A gas-liquid full-mixed phase flooding method in supercritical state

Abstract

The present application belongs to the technical field of oil and gas development, and particularly relates to a gas-liquid full-mixed-phase oil displacement method under supercritical state. The method comprises the following steps: mixing supercritical gas and injection liquid in a volume ratio of 1:1-4 under the conditions of 1.1-1.3 times of the critical pressure and 1.0-1.2 times of the critical temperature of the supercritical gas, and then injecting the mixture into a pump under the conditions of 20-25 MPa and a flow rate of 3-13 m³ / h, determining the injection rate according to the permeability of the oil reservoir, and injecting the mixture into the oil reservoir through a water injection well or a production well, wherein the injection liquid is obtained by diluting the oil displacement agent with water at a ratio of 1-5 parts by weight. The oil displacement method can significantly improve the oil recovery rate, and the oil displacement recovery rate of low-oil-saturation heavy oil reservoirs is improved by 28% compared with traditional water flooding, the oil displacement recovery rate of low-permeability oil reservoirs is improved by 32% compared with traditional gas flooding, and the waste gas storage rate reaches 82%, which has remarkable environmental benefits.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas development technology, specifically relating to a gas-liquid fully miscible displacement method under supercritical conditions. Background Technology

[0002] As oil extraction enters its mid-to-late stages, the development of reservoirs with low permeability, heavy oil, and those following chemical flooding has become a major challenge for the industry. Traditional oil displacement methods have significant drawbacks: liquid flooding is difficult to penetrate small channels, resulting in low oil displacement efficiency; gas flooding suffers from poor bottom displacement due to its low specific gravity and is prone to gas channeling; thermal flooding, chemical flooding, and other technologies are not only costly but also have problems such as demanding start-up conditions and narrow applicability.

[0003] Existing air-to-thermal miscible flooding technology requires external preheating, such as electric heating or steam, with start-up temperatures exceeding 200°C, resulting in high energy consumption and complex operation. Furthermore, conventional thermal miscible flooding technology requires reservoir oil saturation above 50%, making it unsuitable for depleted reservoirs with low oil saturation (30%–50%), such as high-water-cut old oilfields. In addition, traditional oil displacement technologies generally have displacement blind zones, where large and small pores cannot be simultaneously affected, leading to difficulties in effectively extracting remaining oil. Moreover, the lack of specific wellbore safety designs poses a risk of deflagration. Summary of the Invention

[0004] To address the aforementioned issues, this invention proposes a gas-liquid fully miscible flooding method under supercritical conditions. This method solves the problems of gas channeling, inability to reach small channels, difficulty in starting up reservoirs with low oil saturation, high cost, and low safety associated with traditional flooding technologies. It also takes into account environmental protection requirements and significantly improves crude oil recovery.

[0005] A gas-liquid miscible displacement method under supercritical conditions includes the following steps: Supercritical gas and injection fluid are mixed at a volume ratio of 1:1 to 4, under conditions where the critical pressure of the supercritical gas is 1.1 to 1.3 times and the critical temperature is 1.0 to 1.2 times. The mixture is then injected into a pump at a pressure of 20 MPa to 25 MPa and a flow rate of 3 m³ / h to 13 m³ / h. The injection rate is determined based on the reservoir permeability. The mixture is then pumped into the reservoir through existing water injection wells or production wells at the determined injection rate to drive oil flow. Specifically, after mixing supercritical gas and injection fluid, the mixture is injected into a pump at a pressure of 20 MPa to 25 MPa and a flow rate of 3 m³ / h to 13 m³ / h. The injection rate is determined based on the reservoir permeability. The mixture is then pumped into the reservoir through existing water injection wells or production wells at the determined injection rate to drive oil flow. The injection solution refers to the oil displacement agent diluted with water at a ratio of 1 to 5 parts per ten thousand. The critical pressure of the supercritical gas is more than 1.2 times lower than the original formation pressure of the reservoir, and the critical temperature is more than 5°C lower than the reservoir temperature. These conditions ensure that the supercritical state is stably maintained under reservoir conditions.

[0006] In another preferred embodiment, the supercritical gas is any one of nitrogen, carbon dioxide, or industrial waste gas.

[0007] In another preferred embodiment, the industrial gas refers to the emissions from a power plant or steel plant.

