A downhole auxiliary dosing device and method for carbon dioxide huff and puff oil recovery

Abstract

The application belongs to the technical field of oil injection and production, and particularly relates to a downhole auxiliary dosing device for carbon dioxide huff and puff oil production and a method. The device comprises a pipe column and an oil production tree. The pipe column comprises an oil production pipe arranged in a casing. A dosing pipe is arranged in the oil production pipe, and an annular cavity between the two serves as a carbon dioxide conveying passage. The bottom of the dosing pipe is connected to a medicament atomizer. The oil production tree comprises a hanging seal arranged in connection with the pipe column. The hanging seal comprises a first pipe line and a second pipe line. The first pipe line is connected to a medicament heating tank and a dosing pump and communicates with the dosing pipe. The second pipe line is connected to a carbon dioxide plunger pump and communicates with the annular cavity. The above device realizes an injection method in which medicament is atomized in real time downhole and efficiently fused with gaseous or supercritical carbon dioxide, so as to improve the gas-liquid mass transfer efficiency and swept volume and improve the development effect of low-permeability and heavy oil reservoirs.

Description

Technical Field

[0001] This invention belongs to the field of petroleum injection and production technology, specifically relating to a downhole auxiliary chemical dosing device and method for carbon dioxide huff and puff oil production. Background Technology

[0002] Carbon dioxide huff and puff (CO2 huff and puff) technology is an important method for enhancing oil recovery in low-permeability and heavy oil reservoirs. Its core mechanism lies in the fact that carbon dioxide dissolves in crude oil, reducing its viscosity and causing it to expand in volume. Furthermore, it improves crude oil fluidity by extracting lighter components and reducing interfacial tension. However, this technology still faces many challenges in field applications.

[0003] First, due to the inherent heterogeneity of the reservoir, carbon dioxide easily flows along high-permeability channels or fractures, causing the injected gas to break through prematurely and failing to effectively reach low-permeability areas with high oil saturation, thus significantly reducing the huff and puff effect. Second, in conventional carbon dioxide injection methods, the contact between the gas and oil phases is mainly at the macroscopic scale, with a limited gas-liquid mass transfer area. The swelling rate of carbon dioxide in crude oil is slow, making it difficult to achieve the ideal viscosity reduction and expansion effect within the limited well-closing time. This problem is particularly prominent in heavy oil reservoirs.

[0004] To address the aforementioned issues, the industry has been experimenting with co-injecting chemical agents (mainly surfactants, viscosity reducers, oil displacement agents, and temporary plugging agents) during carbon dioxide huff and puff processes, aiming to further improve oil washing efficiency and reduce flow resistance through synergistic effects. Currently, there are three main agent injection methods: Method 1: Injecting a liquid chemical slug into the well from the surface, followed by a carbon dioxide slug. However, the chemical solution often preferentially advances along high-permeability "water-flooded channels" or the bottom high-permeability layer, making it difficult to effectively reach low-permeability zones at the top of the oil layer, edge dead zones, and residual oil in interlayers, ultimately resulting in the awkward situation of "water-flooded areas flooded, oil-rich areas stagnant." Method 2: The agent is usually pre-mixed with liquid carbon dioxide (temperature approximately -18 to -24°C) at the surface and then injected into the wellbore. However, this method has inherent drawbacks: at low temperatures, the agent is prone to crystallization or a sudden increase in viscosity, leading to uneven dispersion. The actual effective concentration of the mixture after injection into the formation is low, and the synergistic effect cannot be fully realized. Method 3: Using nitrogen as a carrier, surfactants are mixed with nitrogen through a high-pressure nozzle at the surface to form an aerosol, which is then injected into the formation, followed by the injection of carbon dioxide. Nitrogen-carried agents have good penetration and gas buoyancy, increasing the contact range between the agents and deep crude oil. Although this method can improve the dispersion of agents, it has two drawbacks: First, nitrogen and crude oil are immiscible and difficult to dissolve in the crude oil. At the same time, it mixes with the subsequently injected carbon dioxide, reducing the carbon dioxide concentration and greatly diminishing the synergistic effect of carbon dioxide and agents swelling crude oil. Second, nitrogen-carried injection requires high construction pressure and a short cycle. Excessive construction pressure can easily cause gas channeling, and the injection of a large amount of agents into the well in a short period of time can lead to excessive saturation of the agents in nitrogen, poor uniformity, and low agent utilization efficiency. Third, it requires additional nitrogen generation equipment and a high-pressure injection system, resulting in large equipment investment, high energy consumption, and limited site space, making it uneconomical and difficult to promote and apply on a large scale. Furthermore, regardless of whether it is slug injection, surface premixing, or nitrogen-carried injection, the mixed fluid is unstable in phase during long-distance wellbore transportation. By the time it reaches the bottom of the well, the mixing uniformity has decreased significantly, the effective utilization rate of the agent is low, and it is difficult to achieve real-time and efficient dispersion and fusion under the high pressure environment at the bottom of the well.

