Optimization method for fire flooding of heavy oil reservoir

Abstract

This invention relates to the field of heavy oil development technology, specifically an optimized method for fire-flooded development of heavy oil reservoirs. The method includes a gas switching stage after the heavy oil reaches a stable combustion state. During this stage, a mixed gas is injected. While maintaining a constant percentage of oxygen in the mixed gas, the percentage of carbon dioxide is gradually increased, while the percentage of nitrogen is correspondingly decreased. This method switches the composition of the mixed gas. The innovative achievement of this invention is a gas injection strategy used after reservoir ignition. This strategy involves switching the composition of the injected gas, gradually adjusting its composition, i.e., while maintaining a constant percentage of oxygen, gradually increasing the proportion of carbon dioxide in the mixed gas, while simultaneously decreasing the proportion of nitrogen. This gas injection strategy significantly reduces or effectively eliminates the problem of ineffective nitrogen circulation within the reservoir.

Description

Technical Field

[0001] This invention relates to the field of heavy oil development technology, and is an optimized method for fire flooding development of heavy oil reservoirs. Background Technology

[0002] Heavy oil reservoirs, due to the high viscosity and poor fluidity of crude oil, present significant challenges to oilfield development due to the low oil-steam ratio, high energy consumption, and large carbon emissions associated with conventional steam injection. Fire-flooding, as an effective means of enhancing oil recovery in heavy oil reservoirs, has been widely used in recent years for the development of ordinary heavy oil and extra-heavy oil reservoirs, but many problems still need to be solved. One such problem is the ineffective nitrogen circulation in traditional air-injection fire-flooding development. That is, in traditional heavy oil reservoir fire-flooding methods, after igniting the oil layer with air, air is continuously injected for further fire-flooding development. Approximately 78% nitrogen and 21% oxygen from the air are injected into the oil reservoir. The oxygen in the air undergoes a series of physicochemical reactions with the heavy oil, producing large amounts of carbon dioxide and small amounts of hydrogen and methane. Nitrogen does not participate in the reaction, and its effect on enhancing oil recovery in heavy oil reservoirs is extremely weak. Nitrogen circulates within the reservoir, occupying injected gas space and consuming injected energy, but it cannot effectively displace crude oil, creating a serious inefficient cycle. Even with high-temperature combustion, the produced gas composition is mainly 12% to 18% carbon dioxide and 78% nitrogen. Furthermore, the produced gas is not currently being fully and rationally utilized, resulting in significant energy waste. This not only increases extraction costs but also contradicts the concept of sustainable development. Simultaneously, in reservoirs after fire flooding, the crude oil within the reservoir layer is almost completely burned off. Although this provides a certain advantage in reservoir sealing, its subsequent utilization value has not been fully explored or rationally planned for a long time, leading to resource idleness.

[0003] Patent application CN114592847A discloses a method for developing heavy oil in thick reservoirs based on combustion-assisted gravity drainage. This method utilizes a production well group consisting of horizontally positioned gas injection wells and horizontally positioned production wells arranged in opposite directions within the reservoir's production area. The wells in the same group are deployed on the same vertical profile, undergoing wet combustion in the vertical direction. Multiple rounds of steam injection and injection are performed between the wells, followed by circulating preheating to achieve thermal interconnection between the two horizontal wells. Oxygen-enriched air is then injected into the gas injection wells and ignited. Once combustion stability is achieved, a mixture of air and water is injected to achieve wet fire flooding in the vertical direction. While this method improves production efficiency through specific well group setup and gas injection methods, the injected gas composition remains primarily conventional air. The problem of ineffective nitrogen circulation is not addressed, resulting in a large amount of nitrogen occupying injection space and consuming energy without effectively displacing crude oil, leading to low gas utilization efficiency and significant energy waste. Furthermore, it does not address the efficient energy conversion after fire flooding or the comprehensive utilization of the reservoir.