[0008] In another preferred embodiment, the industrial gas is further purified before being mixed with the oil displacement mother liquor to remove solid particles, sulfides and harmful components.

[0009] In another preferred embodiment, the specific process of determining the injection rate based on reservoir permeability is as follows: For reservoirs with permeability below 10 mD, a 3m... 3 Intermittent injection is performed at a total injection rate of / h; For reservoirs with permeability higher than 20 mD, using 9m 3 / h~13m 3 Continuous injection is performed at a total injection rate of / h; For reservoirs with permeability between 10 mD and 20 mD, the injection flow rate is 3 m³ / s. 3 / h~9m 3 Continuous injection is performed at a rate of / h.

[0010] In another preferred embodiment, the oil displacement agent is an oil displacement agent or a bio-enzyme chain-breaking viscosity reducer.

[0011] In another preferred embodiment, when the crude oil viscosity is not less than 500 mp.s, the oil displacement agent is a bio-enzyme chain-breaking viscosity reducer; The injection solution is obtained by diluting the biological enzyme solution with water by 1 to 2 parts per ten thousand.

[0012] By selecting biological enzymes as agents for chain breaking and viscosity reduction, excessively high injection fluid viscosity can lead to a deterioration in the displacement mobility ratio, causing severe viscous fingering and swept volume reduction, thereby significantly reducing displacement efficiency. Simultaneously, it can abnormally increase injection pressure, increasing the risk of formation fracturing and near-wellbore blockage, exacerbating inter-layer conflicts, and potentially causing chemical agent retention damage to the reservoir, ultimately leading to decreased recovery and increased development costs. Therefore, injection fluid viscosity is a key technical factor in ensuring effective oil displacement.

[0013] In another preferred embodiment, when the crude oil viscosity is less than 500 mp.s, the agent is an oil displacement agent; The oil displacement agent is diluted with water by 1 to 5 parts per ten thousand to obtain the injection solution.

[0014] In another preferred embodiment, the nitrogen and carbon dioxide gases have a purity of not less than 95% and a hydrogen sulfide content of ≤20ppm.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention premixes supercritical gas with injection fluid at a volume ratio of 1:1 to 4, ensuring high solubility of the supercritical gas in the injection fluid and forming a fully miscible system with suitable viscosity. This system contains both macromolecular oil displacement mother fluid components and small-molecule supercritical gas molecules, combining the high diffusivity and low viscosity of gas with the high density and strong dissolving power of liquid. It can simultaneously affect both large and small pores, eliminating displacement blind zones. The critical pressure of the selected gas must be at least 1.2 times lower than the original formation pressure of the reservoir, and the critical temperature must be at least 5°C lower than the reservoir temperature to ensure stable supercritical operation under reservoir conditions. Injection is performed using existing wells, requiring no new equipment, and 80% of the gas is sealed underground, achieving carbon capture and storage.

[0016] This invention utilizes industrial waste gas and carbon dioxide gas to fully leverage remaining resources, effectively reducing oil recovery costs and making it more suitable for large-scale production. Through thermodynamic and miscibility effects to eliminate interfacial tension, dissolution and viscosity reduction via supercritical fluids, and multi-effect synergy across all molecular scales, this invention significantly improves oil recovery. For heavy oil reservoirs with low oil saturation, the recovery rate is increased by 28% compared to traditional waterflooding; for low-permeability reservoirs, the recovery rate is increased by 32% compared to traditional gasflooding; and the waste gas sequestration rate reaches 82%, demonstrating significant environmental benefits. Detailed Implementation

[0017] The technical solutions of this invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0018] In the following examples, the oil displacement agent and the bio-enzyme chain-breaking viscosity reducer were both purchased from Beijing Huahui Detection Technology Co., Ltd.

[0019] As oilfield development enters its mid-to-late stages, the exploitation of low-permeability layers, heavy oil reservoirs, and reservoirs following chemical flooding has become a key challenge restricting the industry's sustainable development. Existing conventional technologies have significant shortcomings: liquid flooding systems struggle to effectively penetrate micro- and nano-sized pores, limiting their reach; gas flooding, due to its low density, is prone to gas channeling, leading to uneven displacement and difficulty in utilizing bottom crude oil; while thermal and chemical flooding methods are not only costly but also suffer from bottlenecks such as stringent start-up conditions and limited applicability to specific reservoir types.