[0005] In summary, existing carbon dioxide huff and puff combined with chemical reagent injection technologies suffer from problems such as low mixing efficiency, poor reagent dispersibility, high operating costs, and insufficient gas channeling suppression capabilities. Summary of the Invention

[0006] The purpose of this invention is to provide an injection device and method that enables real-time atomization of reagents and efficient fusion with gaseous or supercritical carbon dioxide during carbon dioxide huff and puff operations downhole, thereby improving gas-liquid mass transfer efficiency and swept volume, and enhancing the development effect of low-permeability and heavy oil reservoirs.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A carbon dioxide huff and puff oil well auxiliary chemical dosing device includes a tubing string and a Christmas tree, characterized in that: the tubing string includes a production pipe placed inside the casing, a chemical dosing pipe is provided inside the production pipe, and the annular cavity between the two is a carbon dioxide delivery channel; the bottom of the chemical dosing pipe is connected to a chemical atomizer; the Christmas tree includes a suspension seal connected to the tubing string; the suspension seal includes a first pipeline and a second pipeline; the first pipeline is connected to a chemical heating tank and a chemical dosing pump and is in communication with the chemical dosing pipe; the second pipeline is connected to a carbon dioxide plunger pump and is in communication with the annular cavity.

[0008] The additional technical features constituting the above-mentioned carbon dioxide huff and puff oil well auxiliary chemical dosing device also include: —The carbon dioxide transported within the annular cavity in the region of the atomizer is in a supercritical or gaseous state (the temperature of liquid carbon dioxide at ground level is approximately -18 to -24°C, and the pressure is approximately 1.8-2.1 MPa. After being pressurized by a carbon dioxide pump, the liquid carbon dioxide enters the second channel, and as the ground temperature continuously rises, the carbon dioxide changes from a liquid state to a gaseous or supercritical state). The liquid pharmaceutical agent emitted by the atomizer is formed at least in the form of micron-sized droplets or foam.

[0009] —The dosing tube is a double-layered tube with an insulating hollow layer inside the tube body.

[0010] The atomizer includes a cylinder with a one-way valve at the upper part of the cylinder. The top of the cylinder is connected to the end of the dosing tube by a threaded connection, and the bottom of the cylinder is connected to a sucker rod by a threaded connection. A sucker pump is connected to the lower part of the sucker rod. Several injection holes are arranged circumferentially at the bottom of the cylinder. A wedge-shaped backflow groove is provided on the outer edge of the lower end of the cylinder. The outer surface of the cylinder above and below the injection holes and between the backflow groove has a reduced diameter section.

[0011] The above-mentioned chemical heating box is equipped with a temperature sensor, and the dosing pump and carbon dioxide plunger pump are each equipped with an electronic flow meter. The dosing pump and carbon dioxide plunger pump are also equipped with a pressure sensor. The temperature sensor, electronic flow meter and pressure sensor are all electrically connected to the PLC controller.

