Method for preparing biogas by anaerobic digestion of waste oil reservoir

Abstract

The invention provides a method for preparing biogas by anaerobic digestion of a waste oil reservoir, which comprises the following steps of: determining required anaerobic digestion methanogens and substrates according to geological characteristics, temperature, salinity and residual oil components of a target waste oil reservoir; the abandoned oil reservoirs are screened through the geological parameters and the well pattern mode, and target oil reservoirs are obtained; arranging a well pattern form in which vertical wells and horizontal wells are combined on a target oil reservoir; generating an injection scheme by adopting a physical simulation method; wherein the injection scheme comprises the injection amount and the injection concentration of anaerobic digestion methanogens and substrates of a target oil reservoir; according to the injection scheme, the anaerobic digestion methanogens and the substrate are injected into the target oil reservoir in combination with the well pattern, anaerobic digestion is carried out in the oxygen-free environment, and the biogas is obtained. The natural temperature, mineralization degree, anaerobic environment and other environments of the waste oil reservoir are utilized, substrates and florae are fully stirred and injected into the waste oil reservoir, and then anaerobic digestion is carried out underground through bacteria in an anaerobic environment to generate methane gas.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of oilfield development, and particularly relates to a method for producing biogas by in-situ anaerobic digestion of an oil reservoir through injection of organic matter and methanogen. BACKGROUND

[0002] After years of development, a large number of blocks in China have entered the late stage of high water cut, and the proportion of difficult-to-produce reserves has risen. The demand for resource comprehensive utilization and low-carbon development is urgent. The oil reservoir in-situ anaerobic digestion biogas technology can inject organic matter and methanogen, and through the characteristics of high permeability, high water cut, natural temperature and sealing of the abandoned oil reservoir, anaerobic digestion shows good methanogenesis. At present, the main use of abandoned oil reservoirs is microbial anaerobic fermentation to produce hydrogen gas, which has a long fermentation time and low energy conversion rate, and the performance is generally poor. In thermology, the use of methanogen anaerobic digestion to produce methane is more stable and efficient than microbial hydrogen production. Therefore, a method for producing methane by injecting methanogen and organic matter anaerobic fermentation in abandoned oil reservoirs is needed to improve the demand for resource comprehensive utilization and low-carbon development of abandoned oil reservoirs. SUMMARY

[0003] The purpose of the present application is to first systematically describe a method for anaerobic digestion of biogas in abandoned oil reservoirs, which utilizes the natural temperature, salinity, and anaerobic environment of abandoned oil reservoirs, fully mixes the substrate and bacterial population, and injects them into the abandoned oil reservoir, and then produces methane gas through bacterial anaerobic digestion in an oxygen-free environment underground. By selecting appropriate well network methods and biogas residue treatment methods, oil reservoir plugging is avoided, methane production is improved, and environmental pollution is effectively avoided.

[0004] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows:

[0005] The present application provides a method for anaerobic digestion of biogas in abandoned oil reservoirs, which comprises the following steps: (1) determining the required anaerobic digestion methanogen and substrate according to the geological characteristics, temperature, salinity and residual oil components of the target abandoned oil reservoir; (2) screening the abandoned oil reservoir using geological parameters and well network methods to obtain a target oil reservoir; (3) arranging a well network form combining vertical wells and horizontal wells on the target oil reservoir; (4) generating an injection scheme using a physical simulation method; wherein the injection scheme includes the injection amount and concentration of the anaerobic digestion methanogen and substrate into the target oil reservoir; (5) injecting the anaerobic digestion methanogen and substrate into the target oil reservoir according to the injection scheme combined with the well network to perform anaerobic digestion in an oxygen-free environment, and obtaining biogas.

[0006] Further, in step (3), the well pattern combining vertical wells and horizontal wells specifically includes: arranging the horizontal well at the bottom of the target reservoir as a gas injection-liquid injection well, and arranging the vertical well at the upper part of the target reservoir as a production well; wherein the horizontal well is used to expand the horizontal control range of the reservoir, thereby making the injected organic matter and bacteria evenly distributed and improving the fermentation efficiency; and the vertical well is used to collect the produced methane and other gases.

[0007] Further, the method further includes: (6) separating the obtained biogas on the ground.