[0004] Patent application CN116036817A discloses a method and apparatus for treating associated gas in fire-driven oilfields. The method involves passing the associated gas into a hydrocarbon concentration homogenizer for concentration homogenization, stabilizing the hydrocarbon concentration, mixing it with a certain amount of oxygen-enriched gas, and then conveying it to a regenerative oxidation reactor for regenerative oxidation. The discharged purified gas meets emission standards. This document primarily focuses on the purification of associated gas and ensuring its emission compliance. It does not consider how to efficiently utilize the useful components in the associated gas or optimize gas recycling during the fire-driven process, thus failing to fully exploit gas resources.

[0005] Patent application CN115977584A discloses a method and apparatus for modifying gas storage facilities in heavy oil reservoirs after fire-driven flooding. This includes a three-dimensional spatial distribution analysis of the burned zone in the heavy oil reservoir; analysis of inter-well connectivity across the entire area using gas tracers, production well tail gas composition monitoring, and downhole temperature monitoring; identification of outcrop gas channeling locations within the burned zone and gas channeling pathways outside the burned zone; implementation of gas storage facility sealing and modification; and addition of gas monitoring equipment to all injection and production wells with intact wellbores. While this method addresses the modification of gas storage facilities in reservoirs after fire-driven flooding, it does not connect energy storage and conversion, failing to achieve the rational flow and efficient utilization of energy at different stages. Summary of the Invention

[0006] This invention provides an optimized method for fire flooding development of heavy oil reservoirs, which can significantly reduce the ineffective circulation of nitrogen in the reservoir and improve the recovery rate of heavy oil.

[0007] The technical solution of this invention is achieved through the following measures: an optimized method for fire flooding development of heavy oil reservoirs, comprising:

[0008] Ignition and start-up stage: After air is injected into the heavy oil reservoir from the injection well, ignition is initiated to burn the heavy oil. When the heavy oil reaches a stable combustion state, the gas switching stage begins.

[0009] Gas switching phase: The injected gas in this phase is a mixed gas. The initial injected mixed gas includes nitrogen, oxygen, and carbon dioxide. As time progresses, while keeping the percentage of oxygen in the mixed gas constant at 20% to 21%, the percentage of carbon dioxide in the mixed gas is gradually increased, while the percentage of nitrogen is correspondingly decreased. This method is used to switch the composition of the mixed gas. During the injection process of the mixed gas composition switching, the combustion status in the reservoir and the composition of the produced gas from the production well are continuously monitored. When the percentage of carbon dioxide in the produced gas reaches more than 90%, the mixed gas injected at this time is used as the fixed composition mixed gas, and thereafter, the fixed composition mixed gas is continuously injected into the injection well for fire-driven oil recovery.

[0010] Post-fired oil recovery reservoir gas storage utilization stage: The reservoir area of ​​the post-fired oil recovery reservoir is used as a gas storage facility, and excess nitrogen separated from the air or produced carbon dioxide is injected into the gas storage facility at a set pressure for storage.

[0011] The following are further optimizations and / or improvements to the above-mentioned technical solution:

[0012] Furthermore, during the above ignition, the air injection flow rate is 15 cubic meters per hour to 25 cubic meters per hour per meter of oil layer, and the ignition time lasts for 5 to 15 days.

[0013] Furthermore, during the gas switching phase described above, when initially injecting the mixed gas, the percentage of carbon dioxide in the mixed gas is 10% to 16%, and the percentage of nitrogen in the mixed gas is 60% to 78%.

[0014] Furthermore, in the above gas switching phase, the time interval between each switching of the components of the mixed gas is 5 to 25 days.

[0015] Furthermore, during the aforementioned gas switching phase, when the percentage of carbon dioxide in the produced gas reaches 90% or more, the produced carbon dioxide is collected. At this point, the mixed gas injected into the injection well consists of 20% to 21% oxygen and 75% to 80% carbon dioxide by volume. The fixed-component mixed gas consists of 20% to 21% oxygen and 75% to 80% carbon dioxide by volume. Subsequently, the fixed-component mixed gas is continuously injected into the injection well for fire-driven oil recovery. A portion of the produced carbon dioxide is used to prepare the fixed-component mixed gas required for the injection well, and the remaining carbon dioxide is used for carbon dioxide flooding.

[0016] Furthermore, the aforementioned production wells are equipped with gas composition analyzers.