[0020] Current air-thermal miscible flooding technology relies on external heat sources such as electric heating or steam preheating to above 200°C for startup, resulting in high energy consumption, complex processes, and typically requiring reservoir oil saturation above 50%. This makes it unsuitable for depleted old oilfields with low oil saturation (30%–50%), especially high-water-cut reservoirs. Furthermore, traditional oil displacement technologies generally suffer from micro- and macro-displacement blind spots, failing to simultaneously utilize large channels and micropores, leading to scattered and difficult-to-recover residual oil. Moreover, they lack specific design considerations for process safety, harboring potential wellbore deflagration risks.

[0021] This invention proposes a gas-liquid fully miscible flooding method based on supercritical state. This method utilizes the unique physical properties of supercritical fluids to achieve full miscibility and full sweep of the gas and liquid phases at the pore scale. It has the comprehensive advantages of high-efficiency oil displacement, safety and controllability and environmental friendliness, providing a new technical path for the efficient development of complex reservoirs.

[0022] The following is a detailed description of a gas-liquid miscible displacement method under supercritical conditions.

[0023] Example 1: A gas-liquid fully miscible oil displacement method under supercritical conditions.

[0024] The reservoir conditions in this embodiment are: oil saturation of 35%, reservoir temperature of 85℃, original formation pressure of 18MPa, and permeability of 15mD. It belongs to the depleted reservoir of a high water-cut old oilfield, i.e., oil displacement of a heavy oil reservoir with low oil saturation. The specific process of using the supercritical gas-liquid fully miscible displacement method for this reservoir is as follows.

[0025] S1. The viscosity of crude oil was measured to be 550 mp.s. A bio-enzyme chain-breaking viscosity reducer was selected as the oil displacement agent.

[0026] S2. Dilute the bio-enzyme chain-breaking viscosity reducer with water to 0.02 parts per ten thousand to obtain the injection solution. Mix the injection solution with 96% pure carbon dioxide at a volume ratio of 1:4 using a supercritical mixing device at a working pressure of 8.8 MPa (1.2 times the critical pressure of carbon dioxide 7.38 MPa) and a working temperature of 35℃ (1.13 times the critical temperature of carbon dioxide 31℃). Inject the mixture into the pump at 25 MPa and a pump flow rate of 8 m³ / h. Determine the injection rate based on the reservoir permeability. For reservoirs with permeability below 10 mD, the total injection flow rate is 3 m³ / h. 3 Intermittent injection is performed at a rate of / h; for permeability higher than 20mD, a 9~13m... 3 / h; the injection flow rate increases linearly with the permeability. In this embodiment, the permeability is 15mD, and the flow rate is 5m... 3 The oil is continuously injected into the reservoir via a pump at an injection rate of / h through the original water injection well to drive oil flow.

[0027] After injection using the above method, the crude oil viscosity decreased by 65%, the recovery rate increased by 28% compared with traditional water drive, there was no gas channeling, and there was no safety risk of residual crude oil in the wellbore.

[0028] Example 2: A gas-liquid fully miscible oil displacement method under supercritical conditions.

[0029] In this embodiment, the reservoir conditions are: oil saturation of 42%, reservoir temperature of 90℃, original formation pressure of 22MPa, and permeability of 30mD, classifying it as a low-permeability heavy oil reservoir. The specific process is as follows: S1. The viscosity of the crude oil was measured to be 450 mp.s, and an oil displacement agent was selected as the oil displacement agent.

[0030] S2. Dilute the oil displacement agent with water to 0.05 parts per ten thousand to obtain the injection liquid. Mix the injection liquid with power plant exhaust gas at a volume ratio of 1:4 using a supercritical mixing device. The mixing conditions are: working pressure 12.1 MPa (1.1 times the critical pressure of the mixed gas, 11 MPa) and working temperature 48℃ (1.07 times the critical temperature of the mixed gas, 45℃). Inject the mixture into the pump at 25 MPa and a pump flow rate of 5 m³ / h. The injection rate is determined based on the reservoir permeability. In this embodiment, the permeability is 30 mD, and the injection rate is 9 m³ / h. 3 The gas is continuously injected into the reservoir through the original injection well via a pump at a rate of / h to drive oil flow. The gas used is purified power plant exhaust gas, which mainly contains carbon dioxide and nitrogen after purification. The specific volume percentages of the components are as follows: N2 77~82%, CO2 18%~23%, SO2 0.01%~0.05% (100~500ppm), NO... x The concentration is 0.015~0.05% (150~500ppm), and the particulate matter content is 300mg / m³~1000mg / m³ (≈0.03~0.08%).

