A method for reusing a depleted gas reservoir

Abstract

The application provides a kind of reutilization processing method of depleted gas reservoir, the reutilization processing method is as follows step: (1) using flue gas to carry out natural gas recovery to the depleted gas reservoir, obtain depleted gas reservoir;(2) using flue gas to carry out flue gas storage treatment to the depleted gas reservoir obtained in step (1). The application realizes the efficient recovery of the remaining natural gas in the depleted gas reservoir by injecting flue gas into the depleted gas reservoir, and then utilizes the trap characteristics of the depleted gas reservoir to realize the efficient reutilization of the remaining value of the depleted gas reservoir by playing the remaining value of the depleted gas reservoir and improving the carbon storage efficiency.

Description

Technical Field

[0001] This invention belongs to the field of gas reservoir injection and development technology, and relates to a method for the reuse of depleted gas reservoirs, specifically a method for improving the recovery rate of depleted gas reservoirs using flue gas and its storage method. Background Technology

[0002] In recent years, China's natural gas production and consumption have continued to rise, and it is estimated that by 2030, China's annual natural gas consumption will exceed 6000 × 10⁻⁶. 8 m 3 Domestic natural gas production will exceed 3000×10 8 m 3 Therefore, improving the recovery rate of developed gas fields is of great strategic significance for maintaining the rapid and healthy development of China's natural gas industry. Natural energy depletion-based development and maximizing the utilization of formation elastic energy are the foundation of gas reservoir development. The driving force of gas reservoir development is the magnitude of gas elastic energy, determined by pressure; gas reservoir development is a process of continuous gas pressure depletion. The highly compressible nature of gas dictates that, from an economic and technical feasibility perspective, except for gas reservoirs with high condensate content, all gas reservoirs adopt a one-time natural energy depletion development approach, rather than supplementary energy development. However, as existing major gas fields have successively entered the middle and late stages of development, and the overall recovery rate is low, as of 2022, the cumulative proven geological reserves nationwide were 20 × 10⁻⁶. 12 m 3 Geological reserves of 17×10 12 m 3 Under current economic and technological conditions, the recovery rate of major gas fields ranges from 20% to 80%, with an average recovery rate of only 30.5%.

[0003] Currently, the technical methods for improving gas reservoir recovery mainly revolve around improving the utilization of reserves, improving the pressure drop sweep efficiency, and improving pressure exhaustion efficiency. (1) Technical methods for improving the utilization of reserves include well network densification, well type optimization, sidetracking of old wells, and three-dimensional well network. Well network densification involves adding adjustment wells on the basis of the original well network to reduce the impact of gas reservoir heterogeneity on gas reservoir development and effectively utilize the remaining untapped reserves between wells. Sidetracking of old wells, well type optimization, and three-dimensional well network are achieved by sidetracking to open up untapped vertical layers or by optimizing well types to increase the contact area by adjusting them to directional wells, high-angle wells, and horizontal wells, and by developing different vertical layers in a three-dimensional network with multiple sets of shared networks to increase the effective utilization of untapped reserves in the vertical direction. (2) Technical methods for improving the pressure drop sweep efficiency include reservoir stimulation, optimized well layout, and optimized production system. Reservoir stimulation is achieved by artificially improving reservoir properties to increase the drainage area; optimized well layout and optimized production system are mainly aimed at the adverse effects of water intrusion during the development of water-drive gas reservoirs. (3) Technologies to improve pressure exhaustion efficiency include drainage gas production, surface pressurization, or other techniques to reduce abandoned production. Drainage gas production enhances well lifting efficiency and increases gas production; surface pressurization reduces abandoned pressure, extending well production time and further utilizing formation energy to improve pressure exhaustion efficiency. Improving digitalization and intelligence levels, and localizing tools and equipment, reduces development costs to decrease abandoned production and pressure, maximizing final gas production. Furthermore, laboratory experiments show that CO2 injection can further improve recovery rates in conventional and condensate gas reservoirs. However, for exhausted gas reservoirs in the later stages of development, effective means to further improve recovery rates are lacking.