[0012] This patent also provides a method for auxiliary chemical dosing in carbon dioxide huff and puff oil well production, characterized by being accomplished by the aforementioned auxiliary chemical dosing device for carbon dioxide huff and puff oil well production, and including the following steps. Step 1: Send the tubing string downhole to ensure that the chemical atomizer is located at the target oil layer depth, that is, the chemical atomizer is located in the carbon dioxide gaseous or supercritical state region; Step 2: Start the dosing pump and inject the heated medicine in the medicine heating box into the dosing pipe through the first pipe of the suspended seal; Step 3: Simultaneously start the carbon dioxide plunger pump to inject liquid carbon dioxide into the annular cavity between the dosing pipe and the production pipe through the second pipeline of the production tree; Step four: The agent emitted by the atomizer mixes with carbon dioxide to form a gas-liquid mixed mist (aerosol) or foam state that enters the formation. Step 5: After the well-sealing reaction, remove the suspension seal. Connect the dosing pipe to the polished rod and packing box of the pumping unit, and drive the downhole pump for oil production through the sucker rod.

[0013] The additional technical features constituting the above-mentioned carbon dioxide huff and puff method for downhole auxiliary chemical dosing in oil production also include: The agent includes solutions of surfactants, viscosity reducers, oil displacement agents, and temporary plugging agents, which are heated at a temperature of not less than 60°C in the agent heating chamber.

[0014] The dosing pump injects the agent into the dosing pipe at a rate of 20-140 L / h, while the carbon dioxide plunger pump injects liquid into the annular cavity at a rate of 1-7 t / h.

[0015] —The region of the atomizer within the annular cavity is where liquid carbon dioxide has changed into a gaseous or supercritical state, and the drug within the atomizer is ejected in liquid form.

[0016] Compared with existing technologies, the carbon dioxide huff and puff oil well auxiliary chemical dosing device and method provided by this invention have the following advantages: This patent completely solves the problems of low-temperature crystallization and uneven mixing on the surface. This solution preheats the chemical agent in an independent surface chemical agent heating box and adopts a dual-pipeline distribution method (the first pipeline carries the chemical agent and the second pipeline carries carbon dioxide) and real-time mixing in the well. This avoids contact between the chemical agent and low-temperature carbon dioxide on the surface, eliminating the risk of crystallization blockage from the root and ensuring the activity and effective concentration of the chemical agent when it enters the well bottom. Secondly, it significantly reduces operating energy consumption and equipment costs. This solution abandons the traditional nitrogen-carrying injection process, which requires large-scale nitrogen generation equipment and high-pressure nitrogen injection system. It directly uses the injection pressure of the carbon dioxide plunger pump to disperse the chemical agent in the well. This not only simplifies the surface equipment and significantly reduces energy consumption and land costs, but also overcomes the bottleneck of nitrogen-carrying injection being unable to be promoted on a large scale due to poor economics. Secondly, it significantly improves gas-liquid mass transfer efficiency and contact area. Utilizing the high-temperature, high-pressure environment downhole, carbon dioxide is kept in a gaseous or supercritical state (possessing both high diffusivity and high swelling capacity), and the liquid reagent is broken into micron-sized droplets by a dedicated atomizer at the bottom of the dosing line. Compared to conventional macroscopic gas-liquid mixing, the micron-sized droplets fuse with gaseous or supercritical carbon dioxide downhole to form a gas-liquid mixed mist (aerosol) or foam, exponentially increasing the gas-liquid contact area and greatly enhancing the swelling rate and viscosity-reducing effect of carbon dioxide in crude oil. Furthermore, it effectively suppresses gas channeling and expands the swept volume. The atomized mixed fluid has superior rheological properties and lower transport resistance, allowing it to enter low-permeability pore throats more uniformly, avoiding the fingering and rushing of pure carbon dioxide along high-permeability channels. Simultaneously, the increased seepage resistance of the aerosol fluid in the pores effectively improves the sweep efficiency caused by reservoir heterogeneity, enhancing the utilization of low-permeability and heavy oil reservoirs. Further, it achieves real-time online atomization downhole, avoiding long-distance heat loss. It is independently delivered through the dosing pipe in the production tubing and works in real time with the downhole atomizer. The agent does not need to be premixed on the surface and then transported over a long wellbore. This effectively avoids the viscosity rebound problem caused by heat loss of the mixed fluid along the way in conventional processes, ensuring that the agent performs at its best in the target oil layer.