[0017] The innovative achievement of this invention is a gas injection strategy used after reservoir ignition. The gas injection strategy involves switching the composition of the injected gas, gradually adjusting the composition of the injected gas, that is, while keeping the oxygen percentage constant, gradually increasing the proportion of carbon dioxide in the mixed gas, while simultaneously reducing the proportion of nitrogen in the mixed gas, until the mixed gas injected from the injection well contains no nitrogen or almost no nitrogen. At this point, the gas produced from the production well is more than 90% carbon dioxide. This gas injection strategy significantly reduces the problem of ineffective nitrogen circulation in the reservoir, and can even effectively eliminate the problem of ineffective nitrogen circulation in the reservoir.

[0018] During the gas switching phase, the combustion and oil displacement processes are optimized by gradually increasing the proportion of carbon dioxide in the mixed gas. Carbon dioxide not only participates in complex chemical reactions to drive combustion but also reduces the surface tension of crude oil and improves wettability, greatly enhancing the oil displacement effect. This invention constructs a mixed gas system so that the produced gas is over 90% carbon dioxide, avoiding nitrogen from occupying injection space and consuming energy, thus reducing energy waste. Simultaneously, high-concentration, surplus carbon dioxide can be injected into light oil reservoirs for carbon dioxide-driven oil recovery. This not only reduces carbon emissions but also provides a carbon dioxide source for carbon dioxide-driven oil recovery, aligning with sustainable development principles and effectively mitigating negative environmental impacts.

[0019] Simultaneously, energy storage utilization (such as using it as a gas storage facility) can be carried out on abandoned oil reservoirs after fire flooding. This can revitalize the resources of abandoned oil reservoirs after fire flooding, expand the comprehensive utilization value of abandoned oil reservoirs, and bring new economic growth points to oilfield development. Attached Figure Description

[0020] Appendix Figure 1 This is a schematic diagram illustrating the implementation of the method described in this invention.

[0021] The codes in the attached diagram are as follows: 1 is the injection well, 2 is the production well, 3 is the injected gas, 4 is the produced gas, 5 is the combustion zone, 6 is the transition zone, 7 is the unburned zone, 8 is the plugging zone, 9 is the light oil reservoir, 10 is the carbon dioxide flooding oil recovery, 11 is the surplus gas, and 12 is the released storage gas. Detailed Implementation

[0022] The present invention is not limited to the following embodiments, and the specific implementation can be determined according to the technical solution of the present invention and the actual situation.

[0023] Unless otherwise specified, all percentages in this invention refer to volume percentages.

[0024] Ignition methods include: electric ignition and chemical ignition; electric ignition refers to igniting the fuel layer by inserting an electric igniter; chemical ignition refers to injecting an oxidizer to achieve rapid ignition, such as injecting an oxidizer (or catalyst) to lower the flash point of heavy oil and accelerate the oxidation reaction.

[0025] like Figure 1 As shown, this invention provides an optimized method for fire flooding development of heavy oil reservoirs, comprising:

[0026] Ignition start-up stage: The injected gas 3 in this stage is air. Air is injected into the heavy oil reservoir from injection well 1 at a rate of 15 cubic meters / hour to 25 cubic meters / hour per meter of oil layer. Ignition is then started and continues for 5 to 15 days. After that, the gas injection rate is increased in stages until the gas injection reaches a stable value. Gas injection is then maintained for 10 to 20 days. Once the heavy oil reaches a stable combustion state, the gas switching stage begins.

[0027] During ignition, the air injection rate is 15 cubic meters per hour to 25 cubic meters per hour, which can be 15 cubic meters per hour, 20 cubic meters per hour, 25 cubic meters per hour, or any value between two of these ranges.

[0028] The stable steam injection rate is 35 cubic meters per hour to 45 cubic meters per hour, and can be any value between 35 cubic meters per hour, 40 cubic meters per hour, 45 cubic meters per hour, or any two of these ranges.

[0029] The ignition time can last from 5 to 15 days, specifically 5, 6, 7, ..., 15 days.

[0030] Stable gas injection for 10 to 20 days, which can be 10 days, 11 days, 12 days, ..., 20 days.