[0008] Further, step (6) specifically includes: separating and collecting the biogas through the existing production well of the abandoned reservoir, and the wellhead of the production well is integrated with a multi-stage gas-liquid separation device; wherein the multi-stage gas-liquid separation device includes a gas-liquid cyclone separation device for first-stage separation, a desulfurization reactor for second-stage treatment, and a third-stage treatment unit; the gas-liquid cyclone separation device for first-stage separation is used to efficiently remove ≥98% of liquid and solid impurities in the biogas; the second-stage treatment is the desulfurization reactor, which is used to purify the biogas after removing the liquid and solid impurities in the reactor with an iron-based desulfurizer, so as to reduce the H2S concentration in the biogas to <10 ppm; part of the purified biogas is directly connected to a gas generator set for power generation, and the other part is connected to the third-stage treatment unit; the third-stage treatment unit includes a membrane separation device or a pressure swing adsorption system, which is used to take advantage of the separation characteristics of methane and carbon dioxide to increase the methane purity in the biogas to 85%-90% for use as high-grade fuel gas; in addition, the separated CO2 gas is compressed and injected back into the target reservoir to increase the formation pressure or strengthen the anaerobic digestion reaction environment; and the waste heat generated during power generation is used to heat the anaerobic digestion tank to reduce external energy consumption by 20%-25%.

[0009] Further, the abandoned reservoir is also provided with full-process explosion-proof electrical equipment matched with an intelligent leakage monitoring system, which is used to monitor the gas concentration at the connection of the pipeline and the equipment in real time to ensure the safety and reliability of the biogas separation and collection process.

[0010] Further, in step (2), in the case that the target reservoir does not meet the requirements of the anaerobic digestion methane-producing bacteria and the substrate for temperature, minerals, pH value, and permeability, measures such as heat injection technology, pH adjuster, or fracturing reconstruction are adopted to optimize the target reservoir.

[0011] Further, in the case that the anaerobic digestion methane-producing bacteria are acetic nutrient-type methane-producing bacteria, the substrate is municipal sludge; and in the case that the anaerobic digestion methane-producing bacteria are hydrogen nutrient-type methane-producing bacteria, the substrate is kitchen waste.

[0012] Further, the temperature of the target oil reservoir is 35-70 DEG C, the pH value is maintained at 6.5-7.5, and the permeability is above 1000 mD.

[0013] Further, in step (4), the injection scheme is optimized by using a physical simulation method with the amount of methane production as the optimization basis.

[0014] Further, step (5) specifically comprises: using the existing water injection system of the abandoned oil reservoir, injecting anaerobic digestion methane-producing bacteria and substrates into the target oil reservoir through the well string of the water injection well according to the injection scheme and in combination with the well pattern, so as to perform anaerobic digestion in an oxygen-free environment and obtain biogas.

[0015] Compared with the prior art, the technical scheme provided by the present application has at least the following advantages:

[0016] The present application provides a method for biogas production by anaerobic digestion of an abandoned oil reservoir, which can realize the method for biogas production by anaerobic digestion of an abandoned oil reservoir and improve the in-situ biogas production efficiency. First, the present application uses the oil reservoir to provide a constant temperature for anaerobic digestion, which has simple construction technology, simple and convenient equipment installation, low energy consumption and greatly reduced biogas production cost. Second, the present application optimizes the injection amount and concentration of the injected methane-producing bacteria by using physical experiments, optimizes different injection processes for different oil reservoirs, and improves the biogas production efficiency. Moreover, the well pattern for biogas production by anaerobic digestion of an oil reservoir is a combination of a horizontal well and vertical wells, which has one horizontal well, three vertical wells on each side, and a total of seven wells (one injection and six production) in one well pattern. Under this well pattern, the injected bacteria and organic matter can be injected radially to the surrounding area, covering a large and relatively uniform area, which is beneficial to the full contact between organic matter and microorganisms. The production well is located around the injection well to form a convergence area, and the produced biogas can be quickly collected to the production well to reduce the gas reservoir retention. The pressure control is flexible, and the anaerobic environment (slightly positive pressure) can be maintained by adjusting the injection / production pressure to prevent oxygen intrusion. Moreover, the risk of plugging can be alleviated, and the radial flow around the injection well can disperse the risk of solid deposition. Finally, the present application has a safe discharge system for biogas separation and purification on the ground. The existing production well of the abandoned oil reservoir is used to separate and collect methane, in which the first gas-liquid cyclone separation removes ≥98% of liquid and solid impurities, and the second iron-based desulfurizer reactor reduces H2S to <10 ppm; the purified biogas is used for power generation, and the other part is purified to 85%-90% of methane by membrane separation / PSA for fuel gas, and the separated CO2 is compressed back to the oil reservoir; the system is provided with a waste heat recovery device to utilize the waste heat of power generation to heat the digestion tank, which reduces the energy consumption by 20%-25%. BRIEF DESCRIPTION OF DRAWINGS