[0004] Therefore, it is necessary to provide a method that can efficiently improve the recovery rate of depleted gas reservoirs in the middle and late stages of exploitation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a method for the reuse of depleted gas reservoirs. This invention first achieves efficient recovery of the remaining natural gas in the depleted gas reservoir by injecting flue gas into it. Then, by utilizing the trapping characteristics of the depleted gas reservoir, it leverages the remaining value of the depleted reservoir, improves carbon sequestration efficiency, and thus achieves efficient reuse of the remaining value of the depleted gas reservoir.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] This invention provides a method for reusing depleted gas reservoirs, the method comprising the following steps:

[0008] (1) Natural gas is extracted from the depleted gas reservoir using flue gas to obtain the depleted gas reservoir;

[0009] (2) Flue gas is used to bury the depleted gas reservoir obtained in step (1).

[0010] In this invention, natural gas is first recovered from a depleted gas reservoir by using flue gas, thereby further improving the recovery rate of natural gas in the depleted gas reservoir and obtaining a depleted gas reservoir; then, the depleted gas reservoir is used to store and process the flue gas, thereby realizing the reuse of the remaining value of the depleted gas reservoir.

[0011] As a preferred embodiment of the present invention, the formation pressure of the depleted gas reservoir is ≤10MPa, for example, it can be 10MPa, 9MPa, 8MPa, 7MPa, 6MPa or 5MPa, etc., but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0012] Preferably, the depleted gas reservoir includes a monoclinic gas reservoir or an anticline gas reservoir.

[0013] As a preferred technical solution of the present invention, the concentration of carbon dioxide in the flue gas in steps (1) and (2) is 1-15%, for example, it can be 1%, 3%, 5%, 7%, 9%, 11%, 13% or 15%, etc., but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0014] As a preferred embodiment of the present invention, the natural gas recovery in step (1) includes the following steps:

[0015] (a) Well network conversion and deployment, the well network comprising at least one gas injection well and one production well;

[0016] (b) Inject flue gas into the depleted gas reservoir through injection wells until the N2 content in the gas produced from the production well is >80%, thereby achieving the recovery of natural gas from the depleted reservoir.

[0017] In this invention, by utilizing the structural advantages of the depleted gas reservoir to deploy a well network and continuously inject flue gas to increase formation pressure, the gravity displacement effect of the flue gas can be leveraged to uniformly expand the flue gas swept volume, thereby increasing the natural gas recovery rate by 15-25%. Furthermore, the flue gas used in this invention is waste gas generated from energy, metallurgy, and chemical industries. Compared to injecting gases such as nitrogen or carbon dioxide into the formation, flue gas does not require purification and has advantages such as easy availability and low cost. Moreover, the molecular weights of N2 and CO2 in the flue gas are greater than those of CH4, and the density and viscosity of CO2 are also greater than those of natural gas. Injecting flue gas at relatively low structural locations can prevent flue gas coning, ensuring uniform sweep of the remaining natural gas and thus effectively improving the recovery rate of the remaining natural gas.

[0018] Furthermore, the recovery rate of the depleted gas reservoir obtained after natural gas extraction according to the present invention is higher than 90%, and it has good trapping conditions and burial space of developed reservoirs, providing a place for the geological burial of flue gas, which is conducive to the effective burial of flue gas in step (2).

[0019] As a preferred technical solution of the present invention, when the formation dip angle of the depleted gas reservoir is <3°, the existing well network is fully utilized, and the depleted development well group is deployed as a five-point well network or a nine-point well network.

[0020] As a preferred embodiment of the present invention, when the dip angle of the depleted gas reservoir is ≥3°, at least one injection well is deployed at the lowest position in the updip direction, and production wells are deployed sequentially along the updip direction to form a well network.

[0021] Preferably, the five-point well network includes a central injection well and four production wells surrounding the injection well.

[0022] Preferably, the nine-point well network includes a central injection well and eight production wells surrounding the injection well.

[0023] It is worth noting that the gas reservoirs processed by this invention are depleted gas reservoirs, because the well network conversion and deployment described in step (a) are established on the basis of the original well network. Therefore, the method provided by this invention has the characteristics of low cost.

[0024] In addition, this invention makes full use of the existing well network to implement different deployment methods according to the different dip angles of the gas reservoir structure. In particular, when the dip angle of the formation is ≥3°, gas injection wells are arranged in the low part of the structure and production wells are arranged in the high part of the structure, with the production wells and gas injection wells arranged linearly.

[0025] As a preferred embodiment of the present invention, the injection rate of the flue gas in step (b) is 2 to 3 × 10⁻⁶. 4 Nm 3 / d (N is under standard conditions, at room temperature and pressure), for example, it could be 2×10 4 Nm 3 / d、2.2×10 4 Nm 3 / d、2.4×10 4 Nm 3 / d、2.6×10 4 Nm 3 / d、2.8×10 4 Nm 3 / d or 3×10 4 Nm 3 / d, etc., but not limited to the listed values, other unlisted values ​​within the range also apply.