[0017] This device features a compact structure, strong compatibility, and convenient construction. It only requires adding a chemical dosing pipe to the existing production tubing and modifying the wellhead suspension seal to a dual-channel configuration. No alterations to the existing casing structure or the main Christmas tree are necessary, and it seamlessly integrates with the original well production tubing. Furthermore, this downhole chemical dosing method achieves integrated injection, well shut-in, and oil production operations, simplifying on-site operation and facilitating large-scale deployment. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of a carbon dioxide huff and puff oil well auxiliary chemical dosing device according to the present invention; Figure 2This is a schematic diagram of the chemical atomizer structure of the carbon dioxide huff and puff auxiliary chemical dosing device for oil wells. Detailed Implementation

[0019] like Figure 1 As shown, the structure of this novel downhole auxiliary chemical dosing device includes a tubing string 1 and a tree 2. The tubing string 1 includes a production tubing 11 placed inside the casing 10. A chemical dosing pipe 12 is installed inside the production tubing 11, and the annular cavity 13 between the two is a carbon dioxide delivery channel. The bottom of the chemical dosing pipe 12 is connected to a chemical atomizer 3. The tree 2 includes a suspension seal 4 connected to the tubing string 1. The suspension seal 4 includes a first pipeline 41 and a second pipeline 42. The first pipeline 41 is connected to a chemical heating box 51 and a chemical dosing pump 52 and is connected to the chemical dosing pipe 12. The second pipeline 42 is connected to a carbon dioxide plunger pump 61 and is connected to the annular cavity 13.

[0020] Its working principle is as follows: The core of this device is that the reagent and carbon dioxide are transported independently through dual channels and atomized and mixed in real time at the target layer at the bottom of the well.

[0021] During operation, the surface chemical heating box 51 preheats the chemical agents, which are then pressurized by the dosing pump 52 and enter the dosing pipe 12 through the first pipeline 41 of the suspension seal 4. The chemicals are then transported downwards along the tubing string 1 to the chemical atomizer 3 at the bottom of the well. Simultaneously, the carbon dioxide plunger pump 61 injects liquid carbon dioxide through the second pipeline 42 of the suspension seal 4 into the annular cavity 13 between the production tubing 11 and the dosing pipe 12, allowing it to descend along the annulus to the depth where the chemical atomizer 3 is located.

[0022] The high temperature and pressure environment downhole causes carbon dioxide to change from a liquid state to a gaseous or supercritical state. At this time, the atomizer 3 uses fluid pressure difference or mechanical shearing to break the liquid agent into a mist droplets and spray them into the annular cavity 13. In the annular cavity 13, carbon dioxide collides, shears, and encapsulates the droplets instantaneously. Under high temperature and pressure conditions, the two rapidly fuse to form a gas-liquid mixed mist fluid (aerosol state) or foam system.

[0023] This mixed fluid, thanks to the high diffusivity of gaseous or supercritical carbon dioxide and the extremely large specific surface area of ​​droplets, enters the formation pores under the drive of subsequent injection pressure, achieving synergistic injection of reagents and carbon dioxide.