[0017] One or more embodiments are illustrated by way of example with reference to the accompanying drawings, which are schematic and not intended to be limiting of the embodiments unless otherwise specifically indicated.

[0018] Figure 1 The well network distribution design for anaerobic digestion of waste oil reservoirs to produce biogas provided in this embodiment of the invention;

[0019] Figure 2 A flowchart illustrating the method for producing biogas from waste oil reservoirs through anaerobic digestion, as provided in an embodiment of the present invention.

[0020] In the diagram, 1-horizontal well, 2-vertical well, 3-gas-liquid separator (primary separation), 4-desulfurization reactor (secondary separation), 5-pressure swing adsorption system, and 6-manifold. Detailed Implementation

[0021] To address the problems existing in the prior art, the present invention provides a method for producing biogas through anaerobic digestion of waste oil reservoirs, comprising the following steps:

[0022] (1) Screening of anaerobic methanogens and their substrates

[0023] (2) Preliminary reservoir screening using geological parameters and well pattern: To achieve high methane production efficiency, reservoir temperatures of 35-70℃ are selected to ensure high activity of anaerobic digestion microorganisms. The reservoir pH should be maintained at 6.5-7.5 to avoid acidification of the fermentation system or impact on microbial activity. Reservoir permeability should be above 1000 mD. Simultaneously, the reservoir should possess tight lithological interlayers to prevent leakage of injected materials and generated gases, thereby improving methane recovery and reducing environmental risks. If the reservoir does not meet the requirements for temperature, pH, or permeability, optimization measures such as thermal injection, pH adjusters, or fracturing can be employed.

[0024] (3) Well pattern design: such as Figure 1 As shown, this invention employs a well network combining vertical wells 2 and horizontal wells 1. Horizontal wells 1 are positioned at the bottom of the reservoir as gas-injection / liquid-injection wells to expand the lateral control range of the reservoir, ensuring uniform distribution of injected organic matter and microorganisms and improving fermentation efficiency. Vertical wells 2 are positioned at the top of the reservoir as production wells to collect the generated methane and other gases, and are equipped with multi-stage gas separation and purification devices to achieve efficient gas separation and purification. Through the above screening criteria and well network arrangement, the anaerobic digestion methanogenesis process of this invention can operate efficiently under suitable reservoir conditions.

[0025] (4) Optimization of injection process: The injection amount and concentration of anaerobic digestion methanogens and their substrates were optimized. The optimization method adopted was physical simulation, and the optimization basis was the amount of methanogens produced.

[0026] (5) On-site construction: Using the existing water injection system, the anaerobic digestion of methanogenic bacteria and their substrates are injected into the target oil reservoir through the water injection well tubing according to the optimized injection process obtained in step (3).