[0031] After injection using the above method, the microporous crude oil is effectively displaced, the sweep efficiency is increased by 40%, the recovery rate is increased by 32% compared with traditional gas drive, the waste gas sequestration rate reaches 82%, and the environmental benefits are significant.

[0032] This invention utilizes industrial waste gas and carbon dioxide gas to fully leverage remaining resources, effectively reducing oil recovery costs and making it more suitable for large-scale production. For heavy oil reservoirs with low oil saturation, the oil recovery rate is increased by 28% compared to traditional waterflooding; for low-permeability oil reservoirs, the oil recovery rate is increased by 32% compared to traditional gasflooding; and the waste gas sequestration rate reaches 82%, demonstrating significant environmental benefits.

[0033] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A gas-liquid miscible displacement method under supercritical conditions, characterized in that, Includes the following steps: Supercritical gas and injection fluid are mixed at a volume ratio of 1:1 to 4, under conditions where the critical pressure of the supercritical gas is 1.1 to 1.3 times and the critical temperature is 1.0 to 1.2 times. The mixture is then injected into a pump at a pressure of 20 MPa to 25 MPa and a flow rate of 3 m³ / h to 13 m³ / h. The injection rate is determined based on the reservoir permeability. The mixture is then injected into the reservoir through the original water injection well or production well at the determined injection rate to drive oil flow. The injection solution is obtained by diluting the oil displacement agent with water by 1 to 5 parts per ten thousand. The critical pressure of the supercritical gas is more than 1.2 times lower than the original formation pressure of the reservoir, and the critical temperature is more than 5°C lower than the reservoir temperature.

2. The gas-liquid miscible displacement method under supercritical conditions according to claim 1, characterized in that, The supercritical gas is any one of nitrogen, carbon dioxide, or industrial waste gas.

3. The gas-liquid miscible displacement method under supercritical conditions according to claim 1, characterized in that, The industrial gases referred to are the emissions from power plants or steel plants.

4. The gas-liquid miscible displacement method under supercritical conditions according to claim 1, characterized in that, Before the industrial gas is mixed with the oil displacement mother liquor, it also includes a purification process to remove solid particles, sulfides and harmful components.

5. The gas-liquid miscible displacement method under supercritical conditions according to claim 1, characterized in that, The specific process for determining the injection rate based on reservoir permeability is as follows: For reservoirs with permeability below 10 mD, a 3m... 3 Intermittent injection is performed at a total injection rate of / h; For reservoirs with permeability higher than 20 mD, a depth of 9~13 m is used. 3 Continuous injection is performed at a total injection rate of / h; For reservoirs with permeability between 10 mD and 20 mD, the injection flow rate is 3 m³ / s. 3 / h~9m 3 Continuous injection is performed at a total injection rate of / h.

6. The gas-liquid miscible displacement method under supercritical conditions according to claim 1, characterized in that, The oil displacement agent is an oil displacement agent or a bio-enzyme chain-breaking viscosity reducer.

7. The gas-liquid miscible displacement method under supercritical conditions according to claim 6, characterized in that, When the crude oil viscosity is not less than 500 mp.s, the oil displacement agent is a biological enzyme chain-breaking viscosity reducer; The injection solution is obtained by diluting the bio-enzyme chain-breaking viscosity reducer with water at a ratio of 1 to 2 parts per ten thousand.

8. The gas-liquid miscible displacement method under supercritical conditions according to claim 6, characterized in that, When the crude oil viscosity is less than 500 mp.s, the oil displacement agent is an oil displacement agent; The oil displacement agent is diluted with water by 1 to 5 parts per ten thousand to obtain the injection solution.

9. The gas-liquid miscible displacement method under supercritical conditions according to claim 2, characterized in that, The purity of the nitrogen and carbon dioxide gases is not less than 95%, and the hydrogen sulfide content is ≤20ppm.