[0024] In the structure constituting the above-mentioned carbon dioxide huff and puff oil well auxiliary chemical dosing device —Preferredly, the carbon dioxide transported within the region of the atomizer 3 within the annular cavity 13 is in a supercritical or gaseous state, and the drug emitted by the atomizer 3 is at least in the form of micron-sized droplets. This fusion of two specific states maximizes mass transfer efficiency. Supercritical carbon dioxide possesses both high diffusivity and high swelling capacity. When it encounters micron-sized droplets, the gas-liquid contact surface area increases exponentially, allowing the effective components of the drug to rapidly diffuse into the carbon dioxide phase, forming a highly uniform aerosol system and significantly improving the gas-liquid mass transfer rate. Simultaneously, it enhances drug carrying and penetration capabilities. Supercritical fluids have low viscosity and strong penetrating power, serving as a highly diffusive carrier to efficiently carry micron-sized drug droplets deep into low-permeability pores, preventing premature drug retention or adsorption loss. Furthermore, it improves injection rheology. After fusion with gaseous or supercritical carbon dioxide, micron-sized droplets form a stable mist fluid with superior viscosity and suspension stability compared to pure carbon dioxide. During flow, it propels more uniformly, effectively inhibiting gas fingering along high-permeability channels and expanding the macroscopic swept volume. In short, through the synergistic effect of "highly diffusible carrier + micron dispersion", real-time and efficient atomized phase mixing is achieved downhole, while solving the two major bottlenecks of small contact area and gas channeling.

[0025] —Furthermore, the aforementioned dosing pipe 12 is a double-layered pipe 10, with an insulating hollow layer 121 inside the pipe body. This insulating hollow layer 121 effectively blocks heat exchange between the low-temperature carbon dioxide (especially in the initial stage of surface injection) within the annular cavity and the hot agent within the dosing pipe 12, significantly reducing heat loss during long-distance wellbore transport. This ensures the agent maintains a low-viscosity state after heating and reaches the atomizer smoothly, preventing thickening or crystallization due to temperature drop. Simultaneously, it ensures the stability of the atomized particle size; when the agent reaches the atomizer 3, its temperature and viscosity remain stable, providing a stable physical property input to the atomizer 3. This ensures that the droplets formed by breakup remain stable within the micron range, avoiding increased atomized particle size due to viscosity fluctuations and ensuring efficient gas-liquid contact.

[0026] --like Figure 2 As shown, the above-mentioned drug atomizer 3 includes a cylinder 31. A one-way valve 30 is provided at the upper part of the cylinder 31. In this embodiment, the one-way valve 30 includes an upper baffle 301 and a lower baffle 302. A valve port 3a is provided at the center of the upper baffle 301. A conical valve seat 304 connected by a preload spring 303 is provided at the center of the lower baffle 302. Several discharge holes 3b are provided at the edge of the lower baffle 302. The top of the cylinder 31 is connected to the end of the drug dosing pipe 12 by a thread. The bottom of the cylinder 31 is connected to the sucker rod 9 by a thread. The lower part of the sucker rod 9 is connected to a sucker pump. Several injection holes 32 are provided circumferentially at the bottom of the cylinder 31. A wedge-shaped backflow groove 33 is provided at the outer edge of the lower end of the cylinder 31. The outer surface of the cylinder 31 located above and below the injection holes 32 and between the backflow groove 33 is a reduced diameter section 311.

[0027] The aforementioned atomizer 3 employs a combined design of "internal high-pressure ejection of the agent + external high-speed airflow + surface wedge-shaped backflow groove-induced turbulence," forming an externally mixed gas-assisted atomization (or gas-liquid impact atomization) mechanism that disrupts the originally smooth boundary layer. When high-density carbon dioxide flows at high speed over these wedge-shaped steps, strong boundary layer separation occurs, resulting in high-intensity vortices or turbulence outside the injection orifice 32. This turbulent field provides significant aerodynamic shear force, which can significantly overcome the surface tension of the liquid. When the internally high-pressure ejected liquid agent comes into contact with this turbulence, it is rapidly torn and stretched, breaking into tiny droplets. This achieves a high-flow-rate, high-quality atomization effect within a high-pressure, small space using jet impact and external high-speed airflow assistance.