[0027] (6) Separation of produced gas on the ground: separation and collection of biogas through existing production wells in abandoned oil reservoirs, integration of multi-stage gas-liquid separation equipment at the wellhead of the production well: the first stage separation uses a gas-liquid separator 3, specifically a gas-liquid cyclone separation device, which can efficiently remove ≥98% of liquid and solid impurities, ensuring stable operation of the subsequent treatment unit; the second stage treatment is a desulfurization reactor 4, in which the gas enters a reactor containing an iron-based desulfurizer, reducing the H2S concentration to <10 ppm, meeting the utilization requirements of standard biogas. Part of the purified biogas is directly connected to a gas generator set for power generation, and the other part is connected to a membrane separation device or a pressure swing adsorption (PSA) system 5, which utilizes the separation characteristics of methane and carbon dioxide to increase the methane purity to 85%-90% for use as high-grade fuel gas. The separated CO2 gas is compressed and injected back into the abandoned oil reservoir to increase the formation pressure or strengthen the anaerobic digestion reaction environment. The system is equipped with a waste heat recovery device to use the waste heat generated during power generation to heat the anaerobic digestion tank, reducing external energy consumption by 20%-25%; at the same time, full-flow explosion-proof electrical equipment is used, combined with an intelligent leakage monitoring system to monitor the gas concentration at the connection of the pipeline and equipment in real time, ensuring the safety and reliability of the biogas separation and collection process. The above connection is realized through a manifold 6.

[0028] The application will be described in detail below with reference to the specific embodiments.

[0029] The application provides a method for producing biogas by anaerobic digestion of abandoned oil reservoirs, and a flow chart of the method is shown in Figure 2 The method comprises the following steps:

[0030] (1) Screening of anaerobic digestion methanogenic bacteria, substrates and nutrient solution.

[0031] (2) Screening of oil reservoirs:

[0032] The requirements of anaerobic digestion methanogenic bacteria and substrates for temperature, minerals, pH value and permeability are determined according to step (1) to screen the oil reservoirs.

[0033] (3) Injection process optimization:

[0034] The ratio of anaerobic digestion methanogenic bacteria to substrate, the injection concentration and the injection amount of nutrient solution are optimized, and the degradation of biogas residue is pretreated and optimized. The optimization method is a physical experiment method, and the optimization basis is the amount of methane produced.

[0035] (4) Field construction:

[0036] The existing water injection system of the abandoned oil reservoir is used, and the anaerobic digestion methanogenic bacteria and the substrate are fully mixed according to the optimized injection process obtained in step (3), and then injected into the target oil reservoir layer through the water injection well pipe.

[0037] (5) Separation and collection of biogas:

[0038] In the separation and collection of biogas produced by anaerobic digestion of abandoned oil reservoirs, an integrated multi-stage separation system is constructed at the wellhead through the existing production well: first-stage treatment: after the gas is efficiently removed from liquid and solid impurities (≥98%) through gas-liquid cyclone separation, it enters the second-stage treatment: an iron-based desulfurization reactor reduces H2S to <10 ppm; the purified biogas is used for power generation, and another part is purified to 85%-90% methane through membrane separation or PSA as high-grade fuel gas; the separated CO2 is compressed and injected back into the oil reservoir to maintain pressure or improve the environment; the waste heat from power generation is used to heat the anaerobic digestion tank to save energy, and explosion-proof equipment and leakage monitoring systems are provided to ensure safe operation.

[0039] Example 1:

[0040] A method for producing biogas by anaerobic digestion of abandoned oil reservoirs, comprising the following steps:

[0041] (1) Screening of methanogenic bacteria and its substrate for anaerobic digestion:

[0042] The methanogenic bacteria for anaerobic digestion is acetotrophic methanogenic bacteria, and the substrate is municipal sludge; the selected acetotrophic methanogenic bacteria grows strictly anaerobically, with an optimum growth temperature of 38-40℃ and an optimum pH of 6.8-7.4.

[0043] (2) Reservoir screening:

[0044] The suitable temperature for the bacteria is 39.2℃, the suitable pH is 6.8, and the suitable reservoir parameters are combined to select the appropriate block: the well pattern is a combination of horizontal wells and vertical wells, the reservoir permeability is above 1000 mD; the produced liquid water content is ≥95%; and the target layer has a stable and thick (more than 10 m) dense lithologic barrier.

[0045] The reservoir temperature of block A2 is 42℃, the permeability is 1238 md, the water content of the oil well is 95.8%, the formation water pH is 6.9, and the barrier thickness is 14.6 m.

[0046] (3) Injection process optimization, the optimization method uses physical simulation, and the optimization basis is the amount of methane produced.