[0031] The stable combustion state during the ignition stage is high-temperature combustion, which can be determined by the composition characteristics of the produced gas 4 from well 2. Since the injected gas in this stage is air, the high-temperature combustion means that the carbon dioxide concentration in the produced gas 4 reaches more than 10%, nitrogen more than 78%, oxygen utilization rate greater than 95%, and the high temperature is around 400℃.

[0032] Gas Switching Phase: In this phase, the injected gas 3 is a mixed gas. Initially, the injected mixed gas includes nitrogen, oxygen, and carbon dioxide. At the initial injection, the percentage of carbon dioxide in the mixed gas is 10% to 16%, and the percentage of nitrogen is 60% to 78%. As time progresses, while maintaining the percentage of oxygen in the mixed gas at a constant 20% to 21%, the percentage of carbon dioxide in the mixed gas is gradually increased, while the percentage of nitrogen is correspondingly decreased. This method is used to switch the components of the mixed gas. The time interval for switching the components of the mixed gas is 5 to 15 days. During the component switching injection process... The combustion status of the reservoir and the composition of the produced gas 4 from production well 2 are continuously monitored. When the percentage of carbon dioxide in the produced gas 4 reaches more than 90%, the produced carbon dioxide is collected. At this time, the mixed gas injected into injection well 1 consists of 20% to 21% oxygen and 70% to 80% carbon dioxide by volume. The composition of the fixed component mixed gas is then determined by 20% to 21% oxygen and 70% to 80% carbon dioxide by volume. Subsequently, the fixed component mixed gas is continuously injected into injection well 1 for fire-driven oil recovery. A portion of the produced carbon dioxide is used to prepare the fixed component mixed gas required for injection well 1, and the remaining carbon dioxide is used for carbon dioxide flooding.

[0033] The combustion state of the reservoir can be monitored using existing technologies. For example, it can be determined by the oxygen utilization rate of the gas produced from well 2. The required reservoir combustion state for the gas switching stage is also high-temperature combustion. High-temperature combustion means that the oxygen utilization rate is greater than 95% and the amount of nitrogen produced is more than 80% of the amount of nitrogen injected. The high temperature is also around 400℃.

[0034] During the gas switching process, gradually reducing the percentage of nitrogen injection can simultaneously reduce the space occupied by nitrogen in the reservoir, ultimately significantly reducing the problem of ineffective nitrogen circulation in the reservoir, or even effectively eliminating the problem of ineffective nitrogen circulation in the reservoir.

[0035] Post-fired oilfield gas storage utilization stage: After the fire-driven oil recovery is completed, a large amount of heavy oil in the target reservoir is burned in situ as coke. The reservoir area forms a relatively stable and unique pore structure with good sealing properties. The reservoir area of ​​the post-fired oilfield is used as a gas storage facility. Nitrogen separated from the air or carbon dioxide produced is injected into the gas storage facility as surplus gas at a set pressure for storage. The injected energy can be surplus green electricity. When electricity is needed, the gas well is opened to generate electricity.

[0036] When the reservoir area of ​​the post-fired oil recovery reservoir can be used as a gas storage facility, effective pre-treatment to seal any potential fractures and leakage channels should be carried out according to the specific conditions of the reservoir area. For larger fractures, particulate sealing agents (such as gravel and produced sand) are used; for small pores and leakage channels, gel-type sealing agents (such as acrylonitrile sealing agents) are used. Through effective sealing pre-treatment, the post-fired oil recovery reservoir is ensured to have excellent gas storage conditions. Once the reservoir has met the storage requirements after pre-treatment, gases that meet the storage requirements (such as nitrogen separated from air) are injected into the reservoir area.

[0037] During fire-driven development using this method, the reservoir near the injection well and production well sequentially forms a burned zone 5, a transition zone 6, and an unburned zone 7.

[0038] During the gas switching stage, the oxygen in the mixed gas or fixed-component mixed gas is obtained by separating air. At the same time as separating oxygen, nitrogen can also be separated.

[0039] When storing nitrogen in a gas storage facility, nitrogen separated from the air, besides being used for injection, is used as surplus gas 11 and injected into the reservoir area of ​​the oil reservoir at a set pressure via a gas compressor for storage. During periods of low electricity prices, nitrogen injection continues until the gas storage facility reaches its predetermined storage capacity. When periods of high electricity prices arrive, the stored gas 12 is released, drawing out the nitrogen stored in the gas storage facility and converting its energy into electrical energy via a turbine generator.

