%, CuCl 2 0.04%, ZnSO 4 0.05%, and the pH value is 8.0.

[0377] The Bacillus spore activator consists of glucose, alanine, dipotassium hydrogen phosphate, CaCl 2 and their mass ratio is 0.3:0.05:0.05:0.05.

[0378] The oil washing system consists of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and the formation water of the target reservoir, where:

[0379] Based on the total mass of the wash oil system, the biosurfactant fermentation broth accounts for 10% by mass.

[0380] The preparation steps of the wash oil system are as follows:

[0381] Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37 °C for 72 h to obtain the fermented bacterial liquid. Then heat it to 110 °C and maintain for 20 min. After lysing the bacterial cells, the cell-free biosurfactant fermentation broth is obtained, where:

[0382] The inoculation amount of Pseudomonas aeruginosa is 2%.

[0383] The Pseudomonas aeruginosa fermentation medium consists of the following components in mass percentage: 5% glycerol, 0.8% sodium nitrate, 0.5% dipotassium hydrogen phosphate, 0.5% potassium dihydrogen phosphate, 0.2% NaCl, 0.05% MgSO 4 , 0.03% CaCl 2 , and the pH value is 8.0.

[0384] The injection amount of each slug of the spore profile control system and the wash oil system is calculated and determined by the following formula:

[0385] V = β × k / λ × H

[0386] Where: V—the correlation coefficient value of the injection amount of each slug of the displacement agent;

[0387] k—the reservoir permeability, unit: mD;

[0388] H—the effective thickness of the reservoir, unit: m;

[0389] λ—the viscosity of underground crude oil, mPa·s;

[0390] β—the dosage coefficient, with a value of 4;

[0391] When the V value = 3.6 and 3 < calculated V value ≤ 4, each slug of the spore profile control system and the wash oil system are each injected with 400 m 3 ;

[0392] The volume ratio of the spore agent system to the spore activation and activation system in the spore profile control system is 1:4. The spore agent system is 80 m 3 , and the spore activation and activation system is 320 m 3 .

[0393] (3) Field injection process

[0394] Adopt multi-round and multi-slug injection. In the first slug of each round, use a high-pressure pump to inject 80 m 3 The spore agent into the formation through the injection well, and in the second slug, use a high-pressure pump to inject 320 m 3The spore activation system activates the system and uses a compression pump to accompany the injection of air throughout the process (displacement 50 m 3 / d). After the injection is completed, the water injection well stops injecting for 5 days. The third slug is injected with a high-pressure pump at 400 m 3 of the oil washing system. After the injection is completed, the injection stops for 3 days, and subsequent water flooding is carried out. When the water cut is higher than 95%, the second round of injection is carried out, and the cycle is repeated 7 times.

[0395] After adopting the technical solution of this embodiment, the maximum reduction in water cut is 26%, and the final enhanced oil recovery reaches 31%.

[0396] The above has made a detailed description of the embodiments of the present invention. However, the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention within the knowledge scope of those of ordinary skill in the art.

Claims

1. A method for improving the recovery factor of low-permeability oil reservoirs, characterized in that, it includes the following steps: (1) Screening of target oil reservoirs; (2) Determination of the injection system and injection volume for each slug; (21) Determine the oil displacement system adapted to the reservoir temperature of the target oil reservoir, where: the oil displacement system is composed of a spore profile control system and an oil washing system; the spore profile control system is composed of a spore bacterium agent system and a spore activation and activation system; (22) Determination of the injection volume of the injection system for each slug; (3) Field injection process.

2. A method for improving the recovery factor of low-permeability oil reservoirs as described in claim 1, characterized in that, the screening criteria for the target oil reservoir in step (1) are: underground crude oil viscosity 50 - 250 mPa·s, permeability 0.1 - 50 mD, oil layer thickness 2 - 10 m, reservoir temperature 30 - 75 °C, formation water salinity 5000 - 20000 mg / L, well pattern is an area well pattern, and the maximum injection-production well spacing ≤ 200 m.