[0047] First, the natural core of block A2 is made into a standard core of Φ25x100mm; vacuumize, saturate with block A1 formation water; saturate with target block crude oil; and drive with formation water until the produced liquid water content reaches more than 95%. Then inject anaerobic digestion methanogenic bacteria and its substrate with different injection amounts and injection concentrations, as shown in Table 1. Incubate for 10 days; then measure the amount of methane produced; finally, select the injection process corresponding to the core with the largest proportion of methane produced as the optimized injection process, as shown in Table 2.

[0048] Table 1 Injection concentration optimization of Example 1

[0049]

[0050]

[0051] Table 2 injection volume optimization of Example 1

[0052] No. Inoculum Inoculum amount (PV) Inoculum concentration (%) Methane production amount (mL) 1 Acetoclastic methanogens 0.1 20 1734 2 Acetoclastic methanogens 0.2 20 3656 3 Acetoclastic methanogens 0.3 20 4315 4 Acetoclastic methanogens 0.4 20 4497

[0053] Through physical experiments, it is determined that the injection concentration is 20%, the injection volume is 0.2 PV, and the tank truck is used for regular injection in combination with the production characteristics of microorganisms, and the injection of the bacterial solution and the nutrient solution is concentrated for 3-5 days per month.

[0054] (3) Field construction:

[0055] Ground mixing and preparation of pressurized injection equipment: grinding the substrate to an average particle size of 100 μm, pressurized injection and stirring tank, and fully mixing and stirring the anaerobic digestion methanogenic bacteria and its substrate in the stirring tank to determine the injection concentration of the methanogenic bacteria of 20% and the injection volume of 0.2 PV, and pressurized injection into the target oil layer through the plunger pump. Through the automatic control equipment, the injection concentration and injection volume and other parameters can be automatically adjusted to improve the injection accuracy and ensure the injection quality. Water well injection string: based on the design concentration and injection requirements, the anaerobic digestion methanogenic bacteria and its substrate are injected into the target oil layer by means of the water well string.

[0056] (4) Separation and collection of biogas:

[0057] In the separation and collection of methane through the existing production well of the abandoned oil reservoir, the multi-stage gas-liquid separation equipment is integrated at the wellhead of the production well, the first-stage gas-liquid cyclone separation removes ≥98% of liquid and solid impurities, and the second-stage iron-based desulfurizer reactor reduces H2S to <10 ppm; a part of the purified biogas is used for power generation, another part is purified to 85%-90% of methane by membrane separation / PSA for fuel gas, and the separated CO2 is compressed and injected back into the oil reservoir; the system is provided with a waste heat recovery device to utilize the waste heat of power generation to heat the digestion tank to reduce energy consumption by 20%-25%.

[0058] Field test result evaluation: as of December 30, 2024, the cumulative injection of the bacterial solution and the nutrient system in the A1 oil reservoir is 51,000 m3, the injection cost is about 5 yuan / m3, the production of methane is 623,000 m3, and the sewage treatment is saved, and the input-output ratio is greater than 1:2.8.

[0059] Example 2

[0060] A method for producing biogas by anaerobic digestion of an abandoned oil reservoir, comprising the following steps:

[0061] (1) Screening of anaerobic digestion methanogenic bacteria and its substrate:

[0062] The anaerobic digestion methanogen is a hydrogenotrophic methanogen, and the substrate is kitchen garbage. The selected hydrogenotrophic methanogen grows strictly anaerobically, and the optimum growth temperature is 38-40.5℃, and the suitable pH is 6.8-7.4.

[0063] (2) Reservoir screening:

[0064] The suitable temperature of the strain is 40.1℃, the suitable pH is 7, and the suitable reservoir parameter requirements are combined, that is, the well pattern belongs to the combination of horizontal well and vertical well, the reservoir permeability is above 1000mD, the produced liquid water content is above 95%, and the target layer has a stable and dense lithologic barrier with a thickness of more than 10m to screen the suitable blocks.

[0065] The reservoir temperature of block A1 is 40℃, the permeability is 1360md, the oil well water content is 96.1%, the formation water pH is 7.2, and the barrier thickness is 15m.

[0066] (3) Injection process optimization, the optimization method adopts physical simulation method, and the optimization basis is the amount of produced methane.