[0040] The gas storage tank stores carbon dioxide. The carbon dioxide produced from well 2 is injected into the gas storage tank as surplus gas 11 for sealing. When needed, the stored gas 12 is released, for example, it can be used for carbon dioxide flooding oil production 10.

[0041] The production well is equipped with a gas composition analyzer.

[0042] During the gas switching injection process, the combustion status within the reservoir and the composition of produced gas 4 are continuously and closely monitored. The composition of produced gas 4 (measured by a gas composition analyzer) is analyzed in real time, and the injection ratio of oxygen and carbon dioxide in the mixed gas is flexibly adjusted based on the actual analysis results. For example, if the carbon dioxide content in produced gas 4 is 85%, lower than the expected 90%, the carbon dioxide injection rate can be appropriately increased by 5%; if the oxygen content in produced gas 4 is too high, leading to incomplete combustion, the carbon dioxide injection rate is reduced to ensure stable combustion and that produced gas 4 meets the subsequent oil displacement requirements.

[0043] The present invention will be further described below with reference to embodiments:

[0044] Example 1

[0045] In a certain heavy oil reservoir area, the permeability of the reservoir is relatively high, about 800 millidarcy, the viscosity of crude oil at 50°C is as high as 5000 mPa·s, the oil layer thickness is 12 meters, the porosity is 22.5%, and the oil saturation is 55%.

[0046] Ignition and Start-up Phase: Based on the reservoir size and properties, air is injected into the reservoir at a flow rate of 25 cubic meters per hour per meter of oil layer, using electric ignition. The ignition process lasts for 5 days, with the injection rate gradually increased until it reaches 45 cubic meters per hour per meter of oil layer. After stabilizing the injection for 10 days, the composition of gas 4 from well 2 is monitored to confirm that the reservoir has reached a high-temperature combustion state. At this time, the carbon dioxide concentration of gas 4 produced from well 2 is 13.5%, and the nitrogen concentration is 78%, indicating that the combustion reaction is proceeding normally. Once stable combustion is achieved in the reservoir, the gas switching phase begins.

[0047] Gas switching phase: The flow rate of the mixed gas injected into injection well 1 is 40 cubic meters per hour per meter of oil layer. The initial injected mixed gas includes carbon dioxide, oxygen, and nitrogen. The gas switching process is as follows:

[0048] Step 1: In the initial injection of the mixed gas, the injection flow rate of carbon dioxide is 4 cubic meters / hour per meter of oil layer, the injection flow rate of nitrogen is 28 cubic meters / hour per meter of oil layer, and the injection flow rate of oxygen is always 8 cubic meters / hour per meter of oil layer. The carbon dioxide concentration is gradually increased to more than 15% before proceeding to the next step.

[0049] Step 2: After 5 days, increase the carbon dioxide injection flow rate to 8 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 24 cubic meters per hour per meter of oil layer. Increase the carbon dioxide concentration to over 25% before proceeding to the next step.

[0050] Step 3: After 7 days, increase the carbon dioxide injection flow rate to 12 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 20 cubic meters per hour per meter of oil layer. Increase the carbon dioxide concentration to over 45% before proceeding to the next step.

[0051] Step 4: After 10 days, increase the carbon dioxide injection flow rate to 20 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 12 cubic meters per hour per meter of oil layer. Increase the carbon dioxide concentration to over 55% before proceeding to the next step.

[0052] Step 5: After 15 days, increase the carbon dioxide injection flow rate to 25 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 7 cubic meters per hour per meter of oil layer. Increase the carbon dioxide concentration to over 65% before proceeding to the next step.

[0053] Step 6: After 20 days, increase the carbon dioxide injection flow rate to 30 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 2 cubic meters per hour per meter of oil layer. Increase the carbon dioxide concentration to over 75% before proceeding to the next step.