3. A method for improving the recovery factor of low-permeability oil reservoirs as described in claim 1, characterized in that, in step (21): the spore profile control system is composed of a spore bacterium agent system and a spore activation and activation system, and their volume ratio is 1:2 - 1:5, where: the spore bacterium agent system is composed of spore bacillus powder of bacillus and the formation water of the target oil reservoir, where: calculated based on the total mass of the spore bacterium agent system, the spore bacillus powder accounts for 30 - 50% by mass; the spore activation and activation system is composed of a spore activation activator and the formation water of the target oil reservoir, where: calculated based on the total mass of the spore activation and activation system, the spore activation activator accounts for 0.15 - 0.45% by mass.

4. A method for improving the recovery factor of low-permeability oil reservoirs as described in claim 3, characterized in that, the spore bacillus powder is prepared by freeze-drying the spore-producing fermentation broth of bacteria belonging to the genus Bacillus, where: the bacteria belonging to the genus Bacillus are one of Bacillus subtilis, Bacillus licheniformis, Bacillus cereus, Bacillus coagulans, and Bacillus thermophilus.

5. A method for improving the recovery factor of low-permeability oil reservoirs as described in claim 4, characterized in that, in step (21), select the type of spore bacillus powder and the spore-producing fermentation medium of spore bacillus adapted to the reservoir temperature of the target oil reservoir according to the following specific selection criteria: When the reservoir temperature of the target oil reservoir is 70 - 75 °C, select Bacillus thermophilus; When the reservoir temperature of the target oil reservoir is 60 - 69 °C, select Bacillus licheniformis; When the reservoir temperature of the target oil reservoir is 50 - 59 °C, select Bacillus coagulans; When the reservoir temperature of the target oil reservoir is 40 - 49 °C, select Bacillus cereus; When the reservoir temperature of the target oil reservoir is 30 - 39 °C, select Bacillus subtilis.

6. A method for improving the recovery factor of low-permeability oil reservoirs as described in claim 5, characterized in that, The sporulation fermentation medium of Bacillus subtilis is 2-3% starch, 1.0-2.0% yeast powder, 0.2-0.3% dipotassium hydrogen phosphate, 0.03-0.05% MnSO 4 4, 0.01-0.02% CuCl 2 2, 0.01-0.03% ZnSO 4 4, and the pH value is 7.0; The sporulation fermentation medium for Bacillus cereus is 3-4% dextrin, 1.5-2.0% beef extract, 0.3-0.4% potassium dihydrogen phosphate, 0.02-0.05% MnSO 4 4, 0.01-0.02% CuCl 2 2, 0.02-0.03% ZnCl 2 2, and the pH value is 8.0; The sporulation fermentation medium for Bacillus coagulans is 3-4% molasses, 1.2-1.5% corn dry powder, 0.4-0.5% dipotassium hydrogen phosphate, 0.01-0.03% MnSO 4 0.01-0.03%, 0.02-0.04% CuCl 2 0.02-0.04%, 0.01-0.05% ZnSO 4 0.01-0.05%, and the pH value is 8.0; The sporulation fermentation medium of Bacillus licheniformis is composed of 3.2 - 4.1% cellulose, 2.5 - 3.0% ammonium sulfate, 0.25 - 0.35% dipotassium hydrogen phosphate, 0.03 - 0.05% MnSO 4 0.03 - 0.05%, 0.02 - 0.04% CuCl 2 0.02 - 0.04%, 0.01 - 0.04% ZnCl 2 0.01 - 0.04%, and the pH value is 8.0; The spore-forming fermentation medium for Bacillus thermophilus contains 2.5-3.5% xylose, 1.8-2.1% ammonium nitrate, 0.35-0.43% potassium dihydrogen phosphate, 4 0.03-0.05% MnSO 4 0.01-0.03%, 0.01-0.04% MgSO 2 ZnCl, and the pH value is 8.

5.

7. A method for improving the recovery factor of low-permeability oil reservoirs as described in claim 6, characterized in that, The spore-producing fermentation broth of the Bacillus bacteria is obtained by fermenting Bacillus bacteria at 30-75 °C for 48-96 h in the spore-producing fermentation medium of the corresponding Bacillus bacteria, where: The fermentation temperature of Bacillus thermophilus is controlled at 70-75 °C; The fermentation temperature of Bacillus licheniformis is controlled at 60-69 °C; The fermentation temperature of Bacillus coagulans is controlled at 50-59 °C; The fermentation temperature of Bacillus cereus is controlled at 40-49 °C; The fermentation temperature of Bacillus subtilis is controlled at 30-39 °C.