[0026] Preferably, the injection intensity of the flue gas in step (b) is ≤2000m³. 3 / m, for example, could be 2000m 3 / m、1800m 3 / m、1600m 3 / m、1400m 3 / m、1200m 3 / m or 1000m 3 / m, etc., but not limited to the listed values, other unlisted values ​​within the range also apply.

[0027] Preferably, the injection-production ratio of the flue gas in step (b) is 1.0 to 1.2, for example, it can be 1.0, 1.05, 1.1, 1.15 or 1.2, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0028] This invention ensures stable flue gas injection by controlling the injection rate, injection intensity, and injection-production ratio of flue gas, leveraging the gravity differentiation effect of flue gas to achieve near-piston displacement of CH4 by flue gas, with a 1PV displacement efficiency greater than 85%.

[0029] More specifically, the injection rate of the flue gas is 2 to 3 × 10⁻⁶. 4 Nm 3 / d, if the injection rate is too fast, it will lead to uneven flue gas flow and reduced recovery rate; if the injection rate is too slow, it will lead to longer production cycle and reduced economic benefits.

[0030] The injection intensity of the flue gas is ≤2000m. 3 / m, if the injection intensity is too high, it will lead to uneven flue gas wave and reduced recovery rate;

[0031] The injection-production ratio is 1.0 to 1.2. If the injection-production ratio is too low, it will lead to a decrease in formation energy and a shorter stable production time. Conversely, if the injection-production ratio is too high, it will lead to uneven displacement of flue gas and a decrease in recovery rate.

[0032] As a preferred technical solution of the present invention, the burial treatment in step (2) includes: injecting flue gas into the depleted gas reservoir in step (1) through a gas injection well, and monitoring the formation pressure in real time.

[0033] In this invention, the burial treatment utilizes the gas injection well deployed in step (1) to continuously and rapidly inject flue gas, strictly monitor the formation pressure, and close the gas injection well when the formation pressure reaches the upper limit pressure; during the injection process, CO2 in the flue gas will undergo a series of water-rock reactions with rocks and formation water, ultimately achieving efficient burial of flue gas from depleted gas reservoirs.

[0034] As a preferred technical solution of the present invention, the concentration of carbon dioxide in the flue gas in step (2) is 1 to 15%, for example, it can be 1%, 2%, 4%, 6%, 8% or 10%, etc., but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] As a preferred technical solution of the present invention, the endpoint of the burial treatment in step (2) is: the formation pressure is 1.0 to 1.1 times the original formation pressure, for example, it can be 1.0 times, 1.02 times, 1.04 times, 1.06 times, 1.08 times or 1.1 times, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] In this invention, the natural gas recovery described in step (1) reduces the amount of natural gas in the depleted gas reservoir to obtain a depleted gas reservoir. The depleted gas reservoir itself has strong sealing properties, and the reservoir rock sealing properties are suitable for carbon burial. Furthermore, the flue gas injected in step (1) optimizes the underground gas seepage conditions, increases the movable gas-bearing pore space, and reduces the gas injection resistance. At the same time, the use of low-concentration CO2 flue gas as the burial medium eliminates the need for CO2 capture through ammonia carbon capture technology, significantly reducing energy consumption and costs, and achieving efficient carbon burial.

[0037] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] (1) This invention utilizes the structural advantages of gas reservoirs to deploy a "low injection and high production" well network, which quickly replenishes formation pressure, fully leverages the gravity differentiation effect of flue gas, and expands the flue gas sweep volume; at the same time, it utilizes the depleted gas reservoir trap structure to replace the injected flue gas and achieve carbon burial.

[0040] (2) This invention reuses depleted gas reservoirs. In the early stage, by utilizing the reservoir structure and controlling the gas injection rate, the flue gas is uniformly pushed upward, and the gravity differentiation effect is utilized to uniformly displace natural gas and improve the recovery rate of remaining natural gas by more than 90%. In the later stage, by utilizing the depleted gas reservoir trap structure, the flue gas injection rate is strictly controlled and the formation pressure is monitored to achieve safe and efficient burial of flue gas in the entire gas reservoir.