[0028] —As a preferred implementation, the aforementioned agent heating box 51 is equipped with a temperature sensor 71, and the dosing pump 52 and carbon dioxide plunger pump 61 are respectively equipped with an electronic flow meter 72 and a pressure sensor 73. The temperature sensor 71, electronic flow meter 72, and pressure sensor 73 are all electrically connected to the PLC controller 8. This allows for closed-loop real-time control of injection parameters via the PLC. Each sensor collects temperature, pressure, and flow signals in real time and feeds them back to the PLC controller 8. The controller adjusts the pump pressure and pump flow rate according to preset thresholds and downhole conditions, and adjusts the heating power of the heating box according to the agent injection temperature at the suspension seal, forming a closed-loop control of "monitoring-feedback-adjustment" to ensure that the agent temperature, carbon dioxide injection volume, and agent atomizer 3 operating pressure are always within the optimal range. Secondly, to ensure the stability of the atomized particle size, the PLC controller 8 adjusts the surface plunger pump pressure and flow rate based on wellhead pressure and formation temperature conditions to ensure the atomizer... The carbon dioxide in the well section is always maintained in a gaseous, supercritical, or gaseous state, and the injection pressure difference of the reagent is constant, thus stabilizing the micron-level atomization effect. At the same time, the precise ratio of reagent to carbon dioxide is achieved. The electronic flow meters on the dosing pump 52 and the carbon dioxide plunger pump 61 are linked and controlled by PLC. The injection ratio of the two fluids can be adjusted in real time according to the formation absorption capacity to avoid excess waste or insufficient concentration of reagent and improve the effective utilization rate of reagent. The whole system runs automatically through the PLC preset program, without the need for frequent manual adjustment of ground parameters, reducing operation error and labor intensity, while improving the safety and repeatability of the auxiliary dosing process.

[0029] The following is a further detailed explanation of the steps and working principle of the carbon dioxide huff and puff auxiliary chemical dosing method in oil wells, based on the aforementioned dosing device.

[0030] It should be noted that this embodiment is based on the following reservoir conditions: The target oil layer is a low-permeability heavy oil reservoir, with a burial depth of 1500–2000 m, a layer thickness of 8–15 m, an average porosity of 12%–18%, and an average permeability of (1.5–8.0) × 10⁻³ μm², classifying it as a typical low-permeability reservoir. The original formation pressure was 15–20 MPa, and the formation temperature was 65–85℃, but the current formation pressure has decreased to 3–10 MPa. The crude oil's surface degassed viscosity is 150–800 mPa·s (50℃), falling within the range of ordinary to extra-heavy oil. The reservoir lithology is predominantly fine sandstone with a moderate clay mineral content (5%–12%), exhibiting certain water and rate sensitivity. The target well is a vertical or highly deviated well, with perforated intervals concentrated in the middle of the target oil layer, a perforation density of 16–20 holes / m, and a phase angle of 60° or 90°.

[0031] This patented method for downhole chemical dosing in carbon dioxide huff and puff oil production includes the following steps: Step 1: Send the tubing string 1 downhole to ensure that the chemical atomizer 3 is located at the target oil layer depth (approximately 1000 meters deep), that is, the chemical atomizer is located in the carbon dioxide gaseous or supercritical state region (according to the carbon dioxide pressure-temperature phase diagram, when the temperature reaches above 31.4℃ and the carbon dioxide pressure is greater than 0, only the gaseous and supercritical states exist). Step 2: Start the dosing pump 52 and inject the heated agent in the agent heating box 51 into the dosing pipe 12 through the first pipe 41 of the hanging seal 4. The agent includes surfactant, viscosity reducer and / or oil displacement agent solution. Generally, a surfactant solution with a concentration of 0.3% is selected. The agent heating box 51 is heated to 80°C. Step 3: Simultaneously start the carbon dioxide plunger pump to inject liquid or supercritical carbon dioxide into the annular cavity 13 between the dosing pipe 12 and the production pipe 11 through the second pipeline 42 of the production tree 2. Under the above reservoir conditions, the dosing pump 52 injects the reagent into the dosing pipe 12 at a discharge rate of 70-80 L / h, while the carbon dioxide plunger pump injects liquid carbon dioxide into the annular cavity 13 at a discharge rate of 3-5 t / h.