8. The method for improving the recovery rate of low-permeability oil reservoirs according to claim 7, characterized in that the plate colony counting method is used to evaluate the spore production rate in the spore-producing fermentation broth of the above-mentioned Bacillus bacteria. If the spore production rate reaches more than 80%, it is a qualified fermentation broth and can be used for freezing to prepare Bacillus spore powder.

9. The method for improving the recovery rate of low-permeability oil reservoirs according to claim 3, characterized in that The spore activation activator is composed of glucose, alanine, dipotassium hydrogen phosphate, and CaCl 2 in a mass ratio of (0.1 - 0.3):(0.02 - 0.05):(0.02 - 0.05):(0.01 - 0.05).

10. The method for improving the recovery rate of low-permeability oil reservoirs according to claim 1, characterized in that in step (21), the oil washing system is composed of a cell-free biosurfactant fermentation broth prepared by fermenting Pseudomonas aeruginosa and the formation water of the target oil reservoir, where: Based on the total mass of the oil washing system, the biosurfactant fermentation broth accounts for 5-10% by mass.

11. The method for improving the recovery rate of low-permeability oil reservoirs according to claim 10, characterized in that the preparation steps of the oil washing system in step (21) are as follows: Inoculate Pseudomonas aeruginosa into the Pseudomonas aeruginosa fermentation medium, and then ferment at 37 °C for 72 h-96 h to obtain the fermented bacterial liquid, and then heat it to not less than 100 °C and keep it for 20-30 min. After lysing the bacterial cells, a cell-free biosurfactant fermentation broth is obtained, where: The inoculation amount of the Pseudomonas aeruginosa is 2-5%.

12. The method for improving the recovery rate of low-permeability oil reservoirs according to claim 11, characterized in that Pseudomonas aeruginosa fermentation medium, by mass percentage, consists of the following components: 3-5% glycerol, 0.8-1.2% sodium nitrate, 0.4-0.6% dipotassium hydrogen phosphate, 0.4-0.6% potassium dihydrogen phosphate, 0.1-0.2% NaCl, 0.02-0.05% MgSO 4 , 0.01-0.03% CaCl 2 , the balance is water and the pH value is 8.

0.

13. The method for improving the recovery rate of low-permeability oil reservoirs according to claim 1, characterized in that In step (22), the injection amount of each slug profile control system and oil washing system is calculated and determined by the following formula: V = β × k / λ × H where: V—the correlation coefficient value of the injection amount of each slug injection system; k—the permeability of the oil reservoir, unit: mD; H—the effective thickness of the oil reservoir, unit: m; λ—the viscosity of underground crude oil, mPa·s; β—the dosage coefficient, with a value of 2-5, where: When the calculated result V value ≤ 1, each slug of the spore profile control system and the oil washing system is injected with 100 m 3 ; When 1 < calculated result V value ≤ 2, each slug bacillus profile control system and oil washing system are injected with 200 m 3 ; When 2 < calculated result V value ≤ 3, each slug spore profile control system and oil displacement system are injected with 300 m 3 ; When 3 < calculated result V value ≤ 4, each slug bacillus profile control system and oil displacement system are each injected with 400 m 3 ; When the calculated result of the V value > 4, each slug spore profile control system and oil displacement system are each injected with 500 m 3 .

14. The method for improving the recovery rate of low-permeability oil reservoirs according to claim 1, characterized in that the specific steps of step (3) are as follows: Adopt multi-round and multi-slug injection. In the first slug of each round, the Bacillus bacteria agent system is injected into the formation through an injection well using a high-pressure pump. In the second slug of each round, a Bacillus activation activator is injected using a high-pressure pump and air is accompanied by injection using a compression pump throughout the process. After the injection is completed, the injection well is shut in for 3 d-5 d. In the third slug, the oil washing system is injected using a high-pressure pump. After the injection is completed, it is shut in for 1 d-3 d, and subsequent continuous water flooding is carried out, where: When the water cut is higher than 95%, the second-round injection is carried out, and it is circulated for 5-10 rounds.

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