[0041] (3) The method provided by the present invention injects flue gas into the depleted gas reservoir, takes advantage of the structural advantages and the old well network of the oilfield, requires less investment in surface facilities, and the flue gas is easy to obtain and does not need to be purified into CO2 or N2. The operating cost is low, the recovery rate of the remaining natural gas is greatly improved, and the production benefits are high. By utilizing the trapping characteristics of the depleted gas reservoir, the remaining value of the depleted gas reservoir is brought into play, and the carbon storage efficiency is improved. Detailed Implementation

[0042] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0043] In one specific embodiment, the present invention provides a method for reusing depleted gas reservoirs, the method comprising the following steps:

[0044] (1) Natural gas is extracted from the depleted gas reservoir using flue gas to obtain the depleted gas reservoir;

[0045] The formation pressure of the depleted gas reservoir is ≤10MPa;

[0046] The natural gas recovery includes the following steps: (a) well network conversion and deployment, the well network including at least one injection well and one production well;

[0047] When the stratum dip angle is <3°, make full use of the existing well network and deploy the depleted development well group as a five-point well network or a nine-point well network;

[0048] When the formation dip angle of the depleted gas reservoir is ≥3°, at least one injection well shall be deployed at the lowest position in the updip direction, and production wells shall be deployed sequentially along the updip direction to form a well network.

[0049] The five-point well network includes a central injection well and four production wells surrounding the injection well; the nine-point well network includes a central injection well and eight production wells surrounding the injection well.

[0050] (b) Inject flue gas into the depleted gas reservoir through injection wells until the N2 content in the gas produced from the production well is >80%, thereby achieving the recovery of natural gas in the depleted reservoir.

[0051] The concentration of carbon dioxide in the flue gas is 1-15%, and the injection rate of the flue gas is 2-3 × 10⁻⁶. 4 Nm 3 / d, injection intensity ≤2000m 3 / m, injection-production ratio is 1.0 to 1.2;

[0052] (2) Inject flue gas into the depleted gas reservoir obtained in step (1) through a gas injection well until the formation pressure is 1.0 to 1.1 times the original formation pressure;

[0053] The concentration of carbon dioxide in the flue gas is 1-15%.

[0054] Example 1

[0055] This embodiment provides a method for reusing a depleted gas reservoir, the method comprising the following steps:

[0056] (1) Natural gas is extracted from the depleted gas reservoir using flue gas to obtain the depleted gas reservoir;

[0057] The formation pressure of the depleted gas reservoir is 8 MPa.

[0058] The natural gas recovery includes the following steps: (a) deploying a well network, which includes injection wells and production wells; the formation dip angle of the depleted gas reservoir is 2.68°, and the well network includes at least one five-point well network; the five-point well network includes an injection well located at the center and four production wells surrounding the injection well;

[0059] (b) Inject flue gas into the depleted gas reservoir through injection wells until the N2 content in the gas produced from the production well is 81%, thereby achieving the recovery of natural gas from the depleted reservoir.

[0060] The concentration of the flue gas is 6%, and the injection rate is 2.5 × 10⁻⁶. 4 Nm 3 / d, injection intensity of 1500m 3 / m, injection-production ratio is 1.1;

[0061] (2) Inject flue gas into the depleted gas reservoir obtained in step (1) through a gas injection well until the formation pressure is 1.1 times the original formation pressure;

[0062] The concentration of carbon dioxide in the flue gas is 6%.

[0063] Example 2

[0064] This embodiment provides a method for reusing a depleted gas reservoir, the method comprising the following steps:

[0065] (1) Natural gas is extracted from the depleted gas reservoir using flue gas to obtain the depleted gas reservoir;

[0066] The formation pressure of the depleted gas reservoir is 6 MPa, and its formation dip angle is 3.8°.

[0067] The natural gas recovery includes the following steps: (a) deploying a well network, arranging injection wells in the low structural areas and production wells in the high structural areas, with the production wells and injection wells arranged linearly;

[0068] (b) Inject flue gas into the depleted gas reservoir through injection wells until the N2 content in the gas produced from the production well is >80%, thereby achieving the recovery of natural gas in the depleted reservoir.

[0069] The concentration of carbon dioxide in the flue gas is 10%, and the injection rate of the flue gas is 2 × 10⁻⁶. 4 Nm 3 / d, injection intensity of 1000m 3 / m, injection-production ratio is 1.0;

[0070] (2) Inject flue gas into the depleted gas reservoir obtained in step (1) through a gas injection well until the formation pressure is 1.0 times the original formation pressure;

[0071] The concentration of carbon dioxide in the flue gas is 4%.