[0054] Step 7: After 25 days, increase the carbon dioxide injection flow rate to 32 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 0. The produced gas 4 from well 2 contains 90% carbon dioxide. Thereafter, the injection ratio of oxygen and carbon dioxide in the mixed gas will not be adjusted. A fixed-component mixed gas will be injected into well 1 with an oxygen injection flow rate of 8 cubic meters per hour per meter of oil layer and a carbon dioxide injection flow rate of 32 cubic meters per hour per meter of oil layer. More than 90% of the carbon dioxide produced from well 2 will be used to prepare the fixed-component mixed gas and reinjected into well 1. Throughout the gas switching process, the combustion status in the reservoir and the composition of produced gas 4 will be closely monitored.

[0055] If the volume percentage of oxygen in the gas produced from well 2 is consistently below 5%, it indicates complete combustion. If the volume percentage of oxygen is above the safe combustion threshold of 5%, the carbon dioxide concentration (volume percentage) is readjusted until stable combustion is achieved while ensuring the highest carbon dioxide concentration. During this stage, the oil recovery rate is significantly improved, and this embodiment improves it by more than 15% compared to traditional fire-driven development.

[0056] The carbon dioxide produced from well 2 will be reinjected into well 1, and any excess carbon dioxide gas will be collected and stored in a gas storage facility or used directly for carbon dioxide flooding oil recovery.

[0057] Carbon dioxide flooding for oil recovery: In reservoirs at a depth of 3000 meters with a permeability of 50 mD and crude oil viscosity below 100 mPa·s, the carbon dioxide injection pressure is adjusted to 20 MPa, and corrosion-resistant materials are used in the injection pipelines to ensure stable system operation. This significantly improves oil recovery, increasing it by 10% compared to traditional water injection development.

[0058] Post-fire flooding reservoir energy storage and utilization stage: Analysis of the reservoir's internal geological structure and fracture distribution, previous oil recovery rate, and injection status is conducted to estimate the maximum gas storage capacity under reservoir fracture pressure. Wells that experienced gas channeling in the early stages are pre-treated using plugging methods. After pre-treatment, during periods of low electricity prices, leveraging the abundant solar energy resources in the desert region, a solar power station is cleverly integrated into the injection end of injection well 1. Excess electricity is used to inject nitrogen separated from air into the reservoir at a set pressure until the gas storage tank reaches the predetermined storage capacity. During periods of high electricity prices, the power generation equipment is activated to convert the stored nitrogen energy into electrical energy, achieving efficient energy utilization and improving the overall benefits of fire flooding development by 5%.

[0059] Example 2

[0060] A certain heavy oil reservoir has a permeability of 1200 millidarcy, a crude oil viscosity of 3000 mPa·s, an oil layer thickness of 10 meters, a porosity of 25%, and an oil saturation of 60%.

[0061] Ignition and Start-up Phase: Based on the reservoir size and properties, air is injected into the reservoir at a flow rate of 15 cubic meters per hour per meter of oil layer, using electric ignition. The ignition process lasts for 15 days, with the injection rate gradually increased until it reaches 35 cubic meters per hour per meter of oil layer. After 20 days of stable injection, the composition of gas 4 from well 2 is monitored to confirm that the reservoir has reached a high-temperature combustion state. At this point, the carbon dioxide concentration in produced gas 4 is 14%, and the nitrogen concentration is 78%, ensuring that the combustion reaction proceeds normally. Once stable combustion is achieved in the reservoir, the gas switching phase begins.

[0062] Gas switching phase: The flow rate of the mixed gas injected into injection well 1 is 38 cubic meters per hour per meter of oil layer. The initial injected mixed gas includes carbon dioxide, oxygen, and nitrogen. The gas switching process is as follows:

[0063] Step 1: In the initial injection of the mixed gas, the injection flow rate of carbon dioxide is 5 cubic meters / hour per meter of oil layer, the injection flow rate of nitrogen is 25 cubic meters / hour per meter of oil layer, and the injection flow rate of oxygen is always 8 cubic meters / hour per meter of oil layer. The carbon dioxide concentration is gradually increased from before the switch to more than 20% before proceeding to the next step.

[0064] Step 2: After 7 days, increase the carbon dioxide injection flow rate to 15 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 15 cubic meters per hour per meter of oil layer. Increase the carbon dioxide concentration to over 40% before proceeding to the next step.