[0032] Step 4: The agent emitted by the agent atomizer 3 mixes with carbon dioxide to form a gas-liquid mixed mist (aerosol) that enters the formation. In this implementation, the chemical agent is a composite surfactant system suitable for the crude oil of the target reservoir (such as an anionic compound viscosity reducer and oil displacement agent). The ground agent heating box 51 preheats the agent to 60-80 ℃. After preheating, the agent viscosity drops to 50-200 mPa·s to ensure pumping and atomization performance.

[0033] During the injection phase, the dosing pump 52 injects the heated reagent into the dosing pipe 12 via the first pipeline 41 at a rate of 80 L / h, which is then delivered to the downhole atomizer. Simultaneously, the carbon dioxide plunger pump is started, injecting liquid carbon dioxide (surface temperature -18 to -24℃) into the annular cavity 13 between the production pipe 11 and the dosing pipe 12 via the second pipeline 42. The carbon dioxide injection rate is set to 4 t / h. As the liquid carbon dioxide descends along the annulus, it is heated by the formation temperature and its own throttling effect, transforming into a supercritical state (pressure ≥ 7.38 MPa, temperature ≥ 31.4 ℃) by the time it reaches the depth of the reagent atomizer 3. This state has a density of 200–800 kg / m³ and a viscosity of 0.03–0.10 mPa·s, exhibiting both high density and low viscosity.

[0034] The atomizer 3 uses the downhole high pressure differential and eddy current effect to break the agent into micron-sized droplets (droplet size 10-100 μm). These droplets collide, shear, and encapsulate with supercritical carbon dioxide in the annular cavity 13, fusing to form a gas-liquid mixed mist fluid (aerosol) system. Subsequently, driven by the injection pressure, the system enters the perforated section and formation pores.

[0035] Step 5: After the well simmering reaction, remove the hanging seal 4, connect the chemical dosing pipe 12 to the polished rod and packing box of the pumping unit, and drive the downhole pumping unit through the sucker rod 9 to carry out oil production operations.

[0036] The total amount of carbon dioxide injected in a single round is determined based on the oil layer thickness and treatment radius, with a designed injection volume of 400-500 tons of liquid carbon dioxide equivalent. The total amount of reagent injected is determined based on a carbon dioxide to chemical agent mass ratio of 1:50-1:60, corresponding to a reagent injection volume of 7-10 tons, and an injection time of 5-6 days. After injection, the well is shut in and allowed to simmer for 3-15 days to allow the supercritical carbon dioxide and micron-sized atomized reagent to fully diffuse, swell, and undergo mass transfer with the crude oil—carbon dioxide dissolves in the crude oil, causing the crude oil volume to expand by 10%-25% and its viscosity to decrease significantly. After the simmering period ends (no less than 240 hours in this example), the suspended seal is removed, and the dosing pipe is connected to the polished rod and packing box of the pumping unit. The downhole pump (pump diameter Φ32mm or Φ38mm) is driven by the solid sucker rod to carry out oil production operations. Initially, low-parameter controlled fluid production is used (pump stroke 3-4.5 m, stroke rate 3-6 times / min), gradually optimizing to normal production parameters.

[0037] Expected Results: Before implementation, the daily oil production was 0.5 t / day. After adopting the above-mentioned equipment and parameters for downhole auxiliary chemical injection in carbon dioxide huff and puff production, the average daily oil production increased to 4.1 t / day, with an effective period of 355 days. The expected single-well, single-cycle oil production increase is 1280 t, with an oil turnover rate (increased oil production / carbon dioxide injection volume) of 2.56–3.2 t / t, and an effectiveness rate exceeding 90%. Compared to conventional surface premixed injection methods, atomized mixed fluid can more uniformly enter low-permeability pores, significantly suppressing carbon dioxide channeling and expanding the swept volume, making it particularly suitable for heavy oil reservoirs with deep burial, low permeability, and difficulties in conventional thermal recovery. Compared to traditional carbon dioxide huff and puff, this well initially increased daily oil production by 40%, decreased water cut by 24%, and extended the effective period by 2 months.