[0072] Example 3

[0073] This embodiment provides a method for reusing a depleted gas reservoir, the method comprising the following steps:

[0074] (1) Natural gas is extracted from the depleted gas reservoir using flue gas to obtain the depleted gas reservoir;

[0075] The formation pressure of the depleted gas reservoir is 10 MPa, and its formation dip angle is 3°.

[0076] The natural gas recovery includes the following steps: (a) deploying a well network, arranging injection wells in the low structural areas and production wells in the high structural areas, with the production wells and injection wells arranged linearly;

[0077] (b) Inject flue gas into the depleted gas reservoir through injection wells until the N2 content in the gas produced from the production well is >80%, thereby achieving the recovery of natural gas in the depleted reservoir.

[0078] The concentration of carbon dioxide in the flue gas is 15%, and the injection rate of the flue gas is 3 × 10⁻⁶. 4 Nm 3 / d, injection intensity ≤2000m 3 / m, injection-production ratio is 1.2;

[0079] (2) Inject flue gas into the depleted gas reservoir obtained in step (1) through a gas injection well until the formation pressure is 1.05 times the original formation pressure;

[0080] The concentration of carbon dioxide in the flue gas is 8%.

[0081] A comprehensive analysis of Examples 1-3 shows that the method provided by this invention can not only increase the recovery rate of remaining natural gas by more than 90%, but also utilize the closed structure of the depleted gas reservoir to achieve safe and efficient storage of flue gas in the entire reservoir, thereby maximizing the remaining value of the depleted gas reservoir.

[0082] Example 4

[0083] This embodiment provides a method for reusing depleted gas reservoirs, the only difference between this method and Embodiment 1 is that:

[0084] In this embodiment, the endpoint of injecting flue gas in step (1) is adjusted to: until the N2 content in the gas produced from the production well is 75%.

[0085] Compared with Example 1, this example reduces the N2 content in the gas produced from the production well at the end of natural gas production, resulting in incomplete extraction of natural gas in the original depleted gas reservoir. This leads to a lower recovery rate of the remaining natural gas in the depleted gas field, which in turn affects the subsequent storage of flue gas.

[0086] Example 5

[0087] This embodiment provides a method for reusing depleted gas reservoirs, the only difference between this method and Embodiment 1 is that:

[0088] In this embodiment, the injection rate of the flue gas in step (1) is adjusted to 1.2 × 10⁻⁶. 4 Nm 3 / d.

[0089] Example 6

[0090] This embodiment provides a method for reusing depleted gas reservoirs, the only difference between this method and Embodiment 1 is that:

[0091] In this embodiment, the injection rate of the flue gas in step (1) is adjusted to 3.6 × 10⁻⁶. 4 Nm 3 / d.

[0092] Compared with Example 1, the low flue gas injection rate in Example 5 leads to a longer production cycle and reduced economic benefits; the high flue gas injection rate in Example 6 leads to uneven flue gas distribution, reduced natural gas recovery rate, and consequently affects the subsequent flue gas storage volume.

[0093] Example 7

[0094] This embodiment provides a method for reusing depleted gas reservoirs, the only difference between this method and Embodiment 1 is that:

[0095] In this embodiment, the injection intensity of the flue gas in step (1) is adjusted to 2200 m³ / s. 3 / m.

[0096] Compared with Example 1, the injection intensity of flue gas in this example is higher, which will lead to uneven flue gas sweep and reduced recovery rate, thereby affecting the subsequent storage of flue gas.

[0097] Example 8

[0098] This embodiment provides a method for reusing depleted gas reservoirs, the only difference between this method and Embodiment 1 is that:

[0099] In this embodiment, the injection-production ratio described in step (1) is adjusted to 1:5:1.

[0100] Example 9

[0101] This embodiment provides a method for reusing depleted gas reservoirs, the only difference between this method and Embodiment 1 is that:

[0102] In this embodiment, the injection-to-production ratio described in step (1) is adjusted to 0.9:1.

[0103] Compared with Example 1, the injection-production ratio in Example 8 is too high, which will lead to uneven gas displacement and reduced recovery rate; the injection-production ratio in Example 9 is too low, which will lead to reduced formation energy and shorter stable production time.

[0104] Example 10

[0105] This embodiment provides a method for reusing depleted gas reservoirs, the only difference between this method and Embodiment 1 is that:

[0106] In this embodiment, the endpoint of the burial treatment in step (2) is adjusted to be: the formation pressure is 1.2 times the original formation pressure.