[0065] Step 3: After 10 days, increase the carbon dioxide injection flow rate to 20 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 10 cubic meters per hour per meter of oil layer. Increase the carbon dioxide concentration to over 55% before proceeding to the next step.

[0066] Step 4: After 15 days, increase the carbon dioxide injection flow rate to 25 cubic meters per hour per meter of oil layer, and simultaneously reduce the nitrogen injection flow rate to 5 cubic meters per hour per meter of oil layer. Increase the carbon dioxide concentration to over 65% before proceeding to the next step.

[0067] Step 5: After 20 days, the carbon dioxide injection flow rate is increased to 30 cubic meters per meter of oil layer, while the nitrogen injection flow rate is simultaneously reduced to 0. The gas composition of well 2 is 90% carbon dioxide. Thereafter, the injection ratio of oxygen and carbon dioxide in the mixed gas is no longer adjusted. A fixed ratio of 8 cubic meters per meter of oil layer oxygen and 30 cubic meters per meter of oil layer carbon dioxide is injected into well 1. The 95% carbon dioxide produced from well 2 is used to prepare the fixed-composition mixed gas and reinjected into well 1. Throughout the gas switching process, the combustion status within the reservoir and the composition of the produced gas are closely monitored. During this stage, the oil recovery rate is significantly improved, increasing by 22% compared to traditional fire-driven development.

[0068] Carbon dioxide flooding for oil recovery: In the light oil reservoir at a depth of 2500 meters, the carbon dioxide injection pressure was adjusted to 18 MPa, and corrosion-resistant materials were used in the injection pipeline to ensure stable system operation. Oil recovery rate was significantly improved, increasing by 9% compared to traditional water injection development.

[0069] Post-fire flooding reservoir energy storage and utilization stage: The excess carbon dioxide produced by well 2 will be sealed in a gas storage facility, further expanding the comprehensive utilization value of the reservoir.

[0070] Comparative Example 1: The difference from Example 2 lies in the gas switching time during the gas switching phase: 3 days later in step 2; 7 days later in step 3; 8 days later in step 4; and 9 days later in step 5. During this phase, the oil recovery rate of Comparative Example 1 was 4% higher than that of conventional fire-driven development, but the oil recovery rate of this comparative example was significantly lower than that of Example 2.

[0071] Comparative Example 2: The difference from Example 1 lies in the gas switching method during the gas switching phase: A mixed gas is initially injected into the injection well. In step 1, the volume ratio of carbon dioxide to nitrogen is 1:9; in step 2, the volume ratio is adjusted to 2:8; in step 3, it is adjusted to 3:7; in step 4, it is adjusted to 4:6; in step 5, it is adjusted to 5:5; in step 6, it is adjusted to 6:4; in step 7, it is adjusted to 7:3; in step 8, it is adjusted to 8:2; in step 9, it is adjusted to 9:1; and in step 10, the nitrogen concentration is adjusted to 0. This phase involves a complex switching mode, increasing costs, and the volume ratio of carbon dioxide to nitrogen in step 1 differs from that in Example 1. The oil recovery rate of this comparative example is 5% higher than that of traditional fire-driven development, significantly lower than that of Example 1.

[0072] As can be seen from the above, the crude oil recovery rate of the embodiments of the present invention is significantly higher than that of the comparative example.

[0073] The above technical features constitute various embodiments of the present invention, which have strong adaptability and implementation effect. Unnecessary technical features can be added or removed according to actual needs to meet the needs of different situations.