[0067] Firstly, the natural core of block A2 is made into a standard core with a diameter of 25mm and a length of 100mm; vacuumizing, saturating with the formation water of block A1; saturating with the target block crude oil; and driving the formation water to the produced liquid water content above 95%. Then, the anaerobic digestion methanogen and its substrate with different injection amounts and injection concentrations are injected, as shown in Table 3. After 10 days of static culture, the amount of produced methane is determined, and finally the injection process corresponding to the core with the largest proportion of methane production is selected as the optimized injection process, as shown in Table 4.

[0068] Table 3 Injection concentration optimization of example 2

[0069] No. Inoculum concentration (%) Inoculum Inoculum amount (PV) Methane production amount (mL) 1 10 Hydrogenotrophic methanogens 0.1 1354 2 15 Hydrogenotrophic methanogens 0.1 1681 3 20 Hydrogenotrophic methanogens 0.1 2015 4 25 Hydrogenotrophic methanogens 0.1 2134

[0070] Table 4 Injection amount optimization of example 2

[0071] No. Inoculum Inoculum amount (PV) Inoculum concentration (%) Methane production amount (mL) 1 Hydrogenotrophic methanogens 0.1 20 1853 2 Hydrogenotrophic methanogens 0.2 20 3743 3 Hydrogenotrophic methanogens 0.3 20 6453 4 Hydrogenotrophic methanogens 0.4 20 8343

[0072] Through physical experiments, it is determined that the injection concentration is 20%, the injection amount is 0.3PV, combined with the production characteristics of microorganisms, the tank truck is used for regular injection, and the bacteria liquid and nutrient liquid are injected for 3-5 days every month.

[0073] (3) Field construction:

[0074] Ground mixed preparation pressurized injection device: the substrate is ground to an average particle size of 100 μm, pressurized injection into a stirring tank, and the anaerobic digestion methanogen and its substrate are fully mixed and stirred in the stirring tank to determine the injection concentration of the methanogen at 20% and the injection amount at 0.3 PV, and the methanogen is pressurized and injected into the target oil layer by a plunger pump. Through the automatic control device, the injection concentration and injection amount and other parameters can be automatically adjusted to improve the injection accuracy and ensure the injection quality. Water well injection string: according to the design concentration and injection requirements, the anaerobic digestion methanogen and its substrate are injected into the target oil layer by means of the water well string.

[0075] (4) Separation and collection of biogas:

[0076] Methane is separated and collected through the existing production well of the abandoned oil reservoir. A multi-stage gas-liquid separation device is integrated at the wellhead of the production well. In the first stage, gas-liquid cyclone separation is used to remove ≥98% of liquid and solid impurities. In the second stage, an iron-based desulfurizer reactor is used to reduce H2S to <10 ppm. The purified biogas is used to generate electricity, and another part is purified to 85%-90% methane by membrane separation / PSA for fuel gas. The separated CO2 is compressed and injected back into the oil reservoir. The system is equipped with a waste heat recovery device to utilize the waste heat generated during electricity generation to heat the digestion tank, reducing energy consumption by 20%-25%.

[0077] On-site test result evaluation: As of December 30, 2024, A2 oil reservoir has cumulatively injected 120,000 cubic meters of bacteria liquid and nutrient system, with an injection cost of about 6 yuan per cubic meter, and 1.8 million cubic meters of methane has been produced, and sewage treatment is saved, with a payback ratio of more than 1:4.

[0078] Those skilled in the art can understand that the above embodiments are specific examples for implementing the present application, and in actual application, various changes can be made in form and detail without departing from the spirit and scope of the present application. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the present application, therefore the protection scope of the present application should be limited by the scope defined in the claims.

Claims

1. A method for producing biogas by anaerobic digestion of waste oil reservoirs, characterized by, The method comprises the following steps: (1) According to the geological characteristics, temperature, salinity and residual oil components of the target abandoned oil reservoir, determine the required anaerobic digestion methanogens and substrate; (2) Use geological parameters and well pattern to screen the abandoned oil reservoir to obtain the target oil reservoir; (3) The well pattern form of vertical well combined with horizontal well is arranged on the target oil reservoir; (4) The injection scheme is generated by using physical simulation method; wherein, the injection scheme includes the injection amount and concentration of anaerobic digestion methanogens and substrate into the target oil reservoir; (5) According to the injection scheme, anaerobic digestion methanogens and substrate are injected into the target oil reservoir by combining the well pattern, so as to carry out anaerobic digestion in an oxygen-free environment to obtain biogas.