[0107] Example 11

[0108] This embodiment provides a method for reusing depleted gas reservoirs, the only difference between this method and Embodiment 1 is that:

[0109] In this embodiment, the endpoint of the burial treatment in step (2) is adjusted to be: the formation pressure is 0.9 times the original formation pressure.

[0110] Compared with Example 1, the formation pressure in step (2) of Example 10 is too high, which may damage the integrity of the gas reservoir trap and thus cause the gas burial effect; the formation pressure in step (2) of Example 11 is too low, which will lead to a decrease in the final carbon burial amount.

[0111] Comparative Example 1

[0112] This comparative example provides a method for reusing a depleted gas reservoir, the only difference between this method and Example 1 is that:

[0113] This comparative example adjusts the flue gas described in step (1) to carbon dioxide gas.

[0114] Compared to Example 1, when carbon dioxide is used for burial, the burial gas needs to be purified, which greatly increases the burial cost.

[0115] Comparative Example 2

[0116] This comparative example provides a method for reusing a depleted gas reservoir, the only difference between this method and Example 1 is that:

[0117] This comparative example omits the burial process described in step (2).

[0118] Compared with Example 1, this comparative example omits the burial process, which leads to the waste of depleted gas reservoirs and prevents them from realizing their remaining value.

[0119] In summary, this invention achieves efficient recovery of remaining natural gas in depleted gas reservoirs by injecting flue gas into them. Then, by utilizing the trapping characteristics of depleted gas reservoirs, the remaining value of the depleted gas reservoirs is brought into play, carbon storage efficiency is improved, and thus the remaining value of depleted gas reservoirs is efficiently reused.

[0120] The applicant declares that the specific embodiments described above further illustrate the purpose, technical solution and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for reusing a depleted gas reservoir, characterized in that, The reuse process involves the following steps: (1) Natural gas is extracted from the depleted gas reservoir using flue gas to obtain the depleted gas reservoir; (2) Flue gas is used to bury the depleted gas reservoir obtained in step (1).

2. The method for reusing a depleted gas reservoir according to claim 1, characterized in that, The formation pressure of the depleted gas reservoir is ≤10MPa; Preferably, the depleted gas reservoir includes a monoclinic gas reservoir or an anticline gas reservoir.

3. The method for reusing depleted gas reservoirs according to claim 1 or 2, characterized in that, The concentration of carbon dioxide in the flue gas described in steps (1) and (2) is 1-15%.

4. The method for reusing a depleted gas reservoir according to any one of claims 1-3, characterized in that, The natural gas recovery in step (1) includes the following steps: (a) Well network conversion and deployment, the well network comprising at least one gas injection well and one production well; (b) Inject flue gas into the depleted gas reservoir through injection wells until the N2 content in the gas produced from the production well is >80%, thereby achieving the recovery of natural gas from the depleted reservoir.

5. The method for reusing a depleted gas reservoir according to claim 4, characterized in that, When the formation dip angle of the depleted gas reservoir is less than 3°, the existing well network should be fully utilized, and the depleted development well group should be deployed as a five-point well network or a nine-point well network.

6. The method for reusing a depleted gas reservoir according to claim 5, characterized in that, When the formation dip angle of the depleted gas reservoir is ≥3°, at least one injection well shall be deployed at the lowest position in the updip direction, and production wells shall be deployed sequentially along the updip direction to form a well network.

7. The method for reusing a depleted gas reservoir according to claim 5, characterized in that, The five-point well network includes a gas injection well located at the center and four production wells surrounding the gas injection well. Preferably, the nine-point well network includes a central injection well and eight production wells surrounding the injection well.

8. The method for reusing a depleted gas reservoir according to any one of claims 4-6, characterized in that, The injection rate of the flue gas in step (b) is 2 to 3 × 10⁻⁶. 4 Nm 3 / d; Preferably, the injection intensity of the flue gas in step (b) is ≤2000m³. 3 / m; Preferably, the injection-production ratio of the flue gas in step (b) is 1.0 to 1.

2.

9. The method for reusing a depleted gas reservoir according to any one of claims 1-3, characterized in that, The burial treatment in step (2) includes: injecting flue gas into the depleted gas reservoir described in step (1) through a gas injection well, and monitoring the formation pressure in real time.

10. The method for reusing a depleted gas reservoir according to claim 9, characterized in that, The endpoint of the burial treatment in step (2) is that the formation pressure is 1.0 to 1.1 times the original formation pressure.