Claims

1. An optimized method for fire flooding development of heavy oil reservoirs, characterized in that, include: Ignition and start-up stage: After air is injected into the heavy oil reservoir from the injection well, ignition is initiated to burn the heavy oil. When the heavy oil reaches a stable combustion state, the gas switching stage begins. Gas switching phase: The injected gas in this phase is a mixed gas. The initial injected mixed gas includes nitrogen, oxygen, and carbon dioxide. As time progresses, while keeping the percentage of oxygen in the mixed gas constant at 20% to 21%, the percentage of carbon dioxide in the mixed gas is gradually increased, while the percentage of nitrogen is correspondingly decreased. This method is used to switch the composition of the mixed gas. During the injection process of the mixed gas composition switching, the combustion status in the reservoir and the composition of the produced gas from the production well are continuously monitored. When the percentage of carbon dioxide in the produced gas reaches more than 90%, the mixed gas injected at this time is used as the fixed composition mixed gas, and thereafter, the fixed composition mixed gas is continuously injected into the injection well for fire-driven oil recovery. Post-fired oil recovery gas storage utilization stage: The reservoir area of ​​the post-fired oil recovery reservoir is used as a gas storage facility, and the excess nitrogen separated from the air or the produced carbon dioxide is injected into the gas storage facility at a set pressure for storage.

2. The optimized method for fire flooding development of heavy oil reservoirs according to claim 1, characterized in that, During ignition, the air injection flow rate is 15 cubic meters per hour to 25 cubic meters per hour per meter of oil layer, and the ignition time lasts for 5 to 15 days.

3. The optimized method for fire flooding development of heavy oil reservoirs according to claim 1 or 2, characterized in that, During the gas switching phase, when the mixed gas is initially injected, the percentage of carbon dioxide in the mixed gas is 10% to 16%, and the percentage of nitrogen in the mixed gas is 60% to 78%.

4. The optimized method for fire flooding development of heavy oil reservoirs according to claim 1 or 2, characterized in that, During the gas switching phase, the time interval between each switching of the components of the mixed gas is 5 to 25 days.

5. The optimized method for fire flooding development of heavy oil reservoirs according to claim 3, characterized in that, During the gas switching phase, the time interval between each switching of the components of the mixed gas is 5 to 25 days.

6. The optimized method for fire flooding development of heavy oil reservoirs according to claim 1, 2, or 5, characterized in that, During the gas switching phase, when the percentage of carbon dioxide in the produced gas reaches more than 90%, the produced carbon dioxide is collected. At this time, the mixed gas injected into the injection well consists of 20% to 21% oxygen and 79% to 80% carbon dioxide by volume. The fixed-component mixed gas consists of 20% to 21% oxygen and 79% to 80% carbon dioxide by volume. Thereafter, the fixed-component mixed gas is continuously injected into the injection well for fire-driven oil production. A portion of the produced carbon dioxide is used to prepare the fixed-component mixed gas required for the injection well, and the remaining carbon dioxide is used for carbon dioxide flooding.

7. The optimized method for fire flooding development of heavy oil reservoirs according to claim 3, characterized in that, During the gas switching phase, when the percentage of carbon dioxide in the produced gas reaches more than 90%, the produced carbon dioxide is collected. At this time, the mixed gas injected into the injection well consists of 20% to 21% oxygen and 79% to 80% carbon dioxide by volume. The fixed-component mixed gas consists of 20% to 21% oxygen and 79% to 80% carbon dioxide by volume. Thereafter, the fixed-component mixed gas is continuously injected into the injection well for fire-driven oil production. A portion of the produced carbon dioxide is used to prepare the fixed-component mixed gas required for the injection well, and the remaining carbon dioxide is used for carbon dioxide flooding.

8. The optimized method for fire flooding development of heavy oil reservoirs according to claim 4, characterized in that, During the gas switching phase, when the percentage of carbon dioxide in the produced gas reaches more than 90%, the produced carbon dioxide is collected. At this time, the mixed gas injected into the injection well consists of 20% to 21% oxygen and 79% to 80% carbon dioxide by volume. The fixed-component mixed gas consists of 20% to 21% oxygen and 79% to 80% carbon dioxide by volume. Thereafter, the fixed-component mixed gas is continuously injected into the injection well for fire-driven oil production. A portion of the produced carbon dioxide is used to prepare the fixed-component mixed gas required for the injection well, and the remaining carbon dioxide is used for carbon dioxide flooding.

9. The optimized method for fire flooding development of heavy oil reservoirs according to claim 1, 2, 5, 7, or 8, characterized in that, A gas composition analyzer is installed in the production well.

10. The optimized method for fire flooding development of heavy oil reservoirs according to claim 6, characterized in that, Gas composition analyzers are installed in the production wells.

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