Claims

1. A carbon dioxide huff and puff wellbore auxiliary chemical dosing device, comprising a tubing string and a Christmas tree, characterized in that: The tubing string includes a production tubing placed inside the casing, with a dosing pipe inside the production tubing and an annular cavity between them serving as a carbon dioxide delivery channel. The bottom of the dosing pipe is connected to a chemical atomizer. The tree includes a suspension seal connected to the tubing string. The suspension seal includes a first pipeline and a second pipeline. The first pipeline connects to a chemical heating chamber and a dosing pump and is in communication with the dosing pipe. The second pipeline connects to a carbon dioxide plunger pump and is in communication with the annular cavity.

2. The carbon dioxide huff and puff oil well auxiliary chemical dosing device according to claim 1, characterized in that: The carbon dioxide transported within the annular cavity in the region of the atomizer is in a supercritical state or in a gaseous state, and the drug emitted by the atomizer is at least in the form of micron-sized droplets.

3. The carbon dioxide huff and puff oil well auxiliary chemical dosing device according to claim 1, characterized in that: The dosing tube is a double-layered tube with an insulating hollow layer inside the tube body.

4. The carbon dioxide huff and puff oil well auxiliary chemical dosing device according to claim 1, characterized in that: The atomizer includes a cylinder with a one-way valve at the upper part of the cylinder. The top of the cylinder is connected to the end of the dosing tube by a threaded connection, and the bottom of the cylinder is connected to a sucker rod by a threaded connection. A sucker pump is connected to the lower part of the sucker rod. Several injection holes are arranged circumferentially at the bottom of the cylinder. A wedge-shaped backflow groove is arranged on the outer edge of the lower end of the cylinder. The outer surface of the cylinder above and below the injection holes and between the backflow groove has a reduced diameter section.

5. The carbon dioxide huff and puff oil well auxiliary chemical dosing device according to claim 1, characterized in that: The above-mentioned reagent heating box is equipped with a temperature sensor, and the dosing pump and carbon dioxide plunger pump are respectively equipped with electronic flow meters and pressure sensors. The temperature sensor, electronic flow meter and pressure sensor are all electrically connected to the PLC controller.

6. A method for auxiliary chemical dosing in carbon dioxide huff and puff oil wells, characterized in that: The process, accomplished by the carbon dioxide huff and puff downhole auxiliary chemical dosing device as described in claims 1 to 5, includes the following steps: Step 1: Send the tubing string downhole to ensure that the chemical atomizer is located at the target oil layer depth; Step 2: Start the dosing pump and inject the heated medicine in the medicine heating box into the dosing pipe through the first pipe of the suspended seal; Step 3: Simultaneously start the carbon dioxide plunger pump to inject liquid carbon dioxide into the annular cavity between the dosing pipe and the production pipe through the second pipeline of the production tree; Step four: The agent emitted by the atomizer mixes with carbon dioxide to form a gas-liquid mixed mist or foam that enters the formation. Step 5: After the well-sealing reaction, remove the suspension seal. Connect the dosing pipe to the polished rod and packing box of the pumping unit, and drive the downhole pump for oil production through the sucker rod.

7. The method for auxiliary chemical dosing in carbon dioxide huff and puff oil wells according to claim 6, characterized in that: The agent includes solutions of surfactants, viscosity reducers, oil displacement agents, and temporary plugging agents, which are heated at a temperature of not less than 60°C in the agent heating chamber.

8. A method for auxiliary chemical dosing in carbon dioxide huff and puff oil production wells according to claim 6, characterized in that: The dosing pump injects the agent into the dosing pipe at a rate of 20-140 L / h, while the carbon dioxide plunger pump injects liquid carbon dioxide into the annular cavity at a rate of 1-7 t / h.

9. A method for auxiliary chemical dosing in carbon dioxide huff and puff oil wells according to claim 6, characterized in that: The area of ​​the atomizer within the annular cavity is where liquid carbon dioxide has been transformed into a gaseous or supercritical state (pressure not lower than 7.38 MPa and temperature not lower than 31.4°C), and the drug within the atomizer is sprayed out in liquid form.

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