2. The method of anaerobic digestion of waste oil reservoir for biogas production according to claim 1, characterized in that, In step (3), the well pattern form of vertical well combined with horizontal well specifically includes: arranging the horizontal well at the bottom of the target oil reservoir as a gas-liquid injection well, and arranging the vertical well at the upper part of the target oil reservoir as a production well; Wherein, the horizontal well is used to expand the lateral control range of the oil reservoir, so as to make the injected organic matter and bacterial flora uniformly distributed and improve the fermentation efficiency; the vertical well is used to collect the generated methane and other gases.

3. The method of claim 1, wherein the waste oil reservoir is anaerobically digested to produce biogas. The method further comprises: (6) separating the obtained biogas on the ground.

4. The method of claim 3, wherein the anaerobic digestion of the waste oil reservoir to produce biogas is characterized by, Step (6) specifically includes: separating and collecting the biogas through the existing production well of the abandoned oil reservoir, and the wellhead of the production well is integrated with a multi-stage gas-liquid separation device; Wherein, the multi-stage gas-liquid separation device includes a gas-liquid cyclone separation device for first-stage separation, a desulfurization reactor for second-stage treatment, and a third-stage treatment unit; The gas-liquid cyclone separation device for first-stage separation is used to efficiently remove ≥98% of liquid and solid impurities in the biogas; The second-stage treatment is a desulfurization reactor, which is used to purify the biogas after removing liquid and solid impurities in the reactor with an iron-based desulfurizer, so as to reduce the H2S concentration in the biogas to <10 ppm; part of the purified biogas is directly connected to a gas-fired generator set for power generation, and the other part enters the third-stage treatment unit; The third-stage treatment unit includes a membrane separation device or a pressure swing adsorption system, which is used to separate methane from carbon dioxide to improve the methane purity in the biogas to 85%-90% for use as high-grade fuel gas; in addition, the separated CO2 gas is injected back into the target oil reservoir after compression to increase the formation pressure or strengthen the anaerobic digestion reaction environment; and the waste heat generated during power generation is used to heat the anaerobic digestion tank to reduce external energy consumption by 20%-25%.

5. The method of claim 4, wherein the anaerobic digestion of the waste oil reservoir to produce biogas is characterized by, The abandoned oil reservoir is also provided with full-process explosion-proof electrical equipment matched with an intelligent leakage monitoring system, which is used to monitor the gas concentration at the connection of pipelines and equipment in real time to ensure the safety and reliability of the biogas separation and collection process.

6. The method of anaerobic digestion of waste oil reservoir for biogas production according to claim 1, characterized in that, In step (2), if the target oil reservoir does not meet the requirements of the anaerobic digestion methanogens and substrate for temperature, minerals, pH value and permeability, measures such as heat injection technology, pH adjuster or fracturing reconstruction are adopted to optimize the target oil reservoir.

7. The method of anaerobic digestion of waste oil reservoirs for biogas production according to claim 6, characterized in that, In the case that the anaerobic digestion methanogens are acetic acid trophic methanogens, the substrate is municipal sludge; In the case that the methanogen in the anaerobic digestion is hydrogenotrophic methanogen, the substrate is kitchen garbage.

8. The method of anaerobic digestion of waste oil reservoirs for biogas production according to claim 7, characterized in that, The temperature of the target oil reservoir is 35-70 DEG C, the pH value is maintained at 6.5-7.5, and the permeability is above 1000 mD.

9. The method of anaerobic digestion of waste oil reservoir for biogas production according to claim 1, characterized in that, In step (4), the injection scheme is optimized by physical simulation method with the amount of methane production as the optimization basis.

10. The method of anaerobic digestion of waste oil reservoirs for biogas production according to claim 1, characterized in that, Step (5) specifically comprises: injecting anaerobic digestion methanogen and substrate into the target oil reservoir through the injection well string by using the existing water injection system of the abandoned oil reservoir, combining the well pattern according to the injection scheme, so as to carry out anaerobic digestion in an oxygen-free environment and obtain biogas.

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